THE ENGINEERING GEOLOGY OF THE COARSE CLAY AT THE NATIONAL SOFT CLAY RESEARCH SITE, BOTHKENNAR

It is possible to subdivide the Carse clay deposits above the Bothkennar gravel into bedded, laminated, mottled and weathered facies. The first is composed of silty clays which retain original sedimentary structures from quiet water tidal deposition. The second records locally more energetic conditions which gave rise to the deposition of interbedded silt laminae. Reduced rates of sedimentation at times allowed burrowing organisms to rework the sediment and hence to produce the mottled facies. The last facies is a near-surface zone which has been affected by desiccation, oxidation and modern pedogenesis. Most of the succession above the Bothkennar gravel can be correlated with the Claret beds which occur widely in the area. The lowest part of the sequence has some similarity to the Letham beds, but the exact correlation is suspect. The Claret beds are succeeded by modern tidal flat deposits, which rest on them with marked disconformity. Radiocarbon dating indicates their age to 5000-3000 BP. The borehole depths and the sea level curve together suggest that the Claret beds accumulated offshore in water whose depth reduced from about 20 m to 5-10 m, which agrees broadly with faunal data. The grain size distribution shows little variation with depth. The principal clay mineral is illite, but the plasticity appears enhanced by organic residues. The principal ions in the soil water are sodium, chloride and sulphate: their concentration appears similar to that of the adjacent estuary water. The soil has an electrical resistivity of 1-4-OMEGA-m, controlled by the water content and pore water solutes. The P wave velocity is isotropic and equivalent to that in water.