Abstract:In the design of major construction works, the better the ground conditions are known, the more control there is on the assessment of risks for construction, contract and personnel, and ultimately on final costs. Understanding of the ground conditions is usually expressed as a conceptual ground model that is informed by the results of desk study and of dedicated ground investigation. Using the GSI3D software, a 3D geological model (a model comprised of attributed solid volumes, rather than of surfaces) can be constructed that exactly honours geologists' interpretations of the data. The data is used in its true 3D position.The 3D model of faulted Lambeth Group (Palaeogene) strata in the area of the proposed new Crossrail Farringdon underground station, in central London, has several types of benefit. These include enabling optimum use of available GI data, including third party data, with confidence.The model provides an understanding of the local geological structure that had not been possible using other commonly-used methods: in particular, it shows the likely distribution of numerous water-bearing coarse deposits and their faulted offsets, which has potentially significant effects on groundwater control. The model can help to target ground investigation, constrain design, and control risk.
One of the largest geotechnical survey and site investigations ever carried out in London, costing approximately £3 million, was undertaken prior to the design of the underground sections of CrossRail. This paper outlines the procurement, management and technical aspects of this survey, the application of the results to the prediction of ground movements arising from tunnelling and the design of building protective works.
Excavation of the western ticket hall box at Tottenham Court Road station represented a critical path activity of the Crossrail railway project in London, UK. Base slab construction and preparatory sprayed concrete lining works needed to be completed before the arrival of the two tunnel boring machines. A full observational method (OM) design was implemented to eliminate the lowest level of temporary propping, resulting in a 13 m prop-free excavation at the bottom of the 30 m deep diaphragm wall station box. This paper describes the design and successful implementation of the OM, which eliminated the need to fabricate, install and remove the fifth level of temporary propping. It covers the comprehensive review of the original design input parameters, back analysis of the most probable geotechnical design parameters and comparison of wall deflection and forces in the temporary props from early stages of the construction works. The robust strategy formulated as part of the observational method design to control the subsequent excavation is described: this included trigger criteria, a review process and predetermined contingency measures to ensure safe execution of the excavation works. A genuinely collaborative effort between client, permanent and temporary works designers, independent design checker and the contractor is elaborated. Cost and programme savings achieved by implementation of the observational method approach are outlined.
Line 1 of the Crossrail project is planned to cross the River Thames via twin tunnels at Woolwich in east London. This paper reports on the fascinating investigation into the riverbed of Woolwich Reach, involving integration of a wide range of data from historical, geophysical and borehole sources. It illustrates how combining detailed desk studies and focused borehole investigations with over-water seismic, sonar and magnetic gradiometer surveys can assist civil engineers in the design of major structures within historic urban and tidal river environments.
This paper describes the interaction of the Crossrail project to deliver London's new Elizabeth line in the UK with a building situated directly over the machine-bored tunnels, close to Paddington station. The initial scheme called for the demolition of the existing building at 4–18 Bishop's Bridge Road, including extraction of existing piles, which would have had a considerable impact on the local area. Changes in the design of the Elizabeth line station at Paddington allowed a revised strategy to be considered, whereby the building could be kept, reducing construction costs to the project considerably. A combination of desk study and ground investigation works was used to identify the likely extent of the piling under the building, and to predict the risk of clashes with the tunnels. The results of analyses considering the impact of construction works on the building, and associated monitoring results are presented. The decision not to demolish produced numerous benefits aside from the reduction in cost, including a considerably reduced impact on those using the building and adjacent areas.
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