Materials and Methodology 64 followed by application of cell pressure (CP) of 15-20 kPa and simultaneous release of vacuum pressure (Ishihara, 1993). The cell pressure was applied in this manner to prevent the intrusion and flow of CO2 through the peripheral interface of rubber membrane and specimen boundary.
The specimen preparation was followed by subsequent saturation and consolidation stages.
Fig. 3.16 (a) Mould for the preparation of RS specimen (b) Mould ready to be filled with water-mixed soil from one end, while the other end is fixed with the collar (c) Prepared RS
specimen
Materials and Methodology 65 ratio of the increase of pore water pressure ‘∆u’ to the cell pressure increment ‘∆σc’. To determine the B-value, a cell pressure increment is applied while keeping the drainage valves closed (undrained condition) and the corresponding increment in the pore pressure is measured.
When the increment of pore water pressure attains an equilibrium state, the B-value is calculated. B-value greater than or equal to 0.95 is an indication of degree of saturation nearly equal to 100% (Head, 1992). The B-value and corresponding degree of saturation is affected by compressibility, soil-texture, permeability, density and compactness of packing of the soil (Karg and Haegeman, 2009).
In the present study, in order to expedite the saturation process, the specimens were flushed with CO2 for 10-15 minutes at a pressure lower than the applied initial cell pressure (~15-20 kPa) (as per Ishihara et al., 1978a). CO2 was made to flow into the specimen through the bottom inlet pressure line of triaxial cell base plate, which then passes through the sample to reach the upper pressure line (connected to the bottom outlet pressure line of triaxial cell base plate via top cap). Since CO2 is heavier, it replaces air present in the voids of the sample and is easily soluble in water than air. Subsequently, de-aired water was allowed through the CO2
flushed specimen. The height of de-aired water tank was so maintained that the pressure head should be lower than the initial cell pressure of 15-20 kPa. To attain the saturation, the cell pressure (CP) and back pressure (BP) were then gradually increased in stages while maintaining an almost constant differential pressure of 10 kPa (Head, 1992). After each increment of CP, the B-value was estimated to check the saturation status. The specimen was considered to be completely saturated when the B-value was obtained to be greater than 0.95. The consolidated specimens were then subjected to monotonic and cyclic loading.
For cohesive soil
In order to expedite the saturation process, the specimen was flushed with CO2 for 45 minutes to 1 hour at a pressure lower than 15-20 kPa, the initial CP (Ishihara, 1993). Subsequently, de-
Materials and Methodology 66 aired water was passed through CO2 flushed specimen. The water pressure head was maintained less than the existing cell pressure of 15-20 kPa. To attain the saturation, CP and BP were then gradually increased in stages while maintaining an almost constant differential pressure of 10 kPa. After each increment of CP, the Skempton’s pore-pressure parameter (B) was estimated to check the saturation status. The specimen was considered to be completely saturated when the B-value was obtained to be greater than 0.96. The time taken in saturation for one specimen was 4-5 days, which is, from the application of CO2 to achieving the B-value greater than 0.96.
After attaining saturation, the specimens, both cohesionless and cohesive soils, were isotropically consolidated to a targeted σʹc by increasing CP while maintaining a constant BP.
During the consolidation process, the specimen can be maintained at an isotropic or anisotropic state of stress according to the test requirement. For anisotropic consolidation, an additional axial stress is to be applied on the specimen. The effective stress can be applied either by increasing CP, or by decreasing BP, or by the combination of both (Head, 1992). In this study, the specimen was isotropically consolidated with increased CP while keeping constant BP, by allowing water to drain out into AVC system. After stabilization of incremental pore water pressure, the drainage valve was opened to allow the outflow of water. The drainage of water results in the decrease in volume and increase in the effective stress. During consolidation, the effective stress was estimated as the difference between CP and the residual pore pressure in the specimen, as shown in Fig. 3.17 (Head, 1992).
Theoretically, at the end of consolidation, the residual pore water pressure should become equal to BP; however, in experimental practice, such idealization is not achieved. Therefore, the desired effective confining stress on the specimens was considered to be achieved, when the difference between CP and pore pressure becomes equal to targeted value. The consolidation stage for cohesionless soil was considered to be completed when the excess pore water pressure nearly attained the applied BP. For cohesive soil, as the time required for excess
Materials and Methodology 67 PWP to achieve BP is significantly high, the consolidation is considered to be completed if the volume change becomes constant or when 95% dissipation of the excess PWP is attained. The time needed in consolidation depends on the type of soil and its permeability.
Fig. 3.17 Consolidation schematic of test specimen (after Head, 1992)