“The rock anchors are there for when the structure is empty. When it’s empty and the ground water table is above the bottom of the structure, if the rock anchors aren’t there, it could push up out of the ground like a boat. These buoyant forces are exactly what the rock anchors are there to resist,”

explains UMA’s Senior Engineer/Estimating Manager Mitch Crayton.

Cherokee County Water and Sewerage Authority outsourced this pump station improvement project to Atlanta-based Lakeshore Engineering, a heavy civil contractor that focuses on industrial, municipal, and environmental projects. UMA served as the
geotechnical subcontractor.

Located within proximity to Blankets Creek in Canton, Georgia, the water table is known to fluctuate. UMA’s sole function was to install a rock anchor system to keep the pump station’s concrete slab pinned down. The components to be built on top of the slab
would be a diesel engine-driven centrifugal pump and a concrete cast-in-place
emergency storage tank.

Working Down in the Hole

One of the biggest challenges for UMA’s team was working in a 23-foot-deep-by-
56-foot-wide excavation. Lakeshore Engineering had excavated the pit and installed a soldier pile wall with wood lagging. Tied-off workers accessed the pit via ladders.

“We’re working in a pretty tight area compared to what we’re normally used to on highway projects. It’s not the smallest footprint we’ve worked in, but it’s a lot smaller than our usual working space,”

says Crayton.

Lakeshore Engineering installed a mud mat to provide a stable working surface for UMA’s drilling rig and to keep the worksite clean. There was no air movement so the air was pumped in from ground level and circulated with fans to keep workers from breathing in diesel fumes.

The working area had to be dewatered also due to the proximity of Blankets Creek and a water table approximately five feet below the ground. While the failure of
dewatering pumps would not have been cataclysmic, the potential slow accumulation of
water in the working area could have damaged equipment.

One of the visual spectacles on the project was Lakeshore Engineering using a crane
with rigging to lower a Beretta T46 Drilling Rig into the excavation as UMA’s team
provided direction.

“It’s a smaller machine, but we selected it for this purpose. It doesn’t quite get the production that we’re typically used to, but it can hold its own with this smaller hole size,”

Crayton explains.

UMA’s crew was seasoned and used to working together. They were also familiar with the equipment and how to rig it based on past projects.

“We shut down around Christmas every year for safety and for training. We alternate between first aid, forklift certification, and crane and rigging so that everyone is safe on the job site. That’s always our number one concern,”

Crayton says.

Drilling the Rock Anchors

UMA was tasked with installing 24 rock anchors, evenly spaced every 10 feet vertically and 8 feet 8 inches horizontally. Skyline Steel supplied the anchors, which were 1-inch-diameter Grade 150 ksi DCP (double corrosion protected) bar. The owner’s representatives provided the grid layout with specific locations required to support the pump station equipment.

“It was critical to making sure these (rock anchors) worked because it would be very cost-prohibitive to move them. If even one failed, they would have to change reinforcing plans and the connection details of the slab foundation to the actual structure,”

says Crayton.

The Beretta was drilling a minimum 4.25-inch-diameter hole 15 feet down to bedrock with a permanent casing supplied by Star Iron Works. The drilling crew had to focus closely on ensuring that the anchors were plumb and in the exact location specified.

The material was higher quality partially weathered rock, although the crew hit a high-quality bedrock in a handful of areas. The team then drilled into bedrock with a 10-foot
rock socket.

“Our drillers are schooled enough to distinguish between the materials they’re drilling through. One drills a lot slower than the other and they can hear and feel the differences,”

says Crayton.

The Beretta was equipped with a down-the-hole hammer with a 4½-inch bit to drill the rock sockets. This tooling fit perfectly inside the casing, allowing the cuttings to be extracted onto the mud mat before being plowed away with Lakeshore Engineering’s Takeuchi skid steer loader.

“We drill down into a specific rock socket, place our anchors, and lock them all to that slab. That’s how we end up with our rock anchor system,”

Crayton explains.

Testing Anchor Strength and Movement

UMA conducted two extensive performance tests to ensure that the anchors were of sufficient strength to support the pump station. The movement was essential also.

“These anchors are basically like high-strength rubber bands. Anything that moves has some sort of desire to go back to where it was. When they flood this pit with water, the anchors will keep the slab and pump station from rising,”

Crayton says, noting that the anchors are designed to move.

Testing requirements placed some restrictions on the schedule. UMA was only permitted to install two anchors on the first couple of days and conduct a load test to demonstrate that capacity was achieved before installing the third. This pattern continued throughout the project.

UMA was installing multiple corrosion-protected anchors. The corrosion protection is granted by several components within the anchor, in addition to the grout itself. The
grout was then pumped to grade until clean grout was observed.

“We worked with Lakeshore closely to help speed up their construction process and get out of the way so they could finish their job. The main way we expedited our work was to lock off the anchor to a mud mat using a secondary bearing plate,”

Crayton says.

The double bearing plate system had one bearing plate locking the anchor to the mud mat and one suspended in the air to transfer the loading to the foundation. UMA was able to get all of the testing completed at one time under the initial mobilization. This permitted Lakeshore Engineering to set a bearing plate at a certain elevation and pour the structural slab without having to wait on remobilization for final stressing. This allowed a majority of the elastic elongation to occur prior to the final placement of the slab, ultimately saving the contractor time and money.

Batching and Grouting to Lock in Anchors

UMA grouted the rock anchors in place. The material was mixed in a batch plant on-site
with a specific gravity of 1.85 to 1.86. Its team is trained to check specific gravity prior to pumping, and a third-party contractor was retained to take samples and ensure quality control.

This standard mix was one that UMA was accustomed to and achieved the strength that
was required on this project. The spec called for a minimum of 3,500 psi at three days and 5,000 psi at 28 days.

Most of UMA’s soil nail projects require larger material quantities and hole diameters, and also deeper drilling.

“The crew that was doing this job came off of a year-long soil nail project so they were used to drilling 30-foot-long, six-inch diameter holes, and four batches of grout at a time. There’s a pretty big difference between a rock anchor job and a soil nail job, and our crew adapted well to the switch,”

says Crayton.

The grout was pumped through a tremie pipe to the bottom of the hole from within the casing. The casing sections were installed with the Beretta drilling rig, which was equipped with a special adaptor. UMA was performing a duplex drilling process, which involves drilling the casing and the hole concurrently. Once the rock was hit, the casing was halted, and drilling continued with the inner rod.

UMA workers fed casing to the drilling rig by hand.

“Normally we would use an EZ Spot UR attachment on the excavator to handle large casing sections. We had to use smaller pieces on this job because we didn’t have the real estate to put them in the pit with us,”

Crayton says.

The majority of UMA’s work on the Blankets Creek Pump Station project was done within a week and completed on schedule. Despite the technical nature of the work, the distance between the Jobsite in Georgia and UMA headquarters in Colfax, North Carolina was the true challenge.

“Making sure that everyone who needed to observe different processes was onsite during the installation and load testing was our greatest challenge,”

says Crayton.

That’s generally the hardest aspect of most of our projects: the distance between the field where the actual work is done and the office where the computers are required to do the analysis.”

The post UMA Goes Deep to Anchor Georgia Pump Station appeared first on North America FarmQuip Magazine.

UMA was a subcontractor to Colorado-based Flatiron Construction Corp., which had been retained by the North Carolina Department of Transportation (NCDOT) to build a 1.5-mile divided highway around Greensboro between Battleground Avenue and Lawndale Drive.

President Brian DeSpain, adding that the company utilized licensed engineers on staff says:

“The soil nail walls were designed in-house by UMA. The state identified the locations and it was up to us to figure out how to design the most cost-effective system.”

UMA submitted a proposal in November 2016 and began work in mid-2017. Its engineers submitted drawings for Wall 4 in 2018 and finished construction in April 2019. UMA is no stranger to working closely with the NCDOT, which represents the majority of its public sector work.

No Average Retaining Wall

UMA’s scope on Wall 4 was straightforward. The job was essential to install soil nails and temporary shotcrete facing to allow Flatiron to finish it off with a cast-in-place concrete wall. However, due to adverse geotechnical soil conditions, a simple install became increasingly more challenging. In a budget-saving triumph, UMA’s proven history and competent personnel made light work of the challenges. At just over 30 feet, the height of Wall 4 towers over you when standing at its base. Once completed, it is 42,000 square feet and just over a quarter-mile long.

To provide context, 12,000 square feet is an average-sized wall. Of the seven other walls UMA built on the Greensboro Western Loop, the average size was 3,700 square feet. At 42,000 square feet, Wall 4 accounted for 55 percent of the design-build project’s total square footage of 76,000.

“Most project prequalifications that enable your company to bid on a soil nailing project require that you have completed 2,000 soil nails in the last five years. This one wall is in excess of that requirement,”

says DeSpain.

Working in Lifts

How did UMA install soil nails on a wall that was higher than 30 feet in some locations? In lifts, of course. Flatiron handled excavation and the cast-in-place concrete wall, which required UMA to work between its crews for most of the project.

Flatiron’s first step was to cut the hill down 30 feet to reach the road base with an impressive fleet of four Komatsu excavators and 18 articulated loaders. This mass excavation ultimately exposed Wall 4.

UMA’s drilling crew worked the wall from one side to the other. Each lift the excavation crew cut out provided a successively lower working platform for the drilling crew.

 The Equipment Spread

UMA completed most of Wall 4 with a single drilling rig – a Casagrande C7 – one of the largest soil nail rigs on the market. The supporting equipment included concrete pumps, forklifts, grout plants, and a water truck. Despite having a single crawler drilling rig doing the work, UMA’s crew kept pace with Flatiron’s top-notch excavation team.

A balance was required to work in between Flatiron’s crews. The excavation crew, hindered by wet weather and cold temperatures, persevered to provide UMA with as much access as conditions permitted. The cast-in-place wall crew, on the other hand, required more time to handle formwork, pour concrete, wait for it to cure, and remove the formwork due to the sheer magnitude of this enormous wall.

Using a single rig on this massive wall was intentional, according to DeSpain.

“We designed the wall around the 30-foot stroke of the machine,”

he says, noting that this simplified the operation by allowing the operator to work alone without support from others.

“There were days where he was drilling 100 soil nails in a day by himself.”

UMA bought the Casagrande C7 Drilling Rig specifically for the Greensboro Western Loop project. It was the first of two C7s that UMA has purchased from North Carolina-based International Drilling Equipment. DeSpain says:

“We felt that it was appropriate to buy a new machine because of the size and the length of the project. We basically had one crew on the entire project for two years moving from wall to wall.”

The Drilling Process

It took more than 2,000 soil nails to complete Wall 4. The length ranged from 15- to 50-foot-long, although the vast majority were 30 foot. The C7’s 30-foot stroke allowed UMA to drill most holes with a single stroke. A chevron bit was used to drill and the air was used to extract the cuttings, leaving an open six-inch-diameter borehole.

The rig has no carousel to hold additional drill rods so deeper holes required the support of an EZ Spot UR grapple mounted on a mini excavator. The C7 would drill the 30-foot-hole and the grapple was used to set another drill rod to accommodate the additional depth.

UMA used approximately 61,000 linear feet of Grade 75 epoxy-coated threaded bar produced by Skyline Steel. The temporary casing was required in areas with collapsing soil and rock, but that was only about 1,400 linear feet. Of the roughly 61,000 linear feet of bar installed, about 5,500 linear feet were in rock and the remainder in soil.

Almost 800 tons of Type I/II cement was supplied by Roanoke Cement and mixed on-site with a Colcrete Grout plant. Aiming for a 1.85 specific gravity, a mud balance was used to measure the specific gravity of 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 vertical 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 friction 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 beveled 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. DeSpain says:

“The shotcrete behind the cast-in-place facing is ancillary to this type of design, 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. he explains:

“The cast-in-place wall is like an aesthetic finish. It basically finishes the wall.”

Unlike some walls UMA has designed and built with sculpted shotcrete, 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 Carolina. 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 fractured 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, DeSpain says:

“There were only four people assigned to the other walls on the project. 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 foremen 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 an assistant project manager for adequate material procurement.  A project manager kept the work on schedule and maintained the expected budget.

UMA’s production approach was to create repetition and eliminate uncertainty 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 appropriate 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 project 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 UMA and Flatiron will remember the smooth partnership that turned a large hill into a new stretch of highway.

 The article was written by Brian M. Fraley Fraley Construction Marketing

Source: Ransome Attachments

The post [Job story] UMA Scales up Production on unusually large soil nail wall appeared first on North America FarmQuip Magazine.