Civil Engineering Reference
In-Depth Information
5.11 Pipe jacking and microtunnelling
5.11.1 Introduction
Pipe jacking is one of a number of techniques (similar to jacked box
tunnelling) for creating smaller diameter tunnels, for example sewers and
other conduits, generally up to 3 m in diameter, which attempt to minimize
excavation from the ground surface (open-cut). These are often known
as trenchless technologies. Although not commanding the glamour of more
high profile transportation tunnels, smaller tunnels such as sewers, storm
water drains and other conduits such as high voltage cable ducts are an
important part of our underground infrastructure. A brief description of
the pipe jacking technique is provided in this section. More extensive details
on this, and other trenchless techniques, are given in a number of topics
devoted to this subject, for example Thomson (1995), PJA (1995), FSTT
(2004), Najafi and Gokhale (2004) and Stein (2005).
Pipe jacking is often used to install tunnels under highways, railway cross-
ings and canals, i.e. where access to the ground surface is restricted, or
where open-cut trenching would create a high level of disruption. In contrast
to conventional tunnels where the tunnel lining is constructed directly
behind the excavated face, in pipe jacking the complete tunnel lining sections
are precast and are pushed into place from a shaft. This forms a string of
pipe sections which are all moved through the ground until the desired
length of tunnel has been reached.
When constructing pipe jacked tunnels, it used to be the case that
individuals working in confined spaces would excavate the face within a
shield at the front of the tunnel (see Figure 5.6a in section 5.4 on shield
tunnelling). As with larger diameter tunnelling, however, more mechanized
methods have been introduced, this being a necessity for tunnels less than
0.9 m diameter (i.e. non-man entry size). For these very small diameter
tunnels miniature TBMs have been developed. This small mechanized TBM
development led to the term microtunnelling. In its simplest form,
microtunnelling is the use of a remotely-controlled, computer assisted,
miniature, TBM (EPBM or STM), which is advanced by pipe jacking (Kuesel
and King 1996).
There are a number of key issues when using the pipe jacking technique.
In particular the fact that the tunnel lining sections are all pushed through
the ground. This means that these lining sections have to take consider-
able axial compressive forces, which consequently dominate their design
(Milligan and Norris 1999). In addition, during the installation, large
friction forces can develop between the lining sections and the ground and
hence the force required to push them through the ground increases as the
length of the tunnel increases. These frictional forces can be reduced in a
number of ways. The tunnelling shield or machine is made a few centimetres
larger in diameter than the installed tunnel sections. This creates a gap into
