Pre-support in tunnel construction is becoming increasingly important as transportation systems expand and more and more roads and railways have to be run through areas of unstable ground. In these areas, tunneling can cause ground settlement, endangering buildings and other structures on the surface as well as the people working inside the tunnel.
The answer is pipe roofing – the technique of pre-reinforcing the ground ahead of the tunnel face to ensure that the excavation can proceed safely until permanent support structures can be installed.
To meet these demands, Atlas Copco has developed The Pipe Roof System – a complete system for pipe roofing including everything from consumables to drilling and grouting equipment.
At the heart of the package is the Symmetrix casing system (see page 18) coupled with Atlas Copco’s wide range of Boomer drill rigs and Unigrout grouting units.
The steel pipes are installed ahead of the tunnel face and arranged like an umbrella or canopy around the tunnel profile. The umbrella stabilizes and protects the roof and face of the tunnel by increasing the load bearing capacity of the ground through better load distribution.
Generally, pipes with an outer diameter of 76–140 mm and with a wall thickness of 6–10 mm are installed using standard tophammer tunneling equipment. In specific cases, larger diameter pipes might be installed using special rigs and the Down-The-Hole (DTH) drilling method.
The pipe roof is commonly 9–18 m in length and, due to the overlap, the excavated length beneath is around 3–6 m shorter.
Ebrahim Nikafroozi, Atlas Copco Product Line Manager, explains: “As this method requires only standard equipment, no special training or other machinery is required, making it very easy and cost-effective. By reinforcing the ground ahead of the excavation, the ground is never without support which enables the installation of permanent tunnel support to be carried out more effectively and with increased safety for the operators and improves the risk management for a project in difficult ground.”
Experience gained from several tunnel projects around the world indicate that the method not only increases the stability of the tunnel itself as well as the working face, but also significantly reduces subsidence caused by excavation.
Using the same principals, installation method and equipment, perforated PVC and steel pipes can also be installed for drainage purposes, bringing added value in controlling the ground water in extreme conditions.
Epiroc operated under the trademark “Atlas Copco” prior to January 1, 2018.