Abstract

Circular shafts are an integral component of tunnelling projects. However, there are few well-documented case studies of circular shafts which make it difficult to accurately predict their performance or estimate the magnitude and extent of surrounding ground movement. A cautious approach often adopted by designers can have significant impact on the shaft construction costs and the protective measures implemented for nearby buildings and services.

Field observations of circular shaft construction were assembled from three major tunnelling projects in London. The shafts had a range of geometries and construction methods. In addition, well-controlled centrifuge model tests were conducted of circular shaft excavation. New apparatus and new centrifuge testing procedures were developed to extract small quantities of soil from the centre of the shaft in-flight. The centrifuge model was well-instrumented to monitor bending and hoop strains in the shaft lining and adjacent ground surface movements during excavation.

This presentation describes the new apparatus and the behaviour of circular shafts and the adjacent ground based on a comprehensive review of the field observations and centrifuge test results.