Geophysics for Shatin sewer Upgrade | China

Aged sewer systems

Aged sewerage systems are common in congested urban areas of Hong Kong, not only in old towns like Sheung Wan and Mong Kok, but also new towns such as Shatin and Sai Kung.

Ground settlement is one of the visible impacts induced by aged sewers. A regular monitoring schedule can improve the performance of sewer systems and hence secure public safety and environmental hygiene. Maintenance, however, is not an easy task for underground utilities with a heavy traffic load above. An alternative option is to upgrade the sewer. A trenchless method using Tunnel Boring Machine (TBM) is deployed to construct a new sewerage pipe alongside the existing sewerage system.

Challenge and solution

Sixense mission was to detect the location of the piles beneath Dragon Bridge. The piles are positioned at a distance of up to 2m from the planned route of a TBM used to construct a new sewer.

Precise information on the pile locations was needed to ensure the TBM route is carefully controlled and remains centralised in the 4.75m spacing between piles.

Two geophysical methods were implemented for pile detection: borehole magnetometry (MAG) and parallel seismic method (PSM), which were performed in 6 inclined boreholes.

The main conclusions of the borehole geophysical survey were:

• The alignment (centreline) of the detected nearby piles has been determined where the TBM excavation will take place
• The horizontal distance between the TBM excavation line and the closest interpreted pile edge was calculated

The findings of the survey assisted our client to propose a new safe design for the TBM route.