A drainage tunnel believed to have caused abnormal land subsidence in Tseung Kwan O is to undergo several months of strengthening work, officials said yesterday. It is thought the unusual settlement on reclaimed land in the new town's centre and industrial estate resulted from unexpected changes in underground water levels due to the construction of a Strategic Sewage Disposal Scheme tunnel. Speaking at the tunnel's breakthrough ceremony yesterday, Secretary for Works Lee Shing-see said investigations to find the exact cause would continue for months. 'With excavation of the tunnel now completed, internal wall strengthening will be carried out in the next few months to prevent further underground water leaking through,' he said. His comments came after residents at an estate reported cracks in walls, ceilings and floors. The Drainage Services Department confirmed that a section of seawall around the typhoon shelter in Chai Wan, where a tunnel for the same sewerage project had been built, also reported unusual settlement.