Under Control

The City of Hot Springs, Ark., is known as “America’s First Resort Town” and anyone who has visited there knows it is a city with immense scenery and history. And as is usually the case, any city with a “history” usually is also experiencing an aging infrastructure in need of repair. Such is the case with the City of Hot Springs as it entered into a Consent Administrative Order in 2008.

Since that time, the City has responded with an aggressive campaign to address infrastructure problems within the sanitary sewer system that had led to excessive inflow and infiltration (I/I) problems, which were directly tied to sanitary sewer overflows (SSOs).

The City consists of a complex sanitary sewer system that conveys flow to two treatment plants. Part of that complexity is the extensive pressurized sewer system that exists and how it interacts along with the gravity system. The system consists of approximately 2.2 million ft of gravity sewer along with 1.1 million ft of force mains, which receives its flow from approximately 130 pump stations and 3,200 grinder pumps.

Positive Dye Test

Study Phase

Beginning in 2009, the City of Hot Springs retained RJN Group Inc. (RJN) to conduct an extensive sanitary sewer evaluation survey (SSES) of the existing system. This consisted of city-wide flow monitoring (divided into 65 sub-basins), manhole inspections for all 12,000-plus manholes and smoke testing of approximately 85 percent of the gravity system across the City. In addition, a hydraulic model was constructed and calibrated to identify capacity constraints. Along with the aforementioned study tasks, evaluations of each pump station were conducted.

During the smoke testing of the gravity system, a couple of direct connections to the storm system were uncovered and were immediately disconnected. However, there was a much greater effect of overall I/I entering the system from multiple indirect cross connections to the storm sewers. Indirect cross connections refers to runoff that enters the storm system, leaves the storm system through faulty joints or pipe segments and enters the sanitary system below through aging pipe with multiple defects.

Each of these positive cross connections were then dye-tested by flooding the storm sewer with dyed water and observing the downstream sanitary manhole for the dyed water. Quantifications were taken of each defect by measuring the depth and velocity before the test was conducted and then calculating a flow after the dyed water entered the sanitary system. This allowed RJN to quantify how much inflow was entering the system for each confirmed direct or indirect cross connection across the City. In addition to dye testing cross connections, main line defects of the gravity system were also dye tested. In total, 192 dyed water flooding tests were performed. An example of a positive dye test identified during the flooding is shown in Figure 1.
The known quantification of each public defect to the sanitary system allowed for the prioritization of sewer lines to undergo rehabilitation or replacement and make the biggest overall impact to the system in regards to I/I removal.

Design and Construction Phase

After the evaluation of the sanitary system had been completed, the City of Hot Springs underwent strategic planning of how best to tackle the I/I issues along with capacity constraints across the City to help remove the SSOs. Through two aggressive bond initiatives, the City and its consultants have laid out a plan and have begun to implement that plan to meet the mandates of the Consent Administrative Order (CAO) by 2018 for wet weather overflows.

Trenchless Method of Pipe Bursting

The City continues to make progress in its concerted effort to remove I/I from the system. Along with the I/I infrastructure projects the City has undergone major pump station upgrades along with capacity enhancements across its collection system. Approximately 16,000 lf of sewer pipe has been replaced or rehabbed from projects designed by RJN, most of which were directly tied to being the highest contributors of I/I across the collection system.

The focus of the design on sewer line rehabilitation projects was to use trenchless construction methods wherever possible and specific focus was given to pipe bursting technologies for several different reasons.
One such reason was that a rather high percentage of lines that were high contributors of I/I (approximately 25 percent) were 6-in. diameter lines. Therefore, the decision was made to increase these lines to 8-in. diameter lines by using enlargement applications from pipe bursting. In addition, many of these “high contributing” lines were located in areas affected by traffic concerns, and the majority of the lines were fairly deep. The overall depth of the lines would have increased the cost of open cut replacement through extra excavation, shoring and possible dewatering.

Another design component implemented was the installation of clay dams on the downstream end of the segment or series of segments to mitigate the migration of groundwaters to segments further downstream that were not part of the construction contract. In many cases where clay dams are not used to stop the migration of groundwater, the existing trench can act as a French drain and simply move the location of where I/I enters the sanitary system.

Along with pipe bursting a smaller portion of lines (approximately 12 percent) were rehabilitated using cured-in-place (CIPP) technologies. This application was applied where excavations for entry or exit pits for pipe bursting were not applicable to their existing locations. Additionally, these segments had favorable existing grade and the overall condition of the host pipe was in fair shape. Most of these lines experienced multiple leaking joints or cracks within the pipe, which were contributing high amounts of I/I.

Heller Co. Inc., the construction contractor for the project, used a pneumatic system for the pipe bursting in the City. In a pneumatic system the bursting head is powered by compressed air. An expander was situated at the front of the pneumatic displacement hammer and was launched into the host pipe through an entry pit. The head is connected to a constant tension winch located at the receiving pit, which pushes the new pipe into place on the same alignment as the existing host pipe, while the host pipe is broken and displaced into the soil surrounding the new high density polyethylene (HDPE) pipe. A diagram of the pneumatic bursting operation and a picture of the process is shown in Figures 2 and 3.

Pneumatic Bursting Operation Results

As mentioned previously, the first step of the study phase was city-wide flow monitoring conducted in 2009. This established flow rates for 65 sub-basins across the collection system area for both dry- and wet-weather flows. Peaking factors along with I/I rates were developed for each sub-basin. This provided a baseline of established flows to compare to post rehabilitation flows after construction projects were completed.

The aforementioned sewer line project began in 2011 along with a manhole rehabilitation project awarded to Kim Construction Co. Both projects addressed I/I defects identified during the study phase and the two projects overlapped each other in 12 of the 65 basins. Both rehabilitation projects only addressed public defects, however all known private defects were documented during the study. Once construction had been completed for both projects, RJN conducted post-rehabilitation flow monitoring in the spring 2013.

Results identified an overall 22 percent reduction in peak inflow rates, along with a 32 percent overall reduction in peak infiltration rates for the twelve basins. Along with these results, several basins identified lower dry weather flows, which could be the result of a reduction in permanent infiltration entering the system.
Additionally, the main measuring stick is the reduction of SSOs resulting from I/I entering the system from these 12 basins. Repetitive overflows previously resulting from 1 in. or less rain events have since been observed to be contained within the collection system for rain events exceeding 2 in.

Daniel Jackson, P.E., is project manager at RJN Group Inc.
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