Assistant professor, University of Massachusetts at Amherst, Amherst, MA
Principal research associate, Massachusetts Institute of Technology, Cambridge, MA
President, Advanced Machine Technology, Inc., Cambridge, MA
This paper describes the design and implementation of an automated electropneumatic control system for direct simple shear testing of cohesive soils. The hardware and software used to perform automated application of consolidation increments and for subsequent constant volume shear of soil specimens is described in detail. Two servo-control algorithms are used to conduct a test, one for application of consolidation increments and another for maintaining constant volume during undrained shear. It is shown that a servocontrol system using a proportional controller can successfully control stress during consolidation increments but cannot control displacement by varying stress as required during undrained shear of cohesive soils. Results are presented which show that while the proportional controller can successfully perform displacement control for mechanical and hydraulic systems it cannot do so for more compliant pneumatic systems. This is primarily due to the fact that pneumatic systems are significantly less rigid due to the high compressibility of air as compared to the compressibility of mechanical systems and hydraulic fluids. While attempting to maintain constant height of the specimen, as is necessary to maintain constant volume during undrained shear, the pneumatic-based system falls into an irrecoverable oscillation of the vertical stress. It is shown that the undrained shear portion of a test required the implementation of the more sophisticated proportional plus integral plus derivative (PID) controller. Unlike the proportional controller, the PID controller easily maintains constant height of the specimen during undrained shear without instability in the vertical stress. Results are given comparing the performance of both the proportional controller and the PID controller during undrained shear of a cohesive soil specimen.
Paper ID: GTJ10202J