The objective of this report is to present an approach to designing pile foundations, embedded at depth in a stiff substratum and influenced by adjacent loads applied on the surface of soft superficial soils. The effect of lateral thrust on the piles in an upper soft clay layer due to simulated embankment construction is examined, and soil-pile interaction mechanisms are identified herein for behaviour both at working load and at ultimate lateral capacity. A combination of centrifuge model testing and three dimensional finite element analysis was used to investigate the performance of a row of free headed piles and of a pile group, for different pile and foundation geometries, in terms of changes of bending moment, deflection and lateral pressure due to a uniform surcharge. An approximate formula for lateral thrust in the soft clay layer is developed, based on the differential movement between the piles and the surrounding soil, which accounts for pile spacing, relative pile-soil stiffness and the degree of soil strength mobilisation. This loading function has been incorporated in a computer program, SIMPLE, which calculates the pile bending moment and deflection profiles for long piles and pile groups. The algorithm has been calibrated against the experimental and numerical results, and design charts are produced for the free headed pile case. A design procedure is recommended, and illustrated by a worked example, for piled full-height bridge abutments and other facilities which feature passive lateral loading of piles by a nearby surcharge. The work described in this report was carried out under contract by Cambridge University for the Transport and Road Research Laboratory (TRRL).

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