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Installation and load testing of helical piles in a sensitive fine-grained soil Weech, Christopher Nathan

Abstract

The purpose of this research was to determine how soil disturbance caused by the installation of helical piles in sensitive fine-grained soils affects the mobilization of axial pile capacity at different times after installation. Six instrumented, full-scale helical piles were installed in lightly overconsolidated, highly sensitive, marine silt and clay at the Colebrook test site in South Surrey, B.C. Prior to pile installation, a detailed in-situ testing program was carried out using field vane shear tests and seismic piezocone penetration testing which included pore pressure dissipation tests. The excess pore pressures within the soil surrounding the piles was monitored during and after pile installation by means of piezometers located at various depths and radial distances from the pile shaft, and using piezo-ports which were mounted on the pile shaft. The changes in pore pressure during pile installation were indicators of the soil deformations caused by pile installation. The observed pore pressure dissipation around the piles indicated that primary reconsolidation of the soil was complete after about 7 days. After allowing a recovery period following installation, which varied between 19 hours, 7 days and 6 weeks, piles with two different helix plate spacings were loaded to failure under axial compressive loads. Strain gauges mounted on the pile shaft were monitored during load testing to determine the distribution of loading throughout the pile at the various load levels up to and including failure. Load-settlement curves were generated for different pile sections at different times after installation. The piezometers and piezo-ports were also monitored during load testing and the distribution of excess pore pressures was used as an indicator of the distribution of soil deformations caused by pile displacement. The undrained shear strengths mobilized by the different sections of the piles were backcalculated from the measured loads using published formulations. An "index of soil destructuring" is proposed which relates the ratio of the mobilized undrained shear strength to the in-situ vertical effective stress at the start of load testing to the corresponding strength ratios of the soil in its intact state and in a completely destructured state. The index of soil destructuring is proposed as the basis for a proposed capacity prediction method that is based on the undrained strength ratio.

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