Thanks for the comments from Ross Henderson and Morten
Rasmussen. Don't get me wrong but I am a big supporter of
trenchless techniques, including horizontal directional
drilling (HDD). However, I must point out again that, all
trenchless techniques are (good) tools for pipeline
installer. When used properly, they will install the pipe
safer, quicker and cheaper then open cut.

One of the problems of trenchless techniques is the
associated ground movement. From my experience, the soil
movement and it's influence on the road pavement and other
nearby services are not fully understood and has been
over-looked. We have developed a damage control procedures
for pipe bursting for Transco (gas) and the water industry,
but not many people (especially Contractors) aware of it or
use it. The promoters of trenchless techniques will always
tell you the advantages of their system, how powerful the
rigs are, how long the bore can go, but rarely the
limitations and precautions of using the system were
discussed.

HHD using dry boring will create significant ground movement.
The ground movement is small when using fluid-assisted
drilling, but not completely eliminated. Various case studies
of HDD all reported some ground movement immediately around
the pipe (hole) and at ground surface.

My job is to protect the existing gas distribution network
owned by Transco. About 45% of the network is made up of cast
iron pipe, which is very sensitive to ground movement.
Developing a damage control procedures for HHD would be my
goal, but at the moment, I need to have some feel of the
likely ground movement.

The driven pile example is equal to a cavity expansion from
zero to the final bore diameter, which will give me an upper
bound for the likely ground movement. I agree with Morten
that the problem is 3-D, just like all tunnelling problems.
But in this preliminary study, I think it is appropriate to
simplify the problem to 2-D, in which the pile driving case
(axisymmetric) will give me an approximate solution.

I agree that the problem is not simple and a more rigorous
approach is required in a later stage.

Regards

Paul

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Dr Paul C. F. Ng
Senior Geotechnical Engineer
Advantica Technologies Ltd
Ashby Road
Loughborough
Leicestershire
LE11 3GR
UK

Tel:    +44 (0)1509 282032
Fax:    +44 (0)1509 283118
E-mail: [log in to unmask]
http://www.advanticatech.com
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