Siesmic Response of Single Pile Soil Structure Interaction Effect of Cordera bridge foundation

The response of single pile foundations to the dynamic loads is considered in this paper. The behaviour of such foundation is important specifically in case of earthquake excitation through the supporting soil medium. An axisymmetric finite element model has been implemented to simulate the behaviour of pile in relatively clayey soil deposit using Abaqus software including the soil structure interaction effect soil system. Eight node axisymmetric quadrilateral element CAX8R used to simulate the soil continuum. Contact behaviour between the single pile part and the part of soil was simulated using the ‘surface to surface’ contact method with master-slave concept. Furthermore, the pile behaviour material has been simulated with a linear elastic model while, soil material has been simulated with an elasto-plastic model “Mohr-Coulomb failure criterion”. Two different excitation have been adopted during the analysis: El-Centro and Ali-Algharbi earthquakes in order to investigate the effect of various intensities. The results of the analysis demonstrate alteration in the relative displacement and stresses along the pile with different soil layer and different stiffness with both earthquake excitations.


Introduction
Generally, finite element method or 'Numerical analysis' shows a major role in case of investigating the behavior of many geotechnical structures. It can illustrate the effect of dynamic loading on such structures and also highlighting the aspects which are important in engineering practice, [1] In general, dynamic analysis, Soil structure or soil pile structure analysis is considered as very important aspect as it containing two components affecting the behaviour of the system, both components arisen during seismic events: kinematic interaction and inertiall interaction. Where the kinematic soil-pile interaction refers to pile loading developed by the soil displacement from the seismic wave propagation, whilen the second component is reffered to as Inertials interaction which results from feorces caused by superstructure actuations by the kinematic interactionr [2]. Seismic soil-structure interaction analysis involving pile foundations is one of the more complex problems in geotechnical earthquake engineering. The analysis involves modeling soil-pile interaction, pile-topile interaction, inertial interaction and the nonlinear hysteretic behaviour of the soil. Most of the current methods for analyzing pile foundations can be categorized into two main groups. One is based on the elastic continuum models and the other on the Winkler springs model. The elastic continuum models are suitable for studying the response under low excitations only when the dynamic response is approximately elastic. They are not suitable for analyses under strong shaking. The reduction in soil stiffness and the increase in damping associated with strong shaking are sometimes modeled crudely in these elastic methods by making arbitrary reductions in shear moduli and arbitrary increases in viscous damping. Seismic soil-structure interaction analysis IOP Publishing doi:10.1088/1757-899X/1076/1/012126 2 involving pile foundations is one of the more complex problems in geotechnical earthquake engineering. The analysis involves modeling soil-pile interaction, pile-to-pile interaction, inertial interaction and the nonlinear hysteretic behaviour of the soil. Most of the current methods for analyzing pile foundations can be categorized into two main groups. One is based on the elastic continuum models and the other on the Winkler springs model. The elastic continuum models are suitable for studying the response under low excitations only when the dynamic response is approximately elastic. They are not suitable for analyses under strong shaking. The reduction in soil stiffness and the increase in damping associated with strong shaking are sometimes modeled crudely in these elastic methods by making arbitrary reductions in shear moduli and arbitrary increases in viscous damping. Seismic soil-structure interaction analysis involving pile foundations is one of the more complex problems in geotechnical earthquake engineering. The analysis involves modeling soil-pile interaction, pile-to-pile interaction, inertial interaction and the nonlinear hysteretic behaviour of the soil. Most of the current methods for analyzing pile foundations can be categorized into two main groups. One is based on the elastic continuum models and the other on the Winkler springs model. The elastic continuum models are suitable for studying the response under low excitations only when the dynamic response is approximately elastic. They are not suitable for analyses under strong shaking. The reduction in soil stiffness and the increase in damping associated with strong shaking are sometimes modeled crudely in these elastic methods by making arbitrary reductions in shear moduli and arbitrary increases in viscous damping. Seismic soilstructure interaction analysis involving pile foundations is one of the more complex problems in geotechnical earthquake engineering. The analysis involves modeling soil-pile interaction, pile-topile interaction, inertial interaction and the nonlinear hysteretic behaviour of the soil. Most of the current methods for analyzing pile foundations can be categorized into two main groups. One is based on the elastic continuum models and the other on the Winkler springs model. The elastic continuum models are suitable for studying the response under low excitations only when the dynamic response is approximately elastic. They are not suitable for analyses under strong shaking. The reduction in soil stiffness and the increase in damping associated with strong shaking are sometimes modeled crudely in these elastic methods by making arbitrary reductions in shear moduli and arbitrary increases in viscous damping. Seismic soil-structure interaction analysis involving pile foundations is one of the more complex problems in geotechnical earthqu

Material and overall Properties for the Simulated Model
Single pile selected from foundation of bridge site belongs to khanaqeen district within diyala governoratem which lies northeast of Baghdad in Iraq adopted as a case study, detailes of the simulated model is described in the current next sections.

case study
Cordera bridge Pile foundation located in 50 kms North Ba'qubah region in Diyala Governorate, northeast of Baghdad in Iraq has been concerned in this study. The finitet element modeling has been performed based on available design drawings for the pile and also the site survey and geotechnical reports (supplied from the consultant bureau-Diyala University).

Pile and soil model description
The geometricd modeling consists of simulating the soil continuum and the pile foundation as separated parts then ABAQUS gathering the parts in the assemble module as an axisymmetric modelf Pile foundation as a concrete structure with the circular section with diameter of pile is 1.5 m and depth of the pile is 30 m. The soil and pile assembly are shown in Figure 1. The simulation of the model was done in axisymmetric model. Ant axisymmetric modeling provides reasonable simplification for modeling piled-soil systems. The axisymmetric modelx has the advantage of simplifying the problem by reducing the size of the model. Subsequently, considerable saving in computational time can be achieved; The element used to simulate the soil is an axisymmetric quadrilateral element named CAX8. Axisymmetric elements provide for the modeling of bodies of revolution under axially symmetric loading conditions, finite element mesh is as showed in Figure 2.

Constitutive Models
For most general 3D simulating models through the Abaqus program, soile mass consisting of three layers is typically modeleds as an isotropic homogenous rectangular part, with approximately width and depth (10m x 30 m). The concreted pile bodyz is modeledq in term of linearx elastic model.  Table 1.

Contact
Interactions most interesting numerical methods approved in econtact problemse s include the lawss formulation, the geometries and also interfacesd . Generally, many formulations haved been established which can simulateg eproblems of the interaction, especiallyz fore d the soil -pilem einteraction systems. Ford modeling the contactz behavior betweenz the soil and pile material, frictionz coefficientx which is mainly a functiong of the surface roughness, adhesion and porosity factors hase n been well-defined through two mechanisms: the normal and frictional mechanisms, thuse s, many formulations haven been advanced for simulating interaction between soil and piles systems c [6].
Current work adopt the master slave concept with method of the surface-to-surface contact is used which developed by Wirggers in 1995, wich is broudly used due to its capability in representing larg deformations within the area of contact.

Load Condition
In general most structures are subjecteds to static loads and also can exposed to dynamic loading event during there lifetime. Suche s as earthquakes as an instantious loading. Currentz study include two different shaking/earthquakes records, firstly, the earthquake of El-Centro (1940) of a magnitudec ( 5.4 ) and peak ground accelerationz PGA = 3.50 m/sec2. The secondd is Ali-Al Gharbi / south of Iraq (2012) with magnitudez of (4.9) and PGA=1.04 m/sec2. In order to check the response of the system, these dynamic loading conditions as well as the gravity loads (static loads) was applied to the pile soil system. The timed history records of both earthquakes are shown in the Figure 4 and 5 respectively.

Result and Discussion
Current work is curried out to make assessment and determination of the response of single pile system to earthquake loading in addition to static loading (dead and live loads) considering the site condition effect. The work is describing the relative displacements developed ine the pile-soil interface (points in the top, mid, bottom of soil were considered) The analysise z steps were parted intow two models (i.e. model 1 (M1) is under Ali-Algharbi EQ and model 2 (M2) is under El-Centro earthquake. In general, the deformed shape of the displacementz under both loading cases is shown in Figure 6. Ite d is found that the maximum rrelative displacement in the horizontal direction at the surface point is (0.00368 m) as shown in Figures 7 and 8 respectivelyx . They stress response also conducted from the analysise s with maximum value of (280.4 Pa) at the soil as shown in Figures 9 and 10. Additionallys , the case M2 showed that the relative displacements results was closesd in behaviour with M1 but withe different magnitude of (-0.01161 m) at surfaced at the end of loadings as shown in Figure 11 and 12. In the other hand, for case M1 as shown in Figure 13, ther stresses changet from maximum value of (330.2 Pa) at top region and decrease wheny measure at soil bottom with value of (142.3 Pa). Generallys , for the overalld response of the system, underd the consideration of properties of soil layers, the responsef of pile part wich embedded in softf soil layer varied from the inputx motion and this variation reduceds with increase in soil stiffness alongx the depth of pile. Ther response of the portion of soil nearx pilem imbedded in the stifferf layer was almost the sameq as the input motion. Additionallyt , the maximum deflectionz along pile depthz for the two earthquakesx increased due to inertialg soil structure interaction effectd along the head parth of the pile and its effectz decreasem with pile depth. It also can be seen that displacementh for an input seismicx case M2 increased somehowe more than the case M1 peak groundz acceleration increased.

Conclusions
The singled pile-systemz foundation hase n been successfully modeled using Abaqus programz , with actual datas froms the existings bridged sitez in Iraq, the following conclusion has been obtained. 1. The largez influencez of soil-pile systemz with differents properties with different seismic loading.
2. The selectionz of governing modelz is directly influencing the performance of system as seems thatz variation of stresses in soil mass and also the deformation pattern becom different and largly changed as soil stiffness, and such step should be takens with cure intow account at the beginning of analysis and designe of and structural. 3. Important effect of the soil-pile-structurez interaction (SPSI) hase n depected in the dynamic analysis and seismicx designz of structurese s and foundations based the interactive response between soil and pile. 4. It can Obtaining better insight to the soil and pile performance expectation through this analysis as varying the peak ground acceleration.