Wick Drains Used for HWY Swamps

Growth in commuter and recreational use in this corridor has prompted MTO to expand Highway 69. A strategic link in the Trans-Canada Highway System, the existing two-lane roadway serves to connect northern and southern population centers. One segment of the Highway 69 expansion project is the four laning from Highway 537, northerly for a distance of 8.8 km. This stretch of Highway 69 involves a number of swamp crossings that requires the design and construction of embankments, some as high as 26 meters over weak compressible clay soils. The application of wick drain technology, or Prefabricated Vertical Drains (PVDs), was chosen as a cost effective alternative to the conventional procedure of excavation and backfilling or commonly termed as "dig outs". The wick drain design methodology is part of a 'floatation' design approach that leaves the compressible soils in the ground. This saves considerable time and expense in the excavation and removal of the native soil material and also in the time and material to backfill the excavation. The cost savings from installing wick drains at the 5 swamp sites amounted to approximately 25 million dollars.

Leaving the native soils in place also offer environmental advantages since disturbance to the natural environment is minimized and haulage and disposal is avoided.

The design includes the preloading and/or surcharging the embankment and the use of wick drains to accelerate the rate of settlement. Typically an additional temporary surcharge fill of 2 meters is placed at the top of the embankment that is removed once the embankment settlement is achieved. Staged construction has been specified to prevent embankment failure during the construction of the embankments and a comprehensive geotechnical monitoring program has been designed to control the rate of construction and to monitor the performance of the embankments.

When an embankment is built on compressible soil, it can take several years before the embankment stops settling. Using wick drains reduces the settlement duration to weeks or months. Forcing soil consolidation to occur at the outset of the project and in the shortest possible time during construction is desired to avoid expensive construction problems and delays and also to improve post-construction performance of the roadway. On the Hwy 69 projects, the predicted rate of settlement without wick drains ranged from 2 to 5 years compared to 6 to 13 months with wicks.
The wick drain technology was chosen at 5 of a total 10 swamp locations within the project. Wicks were selected at the sites where the clay thicknesses exceeded 10 meters and where embankment heights exceeded 6 meters. Wicks were designed at a 1.5 m triangular grid pattern to depths ranging from 10 to 25 meters below original ground. (Figure 1) The total length of wick drains is 785,000 meters, approximately equivalent to a round trip between St. Catharines and Sudbury. Wick installation commenced in the summer of 2006 under Contract 2006-5150 awarded to Pioneer Construction and currently scheduled for completion in summer 2009. On this project wicks are being installed within a temporary steel rectangular casing that is pushed into the ground using a backhoe modified with a vertical lead. (Figure 2) The procedure is analogous to a sewing machine whereby a spool of wick drain is fed into the mandrel as the mandrel is pushed into the ground to the design depth.

The measurable success of wick drain technology on MTO projects has revolutionized the methodology of embankment construction over swamps. The advent of wick drain technology has resulted in more efficient construction methods, increased cost savings and minimal disturbance to the environment.

On this project, the development of the wick drain design illustrated a successful partnership between the Prime Consultant (URS Canada Inc), the Foundations Engineering subonsultant (Golders Associates), Northeastern Region Planning and Design and the Pavements & Foundations Section.