Earthquake Damage Resulting From Slab-On-Ground Trenching

Abstract eng:
A code review of current requirements for trenching, or sawcutting of existing slabs-on-ground to accept utilities, was performed for buildings in seismic zones and areas susceptible to liquefaction and differential settlement. The resulting review shows that there are few requirements or recommendations for this practice for existing buildings. A generalized code amendment is suggested in an effort to ensure that proper detailing and design are implemented. Architects, mechanical engineers, and plumbers are often performing trenching procedures with very little input from structural engineers. The resulting trench results in slabs that may perform poorly in large earthquakes where soil settlement occurs and the ability of the slab to accommodate differential settlement is compromised due to the poorly detailed slab and underlayment at trenching locations. Common details currently used by structural and other engineers are evaluated and critiqued for resistance to liquefaction and differential settlement, and a detail is suggested. This paper stems from the authors’ experience in investigating damage following earthquakes, including the recent Napa earthquake in California, where significant (and costly) damage was encountered due to poorly detailed subbase/subgrade systems and patches in slabs-on-ground at trenching locations.

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