Abstract

With the large amount of literature on London Clay, it is generally assumed that its properties are both well understood and well documented. However, there are few reports of how man-made features, such as tunnels, can alter the in situ horizontal stress field and stiffness of London Clay. This paper describes the self-boring pressuremeter tests carried out at a site in north east London at locations in close proximity and remote from a London Underground Ltd tunnel. Two aspects are considered; firstly, the analysis, assessment and interpretation of data and, secondly, the effect of the tunnel on the profiles of in situ horizontal total stress and coefficient of earth pressure at rest.

The analysis of the test data has indicated that interaction between the instrument and the soil may effect the measured ‘lift off’ pressure. Methods of interpretating the test data are suggested. The variation with depth of the ratio of shear modulus (at a particular cavity strain) to mean effective stress proved to be a useful method of checking the reliability of the test data.

During the pressuremeter tests unload-reload loops were performed to measure shear modulus. Because of the nonlinear stress-strain behaviour of London Clay it is recommended that the magnitude of cavity strain imposed during the unload-reload loop is taken into account during the assessment of shear modulus. Finally, in situ stress and stiffness from the self-boring pressuremeter tests are compared with case history data.