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It matters

Understanding the structure of peat to mitigate the risks it poses to construction projects is vital to the geotechnical engineer. An Irish/Dutch exercise has looked into organic soil behaviour.

The presence of peat and organic soils in construction projects often presents difficulties for the practicing geotechnical engineer. This natural material with characteristic large water content, high compressibility and variability in organic matter leads to problems with designing foundations, large differential settlements and slope stability issues.

In the past, different soil improving techniques were applied as mitigating measures to overcome these problems including complete excavation, soil mixing and piled foundations.

Researchers François Mathijssen at Royal Boskalis Westminster nv\Delft University of Technology (TU-Delft) in the Netherlands and Noel Boylan at University College Dublin (UCD) in Ireland are co-operating on research around several aspects of organic soil behaviour.

The increased level of infrastructure development in both the Netherlands and Ireland and the need to mitigate against the risk of peat slope failures in Ireland, requires an improved understanding of organic soil behaviour. Restrictions in space, budget or allowable environmental impact pushes towards a reassessment of solutions that use the material's natural mechanical properties.

Co-operative projects to assess the role of sample disturbance on the engineering properties of organic soils as well as in-situ characterisation using innovative full flow penetrometers are being undertaken. Both institutes are also developing laboratory test devices to improve the understanding of the mechanical behaviour of peat.

During December 2006 to January of this year, the co-operative carried out site investigations at two locations in the Netherlands. These sites – near Vinkeveen and Bodegraven – represent a range of organic soils from organic silt\clay to peat. The investigation included traditional cone penetration, full flow penetrometer and in-situ vane tests. To assess the effect of sample disturbance on the engineering parameters of organic soils, sampling was done using a range of techniques.

While the effects of sample disturbance are relatively well understood for mineral soils, little research has been carried out in this area for organic soils. During site investigation, site workers used several samplers representing traditional Irish and Dutch practice. These included thin walled piston sampling, Begemann tube sampling, hollow auger sampling and high-quality Sherbrooke block sampling.

Laboratory testing is being done at both institutes using a range of strength tests (such as triaxial compression, direct shear, direct simple shear and ringshear) and compressibility tests (such as incremental load and constant rate of strain oedometers). The results will be used to assess the effects on strength and compressibility properties because of the different sampling techniques.

Traditional cone penetration testing in peaty soils suffers from problems such as difficulties measuring resistance in soft soil, temperature effects on measurements and variable interaction with fibres. These often lead to erroneous soil classification results and highly variable engineering parameters correlated from the cone data.

To overcome these difficulties, full flow penetrometers, such as the T-bar and Ball, originally developed for offshore site investigation, are being used. These devices have larger bearing areas than the cone (typically 1m2 compared with the cone's 100mm2) that significantly improves the resolution of the measured resistance by the penetrometer. The full flow mechanism around the probe also significantly reduces the level of corrections to the measured resistance.

Research carried out on several peat sites in Ireland (Boylan and Long, 2006) shows that these penetrometers overcame difficulties measuring resistance in peaty soils, yielding a more uniform and highly repeatable measure of resistance than the cone penetrometer. Both institutes are developing relationships between the resistance measured by these probes and strength parameters for organic soil as derived from laboratory tests.

In parallel to the co-operative projects, both institutes are developing apparatus to improve the understanding of the mechanical behaviour of peat. A direct simple shear/axial shear device is being developed at GDS Instruments in co-operation with TU-Delft. This aims to measure anisotropy of peat samples in combination with the shearing resistance according to the principle of Molenkamp (1998). Research at UCD focuses on developing a direct simple shear apparatus with a facility to monitor deformation of peat during shearing.

Special image analysis techniques are being employed to monitor how the peat mass deforms during shearing. The objective is to improve understanding of how peat derives its strength and why particular types of peat are more susceptible to landslide events.

The co-operative research hopes to improve geotechnical design on organic soils by developing understanding of its behaviour and the effects of sample disturbance on it. This is being done with improved site investigation methods and advanced laboratory testing techniques.

The co-operation has had assistance from several organisations including Boskalis Westminster nv, the Public Works Department, Rotterdam, the Norwegian Geotechnical Institute, Geodelft, Lankelma, Concept Consultants (Acton, UK), Fugro, GeoMil, GDS Instruments and the contractor combinations Consortium N11 & A2, Holendrecht – Maarssen.

References
Molenkamp, F. (1998) Principal of axial shear apparatus, Géotechnique, Vol. 48(3) pp. 427-431.
Boylan, N., Long, M. (2006) Characterisation of peat using full flow penetrometers. Proceedings of the Fourth International Conference on Soft Soil Engineering, Vancouver, Canada, pp. 403-414.


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