Improving the Seismic Performance of Nonlinear Rocking Soil-Structure Systems using Soft Diaphragm Walls

A growing body of evidence suggests that the possibility of nonlinear rocking oscillations can protect the structure from serious damage. One of the potential concerns about this design approach is the magnitude of residual settlement and maximum rotation. Several improvement techniques have been proposed to ameliorate the nonlinear behavior of rocking foundations based on the vertical factor of safety. Rocking systems with a large factor of safety against vertical load are more prone to toppling collapse under severe ground motions. This research explored the effectiveness of using soft walls next to a rocking foundation for mitigating seismic risk. The vital advantage of this improvement technique is that it is a feasible strategy for both new construction and existing structures.
The soil-foundation-structure system has been analyzed using the numerical finite element (FE) method that takes the material and geometric nonlinearities into account. In this case, the nonlinear response mainly involves the interaction between footing uplift and soil failure, which may induce additional (gravitational) aggravating moment (P-Δ effect).
In the first step, a three-dimensional (3D) numerical model has been constructed for the rocking foundation-soil system experiment. In order to verify the accuracy of the simulation, the numerical modeling and the experimental test results have been compared. The results of the centrifuge physical testing conducted were used to validate the numerical simulation. All FE analyses were performed in Abaqus software. This software has been utilized by a number of researchers to study complex soil-foundation–structure interaction phenomena.
Because the initial conditions play an important role in simulating geotechnical problems, a staged analysis procedure has been adopted. In the dynamic analysis stage, an incremental-iterative procedure was used to integrate the equations of motion. The Hilber-Hughes-Taylor algorithm was used to conduct the transient analysis phase. The modified Newton–Raphson method was employed to decrease the calculation cost needed for the great number of degrees of freedom of the model.
Finely refined 3D FE mesh was used to precisely reproduce the mechanism of bearing capacity failure and the rocking behavior. The soil medium and foundation were discretized into eight-node hexahedral continuum elements (C3D8 element type). Two-node linear beam elements were used to model the superstructure (B31 element type). A special surface-to-surface contact formulation between the foundation and soil was used for the realistic simulation of possible uplift and sliding of the foundation. Surface-to-surface contact can calculate contact stresses accurately by reducing the possibility of large-localized penetration of the two surfaces. The properties of the contact element were defined by the interface stiffness in the normal and the tangential directions.
Nonlinear soil behavior was modeled using a kinematic hardening model with the Von Mises failure criterion and associated plastic flow rule. This simplified constitutive model is applicable for the prediction of the undrained behavior of clay as normal pressure independent. In contrast to soil, the behavior of the structure-foundation model is assumed to be linear elastic. A parametric study was carried out to explore the sensitivity of the geometric design variables of the diaphragm wall, such as height and thickness on the behavior of the isolated foundation-soil system.
It should be mentioned that the values of model parameters were determined based on their practical feasibility. The distance of walls has been determined far from the edge of the foundation to avoid static bearing capacity failure.
The results showed that the placement of vertical soft walls next to the foundation could limit the transmitted forces onto the superstructure. This could be lessened the maximum motion of the structure and the following overturning moment of the foundation. Hence, adequate safety margins against toppling collapse could be easily achieved under strong motions.