Behavior of Soft Clays Under Earthquake Loading Conditions
Authors: Idriss I.M. , Dobry R. M. , Doyle E.H. , Singh R.D. 
Published: October 2008

A new method is presented to compute the earthquake response of soft clay deposits. In this method, the change of properties of that soil during cyclic loading is directly incorporated into a nonlinear response calculation in, the time domain. The parameters of the stress strain model proposed for the clay are obtained from cyclic shear tests. A sample problem of earthquake response of a soft clay offshore site is discussed.

INTRODUCTION

Development of seismic design parameters for offshore structures, such as fixed platforms, is a complex problem that requires incorporation of the effects of local site conditions, structural characteristics, and water. An important consideration in developing these parameters is the response in the free field (ie, assuming no structure present). Studies related to free field response during-a postulated seismic event provide results relevant to:estimates of ground motions at sea floor or at some depth below sea floor these motions may then be used as a basis for the seismic design of the structure;estimates of stresses and displacements induced in the soil by these motions the estimated displacements may then be used as a basis for assessing the performance of pile foundations; andevaluation of ground failure potential of the soil, which may be manifested in phenomena such as liquefaction of sands and increase in pore pressures or remolding of clays during seismic loading.

A previous publication (1) included a review of geotechnical aspects related to the seismic design of offshore structures; that reference also discussed available observations and analytical procedures used to estimate ground response to an assumed base rock motion. For a site where subsurface conditions consist of horizontal, or almost horizontal, parallel soil layers, the response of the soil column to horizontal base rock excitation can be reasonably considered as that of a shear beam. Therefore, calculation of this response reduces to the solution of one dimensional vertically propagating shear waves. Computer programs like SHAKE can solve the wave propagation equation and compute the desired ground response parameters; an equivalent linear model of the profile is used, with the damping and stiffness properties of the soil adjusted to the seismic strain level, to approximate the actual nonlinear hysteretic behavior of the soil (2). Most soil response analyses to date, onshore or offshore, have been done using the equivalent linear method.

The results of cyclic loading tests on soft clays, such as found in many marine deposits, indicate a degradation in soil stiffness due to cyclic loading, which may be important for the seismic design of an offshore structure located at a soft clay site. This effect is associated with an increase in pore pressure during cyclic loading and also with deterioration of the structure of the clay due to remolding (3, 4, 5, 6). Modulus degradation of the soil may affect the response characteristics of the site and therefore should be considered in the selection of seismic design parameters for the offshore structure and its foundations.