Self-boring pressuremeter in situ lateral stress assessment in London Clay

Abstract

Abstract Recently a considerable amount of self-boring pressuremeter testing has been carried out in London Clay. This paper considers the results of tests from four sites and the methods for obtaining the in situ total lateral stress, undrained shear strength, limit pressure and shear modulus are reviewed. In order to provide some means of checking the consistency of results produced from self boring pressuremeter tests, the normalized parameters C u / P I (shear strength/limit pressure) and P 0 / P 1 (in situ total lateral stress/limit pressure) have been assessed and found to be reasonably consistent with depth within the London Clay for a wide range of shear strengths. On all of the sites considered, the measured lateral pressures were generally higher than would have been assumed from published information. During the course of the erosion of what is generally considered to have been several hundred metres of material, it is possible that passive conditions could have existed and may have had a controlling influence on in situ lateral pressures both in the past and at present. The self-boring pressuremeter tests generally indicate that plastic deformation of the soil commences at pressures only slightly higher than the measured in situ lateral stress. Also, the values of passive pressure calculated using the pressuremeter are similar in value to those stresses. It is concluded that at some locations London Clay may be at, or near to, the passive condition. It is recommended that long-term monitoring of ground pressures on deep structures is undertaken to provide independent confirmation of the findings of this paper. Also, a field study of the effects that self-boring can have on the ground, particularly in very stiff and hard clays is needed. This would clarify the influence these effects may have on the test results.