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. The initial excavation of the pilot and service tunnels was Alternated layer carried out with full face excavation machines (TBMs). However, these 4 50-95 Brown mudstone Main machines did not cope properly with the drastic changes in geology and left Silty tuff Alternated layer 5-9 inadequate space for injection of solidifying agent into the face. Thus, use of Tuff Alternated layer the TBMs was given up before long. The main excavation method was Sandy tuff Alternated layer blasting, except in parts of the tunnels where a boom-type excavator was used. 3 45-100 Coarse grained tuff Blocky 7-13 Silty tuff Pumiceous tuff The main tunnel was excavated from connecting shafts bored about 600 m Brown mudstone apart along the length of the service tunnel. The primary methods used in Hard block excavating the main tunnel included bottom drift method, upper face cutting 2 165-170 Brown mudstone Main, cracky 40 method, and side drift method, chosen according to local geological Gray mudstone Alternated layer characteristics and other factors. A horseshoe-shaped cross section was Sandy tuff Alternated layer 3-6 chosen where the geology was stable, and a circular one where the ground was Tuffy sandstone Alternated layer 7-8 very swelling 1 200-270 Green tuff Main Tuff breccia Main When excavation of the main tunnel was started, a vertical shaft was bored at Lapilli tuff Alternated layer 10-14 each shore for the conveyance of workers, cement, aggregates, and other Sandy tuff Alternated layer materials. Each shaft had a circular section with an internal diameter of 6.5 m, and had a vertical depth of about 200 m. Fukuyama Welded tuff Sandy tuff Alternated layer Fine grained tuff Alternated layer Andesitic tuff 30-100

Cross section of the pilot, service, and main tunnels

Distance from start point (kml rota M E ; _ 27 I . 28 I __ 29I__ Legend G: Connecting shaft MF: Main tunnel face J: Junction of pilot and service tunnels RG Reverse connecting shaft SF: Service tunnel face SP: Service tunnel pumping station V: Ventilation shaft PF: Pilot tunnel face PP: Pilot tunnel pumping station Fig.3. Schematic Diagram of the Undersea Section (YoshiokaSideasofSept. 1979)

13 Ventilation shafts at intervals of aboul 1 km along the tunnel route connect the Geological Survey and Pilot Boring pilot and service tunnels, allowing air circulation from the inclined shaft to the pilot tunnel and reach the service tunnel through the ventilation shafts. Air circulating in this way also made its way to the main tunnel through the Preliminary investigations of the undersea geological features took place from connecting shafts, and was exhausted through the vertical shafts. This 1946 to 1963 in order to know accurately the distribution and structure of the circulation system ensured sufficient ventilation for all work areas. The seabed geology along the planned tunnel route. These involved dredging of intricate arrangement of tunnels under the seabed is schematically illustrated in the seabed, sonic surveys, submarine boring, observations using a mini Fig.3. submarine, seismic surveys, and magnetic surveys. The result of these investigations was a geological map to a scale of 1:20000. The undersea section of the tunnel was supported, for the most part, with H- shaped steel arch supports, which were coated with shotcrete immediately after In order to ensure safety during actual execution of tunnel excavations and excavation. Rock bolts were used where required. The final finish of the pilot make planning of construction method, a more accurate and detailed and service tunnels was a 20 cm lining of shotcrete, while the main tunnel was understanding of the geology and groundwater conditions in the area was covered with about 70 cm of concrete as a final lining. required. For that purpose, horizontal pilot boring was chosen as the surest method, and it was carried out in each pilot tunnel and the service tunnel. Boring bases were located in side adits excavated from the sides of already excavated sections, allowing simultaneous implementation of tunnel excavation and pilot boring work. In principle, two pilot boreholes were made, one on each side of the tunnel center, and the forward end of the borehole always preceded the tunnel face. Thus, the boring base was alternately moved from the left to right side of the tunnel as work proceeded. The pilot boring arrangement is schematically illustrated in Fig.4.

Pilot bore Side adit for pilot bores Service tunnel

~ / l / Connecting shaft Grouted area _I I___ _ \ Main tunnel EX Excavation of drifts Grouted area Fig.4. Layout of Pilot Bores

Section A-A

Forward grouted area

RT: Radius of tunnel Radius of necessary grouted zone RO: Radius of extra grouted zone LE: Length of excavation LR: Residual length Length of 1 -stage grouting Length of 2-stage grouting Length of L-stage grouting Fig.5. Schematic Drawing of Excavation and Forward Grouting

14 Grouting and Excavation Location and Details of Influxes

During tunneling, a method called forward rock grouting system was chosen, Regarding geological features, influxes T1 and T2 took place where andesite where the rock to be excavated was first grouted to ensure proper excavation. and tuff, respectively, constituted the bedrock. Both locations were Correct implementation of this forward rock grouting system required dividing characterized by crushed and fragile ground caused by the intrusion of andesite the tunnel into sections short enough to be grouted from the face. After each or basalt. Y1 and Y2 both took place in soft ground with tuff crushed by such section was grouted, excavation was carried out within the grouted faults. Those locations were roughly 180 to 240 m vertically below the sea length. This system is illustrated in Fig.5. All the tunnels were excavated surface, with a coverage of about 100 to 215 m. The hydraulic pressure at using this sequence of grouting and then excavation. these locations, as measured by pilot bores or in the grouting holes, was roughly equal to the water head corresponding to the vertical distance below Issues to be resolved when implementing the forward rock grouting system the sea surface, irrespective of the covering. Thus the hydraulic pressure was include the position of the face when grouting takes place, the length of very high, and the fragile or soft ground yielded to cause excessive influx of grouting, the range of earth around the tunnel to be grouted, the length to be water. excavated after grouting, the length of the grouted zone that should remain unexcavated, and the length and location of grouting holes. In addition to The water flowing into the tunnel is analyzed in Table 4. As this table shows, these geometrical factors, the order in which grouting holes are bored and water at the Y2 site was quite similar in nature to sea water, indicating that the grouted, the blend of grouting materials, and the grouting pressure, etc. is also period of contact between sea water and ground was short. Water at the Tl, considered. These matters were decided through study based on information T2, and Y1 sites, however, was rather different from sea water, indicating from pilot borings, preceding forward rock grouting, and actual excavation longer contact between water and ground than in the Y2 case. In all cases, conditions. Table 2 shows the general tendency of grouting and geological though, the influx was attributed to sea water that had penetrated into the characteristics faced in grouting. Where geological and groundwater ground. conditions were particularly poor, a more intensive grouting method was adopted.

Table 2. Conceptual Guidelines for Grouting vs. Geological Conditions Table 4 Nature of Influx Water Geological conditions Good Poor Accident T1 T2 Y2 Y1 Max. inflow (m^)/min 16 10 70 12 (Ref.) Long Short Tappi LE Water temp. (°C) 19 17 19 seawater LR Short Long pH 8.5 7.6 8.0 8.4 8.4 Ionic composition Na+ 405 309 405 435 478 Rl Small Large K+ 7 6 10 6 11 Ca++ 40 41 36 50 20 Number of grouting holes Few Many Mg'1-*- 109 72 137 87 105 ci- 501 387 532 522 552 Number of stages Few Many SO4 - 41 41 58 49 52 Note: Water temperature of Y2 dropped to 14°C Note: LE, LR, and Rl correspond to those in Fig. 5 Ionic composition unit: epm

EXTRAORDINARY WATER INFLUX ACCIDENTS

As indicated in Table 3, four influxes of water inundated the excavations with Engineering Countermeasures at Influx Points water from the face. These extraordinary influxes occurred not in the main tunnel, but in the inclines and service tunnels, both of which were bored to The engineering approaches in use at each influx location before the influx gain a preliminary understanding of rock conditions and to study any needed occurred are outlined in Table 5. This suggests that the number of pilot bores engineering countermeasures. They also took place at an early stage—within used for preliminary investigation of the rock in question was inadequate. the first five years of the investigative work and main tunneling work—when Since there was some discrepancy between the pilot bore results and the details of execution methods and systems had not yet been properly ground conditions actually encountered, there is also evidence that preliminary established. The experience of handling these influxes yielded valuable ground surveys were of inadequate accuracy. Together, these facts indicate information and data, resulting in the establishment of a proper engineering that the pilot boring technique was not properly developed yet during the approach. No major water influxes occurred in any work after these early earlier stages of the work, that the drilling speed was slow, and that it was problems. difficult to orient the boreholes where desired.

Table 3. Extraordinary Influx Accidents at Excavation Face Accident Tl T2 Y2 Y1 Occurrence date Feb. 13, 1969 Dec. 5,1974 May 6, 1976 Jan. 8, 1974 Section affected Tappi Tappi Yoshioka Yoshioka Tunnel affected Inclined shaft Service tunnel Service tunnel Service tunnel Location of influx point Inclined shaft lk223m 16k890m 31k669m 32k747m Sea depth at influx point 25 m 78 m 76 m 58 m Earth cover at influx point 215 m 102 m 128 m 134 m Andesite Tuff Tuff Tuff Geology at influx point Fault fracture zone Basalt intrusion Fault fracture zone Fault fracture zone Note: Location of influx points in the service tunnel are given as distances of the main tunnel.

15 The excavation and support method in use at the influx sites was identical to remained in the caved-in section, pressurizing the face and contributing to that in other areas. This implies that the excavation and support methods were slowing the influx of water. The T2, Y1, and Y2 influxes occurred in tunnels not a direct cause of the influxes. However, as to forward rock grouting, with a gradient of 12 in 1000. On the other hand, T1 happened in the Tappi reports suggest that it was extremely difficult to bore grouting holes and to inclined shaft, where the gradient was 1/4, so here these effects were much inject grout at the influx sites because of the high clay content of the ground. greater than at the other locations. This is bome out by the damage survey As a consequence, no grouting was done at the Y 1 site, and at the T1, T2, and results: the volume of sediment and the submerged length of the shaft was less Y2 sites the amount of grout used was 300 to 400 m3, much less than in this case, though the influx was the second largest of the four. elsewhere. In addition, as shown schematically in Table 2, there was no clear engineering view regarding how to modify the grouting method to suit The damaged areas were restored as follows. First, the water remaining in the changing ground conditions. It was therefore concluded that the direct cause caved-in section of tunnel was drained, along with water still inflowing from of the water influxes was the inability grout properly in ground that was the ground, thus shortening the length of the inundated section as far as crushed and extremely fragile or soft; that is, in ground holding a lot of water possible. Then an area including the influx point was sealed with a bulkhead. and under enormous hydraulic pressure. The length of tunnel between the influx and the bulkhead, as well as the surrounding ground, was solidified and water-sealed using concrete, mortar, cement, and LW (Labiles Wasserglass: a mix of water glass and portland Scale of the Influxes cement). When this was finished, the tunnel was excavated again through the influx point to the further ahead. The new excavation was done in one of two ways; either a method called straight excavation where the previously planned The damage caused by the four extraordinary influxes is quantitatively listed in line was followed or a method called detour excavation where a detour was Table 6 to indicate the scale of each influx. When such an influx occurs, the made to avoid the influx point. A straight excavation along the original line ground around the work face is concurrently loosened. As a result, the ground entailed intensive and wide-range cementing and water-sealing in the vicinity collapses and joins the rush of water into the tunnel. At the four influx of the influx point and the surrounding softened ground. This resulted in an locations in question, once a certain amount of loose earth and rock had increase in the amount of work. On the other hand, a detour only required accumulated, it helped stop further collapse of the face. In each case, the solidifying and water-sealing work at the influx point itself and in the tunnel slanted downward towards the course of progress, so sedimentation of surrounding ground, resulting in less amount of work. However, a detour the crumbling earth and rock added to the holding effect. The inflowing water was only possible where tunnel alignment were less restrictive.

Table 5. Construction Conditions Before Influxes at Influx Points

Accident T1 T2 Y2 Yl Pilot boring Number 1 1 2 0 Inflow amount (1/min) 1500 0 3800 ... Amount of forward grout used (m3) 287 310 378 0 Sectional shape of tunnel Horseshoe Circular Horseshoe Horseshoe (Circular) Excavation method Full-face blasting Full-face picking Full-face picking Full-face blasting (Picking) Support Steel used (mm) 150H 150H 150H 150H (200H) Support interval (m) 1.3 0.7 0.7 0.5 (0.7)

Note: In the case of Yl, terms in parenthesis are those adopted after revision due to deformations caused by earth pressure.

Table 6. Scale of Influxes

Accident T1 T2 Y2 Yl Total amount of influx water (m3) 183,000 188,000 1,845,000 13,000 Max. amount of water in tunnel (m3) 5,300 1,300 121,000 9,000 Max. length of tunnel inundated (m) 196 130 3,015 880 Amount of caved-in earth and rock (m3) 300 1,600 1,000 1,100 Length of buried section (m) 15 70 74 60 Days required for recovery 214 172 162 362 Breakdown Drainage 12 8 19 15 Preparation for grouting, excavation, etc. 125 30 56 23 Grouting and excavation 77 134 87 324 Excavation after recovery from influx Straight excavation Detour excavation Detour excavation Straight excavation

Note: Recovery from an influx covers the work up to the point where the resumed excavation passes through the caved-in points.

16 WORST INFLUX Conditions Before Influx

Of the four extraordinary influxes experienced in the construction of the Seikan During excavation in the Knl layer, water constantly seeped out of the face. Tunnel, Y2 was the last and most serious. This chapter gives a detailed Until the moment of the sudden influx of water, conventional forward rock account of it as a representative case. grouting was being carried out to ensure stable excavation. Figure 7 and Table 7 show how groutihg work was being implemented until the moment of influx. The influx occurred at the time of the 20th grouting stage. The major Conditions Around the Influx Point difference from previous grouting stages, and especially the 19th, was the large volume of water inflow during drilling for grouting; many blockages occurred during drilling around the inflow spots. To compensate for this, a This influx occurred in a service tunnel about 4km500m away from its branch corresponding increase was made in the amount of grout injected. from the Yoshioka inclined shaft. As the service tunnel was excavated away from the branch point, the depth of water above it went from 0 m to about 75 Following the 20th grouting, the 20th excavation, about 70 m long, was m at the influx point, while the earth cover changed from about 150 m to 130 scheduled to be executed. During the 20th excavation, about 3.7 m'/min of m. The tunnel was in the Kunnui (Kn) layer, which has a lower permeability water poured out from the B11 pilot bore and about 4 m3 of earth was drained and is composed of sedimentary rock with tuff as the main component. out. Considering the poor condition of the face and the conditions experienced during the 20th grouting, it was decided to give up the 20th excavation and The tunnel proceeded from the second Kunnui (Kn2) layer into the first proceed with the 21st stage of grouting. This was intended to supplement the Kunnui (Knl) layer at a point about 1 km ahead of the influx point. Knl 20th grouting and to solidify the area judged still soft. However, no comprises lapilli tuff mixed with sandy tuff and tuff breccia. Samples taken significant water inflow occurred and much less grout was able to be injected from these rocks for testing yielded a uniaxial compressive strength of 80 to than predicted during this subsequent grouting. 100 kgf/cm2. The geology of the ground around the influx point is mapped out in Fig.6; these data were gathered during in-depth investigation following Excavation resumed upon completion of the 21 st grouting, but the condition of the influx problem. As illustrated in the figure, a strike crossed the tunnel the tunnel face became worse, and a method such as forepoling and temporary center line at an angle of about 10 degrees some way distant from the influx supports to the face were required to secure safe excavation. Further point, with a dip in the direction of excavation at a gradient of about 20 excavation led to a greater water inflow from the face, requiring drilling with degrees. However, near the influx point, both strike and dip underwent picks and the use of steel-support rib-arch timbering at reduced intervals. drastic changes, with the dip increasing to a gradient of about 40 degrees. These efforts were still underway to support the softened ground around the Softening of the ground was also aggravated by secondary faults and a number face when the excavation reached the influx point (at 31km669ml5cm). of cracks.

Legend t ; Service tunnel excavated before the influx Lapilli tuff ~~ V- : Detoured service tunnel after the influx Sandy tuff ------: Central line of main tunnel Breccia tuff C F : Face where influx occurred Fault fracture zone S B : Side base for pilot bores Fig.6. Geology around the Y2 Influx

Table 7 Grouting Records Near the Y2 Influx

Grouting stage 19th 20th 21st No. of grout holes (pcs.) 19 29 9 Total length of grout holes (m) 1582 2360 525 Total water inflow from the holes (1/min) 1235 4843 72 Amount of grout (m3) 176 290 92

Note; The locations and times of grouting stages are shown in Figs. 7 and 8, respectively.

17 Plane

Influx of water from the grouting hole Legend FG: Grouting face 30- 59 k/min EX: Length excavated between grouting stages 0 50-100 CF: Face where the influx occurred 100-200 SB: Route of pilot bore 200-400 xxxxx: Location in pilot bore where the water influx of 3.7 m3/min. occurred, accompanied by a Envelope of the grouting holes collapse of rock and mud of 4 m3 situated farthest from the tunnel Fig.7. Grouting Work Near Influx Point

Typical section of service tunnel Arch-type support C L Lining 150H steel ' 0.2 m thick

Excavation direction C|F Interval between supports 0-J______1.0

Longiludinal Age ' to CDe CO CO Excavation progress Condition of the face Date: 20th grouting stage Apr. 25, 1976 20th excavation stage Fragile and partly soft ground

Apr. 28 21st grouting

Ma y 1 21st excavation The number of cracks and collapses Forepoling and face increased, causing partial collapse supports started of the face Excavation with picks started Wetting began May 6 A flow of 30 l/min. began Fig-8. Progress of Excavation until the Occurrence of the Y2 Influx

18 The Moment of Influx and Its Progress The water was directed through the fifth connecting shaft at the 33km500m point into the fifth main tunnel, which was then still uncompleted, in order to decrease the flooding speed. This measure \fcas aimed at delaying the flooding Table 8 outlines the moment of the influx and its aftermath. A supplementary of the pilot tunnel so that the capacity of the pump station at the bottom of the account is given in this section. inclined shaft could be increased to drain the rising waters. Despite all these efforts, the flood level continued to rise gradually for a while. The level was When an inflow of about 15 l/min. from the top of the face began at the face finally held constant when a temporary drainage system with a capacity of 32 during shoring work to the face, an inflow measuring about 30 l/min. from a m3/min. which was set at approx. 34k685m point started operating. The spot near the 31 km673m point slopped flowing. Later, water also flowed out conditions at this time are illustrated in Fig.9. of the bottom, adding to the lotal volume of inflow, and exceeded the drainage capacity at the face. The tunnel then filled with water up to ihe level of the It was found during a later investigation after dewatering of the flooded section relay drainage system, which had a drainage capacity of 4 m3/min., located was completed, that rock and earth crumbled in from around the face by or about 100 m backward from the face. The area affected by the deluge was together with the influx and accumulated till it blocked the tunnel, as illustrated maintained constant by this facility, and this constant volume of inflow in Fig. 10. It is thought that this sedimentation prevented expansion of the continued for about 10 hours. cave-in around the face.

Then, suddenly, the ground roared and the water level rapidly rose above the The ionic constituents of the influx water were very similar to those of sea relay drainage facility. This sudden surge of water was later estimated to be water. The water temperature was 17°C at first, then dropped gradually to about 70 m3/min. in volume. Work to stop the surge of water began; this 14°C. Following the completion of a bulkhead, and immediately after the included the construction of a cut-off wall and the closing of the emergency drainage valve was closed, the hydraulic pressure was recorded at about 8 flood-control door located at the 33kml 56m point. However, the force of the kgf/cm2, rising to about 20 kgf/cm2 twelve hours later. This indicates that the flood aborted these efforts, and the water level rose despite both attempts to influx originated directly from the sea. contain it. Although the rate at which the water level was rising dropped in half, the flood water showed no sign of subsiding.

Table 8 Occurrence and Progress of the Y2 Influx

Day Time Influx conditions 6 02:25 A flow of about 15 l/min began from the crown of the face 03:00 The inflow of water from the crown as well as the bottom increased to about 500 l/min 03:30 The flow further increased to about 4 m3/min, being unable to drain at the face The tunnel flooded for about 100 m and then the problem stabilized by the drain there 14:40 The flood level suddenly rose and the ground rumbled (recording a water inflow of about 70 nvtymin Cut-off walls were built at the 31km912m, 32kml06m, and 32km483m points, but all failed to stop the rising waters 7 01:00 The water level rose above the emergency door at the 33km 156m point as it was being closed 05:30 Flood water was guided into the fifth main tunnel through the fifth connecting shaft located at around 33km500m 9 22:30 A temporary drainage system at around the 34km685m point started operation, finally halting the rising flood level

Legend G: Connecting shaft VC: Closed vertical shaft V: Ventilation shaft WG: Gate CF: Face where the influx occurred WL: Flooded area PF: Pilot tunnel face WO: Collapsed earth and rock B: Bulkhead SP: Service tunnel pumping station WS: Cut-off wall PP. Pilot tunnel pumping station Fig.9. Flooding Resulting from the Y2 Influx

19 Recovery of the Damaged Area and Excavation of the A bulkhead was constructed to seal the deeper section of the tunnel filled with Fracture Zone Containing the Influx sediment. Upon completion of the bulkhead, a cementing agent was injected into the sediment and the surrounding ground to reinforce the area and stop water inflow. When the flood level, held constant by drainage equivalent to about 20 m3/min. near the 33km685m point, showed signs of gradual decrease, the Before resuming excavation through the fracture zone containing the influx drainage systems for the flooded area installed by that time were used to drain point, geological surveys were carried out to study the local geological the area. During the drainage operation, the inflow successively decreased, conditions, the various possibilities for grouting and excavation, the route of a with fluctuations of + 1 to -I nvtymin., finally waning to about 15 m3/min. The possible detour around the influx point, and the duration of the extra inflow volume and the length of tunnel flooded are shown in Fig. 11 as they excavation work. This study ended with a decision lo make a detour 50m changed over lime from the moment of influx to the completion of drainage. away from the influx point, as shown in Fig.6. The revised plan was When the water level subsided to the 31km742m point, sedimented rock and implemented and excavations successfully went through the fracture zone. earth was found, as shown in Fig. 10.

Cross section

C: Face containing the influx point D Caved-in section (55.5 m) buried under collapsed mud and rock T: Eighteen supports found to be undamaged during remedial work S: Sandbags B: Bulkheads E Collapsed earth and rock Fig. 10. Cave-ins Caused by the Influx

Rate of inflow ( mVnin ) 70 SO Rate of inflow Length of flooded section of the service tunnel 50 Length of flooded section of the main tunnel 40 30

2 0 o •S 1 0

Ü 0 0 -----;---- Time since May 6 ,00:00(logarithmic scale) 3 6 15 - 36 15 ! ! ! i ! hour 2 3 4 5 1 0 2 0 50 1 0 0 ~TT T 1 P \ " 0 © © ©© ® : Water rose beyond the level of the emergency door at 33,156 m in the service tunnel © : Flood water was guided into the fifth main tunnel. © : A temporary drainage system with a capacity of 32 m^/min came into operation. © : Draining of the flooded area started. © : Drainage of the service tunnel completed down to the 31 km742m point and the construction bulkheads began © ; Closure of the caved-in section was completed when bulkheads were built at the 31km742m point. ® : Excavation of the detour tunnel started from the service tunnel at the 3 lkm793m point. Fig 11. Changes in Water Inflow Rate and Hooded Length in Y 2 Influx

20 COUNTERMEASURES AGAINST EXTRAORDINARY Grouting in Areas with Danger of Influx INFLUXES

The type of ground most likely to result in an abnormal influx is fragile rock or Based on the experience of the exlraordinary water influxes into the Seikan soft fractured zones, or a layer with an extremely loose matrix, either itself Tunnel work, the following issues were studied and countermeasures were amply water-logged or in amply water-logged surroundings. If this type of developed to cope with possible extraordinary water influxes by their ground is grouted prior to excavation work from the face forward, the implementation. resulting strength and water-tightness not only prevents abnormal influxes of <1> How to predict ground conditions ahead of the excavation water, but also allows the construction of a stable tunnel. However, for this <2> How to grout the ground where there was a danger of influx purpose, a much greater volume should be grouted than in normal grouting <3> How to react to visible signs of extraordinary water influxes at the face with greater uniformity and a dense matrix. This was ensured by using the <4> How to react to exlraordinary inflows of water following procedures: <1> Classification of ground conditions according to Table 2, and After each influx, these countermeasures were reviewed and then partially implementation of the requirements given in the table. replaced with new countermeasures or improved. The definitive <2> Grouting in steps to each hole rather than all at once. couniermeasures took shape afler the occurrence of the Y2 influx. After that <3> Control over correlations between material density, pressure, and incident, excavation proceeded deeper, toward the mid-point of the undersea injection rate during grouting. section; here, the tunnel was deeper below sea level and the earth coverage <4> Development of a grout material with good permeability and post- above the tunnel was thinner. Fragile rock and soft fractured zones and layers solidification strength. of extremely weak matrix adhesion were also encountered. However, the successful introduction of the completed countermeasures against Ensuring that the grouting process took place as designated was also an extraordinary water influx prevented any further unusual influxes in the important factor. This included grouting at the correct location, which was undersea section until the completion of the Seikan tunnel work. ensured by fitting a stabilizer to the boring rod to prevent it deviating from the correct bore direction and by analyzing a few bored holes to gain information useful in determining the final bore direction. It was also necessary to ensure Prediction of Ground Conditions Ahead of Excavation correct mixing and desired chemical reaction of the grout materials in the grouting pipe. This was achieved by developing a pump which allows proportional force-feeding of the two grout components and by improving the As soon as tunneling began below the seabed, pilot bores preceded the main pipe design to ensure proper mixing. face to obtain information on the condition of the ground in advance. The greater the number of pilot bores used, the more accurately the nature of the After grout injection, it was necessary to check that the ground was properly ground can be ascertained, and this is especially true in waterlogged ground hardened. To do this, a hole was drilled into the hardened area; if where there is a risk of abnormal influxes of water. In the early stages, measurements were inconclusive, additional boring was carried out. TTiis however, excavation had to be done without sufficient information on the local work of surveying and observing the grouted ground was subject to geological conditions due to technical problems in horizontal pilot boring that it discussion and a final decision by a committee including supervisory experts was difficult to execute pilot boring fast, long, and to the designed position. on excavation and grouting. As a result, lack of information on the ground conditions ahead of excavation was one of the major obstacles to preventing abnormal influxes. Signs of Extraordinary Influxes on the Excavation Face In response to this problem, the technique adopted for boring pilot holes, and the machinery used, were improved so that quick and long boring was possible to the nearest designed location and also where the ground was poor. When the ground to be excavated was thought to present the danger of This work bore fruit, and the problems encountered with horizontal boring abnormal influxes, it could only be excavated after the go-ahead was given were overcome to a certain extent as represented by the record pilot bore length following an investigation and subsequent measures as explained above. This of 2,150 m. The accuracy with which ground conditions could be predicted decision to go ahead was not based on a final solution of all the problems improved greatly. Furthermore where the information gained from pilot involved; rather, it relied on partial data and empirical evaluations. Given this boring, grouting, and excavation was judged insufficient to allow a proper situation, an abnormal influx of water was possible any time, so measures prediction of conditions in the ground ahead, an additional short bore was were also devised to cope with the appearance of signs of influx and an driven into the ground from the face to obtain supplementary data. extraordinary influx that could happen during the work.

Legend ÏTJJJ1 : Waterlogged fragile or soft fracture zone © Construction of shoring or bulkheads © Reinforcement of supports: Length of flooded section of the main tunnel : Inflow or drainage © Construction of wall to block the inflow of earth, rock, and water © Construction of bulkheads to close off the tunnel Support © Additional submersible pumps to drain water from the face area © Boring of drainage holes to reduce the inflow of water from the face I ^ A I : Movable submersible pump © Installation of movable drainage systems in preparation for the increased inflow of water Fig. 12. Emergency Measures to be Taken upon Signs of Influx

21 It is empirically understood that an influx is preceded by changes in the water which occurred during construction. The fourth incident, the greatest influx of inflow points on the face or an overall increase in ihe number and volume of water, almost made the construction a desperate situation for a time. Luckily, inflow, concurrent with relaxation of the ground around the face. Some though, the repeated cave-ins and large flows of water caused the influx point influxes may be controlled through proper treatment at this stage. The to be gradually blocked by sedimented debris. As a result, the situation, emergency measures described in Fig. 12 were implemented when such signs though still dangerous, stabilized without further deterioration. Appropriate appeared. They were intended to improve the ability of the face to support measures for recovery learned from the past experience also proved useful. itself by reducing the amount of influx water flowing towards the face and Thus, ultimately, the worst scenario could be avoided. releasing the pressure of water on the surrounding ground, to reduce loosening of the ground near the face by preventing collapse of the face, and to drain Although, we feel it would be quite difficult to guarantee protection against flood water that would hinder works carried out at the face. critical failures, we believe it would be beneficial to suggest the following points in order to prevent influxes as much as possible and to respond to them correctly should they unfortunately occur. Handling of Extraordinary Influxes [1] Construction work • Preliminary geological survey, especially an investigation of ground The first action against a sudden water inflow is reinforcing and sealing of the strength and a prediction of the amount of inflow influx point or at the section containing it together with the surrounding • Meticulous study of the earth cover during the design stage ground, while the influx water is drained. The second action is to construct a • Acquisition and integration of precise information about the rock mass bulkhead as close as possible to the influx point to seal off the tunnel. to be excavated and grouting conditions. Sometimes the flow of water is loo strong to give time for these actions, • Constant review of the above information by experts however. In such cases, one of the gates for emergency closing installed at • Catch the signs that indicate the potential danger of an influx or cave-in designated positions along the length of the tunnel is to be closed, sealing off the flooded section. A drainage system, designed to have a capacity of about [2] Response to an accident 100 m3/min, was used to keep the basic facilities of the Seikan Tunnel ■ Establishment of a system to integrate all information and clarification servicing the whole construction away from flooding. Additional temporary of the chain of command support materials and bulkhead construction materials, as well as equipment • Establishment of a system for the quick and accurate acquisition of all and materials for drainage, were always available in the tunnel in readiness for information about the accident any sudden influx. Instructions and manuals related to these measures were • Full knowledge about the capacity and availability of current equipment also prepared to facilitate systematic, rapid, and trouble-free management of and materials in order to quickly decide what possible measures can be equipment and personnel to deal with a sudden water influx. taken • Selection of critical equipment and facilities to be saved or strengthened to survive in the last resort, and how to gain the time required to CONCLUSION achieve this • Planning and preparation of measures against secondary incidents

Since there always exits an unlimited reservoir of water above an undersea It might be said that the construction of Seikan Tunnel is typical of sealing and tunnel, its construction work is highly susceptible to fatal damage if any kind grouting work against water. of cave-in occurs. Since the large influx of 1976, no incidents interfered with tunnel construction Except for the case where the geological conditions are relatively good, one of work for the remaining 10 years or so until the breakthrough of the tunnel. the keys to successful undersea tunneling is the guaranteed ability to stop any This is due largely to the fact that everyone involved in the work remained inflows of ground water during excavation under the seabed. However, it is aware of the potential danger at all times. We always kept in mind that we impossible to ignore economics and the need to finish the work on schedule. should work with nature, not trying to conquer it but acknowledging its And even more problematic, it is never possible to judge whether the measures strength. This lead to the successful breakthrough of the tunnel. taken for sealing the tunnel are going to work or not. So it can be said that it It is our hope that this report will be useful as an instrumental reference in the was very lucky that the Seikan Tunnel survived the four major inundations execution of similar projects in future.

22