Cities and urban areas around the world are continuously being modernized with infrastructures developed to match increasing traffic flows. As a result, tunneling projects are moving into more challenging conditions such as soil, weak rock, and shallow overburden in already developed areas. No matter what challenges a tunneling project is facing, compromises can never be made on safety and economic efficiency.
This calls for innovative and reliable methods of ground support such as pipe roofing to prevent settlements from excavation and protect nearby structures and people working inside the tunnel.
Tunnel excavation causes stress to the ground that may result in settlements. To address these challenges, pre-support is used, a method of pre-reinforcing the formation ahead of the tunnel face to ensure that the excavation can proceed safely until heavier and permanent support structures have been installed.
Technical developments have led to an increase in different pre-support technologies, preventing unwanted accidents and protecting surrounding structures from damage. Some methods, such as freezing and jet grouting, are recognized as both cost and time consuming, while the method known as pipe roofing has become increasingly popular because of its many economic advantages.
Experience gained from tunnel projects indicates pipe roofing support not only increases the stability of the tunnel itself as well as the working face, but also significantly decreases subsidence induced by the excavation. The popularity of this pre-support method has led to many different names: umbrella, umbrella arch, tube umbrella, pipe canopy, fore poling, or pipe roofing.
TerraRoc offers a complete package for pipe roofing, including everything from consumables to drilling and grouting equipment.
Renowned for its effectiveness, the Symmetrix casing advancement system is the cornerstone of the package. This system provides a comprehensive solution for concentric drilling and simultaneously advancing of pipes and offers several benefits in pipe roofing.
Straightness
In pipe roofing applications, maintaining straightness is crucial from both design and economic perspectives. The Symmetrix system, with its centric design and smooth drilling process, has demonstrated a consistent ability to produce exceptionally straight holes.
Internal Flushing
The TerraRoc Symmetrix system is ingeniously designed to utilize a flushing medium, primarily water, to transport drill cuttings from the bit face back inside the pipes. This feature is particularly beneficial when drilling in poor or unconsolidated ground conditions, as it enables the drilling of pipes without causing any disruption to the surrounding ground.
Efficiency and Productivity
Symmetrix systems can drill at high speed in virtually any ground. Customers can rest assured that the pipes will be installed successfully and efficiently independent of the encountered ground formation. The easiness to unlock and lock the pilot bit from the ring bit set also adds to the high productivity offered by the Symmetrix system.
Less Torque
The Symmetrix systems advances the pipes by impact energy simultaneously to drilling a hole slightly larger than the outer diameter of the pipes. This means, the pipe is not rotated while drilling, requiring less torque than alternative solutions. Customers can thereby utilize relatively small drilling rigs.
The Symmetrix system components
– A pilot bit drills the center part of the hole and transfers the impact energy to the ring bit.
– A ring bit set, which is an integrated ring bit and casing shoe, welded to the front of first pipe, known as the starter pipe. The ring bit, which is connected to the pilot bit through locking mechanism, drills the void needed to advance the pipe into the ground simultaneous to drilling.
Steel pipes
The pipes included in the TerraRoc pipe roof system offer are:
Threads
All pipes have trapezoidal thread with double start and 10mm entrance and thread ran out to assure easy make-up of the joints.
Welding
The Symmetrix ring bit set is welded to the front of the starter pipe using an automated robot to enhance the quality of welding and quality consistency.
Optional injection valves
Pipes can be delivered with 12 mm or 16 mm injection valves for respectively 3-5 bar or +10 bar injection.
Accessories
To complete the offer, a range of different tools and accessories including suitable wrenches, centralizers and injection caps can be provided when necessary.
Pipe Roof System – range
TerraRoc offers systems with
pipe diameters ranging from 76.1
to 139.7 mm and wall thickness
ranging from 6.3 to 10.0 mm.
Pipe quality and steel grade
Pipes or casings in the TerraRoc Pipe Roof standard assortment are made of high-quality, first-hand steel pipes longitudinally welded with removed welding seam. This ensures a completely round shape and good straightness along the pipe. Steel grade and other characteristics are shown in below table.
Steel grade and properties
| EN 10025 | DIN 17100 | Yield min. (N/mm2) | Tensile strength (N/mm2) | Elongation min. (%) | C max | Mn max | P max | S max | Si max |
| S355 | St 52-3 | 355 | 490 – 630 | 22 | 0.2 | 1.6 | 0.035 | 0.035 | 0.55 |
Note: Other Steel Grades can be provided upon request.
Steel pipes are installed ahead of the tunnel face, arranged like an umbrella or canopy around the excavation line. The umbrella stabilizes and protects the ceiling and face of the tunnel by increasing the load-bearing capacity of the ground. Thanks to the rigidity of the steel pipes, distribution of loads are improved, and the critical spot can be moved further ahead of the working face.
Requiring only standard equipment and little training of your operator crew, your investment in employing special machinery or contractors is notably reduced.
By reinforcing the ground ahead of the excavation, the ground is never without support. This means that the installation of regular tunnel support can be carried out more effectively and with increased safety for the operators.
Using the same principles, installation method, and equipment, perforated PVC and steel pipes can be installed for drainage purposes, bringing added value in controlling the groundwater, especially in extreme conditions.
Engineers consider various measures for design, including ground properties, overhead height, tunnel geometry, load analysis, and the surrounding structures, especially above the tunnel with regards to settlement analysis. The most critical specifications of the pipe roof, from a design perspective, are the outer diameter of the pipes, wall thickness, length of pipes and overlap, and spacing of the pipes.
Generally, pipes with outer diameter of 76-140 mm and wall thickness of 6-10 mm are installed using standard tunneling equipment. In some specific cases, larger diameter pipes might be installed using special rigs and Down-the-Hole drilling method.
The pipe roof length is commonly 9-18 metres and due to the overlap, the excavated length underneath is around 3-6 metres shorter.