The flood plain of the tidal Thames is underlain with soft clays, peats, silts and sands which in turn rest on gravel. These deposits vary in thickness from a few to tens of metres. As with all soft deposits considerable care is required in order to obtain good quality samples for geotechnical testing. The wide range of soils which often occur down single soil profiles and from location to location can make it very difficult to assign soil parameters for design purposes and careful categorization is required. The presence of varying percentages of organic matter as well as silt and sand lenses in the clays can lead to difficulties in the interpretation of in situ tests such as the field vane.
While the foundations for large structures are taken down to the underlying gravel and chalk, problems arise from services and smaller foundations. Stability problems arise during the design and construction of road and flood embankments. Since the gravel aquifer is exposed in the river channel along much of the length, pore water pressures transmitted through gravel during high tides can affect the stability of flood banks. High pore water pressures in the gravel can also lead to instability of excavations unless the pressures are relieved by groundwater lowering. When it is necessary to lower the water pressures for construction purposes consideration needs to be given to the additional settlement of adjacent ground and small structures founded on the alluvium.
The paper looks at the distribution of the various soil types, gives typical geotechnical properties and discusses some engineering implications.