For normally consolidated clay, several researchers have developed a number of theoretical time factors to determine the coefficient of consolidation from piezocone test results. However, depending on assumptions and analytical techniques, it could vary considerably, even for a specific degree of consolidation. Furthermore, the coefficient of consolidation determined by the common methods makes the back-calculated dissipation curve to match the measured dissipation curve only at 50% degree of dissipation. Therefore, it inevitably overestimates and underestimates pore pressures at low degree of dissipation and high degree of dissipation, respectively. Hence, such methods cause some troubles in predicting the long-term behavior of in-situ soft deposits by a linear consolidation theory with the predicted coefficient of consolidation.
In this dissertation, a new method is proposed to determine a consistent coefficient of consolidation by applying the concept of an optimum design technique over input degree of dissipation range. Initial excess pore pressure distribution is assumed to be capable of being obtained by the successive spherical cavity expansion theory. The dissipation of pore pressure is simulated by means of a two-dimensional linear-uncoupled axi-symmetric consolidation analysis. The minimization of objective function which is defined by differences between measured and predicted excess pore pressures was carried out by the BFGS unconstrained optimum design algorithm with a one-dimensional golden section search technique. By analyzing numerical example and practical dissipation test results, it was found that the adopted optimum design technique gives consistent results at various degrees of dissipation and convergent results over 40 ~ 50% degree of dissipation irrespective of the input degree of dissipation. And the predicted pore pressure dissipation curve matches well with the measured one over the entire dissipation range.
Based on the results that the coefficient of consolidation could be reasonably estimated by the optimization technique, it is also tried to predict more realistic excess pore pressure at high degree of dissipation. Applied to some real examples, it can be shown that the excess pore water pressure at high degree of dissipation can be well predicted if the proposed method uses the input dissipation data up to around 50% degree of dissipation. And hence it is expected that the proposed prediction method saves time and expenses in conducting the field dissipation test.
