Civil and Building Engineering.

Several other types of tests for measuring the shearing properties of soils have been developed in addition to the direct shear and triaxial shear tests. Short descriptions of several of these tests follow.

One type of test is the direct simple shear test. In this test, the soil specimen is distorted by rotation of the sides of the shear box. Rotation of the specimen is accomplished by confining the test specimen in a wire-reinforced membrane, an articulated shear box, or a stack of thin rings that can slide. In each of these configurations, the soil specimen is distorted to develop shear strain within it. This type of test has had limited use by consultants and is usually used in conjunction with triaxial shear tests and consolidation tests to develop constitutive parameters for advanced models of material behavior.

Another type of shear test is the ring shear test, in which the shear box is composed of two horizontal rings. The soil properties measured in this test are the same as those measured in the direct shear test. However, the ring shear test is not limited in its range of deformations because one ring is driven in shear using a worm gear system. Ring shear devices are very useful in measuring the residual shearing resistance on well-formed shear planes, such as those that might develop in a slope failure. These devices are commercially available, but have the limitation that a large-diameter soil sample must be used for testing undisturbed soils.

Other types of tests that are used to measure the dynamic shearing properties of soils are the cyclic triaxial test and the resonant column test. The results of these tests are used to solve problems in earthquake engineering or mechanical vibration for which dynamic soil properties are important.

In the cyclic triaxial test, the soil sample is prepared in the same manner as for a conventional triaxial test but is sheared using various levels of cyclic loading. The main property of interest measured in the cyclic triaxial test is the number of cycles of loading needed to cause the soil specimen to fail.

In the resonant column test, a cylindrical soil specimen is placed under a confining stress and permitted to consolidate. After consolidation is complete, the specimen is vibrated in torsion. The resonant frequency of the specimen is measured as a function of the shearing strain amplitude developed in the specimen, and after vibration ceases, the decay of vibrations is measured. The principal measurements made using the resonant column test are shear wave velocity, shear modulus, and material damping ratio.

Lastly, the performance of stress-path triaxial tests is possible. In this type of test, computer-controlled feedback testing is used to force the stress in the test specimen to follow a prescribed path. A stress path is the trajectory or path of stress states on a Mohr-Coulomb diagram. Stress-path triaxial testing is available at only a few commercial testing laboratories and is usually used only for major projects for which benefits might be realized. Modern computer-controlled triaxial equipment like that shown in Figure 3.63 is capable of performing stress-path testing. However, the foundation engineer must first perform advanced modeling of a problem to determine the desired stress path to be followed.