EXTENSIVE INSTRUMENTATION of embedded pile retaining walls helped engineers produce a more cost effective design for the Norwich Millennium Library.
Norwich-based geotechnical contractor May Gurney (Technical Services) installed inclinometers within piles on three sections of the retaining walls with boreholes alongside to correlate design assumptions and soil parameters.
Construction began on the £60M project in June. The horse-shoe shaped, glass roofed building will cover more than 1ha in the city centre.
The first stage is to excavate up to 9m of granular overburden into the underlying chalk. This is supported by more than 400 reinforced concrete contiguous piles up to 17m deep.
Inclinometers 1.5m below pile toe level produce a profile of the rotation and deflection as excavation proceeds. This will determine the position and magnitude of the maximum bending moment.
May Gurney says observations of other embedded pile walls indicated measured deflections between 30% and 50% of the values predicted by finite element analysis using conservative soil parameters.
First readings were taken at an early stage of excavation and the finite element model run for a shallower excavation to allow comparison of predicted deflect- ions and those measured on site.
Actual deflections were between 15% and 33% of predicted values and allowed May Gurney to reduce pile diameters from 900mm to 600mm where the excavation runs next to a road.