One of the tests of good products, design, and construction is that the final result makes it almost impossible for anyone who wasn’t involved in the project to tell how much went into it, in spite of the challenges.
The following three retaining wall projects all took place along roadways and overcame a variety of challenges, from extreme weather and grade changes to time and space constraints.
Highland Valley Copper Mine
First, frost heaved up the compacted base where the retaining wall was to be built. Then it heaved the base up again, this time through a thick insulating layer of loose fill. In the end, more than 100 men had to work in 8- and 12-hour shifts, around the clock, just to keep ahead of the problems caused by the weather.
“There was a cold snap, and we ended up in it,” says Loic Hersco, the owner of TLC Stoneworks Ltd. in Comox, BC, who managed the construction of the walls for Highland Valley Copper Mine in Logan Lake, BC. “It was definitely a challenge.”
The open-pit mine, the largest base metal mine in Canada, covers about 6,000 hectares of land and operates 24 hours a day, 365 days a year. In 2007 alone, it produced 300 million pounds of copper and 4 million pounds of molybdenum. Waste rock is hauled out of the pit by gravel trucks that weigh 500 tons when they’re loaded and is stored at a dump onsite. In 2009, this became a problem: The dump was filling up, and the only place for a new one was across the entrance road to the mine.
“They couldn’t have those trucks going across the entrance road because they’re so big,” Hersco says. “The tires alone are 12 feet high.” If one of the trucks ran over a smaller vehicle, it would feel to the truck driver like running over an empty soda can. The most economical solution was to build an overpass about 50 meters (150 feet) long over the entrance road to allow the haul trucks to cross without interfering with traffic to and from the mine.
The mine chose Lock+Load panels, manufactured by Pacific Lock+Load, for the wing walls of the overpass. The panels are fast to install, strong, and forgiving, Hersco says. Crews also used the geofabric Miragrid, by Tencate, to build the geosynthetic reinforced soil (GRS) walls.
The tunnel under the overpass was of steel multiplate manufactured and supplied by Armtec, says Hersco, who managed the construction of the 8-meter- (about 26-foot-) high wing walls at the end of the tunnel. By November 2009, the project had already been delayed, and time was running out.
“By the end of January [2010], the dump where they were storing the waste would be full, and they’d have to take it across the entrance road,” he says. “If the new road didn’t open February 1, they’d have to shut the mine down.”
The mine is about 75 miles southwest of Kamloops in the Highland River Valley. The soil is glacial till, and the land is hilly, he says. “Five valleys meet there, plus the mine is a hole in the ground 750 meters [about 2,500 feet] deep. It almost creates its own weather pattern.”
Hersco’s first day on the job was a cool -11°Celsius, or 12°F. The site had already been excavated, and Newport Structures, the general contractor for the job, had put a culvert pipe at each side and backfilled and compacted the base course. But a month had passed since that work was done, and during that time it had rained-a lot. By the time Hersco and his employees arrived, the frost had heaved up the base course.
Before they began work on the walls, they did a trial run to find the best way to compact the fill. They couldn’t use fines or any material smaller than 3-inch granular material, because these materials would have expanded in the freezing weather. They couldn’t add water, which also would have aided compaction, because water also expands in freezing temperatures. The average moisture content was about 2.5% to 3%, far below normal.
“Compaction was very hard to gain, but we managed to do it,” he says.
He and his crew excavated the ruined base course down to about 1 meter and 20 centimeters (about 4 feet), and replaced it with drain rock. Then they ran into another problem.
“It’s unbelievable how the cold affects the material,” Hersco says. “Before we got working full-tilt, we did a little bit of basework. It wasn’t freezing, so we left it out at night with a loose layer of 8-inch gravel to insulate it, crushed and raked and leveled. When we came in at 8 a.m., the frost had heaved up the gravel. There was a two-and-half- to 3-inch gap between both materials. Just by walking on it, we made holes in it. We had to excavate the basework, and we couldn’t use the material again because it had ice in it. There was nowhere to thaw it out.”
In warmer weather, they would have needed 15,000 cubic meters (about 530,000 cubic feet) of backfill material. For this project, they got 25,000 (more than 880,000 cubic feet) and used almost all of it.
It snowed lightly for a few days, but at one point it was so cold they had to shut down for two days. With the wind-chill factor, the temperature sank to -36°C (-32°F). “We put two and a half to 3 feet of loose material on the whole job site-150 feet by 100 feet,” he says. “We parked all the equipment inside the heated tunnel. When we started up again, it took a full day just to haul away all the material. It was a huge expense.”
Crews built the wing walls by placing a layer of the geotextile over a layer of blocks and covering it immediately with a 20- to 24-inch-thick layer of uncompacted 3-inch gravel as insulation. The frost usually penetrated down 8 to 10 inches before crews were ready to place another layer of geotextile. When they were ready, they stripped off some of uncompacted gravel, discarded it, and compacted the rest until they had an 8-inch layer of compacted backfill on the geotextile. They placed another layer of the geotextile and a layer of panel over that. Each row sits on backfill, Hersco says.
They repeated the process 44 times, using 44 layers of the geotextile. “It really helped confine the loose material,” he says.
Crews first tried compacting with rollers, but all that did was move the material around, Hersco says, and the colder it got, the more passes they had to do. In the end, they used a 1,000- to 1,200-pound plate compactor, which vibrates and forces the backfill straight down. They compacted to a little higher than the top of the Lock+Load, then scraped it down to make it even.
“All the compaction was done at least half travel speed to get the energy where it had to be,” he notes. “On a dry summer day, it would take three or four passes at full speed. Down here, at half speed, it took up to 12 passes.”
Installed, Lock+Load panels consist of three parts: a steel- and polyolefin-fiber-reinforced solid concrete panel, a steel-reinforced loop that is integral to the back of the panel, and a concrete counterfort that is locked to the panel by a loop. The counterfort extends 26 inches into the backfill, so compaction makes the walls stronger. In addition, Hersco says, “You can actually touch the panels with the compactor and it doesn’t move the walls at all.”
The panels are independent of each other, so if there’s a soft spot in the backfill, it’s easy to make the row even. With some other products, people put in shims, which can cause a structural problem, he says.
The project took three and a half weeks of 24-hour days. If the crews had worked only 12-hour days, the frost would have penetrated the ground, even with the insulation.
There were a lot of people to organize, Hersco says. Crews from one company worked three 8-hour shifts and those from two companies worked two 12-hour shifts. There were more than 100 people in all, about 30 per shift. They worked under lights most of the time because it got dark around 4:30 in the afternoon and didn’t get light again until nearly 8:00 in the morning.
“It was definitely testing,” he says. “You’re outside doing stuff you can’t wear gloves for. It’s cold. The guys were getting really burned out. We did everything we could. We pulled a lot of material out that we put in because we thought frost might be in there. The client was completely on the same page. They wanted to do everything properly.”
If it had been done in the summer, the work probably would have gone twice as fast and cost 40% less, he says.
“But we did it, and it looks good.”
I-76 in Denver
A very challenging retaining wall project began in the summer of 2009 in Denver, CO. When the eastbound and westbound lanes on Interstate 76 were widened, two new bridge structures were required: one to carry traffic over 74th Avenue and one to carry it over a railroad line. Slaton Bros. Inc. of Centennial, CO, which specializes in the design and construction of mechanically stabilized earth (MSE) retaining walls as well as foundation improvement systems, has been building the retaining walls for the new bridges.
“Space was very limited. The job had to be done in three phases, starting with the middle section of the bridges,” says Scott Sothen, a project manager for the company. Another challenge was that 74th Avenue wasn’t perpendicular to I-76 passing overhead. Because of the acute angles that formed between the retaining walls and the bridge, there was no room for traditional soil-reinforcing elements to be installed for the phased construction.
“We selected Tensar and the Mesa retaining wall system because of the unique design and construction challenges created by the phased construction,” Sothen says. “They were really the best option for this project.”
This section of the interstate runs through an industrial area just north of the city and never shut down during the project, which was also a challenge.
Overall, the terrain is fairly flat, Sothen says, although some areas are hilly. The soil is a combination of well-draining silt, sand, and clay. The average annual rainfall in the area is roughly 16 inches per year, but the average annual snowfall is on the order of 60 inches. Stormwater flows into the nearby Platte River.
There were four bridges before the project began: One eastbound and one westbound over 74th Avenue, and one eastbound and one westbound over the railroad. These were replaced by two wider bridges. One, which is almost 200 feet long, carries both eastbound and westbound traffic over 74th Avenue. The second is roughly 180 feet long and carries both eastbound and westbound traffic over the railroad. Tensar provided the final design of the retaining walls for the project.
The first phase of the project began in the summer of 2009 and ended in the fall of 2009. Slaton Bros. built the middle portion of the new bridge structure between the existing bridges over 74th Avenue. The walls fronting the new bridge over the railroad were fairly short cast-in-place walls built by others. Traffic was maintained on I-76 during the entire project, including on the existing on and off ramps at 74th Avenue. Once the first phase was complete, westbound traffic was diverted to the newly constructed bridges at both 74th Avenue and the railroad. Eastbound traffic continued on the existing bridges.
In the project’s second phase, which began in November 2009 and ended in early 2010, the existing bridges for the westbound lanes of I-76 at 74th Avenue and the railroad were demolished. The general contractor built new bridge portions next to the middle sections built earlier and tied them together. Eastbound and westbound traffic was diverted to the newly constructed portions of both new bridges.
During the third phase, which began in early 2010, the existing bridge structures for the eastbound lanes of I-76 over 74th Avenue and the railroad were demolished. The remaining portions of the two new bridges will be constructed on the southern side of the diverted eastbound I-76. Two additional retaining walls will also be constructed. One comes into the new bridge over the railroad to support the widened portion of I-76, and the second comes off of the bridge at the railroad, again to support the widened section of I-76 and also the off-ramp of I-76 to 74th Avenue.
“The process was similar in all three phases,” Sothen says. “We did our wall, they did the bridge, then we moved on to the next phase.”
The excavation was done by the general contractor. Because of the unevenness of the grade, the depth of the excavation varied. The maximum cuts were about 20 feet deep, while some were next to nothing, Sothen says.
Slaton Bros. began building the walls by pouring a concrete leveling pad along the wall layout line and laying a base course of Mesa blocks. The blocks have slots in the tops for inserting the Mesa fiberglass-reinforced connectors, locking the blocks together and also providing a positive mechanical connection of the geogrids to the blocks for structural integrity.
Crews placed uniaxial geogrids from Tensar over and behind the blocks at required design elevations; backfilled with CDOT Class 1 structural backfill, which is generally a free-draining material; and then compacted the material to required densities. They continued to add rows in running bond fashion and installed geogrids at specified design elevations until they reached the desired height.
“What would have made the phased construction easier was if the roadway was perpendicular to the bridge,” Sothen says. “It’s more difficult to construct retaining walls at acute angles, primarily because the geometry creates limited working area for design and construction purposes.”
Because the shoring walls were at an acute angle to the permanent retaining walls, there was no room for full-length soil reinforcing elements to support them. Instead, crews built pressure-relief walls and used alternative design methods and soil reinforcement materials to stabilize the acute corner areas. For example, they used Tensar’s biaxial geogrids and a very clean crushed rock fill about three-quarters of an inch to 1 inch in size that would lock up better.
For drainage, Slaton Bros. installed a 30-mil PVC liner across the top of the backfill at the wall top to collect surface water and send it into a subdrain system and piping that leads it beyond the wall structures.
“The wall construction has been slower than a typical project because of the construction complexities involved, as well as the phasing,” Sothen says, “but overall it’s gone well.”
McCarren Boulevard
Sometimes retaining wall projects need retaining walls of their own.
When a section of McCarren Boulevard in Reno, NV, was widened in 2009, Tom Printz, president of Printz Engineering Services LLC and the West Coast engineering manager of Williams Form Engineering, faced a challenge: He had to design a vertical shoring wall between the road and the steep slope up to nearby residential property lines so the construction company could pour a permanent retaining wall along the widened road.
“There were right-of-way issues,” Printz says. “The general contractor, Q&D Construction, couldn’t go outside the right of way, but they still needed room to pour the wall that lines the boulevard.” He used the earth-anchor system Manta Ray, by Foresight Products in Commerce City, CO, both to prevent soil erosion and to ensure that the environment was safe for the workers pouring the wall.
Williams Form Engineering performs erosion control and rockfall mitigation and is a distributor for Manta Ray, which is used to secure wall faces and for soil reinforcement.
Q&D, which is based in Sparks, NV, and has won a number of awards for safety and construction, built both walls. The company wanted an inexpensive shoring wall it could install itself, Printz says, and the Manta Ray system is user-friendly. The soil in the Reno area is ideal for the earth anchor.
“It’s a very dense sand with just the right grain-size distribution,” he says. “These soils lock together under the load, so you can get quite a bit of load.” Manta Ray anchors are made of hot dip galvanized ductile iron and have a capacity of up to 20 tons, and because they’re driven into the ground, they compact the soil around themselves, making the connection even more secure.
Q&D built two walls using Manta Ray, together about half a mile long, on the west side of McCarren Boulevard north of I-80. The area is hilly and built up, and rain can be heavy at times. In January, the project shut down because of the amount of moisture. The road stayed open during construction, but fortunately there was enough room to work, Printz says.
Time was another constraint. Once a section of slope was excavated, there was little time to secure it because the surface lost moisture quickly and tended to ravel. Typically, two operations went on simultaneously: excavating and driving in the anchors, and installing the wall face coverings and locking the anchors.
The work began at the top of the slope. One Q&D crew excavated the first 5 feet of material vertically; threaded the anchors, which were up to 15 feet long, into the blocks; and drove the anchors into the slope, either 4 or 5 feet on center, horizontally and vertically.
The second crew followed behind, using bent rebar to hold the geotextile, Mirafi 140N by TenCate, to the top of the slope and draping the fabric and a standard chain link fencing over the anchors. They cut holes in the fabric with a utility knife to expose the anchors, explains Brud Beaudoin, the general superintendent with Q&D, then loaded them with a driving tool, positioned them at the proper location and angle, and drove them to the proper depth. They removed the driving tools, put load lockers on the anchors, and pulled the load lockers back with a hydraulic pump, tipping the anchors into place like toggle bolts, for the exact capacity that was required. A gauge on the pump measures the strength of the hold.
“You have to make sure to maintain a decent force,” Printz says. “We were locking them off at 15,000 pounds per anchor.”
Making the inclination angles 15 degrees was also important on this project, he says.
“The 15-degree angle contributes more to the horizontal resistance, to resist horizontal earth pressure. The steeper the angle becomes, the more the load is in the vertical direction. A flatter angle would have been ideal, as all of the load is in the horizontal direction; however, we needed a minimum overburden depth-the depth of earth cover from the top of cut to the anchor-to achieve the loads required for resisting the earth pressure.”
The crews repeated this process until the walls were at the desired height, which ranged from 8 feet to 15 feet. If it had been less, Beaudoin says, the shoring wall might not have been necessary.
Another challenge was the elevation change on the site. The grades of the roadbed and the top of the slope dropped, although not by the same amount, so the elevations of the tops of the walls and the footings both dropped as well. The footings were staggered as the grade dropped away, and an additional row of anchors was placed at each step, says Beaudoin.
“It almost looked like a stair step at the end of the day.”
This was Q&D’s first experience using the Manta Ray system, and it worked well, he says. Once a section was complete, crews poured the retaining wall.
A drainage pipe that connects to the city’s stormwater system is located at the bottom of the wall, and drainage backfill, Class D crushed rock, fills the space between the walls.
The shoring wall project began in September 2009 and was complete by the end of October. The costs for the Manta Ray system were just under $20.00 per square foot, and the quote for soil nailing was roughly $30 per square foot.
“Everything’s holding up great, even with the harsh weather,” Printz says.