Double wall triaxial cells for unsaturated tests

Description

Double wall triaxial cell, inner wall made from glass, complete with acces ring for transducer cables. The cell has to be completed with the base pedestal with High Entry Stone. See accessories. Click here for a Axis translation method: configuration of test system.

Introduction
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.

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.

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.
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.

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.

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

Technical Specs

Accessories

Double-wall triaxial cell unsaturated pedestal
Pedestal set for unsaturated cell, comprising pedestal, High Air Entry Stone sealed into an aluminium ring and a height compensation ring. Using specific adaptive pedestals.

It is possible to test specimens with different diameters in the same triaxial cell – see the table below.

Example
28-WF4171/70
Unsatured test set for 70mm dia.sample to be fitted in double wall triaxial cell mod. 28-WF4171 comprising:  base pedestal, High Air Entry Stone sealed  on aluminium ring and compensation ring.

High Air Entry Stone (HAES) 
High Air Entry Stones (HAES) An HAES is included with the pedestals as standard but it can be easily replaced with stones of other capacities for 50, 70 and100 mm diameter pedestals – see the table below.

Example
28-WF4170/5B
5 bar High Air Entry Stone sealed on alluminium  ring, for 70 mm dia. sample