Lateral Forces on Pile Due to Soil Slope Creep-Paper

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A Los Angeles home built on a slope showed minor distress despite being supported by concrete piles embedded in bedrock. Cracking and separation in nearby streets, walls, and concrete features suggested ongoing slope creep. Two years of optical and manometer monitoring confirmed that the home was moving downslope.<br /><br />To assess the risk to the foundation, the authors modeled the piles using the GROUP computer program and soil properties from site investigations and laboratory testing. Because prior research showed that creep-related soil displacement varies considerably with depth, they tested several displacement profiles. A simple linear profile—maximum movement at the ground surface, diminishing to zero at the assumed depth of creep—best matched the measured structural response.<br /><br />The model predicted a pile-head rotation of 0.10 degrees, consistent with both optical measurements of horizontal movement and manometer measurements of differential floor movement. Based on an estimated total surface displacement of 1.9 inches over 12 years, the maximum moment in a pile at the home’s northeast corner was estimated at about 210 ft-kips. This exceeded the estimated 185 ft-kip capacity calculated using an assumed concrete strength of 2,500 psi, suggesting the pile was likely approaching its ultimate capacity, although the actual concrete strength may be higher.<br /><br />The authors conclude that a linearly varying displacement profile can provide a reasonable initial basis for estimating pile forces from slope creep, with analyses performed using programs such as GROUP or LPILE. Their results also indicate that pile moments and shears reach a maximum when soil displacement is about 10% of the pile diameter.

Keywords

Los Angeles slope creep

foundation distress

concrete piles

bedrock support

pile rotation

GROUP program

soil displacement profile

structural monitoring

pile moment capacity

slope stability