The Earthquake Engineering Online Archive

Representation of irregular stress time histories by equivalent uniform stress series in liquefaction analyses

Seed, H. Bolton; Idriss, I. M.; Makdisi, Faiz I.; Banerjee, Nani G.

UCB/EERC-75/29, Earthquake Engineering Research Center, University of California, Berkeley, 1975-10, 40 pages (475/S413/1975)

The report presents a rational yet simple procedure for determining the uniform cyclic stress series which is equivalent, from the point of view of soil liquefaction, to any given irregular time history of stresses developed by an earthquake. It is believed that the procedure described will serve this purpose with an adequate degree of accuracy for practical purposes. A study of the stress histories developed by earthquakes with magnitudes ranging from about 5 to 6.3 indicates that a cyclic stress series involving 5 cycles at a uniform stress level of 0.65 tau subscript max provides an adequately conservative representation of such events, when used in conjunction with a conservative choice of design ground motion, suggesting that the use of larger numbers of stress cycles than this would lead to overly conservative estimates of liquefaction potential for most sites subjected to earthquakes of such moderate magnitudes. Appropriate numbers of equivalent uniform stress cycles for earthquakes with various magnitudes are analyzed statistically, and recommendations are made for appropriate numbers of cycles for use in engineering design.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-75-29.pdf (1 MB)