Foundation Settlement Report
Content
FOUNDATION SETTLEMENT
By SEEMA S M U07CE043 B.TECH IV
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INDEX Sr. No. 1. 2. 3. 4. CONTENTS OBJECTIVE INTRODUCTION ANALYSIS OF FOUNDATION SETTLEMENT CASE STUDIES Pg no. 3 4 14 20
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1. OBJECTIVE:
The objective of the seminar is to understand the sources of foundation settlement, do analysis of settlement of foundations and to find appropriate solutions.
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2. INTRODUCTION 2.1 Foundation Settlement
All foundations settle to some extent as the soil around and beneath them adjusts itself to the loads of the building. Foundations on bedrock settle a negligible amount. Foundations on certain types of clay may settle to an alarming degree, allowing buildings to subside by amounts that are measured in feet or meters. Foundation settlement in most buildings is measured in millimeters or fractions of an inch. If settlement occurs at roughly the same rate from one side of the building to the other, it is called uniform settlement, and no harm is likely to be done to the building. If large amounts of differential settlement occur, in which the various columns and load bearing walls of building settle by substantially different amounts, the frame of the building may become distorted, floors may slope, walls and glass may crack, and doors and windows may refuse to work properly. Accordingly a primary objective in foundation design is to minimize differential settlement by loading the soil in such a way that equal settlement occurs under the various parts of the building. This is not difficult when all parts of the building rest on the same kind of soil, but can become a problem when a building occupies a site with two or more areas of different types of soil with very different load bearing capacities. Most foundation failures are at tributable to excessive differential settlement. Gross failure of a foundation, in which the soil fails completely to support the building, is extremely rare.
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2.2 What Causes Foundation Settlement?
The causes of foundation settlement are rarely due to the design (or under-design) of the structure itself. More commonly, damage is caused as changes occur within the foundation soils that surround and support the structure. The following paragraphs are brief explanations for a few of the more common causes of foundation settlement.
Weak Bearing Soils
Some soils are simply not capable of supporting the weight or bearing pressure exerted by a building's foundation. As a result, the footings press or sink into the soft soils, similar in theory to how a person standing in mud sinks into soft, wet clay. In such cases, footings may be designed to spread the load over the weak soils, thereby reducing potential foundation settlement. However, the majority of settlement problems caused by weak bearing soils occur in residential construction, where the footings are designed based upon general guidelines and not site-specific soil information.
Poor Compaction
Placement of fill soils is common practice in the development of both commercial and residential subdivisions. In general, hilltops are cut down and valleys are filled in order to create buildable lots. Properly placed and compacted fill soils can provide adequate support for foundations. When fill soils are not adequately compacted, they can compress under a foundation load resulting in settlement of the structure.
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Changes in Moisture Content
Extreme changes in moisture content within foundation soils can result in damaging settlement. Excess moisture can saturate foundation soils, which often leads to softening or weakening of clays and silts. The reduced ability of the soil to support the load results in foundation settlement. Increased moisture within foundation soils is often a consequence of poor surface drainage around the structure, leaks in water lines or plumbing, or a raised groundwater table. Soils with high clay contents also have a tendency to shrink with loss of moisture. As clay soils dry out, they shrink or contract, resulting in a general decrease in soil volume. Therefore, settlement damage is often observed in a structure supported on dried-out soil. Drying of foundation soils is commonly caused by extensive droughtlike conditions, maturing trees and vegetation and leaking subfloor heating, ventilation, and air conditioning systems.
Maturing Trees and Vegetation
Maturing trees, bushes and other vegetation in close proximity to a home or building are a common cause of settlement. As trees and other vegetation mature, their demand for water also grows. The root systems continually expand and can draw moisture from the soil beneath the foundation. Again, clay-rich soils shrink as they lose moisture, resulting in settlement of overlying structures. Many home and building owners often state that they did not have a settlement problem until decades after the structure was built. This time frame coincides with the maturation and growth of the trees and vegetation.
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Soil Consolidation
Consolidation occurs when the weight of a structure or newly-placed fill soils compress lower, weak clayey soils. The applied load forces water out of the clay soils allowing the individual soil particles to become more densely spaced. Consolidation results in downward movement or settlement of overlying structures. Settlement caused by consolidation of foundation soils may take weeks, months, or years to be considered "complete."
2.3 Other Causes of Slab Foundation Settlement
Poor soil preparation. Plasticity of clay soil. Poor drainage.
2.4 Structures settle into the ground for many different reasons.
The number one reason structures settle and foundation repair is later needed is because of poor soil preparation when the structure was built. The soil is not prepared before the concrete is poured. The soil, once it is graded, no longer remains at it's NATURAL DENSITY. It is now disturbed, and is loose and porous. The property is graded to level out the lot, sometimes cutting into a hill at one side and filling up the low spots at the other side. The soil is rarely compacted to it's natural density.
What has happened is that the loose un-compacted soil compacted on its own, and the slab settled into the ground. Un-compacted fill dirt will compact when it gets dry or when it gets wet, bringing the structure down with it. Once a Bedrock foundation repair specialist installs supports deeper into the ground to reach undisturbed soil, soil that is naturally compacted at it's natural density, it is then strong enough to support most structures. On loose soil that is un-compacted, even a light small shed will soon settle. There maybe water and moisture inside the trenches before the concrete is poured, and they pour the concrete anyway. It looks good at the time, and they didn't have to spend any time and money digging the wet muddy soil out of their trenches. No damage is done there are cracks and damage. The soil again was not prepared properly before the concrete was poured.
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The number two reason foundations settle is because of the plasticity of clay soils. The clay will swell when it gets wet, and it will shrink when it dries. This swelling and shrinking will work the weight of the structure down into the ground. Over time, the foundation will remain more settled than it has risen. Plasticity is the ability of the soil to shrink and swell. The higher the plasticity of the soil, the more apt the soil is to rise up and then settle down. One side of the structure may have more clays than the other side, and foundation may be sitting on more clay than the neighbouring buildings, which accounts for some structures experiencing more foundation problems than others. The third main reason for foundation settlement is poor drainage. Water must never pond around a foundation. The soil around the foundation must always shed water away from the structure. To fix the foundation you must first fix the drainage. Water is the enemy. It can wash out around foundation supports, but it can also cause the clays to swell and shrink, damaging the foundation. Also, if there is too much moisture in the soil, it can reach a saturated point where it will no longer support the weight above it.
2.5 Factors Influencing Settlement
1. 2. 3. 4. 5. 6. Size of footing Depth of footing Rigidity of footing Tolerable settlement Water table Rate of settlement
2.6 Danger Signs
-Visible signs of cracking or deterioration in the foundation itself - Garage door leaning - Large cracks along brick facing - Exterior window and wall separation - Cracks in interior's wall - Doors out of square that no longer shut tightly
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2.7 SOLUTIONS - How to prevent foundation problems
A foundation/structural engineer and an architect should be involved in the building process from the beginning of construction to protect the future owner. The soil should be properly compacted before allowing any concrete to be poured. The structure should high enough to insure proper water drainage. The trenches should be properly inspected for standing moisture and wet spots, and the wet soil should be removed and the area properly compacted before concrete is installed.
2.8 Foundation Settlement: crack patterns, other evidence
The photograph shows a significant settlement crack in a poured concrete foundation of a new (modular) home. This crack appeared first as a fine hairline crack. A combination of poor site preparation of soils below the building footings (uncompacted fill), portions of footings sitting on bedrock, and nearby blasting led to differential settlement that produced this damage. Also some reinforcing steel may have been omitted from construction of the foundation wall. A settlement crack is more likely to be wider at top than its bottom as the foundation "bends" over a single point (or as one section of footing tips downward from its neighbor), allowing differential settlement; it is possible for a settlement crack to appear fairly uniform however if a foundation breaks vertically and then pursues differential settlement. Settlement cracks need to be separated into initial settlement due to construction or site factors and ongoing settlement due to site factors.
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Settlement cracks are usually wider at the top of the crack, usually continuous, and may occur multiple times in a wall
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Direction of downwards movement: If you draw an imaginary line orthogonal (at a right angle to) the line of a diagonal foundation crack, usually the line you've drawn points to the direction of downwards movement in the wall. However a diagonal crack may also indicate upwards wall-lift such as by frost or expansive clay soils which are more active under one portion of a foundation wall than another. Sink holes and foundation cracking: Settlement cracks may appear at the opposite end of a wall when a reinforced masonry foundation is settling due to presence of a sinkhole (for example in Florida). This condition can be distinguished by observing or measuring the directions of floor or wall slope. Multiple settlement cracks in walls: Multiple cracks of either type (settlement or lifting) may occur in a given area. Usually settlement cracks are visible both outside and inside of the foundation wall if the wall material is exposed to view at all.
2.9 Differential Settlement vs. Uniform Settlement in a Foundation:
(A）Building Before Settlement Occurs（B）Uniform Settlement (C) Differential Settlement
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2.9.1 Differential settlement in a foundation: We use the term "differential settlement" to describe a condition in which one portion of a building foundation is moving down, (or up and down) at a different rate or in a different amount from other portions of the foundation or wall. Differential settlement will damage the foundation or wall by producing (usually vertical, possibly diagonal or stair stepped) cracks and other symptoms of wall movement. The large foundation crack in this poured concrete wall was caused by differential settlement in a new foundation wall. All of this movement occurred during the first 13 months after the home was built.
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2.9.2 Differential settlement in a building slab or supporting piers can produce significant and recurrent cracks in the living area as drywall and trim are torn and dislocated during building movement. This photograph shows a significant crack in an upper floor interior wall which was bending as the center of this home settled downwards. The home had been built over the site of a previous stream bed. Its basement floor slab and supporting piers was settling downwards at its center. There was some additional movement in some of the building perimeter foundation walls, but the most significant settlement was at the center of the basement floor.
2.9.3 Uniform settlement in a foundation: Some buildings may settle so uniformly that the entire building moves down without producing cracking in the building's foundation or walls. For example, while it has a few settlement cracks, the Empress Hotel in Victoria BC, a very large masonry structure built on pilings on what was originally a marsh, has settled rather uniformly down over many decades, so that now visitors enter the hotel's "lobby" on what was originally its second floor.
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2.10 SHRINKING SOIL AND FOUNDATION SETTLEMENT
Concrete slab foundations are supported by the load bearing soils beneath them. When this support is reduced, the foundation adjusts accordingly. In parts of the country where highly expansive clay soils predominate, structural problems are common when the foundations are not constructed to take into consideration changes in the characteristics of the soil. One such situation involved a single-family residence with extensive cracking to interior sheetrock and exterior brick veneer. An engineering evaluation was requested to determine the cause of the cracking. A slab deflection survey was performed to determine the slope and curvature of the foundation. The results showed localized settlement toward the front of the house such that the slab sloped at a gradient of nearly two inches over 10 feet. The observed cracking of the sheetrock and brick veneer was consistent with the measured slopes of the foundation. A large oak tree was located fifteen feet from the foundation. It is known that most trees will project root systems beyond their canopy (drip line) to seek moisture. It was noted from the survey that the region of greatest settlement coincided with the observed tree canopy and the probable extent of the root system. The slab deflections were not indicative of any water leaks near or beneath the foundation. Water leaks will cause excessive swelling of clay soils and ultimately heave the slab upward in the affected area. In this case, however, the slab had not heaved, but had suffered differential settlement at the side of the house where the oak tree was located. The observed differential settlement was caused by subsiding clay soils that shrank over time and no longer provided adequate support to the slab. The culprit was the tree which had, by way of its root systems, drawn water from the soil beneath the slab. Soil contraction because of moisture extraction was expected to continue as long as the tree roots remained viable beneath the slab.
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3. ANALYSIS OF FOUNDATION SETTLEMENT
Total Settlement (St) = Si + Sc + Ss
Eq.(1.1)
Si – Immediate settlement Sc – Consolidation Settlement Ss – Settlement due to secondary consolidation Skempton and Bjerrum
2 Si = 1 Bµ I s q− o
Es
Eq.(1.2)
q0 - Contact pressure at the base of footing B - Least lateral dimension of footing μ - Poisson’s ratio Es - Soil Modulus Is - Shape factor Es - From Stress strain curve obtained from triaxial CU test Janbu, Bjerrum, Kjearnsli
Si = µ 0 µ1q0 B 1− µ 2 Es
Eq.(1.3)
µ 0,and µ 1 are coefficients to be obtained from chart for given depth and width of foundation and depth of hard layer
Settlement of Rectangular foundations on soil layer
The settlement is given by Si = q0B I E Eq.(1.4)
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Dimension less coefficient depending on ν , L/B, H/B
3. 1 SETTLEMENT OF SHALLOW FOUNDATIONS 3. 1.1 Types of foundation settlement
Foundation settlement under load can be classified according to two major types: immediate or elastic, settlement, Se and consolidation settlement, Sc. Elastic settlement of a foundation takes place during or immediately after the construction of the structure .Consolidation settlement is time dependent and takes place as the result of extrusion of the pore water from the void spaces of saturated clayey soils . The total settlement of a foundation is the sum of the elastic settlement and the consolidation settlement . Consolidation settlement comprises two phases: primary consolidation settlement and secondary consolidation settlement. Secondary consolidation settlement occurs after the completion of the primary consolidation that is caused by slippage and reorientation of solid particles under sustained load. Primary consolidation settlement is more significant than secondary settlement in inorganic clays and silty clay soil. However in organic soil, secondary consolidation settlement is more significiant.
3. 1.2 Elastic settlement based on the theory of elasticity
The elastic settlement of a shallow foundation can be estimated by using the theory of elasticity. From Hooke’s law,
H H
Se = ∫ εzdz = 1 ∫ (∆σz – μs∆σx - μs∆σy)dz o Es o
Eq.(1.5)
Se =elastic settlement Es =modulus of elasticity of soil H=thickness of the soil layer μs =Poisson’s ratio of the soil ∆σx , ∆σy, ∆σz = stress increase due to the net applied foundation load in the x,y and z directions, respectively.
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Theoretically, if the foundation is perfectly flexible, the settlement may be expressed as Se = qo(αB’) 1- μs² Is If Es Eq.(1.6)
qo = net applied pressure on the foundation μs = Poisson’s ratio of the soil Es = average modulus of elasticity of soil under the foundation, measured from z=0 to about z=4B B’= B/2 for center of foundation = B for corner of foundation To calculate settlement at the center of foundation, we use m’ = L B Eq.(1.7)
The elastic settlement of a rigid foundation can be estimated as Se (rigid) = 0.93 Se flexible,center) Eq.(1.8)
Due to the non-homogeneous nature of the soil deposits, the magnitude of E may vary with depth. Es = Ʃ Es(i) ∆z
z
Eq.(1.9)
Es = soil modulus of elasticity within a depth Z = H or 5B, whichever is smaller
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3.1.3 Elastic settlement of foundations on saturated clay
Janbu et al proposed an equation for evaluating the average settlement of flexible foundations on saturated clay soils. Se = A1A2 qoB Es A1 is a function of H/B and L/B and A2 is function of Df/B. Eq.(1.10)
3.1.4 Elastic settlement of sandy soil: Use of strain influence factor
The settlement of granular soils can also be evaluated by the use of a semi empirical strain influence proposed by Schmertmann et al (1978). According to this method, the settlement is
Z2
Se = C1 C2 ( q-q )Ʃ Iz ∆z 0 Es
Eq.(1.11)
Iz = strain influence factor C1= a correction factor for the depth of foundation embedment=1-0.5 [q/ ( q-q )] C2 = a correction factor to account for creep in the soil = 1+0.2 log (time in years/0.1) Q= stress at the level of the foundation Q= γ Df For square or circular foundations Iz =0.1 at z = 0 Iz =0.5 at z = z 1= 0.5B and Iz =0 at z = z2 =2B
Similarly, for foundations with L/B>10,
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Iz = 0.2 at z=0 Iz =0.5 at z = z1 =B Iz = 0 at z = z2 =4B Where B = width of the foundation and L=length of the foundation .Values of L/B between 1 and 10 can be interpolated. The use of Eq.(1.11) first requires an evaluation of the approximate variation of the elasticity with depth. This evaluation can be made by using the standard penetration numbers or cone penetration resistances. The soil can be divided several layers to a depth of z = z2 and the elastic settlement can be estimated. The sum of the settlement of all layers equals Se.
3.1.5 Range of material parameters for computing elastic settlement
Several investigators have correlated the values of the modulus of elasticity Es, with field standard penetration number, N60 and the cone penetration resistance, qc.Mitchell and Gardner (1975) compiled a list of these correlations. Schmertmann(1970) indicated that the modulus of elasticity of sand may be given by Es = 8 N60 pa N60 = standard penetration resistance pa = atmospheric pressure =100kN/m² Es = 2qc Eq.(1.13) Eq.(1.12)
Where qc = static cone penetration resistance Schmertmann et al.further suggested that the following correlations may be used with the strain influence factors Es =2.5 qc (for square and circular foundations) Es =3.5 qc (for strip foundations)
3.1.6 PRIMARY SETTLEMENT
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Primary consolidation settlement relationships
As mentioned before, consolidation settlement occurs over time in saturated clayey soils subjected to an increased load caused by construction of the foundation. On the basis of the one-dimensional consolidation settlement, Sc = ∫εz dz Where Εz = vertical strain = ∆е 1+ еo ∆е = change of void ratio = f (σ’o, σ’c and ∆ σ’) Eq.(1.14)
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4. CASE STUDIES:
Geotechnical studies for foundation settlement in alluvial deposits in the City of Rome (Italy) University of Roma “Roma TRE”, Dept. of Geological Science, L.go San Leonardo Murialdo, 1 00146 Rome, Italy Via degli Scaligeri, 29 00164 Rome, Italy Received 14 March 2006; Revised 31 July 2006; Accepted 7 August 2006. Abstract This study identifies units characterized by specific geomechanical behaviours within some Holocene alluvial deposits in the City of Rome. In particular, the highly compressible units, which may be responsible for subsidence and settlement phenomena below urban structures, have been identified. Investigations carried out during this study have interpreted nearly 800 stratigraphic sections and physical– mechanical features of the alluvial deposits of the Tiber and those of its right- and leftbank tributaries. The analysis allowed to define, for each investigated deposit, a representative series subdivided into lithotechnical units. When comparing the stratigraphic and geologic features among the series, a remarkable difference in geomechanical behaviour in the deposits from the Tiber's left- and right-bank tributaries and within the river's deposits themselves could be recognized. A yield hazard zoning for urban structures related to recent alluvial deposits has also been defined. 4.1. Introduction The City of Rome is located in an area where there as been long-term human activity and continuous transformations of the original terrain have been substantial. The hydrographic networks of the Tiber's right and left banks have been modified more than once during historical times; sometimes, it has even been obliterated by urban growth. These changes mean that nowadays, it is very difficult, at least in the historical centre, to recognize the original terrain. Many of Rome's streets run along the ancient courses of the Tiber's tributaries and many buildings are situated on alluvial deposits now buried by anthropogenic debris. Many of these structures overlying alluvium and debris have been damaged by subsidence and seismic wave amplification. The effects are visible in the buildings' uniform or differential settlement. In the past, the alluvial deposits were considered as continuous bodies consisting of clayey–silty and sometimes sandy sediments. Instead, the formations were found to be
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mainly heterogeneous. The geotechnical characterization of these units is mandatory for evaluating the intrinsic geological environment's hazard in urban areas, where the risk can reach very high values. This study has been carried out through the analysis of borehole and geotechnical data from three left-bank tributaries and three right-bank tributaries of the Tiber River. Data for the Tiber's alluvial deposit from various parts of the city were also included. Based on this geotechnical analysis, a subdivision into lithotechnical units has been created for each deposit to make correlations and comparisons among the different deposits. The same level of detail was not possible for all of the stream valleys, since it was not always possible to obtain geotechnical data. Nonetheless, it has always been possible to define a stratigraphic series that would represent the examined deposit by subdividing it into units after stratigraphical and sedimentological observations.
4.2 Geological, geomorphological, and hydrogeological setting in the City of Rome The dominant feature on the Tiber River's right bank is the “Monte Mario” ridge. It lies parallel to the Tiber valley and reaches nearly 140 m a.s.l.; to the south, it is 60 m a.s.l. Monte Mario is made up of marine deposits .The “Monte Vaticano” Unit, (blue clays) affects the whole structural setting of the City of Rome and is remarkably thick, consolidated, and is the aquatard for hydrogeological units of Rome. These marine deposits are exposed at the base of the “Monte Mario” ridge. To the east and west of the ridge “Monte Vaticano” lies at various depths in paleogeomorphic depression in tectonically deformed “bedrock.” Above those marine sequences we can follow the change from marine to coastal, then to continental sedimentation. The continental and coastal facies trace the flow of the Paleotiber, which is parallel to the coast running SE along a continuously subsiding belt, producing conglomerate deposits tens of metres thick. From the Middle Pleistocene ash and other airborne products from an intense volcanic activity of the “Sabatini” and “Albani” volcanic districts reached the area of Rome. The geomorphologic and hydrogeologic setting was deeply modified by this volcanic activity. The Paleotiber's course was deviated arriving to the present-day position within the boundary between the Albani and Sabatini products. Later, the last glacial lowstand was 120 m below today's level. This produced a deepening of the Tiber's course and its tributaries' as they eroded the volcanic deposits first, followed by the continental Paleotiber's deposits then, and later carving deeply into the underlying and well-consolidated “Monte Vaticano” Unit. When the sea level rose, the deep gorges were in-filled with alluvial deposits, which are the object of this paper.
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The succession of these events created the present-day terrain of the City of Rome: the area consists of a central plain, which is the Tiber's alluvial plain; the relief on the river's right bank is the “Monte Mario”–“Monte Vaticano” structural high; at the Tiber's left bank lies an articulated area that, in the historical city centre, corresponds to the “Seven Hills”
Fig. 2. Geological map of Rome
4.3 Methodology This study is predominantly concerning areas where growth of the modern City of Rome has occurred over the last 50 years and where it will presumably developed in the future. Several hundreds of recent drillings carried out from geological–technical investigations were analyzed and interpreted using previous data from published stratigraphies. Initially, each alluvial deposit was subdivided into units based on physical data from stratigraphic descriptions (colour, sedimentological features, organic matter content, consistency, etc.). Then, for each of these subdivisions, the physical–mechanical parameters from laboratory or on-site tests were determined; this gave a representative series for each alluvial deposit by subdividing them into “geotechnical units”.
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4.4 Lithostratigraphic characterization of alluvial deposits at the Tiber River's left bank 4.4.1 Alluvial deposit of the “Fosso di Grotta Perfetta” stream 4.4.1.1 Geographic setting The “Fosso di Grotta Perfetta” stream flows in a secondary valley and is a left-bank Tiber tributary. For its low-energy, marsh-environment characteristics this stream was considered to be a swamp in the past, the area is long and narrow. It is 8.5 km long and 2 km wide. The lower basin of the stream is now almost completely urbanized, the final branch was channeled during historical times. As of today, the area is known for its instabilities for constructions, principally related to settlement and rigid rotation of structures, caused by the soil's bad geomechanical characteristics and inadequate foundations. The examined area follows the Grotta Perfetta stream course for about 3 km, and a tangential band of about 1 km into the valley of the Tiber. 4.4.1.2. Subdivision into depositional units and geotechnical parameters Based on the physical characteristics of the soil, within the alluvial deposits and beneath the anthropic cover, six main units in the central portion of the valley can be distinguished and linked to geotechnical data from nearly 218 undisturbed samples. About 100 new in situ geotechnical tests were performed. The following are descriptions of the analyzed units and their physical–mechanical parameters. Beneath the anthropic cover, from the most recent to the most ancient unit, deposits from the last historical flood can be observed, which are 3–4 m thick, clayey–silty sands with floated anthropic inclusions. The soils are of pyroclastic origin with some tuff element. The geomechanical behaviour is definitely frictional, the cohesion is almost zero; the gravel–sand fraction percentage is higher than in the overlying anthropic cover. Underlying the historical flood deposits are the brown clays.They are silty clays and clayey silts; consistency is medium at the top and gradually decreasing with depth. Water content and Atterberg's values are rather high; the consistency index is medium to low, decreasing with depth, typical of plastic to soft/plastic physical states. Deeper, the first brown–blackish, high organic matter content unit can be observed. Locally, greyish, brown, and/or greenish lenses are present. Geotechnical analysis highlighted particular parameters that are commonly identified with geotechnically poor soils. The dominant feature is the high organic matter content, which reached 15–25% of the total weight. Densities are particularly low, void ratio being sometimes higher than 4.00. Often the moisture content is higher than the liquid limit; the liquidity index is > 1 indicating liquid–plastic states.
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Such characteristics are typical of metastable or sensitive soils, for which stable states are only possible onsite (with confining pressure); whenever they were disturbed by both static and dynamic tension, they lose their strength. In particular, when subjected to dynamic loading, they can produce local collapse phenomena. Shear strength parameters are definitely low and has high compressibility; with values of compression index Cc often more than 2.00. Around − 5 m a.s.l there is a substantial decrease in organic matter which identifies the top of the greenish clays The water content and Liquid Limit, despite still being high, reveal a certain degree of geotechnical improvement compared with the overlying organic clays. This is particularly due to the absence, or minor presence, of organic matter (less than 10% found only in particular horizons). Nevertheless, the shear strength resistance is still low similar to that of the above-mentioned organic clays. Compressibility characteristics reveal compression index Cc values of 0.6–1.5, which, although typical of highly compressible clays, indicate a slight increase in stability compared with the overlying organic clays. Finally, an organic matter-rich unit characterized by physical–mechanical parameters similar to those of the above-mentioned blackish and organic ones is also present. This layer changes from black to grey in colour at depths exceeding 35–40 m indicating a decrease in organic matter and an increase of cohesion where coarse horizons of floated pozzolan fragments are observed. This geotechnical and stratigraphic setting allowed the layer to be subdivided into two sub-units: the upper AO2′ unit and the lower AO2 unit. The AO2′ unit is characterized by parameters similar to that of the AO1 unit. High water content and Atterberg's values were observed, with very low, sensitive and metastable soil characteristics again observable. The AO2 unit instead shows better mechanical parameters. It is also characterized by the presence of diffused floated pozzolan horizons, lens-shaped and decimetre-sized; only locally, these horizons can be up to 1–3 m thick. Their presence improves the geotechnical properties of the clayey unit; the higher permeability allows some local consolidation within the organic clays. The bed of the AO2 unit has geotechnical conditions compared to the AO2′ which is also evident when analyzing the water content, density, and strain values. These characters define the geotechnical parameters representative of the various lithotechnical units in the Holocenic alluvial deposits in the “Fosso di Grotta Perfetta” stream.
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Table 1. Geotechnical parameters summary table of the lithotechnical units identified in the “Grotta Perfetta” deposit
Average thickness (m) Historical floods 3 γn (kN/m3) 16–19 15–19 12–15 12–17 14–16 16–19 WC (%) 32–65 30–93 91– 186 49– 154 42– 184 28–84 LL (%) //–78 43– 123 102– 171 84– 151 63– 164 46– 104 IP (%) NP23 17– 58 33– 93 24– 79 18– 76 17– 65 φ′ C′ (kPa) 0–38 5–35 9–25 3–15 5–25 0–0.3 Cu (kPa)
30– 47 23– 30 13– 22 17– 29 13– 26 25– 30
– 21–74 16–55 7–19 15–33 15–30
Brown clays 7 Organic clays 6
Green clays 8 Organic clays Organic clays 5 15
// indicates no data.
4.5 Lithostratigraphic characterization of the Tiber River alluvial deposit 4.5.1 Introduction When compared with tributary alluvial deposits, there are more studies of the deposits of the River Tiber. These studies are indispensable for individualizing the stratigraphical and evolutionary relationships between the Tiber and its tributaries. The investigated area is located in the Historical Centre and has been characterized by a number of boreholes that often reached down to Pliocenic bedrock. 4.5.2 The stratigraphic series representative of the Tiber River's alluvial deposit and its geomechanical parameters
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Almost all of the studies about the alluvial deposits of the Tiber within the City of Rome agree on both their subdivision and geomechanical characterization. There is also a general agreement on the reconstruction of the depositional evolution of the Tiber during the Holocene. For the Historical Centre, the authors agree on characterization of the following stratigraphical units. Sandy silts and dark-greenish clayey silts; subdivided into two different ones, a silty– clayey (LAV) unit and a sandier one (SLV). S: coarse sands constituting the filling after an erosional phase of the Tiber River. AG: silty clays and greyish clayey silts with thin organic levels, relatively more frequent at an elevation of − 15 a.s.l. SLG: silty sands, grey sandy silts, and coarse sands. G: base gravel: mainly made up of carbonate clasts from the erosional phase. Our analysis corresponds to the above studies, even though the data come from different areas. Summary table of the principal geomechanical parameters representative of the units observed in the Tiber's alluvial deposits
Units Average γn thickness (kN/m3) (m) Greenish clayey silts Greenish silty sands Medium sands Grayish silty clays Grayish silty sands 7 7 8 23 15 18–20 17–20 17–20 15–20 18.5–20 Wn (%) LL (%) IP φ (% ) 12– 18– 41 25 8– 26 // 27– 35 30– 40 C′ (kPa) 15–30 0–20 0–10 5–25 0–15
LAV SLV S AG SLG
20–0 43–71 20– 40 20– 30 20– 60 20– 30 26–65 ND– 27 34–93 24–46
11– 15– 59 26 //– 22 25– 35
// indicates no data.
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The left-bank deposits show physical–mechanical parameters remarkably poorer than those from the right bank and from the River Tiber itself. The differences in shear resistance between the left-bank and right-bank deposits are in the graphs compare three CPTs on the three deposits we examined.
The plasticity conditions can be compared by examining the Casagrande's Plasticity Chart. It highlights the high plasticity of the organic silty deposits from the “Grotta Perfetta” stream. The deposits of the Tiber and of its right-bank streams show normal activity indices (A = 0.4–0.8), whereas the left-bank deposits show higher values (A = 0.8–2), typical of “high-activity” soils. Prominent for the objective of this study is the remarkable deformability of the leftbank deposits when compared with the right bank and the deposits of the Tiber. The deformational features are here compared with three representative oedometric graphs. The organic pelitic–silty deposits of the “Fosso di Grotta Perfetta” stream clearly show a very high compressibility whose natural void index reaches very high values (e0 = 1.5–2.5). Thus, these normally consolidated alluvial deposits show a relatively higher deformability when associated with organic sediments, which are distributed throughout the left-bank deposits. The pelitic–silty beds in particular are potentially prone to subsidence and settlement if loaded by landfill. The Tiber's deposits and the right-bank deposits instead show fairly similar and comparable deformability characteristics.
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4.6 Conclusions The analysis of nearly 800 stratigraphic logs and a number of laboratory tests gave the opportunity to subdivide the alluvial deposits of the Tiber and its tributaries into depositional units and to define their geomechanical characteristics. This division into units is very important for evaluating the hazard in different areas of the territory. Such hazards are derived from the fact that alluvial deposits are subject to seismic amplification and the structure of organic-rich units are more ‘sensitive’ and can be affected by phenomena similar to the liquefaction of silty sands below the water table. Such sediments are mostly underconsolidated, hence, more subjected to settlement. Through data derived some alluvial deposits in the City of Rome (including those of the Tiber), it is possible to identify areas potentially at risk from instabilities caused by the presence of organic matter. Units with high organic matter content, widely dispersed and several metres thick, are mainly within a portion of the stream with shallow gradients. The Tiber's deposits only show thin layers of organic matter and generally have better geotechnical characteristics than deposits underlying the leftbank tributaries. All the deposits that have been subjected to loading by backfills and buildings in the historical centre over thousands of years have completed their consolidation process and are no longer affected by subsidence. Areas with significant organic matter content on the left bank are subsiding now, whereas the right-bank alluvial deposits are no longer subsiding.
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Case study: Reinforced Earth retaining wall approaches Thursday, June 10, 2010, 11:03 Hrs [IST]
Dhananjoy Das and Saikat Chatterjee present an interesting case study of how the highly-flexible Reinforced Earth technology was successfully deployed on a bridge over a canal in Kolkata. Marshy land had made construction difficult. A bridge was to be built over Bagjola Canal to meet the increasing traffic and developmental demands. The banks of the canal were made of filled up soil on a low and marshy lands. For PWD, the construction of main bridge structure was not so critical due to its pile foundations. Whereas, it was hard to construct its approaches in such a marshy land with 'N' value ranging from 2-3 only, up to a depth of 15m from ground level in most of the places. Conventional RCC retaining wall was not a feasible solution in such ground condition. Therefore, the owner of the project opted for highly flexible Reinforced Earth Retaining Wall and the work was awarded to Reinforced Earth India. The solution: It was a challenging engineering task to build Reinforced Earth Walls under such extreme weak ground conditions with very low bearing capacity to support the load of the structure. Therefore, it was imperative to improve the foundation soil to withstand the load to be imposed from the wall. PVD (prefabricated vertical drain) in combination with stage construction preloading was selected as a best possible solution at that location followed by two layers of PET uniaxial geogrids to improve the foundation soil and ensure the safety of the wall under global stability. Proper instrumentation and stage construction were done to monitor settlement and development of additional pore water pressure with each stage of construction and subsequent dissipation during pause period.
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Owner: Main contractor: Construction stage: RE wall area: Engineering Data Facing: Connection type: Reinforcement type: Fill soil properties: Foundation type: Foundation system:
West Bengal PWD Mackintosh Burn Ltd April 2005 to November 2007 5,062 sq. m (as built) 1.5mx1.5m Cruiciform Terraclass panels Galvanised tie strips High Adherence (HA) galvanised steel strips Phi=32 deg., ?min=15KN/m3, ?max=17KN/m3 N' value = 2-3 up to depth 15m Prefabricated vertical drain
PVD work: PVD had been inserted up to a depth of 15m (average), in 1.5mx1.5m square grids, below a depth of 2.0m from existing ground level. The job was done at a very fast pace and the overall PVD work at an approximate plan area of 7,000 sq. metres, had been completed within two weeks. Instrumentation: Four settlement plates and ten Piezometric tubes had been installed at both the approaches to measure the periodical settlement and pore water pressure respectively. Stage construction: Stage construction technique had been adopted to stabilise the foundation soil with every stage of construction by allowing dissipation of pore water through PVD along with very sophisticated sub-surface drainage system. As the wall height was approximately 9.0m, three stages of rest periods had been considered to achieve
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the maximum settlement in stages. At 4.0m, 7.0m and 9.0m, average rest periods of 45 days each had been provided. The gain in shear strength of foundation soil at each stage of construction allowed construction of next stage of RE wall. During this rest periods instruments were monitored at regular basis, and the next stage of construction started after reaching the desired settlement and dissipation of pore water pressure. Below is one example of settlement and piezometer monitoring report at one of the locations. Notable observations: A maximum 319mm settlement had been observed; differential settlements had been observed varying from 99mm to 319mm; and post-construction settlements had also been observed in some places.
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REFERENCES
1.) Braja M. Das, Principles of Foundation Engineering, Thomson, India Edition. 2.) Website: http://www.sciencedirect.com/ 3.) Geotechnical studies for foundation settlement in Holocenic alluvial deposits in
the City of Rome (Italy) Maria Paola Campolunghi Giuseppe Capelli, Renato Funiciello and Maurizio Lanzini University of Roma “Roma TRE”, Dept. of Geological Science, L.go San Leonardo Murialdo. 4.) Soil mechanics and foundation engineering, vol. 43, no. 1, 2006 Settlement analysis based on recommendation of building code 50-101-2004 V. A. Barvashov. 5.) Settlement analysis of foundations.Willmore, M J Proc lnt Conference on Foundations and Tunnels, London, arch 1987 VI, P25-29. Publ Edinburgh: Engineering Technics Press. 1987
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By SEEMA S M U07CE043 B.TECH IV
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INDEX Sr. No. 1. 2. 3. 4. CONTENTS OBJECTIVE INTRODUCTION ANALYSIS OF FOUNDATION SETTLEMENT CASE STUDIES Pg no. 3 4 14 20
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1. OBJECTIVE:
The objective of the seminar is to understand the sources of foundation settlement, do analysis of settlement of foundations and to find appropriate solutions.
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2. INTRODUCTION 2.1 Foundation Settlement
All foundations settle to some extent as the soil around and beneath them adjusts itself to the loads of the building. Foundations on bedrock settle a negligible amount. Foundations on certain types of clay may settle to an alarming degree, allowing buildings to subside by amounts that are measured in feet or meters. Foundation settlement in most buildings is measured in millimeters or fractions of an inch. If settlement occurs at roughly the same rate from one side of the building to the other, it is called uniform settlement, and no harm is likely to be done to the building. If large amounts of differential settlement occur, in which the various columns and load bearing walls of building settle by substantially different amounts, the frame of the building may become distorted, floors may slope, walls and glass may crack, and doors and windows may refuse to work properly. Accordingly a primary objective in foundation design is to minimize differential settlement by loading the soil in such a way that equal settlement occurs under the various parts of the building. This is not difficult when all parts of the building rest on the same kind of soil, but can become a problem when a building occupies a site with two or more areas of different types of soil with very different load bearing capacities. Most foundation failures are at tributable to excessive differential settlement. Gross failure of a foundation, in which the soil fails completely to support the building, is extremely rare.
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2.2 What Causes Foundation Settlement?
The causes of foundation settlement are rarely due to the design (or under-design) of the structure itself. More commonly, damage is caused as changes occur within the foundation soils that surround and support the structure. The following paragraphs are brief explanations for a few of the more common causes of foundation settlement.
Weak Bearing Soils
Some soils are simply not capable of supporting the weight or bearing pressure exerted by a building's foundation. As a result, the footings press or sink into the soft soils, similar in theory to how a person standing in mud sinks into soft, wet clay. In such cases, footings may be designed to spread the load over the weak soils, thereby reducing potential foundation settlement. However, the majority of settlement problems caused by weak bearing soils occur in residential construction, where the footings are designed based upon general guidelines and not site-specific soil information.
Poor Compaction
Placement of fill soils is common practice in the development of both commercial and residential subdivisions. In general, hilltops are cut down and valleys are filled in order to create buildable lots. Properly placed and compacted fill soils can provide adequate support for foundations. When fill soils are not adequately compacted, they can compress under a foundation load resulting in settlement of the structure.
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Changes in Moisture Content
Extreme changes in moisture content within foundation soils can result in damaging settlement. Excess moisture can saturate foundation soils, which often leads to softening or weakening of clays and silts. The reduced ability of the soil to support the load results in foundation settlement. Increased moisture within foundation soils is often a consequence of poor surface drainage around the structure, leaks in water lines or plumbing, or a raised groundwater table. Soils with high clay contents also have a tendency to shrink with loss of moisture. As clay soils dry out, they shrink or contract, resulting in a general decrease in soil volume. Therefore, settlement damage is often observed in a structure supported on dried-out soil. Drying of foundation soils is commonly caused by extensive droughtlike conditions, maturing trees and vegetation and leaking subfloor heating, ventilation, and air conditioning systems.
Maturing Trees and Vegetation
Maturing trees, bushes and other vegetation in close proximity to a home or building are a common cause of settlement. As trees and other vegetation mature, their demand for water also grows. The root systems continually expand and can draw moisture from the soil beneath the foundation. Again, clay-rich soils shrink as they lose moisture, resulting in settlement of overlying structures. Many home and building owners often state that they did not have a settlement problem until decades after the structure was built. This time frame coincides with the maturation and growth of the trees and vegetation.
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Soil Consolidation
Consolidation occurs when the weight of a structure or newly-placed fill soils compress lower, weak clayey soils. The applied load forces water out of the clay soils allowing the individual soil particles to become more densely spaced. Consolidation results in downward movement or settlement of overlying structures. Settlement caused by consolidation of foundation soils may take weeks, months, or years to be considered "complete."
2.3 Other Causes of Slab Foundation Settlement
Poor soil preparation. Plasticity of clay soil. Poor drainage.
2.4 Structures settle into the ground for many different reasons.
The number one reason structures settle and foundation repair is later needed is because of poor soil preparation when the structure was built. The soil is not prepared before the concrete is poured. The soil, once it is graded, no longer remains at it's NATURAL DENSITY. It is now disturbed, and is loose and porous. The property is graded to level out the lot, sometimes cutting into a hill at one side and filling up the low spots at the other side. The soil is rarely compacted to it's natural density.
What has happened is that the loose un-compacted soil compacted on its own, and the slab settled into the ground. Un-compacted fill dirt will compact when it gets dry or when it gets wet, bringing the structure down with it. Once a Bedrock foundation repair specialist installs supports deeper into the ground to reach undisturbed soil, soil that is naturally compacted at it's natural density, it is then strong enough to support most structures. On loose soil that is un-compacted, even a light small shed will soon settle. There maybe water and moisture inside the trenches before the concrete is poured, and they pour the concrete anyway. It looks good at the time, and they didn't have to spend any time and money digging the wet muddy soil out of their trenches. No damage is done there are cracks and damage. The soil again was not prepared properly before the concrete was poured.
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The number two reason foundations settle is because of the plasticity of clay soils. The clay will swell when it gets wet, and it will shrink when it dries. This swelling and shrinking will work the weight of the structure down into the ground. Over time, the foundation will remain more settled than it has risen. Plasticity is the ability of the soil to shrink and swell. The higher the plasticity of the soil, the more apt the soil is to rise up and then settle down. One side of the structure may have more clays than the other side, and foundation may be sitting on more clay than the neighbouring buildings, which accounts for some structures experiencing more foundation problems than others. The third main reason for foundation settlement is poor drainage. Water must never pond around a foundation. The soil around the foundation must always shed water away from the structure. To fix the foundation you must first fix the drainage. Water is the enemy. It can wash out around foundation supports, but it can also cause the clays to swell and shrink, damaging the foundation. Also, if there is too much moisture in the soil, it can reach a saturated point where it will no longer support the weight above it.
2.5 Factors Influencing Settlement
1. 2. 3. 4. 5. 6. Size of footing Depth of footing Rigidity of footing Tolerable settlement Water table Rate of settlement
2.6 Danger Signs
-Visible signs of cracking or deterioration in the foundation itself - Garage door leaning - Large cracks along brick facing - Exterior window and wall separation - Cracks in interior's wall - Doors out of square that no longer shut tightly
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2.7 SOLUTIONS - How to prevent foundation problems
A foundation/structural engineer and an architect should be involved in the building process from the beginning of construction to protect the future owner. The soil should be properly compacted before allowing any concrete to be poured. The structure should high enough to insure proper water drainage. The trenches should be properly inspected for standing moisture and wet spots, and the wet soil should be removed and the area properly compacted before concrete is installed.
2.8 Foundation Settlement: crack patterns, other evidence
The photograph shows a significant settlement crack in a poured concrete foundation of a new (modular) home. This crack appeared first as a fine hairline crack. A combination of poor site preparation of soils below the building footings (uncompacted fill), portions of footings sitting on bedrock, and nearby blasting led to differential settlement that produced this damage. Also some reinforcing steel may have been omitted from construction of the foundation wall. A settlement crack is more likely to be wider at top than its bottom as the foundation "bends" over a single point (or as one section of footing tips downward from its neighbor), allowing differential settlement; it is possible for a settlement crack to appear fairly uniform however if a foundation breaks vertically and then pursues differential settlement. Settlement cracks need to be separated into initial settlement due to construction or site factors and ongoing settlement due to site factors.
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Settlement cracks are usually wider at the top of the crack, usually continuous, and may occur multiple times in a wall
•
•
•
Direction of downwards movement: If you draw an imaginary line orthogonal (at a right angle to) the line of a diagonal foundation crack, usually the line you've drawn points to the direction of downwards movement in the wall. However a diagonal crack may also indicate upwards wall-lift such as by frost or expansive clay soils which are more active under one portion of a foundation wall than another. Sink holes and foundation cracking: Settlement cracks may appear at the opposite end of a wall when a reinforced masonry foundation is settling due to presence of a sinkhole (for example in Florida). This condition can be distinguished by observing or measuring the directions of floor or wall slope. Multiple settlement cracks in walls: Multiple cracks of either type (settlement or lifting) may occur in a given area. Usually settlement cracks are visible both outside and inside of the foundation wall if the wall material is exposed to view at all.
2.9 Differential Settlement vs. Uniform Settlement in a Foundation:
(A）Building Before Settlement Occurs（B）Uniform Settlement (C) Differential Settlement
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2.9.1 Differential settlement in a foundation: We use the term "differential settlement" to describe a condition in which one portion of a building foundation is moving down, (or up and down) at a different rate or in a different amount from other portions of the foundation or wall. Differential settlement will damage the foundation or wall by producing (usually vertical, possibly diagonal or stair stepped) cracks and other symptoms of wall movement. The large foundation crack in this poured concrete wall was caused by differential settlement in a new foundation wall. All of this movement occurred during the first 13 months after the home was built.
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2.9.2 Differential settlement in a building slab or supporting piers can produce significant and recurrent cracks in the living area as drywall and trim are torn and dislocated during building movement. This photograph shows a significant crack in an upper floor interior wall which was bending as the center of this home settled downwards. The home had been built over the site of a previous stream bed. Its basement floor slab and supporting piers was settling downwards at its center. There was some additional movement in some of the building perimeter foundation walls, but the most significant settlement was at the center of the basement floor.
2.9.3 Uniform settlement in a foundation: Some buildings may settle so uniformly that the entire building moves down without producing cracking in the building's foundation or walls. For example, while it has a few settlement cracks, the Empress Hotel in Victoria BC, a very large masonry structure built on pilings on what was originally a marsh, has settled rather uniformly down over many decades, so that now visitors enter the hotel's "lobby" on what was originally its second floor.
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2.10 SHRINKING SOIL AND FOUNDATION SETTLEMENT
Concrete slab foundations are supported by the load bearing soils beneath them. When this support is reduced, the foundation adjusts accordingly. In parts of the country where highly expansive clay soils predominate, structural problems are common when the foundations are not constructed to take into consideration changes in the characteristics of the soil. One such situation involved a single-family residence with extensive cracking to interior sheetrock and exterior brick veneer. An engineering evaluation was requested to determine the cause of the cracking. A slab deflection survey was performed to determine the slope and curvature of the foundation. The results showed localized settlement toward the front of the house such that the slab sloped at a gradient of nearly two inches over 10 feet. The observed cracking of the sheetrock and brick veneer was consistent with the measured slopes of the foundation. A large oak tree was located fifteen feet from the foundation. It is known that most trees will project root systems beyond their canopy (drip line) to seek moisture. It was noted from the survey that the region of greatest settlement coincided with the observed tree canopy and the probable extent of the root system. The slab deflections were not indicative of any water leaks near or beneath the foundation. Water leaks will cause excessive swelling of clay soils and ultimately heave the slab upward in the affected area. In this case, however, the slab had not heaved, but had suffered differential settlement at the side of the house where the oak tree was located. The observed differential settlement was caused by subsiding clay soils that shrank over time and no longer provided adequate support to the slab. The culprit was the tree which had, by way of its root systems, drawn water from the soil beneath the slab. Soil contraction because of moisture extraction was expected to continue as long as the tree roots remained viable beneath the slab.
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3. ANALYSIS OF FOUNDATION SETTLEMENT
Total Settlement (St) = Si + Sc + Ss
Eq.(1.1)
Si – Immediate settlement Sc – Consolidation Settlement Ss – Settlement due to secondary consolidation Skempton and Bjerrum
2 Si = 1 Bµ I s q− o
Es
Eq.(1.2)
q0 - Contact pressure at the base of footing B - Least lateral dimension of footing μ - Poisson’s ratio Es - Soil Modulus Is - Shape factor Es - From Stress strain curve obtained from triaxial CU test Janbu, Bjerrum, Kjearnsli
Si = µ 0 µ1q0 B 1− µ 2 Es
Eq.(1.3)
µ 0,and µ 1 are coefficients to be obtained from chart for given depth and width of foundation and depth of hard layer
Settlement of Rectangular foundations on soil layer
The settlement is given by Si = q0B I E Eq.(1.4)
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Dimension less coefficient depending on ν , L/B, H/B
3. 1 SETTLEMENT OF SHALLOW FOUNDATIONS 3. 1.1 Types of foundation settlement
Foundation settlement under load can be classified according to two major types: immediate or elastic, settlement, Se and consolidation settlement, Sc. Elastic settlement of a foundation takes place during or immediately after the construction of the structure .Consolidation settlement is time dependent and takes place as the result of extrusion of the pore water from the void spaces of saturated clayey soils . The total settlement of a foundation is the sum of the elastic settlement and the consolidation settlement . Consolidation settlement comprises two phases: primary consolidation settlement and secondary consolidation settlement. Secondary consolidation settlement occurs after the completion of the primary consolidation that is caused by slippage and reorientation of solid particles under sustained load. Primary consolidation settlement is more significant than secondary settlement in inorganic clays and silty clay soil. However in organic soil, secondary consolidation settlement is more significiant.
3. 1.2 Elastic settlement based on the theory of elasticity
The elastic settlement of a shallow foundation can be estimated by using the theory of elasticity. From Hooke’s law,
H H
Se = ∫ εzdz = 1 ∫ (∆σz – μs∆σx - μs∆σy)dz o Es o
Eq.(1.5)
Se =elastic settlement Es =modulus of elasticity of soil H=thickness of the soil layer μs =Poisson’s ratio of the soil ∆σx , ∆σy, ∆σz = stress increase due to the net applied foundation load in the x,y and z directions, respectively.
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Theoretically, if the foundation is perfectly flexible, the settlement may be expressed as Se = qo(αB’) 1- μs² Is If Es Eq.(1.6)
qo = net applied pressure on the foundation μs = Poisson’s ratio of the soil Es = average modulus of elasticity of soil under the foundation, measured from z=0 to about z=4B B’= B/2 for center of foundation = B for corner of foundation To calculate settlement at the center of foundation, we use m’ = L B Eq.(1.7)
The elastic settlement of a rigid foundation can be estimated as Se (rigid) = 0.93 Se flexible,center) Eq.(1.8)
Due to the non-homogeneous nature of the soil deposits, the magnitude of E may vary with depth. Es = Ʃ Es(i) ∆z
z
Eq.(1.9)
Es = soil modulus of elasticity within a depth Z = H or 5B, whichever is smaller
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3.1.3 Elastic settlement of foundations on saturated clay
Janbu et al proposed an equation for evaluating the average settlement of flexible foundations on saturated clay soils. Se = A1A2 qoB Es A1 is a function of H/B and L/B and A2 is function of Df/B. Eq.(1.10)
3.1.4 Elastic settlement of sandy soil: Use of strain influence factor
The settlement of granular soils can also be evaluated by the use of a semi empirical strain influence proposed by Schmertmann et al (1978). According to this method, the settlement is
Z2
Se = C1 C2 ( q-q )Ʃ Iz ∆z 0 Es
Eq.(1.11)
Iz = strain influence factor C1= a correction factor for the depth of foundation embedment=1-0.5 [q/ ( q-q )] C2 = a correction factor to account for creep in the soil = 1+0.2 log (time in years/0.1) Q= stress at the level of the foundation Q= γ Df For square or circular foundations Iz =0.1 at z = 0 Iz =0.5 at z = z 1= 0.5B and Iz =0 at z = z2 =2B
Similarly, for foundations with L/B>10,
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Iz = 0.2 at z=0 Iz =0.5 at z = z1 =B Iz = 0 at z = z2 =4B Where B = width of the foundation and L=length of the foundation .Values of L/B between 1 and 10 can be interpolated. The use of Eq.(1.11) first requires an evaluation of the approximate variation of the elasticity with depth. This evaluation can be made by using the standard penetration numbers or cone penetration resistances. The soil can be divided several layers to a depth of z = z2 and the elastic settlement can be estimated. The sum of the settlement of all layers equals Se.
3.1.5 Range of material parameters for computing elastic settlement
Several investigators have correlated the values of the modulus of elasticity Es, with field standard penetration number, N60 and the cone penetration resistance, qc.Mitchell and Gardner (1975) compiled a list of these correlations. Schmertmann(1970) indicated that the modulus of elasticity of sand may be given by Es = 8 N60 pa N60 = standard penetration resistance pa = atmospheric pressure =100kN/m² Es = 2qc Eq.(1.13) Eq.(1.12)
Where qc = static cone penetration resistance Schmertmann et al.further suggested that the following correlations may be used with the strain influence factors Es =2.5 qc (for square and circular foundations) Es =3.5 qc (for strip foundations)
3.1.6 PRIMARY SETTLEMENT
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Primary consolidation settlement relationships
As mentioned before, consolidation settlement occurs over time in saturated clayey soils subjected to an increased load caused by construction of the foundation. On the basis of the one-dimensional consolidation settlement, Sc = ∫εz dz Where Εz = vertical strain = ∆е 1+ еo ∆е = change of void ratio = f (σ’o, σ’c and ∆ σ’) Eq.(1.14)
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4. CASE STUDIES:
Geotechnical studies for foundation settlement in alluvial deposits in the City of Rome (Italy) University of Roma “Roma TRE”, Dept. of Geological Science, L.go San Leonardo Murialdo, 1 00146 Rome, Italy Via degli Scaligeri, 29 00164 Rome, Italy Received 14 March 2006; Revised 31 July 2006; Accepted 7 August 2006. Abstract This study identifies units characterized by specific geomechanical behaviours within some Holocene alluvial deposits in the City of Rome. In particular, the highly compressible units, which may be responsible for subsidence and settlement phenomena below urban structures, have been identified. Investigations carried out during this study have interpreted nearly 800 stratigraphic sections and physical– mechanical features of the alluvial deposits of the Tiber and those of its right- and leftbank tributaries. The analysis allowed to define, for each investigated deposit, a representative series subdivided into lithotechnical units. When comparing the stratigraphic and geologic features among the series, a remarkable difference in geomechanical behaviour in the deposits from the Tiber's left- and right-bank tributaries and within the river's deposits themselves could be recognized. A yield hazard zoning for urban structures related to recent alluvial deposits has also been defined. 4.1. Introduction The City of Rome is located in an area where there as been long-term human activity and continuous transformations of the original terrain have been substantial. The hydrographic networks of the Tiber's right and left banks have been modified more than once during historical times; sometimes, it has even been obliterated by urban growth. These changes mean that nowadays, it is very difficult, at least in the historical centre, to recognize the original terrain. Many of Rome's streets run along the ancient courses of the Tiber's tributaries and many buildings are situated on alluvial deposits now buried by anthropogenic debris. Many of these structures overlying alluvium and debris have been damaged by subsidence and seismic wave amplification. The effects are visible in the buildings' uniform or differential settlement. In the past, the alluvial deposits were considered as continuous bodies consisting of clayey–silty and sometimes sandy sediments. Instead, the formations were found to be
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mainly heterogeneous. The geotechnical characterization of these units is mandatory for evaluating the intrinsic geological environment's hazard in urban areas, where the risk can reach very high values. This study has been carried out through the analysis of borehole and geotechnical data from three left-bank tributaries and three right-bank tributaries of the Tiber River. Data for the Tiber's alluvial deposit from various parts of the city were also included. Based on this geotechnical analysis, a subdivision into lithotechnical units has been created for each deposit to make correlations and comparisons among the different deposits. The same level of detail was not possible for all of the stream valleys, since it was not always possible to obtain geotechnical data. Nonetheless, it has always been possible to define a stratigraphic series that would represent the examined deposit by subdividing it into units after stratigraphical and sedimentological observations.
4.2 Geological, geomorphological, and hydrogeological setting in the City of Rome The dominant feature on the Tiber River's right bank is the “Monte Mario” ridge. It lies parallel to the Tiber valley and reaches nearly 140 m a.s.l.; to the south, it is 60 m a.s.l. Monte Mario is made up of marine deposits .The “Monte Vaticano” Unit, (blue clays) affects the whole structural setting of the City of Rome and is remarkably thick, consolidated, and is the aquatard for hydrogeological units of Rome. These marine deposits are exposed at the base of the “Monte Mario” ridge. To the east and west of the ridge “Monte Vaticano” lies at various depths in paleogeomorphic depression in tectonically deformed “bedrock.” Above those marine sequences we can follow the change from marine to coastal, then to continental sedimentation. The continental and coastal facies trace the flow of the Paleotiber, which is parallel to the coast running SE along a continuously subsiding belt, producing conglomerate deposits tens of metres thick. From the Middle Pleistocene ash and other airborne products from an intense volcanic activity of the “Sabatini” and “Albani” volcanic districts reached the area of Rome. The geomorphologic and hydrogeologic setting was deeply modified by this volcanic activity. The Paleotiber's course was deviated arriving to the present-day position within the boundary between the Albani and Sabatini products. Later, the last glacial lowstand was 120 m below today's level. This produced a deepening of the Tiber's course and its tributaries' as they eroded the volcanic deposits first, followed by the continental Paleotiber's deposits then, and later carving deeply into the underlying and well-consolidated “Monte Vaticano” Unit. When the sea level rose, the deep gorges were in-filled with alluvial deposits, which are the object of this paper.
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The succession of these events created the present-day terrain of the City of Rome: the area consists of a central plain, which is the Tiber's alluvial plain; the relief on the river's right bank is the “Monte Mario”–“Monte Vaticano” structural high; at the Tiber's left bank lies an articulated area that, in the historical city centre, corresponds to the “Seven Hills”
Fig. 2. Geological map of Rome
4.3 Methodology This study is predominantly concerning areas where growth of the modern City of Rome has occurred over the last 50 years and where it will presumably developed in the future. Several hundreds of recent drillings carried out from geological–technical investigations were analyzed and interpreted using previous data from published stratigraphies. Initially, each alluvial deposit was subdivided into units based on physical data from stratigraphic descriptions (colour, sedimentological features, organic matter content, consistency, etc.). Then, for each of these subdivisions, the physical–mechanical parameters from laboratory or on-site tests were determined; this gave a representative series for each alluvial deposit by subdividing them into “geotechnical units”.
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4.4 Lithostratigraphic characterization of alluvial deposits at the Tiber River's left bank 4.4.1 Alluvial deposit of the “Fosso di Grotta Perfetta” stream 4.4.1.1 Geographic setting The “Fosso di Grotta Perfetta” stream flows in a secondary valley and is a left-bank Tiber tributary. For its low-energy, marsh-environment characteristics this stream was considered to be a swamp in the past, the area is long and narrow. It is 8.5 km long and 2 km wide. The lower basin of the stream is now almost completely urbanized, the final branch was channeled during historical times. As of today, the area is known for its instabilities for constructions, principally related to settlement and rigid rotation of structures, caused by the soil's bad geomechanical characteristics and inadequate foundations. The examined area follows the Grotta Perfetta stream course for about 3 km, and a tangential band of about 1 km into the valley of the Tiber. 4.4.1.2. Subdivision into depositional units and geotechnical parameters Based on the physical characteristics of the soil, within the alluvial deposits and beneath the anthropic cover, six main units in the central portion of the valley can be distinguished and linked to geotechnical data from nearly 218 undisturbed samples. About 100 new in situ geotechnical tests were performed. The following are descriptions of the analyzed units and their physical–mechanical parameters. Beneath the anthropic cover, from the most recent to the most ancient unit, deposits from the last historical flood can be observed, which are 3–4 m thick, clayey–silty sands with floated anthropic inclusions. The soils are of pyroclastic origin with some tuff element. The geomechanical behaviour is definitely frictional, the cohesion is almost zero; the gravel–sand fraction percentage is higher than in the overlying anthropic cover. Underlying the historical flood deposits are the brown clays.They are silty clays and clayey silts; consistency is medium at the top and gradually decreasing with depth. Water content and Atterberg's values are rather high; the consistency index is medium to low, decreasing with depth, typical of plastic to soft/plastic physical states. Deeper, the first brown–blackish, high organic matter content unit can be observed. Locally, greyish, brown, and/or greenish lenses are present. Geotechnical analysis highlighted particular parameters that are commonly identified with geotechnically poor soils. The dominant feature is the high organic matter content, which reached 15–25% of the total weight. Densities are particularly low, void ratio being sometimes higher than 4.00. Often the moisture content is higher than the liquid limit; the liquidity index is > 1 indicating liquid–plastic states.
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Such characteristics are typical of metastable or sensitive soils, for which stable states are only possible onsite (with confining pressure); whenever they were disturbed by both static and dynamic tension, they lose their strength. In particular, when subjected to dynamic loading, they can produce local collapse phenomena. Shear strength parameters are definitely low and has high compressibility; with values of compression index Cc often more than 2.00. Around − 5 m a.s.l there is a substantial decrease in organic matter which identifies the top of the greenish clays The water content and Liquid Limit, despite still being high, reveal a certain degree of geotechnical improvement compared with the overlying organic clays. This is particularly due to the absence, or minor presence, of organic matter (less than 10% found only in particular horizons). Nevertheless, the shear strength resistance is still low similar to that of the above-mentioned organic clays. Compressibility characteristics reveal compression index Cc values of 0.6–1.5, which, although typical of highly compressible clays, indicate a slight increase in stability compared with the overlying organic clays. Finally, an organic matter-rich unit characterized by physical–mechanical parameters similar to those of the above-mentioned blackish and organic ones is also present. This layer changes from black to grey in colour at depths exceeding 35–40 m indicating a decrease in organic matter and an increase of cohesion where coarse horizons of floated pozzolan fragments are observed. This geotechnical and stratigraphic setting allowed the layer to be subdivided into two sub-units: the upper AO2′ unit and the lower AO2 unit. The AO2′ unit is characterized by parameters similar to that of the AO1 unit. High water content and Atterberg's values were observed, with very low, sensitive and metastable soil characteristics again observable. The AO2 unit instead shows better mechanical parameters. It is also characterized by the presence of diffused floated pozzolan horizons, lens-shaped and decimetre-sized; only locally, these horizons can be up to 1–3 m thick. Their presence improves the geotechnical properties of the clayey unit; the higher permeability allows some local consolidation within the organic clays. The bed of the AO2 unit has geotechnical conditions compared to the AO2′ which is also evident when analyzing the water content, density, and strain values. These characters define the geotechnical parameters representative of the various lithotechnical units in the Holocenic alluvial deposits in the “Fosso di Grotta Perfetta” stream.
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Table 1. Geotechnical parameters summary table of the lithotechnical units identified in the “Grotta Perfetta” deposit
Average thickness (m) Historical floods 3 γn (kN/m3) 16–19 15–19 12–15 12–17 14–16 16–19 WC (%) 32–65 30–93 91– 186 49– 154 42– 184 28–84 LL (%) //–78 43– 123 102– 171 84– 151 63– 164 46– 104 IP (%) NP23 17– 58 33– 93 24– 79 18– 76 17– 65 φ′ C′ (kPa) 0–38 5–35 9–25 3–15 5–25 0–0.3 Cu (kPa)
30– 47 23– 30 13– 22 17– 29 13– 26 25– 30
– 21–74 16–55 7–19 15–33 15–30
Brown clays 7 Organic clays 6
Green clays 8 Organic clays Organic clays 5 15
// indicates no data.
4.5 Lithostratigraphic characterization of the Tiber River alluvial deposit 4.5.1 Introduction When compared with tributary alluvial deposits, there are more studies of the deposits of the River Tiber. These studies are indispensable for individualizing the stratigraphical and evolutionary relationships between the Tiber and its tributaries. The investigated area is located in the Historical Centre and has been characterized by a number of boreholes that often reached down to Pliocenic bedrock. 4.5.2 The stratigraphic series representative of the Tiber River's alluvial deposit and its geomechanical parameters
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Almost all of the studies about the alluvial deposits of the Tiber within the City of Rome agree on both their subdivision and geomechanical characterization. There is also a general agreement on the reconstruction of the depositional evolution of the Tiber during the Holocene. For the Historical Centre, the authors agree on characterization of the following stratigraphical units. Sandy silts and dark-greenish clayey silts; subdivided into two different ones, a silty– clayey (LAV) unit and a sandier one (SLV). S: coarse sands constituting the filling after an erosional phase of the Tiber River. AG: silty clays and greyish clayey silts with thin organic levels, relatively more frequent at an elevation of − 15 a.s.l. SLG: silty sands, grey sandy silts, and coarse sands. G: base gravel: mainly made up of carbonate clasts from the erosional phase. Our analysis corresponds to the above studies, even though the data come from different areas. Summary table of the principal geomechanical parameters representative of the units observed in the Tiber's alluvial deposits
Units Average γn thickness (kN/m3) (m) Greenish clayey silts Greenish silty sands Medium sands Grayish silty clays Grayish silty sands 7 7 8 23 15 18–20 17–20 17–20 15–20 18.5–20 Wn (%) LL (%) IP φ (% ) 12– 18– 41 25 8– 26 // 27– 35 30– 40 C′ (kPa) 15–30 0–20 0–10 5–25 0–15
LAV SLV S AG SLG
20–0 43–71 20– 40 20– 30 20– 60 20– 30 26–65 ND– 27 34–93 24–46
11– 15– 59 26 //– 22 25– 35
// indicates no data.
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The left-bank deposits show physical–mechanical parameters remarkably poorer than those from the right bank and from the River Tiber itself. The differences in shear resistance between the left-bank and right-bank deposits are in the graphs compare three CPTs on the three deposits we examined.
The plasticity conditions can be compared by examining the Casagrande's Plasticity Chart. It highlights the high plasticity of the organic silty deposits from the “Grotta Perfetta” stream. The deposits of the Tiber and of its right-bank streams show normal activity indices (A = 0.4–0.8), whereas the left-bank deposits show higher values (A = 0.8–2), typical of “high-activity” soils. Prominent for the objective of this study is the remarkable deformability of the leftbank deposits when compared with the right bank and the deposits of the Tiber. The deformational features are here compared with three representative oedometric graphs. The organic pelitic–silty deposits of the “Fosso di Grotta Perfetta” stream clearly show a very high compressibility whose natural void index reaches very high values (e0 = 1.5–2.5). Thus, these normally consolidated alluvial deposits show a relatively higher deformability when associated with organic sediments, which are distributed throughout the left-bank deposits. The pelitic–silty beds in particular are potentially prone to subsidence and settlement if loaded by landfill. The Tiber's deposits and the right-bank deposits instead show fairly similar and comparable deformability characteristics.
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4.6 Conclusions The analysis of nearly 800 stratigraphic logs and a number of laboratory tests gave the opportunity to subdivide the alluvial deposits of the Tiber and its tributaries into depositional units and to define their geomechanical characteristics. This division into units is very important for evaluating the hazard in different areas of the territory. Such hazards are derived from the fact that alluvial deposits are subject to seismic amplification and the structure of organic-rich units are more ‘sensitive’ and can be affected by phenomena similar to the liquefaction of silty sands below the water table. Such sediments are mostly underconsolidated, hence, more subjected to settlement. Through data derived some alluvial deposits in the City of Rome (including those of the Tiber), it is possible to identify areas potentially at risk from instabilities caused by the presence of organic matter. Units with high organic matter content, widely dispersed and several metres thick, are mainly within a portion of the stream with shallow gradients. The Tiber's deposits only show thin layers of organic matter and generally have better geotechnical characteristics than deposits underlying the leftbank tributaries. All the deposits that have been subjected to loading by backfills and buildings in the historical centre over thousands of years have completed their consolidation process and are no longer affected by subsidence. Areas with significant organic matter content on the left bank are subsiding now, whereas the right-bank alluvial deposits are no longer subsiding.
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Case study: Reinforced Earth retaining wall approaches Thursday, June 10, 2010, 11:03 Hrs [IST]
Dhananjoy Das and Saikat Chatterjee present an interesting case study of how the highly-flexible Reinforced Earth technology was successfully deployed on a bridge over a canal in Kolkata. Marshy land had made construction difficult. A bridge was to be built over Bagjola Canal to meet the increasing traffic and developmental demands. The banks of the canal were made of filled up soil on a low and marshy lands. For PWD, the construction of main bridge structure was not so critical due to its pile foundations. Whereas, it was hard to construct its approaches in such a marshy land with 'N' value ranging from 2-3 only, up to a depth of 15m from ground level in most of the places. Conventional RCC retaining wall was not a feasible solution in such ground condition. Therefore, the owner of the project opted for highly flexible Reinforced Earth Retaining Wall and the work was awarded to Reinforced Earth India. The solution: It was a challenging engineering task to build Reinforced Earth Walls under such extreme weak ground conditions with very low bearing capacity to support the load of the structure. Therefore, it was imperative to improve the foundation soil to withstand the load to be imposed from the wall. PVD (prefabricated vertical drain) in combination with stage construction preloading was selected as a best possible solution at that location followed by two layers of PET uniaxial geogrids to improve the foundation soil and ensure the safety of the wall under global stability. Proper instrumentation and stage construction were done to monitor settlement and development of additional pore water pressure with each stage of construction and subsequent dissipation during pause period.
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Owner: Main contractor: Construction stage: RE wall area: Engineering Data Facing: Connection type: Reinforcement type: Fill soil properties: Foundation type: Foundation system:
West Bengal PWD Mackintosh Burn Ltd April 2005 to November 2007 5,062 sq. m (as built) 1.5mx1.5m Cruiciform Terraclass panels Galvanised tie strips High Adherence (HA) galvanised steel strips Phi=32 deg., ?min=15KN/m3, ?max=17KN/m3 N' value = 2-3 up to depth 15m Prefabricated vertical drain
PVD work: PVD had been inserted up to a depth of 15m (average), in 1.5mx1.5m square grids, below a depth of 2.0m from existing ground level. The job was done at a very fast pace and the overall PVD work at an approximate plan area of 7,000 sq. metres, had been completed within two weeks. Instrumentation: Four settlement plates and ten Piezometric tubes had been installed at both the approaches to measure the periodical settlement and pore water pressure respectively. Stage construction: Stage construction technique had been adopted to stabilise the foundation soil with every stage of construction by allowing dissipation of pore water through PVD along with very sophisticated sub-surface drainage system. As the wall height was approximately 9.0m, three stages of rest periods had been considered to achieve
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the maximum settlement in stages. At 4.0m, 7.0m and 9.0m, average rest periods of 45 days each had been provided. The gain in shear strength of foundation soil at each stage of construction allowed construction of next stage of RE wall. During this rest periods instruments were monitored at regular basis, and the next stage of construction started after reaching the desired settlement and dissipation of pore water pressure. Below is one example of settlement and piezometer monitoring report at one of the locations. Notable observations: A maximum 319mm settlement had been observed; differential settlements had been observed varying from 99mm to 319mm; and post-construction settlements had also been observed in some places.
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REFERENCES
1.) Braja M. Das, Principles of Foundation Engineering, Thomson, India Edition. 2.) Website: http://www.sciencedirect.com/ 3.) Geotechnical studies for foundation settlement in Holocenic alluvial deposits in
the City of Rome (Italy) Maria Paola Campolunghi Giuseppe Capelli, Renato Funiciello and Maurizio Lanzini University of Roma “Roma TRE”, Dept. of Geological Science, L.go San Leonardo Murialdo. 4.) Soil mechanics and foundation engineering, vol. 43, no. 1, 2006 Settlement analysis based on recommendation of building code 50-101-2004 V. A. Barvashov. 5.) Settlement analysis of foundations.Willmore, M J Proc lnt Conference on Foundations and Tunnels, London, arch 1987 VI, P25-29. Publ Edinburgh: Engineering Technics Press. 1987
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