Liquefaction Triggering Analysis (Simplified Procedure)

CSR, the cyclic stress ratio, is the seismic demand: the average earthquake-induced cyclic shear stress normalized by the vertical effective stress, computed from peak ground acceleration, overburden, and a depth-dependent stress-reduction factor. CRR, the cyclic resistance ratio, is the capacity: the cyclic stress ratio the soil can withstand before liquefying, estimated empirically from in-situ penetration resistance through field-calibrated curves. Liquefaction triggering analysis compares the two, and the factor of safety is CRR divided by CSR. Where demand exceeds resistance — a factor at or below one — liquefaction is predicted to trigger at that depth.

The cyclic resistance ratio read from the boundary curves is referenced to a magnitude-7.5 earthquake and a confining stress of about one atmosphere. Real design earthquakes differ. The magnitude scaling factor (MSF) adjusts for the different number of significant loading cycles a smaller or larger earthquake imposes; the overburden correction K_sigma accounts for the experimentally observed reduction in cyclic resistance at higher confining stress; and K_alpha corrects for the static shear stress present under sloping ground. Idriss and Boulanger derived updated forms of these factors so that the reference-condition resistance can be transformed into the resistance appropriate to the site's actual magnitude and stress state.

No. The simplified procedure is strictly a triggering analysis: it predicts whether liquefaction initiates in a given layer, expressed as a factor of safety with depth. It does not by itself quantify the consequences — post-liquefaction reconsolidation settlement, lateral spreading displacement, loss of bearing capacity, or flow failure. Those require separate empirical or numerical methods that take the triggering result as input. For sites where large deformations or flow failures are plausible, engineers move beyond the screen to advanced effective-stress dynamic analyses rather than relying on the simplified factor of safety alone.