Keywords: collapse settlement, rockfill dams, reservoir filling, coupled formulation.

Collapse settlement is particularly important in central clay core rockfill dams. Significant settlement in the upstream slope shells of such dams commonly occurs when the reservoir is filled with water for the first time. This settlement can be about or more than one percent of the dam height and sometimes it can excess two percents. The amount of collapse settlement depends on the quality of the rockfill and how well it is compacted during the dam construction. With the increasing use of low grade upstream rockfill undergoes significant settlement due to this factor becoming more common.

Collapse settlement can cause damage to rigid structures on the dam crest, and cause loss of freeboard, but generally it increases the safety of the dam because of the resulting load transfer to the core. This increases the vertical stress in the core thus reduces any tendency for hydraulic fracture. Collapse settlement may also be important in road embankments if they become inundated or in reclaimed land on which the building is to be constructed. A satisfactory technique for its prediction is clearly therefore important.

In the present paper, for the numerical simulation of such settlement including the following aspects are considered. In order to consider the soil-pore fluid interaction a coupled displacement-pressure governing equation system is used. Such formulation is capable to deal with effective stress analysis which is of the most fundamental concepts in geomechanics problems. The finite element method (FEM) is applied to the governing equations for the spatial discretization and to numerically solve that governing equations [1,2]. As one of the important component of numerical computations in gemechanics an appropriate constitutive model is considered for modelling of the materials behaviour. A key feature for the constitutive modelling which is considered in the collapse settlement analysis is that two sets of material parameters are defined: One for the dry material and the other for the wet [3,4,5]. Another key point in such analyses is consideration of the time duration during which the reservoir is filled and also the pattern that water surface level rises. Some researchers have modelled this process in some steps with fixed times and certain water surface levels at those steps. In this paper the pressure boundary conditions on the upstream slope are defined as continuous functions of time such that the time duration and the pattern of impoundment can be fully captured.

The present method with these main aspects:

• the modified coupled formulation for the behaviour of saturated-unsaturated porous media
• different material properties for rockfill material in the wet and the dry conditions
• time-dependent pressure and traction boundary conditions on the face of the upstream slope of the rockfill dam

These are applied in the collapse settlement analysis of the Masjed-Soleyman rockfill dam during its first impoundment. The dam is one of the highest rockfill dams in Iran which has a vertical clay core and rockfill zones both in upstream and downstream slopes.

The reservoir filling has three effects on a rockfill dam: collapse settlement produced by wetting the rockfill, water load on the dam body and buoyancy in the upstream shell were introduced. It is shown that using the proposed approach, the numerical simulation can capture the three mentioned effects and reproduce quantitatively the behaviour of a rockfill dam during the first reservoir filling.

1

O.C. Zienkiewicz, A.H.C. Chan, M. Pastor, B.A. Schrefler, T. Shiomi, "Computational Geomechanics with Special Reference to Earthquake Engineering", John Wiley, 1999.

2

A.R. Khoei, A.R. Azami and S.M. Haeri, "Implementation of plasticity based models in dynamic analysis of earth and rockfill dams: A comparison of Pastor-Zienkiewicz and cap models", Computers and Geotechnics, 31, 385-410, 2004. doi:10.1016/j.compgeo.2004.04.003

3

E. Maranha das Neves, "Advances in rockfill structures", Kluwer Academic Publishers, 1991.

4

D.J. Naylor, "Collapse settlement - Some developments", Application of computational mechanics in geotechnical engineering, Azevedo et al. (eds), Balkema, Rotterdam, 1997.

5

J. Marcelino and E. Maranha das Neves, "Modelling of collapse effects", Application of computational mechanics in geotechnical engineering, Azevedo et al. (eds), Balkema, Rotterdam, 1997.

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