Since 1975 the Transport and Road Research Laboratory has been carrying out case history studies of site investigations for tunnels; the aims are to bring out the essentials of good practice, show where improvements are needed and put on record the construction experience.
The paper reports the lessons learnt from the last five of these case histories. The tunnel projects are the Liverpool Loop and Link Tunnels, the proposed extension of the Jubilee Line, the Cardiff Cable Tunnel, the Dinorwic Diversion Tunnel and the Kielder Aqueduct Tunnels. The geology of the sites varies from Cambrian slates and grits to Eocene London Clay.
The general conclusions are that good use was made of preliminary information on ground conditions, and that the geology of the sites was usually well appreciated. Boreholes, samples and the use of test results were matters where improvements could be made. Geophysical surveying gave useful results, and recording the geology as revealed during construction was done satisfactorily. Use of inclined boreholes, trial shafts and the borehole camera are among the suggestions made.