Assessing the undrained strength of very soft clays in the SPT

Abstract

The log of a SPT in very soft clay may simply indicate a zero blow-count, or present information on the penetration – under self-weight – of the composition (sampler, rods and hammer) as recommended by some standards. The second type of information is often disregarded by design engineers due to the lack of a standard procedure for measuring these penetrations or because the test is regarded as not sensitive enough to give an indication on the undrained shear strength of soft clays. The penetration under the composition’s selfweight, however, can indicate the magnitude of Su, which, along with other more specific and sensitive tests, can help in assessing the spatial distribution of clay consistency in a large deposit. A proposed test procedure and interpretation had been given in an earlier technical note. This note presents an extended formulation and an evaluation of Su via the SPT at a construction site in Rio de Janeiro, including comparisons with results of piezocone and vane tests. The values of Su obtained with the SPT lie between the profiles given by vane tests, corrected by Plasticity Index, and the Critical State Theory, the latter representing a lower bound to the clay strength.