CHAPTER 2.  DIRECT SHEAR TESTING

To evaluate the soil material model, it is important that actual physical tests of the soil be simulated. This chapter describes one of the physical tests used for evaluating the soil model―the direct shear test. Two other tests―a soil modulus failure test and a soil shear failure test―are briefly described in appendix A and detailed by Coon.⁽⁵⁾

A large-scale direct shear testing device was developed at the user's facility.⁽⁶⁾ This device was developed because it replicates one of the most common material tests performed on soil, except on a scale 10 times larger than the standard American Society for Testing and Materials (ASTM) specifications for direct shear testing. The larger scale was needed to accurately capture the direct shear behavior of National Cooperative Highway Research Program (NCHRP) Report 350 strong soil, which is the standard soil for roadside safety crash testing.⁽⁷⁾ The 500-millimeter (mm)-diameter direct shear testing device is shown in figure 1 and detailed by Coon, et al.⁽⁶⁾

Nicholson, through testing, developed a general curve depicting soil performance during direct shear testing (see figure 2).⁽⁸⁾ Results from direct shear testing performed on NCHRP 350 strong soil with an 18.5-kilopascal (kPa) overburden, using the user's large-scale direct shear testing device, are shown in figure 3. Physical test results from NCHRP 350 strong soil compared well with prior laboratory data.

Figure 1. Large-scale direct shear testing device.

Figure 2. General direct shear testing performance.⁽⁸⁾

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