Use of a Downhole Block Sampler for Very Soft Organic Soils

Techniques required to successfully obtain downhole block samples of typical very soft high plasticity organic clay from Ireland are described. The vane shear strength of the material is as low as 4 kPa. These included using a sampler penetration rate three times faster than normally adopted. Comparisons are made between the results of laboratory tests on Sherbrooke block samples, on two fixed piston tube samplers, and on a continuous sampler. In addition idealized tube sampling strains were imposed on block sample specimens prior to shearing (ideal sampling approach). Both approaches confirmed that the material studied could not survive tube sampling undamaged, unlike the findings of a recent study in the Netherlands on Dutch organic soil. Tube sampling was found to have a more significant effect on triaxial test parameters that on those from the one-dimensional compression testing, where the behavior of the block sample specimens and those from one of the two tube samplers were similar to in situ response. Increasing levels of disturbance were associated with progressively more dilatant behavior.