A damaged sewer corroded a gas pipe above it, allowing gas to travel to a building 24 metres away where it exploded, killing two women, according to a report read to an inquest yesterday.
The gas travelled through the half-filled sewer pipe and a disused branch sewer that ended at the pavement outside the Wai King Building in Ngau Tau Kok, where the two elderly victims died in April last year, the report by the Electrical and Mechanical Services Department said.
It then seeped through soil blocking the pipe and through three holes under the building's entrance, accumulated in a void under the entrance passage and seeped through to a pump room, where it was ignited by sparks from pumps.
The explosion blasted flames into the building's lobby, where Eap Kwun, 92, and Liu Lock-chan, 89, died. Eight people were injured.
Senior engineer Fung Kin-yi said the void under the entrance provided a passage for utility pipes, which entered the space through the three holes that allowed in the seeping gas.
He said the main sewer on the other side of Jordan Valley North Road had broken first and water from it corroded the gas pipe on top of it.