Session 2.2 - Piles and Bridge Foundations

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Gapping Effects on the Lateral Stiffness of Piles in Cohesive Soil

S. Pranjoto and M.J. Pender

This paper summaries research on the effect of gap formation adjacent to single piles embedded in cohesive soil when subject to cyclic lateral loading. Pile-soil interaction and gap formation were modelled using detachable Winkler springs at the front and rear of the pile shaft. Linear soil behaviour demonstrates that gap formation produces non-linear pile head stiffness that rapidly approaches half of the stiffness when the springs on both sides of the pile shaft are active. Non-linear soil behaviour shows a gradual approach to a steady state situation with increasing number of cycles. The steady state gap width at the ground surface widens and the gap depth increases as the magnitude of loading and number of cycles increase. Factors investigated in the parametric study are number of load cycles, level of load, load eccentricity, pile size, pile-soil stiffness ratio, and the undrained shear strength of the soil in which the pile is embedded. As a rule of thumb, gapping reduces the lateral stiffness of the pile head to about half that when no gap is present.

Paper 096: [Read][Print]

A Pseudostatic Approach for Seismic Analysis of Piles in Liquefying Soil

D.S. Liyanapathirana and H.G. Poulos

This paper presents a pseudostatic approach for the analysis of piles in liquefying soil, including the contribution of the superstructure to the pile and the interaction between the pile and the soil. The method involves two main steps. First an effective stress based ground response analysis is carried out to obtain the maximum ground displacements along the pile and the degraded soil modulus over the depth of the soil deposit. Next a static load analysis is carried out for the pile, subjected to the maximum free-field ground displacements and the static loading at the pile head based on the maximum ground surface acceleration. The method has been verified using an independent dynamic pile analysis program developed by the authors for the seismic analysis of piles in liquefying soil. The new method is then used to compute the response of pile foundations during Kobe 1995 earthquake and some centrifuge tests found in the literature where extensive soil liquefaction has been observed. Very good agreement is observed between the computed and the recorded pile bending moments.

Paper 021: [Read][Print]

Keywords: pile foundations, soil liquefaction, seismic analysis, beam on Winkler foundation, pseudostatic approach

Development of Analytical Procedure for Estimating Effects of Liquefaction-induced Ground Flow on Bridge Foundation

K. Tamura and T. Azuma

We develop a new technique for estimating the effects of liquefaction-induced ground flow on a bridge foundation. This technique first analyses ground deformation by liquefaction-induced ground flow and then applies it to a bridge foundation. Ground deformation is obtained by a self-weight analysis, in which soil rigidity is reduced according to the degree of liquefaction. Deformation caused by self-weight of the ground is computed by a 2-D finite element method with reduced rigidity, and the resultant deformation is regarded as that caused by ground flow. In the second stage of the proposed technique, a bridge foundation is idealized so that a rigid footing is supported by piles which are supported by the soils. This considers the nonlinear properties of the pile bodies and the ground. The ground deformation obtained by self-weight analysis is statically applied to a bridge foundation, and the deformation and bending moment of the piles are computed.

Paper 114: [Read]

Keywords: liquefaction, ground flow, bridge foundation, seismic design

Performance of Retrofitted Pile Foundations Subjected to Seismically Induced Lateral Spreading

T.H. Abdoun and W. Wang

Experiences from earthquakes and centrifuge models have shown the great importance of the shallow nonliquefiable soil in increasing the forces and moments imposed on the pile cap and pile foundation subjected to liquefaction-induced lateral spreading. This paper focuses on evaluating retrofitting strategies, with emphasis on the placement of a soft or frangible material near the foundation in the shallow nonliquefiable layer. While this shallow soft material reduces the stiffness and the strength of the pile foundation with respect to the superstructure inertia forces, it constitutes an extremely effective way to mitigate the effect of lateral spreading cases in which the resistance to inertia is provided by other foundation elements are one area of application of the proposed retrofitting strategies. Results of three centrifuge tests, Models 2, 2r1 and 2r2, are presented to illustrate the effectiveness of the implemented retrofitting method.

The experimental results for each of the centrifuge pile models are reviewed and compared. After implementing the proposed retrofitting strategies, a dramatic reduction in the maximum bending moment is observed at the upper boundary of the liquefied layer (2 m depth). A reduction of up to 35% in the measured maximum bending moment is also observed at the lower boundary of the liquefied layer (8 m depth). These significant reductions in measured pile maximum bending moments in Model 2r, together with the associated reduction of up to50% in the measured pile head displacement, demonstrate the effectiveness of the implemented retrofitting method.

Paper 054: [Read][Print]

Keywords: centrifuge modeling, liquefaction, lateral spreading, pile foundation, retrofitting of piles

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