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Soil mechanics

Double wall triaxial cells for unsaturated tests

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General description
General description
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.

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.


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Technical specifications

Product code 28-WF4170 28-WF4171
Max diamter sample [mm] 70 100
Sample diamter range [mm] 50 to 70  50 to 100
Maximum working pressure [kPa] 2000 2000
Maximum cell height [mm] 690 795
Cell diamter including valves [mm] 478 535
Weight (approx) [kg] 30 50
No. of inlet ports 6 6
Attachment for vaccum top cap for extension test Included

Ordering info

28-WF4170 
Double wall triaxial cell for unsaturated tests on 70 mm dia. soil samples, complete with 6 ports

28-WF4171 
Double wall triaxial cell for unsaturated tests on 100 mm dia. soil samples, complete with 6 ports

Accessories

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.

Diameter Double wall triaxial cell
  28-WF4170 28-WF4171
50 28-WF4170/50 28-WF4171/50
70 28-WF4170/70 28-WF4171/70
100   28-WF4171/100

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.

  Pedestal diameter 
Maximum Pressure [Bar] 50 70 100
1 28-WF4150/1B 28-WF4170/1B 28-WF4171/1B
5 28-WF4150/5B 28-WF4170/5B 28-WF4171/5B
15 28-WF4150/15B 28-WF4170/15B 28-WF4171/15B

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


Additional Information

Unsatured Triaxial Testing

Most books and courses in soil mechanics assume that soils are fully saturated, but in most parts of the world soils exist in an unsaturated state.
This is particularly true in tropical and arid regions; even in temperate climatic zones, soils above the water table can remain unsaturated.
A saturated soil is one where all the voids between the soil particles are filled with water. An unsaturated soil contains both air and water within the soil voids.
The presence of surface tension forces at the interface between the air and water within an unsaturated soil allows different pressures to exist in the air and the water. In an unsaturated soil in the field, the pore air pressure is usually at atmospheric pressure and the pore water pressure is lower than the air pressure.
Since we normally treat atmospheric pressure as zero pressure, that makes the pore water pressure negative (since it will be less than atmospheric).
We call this negative pressure “suction” since, if the soil is put in contact with water at atmospheric pressure, it will suck water into the soil.
The fundamental difference between the triaxial testing of a soil sample in a saturated condition compared to an unsaturated condition can be summarised as follows:
  • The behaviour of a saturated is controlled entirely by total stress and pore water pressure (through effective stress).
    The positive pore water pressures are pushing the particles apart and hence reduce the strength of the soil.
  • In an unsaturated soil both air and water fill the voids, and surface tension forces create a negative pore water pressure (or suction). This suction pulls the soil particles together and increases the strength of the soil.
    Therefore, to produce the correct test conditions for the unsaturated soil sample we must be able to:

    - Control the pore air pressure within the sample (independent of the pore water pressure)
    - Deal with the negative pore water pressure (or suction) within the sample during test
    - Successfully measure the volume change of the sample.
The unsaturated layer can extend to great depths, which is governed by environmental conditions. The value of suction is what determines the strength of the unsaturated material.
It is when this suction changes that unsaturated soil can behave differently to that expected of a saturated soil, for example collapsible soils, where the change in moisture content can produce a sudden reduction in volume and also have a dramatic effect on the strength of the material.
It is when this suction changes that unsaturated soil can behave differently to that expected of a saturated soil, for example collapsible soils, where the change in moisture content can produce a sudden reduction in volume and also have a dramatic effect on the strength of the material.

Some examples of application
  • Rainfall induced landslide
    Slopes may stand at steep angles when they are supported by suctions and these impart additional shear strength on the soil.
    When rain infiltrates the slope, the suctions reduce and the slope falls due to a loss of shear strength.
  • Swelling soils
    The volume change of expansive soils is controlled by the suction changes that take place as a result of water ingress.
    Swelling causes differential movements in structures, which can cause extensive cracking.
  • Collapsing soils
    Loose clayey soils may be held in a loose stable state by the presence of suction. If water penetrates the soil the suction is reduced and the loose fabric can become unstable.
    Large volume reduction can take place suddenly and these cause disruption and damage to structures.

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