By J A Hudson, TRRL, P B Attewell,University of Durham, J H Atkinson, University of Cambridge, and M P O’Reilly, TRRL
This paper was first published in GE’s April 1976 issue
Some 60 to 100 km of tunnels for civil engineering purposes were constructed annually in this country during the five years 1970-74 inclusive, and tunnelling on a similar scale is expected to continue. A majority of the tunnels were driven in urban areas and many of these, particularly those for sewerage, were located at shallow depths often in poor ground conditions. Settlements resulting from tunnelling can, if large enough, cause damage to existing buildings and services.
To minimise overall project costs and the risk of damage or accident resulting from tunnelling, the engineer designing a tunnel needs to be able to make reliable predictions of the amount and extent of settlement likely to arise from the driving of tunnels in various circumstances. Given reliable forecasts of settlement, he would be in a position to choose between a number of options depending on the particular location which might include for example (i) a longer tunnel in better ground, (ii) chemical stabilisation or freezing of pockets of weak ground on the more direct routes and (iii) underpinning of the foundations of existing buildings together with relocation of water and gas mains.
Although there has been some development in recent years, predictions of ground settlement arising from tunnelling are still in the main guestimates, the reliability of which depends on the skill and experience of the engineer who, considering the complexity and risk of the situation facing him, can hardly be faulted if the tendency is to play safe. This paper is a short progress report on the studies of ground movements being carried out under the aegis of the Transport and Road Research Laboratory as part of its programme of research on tunnel design. The subject has been approached in two ways: (i) model tests in the laboratory to study the mechanisms of ground movement and the validity of prediction methods and (ii) ground movement surveys on tunnel schemes with different ground conditions and excavation techniques. In this way, it is hoped that theory and site experience can be integrated to provide a rationale for predictions of surface settlement in new situations.