New Installation of tunnels, pipelines for gravity systems, conduits and culverts.
Applicable for installation of gas, water, cable, sewer, surface water channels in man-entry diameters up to 3 m or more over lengths limited only by the ground conditions and by the designed jacking pipe strengths and pipe string design. Pipe jacking is suitable for ground conditions from soft but self-supporting soils to rock.
With open-face systems there is a risk of groundwater entry and loss of face support in cohesionless soils under groundwater so closed-face systems or microtunnelling are likely to be preferred in those conditions.
Pipe jacking requires man-entry into the tunnel and all health and safety requirements for confined space working must be observed.
Pipe Jacking is a trenchless technique that allows buried pipelines tunnels and service conduits to be installed without the need to excavate large trenches thereby minimising disruption to traffic, the environment and local populations.
Pipe Jacking is a technique similar to microtunnelling but utilised only at larger, man-entry diameters. The ground excavation machine or shield is advanced through the ground along a pre-determined route by jacking it forward from a launch shaft using specially designed high-strength jacking pipes. Navigation is achieved using specialist systems which may be laser targeting for straight drives or more sophisticated for drives with included curves.
The difference between Pipe Jacking and microtunnelling is that Pipe Jacking is normally undertaken in pipes/tunnels where man-entry is possible such that the control of the excavation shield is achieved from within the shield as opposed to remotely from surface.
Pipe Jacking can be undertaken using closed face or open face tunnelling options with a variety of cutter head options.
Closed Face – This method operates using either a Slurry Face Support and spoil removal medium or an Earth Pressure Balance technique (see microtunnelling). With both systems excavated spoil is to some extent retained in the excavation face cavity. Slurry techniques use a water/bentonite/polymer slurry mix that is pumped through the cutter head to remove excavated spoil. The pressure maintained at the face gives a degree of face support and the thixotropic nature of the slurry can be altered to improve face support characteristics. The Earth Pressure Balance technique keeps excavated spoil under pressure in the cutter head cavity to provide the face support necessary with only sufficient spoil being removed using a screw feed to allow the machine to advance.
A variety of open face options are available for Pipe Jacking works.
- These include:
- Hand Excavation: excavation of the face by hand with shovels or hand-held power tools. Spoil is collected using hand tools or mini excavator diggers for loading onto a conveyor or into transport trucks or rail cars.
- Rotating Cutterhead: similar to the closed face option but an open that does not isolate the face from the tunnel, excavates spoil which is collected at the face foot by gathering arm loaders for placement onto a conveyor or into spoil transport trucks/railcars for removal to the launch shaft and surface.
- Excavator Arm: a backhoe bucket digs the spoil from the face for loading onto a conveyor or into spoil transport trucks/railcars for removal to the launch shaft and surface.
- Roadheader: similar to the excavator arm option but using a roadheader attachment to cut the face.
Advance of the Pipe jacking shield is controlled by a driver positioned in the shield using controls that advance the pipe string from the jacking frame located in the launch shaft.
On longer drives there is the option to install intermediate jacking stations at intervals along the pipe string between new jacking pipes. This allows shorter lengths of the pipe string to be advanced at any one time reducing the jacking forces necessary to move the pipe string forward thereby reducing the necessary load capacity of the jacking pipes and also the thrust capacity of the jacking frame at the launch shaft and is reaction system.