Relationship between groundwater ingress and geology by pre-excavation grouting

Abstract

A study of relationship between groundwater ingress and regional geology on Hong Kong Island by Pre-Excavation Grouting (PEG) has been carried out by examining the changes in groundwater inflow during the tunnel excavation. In this study, I would like to make reference to Harbour Area Treatment Scheme (HATS) Stage 2A project. HATS is a government infrastructure project in Hong Kong commenced in 1990s which aims to improve the water quality of the Victoria Harbour. The goal of this scheme is to implement a fully integrated system that collect and treat all wastewater discharged to the Harbour in an efficient, effective and environmentally sustainable manner. In this report, Tunnels J, K and L, which is one of the main tunnel project, have been chosen for a comparative analysis study. The objectives of this dissertation includes 1) to study the relationship between groundwater ingress and regional geology through deep tunnel excavation which the tunnel is designed at depth of 160m below sea level; 2) to examine the difference in water ingress between on land and below sea level; 3) to study the changes of water ingress before and after PEG; 4) to examine the relationship between water level changes and ground settlement during the excavation stage. Data has been obtained from various sources, including the Hong Kong Geological Survey (HKGS) archives, technical papers and reports from Drainage Services Department (DSD) and records available from the contractors and consultants. The data has been interpreted by using comparative analysis for the three tunnels. Subject to the water inflow measurement during PEG stage, the rate of water inflow can be obtained. Monitoring records can be made reference to analyze the correlation of groundwater draw down and surrounding settlement. By interpretation of groundwater ingress in three tunnels, it shows that high water inflows are induced during excavation stage. Irrespective to the excavation is in land portion or sub-sea portion, water ingress can be measured. Besides, it provides evidence to show that highest water inflow is encountered in sub-sea portion where the tunnel across the Harbour. The trend of geological structures and water ingress measured from PEG are correlated with the mapping records. The results of the study reveal that fault zones significantly influencing the groundwater flow.