Dewatering - Time and Cost Relief by way of Trenchless Construction

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1 North American Society for Trenchless Technology (NASTT) NASTT s 2015 No-Dig Show Denver, Colorado March 15-19, 2015 TA-T3-04 Dewatering - Time and Cost Relief by way of Trenchless Construction Jenn Stillman, P.E, HDR Inc., Denver, CO Benny Siljenberg, P.E., Brierley Associates, Denver, CO 1. ABSTRACT In the great state of Colorado, water quality is paramount due to limited availability and increasing demand, which leads the State to protect the limited resource to the highest degree. As a result of recent construction dewatering regulations (July, 2013), many owners, contractors and engineers have encountered new, unforeseen challenges posed by dewatering discharge limitations related to water quality. The challenge created by encountering groundwater that does not meet the discharge water quality limitations can cause significant cost and schedule impacts, particularly for open cut pipeline projects. The goal of this paper is present an overview of new considerations related to construction dewatering and remediation in Colorado under the new regulations, and to provide a case example where trenchless construction was selected as the most effective alternative to overcome dewatering obstacles. This paper will: Convey awareness of the recent changes to dewatering discharge regulations and potential remediation actions that are available when contaminated groundwater is encountered. Cite a seven-mile sanitary sewer pipeline construction project along the South Platte River and discuss how construction dewatering regulations resulted in the owner s decision to implement two trenchless construction reaches of the pipeline that were originally designed as open-cut segments. Provide a cost and time savings discussion of the trenchless vs. open-cut with treatment options for the above mentioned project to address groundwater contamination concerns. These options included groundwater treatment, trenchless construction, and alternative permitting avenues. Paper TA-T

2 2. Introduction The Colorado Department of Public Health and Environment (CDPHE) recently renewed the general permit requirement for discharging construction water. A major change from the last renewal included specifically authorizing the discharge of groundwater, surface water, and storm water that has come into contact with construction activities. A focus of the renewed permit is to have the project owners perform due diligence in the form of a paper study for the project site prior to dewatering efforts. The intent of this required study is to identify all sources of potential groundwater contamination, including leaking underground storage tanks, open voluntary cleanup sites, environmental covenants or open resource conservation recovery act corrective sites within a half mile of the project site and any superfund sites or national priorities list sites within a one mile radius of the project site. As further discussed in the conclusion of this paper, it is important to only perform testing of the required/suspected potential pollutants. This information will provide the project team with knowledge of the site s history and how it may affect construction dewatering discharge. The data will help CDPHE decide which pollutants need to be tested prior to issuing the discharge permit. The level of pollutants or natural occurring minerals will dictate which permit will need to be issued, either the construction dewatering permit that allows for direct discharge to a water source or a remediation dewatering permit that requires some type of mitigation prior to discharging. The example project provides a case where the source water contained a higher concentration of naturally occurring iron and manganese than the receiving water. In one instance along the pipeline alignment, the discovery was made prior to applying for the discharge permit and consequently, a segment of the pipeline alignment was changed from open-cut construction to trenchless construction while the project was out to bid. In a second instance along the pipeline alignment, the high iron and manganese was discovered during active construction and dewatering. This segment of the pipeline alignment was also changed from open-cut construction to trenchless construction via a change order to the contractor. 3. Example Project In order to convey flows to the Metro Wastewater Reclamation District s new wastewater treatment plant, a new gravity interceptor pipeline was required to collect flows from seven of the District s member agencies and route them to the new wastewater plant located in Brighton, CO. The 7.2-mile interceptor is referred to as the South Platte Interceptor (SPI) and generally follows the South Platte River, crossing it once, as well as crossing under three major roadways and through three existing slurry wall-lined gravel pit reservoirs, see figure 1. The project included seven tunnels that incorporated microtunneling, open face TBM with ribs and boards, open face TBM with jacked pipe and hand mining techniques. The longest of the trenchless installations was over 3,700 feet with an excavated diameter of 78 inches. The largest diameter tunnel is near the northern end of the pipeline with an excavated diameter of 96 inches. The project also included a 992-foot, 66-inch diameter microtunnel under the South Platte River and construction of over 5,700 feet of new slurry wall. Due to groundwater contamination, two segments of the pipeline were changed from planned open-cut construction to trenchless construction (the South Platter River tunnel and the E. 168 th Ave. tunnel). This paper will discuss the E.168 th tunnel to illustrate the benefit of trenchless construction used to mitigate time and cost impacts associated with groundwater contamination. Paper TA-T

3 Figure 1 Project Overview 4. E. 168 th tunnel Construction of the E. 168 th Ave. tunnel began in the fall of Pipeline installation commenced on the north end of the alignment, just south of 168 th Ave. A dewatering well system, consisting of a series of 15 wells spaced at 50 feet apart, was installed at this location and placed into operation under the project s general construction dewatering permit with the CDPHE. Approximately 235 feet of pipeline was installed before construction had to be shut-down at this location on November 22, 2013, due to high levels of iron and manganese discovered in the dewatering discharge to the South Platte River (river). The contamination was detected when a red/orange plume was observed on the river bottom at the location of the dewatering discharge. Although the iron and manganese are most likely naturallyoccurring, the concentrations exceed the in-stream standards for the river. Table 1 summarizes the iron and manganese concentrations anticipated in the contaminated groundwater zone and the in-stream standards for the river. Table 1.Iron and Manganese In-Stream Standards and Sampled Groundwater Concentrations Paper TA-T

4 Average Concentration in Groundwater (1) Water Quality Parameter In Stream Standard for SPI Tunnel Shaft north Segment 15 of River Alignment south of 168 th of 168th Ave. (3) Ave. (2) Total Recoverable Iron (mg/l) Dissolved Iron (mg/l) Dissolved Manganese (mg/l) Notes: (1) Sampling completed in January of 2014 by pipeline contractor. (2) Average value from samples collected from each of the 15 dewatering wells along the SPI alignment south of the tunnel shaft on the south side of 168th Ave. (3) Average value from samples collected from 4 of the 6 dewatering wells in the vicinity of the tunnel shaft on the north side of 168th Ave. 5. Options to Address the Groundwater Contamination Several options were investigated by the project team including: ground freezing techniques, soil-bentonite slurry wall construction, store and release, and treatment. After much discussion, four options were developed as follows: Option A: Tunnel the entire segment of the SPI alignment in the contaminated zone and use water-tight tunnel shafts. No dewatering discharge would be needed to the river and nuisance water from the tunnel shafts would be hauled to an approved offsite location for disposal. Option B: Tunnel a portion of the SPI alignment in the contaminated zone and use open- cut construction for the remaining portion. A temporary water treatment system would be used to remove iron and manganese from the dewatering discharge prior to discharge to the river for the open-cut work. Option C: Construct the SPI in the contaminated zone using the original design with open-cut construction and discharge dewatering flows to the City of Thornton s (Thornton) Tucson gravel pit on the west side of the river. The pit would serve as a detention basin for iron/manganese removal. Option D: Obtain a site specific standard for iron and manganese from the CDPHE, which would allow the dewatering flows to be discharged to the river at concentrations exceeding the stream standard. This approval is based on demonstration (by the District) that the iron and manganese concentrations are not detrimental to the next downstream user on the river, which is the City of Aurora s (Aurora) Prairie Waters well field. The following sections will describe Options A and B in greater detail as Options C and D were ruled out from further evaluation. 6. Further discussion of Options A & B Option A Tunneling Under this option, the entire SPI in the contaminated groundwater zone will be installed by a microtunneling boring machine (MTBM), requiring two tunnels and three tunnel shafts. The 50 foot open-cut connection to the Northern Treatment Plant (NTP) will be eliminated by placing the northern tunnel shaft at the NTP connection point (STA 0+00). From this point, the tunnel alignment will be offset from the original alignment by roughly 20 feet to the east in order to avoid the 235 ft of existing 78-inch SPI that has already been installed. Two water- Paper TA-T

5 tight shafts will be required at the new MH 4 (STA 0+00) and the new MH 5 (STA to be determined). A third shaft will be required at the new MH 6 (STA to be determined), however, this shaft does not need to be constructed water-tight as the dewatering in this area will connect to a dewatering zone that is covered by the project s general dewatering discharge permit. Although the shafts are considered to be water-tight, some nuisance groundwater will need to be removed from the shafts and disposed. Due to the iron and manganese concentrations, direct discharge to the river at the site is not viable for this option. It is anticipated that this nuisance water will be collected in water trucks and hauled to either the District s Hite Facility or another approved facility for disposal. The volume of water to be hauled is estimated between 50,000 and 200,000 gallons. Option B Treatment Under this option, the northern end of the SPI in the contaminated zone will be tunneled from the NTP connection point (STA 0+00) to the new MH 5A (approximate STA 2+50) using water-tight shafts. From this point, the remainder of the SPI up to MH 6 will be installed via open cut construction methods on the original SPI alignment. The open cut portion of the work will require dewatering with wells. In order to remove iron and manganese from the dewatering flows, a temporary water treatment system will be established on site. Although multiple treatment technologies are available, the project team has selected mixed media filtration (MMF) as the basis for this evaluation. A conceptual level MMF treatment design has been provided by the Contractor through their subconsultants. The treatment system will consist of pre-treatment frac tanks for aeration and sodium hypochlorite dosing, five trains of mixed media filters for iron and manganese removal, and post-treatment frac tanks for dechlorination and ph adjustment. The design flow rate for the treatment system is 3,000 gallons per minute (gpm), which is based on flow estimates from the period of time when the dewatering well system was in operation last fall. This option will require that the contractor obtain a remediation permit from CDPHE for the treatment system and dewatering discharge to the river. 7. Cost Savings The project team worked closely with the contractor to develop a conceptual level cost estimates for Options A and B. A list of cost items was developed that is generally categorized as follows: cost credits back to the District, costs expended to date, and forecasted costs. Descriptions of these categories are provided below. Cost credits back to the District - Represented work that was included in the contract that will not be needed under either Option A or B. Costs expended to date - Represented work that was not included in the contract, but the cost of the work was already been incurred by the contractor due to the groundwater contamination. Forecasted costs - Represented work that was not included in the contract, but needed to implement either Option A or B. The cost savings that were realized by utilizing the trenchless method instead of the treatment option were on the order of $150,000. Considerable time savings were appreciated as it is very timely to mobilize a treatment program as opposed to utilize the trenchless contractor s tools that were already on site 8. Conclusion Paper TA-T

6 The District chose the trenchless option as the most economical and time saving method to handle the contaminated groundwater. This project microtunnel installation of the pipe will begin in January For future projects that need to perform dewatering activities, the project team should rely on environmental consultants to inform them if there is reason to suspect groundwater contamination. If there is no reason to suspect groundwater contamination, the project team should only test the groundwater for what is required by the CDPHE. If the CDPHE receives higher than allowable test results for pollutants or minerals that are not required to be tested by the permit application, the CDPHE must enforce remedial activities. Should CDPHE s decision result in remediation dewatering being required, the project team should perform a cost comparison between treating the water before discharging and mitigating the amount that needs to be treated by incorporating trenchless methods to avoid coming in contract with a majority of the contaminated water. As shown above utilizing trenchless methods in contaminated groundwater areas can provide a project cost savings. 9. REFERENCES Colorado Department of Public Health and Environment, Water Quality Control Division (July 2013) - Fact Sheet, Permit No COG070000, sixth renewal Stillman, J., Lowrey,K., Hemmerle, J., Soule, N. (March 2014) Technical Memorandum- Options for Groundwater Contamination, PAR 1088 SPI Construction Phase. Paper TA-T