Characterization of Loess for Deep Foundations-Article

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The study characterizes loess at a Kansas Department of Transportation test site selected for full-scale lateral-load tests of drilled shafts. The investigation combined laboratory testing—including triaxial, direct shear, consolidation, collapse, and index tests—with field SPT, CPT, pressuremeter testing, and continuous soil sampling. Its purpose was to assess the soil’s engineering behavior and how well standard in-situ correlations apply to loess.<br /><br />Laboratory classification identified low-plasticity clay near the surface, nonplastic to low-plasticity silt at intermediate depths, and clay again below. The loess behaved as a frictional soil with genuine cohesion. Friction angle increased from about 22° in the weathered surface soils to roughly 27° at depth, while cohesion generally decreased with depth. Strength was effectively isotropic, and the tested soil showed only slight collapse potential. Elastic modulus tended to increase with depth below the weathered surface layer.<br /><br />Standard CPT classification methods sometimes misidentified the loess as sand or gravel, so soil classification should be confirmed by sampling. Common CPT and SPT correlations substantially overestimated friction angle, apparently interpreting the soil’s cohesive contribution as friction. CPT-derived modulus values were comparatively consistent and showed promise for developing lateral-load p-y curves.<br /><br />The authors conclude that correlations developed for other soil types should be used cautiously in loess, as they may produce significant errors in estimated soil properties. The lateral-load test results and the resulting static and repeated-loading p-y curves were reserved for a separate paper.

Keywords

loess

Kansas Department of Transportation

drilled shafts

lateral-load tests

triaxial testing

SPT

CPT

pressuremeter

soil classification

p-y curves