EVALUATING URBAN HAZARDS: AN EXAMPLE FROM BUILDING DISTRESS CLAIMS ASSOCIATED WITH TWIN-SUBWAY TUNNELING IN LOS ANGELES, CALIFORNIA

The Los Angeles County Metropolitan Transportation Authority’s Red Line Project involved construction of twin-bore tunnels through Quaternary-age sediments between 1993 and 1996. As widely reported by the local and national media at the time, ground subsidence induced by the construction activities was identified as the cause of considerable distress to buildings located along the transit route.

Engineering geologists view building distress features as a record of one or more deformational events that exceeded a certain strain threshold, thus leaving their imprint on the fixed structure. Because building distress commonly is expressed by visible cracks and out-of-level floors and walls, it can be characterized using standard geologic field techniques such as mapping, profiling and measuring. Different types of damaging events leave behind different sets of distress features. Thus, the causes of distress in old buildings that have been subjected to multiple damaging events can be evaluated by analyzing patterns and locations of deformational features from field data, and by establishing a chronology of possible causative processes.

The Red Line tunnels front a large number of historic unreinforced masonry and older concrete buildings that have been subjected to many possible damaging events. Defining the magnitude of ground subsidence and building settlement associated with the tunnel project is important – both in establishing the extent of tunneling-related distress, and also in gaining insight into other causes of distress. We conclude from our post-tunneling investigations of hundreds of buildings that some tunneling-related ground subsidence and building distress did occur locally. However, most of the claimed distress pre-dated the tunnels, a considerable amount of which resulted from 1994 Northridge earthquake shaking.