Summary

The wide variation in composition, fabric and stress history of glacial clays is reflected by large variations in engineering parameters. Undrained shear strengths obtained from laboratory tests can vary by a factor of six or more. In order to make reasonable assessments of the values to use in design it is necessary to study the fabric, composition and pore-water pressure response during shear. Procedures used at the Building Research Station are outlined and illustrated by a field example. Operational parameters determined from deep in situ loading tests on 865 mm diameter plates can differ appreciably from those determined in laboratory tests. Undrained shear strengths determined from laboratory tests can be up to twice the operational values. On the other hand, moduli determined from laboratory tests are often only a fraction of those applicable to the field situation.

Similarly the ratio of the parameters determined from small in situ tests, such as the static cone penetration, pressuremeter and vane tests, can vary from around unity to three. The actual ratios probably depend on the fabric and composition of the clays, but comparisons have not yet been made in a sufficient number of different glacial clays to make even tentative recommendations.