Shallow Soil Improvement for Rocking Foundations (2011-2013)
Part of DARE Research project “Soil-foundation structure systems beyond conventional seismic “Failure” thresholds: Application to new or existing structures and monuments” awarded to Professor George Gazetas
Principal Investigators: Ioannis Anastasopoulos (National Technical University of Athens), Tarek Abdoun (Rensselaer Polytechnic Institute)
The main goal of this research project is to investigate the possibility of allowing “below-ground” support systems to respond to strong seismic shaking by going beyond a number of “thresholds” that would conventionally imply failure and are today forbidden by current seismic codes. According to such “thresholds”, the foundation is not allowed to fully mobilize its strength and plastic deformation is restricted to structural members only. Capacity design is applied to the foundation to guide failure to the superstructure, thus prohibiting mobilization of bearing-capacity, uplifting and/or sliding, or any relevant combination. However, thanks to the cyclic and kinematic nature of the seismic excitation, such mobilization does not necessarily lead to failure.
Recent research findings suggest that soil–foundation plastic yielding under seismic excitation may be advantageous and should be seriously considered in analysis and perhaps allowed in design. As part of this project, an alternative design philosophy is currently being explored, in which soil failure is used as a “shield” for the superstructure (i.e. plastic “hinging” is moved from the superstructure to the foundation soil, exactly the opposite of conventional capacity design).
The effectiveness of such design philosophy has, so far, been explored analytically (through nonlinear dynamic time history analysis) for a SDOF bridge structure [Anastasopoulos et al., 2010a] and for a typical 2-storey 1-bay reinforced concrete frame structure frame [Gelagoti et al., 2010a; 2010b]. In both cases, it has been shown that soil nonlinearity and foundation rocking may provided substantially larger safety margins (i.e. avoidance of collapse) for seismic motions that exceed the design limits. The key conclusions have been confirmed by small-scale pushover and shaking table tests, conducted at the NTUA Laboratory [Anastasopoulos, 2010; Drosos et al., 2010; Anastasopoulos et al., 2010b].
In the framework of the present project, centrifuge tests were conducted at the Center for Earthquake Engineering Simulation at RPI. SDOF systems lying on different soil profiles were subjected to monotonic and slow cyclic lateral loading in order to validate the findings of the small scale 1g tests conducted at the NTUA Laboratory and explore the concept of a shallow soil improvement that will limit the settlement induced due to foundation rocking.