Pullout Test Rig
Pullout Test Rig
Due to the limitations of existing pullout rigs having fixed boundaries, a novel pullout test apparatus with a movable top platen, to allow dilation of the soil, was developed. It consisted of a steel box which has a removable steel top platen with chamfered sides to reduce friction - similar concept to the top platen of the shear box. The front plate of the box has five openings through which a geotextile passes. This geotextile is then clamped a vertical plate which runs on frictionless bearings. To this, a load cell is attached which linked to the drive mechanism to give the required displacement rate.
To enable dilation of the soil during pullout the confining stress is applied via a closed loop pneumatic/hydraulic unit to a load cell, which is suspended just above the top platen. To allow rotation a ball and socket transfer unit is used to bridge the connection, between the load cell and top platen. An air control valve and a dump valve are used to control an air operated liquid pump, which controlled the hydraulic pressure. This is linked to a data acquisition system. Pneumatic pressure is supplied to the pump by a compressor after passing through a regulator, an air pressure valve and a flow restriction valve. This air pressure then opens the pump which increases the hydraulic pressure to the required confining stress. Once this pressure is exceeded a dump valve operated to reduce the pressure and prevent overshoot. Once the stress fall below the target confining stress the valves reverse. Four low voltage displacement transducers (LVDT) positioned at the corner of the steel top platen monitor vertical movement. These are logged by the data acquisition system along with the load cell output. Control of the data acquisition system is carried out by a PC using specialist software.