Keywords: cofferdam, sheet pile wall, exit gradient, seepage.

The benefit of dams to mankind is undoubted. Their earliest role in proving storage for irrigation water formed a major contribution to the development of our civilisation. Later, the generation of renewable energy offered by hydropower excited the world. More recently, various types of dams have been design for construction purposed such as cofferdams. The seepage flow beneath a dam, which has been the main cause of the uplift force, is an important factor in the design of a dam-type structure [1,2]. A few failure cases of various design of dam have been reported in the literature [3]. The failure of Carsington Dam occurs in 1984 is an example [3].

One type of double-wall cofferdam encountered in geotechnical engineering is constructed of driven sheet pile walls. Following dewatering between the walls, these structures allow construction or remedial work to be performed on sites that were initially underwater. Of particular interest to geotechnical engineers are the overall stability of the soil/water system, the flow rate passing through the foundation soil and the exit gradients that might be experienced inside the cofferdams. This paper will focus on the prediction of exit gradients, which have a significant influence on overall stability.

The conventional method of analysis available to engineers for the prediction of such quantities as flow rates and exit gradients has involved the use of flow nets. This trial and error approach can lead to accurate predictions if the flow net is drawn carefully [1,4]. However, its use requires much practice and experience, which has been lack of junior engineers.

Numerous analytical methods for cofferdams have been developed in the past [5,6,7,8]. Among these, the cofferdams with walls of equal length have been studied as the first approximation [5,6,7]. Recently, Griffiths [8] proposed an idealised analytical model to study the steady seepage through cofferdams walls of unequal length. It was shown that the non-symmetric problem can be solved to a reasonable degree of accuracy by use of existing approximation method [5,6,7] without resort to analytical or numerical analysis. A finite element model has been used to validate the analytical method [6]. However, Griffiths' paper [8] was focused on the determination of the optimum barrier position, which is the sprit of his analytical solution.

In this study, we will focus on another important issue. That is, the exit gradient on both sides of cofferdams. With the aid of the computing package, FASTFLO [9], the effects of location of sheet pile walls on the exit gradient will be investigated, and a best design of cofferdams will be sought for engineering application.

1

A. Casagrande, "Control of Seepage Through Foundations and Abutments of Dams", The First Rankine Lecturer, Geotechnique, 11, 161-182, 1961.

2

A.D.M. Penman, "On the Embankment Dam", Geotechnique, 36, 303-348, 1986.

3

A.W. Skempton, P.R. Vaughan, "The Failure of Carsington Dam", Geotechnique, 43, 151-173, 1993.

4

H.R. Cedergren, "Seepage, Drainage and Flow Nets", Chichester, Wiley, 1967.

5

M.E. Harr, "Groundwater and Seepage", McGraw-Hill, London, 1962.

6

A. Verruijt, "Theory of Groundwater Flow", London, Macnillan, 1970.

7

S. Banerjee, A. Muleshkov, "Analytical Solution of Steady Seepage into Double-Walled Coffer-Dams", Journal of Engineering Mechanics Division, ASCE, 118, 59, 1992.

8

D.V. Griffiths, "Seepage Beneath Un-Symmetric Cofferdams", Geotechnique , 44, 297-305, 1994.

9

CSIRO, "Fastflo Tutorial Guide", 1999.

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