Tunnelling

Thrust Boring

Benex Pipelines has been using Thrust Boring technology for the rehabilitation, renewal and upsizing of sewer mains, storm water and reticulation assets since 1998.

Benex has successfully renewed over 40kms of pipes using Thrust Boring utilising 150mm, 225mm, 300mm, 450mm & 600mm internal diameter HDPE pipe sizes.

Benex is currently Thrust Boring and installing over 2km of HDPE pipe annually.

Thrust boring, or auger boring, is a jack and bore drilling method typically used for installing steel pipe casing beneath an existing surface where the risk of hole collapse whilst installing larger diameters has been identified or where the grade is critical. Thrust boring, is typically performed by placing an auger equipped with a cutting head inside a 200mm to 1500mm diameter steel pipe. The auger is then attached to the rotation shaft of a thrust boring machine. Steel pipe casing is typically installed in 6 metre lengths so an entry pit at least 10.6 metres long is required to accommodate the combined length of the pipe and thrust boring machinery.

The thrust boring machine is capable of drilling in a variety of ground conditions from sand through to reasonably hard rock making it a dependable and economical way of drilling.

Process

  • A large rectangular pit is usually dug on each side of the work area to accommodate the steel pipe and machinery used in this procedure. The pipe casing is jacked into place as the drilling is performed with any excess soil transferred out of the pipe by the auger’s blades.
  • Thrust boring is typically performed by placing an auger equipped with a cutting head inside a 220mm to 1500mm diameter steel pipe. The auger is then attached to the rotation shaft of a thrust boring machine. Steel pipe casing is typically installed in 6 metre lengths so an entry put at least 10.6 metres long is required to accommodate the combine length of the pipe and thrust boring machinery. An exit pit is usually dug on the opposite end of the work are although it is not always required. Auger boring can be effectively used to install pipe sections with a maximum combined length of no more than 70 metres.
  • Thrust boring generally works best in soils that are located about the groundwater table. When groundwater is present during a boring operation, special dewatering measures must be taken to prevent the steel pipe casing from being flooded with water. Once the final section of casing has reached the exit pit, the auger and cutting bit assembly is removed. After the thrust boring machinery has been detached from the steel pipe the carrier pipe or conduits are installed through the casing. Where grade pipe is installed, in the case of grade carrier pipes, timber blocks are strapped to the pipe to centralise the carrier pipe and maintain grade and invert levels. Any remaining space is usually filled with a Hi flow grout. Once the installed pipe work is connected to existing or new infrastructure both the entry and exit its can be bedded, haunched and backfilled.

Micro-tunnelling

This is currently the most accurate pipeline installation method available to the civil construction industry. Line and grade tolerances of +/- 10mm at 100m & +/- 40mm at 200m are the micro-tunnelling industry standard.  Benex specialises in Micro-tunnelling using a Laser Guided Boring Machine (GBM).

Ground Conditions account for an estimated 90% of the risk in any micro-tunnelling project. Good geotechnical information is critical to assess the type of equipment required to be used on the job e.g.  the type of cutters, the need for an outer encasing pipe, the possibility of freeboring in stable ground. All these influence feasibility, difficulty, cost and project risk. The best investment a client can make in a micro-tunnelling project is obtain good geo-technical information.

Micro-tunnelling Methods

There are 3 primary installations for micro-tunnelling and pipe installations.

  • Free Pipe – Where the ground has been tested and determined to be self-supporting (without the need for ground support). The Carrier Pipe is slip-lined into the micro-tunnell and the annulus space is grouted.
  • Close Fit Pipe – Where the ground has been tested and determined to be self-supporting (without the need for ground support). The Casing Pipe, which is also the carrier pipe, is slip-lined into the micro-tunnell. Ideal for designs where the between the Casing Pipe OD and the Micro-tunnell is < 25mm.
  • Pipe-in-a-Pipe – Where the ground has been tested and determined to be self-supporting (without the need for ground support). The Casing Pipe is sliplined into the micro-tunnel. The carrier Pipe is sliplined into the casing pipe and the annular space grouted.