For testing of gravel-geosynthetic interface the use of large size direct shear testing devices are needed. The horizontal displacements that can be applied by classical large size direct shear interface test devices are limited due to the change in the contact area and the development of a moment couple during the application of the normal load at large displacements. A new device is developed which allows large horizontal displacements up to two times the size of the sample with no contact area change. The direct shear device is composed of a 0.3 m square upper box travelling over a lower box three times longer. The normal force on the upper box is applied by a set of pneumatic muscles connected to a sliding car on the rails along the side of the lower box travelling together with the upper box. This way the normality of the force is sustained even in large displacements. The long lower box provides constant contact area even at large displacements. The maximum horizontal displacement capability of the equipment is 0.6 meters. The residual interface properties of geosynthetics-soil can be easily determined with the developed equipment. To demonstrate the potential use of the box a smooth and a textured geomembrane are used and the interface properties are determined for number 2 crushed stone to model drainage material overlying geomembrane at the cover of a waste disposal site. The tests are conducted up to 150 mm horizontal displacement at which the textured geomembrane showed a thirty per cent decrease in interface shear trength which cannot be determined by using conventional testing equipment.
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