An Engineering Geological Model for Kaolin-rich Zones and Soil-Pipe Cavities in Volcanics

Abstract

During the project for Site Formation and Infrastructure Works for Public Housing Developments at Pok Fu Lam South, the major soil slope cutting works discovered two sub-parallel natural cavities up to 5m³ and 50m³. The cavities were found deep in altered saprolite and below large corestones of coarse-ash crystal Tuffs within the Repulse Bay Volcanic Group. Associated with these were multiple persistent kaolin-rich zones along relict sheeting joints, each up to 15cm thick. Remote 3D LiDAR scanning was used to show the detailed geometry of the cavities, and no evidence of recent man-made activity or recent groundwater flow was seen. The cavities were backfilled by cement grout and additional ground investigation (GI) works were carried out to verify further the engineering implications to the geotechnical design model. This paper concludes the investigation, findings, and characterization for the local area, and provides recommendations and insights on the distribution of kaolin-rich zones and formation of soil pipe structures.