Pipe bursting is a rapidly spreading method used for trenchless pipe replacement to an equal or greater inside diameter. Pipe bursting can therefore maintain, or even increase, the existing flow capacity. Pits are dug over the existing deteriorated sewer, equipment is put in place, and the existing pipe is replaced with the new pipe. A bursting tool bursts the existing pipe. The replacement pipe must support the full soil loading since the existing pipe is demolished. The great majority of pipe bursting to date has utilized high density polyethylene (HDPE), a plastic product. The City of Phoenix does not utilize plastic pipes in direct bury applications and desired to determine if vitrified clay pipe (VCP) could be used as a replacement pipe in a pipe bursting application.
City of Phoenix Concrete
Sewer Rehabilitation Program
A condition assessment program focusing on large diameter (24-inch (600 mm) and larger) unlined concrete sewer pipe, was undertaken by the City of Phoenix in 1992. This study found that the City's sewer system includes over 116,000 feet (35,360 m) of unlined concrete interceptor sewers. The City recently completed a $4 million construction project that addressed 19,000 feet (5,800 m) of the most deteriorated 24-to 42-inch (600-to 1,100 mm) interceptor sewers. Trenchless construction procedures were utilized to minimize construction cost and disruption to the public.
During the design phase, alternative rehabilitation methods were carefully considered based on capacity and structural strength. The City was adamant that a high portion of the existing pipe capacity be maintained. After careful study, cured-in-place pipe (CIPP) and custom size Hobas slip lining pipe were determined to be the only lining alternatives that could meet the stringent capacity requirements in most of the locations. However, in some locations, only CIPP could provide the desired capacity.
Competitive Methods Desired
& Pipe Bursting Bid as Alternate
One of our goals was to provide alternative rehabilitation methods to be bid against each other and thereby lower construction costs. Pipe bursting is a rapidly advancing method of sewer rehabilitation. Most pipe bursting experience to date has utilized plastic as the replacement pipe material, and as noted above, the City of Phoenix does not accept plastic pipe in a direct burial application. We had witnessed a demonstration by Albuquerque Underground, Inc., in which vitrified clay pipe was utilized for the replacement material. Based on this experience, pipe bursting using VCP was allowed as an alternate to CIPP for the rehabilitation of a 644 feet (196 m) long segment of 24-inch (600 m) pipe. The project was awarded to Insituform, and all project sewers were successfully rehabilitated using CIPP, including the segment for which pipe bursting was allowed.
Pipe bursting can provide competition to both CIPP lining and conventional replacement, either of which may be dictated by capacity considerations. Also, some sewers cannot be replaced using standard methods, e.g., a sewer under a freeway. For these reasons, the City desired to demonstrate the viability of pipe bursting utilizing VCP.
Figure 1 - PIPE BURSTING PILOT PROJECT PLAN & PROFILE
Project Design & Contractor Selection
Figure 1 shows the 521-foot (159 m) section of sewer selected for the Pilot Project. This section had been identified as a Condition Category III in the 1992 condition assessment study, meaning that it had suffered severe corrosion throughout its length, but rebar was not exposed. The sewer is in an alley, resulting in minimal traffic impact. This sewer has approximately twenty service connections. While atypical for the Phoenix interceptor system, this allows demonstration of procedures necessary for disconnecting and reconnecting services.
NO-DIG is the brand name of a VCP developed by MCP Industries for microtunnelling applications. Approximately 30% of American microtunnelling uses NO-DIG VCP. The jacking forces inherent in microtunnelling require that the pipe be very strong. As shown in Figure 2, the joint is flush, or slightly recessed, to the pipe wall. The calculated failure jacking load is 661 tons (5.9 x 10 N).
The typical procedure in pipe bursting is to pull the HDPE pipe behind the bursting tool. This is not possible for a segmented pipe such as NO-DIG VCP.
Selection of Contractor
Albuquerque Underground, Inc., (AUI) has been awarded the Pipe Bursting Pilot Project. We were pleased because AUI had previously demonstrated in their yards the bursting of 8-inch (200 mm) PVC using VCP. AUI uses the TRS method licensed by Trenchless Replacement Services, Inc. This is the method that was used for the Florida Thunderdome project in which segmented 36-inch (900 mm) Hobas pipe was burst into existing 36-inch (900 mm) concrete pipe.
Basic Set Up
Much of the basic set up is common to pipe bursting operations in which the replacement pipe is plastic. A cone-shaped tool is pulled through the inside of the old pipe to be replaced (see Figure 3), breaking it up, and forcing the broken fragments into the surrounding ground. The bursting tool has a slightly larger outside diameter than the new pipe. In most pipe bursting operations, the replacement pipe is pulled into place by the bursting tool. However, for a segmented pipe such as VCP, the replacement pipe must be pushed into place. Figure 4 shows the anticipated set up in which TRS Hydrahaul H 225 machines will be installed on each end of the "pull," one machine to pull the bursting head and to burst the existing pipe, the other to push the pipe and assure that no separation occurs. For clarity, we will refer to these as the "pulling machine" and the "pushing machine." Pits are required on both ends for the pulling and pushing machines and for each service connection.
The bursting head will allow the use of a polymer or Bentonite which will be use to aid in the bursting and pulling process. The general plan is to attach a hose to the back of the bursting head, and roll it up inside the first sections in a fashion which will allow it to unroll as the bursting operation proceeds. The outer end of the hose will have to be disconnected at the end of pushing each joint, to allow the insertion of a new section of pipe.
After all the equipment is installed in the operating pits, the pull pipe is pushed into the existing pipe using the pulling machine, until it encounters the pushing pit and the bursting head is attached. A marker pipe is used for the first joint of the pull string inserted to alert the operators when this joint reaches the pulling pit.
The head will be installed with the initial section of VCP. The pushing machine will provide all of the motive force, bursting the existing pipe and placing the new VCP. The pulling machine will be in operation, only to provide tension on the bursting head, and thereby maintain the alignment. After pushing the pipe section into place, the next pipe section is lowered into the insertion pit, and the process will continue. Eventually, we anticipate the capacity of the pushing machine will be taxed, and the additional motive force of the pulling machine will be needed.
The pulling machine will be operated at a slightly slower speed than the pushing machine. By allowing the pushing machine to provide a substantial portion of the motive force the jacking pipe will automatically follow the bursting head, leaving no possibility of the jacking pipe separating.
The Hydrahaul H 225 has a design capacity of 225 tons (2.0 x 10 N) and a maximum capacity of 237 tons (2.1 x 10 N). The NO-DIG VCP has a 661 ton (5.9 x 10 N) crushing capacity, therefore, a minimum 2.79 safety factor should be maintained. With this amount of force being applied, the bursting head is obviously of very robust construction, with attention paid to details and welding. The pulling unit can provide an additional 225 tons (2.0 x 10 N) of pull to power the forward motion of the bursting head. This will give us a combined total of 450 tons (4.0 x 10 N) of force which can be applied to the VCP, the bursting head, and to overcome soil resistance. TRS estimates that the pushing machine alone will be able push for 200 feet (61 m). The longest burst is 336 feet (102 m) for which a 353 ton (3.1 x 10 N) force is estimated, well within the 450 ton (4.1 x 10 N) capacity of the two machines.
The existing pipe has nearly nine feet (2.7 m) of cover, therefore, ground movement is not anticipated to be a problem. We will be monitoring ground movement for documentation.
The bid price is $188,000. Adjusting for manhole rehabilitation and replacement, service connections and miscellaneous allowances, this project bid for about $11 .26/inch-diameter/foot. NO-DIG, a Division of MCP Industries, Inc., donated the pipe, equalizer ring, stainless steel joint collars and technical support for a savings of approximately $2.00/inch-diameter/foot. This is more than the approximately $9.00/inchdiameter/foot the City typically will pay for replacement projects. We do expect the pipe bursting cost with VCP to drop severely as the process is proven.
Pipe bursting is a rapidly spreading method used for trenchless pipe replacement to an equal or greater inside diameter. Pipe bursting can therefore maintain the existing flow capacity, or even increase it. Pits are dug over the existing deteriorated sewn, equipment put in place, and a replacement pipe is pushed into the existing pipe after the existing pipe is burst by a bursting/pulling tool. The replacement pipe must support the full soil loadings since the existing pipe is demolished. The great majority of pipe bursting to date has utilized HDPE, a plastic material. This project will use NO-DIG VCP as the replacement pipe. This is a special type of vitrified clay pipe which was originally developed for use in micro-tunneling. 521 feet (159 m) of existing 24-inch (600 mm) reinforced concrete sanitary sewer will be replaced in the Pipe Bursting Pilot Project. Approximately twenty services are on this sewer.
Pipe bursting allows the trenchless replacement of an existing pipe to the same or larger diameter. This project is expected to show the viability of NO-DIG VCP as a pipe bursting replacement material. This provides an additional alternative for those communities that prefer to use VCP and provides competition to currently available plastic materials.
The use of pushing and pulling machines also allows the bursting of longer pipe lengths than possible with only a pushing or a pulling machine, thereby providing a possible advantage to the use of segmented VCP.
The authors wish to acknowledge the following people and companies that assisted in the preparation of this paper.