A practical method for construction of p-y curves for liquefiable soils

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Abstract

In practice, the analysis of laterally loaded piles is often carried out using a “Beam on Non-linear Winkler Foundation method” whereby the lateral pile-soil interaction is modelled as a set of non-linear springs (also known as p-y curves). During seismic liquefaction, the saturated sandy soil changes its state from a solid to a viscous fluid like material, which in turn alters the shape of the p-y curve. Typically, p-y curves for non-liquefied soil looks like a convex curve with an initial stiff slope that reduces with increasing pile-soil relative displacement (y) i.e., elasto-plastic softening response. However, recent research conclusively showed that p-y curve for liquefied soil has a different shape, i.e., upward concave with practically-zero initial stiffness (due to the loss of particle to particle contact) up to a certain displacement, beyond which the stiffness increases due to reengaging of the sand particles. This paper presents a practical method for construction of the newly proposed p-y curves along with an example.

Introduction

A reasonable representation of pile-soil interaction is important in the evaluation of response of pile foundation to earthquake shaking. Winkler approach (also known as Beams on Non-Linear Winkler Foundation, BNWF or p-y approach) for analysis of laterally loaded pile is widely used in the practice due to its ease of nonlinearity modelling and mathematical and computational efficiency. Fig. 1(a) schematically demonstrates the BNWF model where the lateral, axial and end bearing soil-pile interactions are modelled by lateral springs (p-y spring), axial springs (t-z spring) and end-bearing spring (q-z spring), respectively. For evaluating the lateral capacity of pile foundation, p-y springs play a vital role, and the backbone force deformation behaviour defined for this spring is known as p-y curve. In a p-y curve, p is the soil reaction per unit length of the pile and y is the corresponding relative pile-soil displacement. For liquefied soil, the p-y curve used in current practice is a factored value from its non-liquefied state, which is found to be in disagreement with the p-y curve observed from full scale, centrifuge and shaking table tests, see for example [1], [4]. Fig. 1(b) and (c) shows the shape of p-y curves in pre and post-liquefaction stage. Further details of the shape of the p-y curves for liquefied and non-liquefied soils can be found in [1], [2], [3], [4], [5], [6], [7]. This paper presents a step-by-step procedure that can be used for constructing p-y curves for liquefiable soils from a typical field bore log data.

Section snippets

Major steps in the construction of p-y curves for liquefiable soils

The construction of p-y curves for liquefiable soils involves four steps, as follows.

  • 1)

    Evaluation of soil parameters from bore-log data

  • 2)

    Consideration of dimensions of pile foundation

  • 3)

    Construction of simplified stress-strain curve for liquefied soil

  • 4)

    Generation of p-y curve for liquefied soil from stress-strain curve obtained in Step 3 above.

Example for calculating p-y curves for liquefied soil from a typical ground profile

Typically for onshore practice, SPT N value are recorded through geotechnical investigation. Fig. 4(a) shows the ground profile along with a pile for which p-y curves are to be constructed under liquefied conditions. Stepwise description is given below to obtain p-y curves for liquefied soil at a depth of 5 m.

Steps to obtain p-y curve for liquefied soil from ground profile:

Step – 1: The soil considered at a depth of 5 m with SPT value N=5 and unit weight=17 kN/m3

  • (a)

    The SPT value corrected for

Conclusion

In this paper a practical method for construction of p-y curves for liquefiable soils is presented. A step by step calculation procedure has been provided with an example considering a standard borelog data. The calculation uses basic soil properties such as relative density, SPT profile, basic pile geometry and material. Other required soil properties are estimated based on proposed empirical relationships. Once the stress-strain behaviour of post-liquefied soil is defined, the p-y curve can

Novelty of submission

  • 1.

    Mechanics based shape of p-y curves

  • 2.

    Proposed p-y curves for liquefiable soils.

  • 3.

    Understanding the parameters required for construction of p-y curves.

References (10)

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