Benefits of the Rock Shear Test:

  • Accurately measures the effective cohesion and effective angle of internal friction of rock.
  • Useful for design of rock slopes or excavations
  • Good for computing the capacity of deep foundations and drilled shafts with rock sockets.

Rock Borehole Shear Test:  While the shear strength of rock can be critical for design of slopes and bearing capacity of deep foundations, it is difficult, if not impossible, to measure in the laboratory.  Rock quality designation (RQD) does not correlate with shear strength.  As a result of the difficulty of measuring rock strength, engineers have been ultraconservative with their designs.  Schmertmann and Hayes (1997) show that engineers use a factor of safety often greater than 10 (Figure 1).

Figure 1: Poor Prediction of Ultimate Rock Socket Capacity

Figure 2: Rock Borehole Device

Dr. Handy developed a rock borehole shear test (RBST) device to measure the shear strength properties of rock (Failmezger, White, Handy, 2008).    The device is quite robust and can apply a normal stress of 80 MPa and a shear stress of 50 MPa.  The device is placed inside of a cored borehole, and the test is conducted using hydraulic pressure to apply the normal stress and to pull the plates for the shear stress.  A shale or siltstone is likely to be smeared during the test and, after each data point, the plates are rotated axially by 45° for the next normal stress, obtaining a maximum of four data sets. With granite, the rock is likely to chip during each shearing.

The rock shear device will probably need to be removed from the borehole and the rock chips cleaned from the device. For good quality rock, a normal stress of at least 5 MPa should be used to ensure that the plates fully embed into the rock.  The device should then be lowered to about 5 mm above the previous shear depth for the next test data set.  Typical rock shear results are shown in Figure 3.

The rock shear test has a great potential for improving the design of drilled shafts socketed into rock.  Osterberg load tests will be needed to evaluate any scaling effects.

Figure 3: Typical Rock Shear Test Results

Figure 4: Assigning rock shear strengths for a slope

Rock slopes generally fail as block failure surfaces.  Failures can be along existing joints and through intact rock.  The shear strength of joints can be modeled as having a frictional resistance (i.e. tan(Φ)) and a cohesive intercept of zero, while the shear strength of intact rock can be modeled as having a frictional resistance and a cohesive intercept.  The engineer or geologist should map the profile of the rock, delineating the zones where the potential failure surface is in the direction of existing joints and where the failure is either through intact rock or against the joints.  Based on the rock borehole shear test results, the shear strength values are then assigned to each layer.  Presented as Figure 4 is diagram showing assigned shear strength parameters for suggested rock slope stability design approach.