Definition - What does Pipe Jacking mean?
Pipe jacking is a trenchless method of installing pipes, conduits, and utility corridors by applying a force which pushes the pipe through the ground while controlled excavation takes place at the face.
The pipe jacking process begins with excavating relatively small entry and exit pits at the beginning and end of the pipe installation (usually at manhole locations). These pits are just large enough to accommodate the tunneling equipment and construction personnel. Once the pits are excavated, the hydraulic jacking rig and microtunneling machine are put into position.
The hydraulic jacking rig then applies a force that "pushes’" the tunneling machine through the wall of the entrance pit and into the ground. Once the machine reaches a predetermined position in the soil, a segment of pipe is lowered into the entrance pit behind the jacking rig and the microtunneling machine. An adaptor ring is typically used to link the pipe segment and the tunneling machine.
Next, the jacking rig, once again, applies a force that forces both the pipe and machine forward on their way to the exit pit. This process continues, with several pipe segments being jacked in sequence until the cutter head reaches the exit pit.
Pipe jacking can also be referred to as pipe ramming.
Trenchlesspedia explains Pipe Jacking
Figure 1: Illustration of the pipe jacking process (source)
Pipe jacking can be performed on various types of pipe material, including concrete, clay, and steel with standard diameters ranging from 150mm to 2,400mm. While pipe jacking is a versatile trenchless installation process, it is not a one size fits all solution.
To get the maximum value out of the pipe jacking process, several key factors need to be taken into consideration, including soil conditions, tunnel length, and pipe diameter.
Soil Conditions Suitable for Pipe Jacking
While pipe jacking can be used in various soil conditions, ranging from soft round to rock, it is recommended to carry out a detailed site investigation to determine the soil characteristics in the vicinity of the excavation. In general, pipe jacking is ideal for locations where it is not feasible to have operators situated inside the machine.
Engineers and contractors should, however, be wary when jacking in extremely weak soils, since there may not be enough soil strength to support the intended alignment. In such cases, strengthening or stabilization methods, such as ground freezing or grouting may be required.
Poor ground conditions may also possess inadequate strength to provide the necessary reaction against which to jack. In this case, piles or other strengthening arrangements may be needed to increase the reaction capability of the thrust wall.
These additional measures, do, however, increase the overall cost of the drive.
Achievable Tunnel Lengths and Pipe Diameters
Jacking lengths of over 1km can be achieved using this trenchless technique. The maximum drive length depends mainly on the engineering properties of the surrounding soil as well as the installed pipe diameter. For example, longer drive lengths may be difficult to achieve when jacking with smaller diameter cutting heads (less than 30 inches) since booster pumps and intermediate jacking stations may not be possible due to size limitations.