Report No. : GEO Report No. 225

Report Title : Review of Sub-Surface Drainage Provisions for Recompacted Fill Slopes (2008), 69 p.

Author : Fugro Scott Wilson Joint Venture

Abstract

This report presents a review of the sub-surface drainage provisions for recompacted fill slopes in Hong Kong. Recent observations have noted some distress on fill slopes upgraded by recompaction, which could have been a result of unusual transient groundwater movements. The key objective of this review was to look into feasible ways to enhance the robustness of the sub-surface drainage provisions to fill slopes to be upgraded by means of recompaction.

For loose fill slopes that are to be upgraded by recompaction, one effective means to enhance the robustness of the sub-surface drainage detailing is the provision of prescriptive filter pipes at the upstream end of the sub-horizontal drainage blankets. For recompacted fill slopes underlain by a significant thickness of untreated old fill, especially where there is a large catchment, designers should consider carefully the adequacy of sub-surface drainage provisions. Prescriptive raking drains may be called for in controlling the possible build-up of groundwater pressures at depth during intense rainfall. The need for these measures should be assessed on a site-specific basis.

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