Sanitation District 1 (SD1) of Northern Kentucky broke ground in September on a 6-mi.-long (9.6 km) tunnel intended to convey sewage across western Boone County to a new treatment plant on the banks of the Ohio River. The gravity sewer project will route flow to a new 20 million gallon-per-day Western Regional Wastewater Reclamation Facility. When completed, ultimate peak hourly flow can reach as high as 177 million gal. per day (gpd), with average daily flow expected to initially be more in the realm of 10 million gpd.
As Jeff Olsen, vice president of HDR Engineering and project manager of this job, explained, the project will provide relief for an overburdened collection and treatment system and can be used to store up to 14 million gallons of excess water after heavy rains.
“There’s a significant wet weather component” to this sanitary sewer, he explained, adding that it is designed to accommodate wet weather issues and alleviate the burden on the Dry Creek treatment plant in Kenton County.
Other benefits include relief to existing interceptor sewers, with added capacity for future growth; operational simplicity, requiring minimal maintenance; and an energy-saving gravity design that eliminates the need for a pump station. This project, a requirement of the SD1 Consent Decree, will allow for the removal of six pump stations.
Not only is it the largest conveyance tunnel in the state, but it may also be the highest tunnel in Kentucky.
Aerial Tunnel
Most of the tunnel will be 300 ft. (91 m) below the surface, but one section will stretch 700 ft. (213 m) above a low valley.
“There are so many unique facets you don’t see on the average tunnel project,” Olsen remarked. “It’s the first tunnel that’s 30 feet in the air — intentionally.”
Because the end points were fixed and, according to Michael Vitale, project engineer and vice president, Midwest Tunnel Practice Lead, Hatch Mott MacDonald Inc. in Cleveland, Ohio, “the geometry conspired against us at Willoughby Creek,” they were left with four options: lower the tunnel in a vertical drop structure east of the creek, run to the plant in a deep, soft ground tunnel and pump up at the plant; lower the tunnel in a vertical drop structure east of the creek, run to the plant in a deep, soft ground tunnel and lower the plant elevation so they could enter at the appropriate grade without pumping; construct an open-cut crossing of the creek, following natural grade, resulting in a siphon under the creek; or construct an aerial crossing.
At one point, the tunnel was slated to be a pump station and open-cut force main, he explained.
“After exhaustive technical and present-worth comparisons of tunnel verses force main, the gravity tunnel was selected. After this, a pump at the end just made no sense and the long-term maintenance and cost made [the first option] unfavorable.”
As always, money played a role in the decision making.
“The cost of a pump station and maintenance led to the plan for tunnel conveyance and storage,” Olsen explained.
Budgeted for $109.4 million, the tunnel is being funded through bonds and other local resources. No state or federal money is being used, although the new treatment plant received some federal funding. Construction of the $69.2 million plant began in June and is expected to be operational in 2013.
Another option would have required a massive earth-moving operation to lower the plant and redesign it and the system hydraulics, plus the additional cost of a deep soft ground tunnel in difficult ground conditions. The siphon option was nixed by SD1 due to its concerns with silt and long-term operation and maintenance issues.
The aerial bridge option posed concerns too.
“HOBAS pipe was desired on the bridge due to light weight and corrosion resistance,” Vitale reported. “However, the saddles on the bridge [and the bridge itself] expand and contract differently than the HOBAS pipe.”
The design had to account for that to prevent the pipe from opening due to temperature swings.
“The HOBAS people tell us that this is the first time their pipe has been used in this type of application.”
An In-Depth Bore
In addition to the 700-ft. pipe bridge, the project includes 32,500 ft. (9,750 m) of 8.5-ft. (2.5 m) diameter pipe to be installed via traditional tunneling methods and 2,500 ft. (762 m) of open cut sewer installation. Tunnel boring will take one and a half years. Simultaneously, rail is being laid for a train to carry employees in and excavated “muck” out. The 8.5-ft diameter pipe will be installed after the tunneling is complete.
The tunnel’s path is sited in Kope Formation Shale, with layers of stronger limestone. While Olsen said it’s easy to bore through, Vitale noted that the shale has a tendency to swell and is highly slakable.
“In other words, if you drop a chunk in a bucket of water, it isn’t long before you have muddy water and no chunk.”
Rock type and depth are key parameters for this tunnel, Vitale indicated, as are rock strength, bedding thickness and the amount of fracturing.
“Gravity always wants to pull down the material in the crown. Thin bedding, together with joints, fractures, desiccation, etc., make the situation worse. When the tunnel gets so deep that the weight of material over the tunnel approaches or exceeds the rock strength, you can get stress-related issues that magnify this problem.”
In order to avoid additional work later, the mining and lining methods must account for this.
Therefore, Olsen said crews have to install initial tunnel support to keep it from crumbling and allow the pipe to go in. They will place steel rings with 4x4s at 5-ft. (1.5 m) intervals, giving it a “whisky barrel” appearance. Three layers of protection keep water where it’s supposed to be: tunnel support, concrete pipe with plastic lining and grout.
The tunnel will be lined with preformed concrete sections featuring a protective lining to prevent corrosion. The concrete tunnel is designed to last 100 years. The pipe, which was chosen with soil conditions in mind, will reduce the size of the excavated hole to an 8.5-ft. diameter. Vitale explained that an internal corrosion-resistant two-pass lining system is necessary because shale issues have a potential for methane and H2S, slaking, invert degradation, overbreak in the crown and elsewhere due to thin bedding coupled with relatively high stresses compared with rock strength and corrosion issues.
The tunnel will feature five shaft structures: one vortex draft structure, one flow control gate shaft and three access shafts. Vitale reported that some of the shafts would have required deep slurry walls or secant pile walls to construct, due to high groundwater and poor soils. During exploration, crews encountered a thin layer of sand under artesian pressure within the shale.
“This was very unexpected [and unusual] and led us to do supplemental geophysics from the surface in an attempt to rule out buried valleys or zones of saturated sand,” Vitale said.
By moving the shafts to areas of shallow soil and good rock, they avoided unnecessary cost and project risk.
Because of the distribution of limestone and other rock types in the shale, Vitale said it was a challenge to inform prospective bidders about what to expect.
“These layers were variable in thickness, strength and extent.”
McNally Kiewit, a joint venture out of Cleveland, Ohio, and Omaha, Neb., respectively, won the bid as general contractor. To get the massive job done, they are using a refurbished 12-ft.-diameter TBM. The choice of an existing machine owned by McNally Tunneling Corporation put crews ahead of schedule, Olsen pointed out.
“It saved us 6 to 9 months of lead time on building a new machine.”
It’s nearly new: 85 percent of the parts have been replaced since it was originally manufactured in 1969.
Powered by four 150 hp (112 kW) hydraulic engines, a cutterhead specifically designed for the soil conditions rotates clockwise 6.45 times per minute and can mine an average of 1,000 ft. (304 m) per week. The 65-ton computer- and laser-guided boring machine is 200 ft. (60 m) long and has 550,000 lbs. (250,000 kg) of maximum thrust capacity, 27 disc cutters and 8 muck buckets. It will cut the ground at an angle, filtering the dirt out of the tunnel via a conveyor belt on top of the machine.
The muck is being hauled via rail to fill a nearby vacant quarry. Because the boring machine arrived onsite early, Olsen said, some of the muck has been used for embankments on the hillside to support ramps for the aerial portion of the tunnel.
“Everything came together at the right time.”
Tunnel Vision
Several elements of the project fell into place. For the most part, right of way was easily obtained in this primarily rural area.
“Half the project area was owned by 3 or 4 people,” Olsen estimated.
For the most part, the tunnel skirts buildings.
“It was a design consideration not to go under existing structures.”
In addition to acquiring easements, there were several regulatory steps.
“The biggest holdup was to get approval of the Regional Facilities Plan Update,” Olsen said.
Plans were updated when the pump station was changed to the tunnel: local regulators weren’t familiar with tunnels.
Bids were expected in the $130 million range, Olsen recalled. Typically, they come in high, but on this project, they came in under. Contractors were selected based on qualifications for this unique project. HDR Engineering is responsible for overall project management, surveying, open cut design and site work. They, along with the design team of Hatch Mott MacDonald, CH2MHill and Thelen Associates Inc., began planning in 2005.
Beginning preparations that early was all about minimizing surprises, Olsen said, which is why they launched a thorough geotechnical investigation. Vitale agree that a geotechnical program is critical, along with early identification of risks, including operational, environmental and more.
“Tunnel projects are all about managing risks,” Vitale emphasized, “even more so than above-ground construction, since the naturally occurring underground has no codes and no uniform or reliable materials like steel or concrete, not to mention groundwater, gas and other unknowns. Driving a tunnel based on widely spaced borings is a bit like driving down the freeway with a blindfold and only opening your eyes every 15 seconds.”
Although groundwater is not expected to be a serious concern on this project, risk identification is ongoing because water flow is “always a wild card in a rock tunnel design.” The important thing, Vitale said, is to make sure to have the right order of magnitude, select tunnel methods and lining systems to cope and produce a fair baseline for estimates.
The entire length of the tunnel must be dug before any pipe is laid, but pipe is already being prepared by a Hanson plant for installation.
“All Hanson is doing right now is pipe for this project,” Olsen said.
Once again, things fell into place: the plant is just north of Cincinnati, so transportation is quick and inexpensive.
Work onsite began June 1 and by the end of August, the bore machine was in the ground; its first cut completed by Aug. 24 and its initial 100-ft. mining drive through shaft 5 was done on Sept. 10.
Anticipated completion date is November 2012. Weather has been great for surface work, Olsen reported. Crews of approximately 20 have been working around the clock in two shifts. They drill in two 8-hour shifts, reserving the third shift for machine maintenance. They will continue to work through the winter because “the tunnel is a critical path.” CEG
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