Determining the effective stress parameter in drying path by MGGP method

The hydromechanical behavior of unsaturated soils depends on their state of stress. In recent years, the use of effective stress as a fundamental variable converting a multiphase medium to a continuum has appealed to researchers’ attention. The determination of effective stress needs estimation of the so-called effective stress parameter, which is a function of other physical variables and essential parameters such as those of the soil water retention curve (SWRC). In the current study, multigene genetic programming (MGGP) has been employed to predict a relationship between the effective stress parameter of soils. The input variables are the net stress, suction, slope of the soil water retention curve, air entry value, and residual and saturated water contents. The comparison of the performance and accuracy of the obtained equation with the available equations as well as the values of effective stress parameters obtained from experimental tests indicate the reasonable adequacy and accuracy of the proposed equation.  For this purpose, 101 data points of effective stress parameters from the literature were gathered and used. The high coefficient of determination obtained for the proposed equation, namely, 0.94, confirmed its reasonable accuracy. The parametric study revealed that an increase in the ratio of net stress to the air entry value will lead to an increase in the effective stress parameter for the same suction levels. However, a decrease in the effective stress parameter values was observed with the increase in the SWRC slope, and with the decrease in the ratio of residual to saturated water content.