BY THE YEAR 2035 particle physicists are planning to use electromagnets to propel proton beams around a 100km tunnel loop, before smashing them into each other with a 100 teraelectronvolt (TeV) collision energy – enough to release subatomic particles that have never been seen before. Such discoveries, which tell us not only about the world we live in, but about the fundamental elements of the universe itself, can then be used to develop tools and technologies that have not yet even been thought of. "It is in the nature of the human race to understand its surroundings and to ask questions and to fi nd answers. Philosophically it is like art or music, in itself there is beauty in it," explains Michael Benedikt, Future Circular Collider (FCC) study coordinator at European particle physics research facility CERN. "Research and education are fundamental building blocks of our educated society. And it is clear that only by pushing it further we can continue keeping our society moving forward."
Moving forward at CERN has meant a series of collider projects which began with small accelerators and has evolved into larger circular pathways. The most recent development, the creation of the Large Hadron Collider (LHC) which opened in September 2008 was credited with the one of the most important discoveries of particle physics, proof of the Higgs boson particle, a fundamental element of the standard model for atomic physics.
Making this discovery began with a major upgrade to an existing circular accelerator called the Large Electron- Positron Collider (LEP). This 27km ring was completed in 1989 following excavation of 37 caverns, 19 shafts and 32.6km of tunnels. "Here in Geneva we have stable, water tight rock. It is an excellent location for housing tunnels like this," says John Osborne, head of the CERN future projects civil engineering team. The next step for the LHC is an upgrade project named High Luminosity LHC (HL-LHC) which will enable scientists to increase the number of collisions observed in the collider. But this will take ten years to be delivered. "Then the machine will run at high performance for typically another 10 years to 2035-2040 region. By which time it will reach the end of its capabilities," explains Benedikt. "It is clear to the whole community that to carry on researching high energy physics you must be prepared to have something new delivered in 2035-2040, so we are very actively looking at options for that era to continue accelerator based high energy physics at the energy frontier."
"Something new" could be the Future Circular Collider and to enable scientists to continue to push the boundaries of high energy physics, engineers will need to build it. "For the last few years we have been looking at what is possible from a geological point of view, at reasonable cost and risk," says Osborne, who has been with CERN for nearly 20 years. This conceptual design study will be complete in 2018 in time to update the next European Strategy for Particle Physics in 2019.
With a circular tunnel length of between 90 and 105km the overall parameters for the scheme make it one of the biggest tunnelling projects in the world. And should CERN opt for a double ring, an option currently being considered, it will become the largest tunnelling scheme ever undertaken dwarfing the 152km of tunnelling for the Gotthard Base Tunnel or the 137km Delaware aqueduct water supply tunnel.
With the size of the accelerator being so huge, engineers had to start by looking at where it could fit. "We approached it purely from a feasibility perspective," says Charlie Cook, a civil engineer at CERN. "The main task at the beginning of the study was looking at the best position for a range of tunnel circumferences. Simply what could we fit in this region and what would be the best position for each site of the collider? We looked at circumferences from 80-100km and that was given as an input from the physicists – who ideally want 100km."
Scaling up
But why does the collider need to be so large? It comes down to the required collision energy. The purpose of particle accelerators is to increase this energy such that the impact between electrons or protons releases subatomic particles. The greater the energy the greater the force of the collision and the more likelihood there is of seeing smaller particles, enabling physicists to identify the fundamental elements of matter – and antimatter. For future research physicists are seeking to create collision energies of 100TeV, over 7 times the current energy level in the LHC. Achieving this requires a combination of greater tunnel length and greater magnet power. Even at 100km the physicists must also double the power of the magnets used today. "This requires significant advances directly in this industry and many domains starting with the performance of the wire that builds the coil of the magnets," says Benedikt noting that industrial production capabilities to build it must develop alongside.
For the engineers then development of a 100km tunnel is at the top of the agenda. Although other lengths have not been ruled out development and optioneering is focussed on the 100km tunnel. And previous tunnelling experience in the area has taught the team that locating it in the molasse rather than the more shallow moraines or more challenging limestone is the preferred tunnelling medium. "We have tried to maximise the amount of FCC in the molasse and minimise the amount in the limestone because in this region the limestone is heavily karsified and the fractures filled with muddy watery materials which caused a lot of problems when they were building the LEP tunnel," explains Cook. The Molasse with its mixture of sandstones, marls and formations is dry and stable and considered a good excavation rock although there is some structural instability anticipated. "It is quite nice tunnelling rock, fairly stable and impermeable," adds Osborne.
But given the size of the tunnel and the topography of the region with its mountainous terrain some limestone excavation seems inevitable. CERN is surrounded with the Jura Mountains to the west, the pre Alps on the east and south east, and the Saleve in the middle of the FCC, south east of CERN. Unlike the rest of the tunnel which will use TBMs the team envisage drill and blast for any tunnelling required in these sections.
One of the defining parameters of the new tunnel is that it must be able to connect with the Large Hadron Collider which will act as a pre-accelerator for the beams. This makes passing beneath Lake Geneva another inevitability. As the lake heads northwards its depth increases (see bathymetry diagram).
"Wherever we crossed the lake had a knock on effect for the depth of the whole ring. We had to try and keep it as far south as possible essentially to keep it as shallow as we could. We considered other options other than staying in the molasse below the lake so depending on tunnelling technology we could go up into the moraine or even higher into the superficial sediments using immersed tube tunnel technology. But for simplicity of the study at this stage we are opting to stay in the Molasse," says Cook. This means passing beneath the lake at around 200m below the water level. "Going higher means more tunnelling risk but you do reduce the shaft depth," adds Osborne noting that this is something that the team could revisit in the future.
Something else which will certainly be revisited, and is already being used elsewhere, is a bespoke tunnel optimisation tool (TOT) that the CERN team developed in collaboration with consultant Arup. "The tool contains a 3D model of the geology that we put together using various datasets including some data that we purchased from the French Geological Society," says Cook explaining that engineers can change the depth, machine size and location of the tunnel and then examine the implications in the model. "Every time you make a change the outputs tell you the shaft depths, the geology at that profile that the tunnel is running through for each option. It has been a very useful tool for us to immediately get feedback on the impact of changes to the FCC and it has allowed us to narrow and determine the best positions very quickly."
The current concept has 16 shafts which are a combination of 12m diameter access shafts, and larger shafts required for the detection and experimental equipment. "Some of the shaft sizes are ridiculously large. We are seeing up to 31m in diameter and 400m deep so that is pretty big," says Osborne.
To date CERN has always opted for vertical shafts but the challenging parameters have led the team to instead consider inclined tunnels to reach the deepest experimental caverns. "On the Large Hadron Collider we had quite a few construction difficulties with the shafts. We used ground freezing because we had glacial deposits with water so we basically drilled freeze pipes down to the rock. However we had a lot of water ingress and had to inject liquid nitrogen for weeks on end. So we are looking at alternative methods for shaft construction if we need to go through the water bearing glacial deposits."
Construction options being considered for the shafts vary according to the ground conditions. They include conventional excavation in the shallow levels of moraines, where lattice girder rings with shotcrete and steel mesh as support will be used in cohesive ground.
For looser or saturated ground piles walls up to 25m or diaphragm walls at greater depths are the likely methods. As the shafts get deeper into the molasses, excavation with hydraulic hammers is recommended. However the team are also considering the use of the newly developed vertical boring machines which are likely to have been more extensively used by the time the project begins construction.
To intersect or not to intersect Another consideration for the 100km loop is whether or not it will intersect in plan with the LHC. Some form of connection between the two is a vital element of the scheme, so the team must evaluate whether or not the paths of the two colliders should cross. "You need a series of accelerators to preaccelerate your beam. It is like when you have a car that you want to drive at very high speeds – you need a gearbox and you start at the beginning with the first gear and then switch from one to the next and have more and more energy. It was always an important part of CERN that it used existing chains as injection machines. The new concept means using existing chain and it is a question of geological optimisation as to how the machines will be linked," says Benedikt.
Having already modelled both an interesting and non intersecting option in the TOT the team have discovered that the optimum intersecting option involves 13.5 per cent of the excavation in limestone and a total 3,211m of shafts, the deepest being 392m. The maximum overburden is 650m and the tunnel passes through the potentially problematic Jura limestone. The proposed non-Intersecting loop avoids the Jura limestone and involves just 4.4 per cent limestone construction but there is a 1,350m overburden in the pre-alps and there is also over 3,000m of shafts required, the deepest being 383m. Without any physical data for the geology in the Prealps region, a comparison between the two options could rely only on geological interpretations created by the local geologists, GADZ. However, a study by engineering consultants, Amberg concluded that the Intersecting option appeared the most favourable from a civil engineering perspective. Therefore, in February 2016 the team decided to adopt the Intersecting alignment as the baseline, whilst allowing for the Non- Intersecting alignment to be revisited when geological data is collected in the future.
The concept design also shows the tunnel moving in a single plane, described by Cook as a flat Frisbee. "We considered having some kinks in the machine to try and raise the level in the south east but the physicists did not like that," he says.
To add yet more complexity to the scheme, the team is also considering making the project a twin bore tunnel taking the total length to a record breaking 200km. "It is a question of safety and distance of certain equipment from the beam," says Benedikt. "It really needs a detailed understanding of man different domains but it will be considered in the next few years."
As the engineers reach the end of the concept design phase they must also produce a preliminary costing for the scheme. "It is too early to say how it will be financed but it would be a global effort. The LHC had contributions from non member states and I would see that in the next phase of technical design we need to think about potential contributions from different regions worldwide," says Benedikt.
CONTRACTORS WANTED
Contractors are currently being invited to prequalify for the tunnelling and shaft construction works of the upgrade of CERN’s Large Hadron Collider, the HL-LHC. The project involves two major construction contracts in two locations. Each will involve excavation of a new 300m long underground gallery (at CERN it is typical to reserve the term ‘tunnel’ for the actual beam tunnel), a service cavern and four cross passages and a 12m diameter access shaft at a depth of 100m alongside the existing 27km tunnel ring.
"We are procuring consultants for design and site management at the moment and we are also looking for contractors," says John Osborne, head of CERN’s future projects civil engineering team.
The location of the two sites, with one in France and the other in Switzerland is behind the decision to split the work into two separate contracts. "Even though the packages are very similar you have to respect the local legislation," says Osborne.
A total of 9 bids are currently being assessed for the consultancy work for award in June. Tendering for the construction packages will begin in early 2017. The construction schedule is determined by the planned shutdown of the collider in 2019. The nature of the works, with induced vibrations from road headers likely to impact on the beam, means that the collider must not be operational when the tunnelling activities get underway. "The LHC is very sensitive; it has detected earthquakes all over the world. There is a risk of the vibration impact disrupting the beam so although we can build the shaft when the machine is running the tunnelling must be carried out during the shutdown," says Osborne. Shaft construction is scheduled to get underway in 2018.
The Large Hadron Collider began operations in 2008 and has been credited with enabling physicists to prove the existence of the Higgs boson particle, a fundamental element of the standard model for atomic physics. The next step for the circular accelerator is the high luminosity upgrade which will increase the density of particles moving around the ring which in turn increases the number of collisions happening in the collider. The density increase is also described as an increase in the luminosity of the beam giving the project its title as the High Luminosity Project. "The LHC upgrade will take around 10 years until 2025. Then the machine will run at high performance for typically another 10 years to 2035-2040 region. By which time it will reach the end of its capabilities," explains Michael Benedikt FCC study coordinator at Cern. At this point the physicists will continue their cutting edge research using the planned Future Circular Collider, which is currently under development
