Performance of a Drainage Tunnel and Subvertical Drain System

摘要

The construction of two drainage tunnels and the associated subvertical drain system at the Po Shanrnarea in Hong Kong was completed in December 2009. This project is the fi rst of its kind to utiliserndrainage tunnels and a network of subvertical drains to regulate the groundwater table to enhancernthe long-term stability of the hillside of Po Shan area.rnThe groundwater levels, monitored by preinstalled piezometers, once exhibited dramatic dropsrnduring drilling of the subvertical drains; however, have recovered from natural replenishment afterrneffectively plugging of the drains. The level of groundwater drawdown is in line with the designrnfunction of the subvertical drains. In order to understand the groundwater fl ow pattern in thernPo Shan area during and after construction and to set out operation schemes for the subvertical drainrnsystem so as to control the groundwater table in the Po Shan area, a post-construction review hasrnbeen carried out to examine the behaviour of the drainage system and its impact on the surroundingrnsubsurface groundwater fl ow.rnThis paper presents a review of the groundwater monitoring data during and after the constructionrnof the drainage tunnel system covering a period of over 3.5 years. A simplifi ed conceptual hydrologicalrnmodel is developed, which has been verifi ed by the monitoring data, to capture the key elements withrnregard to groundwater variations in different stages of the construction. The performance of therntunnel and subvertical drain system as revealed from these fi eld measurements is discussed.