Piston Sampling to Provide Geotechnical Parameters in Soft Clay

A recent ground investigation for a proposed visitor centre on the North Norfolk coast utilised traditional cable percussive boreholes with piston sampling techniques to obtain samples of the very soft fine grained alluvial soils associated with tidal flat deposits. The boreholes were supplemented with Cone Penetration Testing to corroborate the findings.

The piston sampling technique allows recovery of Quality Class 1 samples and uses hydraulic pressure to slowly push stainless steel sampling tubes into the organic silty clay. Sample recovery was excellent, particularly in comparison with that of 'undisturbed' sampling which failed.

It could be considered that the most difficult aspect associated with such a technique is the storage and transportation of the extremely sensitive samples to a laboratory, which needs to minimise vibrations to mitigate disturbance of the soil. AFHA crafted a bespoke storage and transportation solution, padded core boxes, in order to achieve such a feat and on receipt at the lab it was commented that the samples were 'brilliant' with minimal disturbance observed.

The samples were subjected to compressibility and strength testing of the clay in the laboratory to corroborate the geotechnical parameters determined by empirical relationships with in situ Cone Penetration Testing. The results were used to provide an interpretation of the ground conditions with respect to settlement issues associated with construction of an engineered, but cosmetic, earth embankment which will disguise the piles supporting the structure and which will raise it 2-3 m above existing ground level and out of the flood plain.

The results of the geotechnical analysis allowed the formulation of a construction programme and design of a monitoring and maintenance schedule.