Last edited 29 Oct 2020

Trenchless technology


[edit] Introduction

Trenchless technology methods are used mostly for installing utility pipes especially those running under busy roads or rivers.

Trenchless Technology can be defined as ‘the technology for placing new pipe, cable, or conduit in the ground between two defined points without continuous, open cut excavation between them, or for renovating, replacing, and rehabilitating’ (Kramer, Mcdonald et al. 1992).

Trenchless technology emerged in such countries as Japan and Australia where modern sewer networks were needed to serve increasing populations. The early trnchless technologies used in these countries were then adapted in the UK in the 1970s and early 1980s. (Evans, n.d)

Formerly, site investigations and trial pits were used to investigate ground conditions however over the years this has evolved to virtual visualisation. The use of software tools, visual prototyping and robotic devices have reduced the time taken, cost and risk endured in a project. (Beer, 2010)

The advantages of using trenchless technology as opposed to open cut excavation methods include:

(ref Piehl, R. 2005)

There are various methods of trenchless technologies that may be used, and the type of method chosen is dependent upon the pipe size that needs to installed, the depth it needs to be installed at, the soil conditions of the ground and the overall cost of the method.

Method Type of soil Other comments
Pipe Jacking All soil types soil classes 1 to 5 according to DIN 18300 Exception of non-displaceable hard soil and rock
Microtunnelling All soil types Very fast and reliable system
Impact Moling Soft clays and silts Minimum disruption
Auger Boring (soils with sufficient stand-up time) Suitable for shallower depths
Thrust Boring All soil types No limitation.

The two main methods of trenchless technologies are pipe jacking and tunnelling/microtunnelling due to their wide range of use within different soils.

[edit] Pipe jacking

Pipe jacking is used to describe the technique of installing man-entry pipes by adding sections of pipe at the drive pit and jacking the line forward to form the tunnel lining behind the cutting shield.’ (Kramer et al. 1992) Pipe jacking has been adopted widely since its first recorded use in 1892 in USA by the Northern Pacific Railroad Company. It is often preferred because of its simplicity and because it avoids settlement of ground.

[edit] Method:

  • The tunnelling machine has a thicker wall version with a female end fitted with a collar rimmed with a thin ring of fibre board and a male end fitted with a rubber seal. This connection forms a watertight seal between the two ends, minimising friction.
  • Jacks are extended and released to push the tunnelling machine into the earth and the speed of movement is synchronised with the force of the jacks. The pipes used for jacking need to possess the correct strength otherwise there can be failures which can be difficult to rectify.
  • Lubricant is required between the annulus and surrounding earth to minimise friction acting at longer distances.
  • Finally, manholes are built and shafts are backfilled.

Due to the technology used its cost can be relatively high, however it can save on other engineering costs.

[edit] Microtunnelling

This method entails installing pipes into the ground through remotely controlled hydraulic methods without the aid of personnel (unlike tunnelling). It requires the use of microtunnelling boring machines and Kramer defines it as ‘those methods that install pipes with a diameter of less than 36 inches (900mm) to a predetermined line and level by remotely controlling the cutting head.’(Kramer et al. 1992)

[edit] Method:

Microtunnelling uses two shafts and hydraulic jacks to fit the tunnelling system underground allowing permanent utility pipes to be inserted. This is done by the following method:

  • Shafts are sunk at each end of the intended drive, usually at man-hole positions, one end drive shaft and one reception shaft. The shafts must be long enough to accommodate the tunnelling equipment.
  • The microtunnelling machine is then lowered on guide tracks by a crane allowing it to be thrust forward by hydraulic jacks. It is guided by a laser which projects onto the target surface.
  • The excavated material is crushed by the head of the machine and any slurry is brought to the surface by slurry shafts. Any ground movements are eliminated by counterbalanced ground pressures. Progress is monitored via screens above the ground.
  • After the machine has reached the second shaft, excavation is halted and the pipes are disconnected.

[edit] Comparison between Microtunnelling and Pipe jacking

Essentially both methods use the same technique however pipe jacking allows for bigger diameters of pipes to be installed and ‘Microtunnelling has more limitations in terms of ground conditions. The smaller diameters and remote aspect of the operation make Microtunnelling more prone to problems where soil conditions change rapidly or when obstructions are encountered.’ (Kim et al. 2008)

[edit] Impact Moling

Impact Moling is “...a technique in which a percussive mole (soil displacement hammer) is launched from an excavation to displace the soil and form a bore. The new conduit is normally drawn in behind the mole or pulled back into the bore using the hammers reverse action.” (Cambridge New Media, 2005) Thrust

[edit] Boring

Boring is “...a solution for installing new pipes in virgin ground where accuracy is critical. Working from a compact launch pit a rod is thrust into the ground which pulls through a pipe into its proposed position.” (Tomlinson Brothers (Hucknall) Ltd, 2012)

[edit] Issues

Critical risks can occur from trenchless technology such as pipes bursting underground. Other risks include settlement of the ground which is caused by the ground movement during the installation of the tunnelling machines. This can be prevented firstly by maintaining stability of the excavation face and secondly by avoiding inadvertent loss of soil in the tunnel. Microtunnelling is able to avoid this consequence as the pressure acting on the pipe is always counterbalanced with the volume of soil being moved.

Another potential problem is groundwater flowing into the shafts and tunnels formed by the machines. Adequate ground investigation and site surveys are required to fully understand the water table beneath the ground before work begins.

Obstacles such as boulders can be encountered and this can require costly excavations. Some cases of rock encounter can be so severe that the trenchless method has to be stopped and a full open cut excavation method used to carry out the required work.

[edit] Related articles on Designing Buildings Wiki

[edit] External references

Designing Buildings Anywhere

Get the Firefox add-on to access 20,000 definitions direct from any website

Find out more Accept cookies and
don't show me this again