Cost Comparison of Open-cut and Trenchless Methods for Renewing Sewer Lines
Rayman Mohamed, Mohammad Najafi and Behnam Hashemi Oct 01, 2008
The renewal of aging under-ground infra-structure is a major challenge fac-ing municipalities. Traditionally, replacement of underground utilities employed open-cut excavations, which can be expensive. In contrast, trenchless technologies purport to be less expensive than traditional methods. However, no studies have yet corroborated this claim.
This case study examines the claim by comparing the costs of a trenchless method known as pipe bursting with the open-cut method for replacing sewer pipes in the City of Troy, Mich. We found that the pipe bursting method is much less expensive than the open-cut method. We conclude that trenchless methods, such as pipe bursting, could provide considerable cost savings to municipalities as they seek to renew their underground utilities.
This paper is also important because it presents an approach for comparing the costs of other trenchless technologies with the open-cut method and highlights important areas in which more research is needed.
Most existing underground utilities in North America were installed in the postwar construction boom that resulted from the growing economies of Canada and the United States in the 1950s and 1960s (McKim 1997). This period witnessed the rapid expansion of extensive underground systems for water, sewer, gas and power utilities. As McKim (1997) notes, these systems were constructed in greenfields, and the sparsely built environment presented few impedments to large-scale open trenching.
However, over time these systems have deteriorated and now need extensive rehabilitation. Indeed, the America Society of Civil Engineers (ASCE) estimates that it will cost $1.3 trillion over the next five years just to maintain current underground infrastructure systems. Extensive rehabilitation will be even more expensive, and since open-cut construction is the preferred method, approximately 70 percent of the costs will simply be for replacing the ground dug up in this process (Najafi and Gokhale, 2005).
In response to these costs, a different approach to rehabilitating underground infrastructure has emerged. Referred to as trenchless technology, it is defined as ‘‘techniques for utility line installation, replacement, rehabilitation, renovation, repair, inspection, location and leak detection, with minimum excavation from the ground surface” (North American Society for Trenchless Technology). According to proponents of these technologies, they should provide significant cost savings over traditional open-cut methods (Najafi and Gokhale, 2005).
One approach to trenchless construction is known as pipe bursting. Using a winch and a hammer (that might be pneumatic or static), crews pull a new pipe commonly made of high-density polyethylene (HDPE) through the old pipeline. Simultaneously, the old pipe is shattered and the pieces pushed into the surrounding soils. This technique, however, can be used only in conditions where the old pipe can be broken into small fragments and the soils around the old pipe can absorb the broken fragments. For more information on pipe bursting, refer to Najafi and Gokhale (2005) and Chung et al. (2004).
Because of the minimal disturbances to the surface, it appears that trenchless technologies will be less expensive than traditional open-cut methods. However, since these technologies are fairly new, few studies have compared their costs to those of the open-cut method, and even fewer studies have been conducted using real-world scenarios. This case study presents such a comparison by examining potential costs for replacing sewer lines in the City of Troy, Mich., using the trenchless pipe-bursting method versus the traditional open-cut method. Although new technologies for replacing sewer lines have been the subject of research for some time now (see, e.g., Reyna et al., 1994), this is the first time that such a study is being performed. The approach outlined in this case study will serve as a basis for future research.
The remainder of the case study is as follows: Section 2 introduces the study area, discusses the data, and presents the methodology employed; Section 3 analyzes the costs of using pipe-bursting technology; Section 4 analyzes the costs of using the traditional open-cut method. Section 5 compares the costs of the two methods, and Section 6 concludes.
2. The Study Area, Data, and Methodology
The City of Troy, located in Oakland County, Mich., was chosen for this study for two reasons. First, the city has a reputation in Michigan for adopting novel approaches for infrastructure management. This strategy increases the chances that lessons learned from this study will be applied to future projects. Second, like much of the Detroit metropolitan area, Troy has mostly clay-type soil, making it an appropriate candidate for the use of pipe-bursting technology.
Data for this study came from two main sources. The first source was a geographic information system (GIS) maintained by the city. The GIS provided important data on the extent of and size of sewer pipes used in Troy.
We manipulated the GIS to provide us with information on the length of each pipe segment. (Because we are interested only in ‘public costs,’ we do not consider private sewer lines — commonly called laterals — that run from the public sewer through yards to houses. These laterals are the property of homeowners, and decisions to maintain or replace them are a private matter left up to individual households). Utilizing information provided by the GIS on the diameter of each pipe segment, we were able to determine the total length of pipe for each diameter.
The second source of data was standard cost estimates provided by R. S. Means (2005a and 2005b). These publications are widely accepted and are employed by contractors, engineers and project owners to provide baseline cost estimates for construction projects. From these publications, we obtained information on unit cost estimates for replacing all the sewer pipes in Troy utilizing both the pipe-bursting and open-cut methods. We applied these unit costs to the pipe lengths obtained from the GIS, but with some qualifications as discussed in subsequent sections.
3. Cost for Replacing the Sewer Network Using Pipe-Bursting Technology
Using data from R. S. Means (2005a), Table 1 shows the direct cost for the pipe-bursting method based on the length and diameter of the pipes.
To obtain information on miscellaneous costs for pipe bursting, we utilized Lee et al. (2007, p. 10). The authors presented estimates for “bypassing, pre-inspection, mobilization of equipment, excavation of access pits (including pavement removal), removal of existing cleanouts, pipe fusing, pipe-bursting operation, closing of pits and surface restoration (including pavements), demobilization, final inspection, and construction of manholes.” Based on the data of Lee et al. (2007, p. 10), we determined that miscellaneous costs will be approximately 20 to 40 percent of the direct cost of pipe bursting. The actual percentage we used depended on the diameter and length of the pipe.
Table 2 presents information on each diameter pipe in the study. It consists of information on the length of each diameter pipe (Column 2), replacement cost per foot as obtained from R. S. Means (2005a, Column 3), miscellaneous cost per foot as obtained from Lee et al. (2007, Column 4), the sum of direct and miscellaneous cost per foot (Column 5), and the final cost for each diameter pipe (Column 6). The total cost for replacing the sewer network in Troy using the pipe-bursting method is about $408 million.
4. Cost for Replacing the Sewer Network Using the Open-Cut Method
The cost items for the traditional open-cut method include mobilization, pavement removal and replacement, excavation, sloping, shielding, old pipe removal, pipe bedding, new pipe installation, backfilling, compaction, and demobilization. There are also some indirect cost items such as contingencies, overhead and profit, bonding, and insurance.
Lee et al. (2007) estimated construction costs for the open-cut method using a combination of sloping and shielding. According to their calculations, the cost for the open-cut method using an 18-inch-diameter pipe is $326 per linear foot. To estimate open-cut costs for different pipe sizes, we based our cost estimate on the specifics of this project (the 18-inch diameter pipe) and also used R. S. Means Site Work & Landscape Cost (2005b) to develop Table 3. Based on these analyses, the cost ratio using the open-cut method for each diameter sewer pipe for the City of Troy is shown in Table 4.
Using the numbers in Table 4, Table 5 shows the estimated cost of sewer pipeline replacement using the open-cut method. The total cost for replacing the sewer network in Troy using the pipe-bursting method is about $304 million.
5. Comparison of the Results
When applying the numbers above, the cost for sewer pipeline replacement in Troy using the open-cut method is about $408 million (Table 5). The estimated cost using the trenchless pipe-bursting method is about $304 million (Table 2). The pipe-bursting method costs about $104 million less than the open-cut method.
To calculate the cost per foot per inch of diameter, we multiplied the diameter of each pipe by its respective length and summed the results to arrive at a total inch-length of 22,192,489. Dividing the total cost of each method by the total inch-length gave us the cost per foot per inch of diameter of that method. The open-cut method costs about $18.4 per foot per inch of diameter, whereas the trenchless pipe-bursting method costs about $13.7. Trenchless technology is less expensive than the open-cut method by about $4.7 per foot per inch of diameter.
6. Conclusions and Recommendations for Future Research
In this study, we determined that the pipe-bursting method would be much less expensive than the open-cut method for replacing the underground sewer network in Troy, Mich. The paper is also important because it provides a template for future comparisons of the costs of other trenchless technologies vs. the open-cut method.
However, a number of caveats and areas for future research need to be noted.
First, this research assumed that the whole pipe network would be replaced at the same time. In practice, the replacement process will depend on the age and condition of specific sections of the network. In turn, the cost estimated in this research would be incurred over a period of time.
Second, we assumed that all replacement and renewal work would be conducted using pipe bursting. Again, in practice, specific neighborhood conditions may dictate that other trenchless methods be employed in certain locations
Third, we used data from a recent study by Lee et al. (2007), in which costs for the open-cut method were calculated for pipe sections of 310 feet at a time. However, Troy’s pipe network is much longer. A longer network could affect the cost per foot because contractors’ unit costs usually fall as length increases. On the other hand, the size factor might not hold for trenchless methods because larger diameters and longer pipes may make the project more challenging and require more experience and more sophisticated equipment. In turn, this could actually increase the cost of trench-less methods. We recommend future research that examines this issue.
Fourth, for simplicity, we did not perform cost calculations for each diameter size of pipe. Instead, we used a ratio based on an open-cut parameter from R. S. Means (2005b, see Table 3) and the results of the study by Lee et al. (2007) for an 18-inch diameter pipe. We recommend that future research determine the cost for each diameter pipe, although our preliminary findings suggest that the results would not be much different from those presented here.
Fifth, we did not examine the cost of reconnecting laterals to the sewer lines. This is less expensive to perform in the open-cut method because the trenching required in this process means that reconnection should be easier. On the other hand, the pipe-bursting method would require that a pothole be excavated to reconnect the laterals to the sewer pipes. This factor should be considered in future research. Again, however, we do not believe that including this factor would change the conclusions of this research, as pothole excavation is minimal using vacuum excavation or other new technologies.
Sixth, we did not consider other conditions that might affect the results. In a subsequent study, we will examine how costs for both the open-cut and pipe-bursting methods can vary with different conditions, such as soil characteristics, the state of decay of the existing pipe, and the density of connections. Finally, we did not consider other ‘social’ costs, such as disruptions to traffic. These are likely to significantly increase the costs of the open-cut method relative to the costs of pipe bursting.
Notwithstanding these limitations, it is clear that trenchless technology offers distinct cost advantages. For the same project, the cost for open-cut excavation will increase in proportion to the depth and length of the pipe while the cost for the trenchless method will not significantly increase in proportion to the depth and length of the pipe. This is because the open-cut method requires continuous excavation and expensive trench-wall protection systems while trenchless technology requires only the excavation of entry and exit pits at widely spaced intervals. We conclude that using the pipe-bursting method instead of open-cut excavation could save municipalities considerable sums of money as they seek to renew their underground utilities.
Rayman Mohamed, Ph.D., is Assistant Professor of Environmental Planning for the College of Liberal Arts & Sciences at Wayne State University, Detroit.
Mohammad Najafi, Ph.D., is Assistant Professor and Coordinator of the Construction Engineering and Management track within the Department of Civil Engineering at the University of Texas-Arlington.
Behnam Hashemi is a graduate student in the Construction Engineering and Management area of Civil Engineering at the University of Texas-Arlington.
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