An Introduction to Pipejacking
The pipejacking Technique
The major applications for pipejacking and microtunnelling include new sewerage and drainage construction, sewer replacement and lining, gas and water mains, oil pipelines, electricity and telecommunications’ cable installation, and culverts. Special applications include the installation of rectangular or circular sections for pedestrian subways, road underpasses and bridge abutments.
The technique can be used to negotiate obstacles such as motorways, railways, rivers, canals, buildings and airfields in the path of pipe laying projects; to minimize the surface disruption frequently associated with open cut pipe laying methods in urban areas; or simply to provide a permanent underground tunnel construction.
Advantages and disadvantages: Trenchless method
- Decreasing cost of surface repair
- Decrease Time of project (Trenchless method is very faster than open trench)
- Crossing under the river and other structures
- Less interface with infrastructures
- Less disturbance in Traffic
- Increasing pollutions
- Environmental advantages
- Ability to work under water
- More Safety
- Earth moving is reduced to minimum
- Construction work in all weather
pipejacking, generally referred to in the smaller diameters as microtunnelling, is a technique for installing underground pipelines, ducts and culverts. Powerful hydraulic jacks are used to push specially designed pipes through the ground behind a shield at the same time as excavation is taking place within the shield. The method provides a flexible, structural, watertight, finished pipeline as the tunnel is excavated. There is no theoretical limit to the length of individual pipe jacks although practical engineering considerations and economics may impose restrictions. Drives of several hundred metres either in a straight line or to a radius or a series of radii are readily achievable. A number of excavation systems are available including manual, mechanical and remote control. Pipes in the range mm to 3000mm, can be installed by employing the appropriate system. Construction tolerances are comparable with other tunnelling methods, and the pipejacking method generally requires less overbreak than segmental tunnels and provides ground support and reduces potential ground movement.
Mechanical excavation methods are similar to those employed in other forms of tunnelling. Shields, excavation and face support can be provided for a wide variety of ground conditions.
In order to install a pipeline using this technique, thrust and reception pits are constructed, usually at manhole positions. The dimensions and construction of a thrust pit vary according to the specific requirements of any drive with economics being a key factor. Pit sizes will vary according to the excavation methods employed, although these can be reduced if required by special circumstances.
A thrust wall is constructed to provide a reaction against which to jack.
To ensure that the jacking forces are distributed around the circumference of a pipe being jacked, a thrust ring is used to transfer the loads. The jacks are interconnected hydraulically to ensure that the thrust from each is the same.
A reception pit of sufficient size for removal of the jacking shield is normally required at the completed end of each drive. The initial alignment of the pipe jack is obtained by accurately positioning guide rails within the thrust pit on which the pipes are laid. To maintain accuracy of alignment during pipejacking, it is necessary to use a steerable shield, which must be frequently checked for line and level from a fixed reference.
For short or simple pipe jacks, these checks can be carried out using traditional surveying equipment. Rapid excavation and remote control techniques require sophisticated electronic guidance systems using a combination of lasers and screen based computer techniques.
pipejacking is an inherently safer method of working than open trench construction or traditional segmental tunnelling. When considering the risks associated with deep, large section, open excavations, Health and Safety Executive guidance suggests these risks should be reduced “if appropriate using ‘trenchless’ technology to avoid the need to excavate the trench in the first place”. Given gang size differences between the techniques and the resulting reduction in man-hours, opportunities for accidents to occur are less with pipejacking. There is also significant reduction in the risk of injury as a result of utility strikes and interface with the public.
There are substantial environmental benefits to be gained by the use of pipejacking techniques when compared with the traditional open trench approach. Typically the ‘trenchless’ method will reduce the quantities of incoming and outgoing materials, with a consequent reduction in tipping of spoil and quarrying of imported stone fill. This in turn leads to reduced vehicle movements and subsequently less associated disruption.
In many cases use of pipejacking techniques instead of open trenching will contribute positively towards workplace safety, the interface with the general public, and the local and wider environment.
Technical benefits associated with pipejacking are:
- Inherent strength of lining
- Smooth internal finish giving good flow characteristics
- No requirement for secondary lining
- Considerably less joints than a segmental tunnel
- Prevention of ground water ingress by use of pipes
- with sealed flexible joints
- Provision of invert channels in larger pipes to contain
- the dry weather flow of a sewer in a combined system
- Less risk of settlement
- Minimal surface disruption
- Minimal reinstatement
- Reduced requirement for utilities diversions in urban areas
Design and construction methods
Prior to embarking on a detailed construction design and method analysis, the client’s engineer will generally have ascertained the basic design parameters to meet the requirements of the scheme.
For a sewerage system these are likely to include:
- Hydraulic requirements
- Preferred route
Following an assessment of engineering, environmental and cost parameters, pipe jack excavation method and shaft construction will also be governed by a number of
factors which include:
- Ground conditions
- Details of existing services and underground structures
- Location of manholes and working areas
- Lengths required
- Diameters of pipeline
Ground conditions will play a major role in determining the type of shaft to be constructed, the pipe jack excavation method and any ground support systems to be used. Each of these may have limitations in terms of either the diameter or length of drive. The interface between these variables, together with physical considerations, such as the location of manholes and the size of working areas, will provide an optimum solution or range of solutions which can then be appraised on the basis of cost and value engineering.