the grout and ensure the proper mix, which included Type I/II Portland Cement, water, and admixtures. Once the tooling was extracted, the hole was filled to capacity with grout pumped through a tremie tube, monitoring volume via a magnetic flow meter. Once the grout emerged from the hole, UMA's crew inserted the bar. Most of the soil nails were anchored by pure friction with the soil. Those in the decomposed Granite areas were bonded to the rock. UMA’s engineers determined that five-foot horizontal by five-foot verti- cal spacing was the most cost-effective means to support the soils and critical structures behind Wall 4. The anchor pattern is based on several factors, according to DeSpain. “It's a combination of the soil properties such as friction angle of the soil and the surcharge loading that's above the wall,” he explains. The surcharge is the outward force exerted against the wall face by the weight of anything that sits above it. The length of the soil nails is partly determined by the failure plane behind the wall, which is the assumed location at which the global stability of the structure could fail. “The nail should be installed past the failure plane to bring the factor of safety up to the FHWA-required 1.5,” DeSpain says. The friction angle determines the internal shear- stress relationship between the individual soil particles; a higher fric- tion angle generally requires less nail length to meet the factor of safety. Applying Shotcrete Facing UMA’s next step was to install a four-inch-thick temporary shotcrete facing. Steel bearing plates were installed over the head of each soil nail while the shotcrete was still wet. That was secured with a bevel washer and hex nut. The corners of the plates had five-inch Nelson studs, equally spaced to transfer the cast-in-place load to the soil nails. The four-inch thick, 4,000-psi shotcrete wall was strong enough to hold the wall in place until the final cast-in-place wall was built in accordance with FHWA guidelines. “The shotcrete behind the cast-in- place facing is ancillary to this type of design,” DeSpain says. “the temporary shotcrete isn’t necessary to the final design of the wall – in this case it was a means to facilitate Flatiron’s cast-in-place pour.” Flatiron’s cast-in-place crew took over at this point. They tied rebar cages to the Nelson Studs and assembled formwork over the shotcrete face. A 12-inch-thick concrete wall was then poured between the two to complete the finished wall. This wall is what motorists will see when driving along the Western Loop, according to DeSpain. “The cast-in-place wall is like an aes- thetic finish,” he explains.” It basically finishes the wall.” Unlike some walls UMA has designed and built with sculpted shot- crete, the finish of Wall 4 was specifically selected to complement the proposed sound barrier walls to bring an aesthetically pleasing view to an otherwise lackluster means of earth retention. Unforgiving Soil Conditions Soil conditions were mostly moderate with some areas proving more difficult, which allowed UMA to concentrate its efforts on the sheer
size of Wall 4. A large portion of the wall was constructed in a low- quality decomposed granite, which is typical in this part of North Caro- lina. The remainder was a Piedmont soil, which is a reddish alluvial soil with silt and clay. “This soil type has some cohesion so when you drill in it, it generally wants to stay open,” says DeSpain, noting that UMA has drilled in challenging soil conditions ranging from the glacial till of southern Pennsylvania to the compressed seashells of coastal South Carolina. “We faced challenges trying to keep the holes open in the fracture rock, but it wasn’t overly difficult due to the experience of our team and the specially selected drilling and grouting equipment for the project.” Production soared because UMA assigned a large crew, “There were only four people assigned to the other walls on the project, says De- Spain. “We added more personnel on Wall 4 to not only deal with the size of this wall, but also to increase our productivity.” Teams were dedicated to drilling, preparing the wall for shotcrete, grout mixing, and applying shotcrete. The chain of command included a superintendent with several foreman supported by a roster of seven field personnel. There was also a field engineer to ensure load testing was completed, and quality control was adhered to as well as assistant project manager for adequate material procurement. A project man- ager kept the work on schedule and maintained the expected budget. UMA’s production approach was to create repetition and eliminate un- certainty to the greatest extent possible,” according to DeSpain. He says, “We asked, how can we design the wall to be built in a way that is a repetitive process and can be constructed faster? We wanted to keep the bar and hole sizes consistent to avoid having to cut anything on site.” UMA’s focus was to make sure crews could seamlessly select the appro- priate materials, set them in place, and move on as effortlessly as possible. Despite the magnitude of Wall 4, not to mention the other seven walls, UMA delivered the project ahead of schedule. One more future proj- ect will complete this beltway around Greensboro. Most drivers won’t notice the massive wall towering alongside the roadway as traffic flows along the new beltway, but UMAand Flatiron will remember the smooth partnership that turned a large hill into a new stretch of highway. It took more than 2,000 15- to 50-foot-long soil nails to complete Wall 4. The C7’s 30-foot stroke allowed UMA to drill most holes with a single stroke.
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november 2020
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