Site Investigation for Rock Slope Excavation and Stabilisation adjacent to a major Highway in Hong Kong

Abstract

Owing to increasing traffic flows, many highways in Hong Kong require widening, improvement or parallel development. This paper describes the site investigations undertaken for a particularly difficult highway widening project, where dual 3-lane traffic flows of typically 9000 vehicles per hour had to be maintained throughout the investigation period, with only occasional night-time lane reductions and Sunday off-peak lane closures permitted. In the area investigated the highway runs below steep cut slopes up to 40m high. Above the cuts are natural slopes rising to an elevation of over 100m above the highway. The slopes generally consist of granite, present in a full range of weathering grades. The major hazards for which the detailed design stage site investigation was designed to assess include boulder falls from the natural slopes, minor rockfalls from the cut faces and major blockslides on sheeting joints. It was envisaged that stabilisation, and in certain locations, excavation would be required to investigate these hazards. The site investigation to evaluate the geometry and condition of discontinuities, involved diamond core drilling (Figure 1), complemented by mapping of exposed cut slope faces. In the natural slopes above the cuts, detailed survey and evaluation of the characteristics of various boulders was also undertaken. The key objectives of the investigations were to: obtain comprehensive face maps of the existing cut slopes; determine orientations of sheeting joints on a scale of greater than 10m using borehole intersections and face mapping; determine major block geometries for potential stabilisation requirements; determine shear strength characteristics of the sheeting joints, evaluating rock to rock, infill only and infill to rock contacts; determine transient piezometric pressure response over at least one wet season, and provide a basis for screening and identifying boulders requiring stabilisation. Much of the mapping was assisted by the use of low-level oblique photographs taken from a helicopter. These photographs were digitised and transformed to permit accurate location of exposed jointing in the rock faces. This paper includes details of the investigation and testing methods used and provides a summary of some of the results obtained including information on sheeting joint and microfabric properties of the rock mass.