UMA Geotechnical Construction Takes On NCDOT Interstate 73 and a Challenging Soil Nail Wall | For Construction Pros

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UMA Geotechnical Construction built all eight required permanent soil-nail walls on the approximately two-mile stretch of the Greensboro Western Loop (Interstate 73) project, but they were all dwarfed by Wall 4.

The North Carolina Department of Transportation (NCDOT) led a $176 million project to reduce congestion and improve traffic to the Piedmont Triad International Airport in Greensboro. Part of the project includes a taxiway bridge to Interstate 73. 

This work is essentially the installation of soil nails and temporary shotcrete surfaces so that Flatiron can complete them with cast-in-place concrete walls. however,

UMA Geotechnical Construction built all eight required permanent soil-nail walls along the approximately two-mile stretch of I-73 along the Greensboro Western Loop to make room for the project. The entire project requires 76,000 square feet of walls, of which 42,000 are wall 4. UMA is a subcontractor of Colorado-based Flatiron Construction Corp., which is retained by NCDOT and built 1.5 miles. The highway is divided between Battlefield Avenue and Lawndale Avenue near Greensboro. 

UMA President Brian DeSpain said: "The soil-nailed wall was designed internally by UMA." He added that the company has hired a licensed engineer.

He said: "The country has determined the location. It's up to us. We will figure out how to design the most cost-effective system." 

UMA submitted a proposal in November 2016 and started work in mid-2017. Its engineers submitted drawings for Wall 4 in 2018 and completed the construction in April 2019. UMA is no stranger to working closely with NCDOT, which works on behalf of most of its public sector. 

The scope of UMA on Wall 4 is simple. This work is essentially the installation of soil nails and temporary shotcrete surfaces so that Flatiron can complete them with cast-in-place concrete walls. However, due to unfavorable geotechnical conditions, simple installation becomes more and more challenging. 

When erected above the wall, the height of wall 4 is 30 feet higher. Once completed, it measures 42,000 square feet and is only a quarter of a mile long. 

To provide background information, the average size is 12,000 square feet. Of the seven other walls built by UMA on the West Loop in Greensboro, the average area is 3,700 square feet. In the area of ​​42,000 square feet, Wall 4 accounts for 55% of the total area of ​​76,000 square feet of the design and construction project.  

DeSpain said: "In order for your company to participate in bidding for soil nailing projects, most project prequalifications require you to complete 2,000 soil nails in the past five years." "This wall exceeds this requirement."  

How does UMA nail nails to walls higher than 30 feet in certain locations? Of course on the lift. Flatiron was responsible for the excavation and cast-in-place concrete walls, which required UMA to work among the staff on most of its projects. 

The first step of Flatiron was to cut down the slope 30 feet and reach the roadbed with an impressive fleet of four Komatsu excavators and 18 articulated loaders. This large-scale excavation finally exposed Wall 4.  

UMA's drilling team processed the wall from side to side. Each elevator excavated by the excavator provides a lower working platform for the driller. 

UMA used a drilling rig, Casagrande C7 (one of the largest soil nail drilling rigs on the market) to complete most of the Wall 4 work. The supporting equipment includes concrete pumps, forklifts, cement slurry plants and sprinklers. Although there is only one crawler rig to complete the work, UMA staff are still keeping pace with Flatiron's top-notch excavation team.  

There needs to be a balance between Flatiron's staff. Due to wet weather and cold weather, the excavators worked hard to provide UMA with as many channels as possible. On the other hand, due to the huge scale of this huge wall, the cast-in-place wall workers need more time to process the formwork, pour the concrete, wait for it to solidify and remove the formwork. 

According to DeSpain, a single drilling rig was intentionally used on this large wall.

"We designed the wall around 30 feet. He said, and simplified the operation by allowing the operator to work alone without the support of others. "Sometimes, he has to drill 100 soil nails a day by himself. "  

UMA purchased the Casagrande C7 drilling rig specifically for the Greensboro Western Loop project. This is the first of two C7s purchased by UMA from North Carolina-based International Drilling Equipment Corporation. "Due to the size and duration of the project, we think it is appropriate to buy a new machine," DeSpain said. "In the entire project, we basically only have one staff member for two years." 

After completing Wall 4, more than 2,000 soil nails were used. The length is between 15 and 50 feet long, although most are 30 feet long. The 30-foot stroke of the C7 allows UMA to drill most holes with only one stroke. Use a herringbone drill to drill the hole and use air to extract the cuttings, leaving an opening with a diameter of 6 inches.  

The rig does not have a merry-go-round to fix additional drill rods, so deeper holes need to be supported by the EZ Spot UR grab mounted on a small excavator. The C7 can drill a 30-foot hole, and the grab is used to set another drill rod to accommodate the extra depth.  

UMA used nearly 61,000 li. Ft, grade 75 epoxy-coated rebar produced by Skyline Steel. In areas where soil and rock collapsed, temporary casing is needed, but this only requires about 1,400 miles. foot. A bar ruler installed about 61,000 li, about 5,500 li. The feet are in the rock and the rest is in the soil. 

Roanoke Cement Company provided nearly 800 tons of Type I/II cement and mixed it on-site with the Colcrete Grouting Plant. In order to achieve a specific gravity of 1.85, mud balance is used to measure the specific gravity of the cement slurry and ensure proper mixing, which includes Type I/II Portland cement, water and admixtures. After the tool is drawn out, the hole is filled with the slurry pumped in through the grouting pipe, and the volume is monitored by an electromagnetic flowmeter. 

After the grout came out of the hole, UMA staff inserted the steel bars. Most soil nails are anchored by pure friction with the soil. Those in the area of ​​the decomposed granite are bonded to the rock.  

UMA engineers determined that 5-foot horizontal by 5-foot vertical separation was the most cost-effective way to support the soil and critical structures behind Wall 4. He explained: "This is a combination of soil characteristics, such as the friction angle of the soil and the additional load on the wall." The additional force is the outward force exerted on the wall by the weight of any object on the wall.   

The length of the soil nails partly depends on the failure plane behind the wall, which is an assumed location where the overall stability of the structure may be damaged. DeSpain said: "The nails should be installed above the failure plane so that the safety factor reaches the 1.5 required by the FHWA."

The friction angle determines the internal shear stress relationship between various soil particles. A higher friction angle usually requires a shorter nail length to meet safety factors.  

The next step for UMA is to install a 4-inch thick temporary sprayed concrete panel. When the shotcrete is still wet, install a steel bearing plate on the head of each soil nail. Secure with angled washers and hex nuts. There are five-inch Nelson studs on the corners of the slab. They are equally spaced to transfer the load of the in-situ pouring to the soil nails.  

The four-inch-thick 4,000 psi shotcrete wall is strong enough to hold it in place until the final cast-in-place wall is constructed in accordance with FHWA guidelines. DeSpain said: “The shotcrete behind the cast-in-place panel is an auxiliary tool for this design.” “Temporary shotcrete is not necessary for the final design of the wall. In this case, it is a way to promote the casting of Flatiron. means."

The active personnel of Flatiron took over this. They tied the steel cage to Nelson Studs and assembled the formwork on the sprayed concrete surface. Then pour a 12-inch thick concrete wall between the two to complete the wall.  

When driving along the West Ring Road, the driver will see this wall. DeSpain explained: "The cast-in-place wall is like an aesthetic decoration." "It basically completes the separation wall." 

Unlike some walls designed and built by UMA with carved shotcrete, the surface of Wall 4 has been specially selected to complement the proposed soundproof wall, giving the otherwise unremarkable soil conservation method an aesthetically pleasing view.  

The soil conditions are usually moderate, and some areas are more difficult, which allows UMA to focus on the absolute size of Wall 4. A large part of the walls are constructed of low-quality decomposed granite, which is a typical feature of the area. North carolina. The rest is Piedmont soil, reddish alluvial soil with silt and clay.  

DeSpain said: "This kind of soil has a certain degree of cohesion, so when you drill in, it usually wants to stay open." He pointed out that UMA's drilling conditions are difficult, from glaciers to southern Pennsylvania to the coast of South Carolina. Compressed shells,. “We faced challenges in maintaining the holes in the cracked rock, but it was not very sad due to the experience of our team and the drilling and grouting equipment specifically selected for this project.”

Due to the large number of personnel assigned by UMA, production has soared, "only four people in the project were assigned to other walls," DeSpain said. "We have added personnel to Wall 4, not only to deal with the size of this wall, but also to increase our productivity." 

The team is committed to drilling, preparing walls for shotcrete, grouting and mixing, and construction of shotcrete. The command system includes a roster consisting of several foremen and one foreman, and seven field personnel. There is also an on-site engineer to ensure the completion of the load test, and insist on the implementation of quality control and assistant project manager to carry out the appropriate material procurement. A project manager kept the work as planned and maintained the expected budget. 

UMA’s production method is to create repetition and minimize uncertainty,” DeSpain said. “We asked, how can we design walls to be built in a repetitive way and build faster? We want to make the rod and Keep the size of the holes the same to avoid having to cut anything on site."

The focus of UMA is to ensure that the staff can seamlessly choose the right materials, place them in the right place, and move forward as easily as possible.  

Despite the huge scale of Wall 4, let alone the other 7 walls, UMA completed the project ahead of schedule. Another future project will complete this ring road around Greensboro. Most drivers will not notice the towering walls along the road along with the traffic on the new ring road, but UMA and Flatiron will remember the smooth partnership that turns a mountain into a road. New highway. 

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