Significant Project Goals and Features:
- Cutoff wall system using both trenched and grouted wall for dam rehabilitation
- Installation of 238.6 linear meters of Soil Cement Bentonite (SCB) cutoff wall
- 95 Jet Grout (JG) columns along with 24 trench probe columns
- Jet Grouting completed across the concrete Service Spillway
- Eliminating the water seepage through a coarse gravel layer
- Achieve a continuous cutoff wall with a minimum of 1.0 meter key in to the aquaclude.
- Stabilizing the spillway and surrounding area.
- Approximately 3,000,000 lbs of cement and 832,000 lbs of bentonite used
Description of Work:
Geo-Solutions was subcontracted for the installation cutoff wall system for the overall rehabilitation project for dam in Canada. The system includes a SCB cutoff wall on both west and east sides of the spillway, joined with a JG wall across the Service Spillway to connect the SCB sections creating a continuous cutoff. .
Cutoff Wall Construction:
The SCB wall was designed to stop the seepage of water that has been occurring through the dam. The cumulative length for both completed cutoff walls finished at 238.6 linear meters with approximate width at 0.9 meters and depths reaching 23.0 meters (75 feet). The total vertical square meters of trench excavated was 5,059.
The project specifications required the use of a SCB trench using slurry trenching method. Geo-Solutions used a bentonite slurry during excavation at approximately 7% by the weight of water. The trench was excavated using a Komatsu PC1250 equipped with a long stick and boom capable of reaching the required 23 meters. The trenching bucket is equipped with specialized ripper teeth designed to key in the mudstone material.
The backfill material that was placed into the SCB wall consisted of
- Minimum 3% bentonite slurry to weight of soil
- Minimum of 8% cement to weight of soil for the west wall
- Minimum of 6% cement to weight of soil for the east wall
- Placed at a slump of 5 to 8”
- Permeability of 5×10-7 cm/sec
- Minimum strength of 100 kPa (15 psi)
Because the cutoff wall was installed on the narrow crest of the dam, the SCB backfill had to be mixed remotely. The trench spoils were loaded into off road articulated haul trucks and removed to a spoil area. A select clay material was excavated from another area of the site and used for the base material for the backfill. This soil was placed in a 40 cubic meter mixing pit, which was lined with steel plates and concrete blocks. Slurried bentonite was added to the pit to incorporate the required minimum bentonite. Likewise, slurried cement was added and the backfill was blended thoroughly. The SCB backfill was then loaded into the articulated dump trucks and transported back to the trench. The backfill was placed into the trench using an excavator to form the permanent cutoff wall.
Jet Grout Tie-In:
The jet grouted (JG) portion of the wall was designed to tie together the SCB trenched cutoff walls and create a continuous cutoff. The JG wall was necessary because the existing concrete Service Spillway was to be left in operation for one additional year. A trenched wall could not be installed across the spillway because of the concrete and a significant elevation change.
A test program was conducted prior to full scale production to establish grouting parameters. The columns were excavated and measured to be approximately 1.25 m in diameter.
The JG columns were installed in double row fashion to ensure continuity. The column rows were spaced 1.0m apart and the columns were 1.0 meter center to center spacing to achieve the specified 900mm minimum thickness for each row.
- 28 Columns on the west wingwall, at approximately 21.6 meters in vertical length
- 31 Columns down in the spillway, at approximately 21.6 meters up to 7.3 meters
- 28 Columns on the east wingwall, at approximately 21.6 meters in length
- Final cumulative area of 1,558 square meters.
The grout used for JG columns installed contained
- 50% Reservoir water
- 10% Bentonite slurry
- 40% Cement
- Permeability of 5×10-7 cm/sec
- Minimum strength of 200kPa (30 psi)
- Identifying of key material at the project start. The key material under the deepest gravel lens made a gradual transition from sandy-silt to silt and clay.
- Komatsu 1250 pushed to its depth limits to reach key.
- Drilling through obstructions and a very dense gravel when installing JG columns.
- Tight spacing in the spillway for installing JG columns.
- Controlling grout return and preventing it from entering any body of water.
- Monitoring the existing spillway for heave during jet grouting.
- Working below the reservoir level of water while in the spillway.
- Columns installed against the vertical concrete walls at the sides of the spillway.