Confined groundwater near the rockhead in igneous rocks in the Mid-Levels area, Hong Kong, China Jiu J. Jiao ⁎ , Guoping Ding, Chi-Man Leung Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region, China Received 18 August 2005; received in revised form 3 February 2006; accepted 14 February 2006 Available online 31 March 2006 Abstract It is customary in Hong Kong to assume that the hydraulic conductivity of weathered igneous rocks decreases with depth or as the rock mass becomes less weathered. Such a hydraulic conductivity pattern can only lead to an unconfined aquifer. This paper presents a case study in the regions in and around the Mid-Levels area in Hong Kong regarding a possible relatively high hydraulic conductivity (K) zone and confined groundwater along the rockhead. The Mid-Levels area is located at the lower part of the north- facing slopes of Victoria Peak on Hong Kong Island and is prone to landslides. Although this site has a long history of geotechnical studies because of extensive urban development along the coast and public concern on slope stability, hydrogeology of the site remains poorly understood. This paper reexamined the hydraulic conductivity data in 7 boreholes conducted in the 1970s and found that 4 of them indicate an increase in K at the rockhead. Groundwater conditions revealed by tunnel construction at the coast suggest that K close to the rockhead is about 10 times greater than above rockhead. A careful analysis of storm response of a piezometer group with tips in different depths indicates that there was an upward flow from the bedrock to the colluvium. A field study of two overflow standpipes conducted by the authors showed that the water level can be 0.64 and 3.73 m above the ground surface, which illustrates that the deep groundwater is significantly artesian. A search of the archived site investigation reports from the government and private companies has led to an identification of about 24 sites with overflow boreholes, which suggests that overflow phenomenon is quite common in the study area. The paper then concludes that in the study area there is a relatively high K zone along the rockhead and the groundwater in the zone is confined. It is recommended that geotechnical engineers should carry out a more careful field study on an overflow borehole because such a borehole indicates a confined groundwater condition important for slope stability study and foundation design. © 2006 Elsevier B.V. All rights reserved. Keywords: Confined aquifer; Artesian groundwater; Hydrogeology; Hydraulic conductivity; Rockhead; Weathered igneous rock; Saprolite 1. Introduction The study area is located at the lower part of the north-facing slopes of Victoria Peak on Hong Kong Island, Hong Kong Special Administrative Region, China (Fig. 1). The study area can be divided into two parts with significantly different modes of development. The upper part of the area (>170mPD) is essentially a natural slope (see the area with topography contour lines in Fig. 1) with minimal urban development. In contrast, the lower part of the area has been extensively urbanized (see the area with street lines in Fig. 1). The study area is centered by the Mid-Levels area (see Fig. 1). A rather comprehensive study on geology Engineering Geology 84 (2006) 207 – 219 www.elsevier.com/locate/enggeo ⁎ Corresponding author. Tel.: +86 852 2857 8246; fax: +86 852 2517 6912. E-mail address: jjiao@hku.hk (J.J. Jiao). 0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2006.02.006