Performance of a propped retaining wall at the CTRL, Ashford

Abstract

This thesis is based on the field monitoring of a propped bored pile retaining wall installed
in an overconsolidated clay. Pile bending moments, prop loads, pore water pressures and
lateral earth pressures were logged automatically at intervals of up to 5 minutes
throughout construction (and for 4 years afterwards) and wall deflections were measured
during construction, making this the most comprehensive instrumentation project of its
kind.
The magnitude of the over-read associated with the use of spade cells (used to measure
lateral earth and pore water pressures) in overconsolidated deposits was determined by
comparing readings from a spade cell aligned to measure vertical stress with the estimated
overburden acting on it as the overburden was excavated. This study adds significantly to
the previous data as spade cells have not previously been \lsed in the Atherfield Clay, and
the performance of spade cells under a known changing load has not previously been
measured in the field.
Analysis of the changes in lateral stress and pore water pressure during the wall
installation process showed significant reductions in horizontal stress during wall
installation, reducing the ratio of effective horizontal to effective vertical stress, K, from
about 1 to nearly the active condition. Following wall installation there was no further
change in horizontal stress over a period of about 10 months, during which time no further
construction work took place.
Analysis of the data yielded good agreement between pile bending moments estimated
from inclinometer and strain gauge measurements in the piles, and the onset of concrete
cracking was identified. The components of strain measured in the reinforced concrete
props due to shrinkage, creep and applied load were also identified, allowing prop loads to
be estimated. A simple equilibrium calculation showed that these agree with the measured
wall bending moments and total horizontal soil stresses, demonstrating the overall
consistency of the data collected.
Simple equilibrium analysis of the behaviour of the wall during construction shows that
the soil stresses measured are compatible with the measured structural loads. The longterm
horizontal soil stresses, bending moments and RC prop loads show no increase over
the 6 years since construction began.

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