Liquefaction Triggering Analysis (Simplified Procedure)

Liquefaction triggering analysis evaluates whether saturated, loose granular soils will lose strength and behave like a fluid during earthquake shaking, using the simplified stress-based procedure that has anchored geotechnical earthquake engineering since Seed and Idriss introduced it in 1971. The method compares demand against capacity: the cyclic stress ratio (CSR) imposed by the earthquake versus the cyclic resistance ratio (CRR) the soil can sustain, both expressed as ratios of cyclic shear stress to effective overburden stress. Capacity is read from in-situ penetration tests — standard penetration test blow counts or cone penetration test tip resistance — through empirical curves calibrated on field case histories of sites that did and did not liquefy. The Youd and Idriss 2001 NCEER consensus report standardized these curves and the correction factors, and Idriss and Boulanger's 2008 monograph refined them. The ratio of resistance to demand gives a factor of safety against triggering at each depth. It is the workhorse first-order screen for liquefaction in routine practice worldwide.