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NPTEL - ADVANCED FOUNDATION ENGINEERING-1
Module 2 2
(Lecture 7) NATURAL SOIL DEPOSITS AND SUBSOIL EXPLORATION
Topics 1.17 CONE PENETRATION TEST 1.18 PRESSUREMETER TEST (PMT) 1.19 DILATOMETER TEST 1.20 CORING OF ROCKS 1.21 PREPARATION OF BORIN LOGS 1.22 DETERMINATION OF HYDRAULIC CONDUCTIVITY IN THE FIELD 1.22.1Open
End Test
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CONE PENETRATION TEST
The cone penetration test (CPT), originally known as the Dutch cone penetration test, is a versatile sounding methodproperties. that can be used in a soil test, profile estimate their engineering This testtois determine also calledthe thematerials static penetration andand no boreholes are necessary to perform it. In the original version, a 60° cone with a base area of 10 cm2 was pushed into the ground at a steady rate of about 20 mm/sec, and the resistance to penetration (called the point resistance) was measured.
The cone penetrometers in use at present measure (a) the cone resistance ( ) to penetration developed by the cone, which is equal to the vertical force applied to the cone divided by its horizontally projected area, and (b) the frictional the frictional resistance ( ) which is the resistance measured by a sleeve located above the cone con e with the local soil surrounding it. The frictional resistance is equal to the vertical force applied to the sleeve divided by its surface area-actually, the sum of friction and adhesion.
and : a. Mechanical friction-cone penetrometer (figure 2.25). In this case the penetrometer tip is
Generally, two types of penetrometers are used to measure
connected to an inner set of rods. The tip is first advanced about 40 mm giving the cone resistance. With further thrusting, the top engages the friction sleeve. As the inner rod advances, the rod force is equal to the sum of the vertical force on the cone and sleeve. Subtracting the force on the cone gives the side resistance.
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Figure 2.25 Mechanical friction-cone penetrometer (after ASTM 1992) b. Electric friction-cone penetrometer (figure 2.26). In this case the tip is attached to a string of steel rods. The tip is pushed into the ground at the rate o off 20 mm/sec. wires from the transducers are threaded through the center of the rods and continuously give the cone and side resistance.
NPTEL - ADVANCED FOUNDATION ENGINEERING-1 Figure 2.26 Electric friction-cone penetrometer (after ASTM 1992) by y Figure 2.27 shows the results of penetrometer tests in a soil profile with friction measurement b a mechanical friction-cone penetrometer and an electric friction cone penetrometer.
Figure 2.27 Penetrometer tests with riction measurement (after Ruiter, 1971) Several correlations that are useful in estimating the properties of soils encountered during an exploration program have been developed for the point resistance ( ) and the friction ratio ( ) obtained from the cone penetration tests. The friction ratio, , is defined as
=
fricti friction on resist resistanc ance e
=
cone resista resistance nce
[2.24]
Lancellotta (1983) and Jamiolkowski et al. (1985) showed that the relative density of normally and can be correlated as consolidated sand, , and
(%) = + log
� ′′
10
Where
, = cons consta tant ntss ′ = vertical vertical effective effective stress stress The values of and are
[2.25]
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A
B
-98
66
Unit of
and ′
metric ton/m2
Figure 2.28 shows the correlations obtained for several sands. Baldi et al. (1982), and Robertson
and Campanella (1983) also recommended an empirical relationship between vertical effective stress ( ), relative density ( ) and for normally consolidated sand. This is shown in figure 2.29.
′
Figure 2.28 Relationship between
and (based on Lancellotta, 1983, and Jamiokowski et al., 1985)
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Figure 2.29 Variation of
′ and for normally consolidated quartz sand (based on Baldi et
al., 1982, and Robertson ad Campanella, 1983)
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Figure 2.30 Variation of
′ and for normally consolidated quartz sand (after Robertson Robe rtson ad Campanella, 1983) 1983)
′
Figure 2.30 shows a correlation between , , and the peak friction angle for normally consolidated quartz sand. This correlation can be expressed as (Kulhawy and Mayne, 1990).
tan− 0.1 0.1 + 0. 0.38 38 lo log g ′ 1
[2.26]
Robertson and Campanella (1983) also provided a general correlation between , friction and , and the type of sol encountered in the field (figure 2.31). Figure 2.32 shows the general range of / for various types of soil.
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Figure 2.31 Robertson and Campanellas’ correlation (1983) between
, , and the soil type
Figure 2.32 General range of variation of / for various types of soil (after Robertson and Campanella, 1983)
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The correlations for as proposed in figures 2.30, 2.31, and 2.32, equations 25 and 26 are for normally consolidated sands. In fact, for a general condition, is a function of , relative density, and vertical and lateral initial effective stress. A more rational theory for that correlation has been provided by Salgado, Mitchell, and Jamiolkowski (1997), and readers may refer to that paper for further information.
′
According to Mayne and Kemper, (1988), in clayey soil the undrained cohesion preconsolidation pressure
′ = ′− 1
,
and the overconsolidation ratio can be correlated as [2.27]
Or
= −
[2.27a]
Where
= bearing bearing capacity capacity factor factor (N
K
= 15 for electric electric cone and and NK = 20 for mechanical mechanical cone) cone)
vertical stress ′ == effective vertic vertical al stress effective Consistent units of , , ′ ,and should be used with equation (27): = 0.2 0.243( ) ↑ ↑ 0.96
MN/m2
[2.28] [2.28]
MN/m2
And 1.01
= 0.3.377 ′− effective ve stress, stress, respective respectively ly Where and ′ = total and effecti
[2.29]
PRESSUREMETER TEST (PMT)
The pressuremeter test is an in situ test conducted in a borehole. It was originally developed by Menard (1956) to measure the strength and deformability of soil. It has also been adopted by ASTM as Test Designation 4719. The Menard-type PMT essentially consists of a probe with three cells. The top and bottom ones are guard are guard cells cells and and the middle one is the measuring cell, as shown schematically in figure 2.33a. The test is conducted in a re-bored hole. The pre-bored hole should have a diameter that is between 1.03 to 1.2 times the nominal diameters of the probe.
NPTEL - ADVANCED FOUNDATION ENGINEERING-1 The probe that is most commonly used has a diameter of 58 mm and a length of 420 mm. the probe cells c ells can be expanded either by b y liquid or gas. The guard cells are expanded to reduce reduc e the end-condition effect on the measuring cell. The measuring cell has a volume ( ) of 535 cm3 . Following are the dimensions for the probe diameter and the diameter of the borehole as recommended by ASTM:
Figure 2.33 (a) Pressuremeter; (b) plot of pressure versus total cavity volume
Borehole diameter Probe diameter (mm)
Nominal (mm)
Maximum (mm)
44
45
53
58
60
70
74
76
89
In order to conduct a test, the measuring cell volume, , is measured and the probe is inserted into the borehole. Pressure is applied in increments and the volumetric expansion of the cell is measured. This is continued until the soil fails or until the pressure limit of the device is reached. The soil is considered to have failed when the total volume of the expanded cavity (V ( V ) is about twice the volume of the original cavity. After the completion of the test, the probe is deflated and advanced for test at another depth. The results of the pressuremeter test is expressed in a graphical form of pressure versus volume figure 2.33b.isinpushed as in the this figure, represents the reloading portion the soilshown around borehole backZone into Ithe initial state (that is, the state during it was which in before
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drilling). The pressure, , represents the in situ situ total horizontal stress. Zone II represents a pseudo-elastic zone in which the cell volume versus cell pressure is practically linear. The pressure, , represents the creep, or yield, pressure. The zone marked III is the plastic zone. The pressure, , represents the limit pressure.
The pressuremeter modulus, infinitely thick cylinder. Thus
, of the soil is determined using the theory of expansion of an
= 2(1 + )( + ) ΔΔ
[2.30]
Where
= Δ = − Δ = − = Pois Poisso son n′ sratio (which may be assumed to be 0.33) The limit pressure, , is usually obtained by extrapolation and not by direct measurement. + 2
In order to overcome the difficulty of preparing the borehole to the proper size, self-boring pressuremeters (SBPMT) have also been developed. The details concerning SBPMTs can be found in the work of Baguelin et al. (1978). Correlations between various soil parameters and the results obtained from the pressuremeter tests have been developed by various investigators. Kuljawy and Mayne (1990) proposed that
= 0.4 0.45
[2.31]
Where
preconsolidation dation pressure pressure = preconsoli Based on the cavity expansion theory, Baguelin et al. (1978) proposed that
= − (
)
[2.32]
Where
= undrained undrained shear shear strength strength of a clay = 1 + i n 3
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Typical values of vary between 5 to 12, with an average of about 8.5. Ohya et al. (1982) (see also Kulhawy and Mayne, 1990) correlated with field standard penetration numbers ( ) for sand and clay as follows:
Clay:
(kN/m ) = 1930
0.63 F
2
0.66 F
(kN/m2 ) = 908
Sand:
[2.33] [2.34]
DILATOMETER TEST
The use of the flat-plate dilatometer test (DMT) is relatively recent (Marchetti, 1980; Schmertmann, 1986). The equipment essentially consists of a flat plate measuring220 mm (length) × 95mm 95mm (width) × 14mm (thickness (thickness)(8.66 )(8.66 in.× 3.74in.× 3.74in.× 0.55 0.5 5 in. in. ). a thin flat circular expandable steel membrade having a diameter of 60 mm (2.36 in.) is located flush at the center on one side of the plate ( figure 2.34a). The dilatometer probe is inserted into the ground using a cone penetrometer testing rig (figure 2.34b). Gas and electric lines extend from the surface control box through the penetrometer rod into the blade. At the required depth, high-pressure nitrogen gas is used to inflate the membrane. Two pressure readings are taken. They are
Figure 2.34 (a) Schematic diagram of a flat-plate dilatometer; (b) dilatometer probe inserted into ground
NPTEL - ADVANCED FOUNDATION ENGINEERING-1 1. The pressure A pressure A to to “lift off” the membrane, and 2. The pressure pressure B B at at which the membrane expands 1.1 mm (0.4 in.) into the surrounding soil The A The A and and B B readings readings are corrected as follows (Schmertmann, 1986)
Contact stress,
= 1.05( + ∆ − ) − 0.05( − ∆ − )
Expansion stress, Where
[2.35] [2.35]
= − − −∆ ∆ 1
[2.36] [2.36]
∆ = vacuum pressure required to keep the membrane in contact with its seating ∆ =
air pressure required inside the membrane to deflect in outward to a center expansion of 1.1mm
= gauge pressure deviation from from zero when vented vented to atmospheric pressure pressure The test is normally conducted at depths 200 mm to 300 mm apart. The result of a given test is used to determine three parameters:
= −− − 2. Horizontal stress index, = ′ 3. Dilatometer modulus, (kN/m ) = 34.7( kN/m − kN/m ) 1. Material index,
1
2
1
2
2
Where
= pore water water pressure pressure ′ = vertical effective stress Figure 2.35 shows the results of a dilatometer test conducted in Porto Tolle, Italy (Marchetti, 1980). The subsoil consisted of recent, normally consolidated delta deposits of the Po River. A thick layer of silty clay was found below a depth of 10 ft ( = 0; 28° ). The result obtained from the dilatometer tests have been correlated to several soil properties (Marchetti, 1980). Some of these correlations are given below.
≈
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Figure 2.35 A dilatometer test result conducted at Porto Tolle, Italy (after Marchetti, 1980) 0.47
a. b. c. d. e.
= =(0.5−) 0.6 1.5
[2.37] [2.38] [2.39]
1.6
′ = 0.22 (for normaly consolidated clay) ′ = ′ (0.5 )1.25 = (1 − 2)
[2.40] [2.41]
Where
= coeffic coefficien ientt of at − rest earth pressure (chapter 5) = overconso overconsolidati lidation on ratio overconsolidated idated soil = overconsol = normally normally consolid consolidated ated soil = modulus modulus of elasticity elasticity
Schmertmann (1986) also provided a correlation between the material index ( ) andthe dilatometer modulus ( ) for determination of soil description and unit weight ( ). This is
NPTEL - ADVANCED FOUNDATION ENGINEERING-1 shown in figure 2.36.
Figure 2.36 Chart for determination of soil description and unit weight (after Schmertmann, 1986); Note: 1 /m3 = 9.81 9.81 kN/m kN/m3
CORING OF ROCKS
When a rock layer is encountered during a drilling operation, rock coring may be necessary. For coring of rocks, a core barrel is attached to a drilling rod. A coring bit is is attached to the bottom of the core bared (figure 2.37). The cutting elements may be diamond, tungsten, carbide, and so on. Table 8 summarizes the various types of core barrel and their sizes, as well as the compatible drill rods commonly used for foundation exploration. The coring is advanced by rotary drilling. Water is circulated through the drilling rod during coring, and the cutting is washed out.
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Figure 2.37 Rock coring: (a) single-tube core barrel; b arrel; (b) double-tube core barel Two types of core barrel are available: the single-tube the single-tube core barrel (figure (figure 2.37a) and the doubletube core barrel (figure 2.37b). Rock cores obtained by single-tube core barrels can be highly disturbed and fractured because of torsion. Rock cores smaller than the BX size tend to fracture during the coring process. When the core samples are recovered, the depth of recovery should be properly recorded for further evaluation in the laboratory. Based on the length of the rock core recovered from each run, the following quantities may be calculated for a general evaluation of the rock quality encountered.
Recovery Recov ery ratio =
length length of core rec recove overed red theo theore reti tica call le leng ngth th of rock rock co core red d
[2.42]
NPTEL - ADVANCED FOUNDATION ENGINEERING-1 Table 8 Standard Size and Designation of Casing, Core Barrel, and Composite Drill Rod
Outside diameter of core barrel bit Casing and core barrel designation
Outside diameter Diameter of drill rod borehole
of Diameter of core sample
Drill rod designation (mm)
(in.)
(mm)
(in.)
EX
36.51
116
E
33.34
AX
47.63
18
7
A
41.28
BX
58.74
216
5
B
47.63
NX
74.61
2
15
N
60.33
2
7
(mm)
(in.)
1
38.1
1 2
22.23
7
1 8
5
50.8
2
28.58
18
18
7
63.5
22
41.28
18
3
76.2
3
53.98
2
5
16
1
1
(in.) (in.) 8
7
5
1 8
8
16
(mm)
si tu Rock Quality and R QD Table 9 Relation between i n si
RQD RQD
Rock quality
0-0.25
Very poor
0.25-0.5
Poor
0.5-0.75
Fair
0.75-0.9
Good
0.9-1
excellent
length th of recov recover ered ed Σ leng
Rock quality designation ( RQD RQD))=
pi piec eces es equa equall to or larg larger er tha than n 10 101.6 1.6 mm mm (4 in.) in.)
the theore oreti tica call leng length th of rock rock co core red d
[2.43]
A recovery ratio of 1 will indicate the presence of intact rock; for highly fractures rocks, the recovery ratio may be 0.5 or smaller. Table 9 presents the general relationship (Deere, 1963) between the RQD the RQD and and the in situ rock situ rock quality.
NPTEL - ADVANCED FOUNDATION ENGINEERING-1 PREPARATION OF BORIN LOGS
The detailed information gathered from each borehole is presented in a graphical form called the boring log. As a borehole is advanced downward, the driller generally should record the following information in a standard log: 1. Name and address of the drilling company 2. Driller’s name 3. Job description and number 4. Number and type of boring and boring location 5. Date of boring 6. Subsurface stratification, which can be obtained by visual observation of the soil brought out by auger, split-spoon sampler, and thin wall Shelby tube sampler 7. Elevation of water table and date observed, use of casing and mud losses, and so on 8. Standard penetration resistance and the depth of SPT 9. Number, type, and depth of soil sample collected 10. In case of rock coring, type of core barrel used, and for each run, the actual length of coring, length of core recovery, and the RQD the RQD This information should never be left to memory, because that often results in erroneous boring logs. After completion of the necessary laboratory tests, the geotechnical engineer prepares a finished log that includes notes from the driller’s field log and the results of tests conducted in the laboratory. Figure 2.38 shows a typical boring log. These logs have to be attached to the final soil-exploration report submitted to the client. Note that figure 2.41 also lists the classifications of the soils in the left-hand column, along with the description of each soil (based on the United Soil Classification System).
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Figure 2.38 A typical boring log DETERMINATION OF HYDRAULIC CONDUCTIVITY IN THE FIELD
Several types of field test are now available to determine the hydraulic conductivity of soil. Two fairly easy test procedures described by the U. S. Bureau of Reclamation (1974) are the open end test and the packer the packer test. Open End Test
The first step in the open end test ( figure 2.39) is to advance a borehole to the desired depth. A casing is then driven to extend to the bottom of the borehole. Water is supplied at a constant rate from the top of the casing, and it escapes at the bottom of the borehole. The water level in the
NPTEL - ADVANCED FOUNDATION ENGINEERING-1 casing must remain constant. Once the steady state of water supply is established, the hydraulic conductivity can be determined as
Figure 2.39 Hydraulic conductivity-open end test (redrawn after U. S. Bureau of Reclamation, 1974)
= 5.5
Where
= hydraulic hydraulic conducti conductivity vity ater to the borehole borehole = constant rate of supply of wwater
[2.44]
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= inside inside radius radius of the casing casing = differenti differential al head of of water Any system of consistent units may be used in equation (44). The head, H, head, H, has been defined in figure 2.42. Note that for pressure tests (figure 2.42c and 2.42d) the value of H of H is is given as
=
(gravity )
+
(pressure )
[2.45]
The pressure head, (pressure ) , given in equation (45) is expressed in meters (or feet) of water 0.102m 2m;; 1 lb/in lb/in..2 = 2.30 2.308 8 ft ft )).. (1 kN/m2 = 0.10 Packer Test
The packer test (figure 2.40) can be conducted in a portion of the borehole during drilling or after drilling has been completed. Water is supplied to the portion of the borehole under test under constant pressure. The hydraulic conductivity can be determined.
Figure 2.40 Hydraulic conductivity determination-packer test (redrawn after U. S. S . Bureau of Reclamation, 1974)
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log (for ≥ 10) = sinh− (for 10 > ≥ ) =
[2.46]
2
1
2
[2.47]
2
Where
hydraulic conducti conductivity vity = hydraulic = constant constant rate of of flow into into the hole = length length of portion portion of the hole under under test = radius radius of the hole hole = diferential diferential pressu pressure re head
Note that the differential pressure head is the sum of the gravity head [ (gravity ) ] and the pressure head [ (pressure ) ]. The packer test is used primarily to determine the hydraulic conductivity of rock. However, as mentioned previously, it and also be used for soils.
