The step-loading model of subsidence induced by ground-water level changes with time : Lay Seng Yaw & Shieh Ming Jyh, in: Land subsidence. Proc. international symposium, The Hague, 1995, ed F.B.J. Barends & others, (IAHS; Publication, 234), 1995, pp 313–321

Abstract

This paper tries to develop a step-loading model for simulating the complex stress of soil deposits, and utilizes the Biot consolidation theory and superposition concept to analyse subsidence phenomena induced by changes in groundwater level with time. Finally, the in situ investigation data of Wang-Kon, which is located in the coastal region of Chang-Hwa, Taiwan, will be checked in order to prove the usefulness of this model. INTRODUCTION The most serious subsidence problems at Taiwan coastal areas is due to overpumping groundwater. The groundwater levels fluctuate repeatedly, because of fluctuations in the amount of groundwater withdrawal, and fluctuations in rainfall. This study looks at the temporal effects on subsidence of the repeated processes of loading, unloading, reloading on soil deposits and the cyclic changes in groundwater levels. Studies of surface consolidation settlement induced by groundwater pumping can be divided into two categories, one is the decoupled approach method based on Terzaghi's (Terzaghi, 1943; Taylor, 1948) consolidation theory, the other is the coupled approach method based on Biot's (Biot, 1941, 1955) consolidation theory. The former first looks at the pore water pressure distribution, then calculates the strain of the soil deposits and total settlement of the ground surface by applying the concept of effective stress. The latter is a couple theory by adopting the displacement of porous media and the pore water pressure as the basic variables, which is based on the interaction of pore water and porous media. This couple consolidation theory is generally recognized to be more reasonable. Therefore, this paper not only uses the couple consolidation theory of Biot to study the subsidence problem, but also develops a step-loading model to simulate the complex stress behaviour of soil deposits due to changing water levels, in order to analyse the effects on land subsidence with time. BIOT COUPLE CONSOLIDATION THEORY The analytical model of land subsidence in this paper is based on the soil couple consolidation theory. The couple consolidation theory concerning soil was firstly presented by Biot (1941), then Verruijt (1969), Bear & Corapcioglu (1981) and Corapcioglu & Bear (1983) etc. studied this theory and derived consolidation models from the viewpoint of flowing groundwater. Their basic assumptions: 314 Lay Seng Yaw and Shieh Ming Jyh (a) fully saturated, (b) pore water is compressible, (c) solid grain is incompressible, (d) follows Darcy's law, (e) isotropic and homogeneous, (f) small strains, (g) linearity of stress-strain relations, (h) the main factor to affect porosity is the effective stress of medium. According to the above assumptions, the basic equation of this coupled consolidation theory can be expressed as follows: GVu+x 1 G ds 2v dx dx •EE =o GV«V + _ ^ _ ^ £ ^ =o y l-2vdy dy in these relations, GVV -kVp + s, V G ds _ l-2vdz ds 0dp dt dt = dldx + d dp _ dz