Direct shear machine testing

Q: We just bought a direct shear machine and want to start ASTM D5321 testing. The pneumatic piston applying the vertical loads for normal pressure has a range from a low of 1,124 pounds to a high of 20,000 pounds. Can we use this direct shear box for landfill cover testing?

A: Unfortunately, your low load of 1,124 pounds equals 7.8 psi, assuming your box has the appropriate 1-square-foot area. This load is far too great to capture the normal pressure range for most landfill covers, which is usually in the range of 0.5 to 3 psi. Remember that the site-specific normal pressure generally is the middle normal pressure of the three-point test.

Since this box appears to be built for the testing of a landfill liner system instead of a cover system, you need to ensure the capacity of the “shear” load cell is operating in the 10%-90% load range. This is why most labs doing accredited direct shear testing use two boxes, with each one configured very differently: one for light loads (cover system) and one for heavy loads (liner systems).