Review of Pseudo-Dynamic Design Approach for Waterfront Retaining Structures Subjected to Earthquake and Tsunami 968 REVIEW OF PSEUDO-DYNAMIC DESIGN APPROACH FOR WATERFRONT RETAINING STRUCTURES SUBJECTED TO EARTHQUAKE AND TSUNAMI Deepankar Choudhury Associate Professor, Department of Civil Engineering, Indian Institute of Technology (IIT) Bombay, Powai, Mumbai–400 076, India. E-mail: dc@civil.iitb.ac.in ABSTRACT: Latest design techniques for waterfront retaining structures or sea walls subjected to combined effects of earthquake and tsunami have been reviewed. Both sliding and overturning modes of failures have been studied to obtain the closed-form design equations for factor of safety using modern pseudo-dynamic approach which considers body waves of earthquake motion, frequency of shaking, duration of earthquake, soil amplification along with horizontal and vertical seismic acceleration profiles with depth and time. These additional considerations of earthquake forces made the pseudo-dynamic approach more reliable and acceptable compared to conventional pseudo-static approach after experimental validation. Stability of sea walls found to decrease significantly with increase in both the earthquake accelerations and tsunami height. Design factors for sliding mode of failure to obtain the safe weight of sea walls under combined effect of earthquake and tsunami have been proposed. Pseudo-dynamic results are found to provide economic design factors than pseudo-static approach. 1. INTRODUCTION Waterfront retaining structures or seawalls like gravity type cellular cofferdams, gravity dams, various types of earth retaining walls, reinforced soil structures etc. are used in the earthquake prone areas around the world. These are the key elements in ports and harbours, transportation systems, lifelines and other geotechnical structures. Seismic vulnerability and the associated detrimental economic implications have been highlighted earlier in a number of damage reconnaissance reports for waterfront structures. Under normal service conditions (i.e. when there is no earthquake) waterfront retaining structures are exposed to wave forces. However, the situation changes when such a waterfront retaining wall which retains a submerged backfill on one side (downstream side) is subjected to an earthquake. In that situation, additional hydrodynamic pressure gets generated with the seismic lateral earth pressure on the downstream side of the wall. One of the most important reasons of failure of these waterfront retaining structures is the hydrodynamic pressure, the effect of which according to the classical Westergaard (1933) approach is neither excessively large nor negligible. Although, in the past, few analytical studies were carried out to examine the effect of hydrodynamic pressure, it still seems that due to the complex overall seismic behaviour of waterfront retaining structures—involving interaction between the structure, the retained soil and the water—a better understanding is needed. Currently the design approach proposed by Ebeling & Morrison (1992) for waterfront retaining structures using pseudo-static approach is widely used however it considers the uplift of the wall also leading to a non-critical condition. Similarly, tsunamis triggered by an earthquake (or otherwise too) cause severe damage to the waterfront retaining structures. The combination of earthquake and tsunami forces on a waterfront retaining wall severely challenges its stability, both in terms of sliding and overturning modes of failure. Due to complexity of the study, this important topic was not thoroughly researched earlier and was generally confined to the consideration of the above mentioned forces individually but not simultaneously. 2. ANALYSES OF WATERFRONT RETAINING WALL A rigid vertical waterfront retaining wall with width ‘b’ and height ‘H’, which is subjected to different forces during an earthquake, is shown in Figure 1a. It retains backfill to its full height on one side, referred to as the ‘downstream side’, and holds water to height of ‘h wu ’ on the other side, called as the ‘upstream side’. The water table on the downstream side of the wall is to a height ‘h wd ’. Depending on the consideration of the forces due to tsunami, two separate cases are chosen viz., (1) Case 1: without tsunami forces and active state of earth pressure based on the direction of wall movement (Fig. 1a) and (2) Case 2: with tsunami forces and passive state of earth pressure based on the direction of wall movement (Fig. 1b). Case 1 details the stability of the waterfront retaining wall under the combined action of seismic and hydrodynamic forces, while in Case 2 more complex nature of forces is taken into account by considering tsunami force as well. 2.1 Computation of Different Forces Acting on the Waterfront Retaining Wall A rigid vertical cantilever retaining wall AB of height H, supporting cohesionless backfill is considered as shown in IGC 2009, Guntur, INDIA