Design and evaluation of a large direct shear machine for geosynthetic clay liners

A large direct shear machine-called the pullout shear machine-for strength testing of geosynthetic clay liners (GCLs) is described. The machine tests rectangular GCL specimens measuring 406 by 1067 mm. The maximum horizontal displacement is 203 mm, which allows for the measurement of both peak and residual shear strengths. The basic design concept for the pullout shear machine is to shear a GCL specimen between a movable pullout plate and a stationary reaction plate, each covered with an aggressive gripping surface. The advantage of this approach is that large specimens can be sheared under high normal stress with negligible friction. Furthermore, the gripping surfaces and the rigidity of the pullout plate enforce uniform shear strain at failure. The paper describes four main components of the machine: (1) the shearing system, (2) the normal stress and vertical displacement system, (3) the specimen hydration system, and (4) the process control and data acquisition system. In addition, a technique is described for the measurement of internal pore pressures within a GCL specimen during hydration and shear. The performance of the pullout shear machine is illustrated using test data for unreinforced and reinforced geotextile-supported GCLs. Finally, the relative merits of the machine are discussed in comparison to the capabilities of other shear testing apparatus currently available.