Conquering the "Leda" Clay

One of the key challenges facing the Ministry of Transportation (MTO) and its private partners on the Highway 416 - Veterans Memorial Highway project has been the difficult and unfavourable soil conditions. 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 transverse 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 techniques have been successfully employed, such as lightweight fill materials (polystyrene, and pelletized blast furnace slag) to reduce the magnitude of settlements. In addition, vertical wick drains were used to accelerate the rate of settlement.

Deep cuts in sensitive clay deposits are generally 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. Deep cuts of up to 11 m were required through a critical one km section of the freeway which is 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 great 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 groundwater cut-off wall, both constructed using the slurry trench methodology.