Prop loads in two large braced excavations

Abstract

This thesis is concerned with temporary props used to support stiff reinforced concrete retaining walls cast in situ. Temporary props are usually designed using loads determined from some idealized method of analysis. There is however a widely-held view within the construction industry that current procedures tend to overestimate the actual prop loads, resulting in uneconomical designs. The aim of the research was to investigate the reasons for this apparent discrepancy by monitoring and analysis of the temporary prop loads developed during the construction of the London Underground Limited Jubilee Line extension stations at Canada Water and Canary Wharf. As well as prop loads, which were measured using vibrating wire strain gauges, prop temperatures were monitored in order to assess their influence on the prop loads. The research has shown that

(i) Vibrating wire strain gauges can accurately and reliably monitor the loads developed in temporary steel props, providing certain basic guidelines are followed.

(ii) Temporary-induced loads, which may account for a significant proportion of the total load, depend on the magnitude of the temperature rise and the degree of restraint provided to the ends of the prop by the structure and the retained soil. The effectiveness of the end restraint will probably be about 50% for high-level props but for low-level props may be as high as 65%. However, the range of temperature experienced by the props is likely to reduce with the depth of the excavation.

(iii) Bending moments generated in the props as a result of temperature gradients and rotation of the wall will probably be of the same order as those due to the self-weight effects.

(iv) Neglecting temperature effects, prop loads close to those measured can be calculated using modified limit equilibrium and finite element analyses, provided appropriate soil parameters and input conditions are assumed.

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