A double walled triaxial cell can be the solution: the same pressure inside and outside of the inner cell wall will produce zero expansion of the inner cell and allow to measure the total volume change from a volume change transducer inserted in the cell pressure line.

The inner wall of the cell above is made from glass: this eliminates the problem of water absorption.

In the traditional triaxial systems, where saturated samples are tested, the volume change measurement is a simple monitoring of the water entering or leaving the sample by a volume change transducer.

A correct measurement requires the volume of water leaving the sample and the total volume change of the sample as shown in the sketch.

With these two measurements, by difference the volume change due to water being squeezed out of the sample and the volume change due to the compressibility of air can be determined.

Cell pressure line supplying pressure to the outer cell and the inner cell via the volume change transducer.

On the contrary, in the unsaturated systems volume change measurements are complicated by the compressibility of air.

If an increase of confining pressure is applied to an unsaturated sample, a movement of water out of the sample will occur but at the same time the size will change due to the compression of the air in the voids.

The cell pressure is applied equally to the inside and outside of the glass wall: this eliminates the problem of expansion.

The total volume change of the sample can then be measured using a standard volume change transducer.