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PROPER PLANNING IS KEY TO EFFECTIVE PIPE BURSTING.

There is a growing trend toward using trenchless methods for sewer restoration. The costs of open cutting and the associated disruption it brings have convinced many municipalities to look at specifying pipe bursting with pneumatic tools for rehabilitating sewer lines.

As with any construction or installation method for city utilities, planning issues must be addressed and solved before writing bid specifications.

Pre-Work Testing. Testing the flow and capacity usage of existing lines is often n necessary and continuous effort. Putting effluent meters down into effluent flow can determine flow depth and velocity, which can translate into capacity usage. Conducting these tests at different points in the line can determine the areas and amounts of I/I into the lines.

Video Inspection. The second testing step is to video the line, often following cleaning by jet spray. Video inspection must be done before writing bid specifications. This is crucial so the city knows what lines need to be repaired, as well as for addressing other issues related to bid specifications.

Breaks and Blockages. First, the video should reveal any breaks in the line; the degree of root intrusion; the location of laterals, sags, and swales; offset joints; and possible blockages from sediment or broken pipe. Knowing the degree of damage to a line is essential to all rehabilitation techniques.

Impediments in the line some-times require extra excavation and repair before pipe bursting. Most pipe bursting operations use the pneumatic tool impact strength to burst through these impediments, but having the video to identify these beforehand gives both the municipality and the contractor a heads up on the problem.

Sags and Missing Pipe Sections. The video should also identify the existence of any significant sags or missing sections in the existing line. The bid specifications may need to spell out the repair of these sections before the pipe bursting operation. This can be done by excavating a sag elimination pit and bringing the bottom of the pipe trench to a uniform grade with the existing pipe.

Prior Line Repairs. Also use video to identify prior repairs on the existing line to be burst. Examine the video to reveal any lining sections or steel repair clamps. Any PVC or PE lining section could impede the pipe bursting operation. Unlike the bursting of a clay, concrete, or castiron line, the bursting head may not burst these plastic sections, but instead push them forward in the line, slowing down the burst run. Steel repair clamps in the existing line could stop the bursting process. All three can be addressed with different bursting head styles, more powerful tools, or excavation (if needed) before bursting.

Concrete Encasements. At times, pipe bursting contractors have run into situations where the old line was encased in concrete. Either the old line was set on a "U" concrete cradle or the line was encased in grout after it had been installed. Bursting this line section usually requires a more powerful pneumatic impact tool, which must be planned for in advance.

Excavation Allowance. Video inspection footage may reveal some or all these impediments to the pipe bursting run. The municipality should allow bidding contractors to view video inspection footage so that all parties know what the situation is in the existing line to be burst. Then, the municipality should also write bid specifications to allow for any needed additional excavations. A line item may be placed in the specifications requesting bids for any open excavations and restorations for specific locations or for excavations in general.

Pipe Upsizing Determination. The most common pipe bursting operations involve bursting and replacing size for size, such as 6-in. pipe replaced with 6-in. pipe, or increasing the diameter one size, such as going from 6-in, to 8-in.

A municipality seeking to increase a line's capacity can predict its flow capacity by this general rule of thumb. A 33 percent increase in a pipe's internal diameter can lead a 67 percent increase in flow capacity. Increase the internal pipe diameter by 41 percent and the flow capacity can increase by 100 percent.

Table 1 identifies some typical sanitary sewer diameters and the percentage increases in capacity that would occur if the lines were upsized.

Increasing the pipe diameter two sizes (e.g., replacing 6-in, pipe with 10-in, pipe) maybe possible, depending on soil conditions.

Soil Conditions. Size for size replacement using pipe bursting is easier than upsizing the line. The soil must be compressible enough to allow the burst head to radially compact the soil sufficiently to pull in the larger size pipe into the same space that occupied a smaller diameter line. Upsizing the line by two pipe sizes requires a compressible soil, otherwise, the pipe bursting process will require a larger more powerful tool or be slowed down considerably.

Most cohesive soil conditions will permit pipe bursting. Pipe bursting in dry sand is difficult because the sand will fall back onto the new replacement line as it is pulled in, adding to the friction on the line. Compact, dry sand allows little radial compaction, slowing down the pace of the bursting operation. If these situations exist, pipe bursting can be accomplished, but plan on shorter runs. If the original pipeline was installed in a trench cut out of rock, using pipe bursting to upsize the line may be impossible since insufficient space exists for the new line.

New Pipe Specifications. Most pipe bursting operations will pull in new high-density polyethylene pipe. This new pipe must have an appropriate SDR rating. Lower SDR ratings mean that the wall thickness is greater for a given diameter pipe. Higher SDR ratings mean that the wall thickness is smaller for any given diameter pipe. Internal utility pressure and ground load will be determining factors in choosing a certain SDR-rated pipe. There are charts available that will help determine the appropriate SDR for a given pipe depth.

Depth of Cover. Precaution must be taken to prevent surface disruption during pipe bursting, especially when the new line is larger in diameter than the existing pipeline. The bursting head and the pneumatic tool will be expanding the area currently occupied by the existing pipe when pulling in the replacement line.

Usually, a cover depth of three ft is sufficient when replacing lines size for size. Use four ft of cover for lines that are being increased one pipe size. Use greater cover if the line is increased by more than one pipe size. The deeper the existing line, however, the less surface disruption is a concern.

Adjacent Buried Utilities. Because of soil compaction, when you replace lines using pipe bursting, maintain a proper distance from adjacent buried utilities. A minimum suggested distance is 24 in., but this depends on soil conditions and the new line's size. If there is any doubt about location, specify excavation to physically locate the buried utilities.

Time Frame of the Bursting Operation. To minimize inconvenience to property owners by any service line disruption, the specified length of a single pipe burst run should include the reconnecting of any laterals so the operation can be done in one day.

Minimizing Insertion or Access Pits. The location and number of insertion or access pits should be planned and approved. Some pipe bursting methods have pneumatic tools that reverse out of manholes, thus minimizing both manhole restoration and insertion pits.

Bypass Sewage. Municipalities often require that the pipe bursting contractor provide for continuous and adequate sewage flow around the pipe section receiving the replacement pipe. This requirement depends entirely on line flow during the burst. If flow is minimal during the bursting time frame, the bypass requirement can be reduced.

Contractor Certification. Municipalities will want to specify that bidding contractors either have pipe bursting experience of a certain pipe size and/or on-site support from the manufacturers of pipe bursting equipment. Raising the standard too high for this relatively new trenchless replacement method, however, may exclude able and quality-conscious contractors from the bidding process. Requiring hundreds of thousands of feet of pipe bursting experience is not necessary, especially when manufacturer or local dealer support is on hand.

Mr. Yach is a Technical Writer based in Des Moines, Iowa.
                   CHANGE IN CAPACITY FROM LINE UPSIZING
Original Upsized  % Change
   ID      ID    in Capacity
  6 in.   8 in.      +77
  8 in.  10 in.      +56
  8 in.  12 in.     +125
 10 in.  12 in.      +44
 10 in.  14 in.      +96
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Author:Yach, Richard
Publication:Public Works
Geographic Code:1USA
Date:Jan 1, 2000
Words:1395
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