Jacking is a term used to describe the application of a force with the aim of pushing or lifting an object. This act is typically done by mechanical means using several different devices.
In the trenchless construction and rehabilitation industry, jacking is used to install subterranean pipes by pushing them through the soil. In this case, a jacking rig applies the jacking force at the entry pit, which helps to advance the pipe through to the exit pit. Jacking is also used in another trenchless technique known as thrust boring. During this procedure, an auger with a cutting head is placed in a steel casing and pushed through the soil via a jacking rig.
In both pipe jacking and auger boring, segments of pipe are gradually lowered into the entry pit where they are all jacked in succession. The jacking force from the rig is transferred from pipe to pipe until the installation is completed at the exit pit.
The term is sometimes used interchangeably with pipe jacking or pipe ramming.
Trenchlesspedia Explains Jacking
For the jacking process to be successful, two main components must be present:
- A mechanism to provide a reaction to the jacking force.
- A means of evenly distributing the jacking force to the installed pipes.
The first, and probably most essential element of jacking is the reaction mechanism. In order for the jacking rig to apply enough force to overcome the soil frictional forces, it needs to be able to produce a reaction that is equal but opposite to the jacking force. This is accomplished by using a thrust wall. The thrust wall is essentially a concrete wall that braces the rig during jacking. As the hydraulic jacks of the rig push against the pipes, the rig is also pushed against the wall, which provides the necessary reaction force.
The other crucial element of jacking is the thrust ring. This component is basically a steel ring with the same diameter as the installed pipe. The thrust ring ensures that the force from the hydraulic rams is distributed to the pipe edges equally. Unequal distribution of the jacking force can result in stress concentrations, which can damage the pipe.
Pipe Jacking vs. Ramming
While these two terms are sometimes used interchangeably, jacking and ramming are two different processes. Jacking involves the use of hydraulic pistons, which apply a uniform or gradual force to the pipe. Jacking essentially pushes the pipe through the soil on its intended path.
Ramming, on the other hand, involves the use of a pneumatic hammer (also known as a ramming tool) to drive the pipe through the ground. This hammer applies successive high-frequency percussive blows to the pipe, driving it from the entrance pit to the exit pit (similar to driving a nail through an object with a hammer).
Furthermore, pipe ramming does not have a navigation system. Some jacking methods, on the other hand, can be laser-guided for better accuracy. This difference makes jacking more suitable for longer drive lengths where tight dimensional control is required.