Chapter 16 535 Pipe Bursting
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Chapter 16 535 Pipe Bursting Chapter 16 Pipe Bursting Introduction Underground service utilities in many American cities have been in place for over 100 years. While existing systems have functioned well beyond reasonably anticipated service life, underground systems are mostly deteriorated and need costly maintenance and repair. Common problems involve corrosion and deterioration of pipe materials, failure or leakage of pipe joints, and reduction of flow due to mineral deposits and debris build up inside the pipe. Damage to existing pipes can also occur by ground movements due to adjacent construction activity, uneven settlement or other ground instability. This leads to infiltration and inflow (I&I) increase in sewer systems. In water systems, it leads to flow and pressure reductions, persistent leakage (up to 30 percent of water provided in some systems), pipe bursts, and poor water quality. These problems tend to increase with the age of the network where maintaining this large network of underground sewer, water, and gas pipelines is difficult and costly. The above problems are compounded by the significant negative impacts (of open cut repair or replacement projects) on the daily life, traffic, and commerce of the area served by and along the pipeline in question. Pipe bursting is a well-established trenchless method that is widely used for the replacement of deteriorated pipes with a new pipe of the same or larger diameter. Pipe bursting is an economic pipe replacement alternative that reduces disturbance to business and residents when it is compared to the open cut technique. Pipe bursting is especially cost-effective if the existing pipe is out of capacity, deep, and/or below the ground water table (GWT). Replacing an old pipe with a larger one is termed upsizing. One-size upsizing is replacing the old pipe with a pipe one standard size larger, for example replacing 8” pipe with 10” one. Similarly, two-size upsizing is replacing the old pipe with a pipe two standard sizes larger, e.g. replacing 8” pipe with 12” one. 535-583.indd 535 1/16/09 10:22:25 AM 536 Chapter 16 Pipe Bursting Pipe bursting conventionally involves the insertion of a cone shaped bursting head into an old pipe. The base of the cone is larger than the inside diameter of the old pipe and slightly larger than the outside diameter of the new pipe to reduce friction and to provide space for maneuvering the pipe. The back end of the bursting head is connected to the new Polyethylene (PE) pipe and the front end is attached to a cable or pulling rod. The new pipe and bursting head are launched from the insertion shaft and the cable or pulling rod is pulled from the pulling shaft, as shown in Figure 1. The bursting head receives energy to break the old pipe from one of the following sources: a pulling cable or rod, a hydraulic source, or an air compressor. The energy breaks the old pipe into pieces and expands the diameter of the cavity. As the bursting head is pulled through the old pipe debris, it creates a bigger cavity through which the new pipe is simultaneously pulled from the insertion shaft. There are many variations to this conventional layout that are presented later in the chapter. History Pipe bursting was first developed in the UK in the late 1970s by D. J. yanR & Sons in conjunction with British Gas, for the replacement of small-diameter, 3- and 4-inch cast iron gas mains (Howell 1995). The process involved a pneumatically driven, cone- shaped bursting head operated by a reciprocating impact process. This method was patented in the UK in 1981 and in the United States in 1986; these patents expired in April, 2005. When it was first introduced, this method was used only in replacing cast iron gas distribution lines; it was later employed to replace water and sewer lines. By 1985, the process was further developed to install up to 16-inch outer diameter (OD) medium-density polyethylene (MDPE) sewer pipe. Replacement of sewers in the UK using sectional pipes as opposed to continuously welded PE pipe was described in a paper by Boot et al. (1987). Up to 2006, approximately 9,000 miles of PE pipe has been installed by bursting (Najafi, 2006). Currently, pipe bursting is used to replace water lines, gas lines, and sewer lines throughout the world. 535-583.indd 536 1/16/09 10:22:25 AM Chapter 16 537 Pipe Bursting Force Air Pressure/Hydraulic Hose New HDPE Pipe Bursting Head Ground Serface Pulling Pit Insertion Pit D Old Pipe d Pulling Cable/Rod D= Depth of cover d= Diameter of the existing pipe Figure 1 The Pipe Bursting Operation Layout Pipe Bursting and Trenchless Pipe Replacement Systems Existing old pipes can be replaced by one of several trenchless techniques developed up to date. There are three basic methods of pipe bursting: pneumatic, hydraulic, and static pull. In addition, there are proprietary trenchless pipe replacement systems that incorporate significant modifications to the basic pipe bursting technique. The basic difference among these systems is in the source of energy and the method of breaking the old pipe and some consequent differences in operation that are briefly described in the following paragraphs. The selection of a specific replacement method depends on soil conditions, groundwater conditions, degree of upsizing required, type of new pipe, construction of the existing pipeline, depth of the pipeline, availability of experienced contractors, and so on. Pneumatic Bursting Systems The most common pipe bursting method is the pneumatic system. In the pneumatic system, the bursting tool is a soil displacement hammer driven by compressed air and operated at a rate of 180 to 580 blows per minute. It is similar to a pile-driving operation going horizontally. The percussive action of the hammering cone-shaped head is also similar to hammering a nail into the wall; each hammer pushes the nail a short distance as shown in Figure 2. With each stroke, the bursting tool cracks and breaks the old pipe, the expander on the head - combined with the percussive action of the bursting tool, push the fragments and the surrounding soil providing space to pull in the new PE pipe. The expander can be frontend (attached to the frontend of the hammer) for pipes smaller than 12” or back-end (attached to the backend of the hammer) for pipes larger than 12”. The frontend expander allows withdrawing the hammer through the PE pipe after removing the expander from the existing manhole at the pulling shaft without damaging the manhole. The tension applied to the cable keeps the bursting head aligned with the old pipe, keeps the bursting tool pressed 535-583.indd 537 1/16/09 10:22:25 AM 538 Chapter 16 Pipe Bursting against the existing pipe wall, and pulls the new PE pipe behind the head. An air pressure supply hose is inserted through the PE pipe and connected to the bursting tool. The bursting starts once (1) the head is attached to the new pipe, (2) the winch cable is inserted through the old pipe and attached to the head, (3) the air compressor and the winch are set at a constant pressure and tension values. The process continues with little operator intervention until the head reaches the pulling shaft at which point it is separated from the PE Pipe. Figure 2 The Bursting Head of the Pneumatic System Static Bursting Systems The second common method of pipe bursting is the static pull system. In the static pull system, a larger tensile force is applied to the cone-shaped expansion head 535-583.indd 538 1/27/09 12:53:03 PM Chapter 16 539 Pipe Bursting through a pulling rod assembly or cable inserted through the existing pipe. The cone transfers the horizontal pulling force into a radial force -- breaking the old pipe and expanding the cavity providing space for the PE pipe as shown in Figure 3. The steel rods, each is about four feet long, are inserted into the old pipe from the pulling shaft. The rods are connected together using different types of connections. When the rods reach the insertion shaft, the bursting head is connected to the rods and the PE pipe is connected to the rear of the head. A hydraulic unit in the pulling shaft pulls the rods one rod at a time, and the rod sections are removed. The process continues until the bursting head reaches the pulling shaft, where it is separated from the PE pipe. If cable is used instead of rod, the pulling process continues with minimum interruption, but the tensile force of a cable compared to a rod section is limited. Pipe Splitting The North American Society for Trenchless Technology (NASTT) defines pipe splitting as a replacement method for breaking an existing pipe by longitudinal slitting. At the same time a new pipe of the same or larger diameter may be drawn in behind the splitting tool (NASTT 2008). Pipe splitting is used to replace ductile material pipes, which does not fracture using the above-cited bursting techniques. The system has a splitting wheel or cutting knives that slit the pipe longitudinally at two more lines along the side of the pipe. An example of splitting head is shown in Figure 4. Figure 3 The Static Pull Bursting Head with Accessories to Cut Reinforcing Steel in RCP 535-583.indd 539 1/16/09 10:22:25 AM 540 Chapter 16 Pipe Bursting Figure 4 Pipe Splitting Head (PIM Corporation 2007) Pipe Reaming Pipe reaming is pipe replacement technique that uses a horizontal directional drilling (HDD) machine with minor modification.