New technique makes work on sewers safer

PUBLISHED : Wednesday, 13 February, 2008, 12:00am
UPDATED : Wednesday, 13 February, 2008, 12:00am

A new technique introduced by the Drainage Services Department greatly reduces the risk to workers involved in the construction of sewage tunnels deep underground.

Known as directional coring, the technique allows the department's experts to identify with greater accuracy geological hazards such as faults, weak zones and groundwater at a depth of 160 metres - the depth of the tunnels - before work begins.

It was applied for the first time in the Harbour Area Treatment Scheme Stage 2A, an HK$8 billion project to clean up water along the western shores of Victoria Harbour.

The process involves drilling a pipe-like tunnel about 5cm to 7.5cm in diameter down to bedrock and then parallel to the proposed sewage tunnel for about 1km. Data about the ground features is collected and risks, such as groundwater inflow, can be predicted before construction starts.

'It greatly increases the extent of tunnel sections that can be investigated. Some faults and weak zones can be detected before construction begins,' chief engineer Chui Wing-wah said.

The technique, however, is much more costly than the conventional borehole method, Mr Chui said. Drilling costs about HK$9,000 per metre with directional coring, while the conventional method costs about HK$3,000 a metre.

Conventional ground investigation is conducted by drilling boreholes vertically down to the tunnel level to collect rock samples.

'In many cases, we have difficulties in finding suitable investigation sites [with the conventional method], particularly at urban areas and on navigation channels.

'But the new technique allows us to conduct our works underground with minimal impact on road traffic and the environment,' Mr Chui said.

The technique has been used for five sections of the sewage tunnel, all of which are to be completed by mid-year. The miniature tunnels - 5km in total - cost HK$47 million.