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In this paper, the numerical solutions for groundwater flow in unsaturated layered soil using the Richards equation are presented. A linearisation process for the nonlinear Richards equation to deal with groundwater flow in unsaturated layered soil is derived. To solve one-dimensional flow in the unsaturated zone of layered soil profiles, flux conservation and the continuity of pressure potential at the interface between two consecutive layers are considered in the numerical model. In addition, a novel method, named the dynamical Jacobian-inverse free method, incorporated with a two-side equilibration algorithm for solving ill-conditioned systems with extreme contrasts in hydraulic conductivity is proposed. The validity of the model is established in numerous test problems by comparing the numerical results with the analytical solutions. The results show that the proposed method can improve convergence and numerical stability for solving groundwater flow in unsaturated layered soil with extreme contrasts in hydraulic conductivity.

Abstract. This study investigates the efficiency of subsurface drainage systems includes drainage wells (vertical shaft with drainage boreholes or horizontal drains) and drainage galleries (longitudinal tunnel with sub-vertical drainage boreholes) for the slope stabilization of Li-Shan landslide in central Taiwan. The efficiency of the subsurface drainages is verified through a series of two-dimensional (2-D) rainfall induced seepage and slope stability analyses without and with subsurface drainages remediation during two typhoon events. Numerical results and monitoring data both show that the groundwater level at B5 monitoring station with subsurface drainages remediation during Toraji Typhoon (2001) is about 40 m lower than that without remediation during Amber Typhoon (1997), and the factor of safety Fs of the first potential sliding surface (1st-PSS, the most critical potential sliding surface) is promoted simultaneously from 1.096 to 1.228 due to the function of subsurface drainage systems. In addition, the Fs values of the three potential sliding surfaces (1st- PSS, 2nd-PSS, and 3rd-PSS) stabilized by subsurface drainage systems are constantly maintained greater than unity (FS>1.0 or FS≥1.217) during rainfalls with return periods increases from 25 to 50 and 100 years. This demonstrates the subsurface drainage systems in Li-Shan landslide are functional and capable of accelerating the drainage of infiltration rainwater induced from high intensity and long duration rainfall and protect the slope of landslide from further deterioration.

This paper presents raft-pile-soil interaction for a vertically loaded flexible piled raft on layered subsoil using a two-dimensional finite difference numerical tool. The subsoil is modeled as a linear elastic material and the raft is modeled as a beam structure under plane strain. In addition, the piles are simulated by a series of pile elements considering the pile/soil interface behavior. In the simulations, the required input parameters of soil, pile and interface are determined by back analyses of pile loading tests. Settlement, bending moment, both in pile and raft, as well as effects of raft flexibility for vertical uniform loading in the subsoil were examined. It is found that even though for vertical uniform loading, a relatively high bending moment may be induced in the piles due to lateral displacement of the stressed subsoil. For the case of a piled raft placed over a soft clay layer at ground surface the contact pressure at the raft-soil interface is merely 4 similar to 6% of that developed in the unpiled raft. Nevertheless, the contact pressure may reach 15 similar to 25% of that of the unpiled raft if the piled raft is resting on a sand layer at the ground surface. This implies that the loading carried by the pile group could be reduced by almost 1/4 of the design load and it could eventually reduce the cost of pile group construction to a certain extent