Inelastic Response Spectrum for Seismic Soil Pile Structure Interaction

Inelastic Response Spectrum for Seismic Soil Pile Structure Interaction

Pavan Kumar Emani (Department of Civil Engineering, Graphic Era University, Dehradun, India), Ritesh Kumar (Tokyo Institute of Technology, Department of Civil Engineering, Tokyo, Japan) and Phanikanth Vedula (Bhabha Atomic Research Centre (BARC), Mumbai, India)
Copyright: © 2016 |Pages: 11
DOI: 10.4018/IJGEE.2016070102
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Abstract

Structures resting on deep foundations like pile groups are subjected to entirely different kind of vibrations than those resting on shallow foundations, due to the inherent variations in the ground motions experienced at various levels of the foundation. The present work tries to generate response spectrum for single-pile supported structures using inelastic dynamic soil-pile interaction analysis. In the numerical model, the soil nonlinearity includes both separation at soil-pile interface and the plasticity of the near-field soil. The radiation boundary condition is also incorporated in the form of a series of far-field dampers which absorb the out-going waves. Inelastic response spectra for the structure, represented by a SDOF system, is generated after applying the synthetic time histories compatible with design (input) response spectra (as per IS 1893:2002-part I) at the base of pile to investigate the effects of ground response analysis including kinematics and inertial interaction between soil- pile system. It is found that a structure supported by pile foundations should be designed for larger seismic forces/ accelerations than those obtained from the design spectrum given in IS 1893:2002-Part I. The verification of the developed MATLAB program is reported towards the end, using results from commercial Finite Element software ABAQUS.
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Introduction

Indian Seismic code IS1893-2002 suggests soil-structure interaction analysis for important and non-trivial structures, since the codal provisions do not consider such interactions in precise manner. The code merely gives the seismic design spectrum for structures resting on various kinds of soil. Design of pile foundations in soils, in general, and in liquefying soils has received great attention in the recent past due to the discovery of new failure modes. Analysis of pile- soil dynamics is especially complicated because of the presence of semi-infinite soil and a limited zone of nonlinearity. This is further complicated due to 1) liquefaction and consequent loss of lateral soil support, 2) passive earth pressures from moving soil mass and 3) destabilizing effect of axial loads on the pile (Madabhushi, Knappette, & Haigh, 2010). Both continuum modeling (Maheshwari & Emani, 2014; Emani & Maheshwari, 2009) and discrete or Winkler spring modeling of soils (Nogami, Otani, Konagai, & Chen, 1992; Wang, Kutter, Chacko, Wilson, Boulanger, & Abghari, 1998) is carried out by researchers for non-linear seismic analysis of soil-pile systems, even under liquefaction conditions (PhaniKanth, Choudhury, & Reddy, 2013) (Maheshwari, Truman, Truman, & El Naggar, 2005). Experimental studies are carried out by Miyamoto, Hijikata, & Tanaka (2004) using mining blast induced vibrations, on structures supported on 2 x 2 pile group, and the same are explained using 1D and 3D numerical models considering inelastic soil-pile-structure interaction analysis.

The present work is undertaken with a motivation to help foundation designer in considering the interactions of pile in inelastic soil for estimating the seismic loads. Using beam-on-nonlinear-Winkler (BNW) modeling, an inelastic design spectrum is developed for structures resting on pile foundations.

Soil-Pile Modeling

The soil-pile system consists of a single 10 m long circular pile of diameter 0.4 m, embedded completely in a nonlinear soil. The soil-pile system is modeled as a beam supported by nonlinear Winkler soil (BNW) springs in the near-field and by a series of dampers in the far-field. The spring stiffness values are taken from Nogami et al (1992) as also the damping of the far-field dampers. The inertia of the pile is consistently modeled while the inertia of the soil system is also appropriately modeled by point masses lumped at appropriate height of the pile beam (Maheshwari et al., 2005). The various aspects of the numerical (Finite Element) model of the soil-pile system are described in detail in the Table 1 below.

Table 1.
Single Pile interaction with non-linear soil
FeatureDescription (Figure 1)
Pile
GeometryCircular pile of length 10 m and 0.4m diameter
MaterialE=25E9 Pa, ρ = 2500 kg/m3
MeshingLinear flexural beam element with consistent mass, and no damping. Ten elements are used in 10m length
Loading and Boundary conditions (described in detail latter in the paper)For inelastic seismic response spectrum: Synthetic acceleration time history is applied to the base of the pile-soil system.
For verification problem: A horizontal displacement load is applied on the top of the pile with a harmonic time variation, while the base is kept fixed
Near-field soil
Geometry and meshingTwo-noded nonlinear springs are used to model the exponential degradation of soil stiffness due to plasticity. Also, lumped mass elements model the inertia of near-field soil. Lumped mass on pile side is 188 kg and on far-field side is 110 kg. No damping in near-field zone
Plasticity (Figure 2)Exponential degradation of strain hardening is modelled as

where, Fy=6000 N, Q=600000N, b=0.3 are used, up is the plastic deformation of nonlinear spring
Far field soil
Geometry and meshingThe stiffness and damping of the far field soil is modelled by spring-dashpot assembly in series. The equivalent stiffness and damping are 1.9066e+007 N/m and 4.9968e++5 kg/s. Mass lumped on pile-side is 35 kg.
Soil – Pile interface
ElementsGap elements are used to model the separation between soil and pile
Figure 1.

FE model of the single pile soil system

Figure 2.

Force- displacement relation of non-linear near-field soil springs

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