| Summary: | 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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