An evaluation of the direct shear method for determining shearing resistance of soils

Abstract

The shearing strength of the soil not only controls the resistance to failure by sliding, but also influences the ultimate compressive loads which may be applied. The shearing resistance depends on the nature of the material and on the conditions of loading. Therefore it is necessary to know certain characteristics of the shearing resistance of the material to predict the ultimate strength under actual conditions. To determine these shearing characteristics experimentally, two methods of testing have been developed - the Direct Shear and the TriAxial Methods. These two methods represent the extreme limits of the stress conditions which could exist. The actual stress conditions within the soil should lie somewhere between these limits. If the some results were obtained by both methods, it would he reasonably certain that these results were correct. The variation of these results would also give an estimate of the accuracy of the theory of failure. This project was an investigation of the usefulness of the Direct Shear Method. This was to be done by comparing the results of the direct shear testing with the results obtained by tri-axial testing. To do this a direct shear machine was designed and built. The toots were performed on carefully controlled samples. During the testing certain difficulties were encountered, and one main error in the design of the machine was discovered. The results of the testing showed that the approach to the problem was sound. Since the testing was limited, the results are only an indication of the final results. For the material tested, however, the comparison between the results obtained by the two methods was good. The accuracy of the results was well within the range of ordinary engineering requirements.