Report No. : GEO Report No. 227

Report Title : Guidelines for Soil Bioengineering Applications on Natural Terrain Landslide Scars (2008), 162 p.

Author : S.D.G. Campbell, R. Shaw, R.J. Sewell & J.C.F. Wong

Abstract

This report outlines a range of soil bioengineering measures, suitable for Hong Kong, that provide a low-cost, environmentally sensitive approach to the repair and protection of disturbed natural terrain. The techniques combine mechanical, biological, and ecological principles to repair erosion gullies, remediate shallow mass movement scars, and to protect the slope. They integrate established engineering practices with ecological principles to provide broad mechanical, hydrological and environmental benefits.

Soil bioengineering measures broadly comprise direct and indirect measures. Direct measures are installed on sites that require repair or stabilisation. They include 'living' (i.e. the planting of herbaceous and woody species) and 'structural' (e.g. timber cribwalls and bender fences) components, although the two components are commonly combined (e.g. live cribwalls). The living approach includes conventional direct planting of grasses, shrubs or trees, and techniques that use the stems or branches of living plants to reinforce the soil. The latter include live stakes, live fascines, brushlayers, hedgelayers, and branchpacking. Following installation, the growth of stems and roots, combined with invasive species, create the major structural components. Each measure offers different immediate depths of effectiveness (prior to rooting), typically between 200 to 1200 mm for natural hillsides. Indirect measures consist of live barriers, which are belts of hardy species (trees and bamboo) that are strategically planted to restrict the movement of landslide debris. They can be planted within drainage lines to restrict the passage of channellised debris flows, or below steep slopes to restrict the movement of debris from open hillslope failures.

These guidelines present standards for the collection and assessment of desk study and site-specific data, guidance on the design and construction of specific soil bioengineering measures, selection of appropriate plant species, the maintenance requirements during the establishment period of the measures, and the subsequent monitoring and evaluation procedures. Many of the techniques described are based on the results of a pilot project initiated by the Geotechnical Engineering Office in 2003 to install various soil bioengineering measures on areas of natural terrain affected by recent, shallow landsliding and subsequent gully erosion. Since a full evaluation of the performance of the various measures requires continuous monitoring over a 3-5 year period, these guidelines should be regarded as a work in progress.

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