Installing a pipeline under a railway poses a number of challenges to contractors, and if any problems are encountered, the project could experience lengthy delays and extra costs. Here, microtunnelling expert Stuart Harrison looks at some of the key challenges and how these can be mitigated by selecting a microtunnelling contractor with the right equipment.
Changing ground conditions
Understanding the prevailing ground conditions is key to achieving a successful microtunnelling installation, as this will determine the tooling used and help the contractor plan.
“The more that the contractor knows about the ground conditions at and around the site, the less chance there is of unexpected complications occurring,” Mr Harrison said.
“This becomes even more important when microtunnelling under infrastructure such as railways because a sudden and unexpected change in ground conditions can cause a lot of problems.
“Imagine you’re microtunnelling under a railway and the ground changes or you come across an obstruction. The whole project has to stop, and in the worst case scenario — if you’re using a machine without a retractable head — you’ve potentially got a situation where the cutter head is stuck under the railway and needs to be dug out somehow, which is a difficult and costly job.”
However, the problem with ground conditions is that it’s not uncommon for contractors and subcontractors to arrive on site and find that they are somewhat different to the geotechnical information provided for a particular job. This is because core samples and the geotechnical inspection are like searching for needles in a haystack; it’s not hard to miss spots where the ground changes.
“This is where having the right equipment will increase the chance of success on projects where a pipeline needs to be installed under railways,” Mr Harrison said.
Saving the day with retractable equipment
Mr Harrison said to have the best chance of mitigating these problems and having a successful installation, you need to identify the difference in the geotechnical information versus what is actually there as early as possible.
“To be able to identify if the geotechnical information is wrong, you need a machine such as the Vermeer AXIS laser guided boring system, which has been designed to retract,” Mr Harrison said.
“Most microtunnelling machines can only move in a forward direction, however, the Vermeer AXIS can perform a pilot line through the ground and if any issues are encountered, it can retract, allowing the contractor to inspect the ground.
“Because the pilot line can be retracted, the contractor can effectively trial different drill heads during the pilot stage and assess which one will be the best for the prevailing ground conditions.
“Once the pilot is complete, the contractor can commence jacking the final product pipe with a far greater likelihood of success, as the ground conditions are now predominantly known.”
The other benefit of using a machine that has the ability to retract part-way through an installation is that no excavation is needed if there is a significant change in the ground.
“By being able to retract the drill head, contractors eliminate the need to excavate over the head in the situation of head failure or ground change,” Mr Harrison said.
“This feature ultimately saves time and money on what could become a difficult situation if the head becomes stuck directly under the railway.”
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