Deb Consolidation
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Consolidation (soil)
From Wikipedia, the free encyclopedia Jump to: navigation, search For compaction near the surface, see Soil compaction; for natural compa ction on a geologic scale, see Compaction (geology). Consolidation is a process by which soils decrease in volume. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume. When this oc curs in a soil that is saturated with water, water will be squeezed out of the soil. The magnitude of consolidation can be predicted by many di fferent methods. In the Classical Method, developed by Karl von Terzagh i, soils are tested with an oedometer test to determine their compressi on index. This can be used to predict the amount of consolidation. When stress is removed from a consolidated soil, the soil will rebound, regaining some of the volume it had lost in the consolidation process. If the stress is reapplied, the soil will consolidate again along a rec ompression curve, defined by the recompression index. The soil which ha d its load removed is considered to be overconsolidated. This is the ca se for soils which have previously had glaciers on them. The highest st ress that it has been subjected to is termed the preconsolidation stres s. The over consolidation ratio or OCR is defined as the highest stress experienced divided by the current stress. A soil which is currently ex periencing its highest stress is said to be normally consolidated and t o have an OCR of one. A soil could be considered underconsolidated imme diately after a new load is applied but before the excess pore water pr essure has had time to dissipate. Contents [hide] * 1 Consolidation analysis o 1.1 Spring analogy o 1.2 Primary consolidation o 1.3 Secondary consolidation * 2 Time dependency * 3 See also * 4 External Links * 5 References
Consolidation analysis Spring analogy
The process of consolidation is often explained with an idealized syste m composed of a spring, a container with a hole in its cover, and wate r. In this system, the spring represents the compressibility or the str ucture itself of the soil, and the water which fills the container repr esents the pore water in the soil. Consolidation spring analogy.jpg 1. The container is completely filled with water, and the hole is cl osed. (Fully saturated soil) 2. A load is applied onto the cover, while the hole is still unopene
d. At this stage, only the water resists the applied load. (Development of excess pore water pressure) 3. As soon as the hole is opened, water starts to drain out through the hole and the spring Consolidometers ASTM: D 2435 D 4546 AASHTO: T-216 Durham Geo offers both the pneumatic loading consolidometer (S-450 Terr aload) and the lever type consolidometer (S-449 dead weight Load Fram e). The following may help in choosing between a pneumatic or lever typ e consolidation frame: The S-450 Terraload Pneumatic Consolidation Device requires less space in the lab than a dead weight device. has a lesser chance of being bumped compared to the lever type. is easier to operate when it comes to adding loads to the sample. Opera tors tend to prefer turning a valve rather than carefully adding a weig ht by hand. The weight on the dead weight device can be added too fast or too slow and affect results. The weight works fine, but takes some o perator experience. requires a source of compressed air. This is an additional expense. The re is also a chance that the power will fail, thus a chance of "loosing the sample". The lever type requires no external source of air, and is independent of electrical power or air compressor failures.
The S-449 Dead Weight Consolidation Load Frame is considered to be more accurate than the pneumatic frame at very low loads. slightly less costly to set up, but the pneumatic frame costs less when multiple units are purchased. This is because one S-45040 Digital Reado ut and pressure transducer can be used with multiple S-450 units. shortens. (Drainage of excess pore water pressure) 4. After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excess pore water pressure. End of consolidation)
Primary consolidation
This method assumes consolidation occurs in only one-dimension. Laborat
ory data is used to construct a plot of strain or void ratio verses eff ective stress where the effective stress axis is on a logarithmic scal e. The plot's slope is the compression index or recompession index. The equation for consolidation settlement of a normally consolidated soil c an then be determined to be: \delta_c = \frac{ C_c }{ 1 + e_0 } H \log \left( \frac{ \sigma_{zf}' }{ \sigma_{z0}' } \right) \ where δc is the settlement due to consolidation. Cc is the compression index. e0 is the initial void ratio. H is the height of the soil. σzf is the final vertical stress. σz0 is the initial vertical stress. Cc can be replaced by Cr (the recompression index) for use in overconso lidated soils where the final effective stress is less than the precons olidation stress. When the final effective stress is greater than the p reconsolidation stress, the two equations must be used in combination t o model both the recompression portion and the virgin compression porti on of the consolidation process, as follows: \delta_c = \frac{ C_r }{ 1 + e_0 } H \log \left( \frac{ \sigma_{zc}' }{ \sigma_{z0}' } \right) + \frac{ C_c }{ 1 + e_0 } H \log \left( \frac{ \ sigma_{zf}' }{ \sigma_{zc}' } \right)\ where σzc is the preconsolidation stress of the soil. [edit] Secondary consolidation Secondary consolidation is the compression of soil that takes place aft er primary consolidation. Secondary consolidation is caused by creep, v iscous behavior of the clay-water system, compression of organic matte r, and other processes. In sand, settlement caused by secondary compres sion is negligible, but in peat, it is very significant. Secondary consolidation is given by the formula S_s=\frac{H_0}{1+e_0} C_{a} \log \left( \frac {t} {t_{90} } \right) \ Where H0 is the height of the consolidating medium e0 is the initial void ratio Ca is the secondary compression index [edit] Time dependency The time for consolidation to occur can be predicted. Sometimes consoli dation can take years. This is especially true in saturated clays becau se their hydraulic conductivity is extremely low, and this causes the w ater to take an exceptionally long time to drain out of the soil. While drainage is occurring, the pore water pressure is greater than normal b ecause it is carrying part of the applied stress (as opposed to the soi l particles).
Consolidometers
ASTM: D 2435 D 4546 AASHTO: T-216 Durham Geo offers both the pneumatic loading consolidometer (S-450 Terr aload) and the lever type consolidometer (S-449 dead weight Load Fram e). The following may help in choosing between a pneumatic or lever typ e consolidation frame: The S-450 Terraload Pneumatic Consolidation Device requires less space in the lab than a dead weight device. has a lesser chance of being bumped compared to the lever type. is easier to operate when it comes to adding loads to the sample. Opera tors tend to prefer turning a valve rather than carefully adding a weig ht by hand. The weight on the dead weight device can be added too fast or too slow and affect results. The weight works fine, but takes some o perator experience. requires a source of compressed air. This is an additional expense. The re is also a chance that the power will fail, thus a chance of "loosing the sample". The lever type requires no external source of air, and is independent of electrical power or air compressor failures.
The S-449 Dead Weight Consolidation Load Frame is considered to be more accurate than the pneumatic frame at very low loads. slightly less costly to set up, but the pneumatic frame costs less when multiple units are purchased. This is because one S-45040 Digital Reado ut and pressure transducer can be used with multiple S-450 units.
From Wikipedia, the free encyclopedia Jump to: navigation, search For compaction near the surface, see Soil compaction; for natural compa ction on a geologic scale, see Compaction (geology). Consolidation is a process by which soils decrease in volume. It occurs when stress is applied to a soil that causes the soil particles to pack together more tightly, therefore reducing its bulk volume. When this oc curs in a soil that is saturated with water, water will be squeezed out of the soil. The magnitude of consolidation can be predicted by many di fferent methods. In the Classical Method, developed by Karl von Terzagh i, soils are tested with an oedometer test to determine their compressi on index. This can be used to predict the amount of consolidation. When stress is removed from a consolidated soil, the soil will rebound, regaining some of the volume it had lost in the consolidation process. If the stress is reapplied, the soil will consolidate again along a rec ompression curve, defined by the recompression index. The soil which ha d its load removed is considered to be overconsolidated. This is the ca se for soils which have previously had glaciers on them. The highest st ress that it has been subjected to is termed the preconsolidation stres s. The over consolidation ratio or OCR is defined as the highest stress experienced divided by the current stress. A soil which is currently ex periencing its highest stress is said to be normally consolidated and t o have an OCR of one. A soil could be considered underconsolidated imme diately after a new load is applied but before the excess pore water pr essure has had time to dissipate. Contents [hide] * 1 Consolidation analysis o 1.1 Spring analogy o 1.2 Primary consolidation o 1.3 Secondary consolidation * 2 Time dependency * 3 See also * 4 External Links * 5 References
Consolidation analysis Spring analogy
The process of consolidation is often explained with an idealized syste m composed of a spring, a container with a hole in its cover, and wate r. In this system, the spring represents the compressibility or the str ucture itself of the soil, and the water which fills the container repr esents the pore water in the soil. Consolidation spring analogy.jpg 1. The container is completely filled with water, and the hole is cl osed. (Fully saturated soil) 2. A load is applied onto the cover, while the hole is still unopene
d. At this stage, only the water resists the applied load. (Development of excess pore water pressure) 3. As soon as the hole is opened, water starts to drain out through the hole and the spring Consolidometers ASTM: D 2435 D 4546 AASHTO: T-216 Durham Geo offers both the pneumatic loading consolidometer (S-450 Terr aload) and the lever type consolidometer (S-449 dead weight Load Fram e). The following may help in choosing between a pneumatic or lever typ e consolidation frame: The S-450 Terraload Pneumatic Consolidation Device requires less space in the lab than a dead weight device. has a lesser chance of being bumped compared to the lever type. is easier to operate when it comes to adding loads to the sample. Opera tors tend to prefer turning a valve rather than carefully adding a weig ht by hand. The weight on the dead weight device can be added too fast or too slow and affect results. The weight works fine, but takes some o perator experience. requires a source of compressed air. This is an additional expense. The re is also a chance that the power will fail, thus a chance of "loosing the sample". The lever type requires no external source of air, and is independent of electrical power or air compressor failures.
The S-449 Dead Weight Consolidation Load Frame is considered to be more accurate than the pneumatic frame at very low loads. slightly less costly to set up, but the pneumatic frame costs less when multiple units are purchased. This is because one S-45040 Digital Reado ut and pressure transducer can be used with multiple S-450 units. shortens. (Drainage of excess pore water pressure) 4. After some time, the drainage of water no longer occurs. Now, the spring alone resists the applied load. (Full dissipation of excess pore water pressure. End of consolidation)
Primary consolidation
This method assumes consolidation occurs in only one-dimension. Laborat
ory data is used to construct a plot of strain or void ratio verses eff ective stress where the effective stress axis is on a logarithmic scal e. The plot's slope is the compression index or recompession index. The equation for consolidation settlement of a normally consolidated soil c an then be determined to be: \delta_c = \frac{ C_c }{ 1 + e_0 } H \log \left( \frac{ \sigma_{zf}' }{ \sigma_{z0}' } \right) \ where δc is the settlement due to consolidation. Cc is the compression index. e0 is the initial void ratio. H is the height of the soil. σzf is the final vertical stress. σz0 is the initial vertical stress. Cc can be replaced by Cr (the recompression index) for use in overconso lidated soils where the final effective stress is less than the precons olidation stress. When the final effective stress is greater than the p reconsolidation stress, the two equations must be used in combination t o model both the recompression portion and the virgin compression porti on of the consolidation process, as follows: \delta_c = \frac{ C_r }{ 1 + e_0 } H \log \left( \frac{ \sigma_{zc}' }{ \sigma_{z0}' } \right) + \frac{ C_c }{ 1 + e_0 } H \log \left( \frac{ \ sigma_{zf}' }{ \sigma_{zc}' } \right)\ where σzc is the preconsolidation stress of the soil. [edit] Secondary consolidation Secondary consolidation is the compression of soil that takes place aft er primary consolidation. Secondary consolidation is caused by creep, v iscous behavior of the clay-water system, compression of organic matte r, and other processes. In sand, settlement caused by secondary compres sion is negligible, but in peat, it is very significant. Secondary consolidation is given by the formula S_s=\frac{H_0}{1+e_0} C_{a} \log \left( \frac {t} {t_{90} } \right) \ Where H0 is the height of the consolidating medium e0 is the initial void ratio Ca is the secondary compression index [edit] Time dependency The time for consolidation to occur can be predicted. Sometimes consoli dation can take years. This is especially true in saturated clays becau se their hydraulic conductivity is extremely low, and this causes the w ater to take an exceptionally long time to drain out of the soil. While drainage is occurring, the pore water pressure is greater than normal b ecause it is carrying part of the applied stress (as opposed to the soi l particles).
Consolidometers
ASTM: D 2435 D 4546 AASHTO: T-216 Durham Geo offers both the pneumatic loading consolidometer (S-450 Terr aload) and the lever type consolidometer (S-449 dead weight Load Fram e). The following may help in choosing between a pneumatic or lever typ e consolidation frame: The S-450 Terraload Pneumatic Consolidation Device requires less space in the lab than a dead weight device. has a lesser chance of being bumped compared to the lever type. is easier to operate when it comes to adding loads to the sample. Opera tors tend to prefer turning a valve rather than carefully adding a weig ht by hand. The weight on the dead weight device can be added too fast or too slow and affect results. The weight works fine, but takes some o perator experience. requires a source of compressed air. This is an additional expense. The re is also a chance that the power will fail, thus a chance of "loosing the sample". The lever type requires no external source of air, and is independent of electrical power or air compressor failures.
The S-449 Dead Weight Consolidation Load Frame is considered to be more accurate than the pneumatic frame at very low loads. slightly less costly to set up, but the pneumatic frame costs less when multiple units are purchased. This is because one S-45040 Digital Reado ut and pressure transducer can be used with multiple S-450 units.
