Abstract
Excavation, temporary support and lining of tunnels in London Clay have undergone repeated development. Innovations depended however on the economic and social climate rather than on technical feasibility. An innovation, which could soften the impact of forthcoming shortages of materials and fuel on tunnelling, is spheroidal-graphite cast-iron. Experimental work in the laboratory and in a temporary tunnel, on bolted segmental linings in both the old and the new kinds of iron is described. It shows that rings built from iron segments are less stiff than intended, but that in service this stiffness is not required. Observations of instrumented rings in the tunnel on over fifty occasions, some whilst other excavations were made nearby, also throw light on the interaction between neighbouring tunnels and between linings and the ground. Measurements were made of displacements in the clay near the advancing tunnel face. A mechanism to explain the observed behaviour is suggested and the conclusion drawn that surface subsidence is best reduced by using an 'expanded' lining (a kind of lining which is built directly against the clay behind the shield) with a versatile tunnelling machine capable of sustaining good progress. Nevertheless, an appreciation of the possible hazards of tunnelling in the London Clay formation made clear by an explanation of its geological history and by the study of the experiences of earlier engineers, indicates that contingencies occur which require recourse to some form of bolted lining. Hence a lining has been designed which embodies the conclusions from the experimental work and which may be built in either the bolted and grouted form or the expanded form. Partly constructed in spheroidal graphite iron, this material is economically used by restricting it to those functions requiring its special properties.