Just before the blast
Thanks to the new Galgenbuck tunnel near Schaffhausen, through traffic in the municipality of Neuhausen am Rheinfall will be greatly reduced. But residents will need nerves of steel during construction of the approximately one-kilometre-long tunnel – it is being blast excavated. We spent a day on site with the vibration monitoring experts from Basler & Hofmann.
Blasting under the suburban area
It is 5.30 a.m. at the Engi installation location in Neuhausen am Rheinfall. Vibration monitoring experts Thomas Rupp and Adriano Manuel are waiting for their ride into the tunnel. Even for Thomas, a man with many years of experience in this job, this is an unusual site. Tunnels are rarely blast excavated underneath suburban areas. But blasting was necessary for the Galgenbuck tunnel, due to the varying geological conditions and because the length of the tunnel – at just over one kilometre – is too short for a tunnel boring machine. The tunnel is being blast excavated under the houses of the municipality of Neuhausen am Rheinfall over the course of two years, with one to three blasts per day in the hours from 6.30 a.m. to 7.00 p.m. Just 20 to 60 metres of rock separate the houses from the new structure below. While the tunnel is slowly being blasted out of the ground, the vibration monitoring experts keep an eye on the effects of the blasts on the buildings above. They monitor the foundations of up to eight buildings at a time.
Today, the vibration monitoring experts want to take a much closer look and are heading into the tunnel to measure the emissions at the source. They get into the transporter.
In the tunnel
Preparation for the blast is well under way when Thomas and Adriano arrive at the face. A cherry picker lifts them to the roof section of the tunnel. They install their measuring devices around 30 metres from the blast site. Time is pressing – the miners want to detonate the explosives. They quickly pack away the tools and hurry to the safety cabin. The blasting horn signals the imminent blast. The sheer force of the explosives tears the rock apart. The safety cabin shakes. After only a short time, the tunnel’s ventilation system has cleared the dust. Thomas and Adriano hurry to their machines in order to check their measurements. The smell of fireworks and damp dust hangs in the air. The measurement was a success – a typical blast graph appears on the laptop screen. For around a week, the blasts will be recorded. The additional measurement data from inside the tunnel will serve the purpose of establishing how much energy from the blast is absorbed by the ground between the tunnel and the surface. If enough energy is absorbed, a further section of the tunnel that is currently planned to be excavated mechanically may prove suitable for blast excavation.
In the apartment
The vibration monitoring experts’ final site is an apartment above the tunnel driving operations. They plan to measure the immissions directly in a room. On entering the building they receive a friendly greeting. “The people don’t make a fuss. They are all looking forward to the new tunnel”, says Thomas. While the next blast is being prepared underground, Adriano and Thomas install their vibration and sound-level meters in the apartment. The blast can be clearly felt in the room, and the sound level jumps to a respectable 70 dB(A) – around the same level as a passing lorry. “Everything is okay here”, concludes Thomas, and, satisfied with the results, closes his laptop.