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Geotechnics Lab

Student working on project

The geotechnical laboratories are equipped for both standard and advanced soil element testing. Apparatus in the lab includes triaxial machines, for testing soil specimens up to 300mm long and 150mm diameter. Specimens may also be tested in the resonant column apparatus (2 machines available), and the 300mm shearbox, the latter being ideal for testing rail track bed materials.

We are able to undertake dynamic as well as static soil testing, having a cyclic triaxial rated at 2 Hz for samples up to 150mm diameter. In addition, the lab is equipped with a hollow cylinder apparatus (HCA), where the inner and outer stresses may be independently controlled, and the sample stresses cycled both axially and in torsion. The latter equipment has mainly been used to support research into rail subgrade materials, where it is possible to study principal stress rotation under rail axle loadings. This is part of our TRACK21 programme grant.

Other advanced equipment includes a resonant column apparatus which allows high pressure / low temperature testing, specifically designed to create and test gas hydrates in the laboratory. Recent acquisitions include a cyclic simple shear, at present being used to study the behaviour of chalk around offshore wind turbine foundations. 

Electro-Mechanical Dynamic Cyclic Simple Shear (EMDCSS)

GDS Instruments Ltd.
The simple shear apparatus is used to shear a 100mm diameter cylinder of soil. The shear conditions can be either constant volume or constant stress. The sample is confined by Teflon coated stainless rings and a rubber membrane. The rings are prevented from moving vertically so the sample cannot bulge and maintains a constant cross section at all times. The top platen is used to apply a force normal to the direction of shearing; the bottom platen is used to apply the shear force. The shear force across the sample is interpreted by the use of very sensitive strain gauges above the top platen. Use of this apparatus enables the determination of the shear behaviour, including the shear modulus, of the sample. It has particular use in testing soft sediments such as soft clays, but has been used for stiffer materials such as chalk.
The dynamic capability of this machine allows testing up to 5 Hz, at small strains (0.005% amplitude), up to larger strains (10% amplitude). Maximum displacement at 5 Hz is +/- 1 mm.
The machine is rated at up to 10 kN applied load (axial and shear)

Gas Hydrate Resonant Column Apparatus (GHRCA)

GDS Instruments Ltd.
This apparatus was developed to model methane hydrates in the laboratory, and to determine dynamic properties of the resulting specimen. These structures are typically found within subsea sediments, and are thought to be responsible for submarine landslides and resultant tsunamis on dissociation. Equally they are considered to be a valuable energy source.
The apparatus consists of a high pressure containment cell (250 bar) located within an environmental chamber (-25 degrees C). By introducing methane gas into the sand specimen, formation of hydrates can be controlled, under similar conditions to natural hydrate bearing sediments.
The cylindrical specimen is fitted with a drive head, consisting of electromagnets which are energised by sinusoidal frequencies. By applying torsion to the specimen over a range of frequencies, parameters such as the shear modulus, G, and damping ratio, D, can be derived. These are influenced by the formation of hydrates within the specimen. Sample sizes are 70 mm diameter by 140 mm high. Planned developments in 2014 include the provision of a loading system, to allow strain or stress controlled loading of the specimen within the containment cell, and further enhance the capabilities of the apparatus.

Hollow Cylinder Apparatus (HCA)

GDS Instruments Ltd.
In this apparatus a hollow cylindrical sample can be subject to multiple stress paths with control of the three major principal stresses. This is made possible with independent control and cycling of the inner and outer cell pressures, axial stress and base rotation or torque. The ability to apply a torque force allows the study of the effect of principal stress rotation (PSR) and soil anisotropy.
PSR is important in considering dynamically loaded foundations, such as railway formations subject to cyclic loading from passing train axles. The influence of PSR is also considered in earthquake design and off-shore foundations or for the analysis of landslides.
The hollow cylinder sample has a 100mm outer diameter, with an inner diameter of 60mm. The apparatus has a load capacity up to 10kN, with 100 Nm torque capability. Silicon oil is used as cell fluid and the maximum confining pressure is 1700kPa controlled with high accuracy controllers.
A complete set of local instrumentation can be applied to the sample for increased accuracy. This includes local axial and radial displacement, circumferential displacement and local pore pressure measurement.

Harvey Skinner

Geotechnics Laboratory
Engineering, University of Southampton
Southampton Boldrewood Innovation Campus 
Burgess Road, Southampton SO16 7QF
United Kingdom

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