Alun Griffiths Contractors Ltd were undertaking a contract to improve the flood relief protection along the River Usk in Newport for Natural Resources Wales. This involved the use of a number of different tracked plant, across a number of different geological conditions.
The River Usk at Newport flows across an alluvial plain, which is generally underlain by soft, compressible fine-grained soils to depths in excess of 20m. However, the shallow soils can vary with the presence of:
- Made Ground soils across the surface;
- Backfilled former drainage ditches – which contain highly organic vegetative materials;
- A Desiccated Crust – a stiffer soil band towards the top of the alluvial soils, which has a highly undrained shear strength and lower compressibility.
In addition, the estuarine alluvium around the Severn Estuary is not homogenous, and has an internal micro-structure which can impact on its geotechnical performance.
At the site of each proposed working platform, we completed a supplementary intrusive investigation comprising trial pits, windowless sampling, plate load testing and the in-situ measurement of undrained shear strength.
A working platform design was then prepared using the BRE design method and information on the soil and groundwater conditions specific to the proposed working platform location, and the loads imparted by the tracked plant specific to that location.
The assessment was extended at some locations check the feasibility of using steel plates to spread the crane loadings. At one location, a crane with outriggers was proposed. The BRE method is only applicable for tracked plant, so an alternative design method was utilised to provide a suitable working platform thickness for the plant at this location.
At the request of the Client, the results of testing on a recycled aggregate were evaluated to assess its suitability for inclusion within the working platform.
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