MONITORING SETTLEMENT IN LONDON CLAY

The effect of underground works in overlying structures is a major consideration in urban tunnelling. To understand better the mechanisms occurring in the development of settlement troughs, the UK company Brown & Root has begun an internally funded project. A monitoring site has been established along the line of an advancing tunnel, to investigate the relationship between tunnel construction and the response of the surrounding ground. The site of the project is a disused timber merchant's yard 400m along the route of the 2.8m diameter cable tunnel being driven between two parts of east London, an electricity substation in West Ham and the Greenwich Peninsula. The tunnel was driven through London Clay for its whole length. This paper provides a case history of the vertical displacements experienced in the ground, as the tunnel passed through the ground under the site, giving an insight into the development of the idealised settlement trough. Nine boreholes were sunk in three arrays perpendicular to the tunnel alignment, to facilitate installation of the sub-surface instrumentation and enable a comprehensive site investigation. The instruments included multi-point extensometers, earth pressure 'spade' cells, piezometers, transducers, and a data logger. Charts show the rates and changes of settlement.