Pipebursting uses systems that break the existing pipe structure, expand the size of the void created to enabling the replacement pipe to be inserted into it. This can be achieved in one of three ways:
- Dynamic Pipebursting (pneumatic machine system)
- Static Pipebursting (hydraulic rod system)
- Expansion shell bursting (hydraulic machine system)
Dynamic Pipebursting utilises equipment that is based on that used for Impact Hammer installations.
The dynamic pipebursting machine consists of a pneumatically powered impact hammer that is fitted with an expansion sleeve that is sized to provide the required void to accept the replacement pipe.
For size for size replacement the expansion sleeve is greater in diameter than the pipe being replaced and for upsizing the sleeve is sized to ensure the replacement pipe can be readily installed.
A winch cable is passed through the existing pipe and is attached to the front of the pipebursting machine. The winch is then used to draw the pipebursting machine into the existing pipe.
As the impact hammer is operated its forward thrust reacts against the force within the winch cable and advances the pipebursting machine forward into the pipeline. It fractures and fragments the old pipe and displaces the fragments radially outwards into the surrounding ground. The soil adjacent to the pipeline is compressed to provide a void of sufficient size to accept the new pipe which is introduced directly behind the pipe bursting device. The winch cable also acts as a guide to ensure that the pipebursting machine maintains its position within the bore of the existing pipe and does not deviate into the surrounding ground.
The replacement pipe is usually attached to the rear of the pipebursting machine and is pulled into position as it advances. Once the pipe bursting machine arrives at the target excavation it is detached from the replacement pipe which is then ready for connection to the pipeline network.
The following should be noted in relation to the operation of this system:
This system operates best in compressible soils such as soft clays. Hard clays and dry well graded sands pose the most challenging conditions as these soils are much more difficult to displace.
The pipe to be replaced should be constructed of a brittle material such as cast iron, vitrified clay, unreinforced concrete, asbestos cement etc. Pipes made from ductile materials such as steel or ductile iron can be replaced but special pipe splitting equipment is required which is detailed below.
It should be noted that there is the potential for the broken shards of the existing pipe to be orientated so that they are not fully pushed into the surrounding ground and any sharp protruding edge or point can cause surface scoring on the outer circumference of the replacement pipe. This is a particular problem when a semi-rigid pipe such as those manufactured in polyethylene are selected as the replacement pipe. Care should be taken in selecting the SDR rating of the replacement PE pipe to ensure that it has adequate wall thickness to operate at its design pressure rating should any minor surface scoring occur during installation. Polyethylene pipes with a hardened outer skin are available which can be used to help overcome this potential problem.
Static Pipebursting utilises a hydraulically powered push/pull rig that can advance and retract metal rods through the existing pipe applying significant tonnage to the rods as they are retracted. The pipebursting rig is set up in a launch access pit or manhole (smaller machines are available to suit this latter application). The replacement process commences with the rig used in push mode to introduce a string of steel rods inside the pipeline to be replaced. Once the reception point has been reached, the bursting head, which is a static steel expander cone, is fixed to the lead rod.
The replacement pipe is then attached to the bursting head and the rods are drawn back to the launch point with the rig in pull mode. This action fragments the existing pipe, pushes the shards into the surroundings ground, creates the required void and simultaneously installs the new pipe. The previously prepared new pipe string which is usually a polyethylene pipe is attached to the bursting head via a swivel. Once the bursting head arrives at the launch pit the assembly is removed and the new pipe is in position and ready to be connected to the rest of the network.
As with Dynamic Pipebursting there may be interaction between the broken shards of the old pipe and the replacement pipe so similar precautions should be taken when selecting the new pipe.
Repair collars that may be in place on the existing pipe can cause a problem as they are usually not made of a brittle material that the bursting head can fracture. However, the bursting head can be designed so that it incorporates cutting blades that will allow these potential obstacles to be overcome. They are designed to cut through these collars rather than break and fracture them, eliminating the threat.
Another form of Static Pipebursting is the use of a cable-based system. Here instead of using bursting rods to apply the bursting force when pulling back a bursting head a wire cable is used. These systems are normally used on small diameter pipes over relatively short distances and they have the following advantages over other rigs:
- They are lightweight and compact
- They have a small footprint and can be used from small excavations or chambers
- They are easier to transport, install and use because once in place, unlike bursting rods, the cable does not have to be handled by the operator during the bursting process.
The operation of this system is very similar to that of rod based systems.
Expansion Shell Pipebursting utilises a specially designed machine which is usually hydraulically powered to provide lateral bursting power, but they have no forward motive power.
The unit must be winched through the existing pipe to allow it to be used to complete the pipe replacement. This system is not favoured for replacing long lengths of small diameter pipes as it is far more time consuming than the dynamic or static pipe bursting systems. Its main application is for sewer replacement as due to the bursting heads short length it can be introduced into the existing pipeline via existing manholes eliminating the need for expensive deep access excavations.
For the operation, a winch cable is passed from the reception point through the existing pipe to the launch access pit or manhole. The expansion shell bursting head is attached to this cable and placed inside the existing pipe. The new pipes which are normally short length with integrated push fit joints are attached to the rear of the expansion shell unit.
A hydraulically-powered pushing unit is used to push the replacement pipes behind the expansion shell unit as it advances. A tensioning system can be used to ensure that the replacement pipe joints remain intact during the replacement process. The expansion shell equipment is pulled into the pipe and the short distance that it is moved into the pipe is a function of its length. The hydraulic mechanism within the bursting head is activated and this causes the shell to expand. This expansion exerts significant lateral force onto the old pipe which fractures and fragments it and pushes the shard into the surrounding ground. The shell is then retracted to its normal position and winched further into the existing pipe by a distance that is equivalent to the length of the shell bursting head and the process is repeated. This procedure is continued over the full length of existing pipeline being replaced until the shell bursting head reaches the winch access pit. The shell bursting head is then removed leaving the new pipe in place ready to be restored to service.
Careful consideration of the ground conditions should be made before selecting this system with the short length pipe option as the following problems can occur:
- In very wet soft conditions, such as running sand, joint sag may occur due to the pipes sinking under their own weight. In addition, expansion shell machines have open channels when in the expanded position and this can allow sand to enter into the machine and stop its operation.
- In conditions where high skin friction is expected, such as very soft clay, there is the potential to damage the replacement pipe joints due to the exertion of jacking forces that are greater than the design joint strength during pipe installation.
Again, as with Dynamic and Static pipebursting, there may be interaction between the broken shards of the old pipe and the new pipe so similar precautions should be taken when selecting the new pipe.