Geotechnical aspects of tunnelling in discontinuous rock, with particular reference to the lower chalk

Abstract

Discontinuities, defined as breaks or interruptions of the mechanical properties of a solid, are recognised in the literature as being of vital importance in controlling the behaviour of rock during tunnelling. This thesis presents a detailed study of an experimental tunnel excavated in discontinuous rock (Lower Chalk) at Chinnor in Oxfordshire. The work falls into three main areas of study. Prior to tunnel excavation, detailed discontinuity surveys were carried out using an orthogonal scanline technique on exposed faces in the Chinnor quarry and in a 3m diameter shaft. This work, supported by subsequent surveys in the tunnel during its excavation- established - in numerical terms - the discontinuity characteristics of the Lower Chalk at Chinnor.p Ground behaviour during tunnel excavation was monitored in boreholes, and in a trench excavated across the tunnel line. Ground movement results, when viewed in the light of data obtained from finite element analyses, have emphasised the non-elastic discontinuous response of the ground. The actual behaviour of the ground at Chinnor was found to be equivalent to that of a material having a deformation modulus that was approximately l/20th of the Young's modulus of chalk determined from laboratory tests on intact samples. This modulus reduction was a direct result of the interaction between the tunnel excavation system and the discontinuous ground. Having established the role of discontinuities in controlling the behaviour of the ground during tunnelling, and recognising the importance of discontinuity spacing as an index of discontinuity frequency, a detailed study of various statistical and geotechnical aspects of discontinuity spacings in rock is presented. The distribution of discontinuity spacing values obtained from measurements in three tunnels in the UK was found to closely follow a negative exponential distribution. The implications of this relation are that a single parameter expression can be, adopted to describe the discontinuity spacing characteristics. By integrating this expression between the appropriate limits, a formula giving a precise value for the RQD (Rock Quality Designation) of the reck exposed can be obtained. In addition, important conclusions concerning optimum sample size and precision can be made once the discontinuity spacing distribution has been described in these simple mathematical terms. Subsidiary to the three main areas of study outlined above, three additional peripheral subjects are considered. These include an evaluation of the hydrological characteristics of the chalk and an examination of chalk cutability. In addition, data concerning the shear strength characteristics of discontinuity surfaces in the chalk (obtained from laboratory tests) are incorporated in an analysis of stability near the tunnel at Chinnor, using a computer program on file at the University of Durham.