Unusual Tunnel Project - Flooding Waters, Special Snorkel
Unusual Tunnel Project - Flooding Waters, Special SnorkelNancy Creek Tunnel overcomes flooding, reaps efficiency with Putzmeister Snorkel
STURTEVANT, Wis. (February 2, 2005) – Over the past few decades, Atlanta and its neighboring communities have seen its population increase at a rapid rate. However the growth of the city’s water and sewer system has not kept pace. As a result, a large-scale upgrade program, which includes constructing deep, large-diameter tunnels, is in progress.
One significant project within the program is the Nancy Creek Tunnel. The new 8-1/4 mile long sewer tunnel is being constructed collaboratively by the City of Atlanta, DeKalb County and Fulton County. The project is intended to reduce the number of sanitary sewer overflows within surrounding communities.
The $131 million project, which includes the construction of a 100 million gallon per day pump station, is being built via the general contractor Nancy Creek Constructors – a joint venture between Tokyo-based Obayashi Corp. and CJB Inc. of Atlanta. It primarily consists of constructing a 16-ft diameter tunnel over 43,700 linear feet long in addition to three construction access shafts and eight intake structures. The intake structures will be used to divert the flow from shallow sewers down to the tunnel.
The project also includes 2,200 linear feet of small-diameter tunnel using drill-blast that will connect the main tunnel to the intakes. These connecting tunnels will be lined with 54- and 60-inch reinforced concrete pipe.
Upon project completion, an estimated 110,000 cubic yards of concrete will have been consumed. The first of which was pumped for the development of three construction shafts. These entry shafts, at 32-ft in diameter and over 150-ft deep, enable access to the tunnel by people and equipment.
To pump the deep shafts, Pioneer Concrete Pumping Services of Atlanta, Georgia utilized their larger 55- and 58-Meter truck-mounted concrete pumps that were needed for their longer reach. Having started in the spring of 2004, it took approximately three months to pump 4800 cubic yards of concrete for the final completion of all three shafts.
Crews then mined the tunnel to 300-ft depths beneath the surface in two drives, using two 18-ft, 4-in diameter tunnel boring machines needed to drill a perfectly circular tunnel shape. Underground, specialty machines are now placing a one-foot layer of concrete as a lining around the tunnel.
Ray Hutton, Project Engineer for Nancy Creek Constructors, was responsible for purchasing the concrete placing equipment for use within the tunnel. He chose Putzmeister products, including two complete set-ups consisting of a BSA 2109 H-E concrete pump, JT 10000 jumbo-sized hopper, and a Snorkel TSV 3-6-150. The models were selected for their unique performance and efficiency characteristics in combination with feasible pricing parameters demanded by the job.
First, the high-pressure BSA 2109 H-E concrete pump was equipped with electric power to avoid exhaust fumes, and it was specially configured for mounting on rails. Although the unit is capable of outputs up to 125 cubic yards an hour (95m3/hr), it was needed more for its high pressure capabilities up to 1390-psi (91 bar) – rod side.
Secondly, the JT 10000 jumbo-sized hopper – with a huge 13 cubic yard (10m3) capacity – is being effectively utilized to receive, store and mix concrete for continuous feeding to the pump between lengthy concrete deliveries. The re-mixing hopper features sloped sides, rubber lined hoppers, and twin augers, which offer variable speed and bi-directional operation for efficiently mixing and feeding the pump. Concrete can be fed to the pump at any speed from 0 to 125 yards per hour to accommodate the varying cycle time between concrete loads.
Finally, unusual to the job was a Snorkel tunnel distributor – a high-speed concrete placing system capable of placing concrete within a full 360-degree radial pattern. Although there are only a handful found on North American projects, the Snorkel offers greater efficiency and is ideal for better operator control in more precisely placing concrete within the tunnel lining forms. This is in contrast to the typical approach of just letting gravity control the concrete flow.
Featured with a concrete pipe and flexible hose, the Snorkel hydraulically extends and retracts to match up to a fitting on the form, which then enables concrete placement via its link to the concrete pump. The unit’s self-elevating “operator’s platform” allows a worker to easily couple the snorkel’s concrete pipe to these form openings.
To line a 16-ft finished full circle tunnel, the contractor is using 240 linear feet of jump form in 30-ft sections. To begin the process, the Snorkel is first coupled directly to the lower half of the form to place concrete from the 4 to 8 o’clock position. Next, it is swiveled to the 10 o’clock and then the 2 o’clock positions for concrete placement. Finally, the snorkel goes to the 12 o’clock position and the top of the circle is completed.
The Snorkel TSV 3-6-150 is capable of working in tunnels from a minimum 10-ft (3m) to maximum 20-ft (6m) diameter. It is powered via a hydraulic power pack and rides on rails.
With this combination of concrete placing equipment, the crew averages over 2.3 cubic yards of concrete per lineal foot in the tunnel, placing at outputs over 80 cubic yards an hour. By working three eight-hour shifts, the goal is to place 210 linear foot a day.
Commenting on the concrete placing equipment, Ray Hutton said, “Obviously with a tunneling project of this size, there was absolutely no other way to place concrete as efficiently.”
Challenges were abundant with the job. Primarily within a residential area, the project had to address and take important steps to minimize noise and disruption. Then, just after the first pour in mid September of 2004, Hurricane Ivan reeked havoc by flooding the tunnel.
Unfortunately what nature had flooded in a mere eight hours took crews ten days to pump out. Over 50 million gallons of water were drawn out of the initially started three-mile long tunnel. Fortunately, it was a very hard rock tunnel – with mylonite of 30,000 to 50,000 psi that’s ten times harder than concrete.
Consequently, the Putzmeister equipment – Snorkel, concrete pump and jumbo-sized hopper – were all sent to the manufacturer’s North American facility in Wisconsin to be “de-watered”. To make the units operational again, employees rebuilt motors, replaced control panels and electrical boxes, drained hydraulic fluid lines, cleaned filters, and basically purged the overall systems to eliminate water and other contaminants.
Once the equipment was back in action, Allied Ready-Mix of Atlanta again started discharging concrete from its ready-mix trucks into a drop hole adjacent to the construction shaft. A hopper above the shaft funneled the concrete down 150-ft where a remixing chamber was positioned to prevent the concrete from hitting so forcefully at the end of the shaft. The concrete went into a train of three 12-yard Moran cars, which are theoretically concrete mixer drums on a train rail.
To maintain a steady cycle, one Moran train would be loading concrete, another train would be in transit, while yet another would be unloading into the jumbo hopper for storing and remixing before being gradually discharged to the pump for continuous concrete placement into the forms. A California switch could divert a car off the track, which assisted with using three trains for a total of 36 cubic yards per cycle.
According to Jamie Bonner, Project Manager for Nancy Creek Constructors, “In theory, the process sounds rather simple, but to maintain a continuous flow of concrete via Moran cars is a complicated task.”
According to Bob Liebermann, Specialty Products Manager for Putzmeister America, “In tunneling, it’s a notably noisy and echoing environment and wireless communications are not available underground. That’s one reason why coordination is so difficult and why equipment reliability plays a paramount role within these underground settings.”
To further complicate the equation is the unusual concrete mix design. Bonner noted, “The concrete is a very sophisticated mix, specifically designed for long tunnel life. With its high slump and highly abrasive nature, it’s not the easiest to pump and extremely hard on equipment. It’s crucial to maintain a steady concrete flow throughout the entire process so the slickline doesn’t get plugged.”
Construction of the tunnel began in July 2002. The first drive from the Clayton to Roswell shaft is 26,212 ft, and the second drive from the Roswell to Johnson Ferry shaft is 16,457 ft. The project is approximately 80% complete as of early February, and a completion date of January 2006 is anticipated.
General contractor: Nancy Creek Constructors –
A joint venture of Obayashi Corp. (Tokyo, Japan) and CJB Inc. (Atlanta, GA)
Pumping contractor: Pioneer Concrete Pumping – Smyrna, GA
Ready-mix supplier: RMC Allied – Atlanta, GA
Equipment: Two complete setups of Putzmeister BSA 2109 H-E high-pressure concrete pump, Putzmeister JT 10000 jumbo-sized hopper, and a Putzmeister Snorkel TSV 3-6-150