Double-Wall Triaxial cells

Double-Wall Triaxial cells

For unsaturated tests

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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


Double-wall triaxial cell unsaturated pedestal
Pedestal set for unsaturated cell, comprising pedestal, 2 bar 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.
Diameter, mmDouble-wall triaxial cell product code
High Air Entry Stones (HAES)
A 2 bar capacity 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.
Maximum air pressure
Pedestal diameter
50 mm70 mm100 mm
1 bar28-WF4150/1B28-WF4170/1B28-WF4171/1B
2 bar28-WF4150/2B28-WF4170/2B28-WF4171/2B
5 bar28-WF4150/5B28-WF4170/5B28-WF4171/5B
10 bar28-WF4150/10B28-WF4170/10B8-WF4171/10B
15 bar28-WF4150/15B28-WF4170/15B28-WF4171/15B
Spare parts
Pedestal diameter, mm5070100
Aluminium plate for testing saturated samples28-WF4150/328-WF4170/328-WF4171/3
Aluminium retaining ring28-WF4150/428-WF4170/428-WF4171/4
Base pedestal28-WF4150/528-WF4170/528-WF4171/5
Typical configuration of a system to perform triaxial tests on unsaturated samples 70 mm dia. with the axis translation method with the double wall cell 28-WF4170
28-WF4170Double-wall triaxial cell for 70 mm diameter samples1
28-WF4170/70Pedestal with HAES for 70 mm diameter samples1
28-WF4072/ATop cap for 70 mm diameter samples1
28-WF4005Tritech triaxial testing machine, 50 kN capacity1
86-D2015Air compressor, 50 L1
28-WF2016/2Air filter/water trap for air compressor1
28-WF4220/ADe-airing tank, 7 L1
86-D2001Vacuum pump1
86-D2005Air drying unit1
86-D2064Rubber tube for vacuum2
86-D0819Silica gel dessicant with indicator1
28-WF4225Valve panel for de-airing tank1
28-WF4331Pressure control panel, 3 lines1
28-WF4330/2Digital gauge for 28-WF4331l1
28-WF4320Bladder air/water cylinder2
28-WF4191Nylon tubing 6x 8 mm diameter., 10 m2
28-WF6353Submersible load cell, 5 kN capacity1
30-WF6208Displacement transducer, 25 mm travel1
28-WF6221Mounting bracket for 25 mm diameter cell ram1
28-WF6300Pressure transducers, 10 bar2
28-WF6310De-airing block2
28-WF4410Automatic volume change apparatus2
30-WF6008Geodatalog 8-channels datalogger1
82-P9008/ELTSet of four cables2
30-WF6042Transducer extension cables6
Nominal sample diameter, mm70100
Sample diameter range, mm50 à 7050 à100
Maximum working pressure, kPa20002000
Maximum cell height [mm]690795
Cell diameter, mm (inc. valves)478535
Weight, kg (approx.)3050
No. of inlet points5 (for top drainage/back pressure, cell pressure and base drainage/pore pressure)
Attachment for vacuum top cap for
extension tests
Ordering informationDouble-wall triaxial cell for unsaturated
tests on 70 mm diameter soil samples,
complete with 6 ports for transducer
Double-wall triaxial cell for unsaturated
tests on 100 mm diameter soil samples,
complete with 6 ports for transducer