ABSTRACT
The central portion of the 9km long M8 tunnel in Sydney, Australia, was developed under the existing Cooks River and a deep palaeochannel filled with Tertiary and Quaternary sand and clay sediments. The palaeochannel was an artifice of regional scale faulting in the underlying Hawkesbury Sandstone bedrock. Grouting from the surface aimed to reduce water inflows into the tunnels during construction. A primary grid of three parallel rows of boreholes spaced 20 m apart were drilled covering a 1300 m long section of the alignment were drilled via percussion methods. Secondary, angled holes were drilled to target specific water bearing features for pre-grouting. Each hole was logged using an acoustic televiewer probe with the interpretation of defects completed using WellcadTM. The holes were interpreted geotechnically, focused on open defects, using defect codes commensurate with tradition logging and the Australian Standard at the time. The key parameters logged were defect aperture and void observations. Defects with >100 mm aperture became the focus for modelling and targeting with secondary grouting holes. Regional faulting was expressed as sub-vertical joints, which formed only part of the overall deformation picture at the site. Detailed Design Phase site investigation also suggested accommodation structures in the form of sub-horizontal to shallow dipping bedding plane shears would be prevalent. The televiewer logging confirmed this. Structures were correlated between the boreholes using VulcanTM as the modelling software. The modelling resulted in significant refinement to the predicted intersection of sub-vertical structures related to the regional fault zone as well as defining 13 individual bedding shear planes that would intersect the tunnel alignment. Subsequent excavation and mapping confirmed the models high accuracy.
