Lateral Forces on Pile Due to Soil Slope Creep-Article

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The paper examines how gradual downslope soil creep can impose lateral loads on building piles. The case study concerns a Los Angeles home supported by concrete caissons socketed into bedrock beneath a thick colluvial layer. Cracking and separation in outdoor improvements, optical surveys of the house, and floor-level measurements indicated ongoing movement consistent with soil creep.<br /><br />The authors modeled a two-pile group using the GROUP computer program and nonlinear p-y soil curves. Because measured creep profiles vary and site data were limited, they assumed soil displacement decreased linearly with depth, reaching zero at the bottom of the creep zone. The model’s predicted pile-head rotation closely matched rotation inferred from structural monitoring. Estimated movement over roughly 12 years was about 1.9 inches (48 mm), producing a peak pile moment of approximately 210 ft-kips—potentially greater than the capacity calculated using the minimum specified concrete strength.<br /><br />The analysis also suggested that pile moments approached their maximum when soil movement reached about 10% of pile diameter. At greater displacement, soil flow around the pile may limit additional loading. The modeled forces substantially exceeded those implied by the commonly used Los Angeles minimum design load of 1,000 lb/ft.<br /><br />The authors conclude that a uniform prescriptive load may underestimate creep effects, depending on soil properties, creep depth, and movement. They recommend site-specific analysis, while noting that creep rates and displacement profiles remain uncertain and require further research. The unusually thick creeping layer and relatively high measured movement rate made this site an exceptional case.

Keywords

soil creep

lateral loads

pile foundations

concrete caissons

Los Angeles case study

colluvial layer

pile rotation

p-y curves

structural monitoring

site-specific analysis