Rate effect of piezocone testing in two soft clays

Abstract

The Onsøy and Sarapuí II soft clays have been studied for a number of years. In particular, piezocone tests with different rates have been conducted at both sites. Since the plasticity index and the coefficient of consolidation—which play important roles in the rate effect—are different in Onsøy and SarapuÍ II clays, an interesting comparison is possible. A bowl shaped curve was obtained for the normalized cone resistance versus penetration rate in the case of Sarapuí II clay. The cone resistance versus rate curve can be explained by the pore pressure trends, especially from the u1 trend. Since only three rates have been used for Onsøy clay, a more complete picture cannot be obtained. However, the use of the normalized rate (or velocity), even using ch from piezocone dissipation data, was not capable to unify the resistance-rate data for the two clays tested. Adaptations in regular rigs, allowing much smaller penetration rates, and small diameter cones, are necessary if drained conditions are to be achieved when conducting piezocone tests in deposits like Sarapuí II or Onsøy clays. No rate effect was found for penetration rates greater than the standard rate, unlike expected, due to the high Ip values in both deposits. Tests with higher rates are necessary to properly evaluate the role of Ip on rate effect on cone resistance.