Seismic Performance of Hillside Fills

Permanent ground deformations in unsaturated, compacted hillside fills under seismic loading conditions are discussed, with emphasis given to fill performance during the 1994 Northridge earthquake. These movements represent a significant yet often unrecognized hazard to developed hillside areas, as relatively modest deformations induced widespread damage totaling hundreds of millions of dollars during the Northridge event. The development of grading standards in the Los Angeles area is reviewed to place the seismic fill deformation problem in context with other issues that have shaped design and construction practices for hillside fills. Field observational data on fill performance during the Northridge earthquake is presented, and typical ground distress patterns are found to include cracking near cut/fill contacts, lateral extension and settlement of fill pads, and bulging of fill slope faces. For most sites, the prevalent mechanism of permanent ground deformation responsible for the fill movements is contractive volumetric strain accumulation within the unsaturated fill soils during strong earthquake shaking (that is, seismic compression).