This paper was presented at the "New products and technology : what the future holds" session. These proceedings are available on CD-ROM. One of the key challenges facing the Ministry of Transportation of Ontario (MTO) and its private sector partners on the Highway 416 project has been the difficult and unfavourable soil conditions. The Ottawa area is known for its weak, sensitive and compressible marine clay soils. Certain geometric constraints due to urban development and commitments made in the Environmental Assessment approval necessitated that one deep roadway cut and several high embankment fills traverse poor clay materials. Large settlements due to compression of clay subsoils were a major consideration in three areas of high bridge approach fills. A number of innovative construction tehcniques have been successfully employed such as lighweight fill materials (polystyrene, pelletized blast furnace slag) to reduce the magnitude of settlements. In additon, vertical wick drains were used to accelerate the rate of settlement. Deep cuts in sensitive clay deposits are generally to be avoided given risks associated with soil disturbance and instability. These soils can be subject to a dramatic loss of strength when disturbed, and can liquefy, resulting in large retrogressive landslides such as the one that occured in June 1993 near Lemieux along the South Nation River, about 50 km east of Ottawa. Deep cuts of up to 11 m were required through a critical 1 km section of the freeway which was adjacent to an established residential area on one side, and a sensitive natural pond area on the other. Safety to road users and potential impacts on adjacent property owners were of paramount concern. In addition to slope stability issues, the effect of lowering the groundwater upon these adjacent areas had to be considered. These issues were successfully addressed through the use of structural concrete diaphragm walls in conjunction with a cement bentonite ground water cutoff wall, both constructed using the slurry trench methodology. These successes will provide a current knowledge base from which to draw when contemplating cost effective and technically feasible solutions for future projects constructed through areas where sensitive clay soils are present. (A)
Abstract