On July 25, 2024, a road surface heave occurred in the Hokusatsu Tunnel on the Hokusatsu Transverse Road in Kagoshima Prefecture, Japan. The heave extended about 50 m in length and reached a maximum height of 40 cm. On the day of the incident, the operator immediately closed the section between Takao IC and Satsuma-Togano IC, including the tunnel. Subsequently, a 30 cm-thick lining concrete section of one tunnel wall collapsed; the collapsed area was 6–7 m wide and 5–6 m high, causing soil and sand to rush into the tunnel and further worsening the disaster. According to the prefectural road maintenance division, the abnormality was reported by a road user. A large amount of groundwater continued to flow into the tunnel from the collapsed wall. The water inflow may be related to deformation of the tunnel structure.

Road surface heave in the tunnel

Location of the lining concrete collapse

The Hokusatsu Tunnel connects Izumi City and Satsuma Town in Kagoshima Prefecture. With a total length of 4,850 m, it is the longest tunnel in the prefecture and has operated with two-way traffic since March 2018. The deformation occurred approximately 1,800 m from the Izumi City-side portal. On July 26, the day after the road surface heave was discovered, the wall collapsed. On July 27, soil and sand from the collapse poured in, almost completely burying the inside of the tunnel.

Soil and sand entering the tunnel

Schematic of the soil and sand inflow

The 2,610 m Izumi City-side section, including the deformed zone, was excavated by the Kumagai Gumi–Seibu Construction–Watanabe Construction–Kamada Construction JV using the New Austrian Tunnelling Method (NATM). To address deformation, the tunnel invert was reinforced with a concrete invert lining. During construction, naturally occurring arsenic was detected in the inflow water, so water treatment equipment for arsenic separation was installed in the tunnel. However, the current inflow volume is so large that the water treatment facility cannot keep up with the treatment demand. Water quality testing conducted on July 25 at the Takao River drainage point showed that the water met environmental standards. However, the results of subsequent water quality tests have not been released, and the current situation remains unclear. During construction of the Hokusatsu Tunnel, the maximum water inflow reached 1,200 t per hour. Even after breakthrough, water inflow of about 600 t per hour continued. Therefore, to reduce water inflow and control water treatment costs, measures were implemented to lower the permeability of the excavated ground.

 Geological profile of the Hokusatsu Tunnel

Tunnel interior before implementation of water inflow countermeasures

Special countermeasures were taken for this water inflow event, mainly targeting a 100 m section within the 1,800–1,900 m range from the Izumi City-side portal, where water inflow is large and arsenic concentration is high. In this countermeasure work, ultra-fine cement with an average particle size of 1.5 μm was injected from inside the tunnel using drilling machines to form a 3 m-wide ring-shaped ground improvement zone. To prevent ground movement and tunnel deformation caused by injection material squeezing, the distance between the tunnel outer ring and the ground improvement zone was set at 3.6 m.

Ground improvement zone established for the water inflow treatment work

At present, it is unclear whether any changes have occurred in the ground improvement zone. Kagoshima Prefecture plans to seek assistance from experts at the Ministry of Land, Infrastructure, Transport and Tourism and other organizations to identify the cause and study countermeasures. Repair work on the tunnel is expected to take several years.