DATE: 2015
MODEL: MC 4 D e MC 8 D
“We used two Comacchio drill rigs of the seven in our fleet to meet the challenging requirements of the project: an MC 4 D and an MC 8 D.”
The Large Hadron Collider (LHC) at CERN is the largest scientific instrument ever designed and built by man for scientific research. Built by the European Organization for Nuclear Research (CERN) from 1998 to 2008, the collider is contained in a circular tunnel located at the Franco-Swiss border near Geneva, with a circumference of 27km, at a depth ranging from 50 to 175m underground.
Since 2010, thanks to the joint effort of a global user community of 7,000 scientists from over 60 countries, the LHC has been exploring the new high-energy frontier. The crowning achievement of these experiments was announced in July 2012, with the first empiric evidence of the long-sought Higgs boson, the cornerstone of the Standard Model (SM) of particle physics. But while particle physics looks beyond this result, pushing the limits of human knowledge about our world and the origin of the universe, studies are underway at CERN for an upgrade of the LHC (called the High Luminosity LHC) aimed to further increase its discovery potential.
One of the crucial aspects of the HL-LHC project is the upgrade of the cryogenic test station used for the test of the LHC prototype magnets, known as SM18 (building 2173). Today, the HL-LHC project requires an upgrade in terms of powering these clusters, which is why the test station will undergo major upgrades in the coming years, so it can test larger prototype magnets with higher currents (>20 kA). The strategy contains modifications of the existing testing facilities and building of new ones for the period 2015-2020.
As a part of this project, a new vertical test station will be built in cluster D of the SM 18 testing facilities in 2015-2016. An upgrade that will require serious civil engineering works. The first part of the works was undertaken in late March 2015 by the Italian engineering company Gruppo Dimensione and its subcontractor Terracon, supervised by CERN project manager Helena Botella.
“We were commissioned to build a retaining berlinese-type wall for the vertical excavation that will be carried out in cluster D of the testing facilities hosted in building 2173, also known as SM 18,” explains Marco Framarin, project manager at Terracon.
The work was carried out entirely inside the building and involved the execution of micropiles of different diameters:
The geotechnical information provided showed an upper layer comprised of backfill material with gravel, pebbles and limestone blocks up to 2m depth, followed by an underlying unit comprised of clay with very hard boulders up to approximately 9.5m and molasse-type sedimentary rocks up to 11.5m depth.
“The molasse sandstone turned out to be very well cemented, which led to an increased consumption of drilling material,” says Framarin. However, this was not the major challenge to tackle. “The main difficulty posed by the project was related to the very strict environmental requirements that were far beyond what we’ve experienced on any previous job site. The equipment was supposed to release zero emissions during operation: no dust, water, exhaust gas, cement dust or any other element that a micropile job site normally involves was allowed to contaminate the space inside the testing facilities”.
The entire job site was confined within a Safety Airlock System (SAS) decontamination chamber, the access to which was controlled through a special decontamination hall. Consequently, all of the equipment and materials (pipes, cement, drilling accessories and tools) were taken inside the SAS chamber with switched-off engines and completely wrapped up (excluding the drill rig). All the transport operations inside the testing facilities were carried out by means of the CERN electrical vehicles, in order to avoid any problems inside the building.
“You need to consider that the testing facilities of SM 18 include some clean rooms where the staff works in a sterile environment as if it were an operating room,” Framarin says.
The drilling had to be performed in confined spaces, part of it with limited overhead space (lower than 240cm), thus the decision to use a Comacchio MC 4 D in the “short mast” version featuring a 2,127mm long mast with 943mm feed stroke.
“We used two Comacchio drill rigs of the seven in our fleet to meet the challenging requirements of the project: an MC 4 D and an MC 8 D.”
Both of these drill rig models feature separated power packs and are specifically designed for operation in confined spaces, inside buildings, basements and small tunnels. The use of separate power packs made it possible to evacuate the exhaust gas of the engines powering the drill rigs. The exhaust gas was vented to the outside, crossing the laboratory, by means of a fixed pipe and aspirator.
“The separated power packs proved essential in order to meet the project requirements, because without placing the engines in a stationary point, it would have been impossible to extract exhaust gas without losing much time in the trimming and set-up phases.”
It was therefore decided that the two machines should work in separate shifts, from 6.00 to 15.00 and from 16.00 to 01.00. Furthermore, an additional inlet ventilation fan was installed within the SAS chamber. The original design of the chamber included only one air outlet to ensure that the job site would remain in negative pressure. Before adding a ventilation inlet, test and measurements were undertaken to check that the room would still remain in negative pressure, ensuring that no dust could be vented outside the SAS.
“The choice of the drilling method offered only one option,” Framarin adds, “To avoid vibrations, only rotary drilling was allowed, even if the most appropriate methodology according to the soil study would have been rotary-percussive drilling. But its use was prohibited both for the vibrations and dust produced, as well as the noise.”
The drilling was performed both with water circulation, using tri-cone and tri-blade type bits, and with augers combined with claw bits. The augers proved to be a more effective technique, and offered the advantage of a simpler handling of the drilling spoils, that were put inside buckets and taken outside when full. When drilling with water circulation, two tanks were used for the collection and decantation of the fluids, as well as a pump station, to evacuate the mud to the outside, where it was further filtered.
“Finding the most effective drilling technique and drilling tools was not an easy task,” Framarin explains. “We managed to find the right solution for the 220mm micropiles thanks to one of our longstanding suppliers of drilling tools. But for the 150mm drillings we had to adapt a tri-blade chevron-type bit with special claws to manage drilling through the harder layers (molasse), but mostly because the gravel at the upper layers was difficult to extract, due to its dimensions.”
As a result of the combination of drilling methods, a peak production of 69m was achieved in a shift with the MC 8 D. In total, 1,272m were drilled between end of March and end of April, including 598m with 150mm diameter and 674m with 220mm diameter (144m of which were in extremely low overhead conditions).
To avoid cement dust, a separate SAS decontamination chamber was dedicated specifically to the cement-mixing equipment, with the CERN staff attending to all supply and transport of materials and buckets full of spoils with its lifting systems and electric carts.
After overcoming all challenges, work was completed by Gruppo Dimensione and its subcontractor Terracon successfully on time and achieved a positive response from the international team of CERN engineers managing the project.