Report No. : GEO Report No. 96

Report Title : Review of Fill Slope Failures in Hong Kong (1999), 87 p. (Reprinted, 2002)

Author : H.W. Sun

Abstract

It is projected that about 6000 fill slopes constructed before 1977 will be registered in the New Slope Catalogue that is currently being compiled by the GEO. Many of these are potentially substandard and require upgrading to current geotechnical safety standards. To facilitate a proper assessment of the range of feasible upgrading solutions, there is a need for a better understanding of the fundamental behaviour of loose fill and the likely failure mechanisms.

Failure initiation of a fill slope may involve shearing failure along a sliding surface, washout of the soil mass caused by concentrated flow of surface water and liquefaction of the loose soils with a metastable structure. Possible failure triggering mechanisms and contributory factors are reviewed in this Report, along with the statistics on fill slope failures in Hong Kong. As liquefaction generally involves sudden collapse of the loose soil structure with little prior warning, resuting in relatively mobile debris which may lead to significant damage, the characteristic behaviour of loose soils that may be prone to liquefaction is examined in detail in this Report.

Since the mid-1980's, much research work has been done overseas and significant advances have been made in the conceptual understanding of the generalised behaviour of collapse of soils in a loose state with a metastable structure. A review of the state-of-the-art international research on the behaviour of loose soils has been carried out. In addition, an effective stress soil model which incorporates the concepts of collapse surface and critical state to allow for the phenomenon of liquefaction caused by water ingress has been developed.

The soil model developed has been implemented in the finite difference computer program FLAC and used in a back analysis of the 1976 Sau Mau Ping failure. Field stress paths corresponding to surface infiltration of rainfall were imposed. Although further work may be carried out to refine the model, the results from the preliminary analyses were able to illustrate the nature of progressive failure involving stress redistributions in a strain-softening material. In addition, the model used to assess the mobility of the failure debris was able to replicate the observed profile of the landslide debris.

Numerical modelling shows that the collapse of a saturated (or near-saturated) loose zone in a slope can cause significant yielding of non-liquefiable materials leading to progressive failure. It is important to note that given unfavourable boundary conditions, only a portion of soil in the slope needs to be metastable and become saturated (or near-saturated) for a flowslide to develop. This suggests that if a loose fill layer is present at some depth that is liable to become wetted up via subsurface seepage from the upslope area or rising groundwater table, a global failure may take place even if the slope has a surface cover to prevent direct infiltration.

Conventional limit equilibrium calculations based on 'generalised' shear strength parameters for fill in Hong Kong (e.g. 33¢X to 35¢X) are not necessarily conservative if the material is prone to collapse with markedly strain-softening behaviour. Some pertinent issues related to investigation of fill slopes are highlighted in the Report.

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