Rivercrossing with GRUNDODRILL 18 ACS in the Swiss Alps
Switzerland is famous for its scenic beauties and represents continuity and down-home pragmatism. Maybe that is due to the mighty natural rock formations as they are to be found in the Bernese Oberland. These conditions are a technical challenge for any drilling rig.
The jobsite was located directly by the river Kander above the Blausee fish ponds, approximately 10 km away from the village Kandergrund, at around 1000 metres in altitude. The Kander valley is a very deep and narrow valley which runs across the Alpine mountain ridge in west-eastern direction. Alternating layers of hard lime and soft marl and clay are characteristic for the soil here. Moreover, blocky rocks from the Kander glacier settled in the ground during Earth history. Some of these rock blocks can be as big as houses.
The major difficulties of this installation job were these soil conditions, not the two HDPE pipes OD 63. Later on, these HDPE pipes are to host fibre optic cables that have been installed on behalf of Swisscom.
The contract was awarded to the company Zemp from Wiggen in Switzerland. The five-headed bore crew has a lot of experience in the application of trenchless technologies using three drilling rigs and two cable ploughs. Now company boss Markus Zemp wanted to apply and test the new GRUNDODRILL 18ACS drill rig which is especially suitable for quickly alternating rocky formations.
The whole project was quite tricky, starting with the transport of the bore rig to the site. Because the truck was unable to pass through the narrow street, a tractor had to transport the tandem trailer with the bore rig. At the end, even that did not work anymore and the bore rig had to leave the trailer and drive the last 100 metres to the place of the bore on its own. The same procedure was repeated with the Bentonite mixing system.
The GRUNDODRILL 18 ACS was positioned at the riverside and aligned towards the target.
156 drilling metres beneath the river Kander had to be mastered altogether.
The 1.55 m long Rockbreaker which was used for the 6½“ pilot bore is an integral part of the rock drilling equipment. It is driven and controlled by the outer tube. The inclination angle is 1.75°. The roller chisels at the tip transfer a torque of max. 2500 Nm at up to 350 rpm via the inner tube which is quickly and firmly plugged together with the plug-and-turn connection. The transmitter mounted behind the roller chisels allows for detection and control of the bore almost right from the start.
The average bore depth was 3 m, yet ranging from 7m to 8 m beneath the riverbed. Depth and position of the Rockbreaker were continuously monitored with a DCI F5 system and readjusted when required.
The bore had to be re-launched several times because the roller chisel was diverted at times when encountering boulders. “This is not unusual and happens quite often,” said Markus Zemp, who was surprised by the bore rig’s smooth running and the low diesel consumption. He was also impressed by the easy to handle manual hydraulics for emergency operation and the low Bentonite consumption of 20 - 50 l/min, even at high boring performance, reducing the cost for procuring, recycling and disposal of the drilling fluid.
Pulling in the pipe was merely a matter of form then. Thanks to its large diameter, the pilot bore did not have to be upsized any further and the pipe could be pulled in using a normal backreamer.