Performance of a propped retaining wall at the Channel Tunnel Rail Link, Ashford
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.