Going to Great Depths for Scientific Research

DUNE Project Aims to Unlock Secrets of the Universe

Scientists from around the world are embarking on an ambitious program to help unlock the secrets of the universe as part of the international Deep Underground Neutrino Experiment (DUNE). The DUNE project comprises a consortium of more than 1,400 scientists and engineers representing over 200 institutions in 36 countries. The U.S. Department of Energy’s (DOE) Fermi National Accelerator Laboratory is the host lab for DUNE, which is on the leading edge of neutrino science.

According to DOE, neutrinos are the most abundant particles that have mass in the universe, yet very little is known about them. Every time atomic nuclei come together or break apart, they produce neutrinos. Neutrinos are produced in vast quantities by the sun, and each second a trillion neutrinos pass through our bodies unnoticed. By studying neutrinos, scientists hope to develop a clearer picture of the universe and how it works. On its website, DUNE lists major scientific goals:

• Find out whether neutrinos could be the reason the universe is made of matter;
• Look for subatomic phenomena that could help realize Einstein’s dream of the unification of forces; and
• Watch for neutrinos emerging from an exploding star, perhaps witnessing the birth of a neutron star or a black hole.

This research comprises three major components: a powerful neutrino beam generated in Fermilab’s facility in Batavia, Illinois, and two state-of-the-art particle detectors. One particle detector will be located near the beam source on the Fermilab site in Illinois, with the second detector in South Dakota 800 miles from the source, with neutrinos produced by the beam traveling directly through the earth. The far particle detector will be housed in the South Dakota portion of the new Long-Baseline Neutrino Facility (LBNF), located at the Sanford Underground Research Facility (SURF) in Lead, South Dakota.

DUNE at LBNF

The particle detector located at the LBNF in South Dakota will be housed at the site of the old Homestake Gold Mine. The mine ceased operations in 2002, and converted to use for physics research beginning in 2006.

Funded in part by DOE and the European Organization for Nuclear Research (CERN), the LBNF program involves expanding the Homestake facility for advanced research. The new particle detector will be housed in three huge caverns – about 7 stories high – located approximately 1 mile below the surface at the old mine’s 4850 Level.

Building on its experience from working on deep underground tunneling for transport and resources projects, as well as other science and energy facilities, Arup is helping to develop all aspects of the facility – both the underground elements and site infrastructure on the surface. The firm is tackling unprecedented engineering challenges to help create the massive underground caverns that will house the detector; designing the cooling and ventilation systems in the new caverns for LBNF; and providing power, communications and life safety systems for the scientists who will be working a mile below ground.

Arup has worked on the design development of LBNF since 2013. In 2017, the firm finished the design of the enabling works package for the pilot tunnel construction, blast door installation, expanded surface site infrastructure and refurbishment of the former waste rock conveyance system.

Arup delivered the final design for the facility in mid-2019 and major excavation began in mid-2021 to create the caverns that will house the experiment. Over the next three years, about 800,000 tons of rock was excavated and removed using the laboratory’s upgraded rock handling system – the project reaching a major milestone earlier this year with the completion of major excavation.

Geotechnical investigation

A comprehensive geotechnical investigation – involving the use of historical observations and existing Homestake records, as well as site investigations and laser scans that collated as-constructed geometric information – formed the basis for Arup’s excavation design. Four long horizontal borings were made through the future cavern site from existing tunnels on the 4850 Level. A robust laboratory testing program helped define the strength and stiffness parameters of the rock.

The rock properties were further validated by an observational and back-analysis exercise that saw the design set of inputs (rock mass strength, in-situ stress and so on) applied to known cases in the mine of both successful excavations (such as the enlarged hoist room at the laboratory’s 4550 Level) and ‘failed’ conditions (such as fractured, hour-glass-shaped rock pillars or slabbing tunnel walls). The in-situ observations were compared against the modeling results for validation, with minor calibration adjustments made prior to undertaking the final design.

The rock conditions are well characterized from over a century of mining, as well as more recent underground civil construction works for SURF. The cavern locations are bounded laterally on three sides by existing tunnels (drifts) and vertically above and below by adjacent mine levels.

The ready access to these areas, along with the information gleaned from the new horizontal boreholes and existing historical records, meant that Arup had far more detailed data than that typically available for a civil-type underground project.

This enabled a well-defined 3D rock mass model to be developed and used in the design, ensuring the caverns could be situated and orientated outside of the influence of any rhyolite zones (brittle, glass-like intrusions that have the potential to shatter upon exposure) and persistent mineral infilled joint veins (historically a source of water transmission). As the joint veins generally run north–south, their influence is minimized by the orientation of the caverns, which are east–west to match the beamline direction.

“The geology was amenable to cavern construction due to the competency of the rock and the lack of groundwater,” said Jarred Dull, site field engineer and geologist for Arup. “The rock displays folding and deformation on a multitude of scales, but without fracturing or strength loss. There is some water, but there is a series of deep dewatering wells that suppress the water to about 1,000 ft below the level we are working, so generally groundwater was not a concern.”

Excavating the Caverns

The facility will be housed in some of the largest caverns ever to be constructed at depth. The five-story-tall neutrino detector will be placed in two 150m x 20m x 30m-high chambers situated to either side of a central utility cavern (190m x 20m x 11m high), with a number of ancillary connecting tunnels and chambers excavated to support operations. The chambers were formed using drill-and-blast, with explosives placed in drilled holes in the rock in a predefined pattern. All muck was removed from the cavern via loaders and taken up the Ross Shaft in rock skips.

“The chambers each began with a pilot bore, situated at what would be the top of the caverns, to confirm that ground conditions matched what was found in the investigation,” said Seth Pollak, Arup project manager for excavation for the LBNF. “At that point, the pilot tunnel was slashed out to form a large central drift followed by a left and right top heading and then we began the benching process.”

At the surface, the rock was crushed and transported 1,280m (4,200ft) using the newly installed overland conveyor system, before being deposited into a former mining area known as the Open Cut. The Arup team strengthened the steel headframe at the top of the Ross Shaft, and designed utilities within the upgraded shaft. A new cage for personnel and equipment, as well as new rock skips for transporting the excavated rock, were designed and constructed by others.

Arup worked alongside Fermilab, excavation contractor Thyssen Mining and CM advisor Kiewit-Alberici JV to assess the condition of the rock in the caverns after each blast against the design. Permanent ground support consisted primarily of 20-ft rock bolts and a 100-mm thick layer of shotcrete reinforced with a combination of wire mesh and fibers.

“You really don’t see many chambers of this size other than perhaps on hydropower projects, but the main difference here was the depth,” Pollak said. “At that depth and under those stresses, rock behavior can change drastically, so you have to apply what you know from shallower, civil structures and incorporate that with learnings from the mining industry. It was quite an interesting combination.”

One of the keys to success according to Arup was the decision to have staff on site full time during the excavation phase of the project to define support types, map the exposed rock and monitor the instrumentation results. A number of instruments, including multiple point borehole extensometers, were installed to measure rock mass deformations at defined depths into the ground in real time and allow for comparison against the 3D numerical design models. This system ensured the ground behavior was consistent with the design as the caverns were opened up to their full dimensions.

“Collaboration between Thyssen and us throughout the whole construction phase was one of the key successes to excavating these caverns,” Dull said. “During the excavation phase, we were able to incorporate their ideas and refine the design as the project evolved.”

The excavation contractor maintained an exemplary safety record working over a million hours without a lost-time accident

Ongoing science experiments were taking place several hundred meters away during the cavern excavation, and these had to remain undisturbed by the blasting and construction process.

“Everything including personnel, materials, equipment and muck had to go up and down one shaft, which was shared by the existing lab and the construction team,” said Josh Yacknowitz, a principal at Arup who has been involved with the project since the firm got involved more than a decade ago. “Because of that, managing the logistics was a challenge and a big part of the success of the project.”

Mining for Scientific Gold

Now that major excavation is complete, work is ongoing for the buildings and site infrastructure contract, which is expected to last another 2 ½ years. That work generally consists of outfitting the caverns, power systems, communications and everything else to enable the experiments to be installed and operate. The goal of the LBNF-DUNE team is to have the first detector operational before the end of 2028.

Creating a modern research campus nearly a mile below ground in an abandoned gold mine originally established in 1877 and added to and expanded over a 125-year period presented quite a design challenge – with scientific work ongoing nearby further adding to the complexity. Arup combined its mining and tunnelling engineering experience with the civil infrastructure design skills required to develop a world-class science and research facility.

When complete, DUNE is set to solve some of the biggest scientific questions relating to subatomic particles, expanding the realms of our knowledge.

Information for this article was sourced from the Fermilab website; interviews with Josh Yacknowitz, Seth Pollak and Jarred Dull of Arup; and the Arup Journal (Issue 1, 2021).