Methods and techniques | Shrink–swell

For each shrink–swell susceptible rock formation, in the UK, BGS are collecting samples to test their geotechnical and mineralogical properties and their shrink–swell characteristics. Over time we will build a comprehensive database for each rock type that will provide planners, engineers and homeowners with information about the hazards posed by shrink–swell.

Currently, research is concentrated on those rock types demonstrating the greatest susceptibility to shrink–swell behaviour and therefore most likely to pose problems in the towns and cities of south-east England. Research into the clays of the Lias Group, and the Reading and Gault formations has been successfully completed. Currently research is concentrated on the London Clay and the clays of the Weald.

The BGS carry out detailed geotechnical and mineralogical investigations into rock types known to shrink, and are modelling their properties across the near surface. This research underpins guidance contained in the national GeoSure dataset, and is the basis for our responses to local authorities, companies and members of the public who require specific information on the hazard in their areas.

SHRINKiT

SHRINKiT

To assist with laboratory testing of undisturbed clay samples, BGS have developed SHRINKiT — an automated measurement system to safely and reliably measure the shrinkage properties of soils.

The standard test, originally devised in the 1950s, used liquid mercury. Not only was this very dangerous, it also produced unreliable results.

To tackle this problem SHRINKiT uses an automated laser to measure the pattern of volume change as a sample is dried on a moving platform. This produces consistent, reproducible and meaningful results, and is safe to use.

3D swelling strain

3-D Swelling strain

The 3D swell strain apparatus was developed at BGS by adapting a design taken from the International Society of Rock Mechanics (ISRM).

The apparatus measure the orthogonal (X, Y and Z) strains of a cube of clay or mudstone immersed in water and subject to swelling. The overall volumetric strain is calculated from the data and, if required, the strain anisotropy (different properties in different directions).

The advantage of the design is that does not require leak-proof seals for the strain gauge armatures, as illustrated in the ISRM method.

1D swelling pressure

1-D Swelling Pressure

The BGS’s automated 1D swelling pressure apparatus was developed by combining a small triaxial load frame with an oedometer consolidation cell.

The swelling of the flooded specimen soil is sensed by a transducer and an electronic feedback system initiates a vertical force sufficient to counteract the swelling strain which is applied via the load frame motor. The load is logged and the results are summarised in a plot of swelling pressure versus time.

The test fulfils the specifications of the British Standards swelling pressure tests.

Clay-rich formations are capable of exerting considerable stresses on foundations and services when exposed to an increase in water content from the desiccated (dry) state. The 1D swelling pressure test enables the susceptibility of these formations to shrink–swell to be determined.

Contact

For further information contact: enquiries@bgs.ac.uk