Impact of soil deformation on phreatic line in earth-fill dams

Abstract

Generally seepage analysis and stress–strain analysis are conducted separately in the analysis of dams with varied water levels, which neglects the impact of soil deformation on seepage. The impact, however, is significant when the water level varies greatly. In this study, a simplified approach for consolidation analysis of unsaturated soil is used to conduct numerical simulations of water-filling in an earth-rock dam. Pore water pressure and phreatic line are simultaneously obtained in addition to stress and displacement within the dam. The computational results show that due to the coupling effect between deformation and pore water pressure, the development of phreatic line within the core-wall of the dam is quicker than that computed from unsaturated seepage analysis without coupling deformation. The variations of pore water pressure are related not only to unsaturated seepage induced by variations of water level, but also to the excess pore water pressure induced by deformation.

Introduction

When a dam is subjected to varied water levels, its seepage analysis and stress–strain analysis are usually conducted separately. Seepage analysis is firstly carried out and subsequently followed by stress–strain analysis. Based on such an approach, the impact of stress–strain properties of soil on seepage is neglected. Seepage analysis is a classic topic in soil mechanics and it can be conducted by using many numerical methods such as the routine finite difference method, the finite-volume method, the boundary-fitted coordinate transformation method, the finite element method, the numerical manifold method, the meshless method etc. (Bathe and Khoshgoftaar, 1979, Darbandi et al., 2007, Desai, 1976, Jiang et al., 2010, Jie et al., 2004, Lam and Fredlund, 1984, Li et al., 2003, Zheng et al., 2005). The soil mass below the phreatic line is under saturated conditions with positive pore water pressure, then seepage theories for saturated soil are applicable. Regarding the soil mass above the phreatic line with negative pore water pressure, seepage theories for unsaturated soil are required and the coefficient of permeability varies with negative pore water pressure (Fredlund and Rahardjo, 1993).

If the variation in water level is small, it is generally believed that the seepage is marginally affected by the stress–strain properties of soil. However, when the water level varies greatly, the impact of stress–strain properties of soil on seepage cannot be neglected and consolidation theories are required. Biot's consolidation theory has been extensively used in the analysis of saturated soil (Biot, 1941, Sandhu and Wilson, 1969). However, if negative pore water pressure exits, the consolidation theories for unsaturated soil will be more suitable.

The consolidation model coupling deformation, pore water pressure and pore air pressure was first proposed by Barden (1965). Closed formulations were derived by using continuity equations of water and gas, Darcy's law, suction state function, Bishop's effective stress equation and the relationship between porosity and effective stress. Other typical consolidation formulations were proposed by Scott (1963), Lloret and Alonso (1980), and Fredlund et al. (Fredlund and Hasan, 1979, Fredlund and Morgenstern, 1976, Fredlund and Rahardjo, 1993).

In this paper, a simplified approach for consolidation analysis of unsaturated soil suggested by Shen (2003) is used to conduct consolidation analysis of an earth-rock dam subjected to water filling. This approach is based on Bishop's effective stress (Bishop, 1959). By introducing the air drainage ratio, pore air pressure can be solved indirectly and is no longer treated as an unknown quantity in governing equations, greatly simplifying the amount of computation and the complexity of programming. It has been successfully used to analyze surface cracks on clay by Deng and Shen (Deng et al., 2003, Deng et al., 2006, Deng and Shen, 2006). Here, this approach is employed to analyze the seepage in an earth-rock dam during water filling and to study the impact of deformation of soil on the development of phreatic line.