Soil Testing

The list below shows the different tests, gives an explanation of each test and links it to apparatus that can perform these tests. All tests can be managed and recorded through GDS’s software GDSLAB and this guide is designed to help you find the right equipment for your purpose. Soil testing includes materials such as clay, silt, sand, gravel, peat, loam and loess. The soils strength is governed by the interaction between its particles. During a test the soil is measured by discrete particle failures occurring in the failure planes. There are a host of different tests you can run on soil from a variety of testing apparatus. Please note: Some apparatus may require additional components or software modules to perform certain tests.
Advanced HCA Loading

Allows all controllable parameters within a HCA to be targeted by the user for a test stage.

The following products can perform this test:-
GDS Small-Strain Hollow Cylinder Apparatus
Axial Extension
Axial or Radial Deformation

Axial deformation is measured by a displacement transducer in all GDS systems (LVDT, LDT or digital gauge).  Radial deformation is calculated as standard by measuring the volume change to 1mm3 directly on the back pressure controller.  For direct measurements of radial strain (i.e. transducers mounted upon the sample, and direct measurements of axial strain in the same way, GDS can supply a Hall Effect local strain set or an LVDT local strain set. Please note: Some apparatus may require additional components or software modules to perform certain tests.

B Check

The B-Check is a pre-test to check that the specimen is sufficiently saturated by measuring the Skempton pore water pressure parameter B. The cell pressure is increased to the specified value whilst the back pressure remains on hold volume (i.e. the undrained state). The system compares the change in pore pressure (Δu) caused by the increase in cell pressure (Δσ3). The value of B is calculated as Δu/Δσ3.  If your GDS triaxial testing system has a cell pressure controller and a pore pressure transducer, then the B-check is automated within the GDSLAB software. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Back Pressure Cyclic Direct Shear Displacement Tests

Enables a direct shear test in which the load is cycled back and forth to be performed on a specimen at dynamic rates with back pressure applied. 

Back Pressure Cyclic Direct Shear Load Tests

Enables a direct shear test in which the load is cycled back and forth to be performed on a specimen at static rates with back pressure applied.

Consolidated Drained (CD) Triaxial

A Consolidated Drained triaxial test (CD test) is a constant rate of strain shearing test performed with back pressure control on the sample (i.e. the back pressure valve is open).  The test must be performed sufficiently slow such that the sample can drain and therefore no excess pore pressures should build up.  For this reason a CD test is usually performed particularly slowly, particularly on samples of low permeability.  The sample has first been saturated and consolidated. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Consolidated Undrained (CU) Triaxial

A Consolidated Undrained triaxial test (CU test) is a constant rate of strain shearing test performed undrained, i.e. with no back pressure control on the sample (i.e. the back pressure valve is closed).  Unlike a CD tests (Consolidated Drained), excess pore pressure will be seen to build up.  The sample has first been saturated and consolidated. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Constant Normal Stiffness

A constant normal stiffness condition may be specified and maintained when testing specimens within GDS direct shear and direct simple shear apparatuses. This condition allows for both the normal stress and normal strain to vary as a specimen is sheared based on the contractive and dilative tendencies of the geo-material – here dilation results in increased normal stress and specimen height, while contraction results in decreased normal stress and specimen height. The magnitude of normal stress and normal strain variation is proportional to the constant normal stiffness value specified by the user, entered in units of kN/mm or kPa/mm. Practical applications of such a test condition include modelling the interface between piles socketed or grouted into a geo-material.

Constant Pressure Source

All GDS controllers are designed for use as constant pressure sources.  The stepper motor constantly adjusts itself to ensure the desired pressure is maintained, with the additional advantage that volume change is measured to 1 cubic mm* (*except the pneumatic controller which only regulates pressure and has no capability for measuring volume change or flow).

Constant rate of Loading (CRL) Consolidation

Provides a controlled ramp of load while the back pressure is maintained. There is also the option to limit the rate of strain if a specified hydraulic gradient across the sample is exceeded.

Constant rate of strain (CRS) Consolidation

Provides a controlled ramp of strain while the back pressure is maintained. There is also the option to limit the rate of strain if a specified hydraulic gradient across the sample is exceeded.

Continuous Infinite Volume Flow (either target or ramp)

Some geotechnical applications require the continuous flow of fluid, either under pressure control or via a rate of fluid flow.  GDS manufacture an Infinite Volume Controller (IVC). By connecting 2 GDS pressure/volume controllers in parallel, the IVC system automatically switches between them when they run out of volume thus providing a seamless supply of pressure with unlimited volume capacity.  The IVC can be used with any 2 pressure controllers from the GDS range, i.e. advanced, standard or enterprise. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Controlled Hydraulic Gradient

Controlled hydraulic gradient means the user can set a maximum pressure gradient across the sample.  In triaxial permeability testing this coule be the difference (i.e. the gradient) between the applied back pressure at the top of the sample and the measured pore pressure at the base of the sample, or in triaxial shearing it could be related to the excess pore pressure generated during a shearing test. If the user defined value is exceeded then the test will be paused until the sample equilibrates enough to bring the difference back to within the stated pressure before continuing.  Effectively by having control on the hydraulic gradient the test will not run too fast for the permeability of the soil. 

The following products can perform this test:-
Optional Test Module 7: Triaxial Permeability Evaluation
Cyclic loading of samples under either load or strain

Cyclic loading refers to the repeated loading of a sample.  All GDS systems can imply this loading either under load control, or under strain (displacement) control.  Load control will be via feedback from the load cell and displacement control will be via the feedback from the displacement transducer.  Load and displacement control can be performed by all GDS system.  The speed of the cyclic loading depends on the frequency performance (i.e. the dynamic performance) of the system in question.  Dynamic stress control is possible, but only available on the most advanced GDS dynamic because of the additional processing required to calculate the sample area during the cycles.  Please note: Some apparatus may require additional components or software modules to perform certain tests.

Cyclic Simple Shear

The Dynamic Cyclic Simple Shear Module allows you to run two tests. 

Test 1: Dynamic control of Axial actuator for either axial displacement (mm) or axial force (kN).
Test 2: Dynamic control of horizontal (shear) actuator for either shear displacement (mm) or shear force (kN).

Cyclic Testing, Slow
Damping Ratio in Flexure

The damping ratio is a measure of describing how oscillations in a system die down after a disturbance i.e. how the shear waves generated in the sample die down after a test is complete. This test can be carried out in torsion or flexure through the resonant column.

The following products can perform this test:-
Resonant Column Apparatus (Stokoe Type)
Damping Ratio in Torsion

The damping ratio is a measure of describing how oscillations in a system die down after a disturbance i.e. how the shear waves generated in the sample die down after a test is complete. This test can be carried out in torsion or flexure through the resonant column.

The following products can perform this test:-
Resonant Column Apparatus (Stokoe Type)
Direct Shear Tests

Direct shear tests: The sample is contained inside a rigid box with a horizontal split line at the mid-height of the sample. The shear force is applied to the sample by moving the two halves of the box horizontally relative to each other. The intension is to induce a shear plane to form between the two halves of the sample. Properties can be obtained for the material prior to shear and post failure or residual. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Dynamic Cyclic Axial Stress Capability

Dynamic cyclic axial stress is an extremely advanced test, only available on dual axis advanced dynamic triaxial testing systems.  During the cyclic loading the area of the sample is continuously calculated allowing the sample stress to be accuratelly maintained at the desired amplitude and wave-shape.

Dynamic HCA Loading

Allows load and displacement, in the axial and rotational directions, to be applied to a HCA specimen at dynamic rates. 

The following products can perform this test:-
GDS Small-Strain Hollow Cylinder Apparatus
Gas Adsorption onto a sample

Gas adsorption is the process of gasses coming into contact with a solid, and part of the gas being taken up by the solid, i.e. being adsorbed (note, different to absorb).

The GDS Adsorption cell is a Twin Cell based system used for the Adsorption of a Gas onto a chosen sample. One chamber will have a gas with a known volume and pressure. The second chamber will hold a sample with known volume under a vacuum. The gas is then introduced to the sample chamber and the process of adsorption begins.

The following products can perform this test:-
CO2 Gas Adsorption Cell
Geo-membrane Shear Test

Direct shear testing of a geo-synthetic material embedded in a soil specimen to determine the shear strength of the geo-synthetic-soil system. 

Local Strain Measurement

Accurate determination of soil stiffness is difficult to achieve in routine laboratory testing. Conventionally, stiffness of a triaxial test specimen is based on external measurements of displacement which include a number of extraneous movements.  By placing transducers directly on the sample, accurate measurements of strain may be obtained, particularly in the small strain area of the sample loading cycle where strain dependancy of soil stiffness (stiffer at low strains) can be otherwise be masked when using external transducers.

Multi-stage Testing

The procedure for conventional triaxial tests requires three separate soil specimens to be sheared to failure under different confining pressures so that Mohr-Coulomb failure envelope can be determined and soil shear strength parameters can be obtained. An alternative procedure is the multi-stage triaxial compression test, which requires only one soil specimen to be tested at three stages of shearing with different confining pressures.


There main advantages of multi stage soil testing is the requirement for fewer soil specimens, as well as reduced testing time.  The advantage of performing the test with 3 separate samples is that if one of the samples is unsatisfactory (perhaps due to sample disturbance for example) then it is likely to be noticed when the Mohr circles are plotted, whereas this could remain unnoticed in the multi stage test. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Non Invasive Shear Wave Velocity

Determination of the shear wave velocity within a soil mass without physically probing the soil with instruments. Please note: Some apparatus may require additional components or software modules to perform certain tests.

The following products can perform this test:-
Pore Water Volume Change

Soil testing involves the application of pressure (stress) and the resulting measurement of strain either by measuring displacements or volume change. GDS pressure/volume controllers can apply pressure and measure volume change, or alternatively can apply volume change and measure the resulting pressure. Essential for soil testing.  Please note: Some apparatus may require additional components or software modules to perform certain tests.

Resilient Modulus tests

A material’s resilient modulus is actually an estimate of its modulus of elasticity (E). While the modulus of elasticity is stress divided by strain for a slowly applied load, resilient modulus is stress divided by strain for rapidly applied loads – like those experienced by pavements.

Resilient modulus is determined using the triaxial test. The test applies a repeated axial cyclic stress of fixed magnitude, load duration and cycle duration to a cylindrical test specimen. While the specimen is subjected to this dynamic cyclic stress, it is also subjected to a static confining stress provided by a triaxial pressure chamber. It is essentially a cyclic version of a triaxial compression test; the cyclic load application is thought to more accurately simulate actual traffic loading. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Resonance in Flexure

Is the deformation of an object through bending, curving, or flexing.  This non-standard can be performed in the GDS resonant column apparatus as it has the ability in the software to reverse the polarity in one of the coils and switch 2 of the other coils to be open circuit, thus resonating the soil sample in flexure (bending).

The following products can perform this test:-
Resonant Column Apparatus (Stokoe Type)
Resonance in Torsion

The standard test performed in a resonant column cell is resonance in torsion.  By establishing the resonant frequency in torsion of a soil sample, the soil shear modulus can be calculated.

The following products can perform this test:-
Resonant Column Apparatus (Stokoe Type)
Shear Modulus Profiling

Determination of the variation in maximum shear modulus of a soil mass with respect to depth from the ground surface.

The following products can perform this test:-
Simple Shear

Simple shear tests: Simple shear samples are restrained by either thin horizontal bands or by wire reinforced membranes. The topcap and pedestal are moved horizontally, relative to each other to apply the shear loading. The sample is allowed to deform in a lozenge shape rather than just forming a horizontal shear plane as for direct shear testing. The simple shear test can be used for carrying out tests that rotate the principal stress direction as well as tests that determine the shear strength / stiffness of the material. 

Slow Speed (less than 2Hz) Torsional Shear)

The damping ratio is a measure of describing how oscillations in a system die down after a disturbance i.e. how the shear waves generated in the sample die down after a test is complete. This test can be carried out in torsion or flexure through the resonant column. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Specimen pre-conditioning

Enables a cyclic axial stress to be applied to the test specimen under a constant confining pressure for a specified number of load cycles before the resilient modulus or permanent deformation tests are started. This conditioning phase helps to minimise the effects of imperfect contact between the system platens and test specimen, as well as eliminate effects due to the time interval between specimen compaction and loading.

The following products can perform this test:-
Resilient Modulus Testing System
Static Displacement

A static displacement test involves applying and maintaining a constant displacement to the specimen. The load response can subsequently be observed. Alternatively this may refer to cases where a displacement, which may vary, is applied to a specimen for tens of seconds or more. Please note: Some apparatus may require additional components or software modules to perform certain tests

Static Load

A static load test involves applying and maintaining a constant load on the specimen. The displacement response can subsequently be observed. Alternatively this may refer to cases where a load, which may vary, is applied to a specimen for tens of seconds or more. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Stress Paths

In a triaxial test, a stress path is described by any change in stress state in any direction on a sample.  To be able to perform a stress path test it is generally considered that control of both the axial stress and the radial stress is required.  With this level of control, linear paths may be controlled of either axial stress and radial stress, p and q (deviator stress), or s and t paths. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Unconsolidation Undrained (UU) Triaxial

An Unconsolidated Undrained triaxial test (UU test) is a constant rate of strain shearing test performed without back pressure control on the sample (i.e. the back pressure valve is closed).  The test is generally quite fast as drainage is not required from the sample.  There is no requirement for the sample to be saturated or consolidated before the shearing stage. Please note: Some apparatus may require additional components or software modules to perform certain tests.

Unsaturated Tests

Triaxial testing following interbational standards is almost exclusively performed with samples in the saturated state.  Unsaturated soil testing is an extension to traditional triaxial testing in that soils from above the water table may be tested under conditions approaching the in-situ stress state and degree of saturation or partial saturation.

The following products can perform this test:-
Unsaturated Triaxial Testing of Soil
User defined waveforms

Dynamic cyclic triaxial testing is usually performed with a cyclic loading pattern which is the shape of a sine wave.  Some experiments require a unique waveshape to be applied to the soil, for example in recreating earthquake loading from a measured seismograph trace.


All GDS advanced dynamic systems allow user defined waveforms to be used, with the enterprise level dynamic systems having user defined waveforms as an optional feature.

Variable Direct Simple Shear

Enables load or displacement to be applied to a simple shear specimen in two horizontal shear directions. 

The following products can perform this test:-
Variable Direction Dynamic Cyclic Simple Shear