The New Brunswick Department of Transportation proposed to construct a 13 metre high approach embankment leading to a single span overpass structure founded on spread footings. Preliminary investigation of the subsurface conditions revealed that the site is underlain by up to 22 metres of compressible soils. Much of this soil was glacial till, which would normally suggest no settlement or stability problems. However, our detailed review of the existing soils information has shown that this glacial till is a so-called flow till or water-laid till, which is typically a relatively soft soil. Significant settlements of up to 600 mm were anticipated. To further compound this problem, the compressible layer was significantly less thick at the north abutment, and the differential settlement between the north and south abutments could be as much as 0.3 metres. The calculated time required to reach 90% consolidation was in the order of 20 years. This magnitude of settlement over a 20-year period was considered unacceptable by the NBDOT. To remedy this problem, vertical drains were used to expedite the rate of settlement. The drains were installed in a triangular pattern with spacings of 1.5, 3.0 and 5.0 metres. The spacing was smallest in the area of the proposed bridge abutments and was increased away from the abutments. Four settlement plates and three piezometers were installed to monitor the settlement and pore water pressure during and after construction of the embankment. Settlement measurements showed primary consolidation ended in less than a year, agreeing well with predicted values.
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