An improved method for estimating the time and strain at the end of the "primary" consolidation of a clayey soil with non-linear creep

For the assessment of the total settlement of a soil mass in 1-D loading, it is usual to separate the "primary" consolidation settlement and the "secondary" consolidation settlement in the calculation. Two conventional curve-fitting methods for the determination of the coefficient of consolidation, that is, Taylor's root time method and Casagrande's log time method, are commonly used to estimate the strain and time at the end of the "primary" consolidation (EOP). However, the estimation using these two methods are generally influenced by the non-linear "secondary" compression, in which the coefficient of the "secondary" compression (Cα) has a non-linear relationship with the logarithm of time. This is especially true for cohesive soils with pronounced creep behavior. As a result, the quantification of the "secondary" settlement and hence the total settlement, become difficult, frequently leading to the underestimation of the total settlement. In this technical note, a new simple graphical method utilizing both Taylor's method and the inflection point method (Cour, 1971; Robinson, 1997; Mesri et al., 1999) is proposed to isolate the "primary" consolidation and the "secondary" consolidation. Procedures of using the method to determine the time and strain at the EOP are described using data from oedometer tests on a Hong Kong marine clayey soil. Values obtained using the proposed new method are compared to those from the two conventional methods.