Effect of underground drainage tunnels on groundwater flow system in the Po Shan area

Abstract

The Po Shan drainage tunnel was built into the hillside above Po Shan Road, Hong Kong, in order to channel groundwater out of the slope so as to improve slope stability. Subvertical spider drains emanate from the tunnel to intercept water at different parts and levels within the slope.

This study aims to investigate the effectiveness of the drainage tunnel in maintaining low groundwater levels such that the slope remains stable even in the event of heavy rainstorms. The responses and variations of the subvertical drain pressures, piezometer levels and tunnel flow rates to rainfall will also be examined.

To achieve this, piezometric readings, pressures of the tunnel subvertical drains, drainage tunnel flow rates and historical groundwater data are reviewed and analysed. A regional two-dimensional numerical model across the Po Shan area was developed to investigate the hydrogeology of the area and the effect of the tunnels to the groundwater system. Water samples are obtained from subvertical drains and weirs in the tunnels.

Observations generally demonstrate the effectiveness of the tunnel and the subvertical drains in lowering pore water pressures in rock, but not the decomposed rock aquifer.It was found that the subvertical drains take much longer to decrease pore water pressures at shallow depths in the semi-confined model compared to the unconfined model. The results of the regional numerical model across the Po Shan area reveal the possibility of a heavily jointed, highly permeable zone at depth below rockhead based on available tunnel flow data. Hydrochemical data obtained from samples obtained in the tunnel reveal elevated levels of radon and nitrates.