Digging Deep

The Louisville Water Company (LWC) celebrated its 150th anniversary in 2010, a history that includes a track record of innovation. Prominent water works professionals George Warren Fuller and Charles Hermany helped advance the state of the art while working in Louisville, Ky., with advances in coagulation and rapid sand filtration.

It is appropriate then, that LWC?s 150th anniversary also marks the beginning of its latest innovative project ? a first-of-its-kind combination of collector well technology coupled with a hard rock tunnel connector designed to meet regulations of the Safe Drinking Water Act (including the Long-Term 2 Enhanced Surface Water Treatment Rule and Stage 2 Disinfection/Disinfection Byproduct Rule ? both of which are scheduled to take effect in 2012). Dedicated in December 2010, the Riverbank Filtration Tunnel and Pump Station is a $55 million project that uses natural riverbank filtration to deliver high-quality drinking water to customers.
The riverbank filtration process involves capturing raw water from an aquifer below the layer of sands and gravels underneath the Ohio River. As water seeps through these layers, it is naturally filtered of particles and contaminants from the raw river water, including sediment, pathogens and organic chemicals. Because the aquifer is supplied by the river above it, the aquifer is continually recharged.

LWC is using collector well technology ? a vertical shaft with horizontal wellscreen laterals radiating outward ? to draw water from the aquifer.? Collector wells ? also referred to as Ranney wells after the inventor of the technology, Leo Ranney ?? have been around since the 1920s. Typically, each well is equipped with a pump and piping to convey captured water. In this case, LWC used a hard rock tunnel to connect multiple wells and convey collected water by gravity to a single extraction shaft located at its B.E. Payne water treatment plant.

The Riverbank Filtration Tunnel and Pump Station construction contract was bid in November 2006 and went online in December 2010. Officials expect the project to meet its goals of regulatory compliance, but they also expect a number of secondary benefits that include system redundancy, improved security, consistent high quality? water. Protection from spills on the Ohio River and a more stable temperature of water in the system, which will reduce the number of water main breaks.

Project Background

LWC officials began investigating the use of riverbank filtration as far back as the 1940s. At that time, LWC commissioned the U.S. Geologic Survey (USGS) to study the aquifer, and the conclusions were profound. USGS identified the link between the aquifer and the river, specifically, that as the water level in the river rose, so too did the groundwater table, and vice versa. At that time it was estimated that 280 million gallons a day (mgd) of naturally filtered river water was available in the aquifer, according to Kay Ball, LWC program manager for the Riverbank Filtration Tunnel and Pump Station project. However, the technology at the time was limited, making access and extraction of the water infeasible, Ball said.

LWC revisited the idea in the 1970s as regulations were introduced involving microbials and disinfection byproducts. The Milwaukee cryptosporidium outbreak of 1993 renewed focus on the idea, leading LWC to implement a phased approach to riverbank filtration.

?The Milwaukee outbreak was something that made water utilities across the country stand up and take notice,? Ball said. ?At that time it was believed that filtering and disinfection was adequate to provide safe water, but that crypto outbreak showed that wasn?t the case and that advanced treatment was needed.?
LWC?s response was to implement a phased approach to implement advanced treatment using riverbank filtration. LWC received board approval in 1995 to move forward with the riverbank filtration program. The first phase was construction of a demonstration collector well? which was completed in 1999. The demonstration well was a typical Ranney collector well that included a separate pump station located 60 ft above the ground to stay above the flood plain.

Since going into service, the demonstration well has pumped an average of 17 mgd from the aquifer. The well proved the viability of riverbank filtration in Louisville and convinced LWC officials to move forward with the second phase of the program, which involved the construction of numerous wells along the river to provide sufficient capacity for its nearly 800,000 customers in the metropolitan area.

Original plans called for the construction of 15 to 20 additional collector wells incorporating traditional above-ground pumps and connected via shallow pipeline. The decision to use collector wells vs. standard vertical wells allowed LWC to increase capacity of each well while maintaining the same footprint. This is a result of the horizontal laterals providing a greater square footage of wellscreen vs. a vertical well.

Although LWC made efforts to design the collector wells and elevated pump stations to be aesthetically pleasing, public outcry led planners to come up with an alternate solution. ?The pump stations were designed to be attractive with brick and wrought iron railing, but it became very apparent that the public was not going to accept it. River Road is part of a National Scenic Byway with historic homes along the river; it didn?t matter what the pump stations looked like, there was to be no above-ground structures.?

Going Underground

Since above-ground pump stations were out of the question, LWC needed to find another solution. ?That?s where the concept of a tunnel came into play,? Ball said. As officials began looking at the tunnel option, other benefits became apparent. By using a deep tunnel to collect the water, gravity can be used to convey water, eliminating the need for separate pump stations for each well. In addition to eliminating above-ground structures, the new plan allowed LWC to build the pump station on the site of its existing plant, minimizing impact on the public while providing an extra degree of security by locating it within a secure fenced area and above the floodplain.

The final vision for the Riverbank Filtration Tunnel and Pump Station project included four collector wells connected to a 7,800 foot tunnel 150 ft below the ground in the bedrock and a pump station to deliver the required 60 million gallons a day to the plant

The water flows by gravity from the river , through the natural sands and gravels into the wellscreen laterals and then into a central caisson to the tunnel. From there the water travels along? the tunnel and is pumped from one common pump station at the end of the tunnel located within the secured area of the existing plant.?? The design allows the wells to be capped at ground level so there are no above-ground structures located along the riverbank.? ?

Jordan Jones & Goulding (now Jacobs Engineering) was selected as the consulting engineer to perform the geotechnical evaluation and design. The project was required to address several issues that were unique to the gravity flow tunnel concept. The type of wells (vertical or horizontal collector wells), the type of tunnel (hard rock or soft soil in the saturated aquifer) and the connection between the collector well and the tunnel needed to be evaluated.? ?

In advance of bidding, LWC and its consultant conducted a thorough subsurface investigation program that included 43 vertical borings to characterize the ground. This information helped in the planning and design process, and? the geotechnical baseline report was made available to prospective bidders to help reduce construction risk.

The investigation showed that the bedrock would be conducive to tunneling. After a value engineering exercise, LWC decreased the minimum diameter of the planned tunnel to 10 ft ID, which allowed for a more competitive bid based in part on the availability of hard-rock tunnel boring machines in that diameter, as well as contractor experience.

Collector wells were chosen over vertical wells based in part on the fact that the larger diameter of the collector wells posed less risk in making connection to the deep collector tunnel and the ability to secure the wells. Hydrogeology consultant Dave Schafer & Associates helped determine the number and spacing of the collector wells.

The project was bid in November 2006 and a design-bid-build contract awarded to Mole Constructors of Beachwood, Ohio, for a bid of $34 million. Notice-to-proceed was issued in March 2007, and the project dedicated in December 2010.

The construction shaft, constructed by Bencor Corp. of America, consisted of a 40-ft diameter slurry wall construction that extends through 100 ft of the sands and gravel and is socketed about 15 ft into the bedrock. This method was selected because of the slope of the bedrock below the aquifer at the construction shaft location. The remainder of the shaft in bedrock was reduced to 25 ft ID and constructed by traditional drill-and-blast for the total construction depth of approximately 200 ft.

The 7,800-ft long tunnel was mined by a main-beam tunnel boring machine (TBM) manufactured by The Robbins Company of Solon, Ohio. The TBM excavated the 12-ft diameter tunnel through the shale layers interbedded in the limestone bedrock. Since the shale would be expected to deteriorate over time if exposed to water, a concrete lining was specified. The lining consisted of unreinforced concrete, poured in-place, resulting in a finished 10-ft diameter tunnel. The TBM averaged about 75 ft per day and the tunnel was mined in about eight months.

The collector wells, constructed by Ranney Collector Wells, consist of a 13-ft ID concrete caisson that extends approximately 100 ft to bedrock. Eight 12-in. stainless steel wellscreen laterals are jacked from the caisson extending into the sands and gravel of the aquifer.

The pump station, constructed by Reynolds Inc., consists of two 20-mgd pumps, one 15-mgd pump and one 10-mgd pump for a total pumping capacity of 65 mgd. The pump station is installed atop the construction shaft with the shaft now acting as the shaft for the pumps.

Looking Ahead

The Riverbank Filtration Tunnel and Pump Station project was dedicated Dec. 10, 2010, capping another innovation in the rich history of the Louisville Water Company. ?We are really proud of this project because we think it is another advancement in the science of treating water,? said Greg Heitzman, president of LWC. ?We paid a premium over more conventional methods, but we are getting a much longer-lasting solution with secondary benefits that we would not have gotten with a project with a lower cost.?

Some of the benefits include preserving the Ohio River viewscape, increased security and high-quality raw water that will help LWC meet drinking water requirements. Additionally, because raw water is being pulled from the underground instead of directly from the water, water temperature will be more consistent.

?Studies have shown that water mains fail at an increasing rate when the water temperature drops below 40 F,? Heitzman said. ?The riverbank water will help us achieve a more stable temperature, and we are anxious to see the impact it has on water main failures.?

The one-of-a-kind approach has garnered the attention of water officials from across the country and internationally, who have come to Louisville to see the project first-hand. Additionally, the project is one of five finalists for the American Society for Civil Engineers? Outstanding Civil Engineering Achievement award, which will be announced on March 31.

?The Riverbank Filtration Tunnel and Pump Station involved a partnership that required all parties to design and build a complex project,? Heitzman said. ?It allows us to deliver a higher-quality product to our customers and achieve reliability for the future as we grow.?

Jim Rush is editor of UIM.

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