Explosion at the Intersection of Hyde and Post Street, San Francisco
Background
Exponent conducted an engineering investigation of a June 1994 gas explosion that destroyed a three story structure at the southwest corner of Hyde Street and Post Street in San Francisco. The owner-occupant had left the building some 30 hours prior to the explosion.
Our Work
As part of a sewer line refurbishment project, a piping contractor had been conducting construction and excavation activities directly in front of the building for several weeks prior to the explosion. Exponent engineers reviewed available photographs, documents and reports about the explosion, inspected the building site and surrounding neighborhood and inspected and tested the gas fueled appliances and gas meter, and associated piping recovered from the subject building. We also performed structural analysis of the building to determine the sequence of the collapse and excavated and inspected the gas service lateral and conducted sewer gas tests to trace the potential fuel for this explosion.
Exponent concluded that the explosion was caused due to a natural gas accumulation from a leak on the inlet flange of the gas meter in the basement. The leak at the meter occurred because the gas service lateral to the subject building was damaged and pulled during the construction activity. Eyewitness accounts confirmed that a large object fell on the exposed service line during sewer excavation, approximately one week before the explosion. Excavation of the lateral revealed a downward deflection of greater than two feet from the supply main. Ignition of accumulated natural gas in the basement destroyed key structural members of the building, causing the complete collapse.
