Double-Wall Triaxial cells

In traditional triaxial systems, where saturated samples are tested, the change in specimen volume is measured simply by monitoring how much water enters or leaves the sample using a volume change transducer.

When testing unsaturated samples however, this is not possible since volume change measurements are complicated by the compressibility of the air in the voids. If an increase of confining pressure is applied to an unsaturated sample, a movement of water out of the sample will occur but an additional change in size will result from the compression of the air in the voids. Correct determination of volume change requires both the volume of water leaving the sample and the total volume change of the sample to be measured.

Axis translation method with High Air Entry Stone (HAES): operating principle.

One of the problems with testing a sample with high suction (negative pore water pressure) is that it will suck the water from the porous stone on the base pedestal and cause cavitation in the pore water pressure measuring system.To prevent this happening the porous disc has been replaced with a high air entry stone. These stones allow water to pass through but air at pressures below the rating of the stone cannot pass. For example, a 5 bar stone will not allow air at pressures under 5 bar to pass through. The stone is cemented into an anodised aluminium ring which fits into a recess in the pedestal with an o-ring seal to prevent water passing around the outside of the stone.

Components of pedestal set for unsaturated cell

1. Pedestal set for unsaturated test
2. High air Entry stone ( HEAS) sealed on aluminium ring
3. Aluminium compensation ring
4. Aluminium plate for saturated soil test
5. Porous stone for saturated soil test