Confined Groundwater Zone and Slope Instability in Hillsides of Weathered Igneous Rock in Hong Kong

Date, Time & Venue
10 January 2001 (Wednesday); 5:30pm, refreshments for 6:00 pm; HKIE

Program Highlights
It is customary in Hong Kong to assume that the hydraulic conductivity of weathered igneous rock decreases with depth or as the rock mass becomes less decomposed. Generally, groundwater flow studies for slope stability purposes treat the saprolite as an aquifer and the ‘rockhead’, notionally the upper surface of a zone dominated by Grade III or less weathered material, as an impermeable boundary. This talk will examine published hydraulic conductivity data and groundwater observations and show that such a model may be inappropriate. Evidence is cited to demonstrate that, at least in some places, a more permeable zone may exist at depth, either in the lower saprolite or at rockhead. The groundwater flow regime in such a high hydraulic conductivity zone may be confined, resulting in higher water pressure rise in response to rainfall than might otherwise be expected. These high pressures may be responsible for a significant reduction in slope stability. The talk will demonstrate that misinterpretation of the hydraulic conductivity profile will lead to incorrect groundwater flow and slope failure models. In addition to slope stability study, the presence of a confined groundwater zone in the weathering profile may have implications for tunnelling, bored pile construction and deep excavations.

Dr Jimmy Jiao obtained his PhD in the Department of Civil Engineering at the University of Birmingham, UK in 1993. From 1994-1996, he worked as a post-doctoral research fellow in the Department of Geosciences at the University of Alabama, USA. He joined the Department of Earth Sciences at the University of Hong Kong as an Assistant Professor in 1997. Much of Dr Jiao's research involves groundwater-related geotechnical and environmental problems. Currently, his research interest includes impact of groundwater flow on slope stability, modification of groundwater regimes by land reclamation, and groundwater contamination due to urban development in the Pearl Delta River region.

Book Prizes
Up to two Book Prizes will be awarded to young members who submit reports on the meeting to Ir Dr J-H Yin and judged by the Geotechnical Division Committee to be of sufficient merits.

Registration & Enquiries
No prior registration is required. For enquiry, please contact Ir Dr P L R Pang, Tel: 2762 5362 or Ir Patrick A Chao, Tel: 2378 4488.

Photos (taken by Dr HC Chan)


Presentation in HKIE


Dr Jiao (right) received a souvenir from Dr YC Chan (left), assistant director of GEO


Dinner after the talk (from left to right): Mr Ian Askew, Dr HC Chan, Dr JJ Jiao, Dr CK Lau, Dr Jack Pappin, Dr R Pang, Mr PA Chao,