When previous repairs to the downstream buttresses of Daniel Johnson Dam did not provide long-term satisfactory performance, waterproofing was required to restore the appearance and watertightness of the structure.
By John Wilkes, Jean-Pierre Tournier, Benoit Durand, Patrice Filion and Kaveh Saleh
Daniel Johnson Dam, owned by Canadian provincial utility Hydro-Quebec, is the largest concrete multiple arch buttress dam in the world. This dam is located on the Manicouagan River about 215 km north of Baie-Comeau city in the province of Quebec. Construction work on Daniel Johnson Dam began in 1961 and ended in 1968. The dam consists of 13 arches and 14 buttresses with a slope of 1:0.6 (vertical/horizontal) for the buttresses. Total length of the dam is 1.3 km, and maximum height is 214 m.
During the last two decades, leakage marks have appeared on the downstream buttresses, and also some degradation of the concrete due to freeze-thaw cycles along the vertical joints was observed. This situation led Hydro-Quebec to replace the joint sealing for enhancing the watertightness and appearance of the downstream face of the dam. But repairing the joint sealing had not provided long-term satisfactory performance. So a new and more effective solution was required to restore the watertightness and appearance of the joints.
The solution chosen in 2008, provided by CARPI, was to use a flexible waterproofing sheet geomembrane system to stop infiltration and freeze-thaw action affecting the buttresses. The system consists of 2.5-mm-thick polyvinylchloride (PVC) geomembrane sheet that was laminated during fabrication to a 500 g/m² non-woven anti-puncture geotextile. A 60-mm-wide x 6-mm-thick stainless steel plate perimeter seal anchors the PVC geocomposite to the concrete face of the downstream concrete buttresses. The project was particularly challenging due to the dam's massive size (third largest reservoir and largest multiple arch dam in the world) making installation access difficult. Installation of the geomembrane system was completed in spring 2012.
|Workers scaled the surfaces of the downstream buttresses to install the geomembrane system.|
Description of the dam
Daniel Johnson Dam impounds water for a power generating station, called Manic 5 and Manic 5-PA, with a capacity of 2,592 MW, thanks to a drop of 141.8 m, with a reservoir that stores 135 km3 of water spanning 1,973 km². Construction of Daniel Johnson Dam began in 1961 and ended in 1968. Filling of the reservoir began in 1964, and it reached its maximum operating height of 359.66 meters (1,180 feet) for the first time in 1977.
The dam itself consists of 13 arches and 14 buttresses. The large central arch is supported by two massive buttresses. The 12 smaller arches are all identical, except for the two arches that are adjacent to the central arch, which share an inclined buttress with the central arch (see photo at left, note the curvature of the two arches adjacent to the central arch). Daniel Johnson Dam has a system of inspection galleries at different levels and another system of tunnels in the abutments on each bank for drainage.
All major joints on the arches have two waterstops located on the upstream side. One waterstop is located closest to the upstream face of the dam and is a rigid copper waterstop formed by a Z-shaped blade. The second waterstop, located just behind the first, is a flexible PVC waterstop.
|Daniel Johnson Dam in Quebec, Canada, is the largest concrete multiple arch buttress dam in the world, with 14 buttresses across its 1.3 km length.|
Degradation of concrete and leakage marks have appeared on the downstream buttresses of the dam along the joints, which led Hydro-Quebec to evaluate the possibility of enhancing the watertightness of the downstream face. Because previous seal joint repairs had not provided satisfactory performance, a new and more effective solution was required to restore the watertightness and appearance of the structure.
Use of a geomembrane system to waterproof upstream joints or cracks on the buttresses was proposed, conjointly by Hydro-Quebec and CARPI, as the solution to the problem.
The use of geomembrane systems is a well-established technology dating back more than 15 years. In the mid-1990s, Hydro-Quebec's IREQ (Research Institute of Hydro-Quebec) conducted a three-year research investigation into how to rehabilitate concrete dams for Canada's cold climate. This research was highly favorable of the PVC geomembrane systems that had been developed in Europe. This type of geomembrane system has been used on both new and old dams for joints or cracks on more than 200 dams, including the Patanoyryssi roller-compacted-concrete dam in Greece in 1998.
In addition, the ISMES laboratory in Italy had certified the geomembrane system proposed in 2010 for Daniel Johnson Dam for water pressures up to 240 m of head. Testing performed for this certification included testing in a pressure vessel that would verify imperviousness of the geomembrane and the associated perimeter seals.
Hydro-Quebec selected CARPI to provide the geomembrane system for Daniel Johnson Dam because the company had installed more than half a dozen geomembrane joint systems dating back to 1995. Daniel Johnson Dam, however, is unique because the geomembrane system is being installed on the downstream face and will not be subject to high water heads. Instead, the purpose is to keep water in the form of rain or snow from entering the joints during freeze-thaw cycles and causing damage.
Thus, there were several different considerations for the Daniel Johnson Dam downstream joint geomembrane system, including the system's ability to prevent water intrusion and withstand high winds and high sun exposure.
|Creative engineering was necessary to navigate this ledge on buttress 7 and access the worksite.|
The solution developed to reduce freeze-thaw damage on the downstream buttresses of Daniel Johnson Dam consists of:
- External waterstop 70 cm wide over the vertical joints in the upper part of the downstream face of the dam on buttresses 2 through 5 and 8 through 13. All of these were straight vertical joints about 430 meters long;
- External waterstop 70 cm wide over the three vertical joints (CJ7-1, CJ7-2, and CJ7-3) in the upper part of downstream buttress 7 over the ledge of the arch onto the lower buttress;
- External waterstop 70 cm wide over the two vertical joints (CJ7-4, CJ7-5) in the lower part of downstream buttress 7. These joints are slanted diagonally on the buttress; and
- External waterstop 3 m wide over the vertical crack in the lower part of downstream buttress 8.
The joints across buttresses 1, 6 and 14 were not treated. Another solution had already been tried on buttress 6, and buttresses 1 and 14 did not require treatment.
The waterproofing system used for Daniel Johnson Dam consists of:
- Waterproofing liner, SIBELON CNT 3750 geocomposite, consisting of a 2.5-mm-thick PVC geomembrane laminated during fabrication to a 500 g/m2 non-woven anti-puncture geotextile. The geocomposite was cut to 70 cm widths during manufacture.
- Vertical and horizontal watertight stainless steel perimeter seal anchoring the PVC geocomposite to the concrete face. The flat profiles are anchored to the face of the dam via chemical anchor bolts 15 cm apart. All flat profiles, nuts, washers, couplers, anchor bolts and other metal items used in the perimeter anchor seal were AISI 304 stainless steel. A rubber gasket was used to ensure even and adequate compression. An epoxy resin was applied to the face of the dam along the perimeter seal.
- Horizontal stainless steel flat profile for the top and bottom perimeter seal. The profile is bolted to the concrete of the crest wall with mechanical anchors. Two strips of geonet are placed under the geocomposite for ventilation. All flat profiles, nuts, washers, couplers, anchor bolts and other metal items used in the perimeter anchor seal are made of type AISI 304 stainless steel.
Installation on 10 buttresses
The design and subsequent installation of the geomembrane system for the downstream face of Daniel Johnson Dam was very straightforward, requiring only hand tools to complete installation.
The major challenge was accessing the work site safely given the size and configuration of the dam. The work on buttresses 2 through 5 and 8 through 13 required developing a safe access method to get the worker over the parapet wall into the swing stage (a work platform suspended by cables to allow access like is used for window washing on skyscrapers), with the ability to access the top of the joint that is underneath the overhang of the crest roadway.
Once access to the buttresses was established, the work progressed according to plan and schedule. Some of the buttress joints were deteriorated requiring minor surface preparation by grinding and backfilling of the voids. The work was performed by:
1. Installing the perimeter seal batten strips with anchor bolts.
2. Installing the geomembrane over the joint and puncturing onto the perimeter anchor bolts.
3. Puncturing the gasket over the anchor bolts.
4. Installing the epoxy mortar underneath the batten strips at the edge of the geomembrane.
5. Partially tightening the batten strips to allow epoxy mortar to set up.
6. Final tightening of the perimeter seal anchor bolts to 30 Nm.
7. Final inspection, including torque testing of bolts.
This work involved installation of 430 linear meters of geomembrane system in 30 working days in fall 2011.
|Due to the shape of the ledge on Daniel Johnson Dam, accessing the site for installation of the geomembrane system proved to be a precarious situation.|
Installation on buttress 7
For all 13 of the arches except center arch 6, the buttresses have constant width and run straight vertically, although at an inclination. Arch 6, however, has buttresses on each side (buttresses 6 and 7) that vary in width and run diagonally relative to the crest of the dam. This diagonal configuration, which leads to diagonal joints, greatly complicates access to buttress 7, which has five joints that needed to be waterproofed.
To restate, the joints on buttress 6 were covered in the past few years with an epoxy system that has been replaced with the geomembrane system for the remaining 11 buttresses to be repaired. Thus, due to this diagonal configuration and the great height of this arch (more than 135 meters) typical swingstage systems do not work.
This situation presented the greatest challenge for the project. Furthermore, the top of arch 6 has a ledge that appears to be small and relatively flat but is significantly slanted and wide enough to require a special method to access this small area. The photo below demonstrates the visual illusion of working in this area. Looking down more than 30 meters from the crest of the dam, this ledge appears to be very narrow and flat. In reality, the ledge is more than 2 to 3 meters wide and inclined severely, beyond what can be accessed without a swingstage.
Furthermore, at this location the slant of the ledge limits the ability of the swingstage to access the joint area from the top and also prevents access to the top of the joint from below the ledge. Finally, the joint below the ledge has 2 short sections to complete two of the joints that need to be waterproofed and then two separate joints that are long (82.13 and 84.47 meters) and far below the ledge.
A different access method was required because there was not a way to drop the swingstage over the ledge so that in some sections the area was accessed from the bottom and from some areas from the top. The installation on buttress 7 required five different access methods, including fixed scaffolding on the crest with swingstages, fixed scaffolding on the ledge, swingstages suspended from the ledge, and a mast climber system, which is a work platform that entails mounting a beam on the face of the dam for the work platform to travel on instead of the cables used to suspend swingstages.
The work to complete the top portion of two joints above the ledge and another joint was completed in less than 10 days in the fall of 2011. This represented 87.86 linear meters of work completed over 15 days.
There was then a delay in work because the mast climber installed on buttress 6 had to be moved to buttress 7, causing a few weeks' delay. Once the scaffolding and mast climber were in place, the two separate joints referenced earlier were completed in 15 days in October and November, as the weather window was quickly starting to disappear at this 50 degree north latitude.
The work resumed in the spring of 2012 with installation of the scaffolding and swingstages, where 60 linear meters of geomembrane joint system were scheduled to be installed in three weeks.
The final result is that of the originally contracted work: all the geomembrane system was completed on 11 buttresses for a total of 851 linear meters of geomembrane joint system, except for 107 linear meters for the lower portion of one joint that was postponed by Hydro-Quebec for technical reasons. There were no significant safety issues for all work done in 2011 and no lost time, with only one minor cut that required first aid.
Although it is too early to draw overarching conclusions on the effect of installing the geomembrane system on the behavior of Daniel Johnson Dam, Hydro-Quebec is optimistic that the product will meet its expectations both in terms of results and durability, based on the results seen so far.
John Wilkes is president of Carpi USA Inc. Jean-Pierre Tournier is a hydroelectric advisor with Hydro-Quebec. Benoit Durand and Kaveh Saleh are researchers and Patrice Fillon is head of dams and civil engineering works at the Hydro-Quebec Research Institute.