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INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING This paper was downloaded from the Online Library of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The library is available here: https://www.issmge.org/publications/online-library This is an open-access database that archives thousands of papers published under the Auspices of the ISSMGE and maintained by the Innovation and Development Committee of ISSMGE. XIII ICSMFE, 1994, New Delhi, India / XIII CIMSTF, 1994, New Delhi, Inde II EXTRAORDINARY INFLUX ACCIDENTS IN SEIKAN UNDERSEA TUNNEL EXTRAORDINAIRES ACCIDENTS INONDES DANS SEIKAN TUNNEL Hidenori Tsuji1 Masamichi Takizawa2 Tomiji Sawada3 'Director, Shinkansen Division, Japan Railway Construction Public Corporation, Tokyo, Japan 2Former Manager, 2nd. Sec., Engineering Div., Japan Railway Construction Public Corporation, Tokyo, Japan 3General Manager, Civil Engineering Department, Civil Engineering Division, Taisei Corporation, Tokyo, Japan SYNOPSIS The Seikan Tunnel’s undersea section runs for about 23.3km under the seabed, the geology around which is characterized by the Neogene sedimentary rocks. The planned undersea route of the tunnel had a maximum depth of 140 m under the water with minimum overburden of 100 m. Excavation of pilot tunnels, service tunnels and main tunnels proceeded in this order from both ends of the route. The construction work encountered four major extraordinary inundation accidents. They occurred at fractured zones embrittled by intrusions of igneous rock dykes or softened by faults. This experience lead to the establishment of measures against extraordinary influx of water. Consequently, no inflows slopped the tunneling work through fragile or soft fractured zones and extremely loose base rocks until the completion of the work. OUTLINE OF THE SEIKAN UNDERSEA TUNNEL construction was made in a manner that the pilot tunnel also served as a service tunnel. Introduction When carrying out undersea tunneling, there occurs influx of water whose origin is sea water as work proceeds. This influx of water must be stopped to prevent deterioration of tunnel stability, loss of work efficiency, increase of The Tsugaru Strait separates Japan's largest island, Honshu, from the second drainage costs, and damage to equipment. The problem was actually solved in largest, Hokkaido. The strait is about 100 km long from east to west, and its the case of the Seikan Tunnel by grouting the ground ahead of the face with a narrowest points, both about 20 km across, form necks at the eastern and solidifying agent prior to excavation of most underground sections. This western ends. Investigations that began in 1946 revealed the undersea solidifying agent, a mixture of water-glass and cement, was injected into the geological features of the strait. According to these surveys, the water was ground mainly to a radius three times the tunnel section. more than 200 m deep at the eastern neck, and the seabed geology consisted of a volcanic zone which was considered difficult to work with. The seabed at the western neck was found to have a ridge-like topography and a maximum Geology depth of about 140m; geological conditions were found to be relatively favorable for tunneling as their origin consists of mainly sedimentary rocks of the Neogene period. Consequently, the western neck of the strait was chosen The main component of the sub-seabed geology through which the Seikan for the route of the Seikan Tunnel which was to cross the Tsugaru Strait. tunnel runs is sedimentary rock formed during the Neogene period. In detail, Following this decision, further investigations were launched to study the it is a mixture of sand, lapilli, and breccia supported in a matrix of mudstone, topography and geoiogy below the western neck. In addition to seaborne silty stone, shale, fine-grain sandstone, and tuff. This is shown in Table 1. surveys, experiment tunnels were also drilled to look into and develop a The main sedimentary layers of this rock are named Fukuyama, Kunnui, technology for construction of an undersea tunnel. Following this work, the Yakumo, and Kuromatsunai in the order of formation (older lo younger) and tunnel route shown in Figure 1 was designed, assuming that it would be each layer is further subdivided. Consolidation is generally better in the older double-tracked tunnel for use by trains of the Shinkansen (Bullet Train) type. layers, but where faults and intrusive dikes are found, consolidation has been considerably reduced by the resulting fractures and metamorphosis. The The planned route ran along the ridge-like seabed topography from Tappi (on consolidation of silly and sandy layers is sometimes poor, depending on the the Honshu side) to Yoshioka (on the Hokkaido side); the maximum water layer's granular distribution. depth along the route was 140 m and the tunnel would go as deep as 100 m below the seabed. The portions of the tunnel under dry land were excavated The geological features of the area are shown in Fig.2; the main syncline axis using conventional mountain tunneling techniques, and only a single main runs along the center of the strait, and the strata become younger towards the tunnel was excavated. On the other hand, more than 11 km had to be dug strait mid-point. There are more than 10 main faults and companion sub­ under the sea from each shore. In addition to the main tunnel, pilot and service faults, and the age of the strata is locally mixed or varied because of the faults, tunnels were constructed in these sections for advance of geological and past foldings and non-conformities. In some places, igneous rock is also engineering studies as well as for drainage, ventilation, and material transport exposed in the form of intrusive dikes of basalt and andesite. purposes. Investigations from aboard vessels on the surface were not expected to yield sufficient detail of the seabed geology, so while excavating Rock crushed by faulting contains many cracks with poor bonding, resulting the pilot tunnels the geological and groundwater characteristics were observed in a fragile base which allows water to flow through. Rock subjected to higher in detail and engineering measures necessary to counter any potential problems levels of crushing has become extremely soft or expansible because of were developed on the basis of these observations. Following pilot tunneling, metamorphism to the base rock. Shear fracture zones of this type are very service tunnels were excavated; the aim of this was to verify the previous prominent on the Yoshioka side. conclusions—which were based on the results of pilot tunneling—and to obtain further detailed data in preparation for main tunnel excavation. Based Intrusive dikes of igneous material contain rock which was crushed and altered on the results of these investigations and studies, excavation was carried out as a result of the intrusions, and adjacent rock has suffered physical fractures for a main tunnel with a cross section five times larger than the pilot or service and metamorphic effects from hydrothermal chemical reactions. As a result the tunnels. For a distance of about 5 km at the center of the undersea section. rock surrounding such dikes is often very fragile and permeable. When 11 intrusive dikes of this type are found penetrating a fault, the rock around them The groundwater pressure in the seabed rock is almost equal to the depth is often soft and expansible. These intrusive dike fracture zones are prominent below the sea's surface multiplied by the unit volumetric weight of water. on the Tappi side. However, the amount of water influx varies greatly depending on the condition of the rock in a particular location. Km — -------4 ------—4^---------^ ^ —--------F--------------------------------- Vertical Section Land Par t of Aomori Si de Undersea Part Land Part of Hokkai do Side Elevation 1*13. 55km L=23. 30km L=17. 00km 3 0 0 — 2 0 0 — p' --------- /p J 211 IS. 1 2 3 12. HOKKAI DO Pl ane Legend — M : i&in Tunnel, — P : Pilot Tunnel (Gradient:3/00), — S : Service Tunnel (Gradient: 12/00), — I : Inclined Shaft R : Section of Plane CircleOMus Ihit: k m ), n /* : Topography, * ^ v/\ : Contour of Sea Bed (Unit: m) cig. 1. Longitudinal Profile of Seikan Tunnel Vertical Section Undersea Part YoshioKfi - t — Lengt h 1 5 ■ 2 0 2 5 3 0 ~35" ‘km Pl ane I Legend v / " : Topography, — M : ifein Tumel , — P : Pilot Tuinel, — : Boundary, — ¡Fault, :Syncline, -J- : Anticline Fig.2. Geology in Undersea Part of SeikanTunnel 12 Table 1. Stratigraphy and Uthology in Undersea Part of Seikan Tunnel CONSTRUCTION METHOD FOR UNDERSEA SECTIONS Thickness Formation Lithology Natural State <*c (m) (MPa) Pilot, Service, and Main Tunnels Kuromatsunai 230-350 Sandy mudstone Main 2-8 Fine grained luff Laminated Sandy tuff Laminated 4 Construction began with the digging of horseshoe-shaped excavations, about 5 Marl stone Laminated m high and 6 m wide, from the shore on both the Honshu and Hokkaido Yakumo 60-113 Hard shale Main, straiificated 43 sides. These inclined downward at a gradient of 1 in 4 to link with the Hard mudstone Main, cracky 19-20 undersea sections. Pilot tunnels were then excavated to the mid-point of the Sandy tuff Laminated seabed at an upward gradient of 3 in 1000 prior to two other tunnels. The pilot Tuff Laminated 6-9 tunnels also had a horseshoe-shaped cross section about 4 m high and 5 m 50-200 Coarse grained tuff Alternated layer 2-10 wide. The service tunnels, which branched off from the inclined shaft at each Kunnui 5 shore, were excavated from points about 120 m above the pilot tunnel Sandy tuff entrances and run parallel to the main tunnels at a distance of about 30m from Lapilli tuff Alternated layer their center line. The service tunnel has almost the same dimensions and shape Silty tuff Alternated layer as the pilot tunnel.

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