Although commonly known by the generic of Pipe Bursting, the family of technologies covering the trenchless pipe replacement sector is more correctly known as Online Pipe Replacement. The technology family is nominally covered by four ‘sub’ technologies that include: Pipe Bursting, Pipe Splitting, Pipe ‘Eating’ or ‘Reaming’ and Pipe Extraction. These will be covered separately in the following.
Pipe Bursting and Pipe Splitting
These technologies can effectively be treated as one Online Pipe Replacement type, because in the systems that are now most commonly used the systems operate in the same way, only on different pipe materials using different types of bursting head.
Pipe bursting originated out of the Impact and Ramming hammer technologies with specially designed bursting head being positioned at the lead end of an impact hammer. The systems worked primarily on non-ductile pipe materials (unreinforced concrete, clayware, cast iron etc) in that the bursting head was hammered into the pipe being replaced with a bursting head that was larger than the diameter of the pipe. The impact energy passed into the pipe material shattering it. The hamer was designed with an expansion cone that pushed the broken shards of pipe out of the hammer path into the surrounding ground. The system does rely on the surrounding ground being compactable so as to enable this to take place. The replacement pipe is shackled to the rear of the impact hammer and is pulled into place in the void created by the advancing hammer as it proceeds through the pipeline.
A development on this technology was the use of a bursting head that is winched through the pipe to be replaced but which then bursts the pipe by using an expanding outer shell that is pushed out circumferentially from inside the pipe. Known as Hydraulic pipe bursting, the system breaks the pipe as in hammer technology but has less impact on the surrounding area as the bursting process does not impart impact energy into the ground so causing less vibration and less likelihood of damage to nearby services and buildng etc.
More recently the Hydraulic rod bursting technique has become increasingly popular. In this system a hydraulic rod pushing unit is placed in a start pit at one end of the pipe being replaced. A string of steel rods is then passed through the pipeline to a reception pit at the other end of the bursting run. A bursting head is then attached to the rod string, follwed by an expansion cone, a swivel and the finally the replacement pipe. Once the assembly is completed the hydraulic rod unit is used to pull the rod string back towards the start pit. This causes the bursting head and the rest of the assembly to be pulleed through the pipeline. The bursting head breaks the pipe up, the expansion head pushes the shards into the surrounding ground and the new pipe is pulled into place in the void created. Again this relies on the pipe being friable and the surrounding ground being compactable.
As many pipe materials are ductile in nature, the standard type of Hydraulic rod bursting system often had difficulties in effectively bursting the pipes insitu causing machines to stop in mid run. There was also found to be a difficulty with repair collars for a similar reason as these tend to be made of steel. So, as a further development to the pipe bursting system, new ‘splitter’ heads were developed. This technique is also now known as Pipe Splitting. Using exactly the same rod pushing/pulling rigs as for standard pipe bursting, the same technique is followed but with the splitter head instead of the bursting head.
The Splitter head works by first scoring the inside of the pipe being replaced, something like when glass is cut. A follow-up blade then slices through the split in the now weakened pipe, tearing it open. The expansion cone, instead of pushing shards into the surrounding ground, in this instance, forces the now split pipe apart to create the necessary void. The new pipe is pulled into the space created. There are a number of different splitter heads. Some
act as described above with a singler cutting action, others use multiple cuts to split the pipe into different segments that are pushed seperately into the surrounding ground in an attempt to reduce the potential for the elastic nature of the ductile pipe to close it back into place after the bursting assembly has passed through. This system is also applicable to asbestos cement, PVC and PE pipes as well as the more usual metal pipes, so can be used on more modern system replacement works as well.
The most recent development in the ‘rod’-based bursting field has been the introduction of the small footprint burster. Aimed at the smaller diameter pipeline market, these units can operate from very small start pits if not from existing manhole. They tend to utilise very strong compacted steel cable instead of pulling rods to apply the bursting force and can be used for both pipe bursting and splitting techniques.
Pipe Eating and Pipe Reaming
Both of these technologies have developed from other existing trenchless techniques.
Pipe Eating is an expansion of the capabilities of the microtunnelling unit. The cutterhead of a microtunneller can be designed to handle and cut a variety of materials including hard materials like concrete and clayware pipe. By special adaptation, a microtunneller can be used to excavate the existing pipeline along its route prodcing the pipe remains as part of the standard spoil material. The new pipe is jacked into position in the usual way behind the microtunnelling shield. The advatages of this system are that the whole of the old pipe material is removed from the ground and the line and level of the new pipe can be accurately controlled.
Pipe reaming on the other hand is a development of the HDD technique and is in many ways similar to rod bursting, but a surface launch variety. Here an HDD rig is set up at one end of the pipe being replaced, the drill string is passed through the pipe to a reception point further along the pipeline. A specially designed reaming head is attached to the drill string which, when pulled back towards the drill rig, can rotate and cut the existing pipe out of the ground. Drilling fluid is used to lubricate the process and remove the old pipe material as spoil. The replacement pipe is attached to the rear of the reaming head and is pulled into place as the reaming process proceeds. The advantage here is that one piece of equipment with relatively minor modifications can be used for two very different trenchless techniques. There is also no need for major start pit excavations.
Pipe Extraction is very much ‘what it says on the label’. Here a winching system is attached to a cable that is designed with either a set of pulling heads embedded in it in some way or a cable that has been grouted into the pipe to be extracted. The winch pulls on the prepared cable/pipe and pulls it out of the ground. The new pipe is either attached to an extension of the pulling cable at the pipes far end and pulled into the ground as the old pipe is pulled out or sufficient cable is left trailing to be able to undertake the new pipe pull-in after the old pipe is removed. The only advantage to doing two separate pulls is that the winch power requirement could be reduced as it has less work to do at any one time.