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

Abstract

The excavation of the Hyper-Kamiokande cavern, 600 m underground, is complete. Measuring 69 m in diameter and 94 m in height, it is among the world's largest rock caverns. A vertically oriented, dome-capped cylindrical design was chosen to optimize cost and performance. Combined with substantial overburden, the geometry posed major engineering challenges. This paper outlines the underground works, main cavern design, excavation plan, and the evolution of support design and construction methods during excavation, namely the information-based (observational) design and construction approach.

Figures (11)

• Figure 1: (Left) Wide-angle photograph of the completed HK main cavern, showing the entire dome from the bottom of the cylindrical section. The HK cavern is located approximately 600 m underground in the mountains of Kamioka Town, Hida City, Gifu Prefecture, Japan. It has a diameter of 69 m, a height of 94 m, and a total volume of approximately 320,000 m$^3$. (Right) Cross-sectional diagrams of major underground rock caverns in Japan and abroad. The span represents the shortest unsupported roof dimension and is the primary parameter controlling stability under in-situ stresses.
• Figure 2: Cross-section of the Hyper-Kamiokande stainless-steel water tank installed inside the 69 m-diameter cavern. The tank is 68 m in diameter and 72 m in height and will be filled with ultra-pure water to 71 m (total 258,000 m$^3$; fiducial 188,000 m$^3$). The detector is partitioned into an Inner Detector (ID) and an Outer Detector (OD); about 20,000 pressure-tolerant 50-cm PMTs (expandable to $\sim$40,000) are mounted on the ID wall. This near-unity aspect ratio is a cost- and performance-driven detector choice that imposes stringent constraints on cavern excavation and stability.
• Figure 3: Overview of the Hyper-Kamiokande underground facilities. The total underground excavation for the project, including access tunnels and auxiliary caverns, is 479,000 m$^3$.
• Figure 4: Excavation sequence of the main cavern. The gray filled area in the dome indicates the spiral pilot heading (outlined with dashed lines), and the gray filled area in the cylinder indicates the vertical muck-drop shaft (outlined with dash-dotted lines). In the cylinder, the spiral working drift is shown only in the right panel as an open area outlined with dashed lines. (Right) Longitudinal cross-section encompassing both the dome and the cylinder. Thick solid lines indicate ring and core boundaries in the dome, and bench boundaries in the cylinder; these boundaries coincide with PS-anchor installation boundaries. (Upper-left) Dome plan. Dome excavation proceeded circumferentially by rings, each starting from the boundary with the previously excavated spiral pilot heading (gray filled area). Thin solid lines indicate individual blast rounds. (Lower-left) Cylinder plan. Each bench was divided radially into four segments, and each segment was subdivided into nearly equal areas (blocks) for bench blasting (thin solid lines).
• Figure 5: Geological and analysis models. Upper panels: final geological model (left: north--south section; right: east--west section) developed from pre-construction investigations and subsequently updated through the information-based (observational) design and construction approach. Lower panels: stability-analysis model (left: north--south; right: east--west) incorporating the analytical geological model (simplified geological classifications, planar weak-layer representation, and parameterized rock-mass properties) together with excavation geometry and boundary conditions. Legends indicating rock-mass classes and weak layers are shown within the figure. Rock-mass class labels used in the geological model denote grouped/mixed geological units based on observations: CH-B (CH--B mixed), CH-1 (aplite--CH mixed), CH-2 (skarn--Inishi mixed), CH-3 (skarn CH), and CH-M (CM and CM-like CH).