Updated liquefaction potential analysis eliminates foundation retrofittingof two critical structures

Developments in geotechnical engineering after 1970 prompted the reevaluati on of the foundations of three large nuclear material processing buildings in 1994. Concerns were raised because the subsurface conditions beneath the m contain a Miocene-age clayey sand stratum with very low standard penetrat ion resistance values. A preliminary liquefaction potential evaluation base d on Seed et al. [The influence of SPT procedures in soil liquefaction resi stance evaluation. Report no. UCB/EERC-84/15, October 1984] empirical proce dures for Holocene-age data showed that based on the low blowcount the form ation was liquefiable, and that foundation retrofitting was in order. Analysis of the soil performance during the Charleston, South Carolina eart hquake indicated that the dynamic strength of sands increases with the age of the deposit [Lewis et al. Liquefaction resistance of old sand deposits. Paper presented at the Pan American Soil Mechanics Conference held in Brazi l, 1999], a finding in agreement with conclusions reached earlier by Skempt on [Standard penetration test procedures and the effects in sands of overbu rden pressure, relative density, particle size, aging and overconsolidation , Geotechnique 36(3) (1986) 425-447], and, confirmed later, by Kramer and A rango [Aging effects on the liquefaction resistance of sand deposits: a rev iew and update. Proceedings of the Eleventh European Conference on Earthqua ke Engineering, Paris 1998, Abstract vol. 184]. Because of the age of the s ands at the subject sites, it was decided to test in the laboratory the pot ential strength gin of the site soils due to aging relative to the values d erived from the empirical chart. The laboratory test results demonstrated a strength increase by a factor between 2 and 3. Based on these findings, it was concluded that there was no need to retrofit the building foundations, thus saving several million dollars. The paper describes the buildings, the geologic setting and the field and l aboratory work performed in this investigation, a summary of field performa nce of sands during the Charleston and Northridge earthquakes, and presents a design-oriented chart to predict the dynamic strength increase with age of sand deposits older than Holocene period. (C) 2000 Elsevier Science Ltd. All rights reserved.