Effect of rising water table in an unsaturated slope

https://doi.org/10.1016/j.enggeo.2010.04.005Get rights and content

Abstract

Recently, climatic changes have caused more extreme weather conditions such as heavy rain falls and droughts. Therefore, climatic changes are expected to produce increased variations in infiltration characteristics and positions of water table in slopes.

A physical slope model has been developed to study the effects of climatic changes in an unsaturated slope. The effect of rising water table in an unsaturated slope was investigated experimentally in the physical slope model. In addition, finite element analyses were carried out to simulate infiltration in slopes under steady-state and transient conditions. A comparison between the results of laboratory model measurements and numerical analyses shows good agreement despite the complex unsaturated soil conditions. Both, experimental data and numerical analyses demonstrate a delayed response in pore-water pressure in the unsaturated zone due to the rising of water table.

A conceptual framework is presented to describe the physically possible lower and upper limits of pore-water pressures in a slope resulting from the rise in water table.

Introduction

Global warming and corresponding climate changes are one of the biggest challenges in this century. Climate history and model predictions for Central Europe show more extreme weather conditions such as heavy rain falls and droughts are to be expected (Christensen et al., 2007). Precipitation is likely to increase in winter but decrease in summer. Furthermore, spring warming will cause earlier snowmelt and will change the soil moisture regime. Therefore, it is to be expected that groundwater levels will be affected directly by the climatic changes with probably higher fluctuations in the future.

Shearing resistance in soil slopes is mainly governed by shear strength, which in turn is controlled by effective stress. Effective stress is defined as total stress minus pore-water pressure. Therefore, a rising water table increases pore-water pressures in the slope, reduces the effective stress and consequently decreases stability of slope. Hence, pore-water pressures play a crucial role in the stability of earth structures. It is important to understand the response of pore-water pressures to changes in groundwater levels in order to prevent damages like slope failures. Numerous studies on the flow of water in soils have been carried out by many researchers. In early seepage analyses through dams, Casagrande (1937) considered only the saturated flow using the well-known graphical method of flow net. Freeze, 1971, Papagianakis & Fredlund, 1984 showed the existence of water flow between saturated and unsaturated zones in soil. For saturated–unsaturated soil systems, the application of the flow net technique is no longer applicable (Freeze, 1971). However, the flow equations can be solved using finite element methods.

For a horizontally layered slope, Rulon and Freeze (1985) pointed out that pore-water pressures have to be calculated by taking into account the unsaturated soil properties. Technical advances in computer technology and software development in the past three decades have allowed advanced seepage analysis to be carried out. Nowadays, sophisticated finite element programs can generate solutions for complex time-dependent saturated–unsaturated soil conditions.

Climatic changes can cause a reduction in matric suction near the ground surface due to rainfall infiltration. Such a reduction can be the triggering factor for shallow landslides, especially for fine-grained soils of low permeability. A significant triggering factor for coarse-grained soils of high permeability like sands is a rise of water table in combination with a decrease in matric suction due to rain fall infiltration. This condition can result in shallow landslides near the ground surface due to a reduction in matric suction or after some time, it can result in deep landslides due to the rising of water table and increasing positive pore-water pressures at deeper depths.

Physical slope models have been used to study rainfall and seepage-induced slope failures on the basis of small scale model studies. Orense et al. (2004) and Tohari et al. (2007) concentrated their research on failure mechanism and indicators for slope failures. They stated that monitoring of volumetric water contents at certain points in a slope can be used as indicators for a landslide warning system. However, changes in volumetric water content depend highly on pore-water pressures and hydraulic properties of the unsaturated zone. In this paper, the effects of a rising groundwater table on pore-water pressures and water content in an unsaturated slope are studied.

A physical slope model is presented. The objective of this research is to analyze the effect of a rising water table and to compare the measurements in the physical experiment with numerical analyses based on unsaturated soil mechanics.

The purpose of the laboratory slope model design is to simulate the response of different climatic conditions on a slope, such as rainfall and groundwater flows and to quantify their influence on slope stability. In addition to the main experiment and its analysis, this study includes the specifications and design details of the model box, preliminary studies, testing of soil properties, setting up and calibration of measuring devices and data acquisition system. The physical slope model is described first. Subsequently, experimental data are presented and compared with independent measurements. The numerical model is then explained and the results obtained are compared with experimental data.

Section snippets

Description of the slope model

A physical slope model was designed to study the effect of climatic changes on a soil slope. Different boundary conditions can be applied to the slope model in order to simulate changes in rainfall patterns and groundwater table due to changes in climate. The main components of the apparatus are a box with one translucent side, a sturdy metal frame, a rainfall simulator and numerous measuring devices as illustrated in Fig. 1, Fig. 2. The box can be tilted back and forth using a hydraulic pump.

Soil properties

The soil used for the experiment was a fairly coarse sand called ‘Sand vom Stuecken’ which was selected from a gravel pit located near Eschenbach SG, in Switzerland. The sand was originated from the Swiss Molasse basin and can be identified as a gravel deposition from the latest ice age in Europe, 115 000 to 10 000 years ago. The geological map characterizes the material as a clean to silty gravel and sand.

Particle sizes passing through a 4 mm sieve were used in the experiments. All the laboratory

Setting up of the experiment

The careful setup and calibration of the measuring devices played an important role. Each instrument was checked and calibrated prior to installation. The following sections describe the instrumentation and setting up of the experiment.

Setting up initial conditions

The main objective of the testing program was to study the effects of rising water table in unsaturated soil slope. The development of phreatic surface during the rising of water table is significantly affected by the unsaturated soil properties in the unsaturated zone (Rulon and Freeze 1985). Therefore, it is important that the unsaturated soil properties are carefully characterized.

As an independent verification of the drying and wetting Soil–Water Characteristic Curves (SWCCs) in the slope

Theory of water flow

The flow of water in saturated and unsaturated soils can be described using Darcy's law (Fredlund and Rahardjo 1993). According to Darcy's law, the rate of water flow through a soil mass is proportional to the hydraulic head gradient described as follows:vw=kwhwywhere vw is the flow rate of water, kw the coefficient of permeability, and ∂hw/∂y the hydraulic head gradient in the y-direction. The total hydraulic head (hw) is the sum of elevation head (y) and the pressure head (uw/(ρwg)):hw=y+uw

Phreatic surfaces

The advancement of the rising water table from the initial condition up to stage 3 is shown in Fig. 11. The markers show the piezometer readings (stage 1 to stage 3) measured during the experiment in the model box at the end of each stage. The end of each stage was defined as the time when no longer changes in piezometer levels were recorded. The corresponding phreatic surfaces of the numerical analysis from the initial condition to stage 3 are indicated with solid lines. The experimental

Conclusions

The physical slope model was used to study the effect of rising groundwater table on pore-water pressures of an unsaturated soil slope. Experimental data were found to lie within the drying and wetting Soil–Water Characteristic Curve (SWCC) obtained from independent measurements in Tempe cell and capillary raise open tube tests.

A numerical finite element model has been used to verify the measurements of pore-water pressures and volumetric water content in the unsaturated soil slope during the

References (19)

  • ASTM

    Standard test method for particle-size analysis of soils (D 422-63)

  • ASTM

    Standard test method for specific gravity of soils (D 854–92)

  • ASTM

    Standard classification of soils for engineering purposes (unified soil classification system) (D 2487-93)

  • ASTM

    Standard test method for capillary–moisture relationship for coarse- and medium-textured soils by porous-plate apparatus (D 2325-68)

  • A. Casagrande

    Seepage through dams

    New England Water Works Association

    (1937)
  • Christensen, J.H., B. Hewitson, A. Busuioc, A. Chen, X. Gao, I. Held, R. Jones, R.K. Kolli, W.-T. Kwon, R. Laprise, V....
  • D.G. Fredlund et al.

    Soil mechanics for unsaturated soils

    (1993)
  • D.G. Fredlund et al.

    Equations for the soil–water characteristic curve

    Can. Geotech. J.

    (1994)
  • R.A. Freeze

    Influence of the unsaturated flow domain on seepage through earth dams

    Water Resour. Res.

    (1971)
There are more references available in the full text version of this article.

Cited by (0)

View full text