UCLA

We describe the characteristics of a simple shear apparatus capable of applying realistic multidirectional earthquake loading to soil specimens. This device, herein termed the Digitally Controlled Simple Shear (DC-SS) apparatus, incorporates features such as servohydraulic actuation and true digital control to overcome control limitations of some previous dynamic soil testing machines. The device is shown to be capable of reproducing sinusoidal and broadband command signals across a wide range of frequencies and amplitudes, although the device has limited control capabilities for very small command displacements (less than approximately 0.005 mm). The small deformation limitation results from noise introduced to the control system from analog-to-digital conversion of feedback signals. We demonstrate that bidirectional command signals can be accurately imparted with minimal cross coupling, which results from an innovative multiple-input, multiple-output digital control system. The capabilities of the device are demonstrated with a series of broadband tests on unsaturated soil specimens subjected to uni- and bidirectional excitation.