A site in central London called One New Change is currently being redeveloped. The new development has a basement that extends to approximately 14 m below ground level. Demolition of the building that formerly occupied the site, excavation of the new basement and construction of the new development have caused unloading and subsequent reloading of the ground. This change of loading has resulted in displacements occurring to the nearby London Underground Central Line tunnels that run parallel to the northern boundary of the site. The crowns of the tunnels vary from approximately 20 to 25 m below ground level. The piles and retaining wall for the proposed works are located at least 7 m clear distance to the south of the tunnel closer to the site boundary. The results of analyses carried out to assess the effects on the tunnels indicated that the imposed distortions caused by the proposed works were within tolerable limits. Monitoring was proposed to check that ground and structural movements were within the range of calculated values, and contingencies were identified in the event of agreed trigger levels being exceeded. Displacements measured during the demolition, excavation and construction phase of the project are presented. Conclusions are drawn from comparing the measured displacements with the tunnel deformation calculations.
An 8·2m diameter, 40m deep shaft for Crossrail is being constructed below the 10m deep basement of the Moorhouse development near Moorgate in the City of London. The depth of the shaft is such that it will penetrate through stiff London Clay and will be founded at the bottom of the Lambeth Group. The shaft is being constructed after the Moorhouse structure has been completed and the design of the Moorhouse foundations places tight constraints on acceptable ground movements due to construction of the shaft. Furthermore, the shaft needs to be designed to accommodate future ground movements associated with construction of Crossrail. The paper describes the complex relationship between the foundations of Moorhouse, the draught relief shaft and the future Crossrail assets. The optimised design includes extensive slip coating and base grouting of the Moorhouse piles, a complex temporary works dewatering system around the shaft and the option to carry out additional dewatering from within the shaft during construction. Control of ground movements through the Lambeth Group was perceived to be a particular problem in relationship to destressing the ground around the Moorhouse piles. To prevent longterm settlement of these piles, provision was made for radial grouting to “restress” the ground should the need arise.
This is the accepted version of the paper.This version of the publication may differ from the final published version. that support Moorhouse and the presence of these foundations placed tight constraints on acceptable ground movements associated with construction of the shaft. The depth of the shaft is such that it penetrates through stiff London Clay and is founded at the bottom of the Lambeth Group. The paper describes the contingency measures to deal with potentially difficult ground conditions including the water bearing layers of the Lambeth
Permanent repository linkGroup. The construction processes included a complex temporary works dewatering system around the shaft with the option to carry out additional dewatering from within the shaft during excavation. Provision was also made for radial grouting to "restress" the ground, to prevent long-term settlement of the Moorhouse piles, should the need arise. The success of the project was due, in no small part, to the detailed planning and consideration of contingency measures to deal with perceived risk.
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