Consolidated Undrained Triaxial Testing
Consolidated Undrained Triaxial Testing of a Fine Grained Soil
Introduction
A consolidated undrained triaxial test was carried out by 16 separate groups to give a broad appreciation of soil behaviour by plotting the resulting effective stress paths.
Results
To determine the value of Skempton’s B, the cell pressure was increased by 100kpa. The value of B indicates whether the sample is saturated or not; a value approaching 1 is expected for a fully saturated material. The change in pore water pressure was 99kpa, giving a B value of 0.99.
Effective stress path
Effective stress path in the triaxial plane
v) There appears to be a period of around 15000 seconds at the beginning of the shearing phase where the loading ram wasn’t in contact with the sample; this would explain the plateau up to 0.025 strain and the noise at the beginning of the A vs strain graph.
The group set of 16 stress paths shows a slight deviation in the Kf line, if any, which would usually be a sign of the existence of a preconsolidation stress. The deviation seems to be located around 550kPa. This point also coincides with a change in the shape of stress paths produced by the triaxial tests. The last 5 paths curve back on themselves at the failure line, which could be a result of pore pressure maintaining a positive gradient; this is a sign of a normally consolidated soil. Although the pore pressure measured against strain stays positive in group 2’s results, it reaches a maximum before reducing and follows a typical over-consolidated soil’s pore pressure graph. With further shearing, the pore pressures may have become negative. Similarly, the plot of Skempton’s A follows a typical graph created by an over-consolidated soil, but without going negative. This suggests a slightly overconsolidated soil, with an OCR of around 2.
The negative change in volume from the consolidation stage is assumed to be
Introduction
A consolidated undrained triaxial test was carried out by 16 separate groups to give a broad appreciation of soil behaviour by plotting the resulting effective stress paths.
Results
To determine the value of Skempton’s B, the cell pressure was increased by 100kpa. The value of B indicates whether the sample is saturated or not; a value approaching 1 is expected for a fully saturated material. The change in pore water pressure was 99kpa, giving a B value of 0.99.
Effective stress path
Effective stress path in the triaxial plane
v) There appears to be a period of around 15000 seconds at the beginning of the shearing phase where the loading ram wasn’t in contact with the sample; this would explain the plateau up to 0.025 strain and the noise at the beginning of the A vs strain graph.
The group set of 16 stress paths shows a slight deviation in the Kf line, if any, which would usually be a sign of the existence of a preconsolidation stress. The deviation seems to be located around 550kPa. This point also coincides with a change in the shape of stress paths produced by the triaxial tests. The last 5 paths curve back on themselves at the failure line, which could be a result of pore pressure maintaining a positive gradient; this is a sign of a normally consolidated soil. Although the pore pressure measured against strain stays positive in group 2’s results, it reaches a maximum before reducing and follows a typical over-consolidated soil’s pore pressure graph. With further shearing, the pore pressures may have become negative. Similarly, the plot of Skempton’s A follows a typical graph created by an over-consolidated soil, but without going negative. This suggests a slightly overconsolidated soil, with an OCR of around 2.
The negative change in volume from the consolidation stage is assumed to be