Description

This is an Automatic Triaxial Testing Machine used to perform consolidated undrained and consolidated drained triaxial test. Consolidated Undrained (CU) Test & Consolidated Drained (CD) Test Peak effective strength parameters (c’ and f’) may be determined either from the results of consolidated undrained (CU) triaxial compression tests with pore pressure measurement, or from consolidated drained (CD) triaxial compression tests. The consolidated undrained/ drained triaxial compression tests are normally performed in several stages, involving the successive saturation, consolidation and shearing of each of three
specimens.

Saturation is carried out in order to ensure that the pore fluid in the specimen does not contain free air. Saturation is normally carried out by leaving the specimens to swell against an elevated back pressure. Back pressure (which is simply an imposed pore pressure) is applied through a volume change gauge to the top of the specimen, while a cell pressure of slightly higher value is also applied. Both cell pressure and back pressure are normally increased in increments, allowing time for equalization at each stage. The degree of saturation can be expressed in terms of Skempton’s pore pressure parameter.

Consolidated Undrained (CU) Test: Once consolidationis complete, the specimen is to be isolated from the back pressure and the rate of vertical movement of the compression machine platen set according to result of consolidation.. During the shear stage the vertical stress is increasedby the loading ram, and measurements are made at regular intervals of deformation, ram load and pore pressure. These are converted to graphs of principal stress difference (s1- s3) and pore pressure as a function of strain, and failure is normally taken as the point of maximum principal stress difference. The effective stress Mohr circles are plotted for the failure conditions of the three specimens which has been subjected to different consolidation level, and the gradient and intercept of a straight line drawn tangential to these circles defines the effective strength parameters c’ andf’.