Conventional Triaxial Apparatus
A widely used apparatus to determine the shear strength parameters and the stress–strain behavior of soils is the triaxial apparatus. The name is a misnomer since two, not three, stresses can be controlled. In the triaxial test, a cylindrical sample of soil, usually with a length-to-diameter ratio of 2, is subjected to either controlled increases in axial stresses or axial displacements and radial stresses. The sample is laterally confined by a membrane, and radial stresses are applied by pressuring water in a chamber (Figure). The axial stresses are applied by loading a plunger. If the axial stress is greater
Schematic of a triaxial cell.
than the radial stress, the soil is compressed vertically and the test is called triaxial compression. If the radial stress is greater than the axial stress, the soil is compressed laterally and the test is called triaxial extension.
The applied stresses are principal stresses and the loading condition is axisymmetric. For compression tests, we will denote the radial stresses sr as s3 and the axial stresses sz as s1. For extension tests, we will denote the radial stresses sr as s1 and the axial stresses sz as s3. The average stresses and strains on a soil sample in the triaxial apparatus for compression tests are as follows:
where Pz is the load on the plunger, A is the cross-sectional area of the soil sample, ro is the initial radius of the soil sample, Dr is the change in radius, Vo is the initial volume, DV is the change in volume, Ho is the initial height, and Dz is the change in height. We will call the plunger load the deviatoric load, and the corresponding stress the deviatoric stress, q 5 (s1 2 s3). The shear stress is t 5 q 2 . The area of the sample changes during loading, and at any given instance the area is
where Ao (5 pr 2 o) is the initial cross-sectional area and H is the current height of the sample. The dilation angle for a triaxial test is given by Equation . The triaxial apparatus is versatile because we can (1) independently control the applied axial and radial stresses, (2) conduct tests under drained and undrained conditions, and (3) control the applied displacements or stresses. A variety of stress paths can be applied to soil samples in the triaxial apparatus. However, only a few stress paths are used in practice to mimic typical geotechnical problems. We will discuss the tests most often used, why they are used, and typical results obtained.