Abstract:
The use of the triaxial test to characterize the strength of soils for civil engineering applications is widespread. These tests are typically conducted with confining stresses in excess of 5 psi. To characterize the
strength of a soil located in the upper layers of the subgrade of an aggregate
surfaced road it is necessary to conduct triaxial tests with low confining stresses (5 psi or less). The development of a method for conducting multistage, consolidated
undrained (CU) tests at low confining stresses (0.5 to 5.0 psi), with back pressure saturation, is presented. Aspects of the test procedure that require special attention are described and recommendations are made including: 1. Compaction of the sample in an atmosphere of carbon dioxide
reduces the time and pressure required to complete back pressure saturation. 2. Seepage force related pore pressures develop during sample flooding. Zeroing of the effective stress transducer should be
completed prior to sample flooding so that it is certain that zero effective stress conditions are present. 3. Back pressure saturation is simplified by the use of a slave regulator (air loaded pressure regulator) that maintains a nearly constant pressure differential between the cell pressure and the back pressure. 4. The stress path method of interpretation is an essential part of multistage triaxial testing. This method simplifies the decision of when to stop each shear stage and the determination of shear strength parameters.
5. The use of a computer data acquisition system that processes data in real time and visually presents test progress simplifies the completion of multistage triaxial tests.
