• 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, mm Double-wall triaxial cell product code
28-WF4170 28-WF4171
50 28-WF4170/50 28-WF4171/50
70 28-WF4170/70 28-WF4171/70
100 - 28-WF4171/100
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 mm 70 mm 100 mm
1 bar 28-WF4150/1B 28-WF4170/1B 28-WF4171/1B
2 bar 28-WF4150/2B 28-WF4170/2B 28-WF4171/2B
5 bar 28-WF4150/5B 28-WF4170/5B 28-WF4171/5B
10 bar 28-WF4150/10B 28-WF4170/10B 8-WF4171/10B
15 bar 28-WF4150/15B 28-WF4170/15B 28-WF4171/15B
Spare parts
Pedestal diameter, mm 50 70 100
Aluminium plate for testing saturated samples 28-WF4150/3 28-WF4170/3 28-WF4171/3
Aluminium retaining ring 28-WF4150/4 28-WF4170/4 28-WF4171/4
Base pedestal 28-WF4150/5 28-WF4170/5 28-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
Code Description Qty
28-WF4170 Double-wall triaxial cell for 70 mm diameter samples 1
28-WF4170/70 Pedestal with HAES for 70 mm diameter samples 1
28-WF4072/A Top cap for 70 mm diameter samples 1
28-WF4005 Tritech triaxial testing machine, 50 kN capacity 1
86-D2015 Air compressor, 50 L 1
28-WF2016/2 Air filter/water trap for air compressor 1
28-WF4220/A De-airing tank, 7 L 1
86-D2001 Vacuum pump 1
86-D2005 Air drying unit 1
86-D2064 Rubber tube for vacuum 2
86-D0819 Silica gel dessicant with indicator 1
28-WF4225 Valve panel for de-airing tank 1
28-WF4331 Pressure control panel, 3 lines 1
28-WF4330/2 Digital gauge for 28-WF4331l 1
28-WF4320 Bladder air/water cylinder 2
28-WF4191 Nylon tubing 6x 8 mm diameter., 10 m 2
28-WF6353 Submersible load cell, 5 kN capacity 1
30-WF6208 Displacement transducer, 25 mm travel 1
28-WF6221 Mounting bracket for 25 mm diameter cell ram 1
28-WF6300 Pressure transducers, 10 bar 2
28-WF6310 De-airing block 2
28-WF4410 Automatic volume change apparatus 2
30-WF6008 Geodatalog 8-channels datalogger 1
82-P9008/ELT Set of four cables 2
30-WF6042 Transducer extension cables 6
Models 28-WF4170 28-WF4171
Nominal sample diameter, mm 70 100
Sample diameter range, mm 50 à 70 50 à100
Maximum working pressure, kPa 2000 2000
Maximum cell height [mm] 690 795
Cell diameter, mm (inc. valves) 478 535
Weight, kg (approx.) 30 50
No. of inlet points 5 (for top drainage/back pressure, cell pressure and base drainage/pore pressure)
Attachment for vacuum top cap for
extension tests
Ordering information Double-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