Utilizing Trenchless Technology to Achieve Watermain Asset Management Objectives

Transcription

1 North American Society for Trenchless Technology (NASTT) NASTT s 2015 No-Dig Show Denver, Colorado March 15-19, 2015 MM-T6-05 Utilizing Trenchless Technology to Achieve Watermain Asset Management Objectives Kristofer J. Knutson, Moorhead Public Service, Moorhead, MN Jade Berube, Apex Engineering Group, Bismarck, ND 1. ABSTRACT As water distribution infrastructure in the United States continues to deteriorate, the need for watermain replacement becomes an important factor in the management of utilities. This importance is highlighted by the fact that for most utilities, nearly one-half of their respective assets are comprised of watermains. Moorhead Public Service (MPS), a drinking water utility serving about 42,000 customers, is aggressively pursuing replacement of nearly 33 miles of aging cast iron watermain over the next years. To meet the goal of replacing 10,000 feet of cast iron pipe per year, MPS and Apex Engineering Group (Apex) have cooperatively pursued several trenchless methodologies and unique bidding strategies to meet replacement goals in the most cost-effective methods possible. This presentation will address the following topics to provide an effective template for the incorporation of trenchless methods into a Watermain Asset Management Plan: utilization of GIS based datasets to develop project prioritization use of alternative bid comparisons between Cured in Place Pipe (CIPP) and Horizontally Directional Drilled (HDD) to maximize quantity of replacement use of alternate materials such as Restrained-Joint Integral Bell (RJIB) pipe in several projects to reduce traffic disruption use of hydraulic modeling and risk based assessment tools in project prioritization By utilizing GIS tools and trenchless methodologies, MPS and Apex quickly developed plans and estimates for watermain projects. Specific case studies of projects completed demonstrate practical lessons learned and how pipebursting, HDD, sliplining, and CIPP can be incorporated into an asset management plan to meet replacement objectives. 2. INTRODUCTION MPS completed a Watermain Asset Management Plan (WAMP) in The plan, which outlines replacement needs of more than two miles of Cast Iron (CI) watermain per year, was approved and implemented in the fall of In both 2013 and 2014, trenchless technologies proved invaluable to meeting the 10,000 foot/year replacement goal outlined in the WAMP. As a part of the WAMP, a Herz Distribution Model was used to calculate the future watermain replacement requirement for three categories of distribution pipe, which is shown in Table 1. The pipe categories were selected based upon average lifespans interpreted from literature, as well as MPS determination of empirical lifetimes. The pipe lifetimes used for the calculation of future watermain replacement costs were decreased slightly from Paper MM-T

2 AWWA recommended lifetimes, presented in Table 2, in order to compensate for the corrosive nature of the soils in the Red River Valley. Table 1. Assumptions utilized for Herz Model. Installation Date Pipe Material Average Lifespan Standard Deviation (Years) (Years) Pre-1940 CI CI / AC Post 1980 C The Herz Model has been used extensively to model the need for watermain replacement in numerous studies and is based on three empirically based fitting parameters (Table 2). Based on the Herz distribution estimate (Equation 1), between 2014 and 2030, a minimum of 8,000 to 9,000 feet will need to be replaced annually until the CI and Asbestos Cement (AC) watermain is replaced. After the CI and AC pipe are removed from the system, the amount of annual replacement will drop significantly due to the longer lifespan of C pipe; however, the need for watermain replacement will not be eliminated. A cost estimate was developed to provide a visual of how much MPS will need to spend to replace the required amount of pipe. Table 2. Fitting Parameters for Development of Herz Model. Title 1920 s Cast Iron Cast Iron/AC C A Fit Factor B Fit Factor C Fit Factor ( a + 1) /( bexp[ b( t c)] F( t) = 2 ( a + exp[ b( t c)]) [1] Replacement Required (feet) 12,000 10,000 8,000 6,000 4,000 2, Year Herz Distribution Future Replacement Needs Figure 1. Herz Model projections for future watermain replacement. Paper MM-T

3 The Herz Model (Herz, 1996) displays a replacement curve exhibiting increased replacement requirements between 2013 and Figure 1 exhibits a shape characteristic of the Nessie Curve. The Nessie Curve, so described because the graph follows an outline likened to a silhouette of the Loch Ness Monster, has been observed in numerous water systems facing similar replacement requirements. 3. UTILIZATION OF GIS FOR PROJECT PRIOTIZATION AND SELECTION To identify the highest priority and most cost-effective future projects, a Watermain Project Priority List (WPPL) was created to categorize and budget for future replacement projects. A scoring matrix encompassing three specific criteria was used in the development of the WPPL to prioritize projects which included: coordination with the road restoration projects conducted by the city, watermain leak history, and watermain material. These data were analyzed simultaneously in GIS to construct the WPPL. The goal of the WPPL was to encapsulate all of the scoring factors into one table so that projects could be appropriately budgeted and scheduled for replacement. An additional benefit of the analysis was that the public could be made aware of possible replacement plans well before the projects occurred. The WPPL is available as a web map at gis.mpsutility.com. As a result of the analysis, identified regions of watermain were scored and ranked for replacement through For 2013 and 2014, a number of trenchless technologies were incorporated to achieve the 10,000-foot replacement goal (Table 3). Table and 2014 Main Replacement Methods and Cost Projects Length Pipe Dia. (in) Pipe type Project Cost Project Replaced (ft) Old New Old New Total $/ft Method 11 th St. S. & 20 th Ave CI C $87,319 $134 OT 17 th St. S. & 20 th Ave CI C $121,385 $169 OT 19 th St. N. & 10 th Ave CI C $104,727 $175 OT 11 th St. S. & Main Ave CI C $137,117 $312 PB C 8 th St. S. & 12 th Ave CI C $310,000 $456 HDD - C 2014 Projects Center Avenue 2, CI C $456,000 $156 SL C 12 ½ St. S. Alley 1, CI C $83,700* $63* PB 12 th / 13 th St. N. 1, ,8 CI C $147,000* $131* OT 14 th St. S. 1, ,8 CI C $182,000* $127* OT, PB, HDD 2 nd Ave. N CI C $118,000* $137* OT, HDD OT = open trench, HDD = horizontal directional drill, C = Contractor, SL = slip line, PB = Pipe Burst, *Estimated cost The most significant project, the Center Avenue Watermain Replacement, was prioritized because six watermain breaks had occurred over the past 10 years, and several mechanical joint failures ensued due to the deterioration of mechanical-joint restraint bolts. The respective size and bury depth of the 16-inch diameter pipe made repairs costly and on several occasions, dangerous to staff, because the excavation pits had to be so large in order to repair the watermain (Figure 2). Paper MM-T

4 Due to the number of watermain replacement areas on the WPPL that coincided with City roadway restoration projects in 2013, the Center Avenue Watermain Replacement was pushed back a year to Figure 2. Center Avenue Main Break Repair. 4. CENTER AVENUE PROJECT BACKGROUND The Center Avenue Watermain Replacement identified in the WAMP consisted of the rehabilitation of approximately 3,000 linear feet of watermain that had encountered multiple breaks along Minnesota Department of Transportation (MnDOT) Trunk Highway No. 10. With MnDOT s proposed reconstruction of a neighboring section of the roadway scheduled for 2015, and improvements on the opposite side of the main included in another 2014 project, MPS elected to proceed with the rehabilitation in the summer of The section of watermain proposed was originally installed in 1952 and consisted of a 16-inch cast iron watermain from the Water Treatment Facility toward the City of Moorhead s Downtown Area (Figure 3). The main initially operated as the primary transmission line from the Treatment Facility into the City s downtown, but with the expansions during the last 60 years additional transmission lines have been installed in the system, reducing the dependency on the aging main. The proposed watermain rehabilitation posed a number of concerns including the location of the main, service connections, other existing utilities, depth of piping, railroad crossings (106 trains per day through the City) and proximity to the neighboring Trunk Highway (22,000 vpd). Paper MM-T

5 Figure 3. Center Avenue Watermain Project Location. The exact location of the existing watermain was also in question. With the original construction in excess of 60 years, record drawings were non-existent. The existing main had been located at various points throughout the years by Public Service Staff when leaks or joint breaks occurred or when service connections were completed. The only record of the initial installation location within the Trunk Highway corridor was the original railroad crossing permit. Although not exact, the permit provided a rough location of the main throughout the center section of the project. Figure 4. Original Railroad Crossing Permit Map. Paper MM-T

6 5. DESIGN ALTERNATIVES FOR THE CENTER AVENUE WATERMAIN REPLACEMENT PROJECT Following the review of the existing conditions, constraints and project needs, various replacement methods were reviewed to consider the appropriate rehabilitation method for the watermain (Table 4). Table 4. Rehabilitation Alternatives Const. Method Directional Drilling Pros New Pipe/ Location Verification Cons Casing installation at railroad (Costly: $500/ft) Service/Hydrant Will require Installation sleeves/couplers and Open Cut Testing Tested per Minnesota Department of Health Requirements Pipe Lining Sliplining Open Cut Trenchless Trenchless New Pipe/Location Verification Large Range of Costs, lack of contractors. Will require sleeves/couplers and Open Cut Tested at 2x working Pressure (110 psi) Loss of Pipe Sizing, relatively new to watermain in area. Will Require Open Cut Tested per Minnesota Department of Health Requirements Casing (railroad), narrow conditions, and utility constraints. All open cut. Tested per Minnesota Department of Health Requirements Internal discussions led to the elimination of the Open Cut alternative because of the difficulties Public Service Staff encountered during previous line repairs and the expenses and timing associated with crossing the railroad. Directional drilling was also eliminated due to the costs and timing associated with permitting the railroad crossing. The slipline piping alternative was one of the two remaining alternatives. The pipe size was reduced to 12-inches from the possible 14-inches because a number of unknowns existed inside the current transmission line and the possibility of modifications in the fusing process. Figure 5. Slipline Pipeline Sizing. The selected 12-inch fusible C pipe was modeled to determine if the reduced pipe size had any effect on pressure in the system. With the age of the existing 16-inch cast iron watermain, a C-Factor of 63 had been used to model previous runs and appeared to be consistent with the static and dynamic modeling when created. The existing cast iron pipe was modeled compared to the proposed 12-inch C pipe with a C-Factor of 135. The models illustrated that Paper MM-T

7 the modification from the existing 16-inch to the proposed 12-inch would not result in a significant reduction in pressure (Figure 6). Figure GPM Fire Flow Pressure Contours Showing Change in Pressure from Exsting 16 Pipe to 12 Pipe. Another alternative to sliplining the main was cured-in-place-piping (CIPP). The CIPP alternative is relatively new to the northern Midwest due to the shortage of contractors. Because of this known factor, three companies (Fer-Pal Construction, Insituform, and Michels) were contacted early in the design process to get feedback, discuss alternatives and construction timing for the improvements. MPS also allowed for an early-season bidding timeframe and open construction schedule to allow the potential contractors sufficient time to complete the proposed CIPP. Following the review of the proposed alternatives, the consensus was that either alternative (sliplining or CIPP) would be sufficient for the rehabilitation. With both alternatives considered, MPS selected to bid both options, and to allow costs to guide the decision-making process to determine the construction method. Bids were received for the project in March of 2014, which was nearly two months prior to the start of typical construction activities within the City of Moorhead. Six bids were received for the proposed improvements; three for sliplining the main, and three for CIPP. The lowest bid was the proposed method of sliplining at almost 50% of the CIPP alternative cost. With the significant cost savings of sliplining, MPS elected to award the rehabilitation to the lowest bid. Paper MM-T

8 6. CONSTRUCTION OF CENTER AVE WATERMAIN REPLACEMENT PROJECT Construction of the Center Avenue Watermain Replacement Project began on July 7, 2014, and was completed by August 29, With a limited number of service connections along the replacement section, the sliplining was completed through six (6) Horizontally Directional Drilled (HDD) pulls ranging from 350 to 800 feet. Pull lengths were determined by making the fewest temporary water service connections, and the need for existing surface rehabilitation in the bore pit locations. In order to minimize the repair of existing surfaces, bore pits were made outside service laterals in open or boulevard areas. Figure 7. Slipline Horizontal Direction Drilling Lengths. Following the installation of the new 12-inch main, the existing 16-inch host pipe was cut, allowing for connection to existing service laterals. Paper MM-T

9 Figure 8. Service Connection. After the service connections were completed, the contractor placed a control-density fill with a cellular agent in the annular space between the host pipe and the new main. The material was pumped into the void at the same locations utilized to install the new pipeline. Upon completion of the installation of the water system, the new main was pressure tested at 150 psi for two hours, and tested per the Minnesota Department of Health for coliform bacteria. The construction consisted of six (6) HDD pulls and the utilization of seven (7) bore pits. To make the necessary service and hydrant connections, the host pipe was carefully cut away from the pulled 12-inch C fusible pipe. 7. CONCLUSIONS The utilization of various trenchless underground alternatives has allowed MPS to complete the goal of 10,000 lineal feet of watermain replacement per year while minimizing impacts to neighboring properties. Trenchless technologies have also reduced the costs of replacement in select areas, allowing flexibility in the budgeting process and project prioritization. 8. REFERENCES Herz, R. K. (1996). Ageing Process and Rehabilitation Needs of Drinking Water Distribution Networks. Journal Water SRT, 45:5:221. Paper MM-T