Analysis and Design of Retaining Structures against Earthquakes

Simplified linear elastic analytical solutions are presented for the seismic pressures against rigid walls which agree closely with the exact solutions presented by Wood (1973). The finite element method is used to extend the analyses to nonhomogeneous elastic materials and to nonlinear soils. The finite element analyses give almost exact solutions for the elastic cases. Some practical guidelines are given for estimating the dynamic pressures for use in practice.

Rigid retaining walls experience both sliding and rocking displacement during earthquakes. Richards and Elms' (1979) method incorporates only sliding displacements in design. A realistic method of computing both sliding and rocking displacements of the walls based on nonlinear soil properties both for the base soil and backfill was developed by Rafnsson and Prakash (1994). A design procedure based on this method was then developed by Prakash et al. (1995b). Comparisons of the displacements of typical walls by both the methods have been made. Also, typical field data from recent earthquakes has been analyzed to compare the observed displacements with the computed displacements.

A pseudo-static model is developed for the prediction of permanent residual displacement of monolithic retaining walls in rocking. The model follows a Newmark approach, and is validated by comparison with experimental data of the behaviour of a large gravity wall subject to earthquake shaking. The experimental data show that rocking of retaining walls comprises two components, an elastic recoverable element and a permanent or residual component. The rocking model showed good agreement with the observed data, matching the total displacement closely after a series of five earthquakes. Comparison with conventional design approaches suggests that there is opportunity to reduce conservatism in the design of gravity walls prone to rocking by using a mechanism-specific model for the wall behaviour, particularly where the nature of the earthquake input motion can be defined.

Full scale dynamic testing is undertaken to monitor the vibrational characteristics of cantilever wall-soil systems. Impact hammer as well as controlled shaker excitation at different force levels are employed. Two different reinforced concrete cantilever wall-soil systems are studied. The employed testing techniques are described and the recorded modal characteristics are presented. These modal characteristics may be used for the purpose of seismic design; and for calibration of computational and/or small scale laboratory centrifuge models.

An analytic method is described for determination of the most critical threshold acceleration for bridge abutments. The method considers the posSibility of sliding, bearing capacity, or mixed modes of failure. The authors have updated, and extended the coupled equations of motion that appear in the literature for predicting seismic induced permanent deformation. Comparisons between observed and computed model responses serve to verify the ability of the proposed algorithms to predict sliding, tilting or mixed modes of deformation. Threshold accelerations are estimated for a representative sample of 40 bridges with free-standing abutments from New York State. It appears that significant numbers are at risk for an earthquake with peak accelerations above 0.15g. This would indicate that extending this easy screening procedure to other areas in the East and Midwest in moderate earthquake zones would be worthwhile.

Simplified methods of analysis are herein proposed for estimating earthquake-induced abutment fill settlements. Primary factors such as excitation history, amplification (or deamplification) within the fill, and nonlinear soil properties and slope geometry which control the settlement have been accounted for in the approach. Settlement caused by the permanent slope rotation has been obtained from a procedure similar to Newmark's sliding block model. Additional settlement caused by grainslip-induced deformation is also incorporated. Readily usable simple relationships have been developed based on the extensive seismic excitation database generated from the 1994 Northridge earthquake in order to evaluate settlements of typical abutment fill heights of 4–10 m subjected to any earthquake excitation.

An empirical equation for estimating the permanent displacements of gravity retaining walls with dry backfill during earthquakes is proposed. It was obtained analyzing the displacements of Newmark's block produced by the horizontal components of 155 earthquakes which took place in various parts of the world. The displacements were correlated with the earthquake destructiveness potential factor,P_D, a parameter representative of earthquake severity introduced by Araya and Saragoni (1984). This parameter, defined as the ratio between the Arias intensity and the square value of zero crossings per second of an accelerogram, synthetizes the major characteristics of the seismic ground motion which can affect the behaviour of the wall. For each record of the earthquakes considered, the potential destructiveness factor and the final displacements of Newmark's block were determined using different values for the resistance factor,N, and, with the multiple regression technique, the equation that best fits the calculated values was determined. This equation was compared with the equation suggested by Wong (1982) and their uncertainties were analyzed. Finally an empirical equation for estimating the displacements of a gravity retaining wall is obtained.