Dynamic Analysis of Anchored Diaphragm Walls

Summary
In this paper, the behavior of diaphragm walls with and without anchor have been studied. In addition to static loads, the walls were subjected to seismic load due to earthquake, using finite element program, PLAXIS 2D. Maximum displacement and bending moment of the wall were compared in different cases.
 
Introduction
Deep excavations in urban areas require not only the stability of retained soil, but also special attentions due to the nearby existing buildings. They must be designed in such a way that the requirements for both ultimate and serviceability limit state for supporting system and neighboring structures are satisfied. Anchored diaphragm walls are one of the safest lateral supports, which help in overall stability of the excavated areas. Diaphragm walls represent in general a considerably more complex problem than gravity or cantilever walls.
 
 Methodology and Approaches
The wall displacement, maximum bending moment and anchor extreme force were calculated using a dynamic PLAXIS finite element program. The soil was considered as elasto-plastic material, using Mohr–Coulomb constitutive model.  The wall and the anchor were considered to behave elastically. Considering the dynamic applied load, absorbent boundaries were assumed to prevent dynamic wave reflection.  Three different historical strong motions namely, Tabas, Bam and Roodbar were applied to the wall. The following outputs were obtained for different excavation geometry and mechanical properties for soil.
 
Results and Conclusions
general results can be drawn as following: 1-Change in the soil stiffness (modulus of elasticity and cohesion) from soft to stiff has caused the maximum wall displacement and bending moment to increase by 76% and 20%, respectively. 2- Under dynamic load, the maximum lateral displacement of the wall is at its maximum value, while the static wall displacement is maximum at about one third the wall height measured from its toe. 3-Peak ground acceleration has direct effect on maximum wall displacement and bending moment, while earthquake frequency has inversely changed these output parameters. Based on the obtained results 30% increase in earthquake acceleration has caused the maximum wall displacement increase by 15%. 4- The maximum displacement and bending moments increased noticeably under earthquake load. Therefore, for diaphragm walls in earthquake prone area, dynamic case should be considered in design. Albeit, building codes, accept higher displacement and bending moments under exceptional loadings.