It’s a big bridge across a big river with a lot of challenges.” This is something of an understatement, coming as it does from Tyler Wilson, recently a private sector engineer and now a chief civil engineer with the Alberta provincial government. Wilson is in charge of the US$112 million (Canadian $148 million) Peace River Bridge twinning project, the largest design-bid-build contract ever for the provincial government. That price tag is only for construction of the bridge and onramps. It excludes land acquisition, substantial soil remediation work and road realignments.
A joint venture of Flatiron Construction and Aecon was selected by Alberta Transportation in the autumn of 2017 to construct a new bridge parallel to the existing Peace River Bridge - 50m upstream and which opened in 1968. Work includes twinning Highway 2 of which around 1.6km runs through the town of Peace River, population around 7,500.
The town is nearly 500km north-west of the provincial capital Edmonton. Also, the townsite is on the edge of the Peace River and nearly 300m below the relatively flat and extremely rich farmland surrounding it. The Peace River region is a major wheat producing area and Highway 2 over the river is a major route for mining, oil and gas. This makes the Peace River crossing a national strategic corridor. Also, in the event of forest fires further north – a common event – the route is essential for moving firefighters and material.
The initial view of the town is an impressive sight as you approach it heading north on Highway 2. Unexpectedly, the road descends a wide gully. Flat farmland suddenly gives way to expansive aspen parkland with views some kilometres along and across the river valley. The town itself nestles along the river’s flat west bank which is basically a flood plain, an important fact that the contractors had to consider daily during the pier construction phase of the new bridge. It meant a potential for serious flooding if the river’s flow were to be constricted, winter or summer.
The river’s name can belie its true nature – the Peace is not always so peaceful. Keeping construction on schedule has necessitated a cat-and-mouse game with the river’s natural rhythms. While beautiful to behold, the Peace River must also be respected. It flows almost 2,000km, originating to the west in the Canadian Rocky Mountains of neighbouring northern British Columbia province. It flows to the north-east and crosses into northern Alberta and eventually empties into Slave River, past the town of Peace River; the annual discharge is a hefty 68.2 billion cubic metres of water. Apart from being the widest river in the province, it has the highest volume.
The real issue for the contractor was to manage the river’s natural velocity and volume through what was going to be a narrowed channel during the construction of piers – the Venturi effect. Also, ice build-up could be a danger to piers, and work crews, and possibly contribute towards flooding the town.
Ice forms on the Peace River from January to often as late as early April. It is mostly frazil ice that eventually creates loose, randomly oriented pans or plates. The pans, as well as breakaway ice from riverbanks, consolidate in the river. This produces a very jagged ice pack surface. Around the town of Peace River, the formation of ice cover can result in a gradual increase in river water levels. The ice and water levels are closely monitored by both Alberta Environment and British Columbia’s BC Hydro whose dams have significant effect on the Peace River's water levels in Alberta.
Partnership
Close cooperation was needed between the new bridge’s construction team, the design engineers, Alberta Environment, the local municipalities, the town and hydro operators. This partnership, says Wilson, was key to seeing the bridge pier work proceed without extensive delays and damage to heavy equipment.
Wilson knows something about partnerships, having spent his engineering career in the private sector until March 2018 (see box). “Through constructive collaboration, the client [project owner], client’s consulting engineer and the contractor have created a team where we all work towards the same goal,” says Wilson. “On larger jobs, unless we are working together and adequately transferring risk to those with the capacity to manage it, you often create a very combative environment. Here, that potential friction in winter in the middle of the river can cost a lot of money and much worse.
“What is unique about this project is the team effort and collaboration that has paid dividends. Despite the challenges we’ve had, we’re still on pace for budget and target completion date. “For what could have been, things have gone really well,” he says.
Some of the first issues encountered on site were geotechnical in nature. The Peace River region is basically the bottom of an ancient lake, which accounts for the good farming land. However, the soil is also very sandy and there is a lot of clay around the river banks itself. There is very little cohesion, especially along the river bank itself: “Everything wants to slide.”
Onsite preparation work started in the autumn of 2017 with wick drains and pre-earthworks on the approach embankments, especially high up on the western slope of the river bank where a new on-ramp was to be constructed. “The wick drain mandrill is like a large sewing machine that shoots the wick, like a needle, into the ground. Moisture rises along the wick and dissipates, allowing the soil to compact and densify,” he says. There was also a lot of mechanically stabilised earth wall needed along the hillside. Down on the valley floor, next to the river, the contractor was setting up shop with material yards, heavy equipment storage and general site operations. It is also where the bridge’s massive steel girders were to arrive for pre-assembly into deck sections before being winched across the piers.
Banana peel
“We were putting a lot of weight on a clay mass, part of the river’s flood plain which consists of silts and sands which have a lot of pore space,” explains Wilson. “Without consolidation of the soil mass which was achieved with the wick drains to remove the water, the entire embankment becomes quite unstable. It’s a bit like standing on a banana peel.
“On the opposite [eastern] river bank, we had issues with weeping springs where, because of the sand lenses within the valley slopes, water would wander wherever it wanted, which eventually induces slope failure. The designer and contractor did an excellent job of foreseeing some of the long-term issues with these springs and we worked together to develop a scheme to avoid potential future stability issues.”
Consolidation on both river banks meant work could start on building outwards into the river a clay berm upon which work on the piers could start. "However, water now has to go through the narrower channel at the end of the berm and the opposite, western bank of the river. When this berm reaches pier 3 in the middle of the river, we have reduced the channel capacity by almost 55%. This effectively doubled the water’s velocity. Much like putting your thumb on the end of a garden hose,” says Wilson.
Faster water velocity meant more scour on the end of the berm that extended outward into faster running water within the constricted channel. Beyond the risk the scour posed to the new bridge berm platform, there was also concern about scour on piers of the old road bridge only metres upstream and the piers of the Canadian National Railway bridge which, similarly, is only metres upstream of the old bridge.
As the water velocity drastically increased with a reduction in channel area, we wanted to wait to go out from pier 2 to pier 3 until the risk of a high flow event was reduced. Led by the contractor, the project team investigated the likelihood of a one-in-20-year flood event, a one-in-30-year event and a one-in-50-year event and evaluated the flows based on those model events,” says Wilson.
They waited to build the berm out to pier 3 until they were in a lower flood risk period – late summer. “The reality is, though, the Peace doesn’t care about your mathematical models.”
The river promptly flooded and heavy equipment had to be hastily removed from the berm before water overran the structure and washed a lot of the top away between pier 2 and 3. “We lost about two to three weeks just getting that back in order.”
“Scour is quite unpredictable, being effected by whatever is in the water at the time – silt, ice, soil runoff along with the river bed substrate, explains Wilson. “The toe of our berm was eroded away before we could get coffer sheets installed so we tried additional rip-rap - large boulders - on the exposed leading edges of the berm to develop some mass. But the Peace simply rolled them away downstream, so powerful now was the current. We did eventually manage to sink down 18m corrugated sheet piles.”
The unexpected
It took two months to sort out the berm issues for pier 3 before work constructing the coffer cell for pier work could commence. On piers 1 and 2 they had problems putting in some of the piles due to unfavourable ground conditions. Then, on pier 3, something totally unexpected happened. “While drilling the bore for pile 9, we hit a pocket of methane. Even though the region is gas-producing, we didn’t expect it from simply boring piles,” he says.
Work was halted and methane was seen to be bubbling up through some water infiltration at the bottom of the pit. “While it was unlike events within the local oil and gas drilling industry, the concern was that a spark could ignite a flash of methane. You never really know at what point that will occur, especially when a drilling rig is working in a mostly dry hole, grinding away with a diamond-tipped cutter blade into rock. A lot could go wrong and quite quickly,” says Wilson.
“There were some oilfield experts around town to provide guidance. Workers in the pit then wore proper air-packs and had a gas monitoring programme. After about a week, we retooled, got the right kit and went back to work.”
The Peace River level has a tendency to periodically rise, thanks to its drainage basin being most of northern Alberta. A period of extended rain can mean a big rise in the river level – a high berm along the river’s bank protects the town site from such events, at least as best it can. But the contractor’s smaller and lower berm running into the middle of the river was fully exposed to extreme rises in water levels.
High-water levels and methane gas issues were overcome, but winter was on the way and the team were still deep in the coffer cell for pier 3. “We were cognisant that ice would be coming downriver and ice build-up was becoming a concern. If the early winter ice flows get jammed around the bridge piers, then all the water will start flowing over them and around them.”
So what do you do? “Pull out and wait for spring thaw, or look for an approach where we could collectively manage risk."
Counsel of wisemen
There was to be no retreat. “In reality we could have just pulled back and gone in next year. However, that would have meant an entire extra year for cost and schedule. So to pull this off, we set up a counsel of wisemen,” he explains.
“I got a team together, the contractor got a team together, the river engineering group for environmental protection put a team together. We had people from BC Hydro and engineers from the town of Peace River. We even commandeered bush planes to fly along the river and its tributaries to check out where ice fronts were.”
The ice front accumulated far downstream and then quickly accumulated and developed upstream at up to 30km a day. There was the risk of the frazil ice pans jamming up at the berm constriction and causing a flood over the town dykes, or there was the risk of the ice front back-water rising quickly as it moved upstream. This was compounded by the need for BC Hydro to set the ice elevation higher so that they could have the river channel capacity to ramp up the flow should the power demand dictate.
“It was quite a feat to the raise the water level downriver at the right time to set the ice in at the higher-than-normal water level. After the elevation was set, BC Hydro then reduced the flow which dropped the river’s winter water level below the underneath of the ice sheet giving them the operational reserve capacity.”
A “trigger-and-action response plan” was created. "The weather experts told us when we could expect water and ice movements." The contractor also had a plan to get their equipment off the berm to safety and lower the berm surface over a 100m distance to let water flow over it, so avoiding ice jams and floods. “We had the statistical information, along with the eyes of the local people and satellite imagery temperature and river flow models. If we knew that the approaching ice would reach us in 15 or so days, we could plan accordingly to move things off the berm and lower it.”
The counsel of wisemen earned its money. “Team meetings went from weekly to every three days to every day to manage this risk. It would be minus 30°C for two weeks in early December and then by Christmas [December 25] it would be plus 4°C. The ice would jam up at the berm and we would pull crews off and then would let go before any build-up of water."
Eventually, by the end of 2019, all the piles were drilled, piers completed with the pier 3 cap remaining. However, with the thermometer dipping to minus 25°C the caps had to be shrouded in heavy tarps under which the contractor ran a series of Herman Nelson portable diesel heaters.
After pier 3 was constructed, the contractor was ready to remove the coffer cell, deconstruct the berm and pull out of the river. But it was noticed that the Peace had been busy scouring the berm allowing the steel cell to move. It created an uneven distribution of stress and had twisted the coffer cell sheets and walers. “The boxes around the piers certainly weren’t square anymore,” says Wilson. “The sheets started zippering up and water was seeping inwards. We had to keep the pumps working in the bottom of the hole.”
Deconstructing the coffer box involved removing the internal walers – the bracing frames. As the walers were being twisted by the external soil pressure, cutting the bracing had to be done very carefully because of the stored energy in the bracing system. “When a brace was cut, this energy had to go somewhere.”
Although some thought it best to just blast the sheets, a less invasive route was taken - simply flood the interior of the coffer cell that surrounded the piers to balance the pressure. Experienced welder-divers were brought in to cut the braces, allowing removal of the coffer sheets.
Deck launch
The girders have now been completely assembled and launched across the piers. Each span consisted of 4.2m-high steel girders. A bridge frame launch was modelled in a wind tunnel at Western University in the province of Ontario. This was done, explains Wilson, to understand the effect high winds could have on unsupported spans during the launch. At 4.2m, they catch a lot of wind and can become unstable. Accordingly, launching over the river took place only when the winds were 20kph or less.
According to Aecon-Flatiron, the project team set a record by being the first-ever to launch a span of this length, twice - span number three from pier 2 to 3 and the fourth span from pier 3 to 4.
Meanwhile, deck formwork is progressing in preparation for concrete pouring this summer and autumn. Work also continues on the underslung pedestrian walkway.
Completion is set for the autumn.
The Old Bridge
The old bridge, opened in 1968, is somewhat quirky in design, explains Wilson. It consists of four tied arches but with the arch on the eastern end smaller than the other three. “It is function over form,” he says. “They were smart, they avoided putting the piers in the deepest part of the river just to gain structural symmetry. So only a smaller arch was needed.” Both the old and new bridges have five piers. “We are mirroring the piers of the original bridge so that we maintain the open navigation channels and maintain the velocity and volume of the river.”
The twinning project
The existing Peace River Bridge sees more than 17,000 vehicles per day. The nearest alternate route across the Peace is 70km away. When opened, the new 563m-long, two-lane bridge will handle traffic heading west and the old bridge will take eastbound traffic. The contract for the Peace River Bridge twinning project was awarded to a Flatiron-Aecon Joint Venture with a modified tender bid price of more than US$112 million.
From private to public sector
Wilson, 38, was originally an ironworker by trade for bridges, industrial plants and buildings. In his mid-20s he enrolled in civil engineering at the University of Alberta in Edmonton. He has worked for several Canadian and global contractors, including civil engineering works for mining projects, in Canada and internationally. He joined Alberta Transportation in March 2018 and looks after major projects across an area the same size as the US state of Vermont.
