The significance of the strain path analysis in the interpretation of piezocone dissipation data

When a piezocone is driven into the ground, pore water pressures are set up as a result of the stress changes. The dissipation of these pore pressures occurs in the subsequent con-solidation process in a manner dependent on the initial stress distribution as well as on the coefficient of consolidation. The process of analysis therefore has two distinct components. The first is to identify the appropriate total stress distribution caused by driving the piezocone, which acts as the initial condition for the consolidation. The second is to solve the consolidation problem itself. This approach has been followed by Levadoux & Baligh (1986, J. Geotech. Engng, ASCE 112, No. 7, 707–726), Baligh & Levadoux (1986, J. Geotech. Engng, ASCE 112, No. 7, 727–745) and by Houlsby & Teh (1988, Penetration testing 1988 (ed. J. de Ruiter), Balkema, Amsterdam, Vol. 2, pp. 777–783). Each of the two sets of authors uses the same consolidation theory for the second stage of the analysis, and a strain path method to identify the initial stress conditions. However, different assumptions about soil behaviour and the relevant soil parameters are made in carrying out the strain path analyses. A real situation will differ from the theoretical one both by differences from the model assumed, and in the choice of numerical values of the relevant parameters. For the two approaches to the piezocone analysis. Houlsby & Teh (1988) used an elastic-plastic model, in which the value of the rigidity index, G/s_u, may be specified in the analysis, while Levadoux & Baligh's solution, with a similar theoretical basis, used parameters specifically appropriate for Boston Blue Clay. This paper discusses the question of an appropriate choice of numerical values of the soil parameters and reports a comparison between the two theoretical predictions and field measurements from two different piezocones deployed in Sarapui soft clay, Brazil. The values of the coefficient of consolidation c_h calculated from the field data by the two methods are then compared with high quality laboratory tests on the Sarapui clay. It is shown that both theoretical predictions provide good agreement for the shape of the dissipation curve, but that the predicted c_h values are different, and that thought should indeed be given to the use of an appropriate value of G/s_u.