Excavation of a 69-m diameter and 94-m high cavern for the Hyper-Kamiokande detector

This paper details the completion and engineering challenges of excavating the 69-meter diameter by 94-meter high underground cavern for the Hyper-Kamiokande detector, highlighting the design evolution and construction methods employed to realize this massive rock cavern 600 meters underground.

The Gist

Imagine you are tasked with building a massive, cathedral-sized swimming pool, but instead of building it on the surface, you have to carve it out of solid mountain rock, 600 meters (about 2,000 feet) underground. That is exactly what the team behind the Hyper-Kamiokande (HK) project just finished doing.

Here is the story of how they pulled off one of the most ambitious underground engineering feats in history, explained simply.

1. The Goal: A Giant "Water Telescope"

The scientists wanted to build the world's largest water tank to catch "ghost particles" called neutrinos. These particles zip through the Earth almost without stopping. To catch them, you need a huge volume of ultra-pure water and thousands of super-sensitive cameras (photomultiplier tubes) lining the walls.

To make this work, they needed a cavern that is:

• 69 meters wide (about the length of two basketball courts side-by-side).
• 94 meters high (taller than a 30-story building).
• Buried deep underground to block out cosmic rays from space that would mess up the data.

This makes the HK cavern one of the largest single underground rooms ever dug by humans.

2. The Challenge: Digging a "Giant Egg" in a Mountain

The mountain they chose (in Kamioka, Japan) is made of hard rock, but it's also under immense pressure from the weight of the mountain above it.

Think of the rock around the cavern like a giant, heavy blanket. When you dig a hole, you remove the support holding that blanket up. The rock wants to collapse back into the hole. Because the cavern is so huge and so deep, the pressure is intense. If the rock fails, it could crack, slide, or even collapse, crushing the equipment inside.

The engineers had to design a shape that could handle this pressure. They chose a cylinder with a dome on top (like a giant soda can with a rounded cap). This shape is naturally strong, similar to how an eggshell is hard to crush from the outside.

3. The Strategy: "The Observational Approach"

In the past, engineers might have designed a tunnel, dug it, and hoped for the best. But this cavern was too big and the risks too high for that.

Instead, they used a method called "Information-Based Design" (or the Observational Approach). Imagine you are walking through a dark forest with a flashlight. You don't know exactly what's ahead, so you take a step, shine the light, check the ground, and then decide where to step next.

• Dig a little: They excavated the rock in small sections (like peeling an onion layer by layer).
• Listen to the rock: They installed hundreds of "ears" (sensors) to listen to the rock. These sensors measured how much the rock was stretching, cracking, or moving.
• Adjust the plan: If the rock moved more than expected, they didn't panic; they just changed the plan. They added more steel cables (anchors) or thicker concrete (shotcrete) to hold it up.

This made the construction process a living conversation between the engineers and the mountain.

4. The Tools: Rock Anchors and Concrete

To keep the cavern from collapsing, they used two main tools:

• Shotcrete: This is like a high-tech, fiber-reinforced concrete spray. They sprayed it onto the walls immediately after digging to create a "skin" that holds the loose rocks together.
• Prestressed Anchors: These are giant steel cables drilled deep into the rock, like tent stakes. They are tightened to pull the loose rock back against the solid rock behind it, effectively "stitching" the mountain back together.

5. The Hiccups: When the Mountain Pushed Back

Even with the best plans, the mountain surprised them.

• The "Key Block" Fear: In the dome section, they worried a giant chunk of rock might detach and fall like a heavy door. They set up special sensors to watch for this. Fortunately, the rock held, and they didn't need to install hundreds of extra anchors.
• The Cracking: In the cylindrical section, the rock moved more than predicted, causing cracks in the concrete "skin." This was a scary moment. The team had to pause, install safety nets to catch falling debris, and drill in more anchors to reinforce the weak spots. It was like realizing a dam had a small leak and immediately reinforcing it before the water could break through.

6. The Result: A Landmark Achievement

By July 2025, the excavation was complete.

• The Size: They removed about 320,000 cubic meters of rock.
• The Safety: Despite the massive size and deep pressure, the cavern is stable. The sensors show that the rock is slowly settling down and stopping, which is exactly what engineers want to see.
• The Future: Now, they will install the giant stainless steel tank, fill it with 188,000 tons of ultra-pure water, and start hunting for neutrinos.

Why Does This Matter?

This isn't just about digging a hole. It's about proving that humans can build massive, safe structures deep underground to answer the biggest questions in physics:

• Why is there more matter than antimatter in the universe?
• How do stars explode?
• Do protons (the building blocks of atoms) ever decay?

The Hyper-Kamiokande cavern is the "home" for these experiments. By successfully digging this giant room and keeping it safe, the engineers have paved the way for the next 20 years of discoveries about the universe. It's a triumph of human ingenuity, turning a mountain into a window for the cosmos.

Technical Summary

1. Problem Statement

The Hyper-Kamiokande (HK) project requires an unprecedented underground facility to house a massive water Cherenkov detector for particle and astroparticle physics. The engineering challenge involves excavating a cavern with a 69-meter diameter and 94-meter height (total volume ~321,000 m³) at a depth of approximately 600 meters underground in the Kamioka mine, Japan.

Key challenges included:

• Scale: The cavern has the largest span (69 m) and height (94 m) among global underground rock caverns, surpassing previous records like the Gjøvik Olympic Hall (61 m span) and Super-Kamiokande (40 m span).
• Geological Complexity: The site is located in the Hida metamorphic belt, characterized by complex lithology (gneiss, skarn, aplite) and high in-situ stress ratios ($\sigma_1/\sigma_3 \approx 3.7$ to $5.4$).
• Stability Risks: The combination of large span, substantial overburden, and high stress ratios creates significant risks of tensile failure in the dome and shear failure/wedge sliding in the cylindrical walls.
• Uncertainty: Pre-construction geological investigations could not fully characterize the rock mass, particularly regarding the continuity of weak layers and exact stress states, making a purely deterministic design insufficient.

2. Methodology

The project employed an Information-Based (Observational) Design and Construction Approach, integrating full 3D sequential elasto-plastic analysis with real-time field monitoring.

• Excavation Strategy:

  • Dome: Excavated using a spiral pilot heading followed by ring-wise enlargement (6 rings).
  • Cylinder: Excavated downward in 19 benches (3–4 m lifts) using a "bench and core" method with muck dropped via a vertical shaft.
  • Support System: A combination of Prestressed (PS) anchors (300 kN and 600 kN) for long-term suspension and confinement, and Shotcrete (primary and secondary layers, total 30 cm) for immediate face support and block stabilization. Rock bolts were used for temporary face support but excluded from long-term stability calculations.

• Numerical Modeling:

  • Software: FLAC3D (v7) was used for full 3D elasto-plastic finite-difference analysis.
  • Modeling: The domain included ~9 million elements. The model incorporated the Mohr-Coulomb failure criterion, strain-softening for tensile failure, and interface (IF) elements to simulate slip along major weak layers.
  • Stress Cases: Analyses were run using a nominal stress case (Case A) and conservative risk cases (Case C for the dome, Case D' for the cylinder) based on in-situ stress measurements.

• Monitoring & Feedback Loop:

  • Instrumentation: Extensometers, PS-anchor load cells, shotcrete stress gauges, and fiber-optic displacement meters were installed to track deformation and load.
  • Geological Mapping: Continuous wall mapping, Measurement-While-Drilling (MWD) data, and borehole television (BTV) surveys updated the geological model in real-time.
  • Management Criteria: Three thresholds were defined (Cause/Review, Countermeasure Implementation, Work Suspension) based on predicted displacement and anchor yield loads to trigger design updates or safety interventions.

3. Key Contributions

• Adoption of 3D Sequential Analysis: Unlike traditional 2D tunnel analyses, this project utilized full 3D modeling to account for the non-axisymmetric stress field and the specific geometry of the dome-capped cylinder.
• Dynamic Design Evolution: The paper documents the transition from a baseline design to the final "as-built" configuration. Key updates included:
  • Reducing the bedrock elastic modulus from 16 GPa to 8 GPa based on observed deformation.
  • Introducing Interface (IF) elements to model slip along weak layers (e.g., Weak Layers A, H, f', X7).
  • Adjusting the dome rise from an initial 14 m plan to 21 m to mitigate deep tensile failure zones under high stress ratios.
• Management of "Key Blocks": The identification and monitoring of a potential massive key block in the dome (formed by the intersection of Weak Layers A, H, and f') using targeted extensometers, which confirmed the risk was manageable without massive additional support.
• Countermeasure for Shotcrete Cracking: A detailed case study of shotcrete cracking in the cylinder due to unexpected weak-layer movement (Weak Layer X7), leading to the implementation of "Priority Monitoring Areas" with conservative design assumptions (neglecting shotcrete shear resistance) and emergency anchor reinforcement.

4. Results

• Completion: The cavern excavation was successfully completed on July 31, 2025, after four years of work.
• Stability Performance:
  • Dome: Despite initial over-prediction of elastic modulus, the final support design (579 PS anchors) successfully stabilized the crown. No catastrophic failure or massive key block movement occurred.
  • Cylinder: Unexpected displacements along Weak Layers A and X7 led to shotcrete cracking. Through the implementation of additional anchors (approx. 90 total in priority areas) and safety nets, the structure was stabilized.
• Creep Behavior: Post-excavation monitoring (Aug–Dec 2025) showed that displacement rates converged to below 1 mm/month two months after completion, indicating the cavern has reached a stable state.
• Model Accuracy: The final analytical models, updated with observed geological data and displacement measurements, showed strong consistency with the actual cavern-scale deformation, validating the observational approach.

5. Significance

• Engineering Milestone: The HK cavern is now the world's largest single-span underground rock cavern (69 m) and one of the tallest (94 m), representing a landmark achievement in rock engineering.
• Scientific Impact: The successful excavation enables the construction of the Hyper-Kamiokande detector, a flagship facility for studying neutrino oscillations, CP violation, nucleon decay, and supernova neutrinos.
• Methodological Legacy: The paper provides a comprehensive reference for future large-scale underground projects. It demonstrates the critical necessity of information-based design where pre-construction models are continuously refined by field data. It highlights the importance of maintaining support element integrity (specifically PS anchors) and the ability to adapt designs dynamically to unforeseen geological conditions without compromising safety or schedule.
• Cost and Schedule Optimization: By using an observational approach, the project avoided over-designing for worst-case scenarios that never materialized while ensuring safety through targeted countermeasures, optimizing both cost and construction time.