Atlanta’s water supply program isn’t just a massive effort towards long-term water storage for the sprawling city’s 1.2 million customers. The project in the Peach State is leading the charge with a unique contract structure, a team that overcame challenges, and a plan to revitalise what has historically been considered a blight on the environment: disused quarries.

Atlanta’s 90m deep, vertically-walled Bellwood Quarry was once mined for granitic gneiss and is over 100 years old. The USD 300M project will turn the inactive quarry into a 9.1 million cubic metre raw water storage facility connected up with the Chattahoochee River and various water treatment facilities. The water network will bolster the city’s emergency water supply – currently at just three days – to 30 days at full use and up to 90 days with emergency conservation measures. The 162ha parcel of land surrounding the quarry will be turned into the city’s largest park.

“This project has precipitated the upcoming use of several disused quarries across the US for water projects. Quarries are being repurposed as reservoirs because droughts are becoming more common,” said Don Del Nero, Vice President of Stantec, serving as the project’s Engineerof- Record.

Construction of the ground-breaking project reached a significant milestone with the breakthrough of a Robbins TBM on 4 October 2018, completing an 8km long tunnel connecting up with an intake at the Chattahoochee River. Boring the tunnel required the same level of ingenuity and ability to overcome the obstacles that have been the project’s hallmark from the start.

A new structure

The Atlanta WSP is only the third underground project in the U.S, and certainly the longest tunnel, to be built under the Construction Manager at Risk (CMAR) contract structure. The CMAR structure allows the contractor to act as a consultant to the owner during the development and design phase and as a general contractor during the construction phase.

“Early on, the city struggled with the decision between Design-Build and CMAR,” said Bob Huie, Project Director for the PC/Russell JV. “Ultimately CMAR was decided on because of the city’s experience with that model at the airport.”

The PC Construction/HJ Russell (PCR) JV, selected as the CMAR for the project, had experience with the model at regional projects including Georgia’s Yellow River Water Reclamation Facility. “We provide construction management, cost analysis and estimates, and scheduling services to the owner. We also take responsibility to hire all subcontractors.”

The designer for the construction works including tunnel and shafts, JP2 JV – consisting of Stantec, PRAD Group, and River to Tap – with a Stantec-specified hard rock TBM. Operation and assembly of the TBM was then sub-contracted to the Atkinson/Technique JV after a significant competitive bidding process.

The ATJV was responsible for the construction of the tunnel and deep rock intake shaft at the Chattahoochee River. Other deep rock shafts and connecting tunnel structures include those located beneath the Quarry Pump Station, River Intake Pump Station, and five blind-bored, 128m-deep pump station shafts at the Hemphill Water Treatment site. Two more shafts, one riser shaft, and one drop shaft are also located along the tunnel alignment at the quarry site.

“The CMAR structure worked well because there were so many moving parts to the project and it allowed PC/Russell to really focus on the management function,” said

Del Nero. ATJV was responsible for the underground works schedule and PC/Russell was responsible for the overall schedule. “Approximately 50% of the contract value was for the tunnel, and 50% for service work.

ATJV holds 50% of the contract value, and PC/Russell the other 50%. A guaranteed maximum of 10% of PC/Russell’s contract value is allocated towards construction work. We wanted to keep them focused on the schedule and management functions, which they did well.”

Del Nero added that with the structure, everyone had a seat at the table and it sped up decision making rather than prolonging it. “Allowance items and contingency items were integrated into the contract. Contract amendments took weeks rather than months.”

Delivery on location

Tunnel work began promptly, per an aggressive schedule that required tunnelling to be complete by the end of 2018. Components for the 3.8m-diameter Robbins Main Beam TBM were shipped in truckloads to the jobsite in summer 2016 to be assembled using Onsite First Time Assembly (OFTA) with assistance from Robbins personnel.

Despite summer temperatures hitting 43°C and 100% humidity, the TBM was ready to launch by October 2016.

Overcoming the odds

Initial geological testing via approximately 30 shallow borings and 25 deep borings showed that the tunnel (at 120m+ deep) contained a wide range of rock strengths and that a schist/gneiss mélange would transition to feldspar, and finally a faulted section at the end of the tunnel in mylonite. The initial section of schist/ gneiss was so hard that it required swapping out of the probe drills. The switch-out reduced probe drill times to two hours from a high of eight hours.

The rock hardness challenged the 19-inch disc cutters from the outset. “There was ground so hard that it would sometimes take eight hours to go 1.5m. It was between 117 and 310 MPa UCS. The beginning of the job was tough,” said Weslowski, but he added that once the learning curve had been overcome “they started breaking project records left and right towards the end. We got a best day of 38.4m; rates just kept increasing.”

Excavation rates ramped up accordingly as ground conditions improved in the sections of feldspar and mylonite. Rates went from 14m per day in the first half of the tunnel to 21m per day in the second half of the tunnel and finally to 30m per day towards the end of tunnelling.

Those rates are based on two eight-hour shifts mining six days per week. The majority of the ground encountered was type A ground, requiring rock bolts at 1.2m centres. The remainder of the ground was supported by either rolled channels and wire mesh (type B) or full ring beams and wood lagging (type C).

“Our original goal was December 2018, and Halloween was our stretch goal. We were not expecting to hole through in early October. We got 183m during our best week,” said Jake Coibion, Tunnel Manager for Atkinson Construction.

Other challenges included groundwater encountered during tunnelling. “We did encounter groundwater contamination that required construction remediation. This remediation work was completed successfully,” said Huie. In late March crews encountered low levels of a type of benzene in a section of tunnel below an old chemical plant site from the 1950s and 60s.

While the concentrations were not at hazardous waste levels, construction remediation was carried out to ensure the safety of the construction staff. “We stopped the machine and took the crew through a 40-hour Hazardous Waste Operations and Emergency Response (HAZWOPER) training program. To enter the tunnel required tieback suits and gas masks, and much of this was in the summer. Crews worked in three-hour shifts as they couldn’t drink or use the restroom in the tunnel. The remediation was successful, we sealed off all contamination and the levels are currently undetectable,” said Coibion. Atmospheric monitoring remains in place in the 300m section, which ultimately took three months to complete. The section will be 100% concrete lined, with additional protection measures in place.

Shaft construction is now complete for all 11 shafts. Five 3m-diameter, 120m+ deep blind bore shafts were drilled from the bedrock surface to tunnel depth through 3.4m-diameter steel casings in the overburden.

Upon completion of the blind bore drilling, a 1.9m ID steel pipe casing with 25mm wall thickness was then lowered into the shaft and grouted in place.

Five 2.4m diameter adits with lengths ranging from 6m to 9m connect the shafts directly to the tunnel. The adits, constructed in heavily jointed rock, were built by blasting out a center berm followed by the perimeter in a delicate process. Adit transition zones are then lined with concrete forms.

By the end of October 2018, the TBM had been removed from the tunnel. “We will likely line somewhere between 55% to 65% of the tunnel with a 300mm thick concrete liner, making the i.d. 3m,” said Huie; discussions to determine the final percentage were active at the time of writing.

“Final lining is expected to be complete by late Autumn 2019. “We’re also extending the tunnel a further 600m using drill-and-blast from the Chattahoochee shaft to the raw water intake, and there is one additional raisebore shaft to be constructed at the intake site.”

Breaking barriers

Del Nero emphasised that the CMAR structure should be considered for more tunnel projects. “It has the built-in flexibility to be adapted to any tunnel project. For example, at Lake Mead (another CMAR tunnel) they needed a minimum of 25% of the CMAR entity’s work focused on construction. That change in self-performance from Atlanta is a major driver in the type of CMAR that pursues the project.

“At Atlanta WSP, we had three different construction sites and multiple trades, so we desired a general contractor acting as the CMAR with intense focus on cost and schedule management.

“We ultimately decided to let the tunnel guys (ATJV) focus on the tunnel. They know mining, that is their bread and butter.”

“The advantage is that the owner gets a team that works together closely,” said Huie of the CMAR structure. “The best way is to get the entire team working together in one location.

“We had 60 people working together at the beginning, from the owner, the contractor, and the designer. We were one team, and that promotes a common goal.”

Huie added that the tunnel represented PC Construction’s first deep rock tunnel project, and “we knew there could be a high level of risk with what you might encounter. Because we were working so closely together the owner, contractor, and designer could each take on certain risks and better control those risks.”

For the duration of the project the underground works Engineer-of Record, Stantec, assumed the design risk while the CMAR assumed construction risk.

As for the influential nature of the project, Del Nero is clear: “There are thousands of old, abandoned quarries in the U.S. alone. The use of quarries in our cities is going to become more in vogue as droughts increase in frequency.

Once cities take a look at the Atlanta Water Supply Program, they will see it is possible to increase emergency water supply by ten times or more.”