GCS Project Summary
Project Name: M.A.P. 199st Pipe Repair
Client: City of Edmonton and M.A.P.
Location: 199st and South of Stony Plain Road in Edmonton, Alberta

The project called for de-watering to repair approximately 40 meters of 48” concrete pipe in silty-clay soils with a high water table. GCS was selected due to our experience in de-watering problem soil types. The pipe repair was located 10 meters below existing grade and in order to achieve a stable bed for the new pipe, de-watering to a minimum of 13 meters below grade was required. Time of the project was August –September with an estimated 56 de-watering days to accommodate draw down and construction. The job was scoped out for a 2-stage well point system using standard 6 meter well points installed with a hollow stem drill auger mounted on a skid-steer. The maximum draw for this type of system is very dependent on soil types and in perfect conditions a system can draw to a depth of 5 meters. On this project, the maximum excavation footprint would allow us no more than 2 stages of system. The initial pre-cut could go down to only 5m due to the water table and our challenge was to de-water the problem soil type another 8-9 meters. 2 vacuum units with 2 exchangers and 200 meters of system ran for 12 days. The 1st stage of de-watering lowered levels to only 3.5 meters and this posed a problem in that we needed to get down a minimum of 4.5m to make this project work. The call was made to excavate to a depth of 4.5m below the first stage. We anticipated very wet conditions at that level and our worry was that the sides slopes of the excavation would begin sloughing in before we could effectively draw water levels down. We had to devise a way to draw the water down while excavating was underway. In order to save this project we used rig mats to support the drill rig, installed and started our system right behind the driller and added to the system as points were drilled. This method, although not normally recommended, takes a high degree of coordination with the construction crew and after some minor sloughing we lowered the water table enough to stabilize the area. We installed 135m of system using this technique and after 12 days, the water table was low enough to commence excavating for the pipe repair. Another problem common with this type of pipe repair is the inflow of water running along the existing pipe bedding material. The traditional well point system has difficulty picking up this flow and we resorted to using a submersible pump encased in a perforated pipe full of washed rock to intercept the flow before it could flood the excavation. The pump was used from the time the existing pipe was exposed until backfill commenced. In order to minimize draw down time, both system stages incorporated a line of well points down the center of the excavation and around the entire perimeter. Each of the 2 stages was divided into 2 systems for a total of 4 vac units and 4 exchangers. Initially, 3 systems were powered by a 70 kVA genset but with the addition of the 4th system, a job shack and light stand, a 25 kVA genset was included. Portions of each system were temporarily removed to allow construction equipment access to the centre of the excavation whenever required. Once drawdown was achieved, the project timeline was determined by the construction schedule. This project called for 56 days of de-watering and with M.A.P.’s help we were on right on target. The volume of water discharged was approximately 430 m3/day for a total of 25000m3. The water was discharged into a 3 compartment settling tank, and pumped into a nearby existing surface drainage channel. Demob of the systems spanned a 2 day period and backfill occurred over 4 days with no water table issues.