Pipe Bursting

Pipe bursting is trenchless pipe replacement technique in which a cone shaped tool (bursting head) is inserted into the existing pipe & forced through it, fracturing the pipe & pushing its fragments into surrounding soil. At the same time new pipe is installed by pulling it into the place the new pipe can be of the same size or larger than the replaced pipe the rear end of the bursting head is connected to new pipe & front end of the same is connected to either a winched cable or a pulling rod assembly. The bursting head & new pipe are launched from the insertion pit. The cable or rod assembly is pulled from reception pit.

The nose of bursting head is often smaller in diameter than the existing pipe to maintain alignment & to insure uniform burst. The base of bursting head is larger than diameter of existing pipe to be burst; it is also larger than diameter of replacement pipe to reduce the friction

Typical Pipe Bursting operation layout.

Main classes of pipe bursting

Pneumatic Pipe Bursting:

     

Pneumatic pipe bursting

·        In this case, head is a cone shaped, soil displacement hammer, driven by compressed air with the rate of 180-580 blows/min.

·        The percussive action of the bursting head is similar to hammering a nail into a wall, where each impact pushes the nail small distance further into a wall. In like manner a bursting head creates a small facture with every stroke, and thus continuously breaks the old pipe.

·        The percussive action of the bursting head is combined with tension from cable, which is inserted through old pipe & attached to front end of bursting head. It keeps the bursting head pressed against the existing pipe wall, pulls the new pipe behind the head.

 Hydraulic expansion:

 In this system the bursting tool consist of 4 or more interlocking segments, hinged at ends & middle. An axially mounted hydraulic piston drives the lateral expansion & contraction of the head. The expansion of the head leads to lateral breaking of pipe. And as tool advances, after contraction, new pipe is replaced connected at the rear end.

Static pull:

       In this system, the tensile pull given to the bursting tool is converted to a radial force due to cone shape, is the main force for breaking of pipe. This can be performed by two ways viz.- by rod assembly & by winch cable assembly. Rod assembly method proceeds in consecutive sequences since rod reached in reception pit have to be removed whereas winch cable is continuous method.

Design consideration

Parameters listed below will prove to be guidelines for selection of replacement pipe, bursting length, bursting system etc

Ground condition:

·        Readily compactable which limit outward ground displacements to a zone closer to  pipe alignment.

·        Relatively stiff which allows the expanded hole to remain open while the new pipe is being installed. It also results in lower drag and reduced tensile stress on pipe while replacement.

·        In ground condition such as sand & crawfish type soil below the GWT may result in change in invert position if pneumatic system is used, which can be eliminated by static pipe system.

·        In densely compacted soil, backfills, dilatants soil, expansive soil &collapsible soil are less favorable condition because they increase the pulling force & zone of influence of ground movement.

·        If hard rock present close to pipe, the bursting head will tend to displace towards softer soil.

·        Variation in properties along the length leads to change in grade &/or alignment.

Groundwater (g.w.) condition: 

Bursting in saturated soil can cause the water pressure to rise around the bursting head, unless soil has permeability to allow water pressure to dissipate quickly. In some cases, g.w. can have buoyant & lubricative effect on bursting operation. If g.w. is removed in large degree densification of soil surrounding the existing pipe can result in increase of bursting forces.

Host pipe:

The material of pipe to be bursted effects significantly as pulling force greatly depends on it. The pipe of materials such as clay, plain concrete, cast iron, asbestos cement is good while of steel are troublesome for operation.

·        It is considered that 8 inch pipe is easier to burst than a 4inch one. An upsizing of replacement pipe up to 30% is generally practiced.

·        The depth of the host pipe affects the expansion of surrounding soil. Insertion & reception pits grow larger & more complex as the depth increases. Change in grade or bends should be given due consideration

Surrounding utilities: 

Utilities that interfere with bursting should be located & exposed prior to burst. Generally, underground utilities in moderate conditions are unlikely to be damaged by vibrations at distances greater than 2.5 feet from bursting head and at distances greater than 2-3 times dia. from the pipe alignment.

Replacement pipe:

HDPE & MDPE are the most common pipe materials having the properties continuity, flexibility & versatility. It is experienced that long term loading governs the selection of pipe wall thickness for non-pressure pipe, rather than pull strength. An additional 10% of thickness requirement can be allowed for sacrificial scarring. Other materials used in pipe bursting are cast iron, vitrified clay. It is preferable that all pipe joints are designed for trenchless installation i.e. to have a nominally flush exterior profile.

Number of pits & length of bursting:

Location of insertion pits should be such that their no. is minimized & length of bursting maximized. The size of existing pipe & the upsizing percentage has an effect on the safe length of bursting. In sewer replacement jobs, the burst length is usually from manhole to manhole.

Effect on nearby structures: 

This is of vital importance. Laboratory studies, field studies, analytical & simulation techniques have been used to predict the soil displacements along the pipe adjacent to the pipe being burst. Local excavations made to relieve induced stresses.

Construction consideration

·        Preliminary work: In this stage of work, Inspection of the line to be replaced is done& then cleaning of that line takes place.

·        Service excavation: The services are usually excavated prior to bursting to provide temporary bypass service & to protect the services during the bursting operation.

Preparation of insertion &reception pit: 

For sewer application insertion & reception pits are usually excavated in front of manhole or manholes are removed and replaced. For replacement of gas & waterlines, service pits can be expanded & used. All pits should be prepared & shored in an approved manner. The insertion pit must be large enough to allow the pipe to be inserted.

Replacement pipe preparation: 

When a replacement pipe is of polyethylene, it is delivered on to the site in segments& butt-fused into a continuous pipe on site trained personnel are employed for butt fusion following the recommended method. Quality control can be achieved by controlling the length of heating, fusing & cooling time as well as temperature & pressure of each joint. The replacement pipe should not be dragged over the ground surface; instead the pipe should be moved over roller or slings for insertion & transportation. The pipe ends should also be capped to prevent entry of foreign matters for water or gas piping.

Equipment installation: 

The winch is placed into an reception pit, and the cable pulled through the pipe and attached to the front of the bursting unit in an insertion pit the winch, cable & cable drum mist be provided with safety cage and supports so that it may be operated safely without injury to persons to a party. For all static rod 7 cable pull machine, the machine should be properly braced to resist the horizontal force necessary for the bursting operation.

Bursting operation: 

When the winch cable connected to front of the bursting tool is pulled the tool advances through a bursting length; cracks & fractures the pipe then new pipe attached at the rear of the tool advances with it & get replaced in the position of old pipe.

Reconnection of services & annular space sealing: 

There should be at least 4 hours relaxation period for pipe after installation. After this time the annular is sealed. Smooth & watertight sealing is extended a minimum of 8 inches into a manhole wall. Service connections are reconnected to the new pipe by saddles made of the material compatible with that of pipe. These connections should be installed in accordance with manufacturers recommended procedure.

Manhole preparation: 

Entry & exit holes from manhole must be enlarge to accept the new pipe as required. The sewer manhole may need modification to allow tool passage.

Troubleshooting in pipe bursting jobs: 

Sometimes bursting head gets stuck at an unexpected obstruction in or around the pipe e.g. steel repair clamps on water pipes.

Effect on surrounding environment

Ground displacements: The soil displacement expands from the source through the soil in the direction of the least soil resistance. They are a function of both time & space. The ground displacement depend primarily on

1.      degree of upsizing

2.    type &compaction level of the existing soil around the pipe

3.    depth of bursting

In the relatively homogeneous soil with no close rigid boundaries, the displacements are likely to be directed upwards at smaller depths (A), while at increased depth they are expected to have more uniform direction (B). As backfill material in a trench is often weaker than the original surrounding soil & hence the displacement is restricted to trench only (C). If the existing trench or pipe bedding is weak the pipe may move downward instead of upward (D). The ground movements generally tend to spread symmetrically around the vertical access through the existing pipe. But proximity of a rigid boundary may break the vertical symmetry & shift the surface heave to the side (E). If pipe is upsized in a loose soil that will be compacted by ground vibration. The diameter increase is compensated by soil compaction within a short distance of the pipe & outside of that zone settlement may occur (F).

Positioning of replacement pipe: The bursting head has a typically a larger diameter than the replacement pipe, a cavity is created in the soil, which allows the replacement pipe to take different position within the cavity, depending on longitudinal bending of the pipe & localized ground movement.

The position of new pipe generally depends on the soil characteristics, site conditions & installation procedure. If the soil displacements are directed primarily upwards, the new pipe has its centerline higher than the original pipe, but matches inverts elevation (A, C). If soil expansion is more uniform in all directions, the new pipe matches the centerline of existing pipe (B). When ground movements are directed primarily downwards, the new pipe matches the crown of existing pipe, but invert is at lower elevation (D). If ground movements are asymmetrical, new pipe may move laterally (E).

If the existing pipe has depth variation along length, may create grade problem for pipes replaced.

Disposition of pipe fragments:

The test carried out at TTC test site shows that the pipe fragments generally tend to- 1. Settle at the sides & bottom of new pipe in sandy backfill or 2. All around the perimeter of new pipe in clay or silt backfill. The fragments tend to locate somewhat away from the replacement pipe, with a typical separation of up to ¼ inch. Orientation of pipe fragment is important when the risk of new pipe perforation is to be estimated. After study it is found that, the small pipe fragments with a 20-degree tip, & oriented at 90 degrees to the top of pipe are more risky.

Ground vibration:

All pipe bursting operation create to some extent vibrations of soil particles in the ground. The study showed that none of the pipe bursting technique is likely to damage nearby utilities if they are at a distance of few feets from bursting head. The vibration levels due to bursting depend on the power applied through the bursting process, & therefore on the size & type of the existing pipe, & degree of upsizing

Effect on nearby utilities:

Ground movements during the pipe bursting operations may damage nearby pipes. From safety point of view both horizontal & vertical distance between the pipe to be burst & the existing adjacent pipe should be at least 2 times diameter of replacement pipe.

Stress in replacement pipe:

Pulling force applied to pipe behind the bursting head produces axial stress in new pipe, which has to be withstanding without failure or damage. If soil dose not immediately collapse around the replacement pipe, the axial stress developed in the pipe is due to friction created by weight of the pipe, otherwise additionally due to friction offered by soil. The stress in replacement pipe can be lowered with the use of lubrication mud, which both delays the collapse of soil around the replacement pipe & reduces the pipe-soil friction coefficient.

Advantages over open cut replacement:

·        Pipe bursting is much faster than open cut.

·        Pipe bursting proves to be more efficient.

·        It is experienced that pipe bursting is often cheaper than open cut.

·        It is environment friendly.

·        It is less disruptive to surface features than open cut.

Limitations:

·        Ductile iron & steel pipes are not suitable for pipe bursting. (pipe splitting is used.)

·        Pipe bursting is not effective in expansive soil.

·        Close proximity of other service lines may create problem.

·        Point repairs, if any, create problem, because it reinforces the existing pipe with ductile material.

·        Pipe collapsed at a section create obstruction to pipe bursting process.

Economic feasibility

The cost advantage is especially notable in sewer line replacement, where an enhanced depth of line increases the cost of open cut replacement through extra excavation, shoring, dewatering, etc., while has minimal effect on cost of pipe bursting. Additional advantages of pipe bursting over the open cut replacement are indirect cost savings, due to-

1.      Less traffic disturbance

2.    Shorter time for replacement

3.    Less business interruption

4.    Less environmental disturbance.

Cost comparison