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Design of Temporary Slopes and Excavations in NC Residual Soils
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  • Alternative Title:
    Design of Temporary Slopes and Excavations in Residual Soils : Final Report
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    Final Report, January 2013-August 2016
  • Abstract:
    In general, current design methods and procedures for temporary slopes and temporary excavation support systems do not consider the suction-induced characteristics of unsaturated residual soils, and therefore may result in overly conservative designs and unnecessary construction costs. The main objective of this research project was investigating the possibility of a more costeffective design of temporary slopes and retaining structures in residual soils based on the incorporation of matric suction in the analyses approach. The components of this research project included: field and laboratory testing program, evaluation and development of predictive models, and analysis of cut slopes and sheet pile walls in excavation and considering water infiltration effect on suction. A full-scale field experimental program was conducted in Greensboro, NC, and included the excavation of three slopes (1:1, 0.5:1 and 0.25:1) and the construction of a cantilever sheet pile wall. Installed sensors included tensiometers for matric suction and moisture content measurements, inclinometer casings, strain gauges and pressure cells. Collected field data included suction measurements, and deformation data and the changes occurring in such data due to three excavation levels (4.6m, 6.1m and 6.7m) as well as due to water infiltration. Laboratory tests were performed on retrieved undisturbed specimens taken from the test site. Laboratory testing included measuring soil water characteristic curves, unsaturated shear strength and the physical properties of the site soils. Models for predicting matric suction as a function of volumetric water content, and shear strength as a function of matric suction were developed based on the experimental results. Utilizing the database of field and laboratory test data, as well as the developed predictive models, slope stability analyses and PLAXIS numerical models of the tested sheet pile wall in the unsaturated site conditions were performed and verified with field measurements. Both stageexcavation and infiltration effect were analyzed. Based on the results of the numerical analyses, a simplified approach for determining the required depth of embedment of a cantilever wall incorporating the effect of matric suction is proposed. The proposed approach is termed Suction Stability Number (SSN) and accounts for the stabilizing influence of matric suction in a similar manner to cohesion in the conventional stability number.

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