A Multiple Purpose Soil Testing Apparatus

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Courtesy of ASTM International

A relatively compact soil testing system that is capable of performing monotonic/cyclic triaxial and torsional simple shear tests has been developed. A high-torque direct-drive, servo/stepper motor is used to provide the driving force. Under the servo mode, the motor could be controlled to provide a constant torque, which is used in stress-controlled tests. Strain-controlled tests are conducted using the stepper control mode. To perform triaxial tests, the rotational motion of the motor is transformed into linear motion using a preloaded ball screw shaft. A preloaded ball spline shaft directly attached to the motor is used to perform torsional simple shear tests. Because there are no gears in the driving system, there is no backlash during reversals of loading directions. The motor is equipped with an encoder and an indexer. The extremely rigid driving system and high resolution of the encoder enable the linear and rotational displacement to be monitored through the encoder/indexer system. Test control and data logging are fully automated using a personal computer. This paper provides details of the apparatus. A set of strain-controlled test data are presented to demonstrate capabilities of the new apparatus.

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