Consolidated Undrained (CU) Compression Test
The purpose of a CU test is to determine the undrained and drained shear strength parameters (su, f9cs, f9p). The CU test is conducted in a similar manner to the CD test except that after isotropic consolidation, the axial load is increased under undrained condition and the excess porewater pressure is measured. The applied stresses are as follows:
Stage 1: Isotropic consolidation phase
Stage 2: Shearing phase:
While the total stress path is determinate, the effective stress path can be determined only if we measure the changes in excess porewater pressures. The stresses on the soil samples and the stress paths in a CU test are illustrated in Figure 10.24. The effective stress path is nonlinear because when the soil yields, the excess porewater pressures increase nonlinearly, causing the ESP to bend. In a CU test, the volume of the soil is constant during the shear phase. Therefore,
which leads to
The axial displacement is measured and ε1 5 Dz/Ho is calculated. The maximum shear strain is
The elastic moduli Eu and (Eu)s are determined from a plot of (s1 2 s3) as ordinate and ε1 as abscissa