latest technology ofunderground rock cavern excavation in japan

Tunnelling and Underground Space Technology 18(2003) 127–144

0886-7798/03/$ - see front matter� 2003 Elsevier Science Ltd. All rights reserved.PII: S0886-7798Ž03.00039-7

Latest technology of underground rock cavern excavation in Japan

Masanobu Tezuka , Tadahiko Seoka *a b,

The Kansai Electric Power Co., Inc., Osaka, Japana

San Roque Power Corporation, San Roque, San Manuel, Pangasinan, Philippinesb

Abstract

Four major projects with large-scale rock underground caverns are discussed in this report; two hydropower stations, oneunderground oil storage facility and one underground museum in Japan. The technology applied to the construction of thesecaverns have some unique characteristics and can be applied to various other underground uses for different purposes and appliedto larger-scale underground rock caverns with different geological and in situ stress conditions. Generally, construction of anunderground cavern is comprised of five major activities: geological investigation; cavern stability analysis; initial rock supportdesign; excavation works; and redesign of rock support according to the observational construction method. Weighted factorsallocated among these factors should be established based on the rock properties and the purpose of the underground cavern use,which would define the major characteristics of the construction project. This report discusses some of major characteristics ofthese projects in Japan.� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Underground cavern; Powerhouse; Oil storage; Underground museum; Observational method

1. Introduction

Typical large-scale rock underground caverns in Japanare seen in underground powerhouses at over 50 loca-tions and oil storage facilities at three locations. Thehistory of underground powerhouses, which started withthe Uryu Power Station(Hokkaido Electric Power Co.,Inc., 51 MW, 1943), extends over some 60 years.Currently Kan-nagawa Power Station(Tokyo ElectricPower Co., Inc., 2700 MW) and Omarugawa PowerStation (Kyushu Electric Power, 1200 MW) are underconstruction. Construction of underground oil storage atthree locations, Kuji, Kushikino and Kikuma, was com-pleted consecutively during the period from 1994 to1995, and are under normal operation. Planning ofconstruction of a LPG storage facility has been under-way employing such technology. However, there hasbeen an increase in the number of underground cavernfor general public use. An example of a newly con-structed underground cavern for general public use isthe Takayama Festival Float Art Museum(Gifu Prefec-ture, 1998). It is a facility for storage and exhibition of

*Corresponding author. Tel.:q63-75-614-2080; fax:q63-75-614-2117.

E-mail address: [email protected](T. Seoka).

traditional art objects such as floats used for the famousTakayama Festival.

This report delineates a summary of discussion of thefollowing four projects to introduce the latest rockunderground cavern construction technology in Japan:

a. Kazunogawa Power Station(Tokyo Electric PowerCo. Inc., 1999).

b. Okutataragi Power Station(Kansai Electric PowerCo. Inc., 1998).

c. Underground Oil Storage Projects(Kuji, Kushikinoand Kikuma) (Japan Underground Oil Storage Co.Ltd., 1995).

d. Takayama Festival Float Art Museum(TobishimaConstruction Co. Ltd., 1998).

Their locations are indicated in Fig. 1(Tezuka, 2001).

2. Kazunogawa Power Station (Tokyo Electric PowerCo., Inc., 1999)

Tokyo Electric Power has been constructing theKazunogawa Pumped Storage Power Station with amaximum output of 1600 MW, maximum discharge of280 m ys and effective head of 714 m in Yamanashi3

Prefecture. The construction project started in January1993, and the first and the second units have already

128 M. Tezuka, T. Seoka / Tunnelling and Underground Space Technology 18 (2003) 127–144

Fig. 1. Location of the underground caverns.

Fig. 2. Cross-section of the powerhouse cavern.

Table 1Rock properties

Test results

Deformation wJack Testx (=10 MPa)3

Tangent elastic modulus Modulus ofLoading Unloading deformation

Average 11.5 7.2 4.0(Range) (5.0–22.5) (2.6–15.0) (1.0–11.1)Good Region E(Unloading) Ave. 12.6=10 MPa3

Weak Region E(Unloading) Ave. 7.7=10 MPa3

Strength wRock Specimen TestxUnconfined Compression Test Ave. 10 MPa6

(35.3–245 MPa)High Pressure Triaxial Compression TestPeak Strengthts12.8 MPaqs tan 578Residual Strengthts2.8 MPaqs tan 488

wRock Shear TestxGood Region(Peak) ts1.5 MPaqs tan 588Weak Region(Peak) ts0.8 MPaqs tan 558Residual Strengthts0.5 MPaqs tan 508

Rock stress wInitial Rock Stress MeasurementxOverburden: 520 ms s14.2 MPa(N178E, 688down)1

s s12.0 MPa(N1918W, 208down)2

s s9.4 MPa(N978W, 98down)3

started commercial operations with an individual outputof 400 MW in December 1999 and June 2000,respectively.

The underground powerhouse cavern is approximatelyin the middle of an 8 km water conduit connectingupper and lower reservoirs and approximately 500 mbelow the ground surface. The underground cavern hasa width of 34 m, height of 54 m and length of 210 m,and its cross-section has a horseshoe profile(Koyamaand Nambu, 1998; Koyama et al., 1997). Cross-sectionalviews of the underground cavern are indicated in Fig.2. The cross-sectional area of excavation is 1500 m ,2

while the excavation volume is 250 000 m .3

2.1. Geology and rock characteristics

The underlying geology surrounding the power stationconsists of Shimanto Terrane of mixed beds of sandstoneand mudstone, which are considered to have depositedduring late Cretaceous period of the Mesozoic era andthe Tertiary Cenozoic era. The site of the power stationis composed of highly developed jointed rocks, wherethe dip of the discontinuity is mainly classified intothree categories. Table 1 indicates the results of the rocktests conducted at the pilot tunnels excavated above thepowerhouse cavern site prior to the construction.

2.2. Observational construction system

Uniqueness of this underground cavern project is theuse of observational construction system(Koyama etal., 1999). The rock characteristics that affect stabilityof the underground cavern such as tangent modulus ofelasticity (12 600 MPa is the average value of thegeologically good zone) remain relatively lower, whilethe geology presents complicated conditions as numer-ous faults exist behind rock surface. The overburden is

500 m, which constitutes a higher initial stress condition(s s12 MPa,s s11 MPa, data used for the stability1 2

analysis). In order to detect the rock behavior duringexcavation, which cannot be detected in the predictionanalysis prior to excavation works, installation of meas-

129M. Tezuka, T. Seoka / Tunnelling and Underground Space Technology 18 (2003) 127–144

Fig. 3. Results of measurement and back analysis in the model cavern.

uring instruments and intensive analysis of the measureddata should be conducted.

One of remarkable features of the observational con-struction system conducted at the site is the prevalentuse of the back analysis. Particularly, it is notable thatprior to the underground powerhouse cavern excavation,a trial of back analysis was conducted by actuallyexcavating the model cavern at a scale of 1y5 to measurethe rock deformation and to provide approximate guid-ance of the initial stress. Fig. 3 indicates the measure-ment results of the model cavern excavation. Study flowof back analysis is shown in Fig. 4. The initial stressobtained from the back analysis under the assumptionthat the vertical stress is equivalent to the overburden,s s12.5 MPa,s s9MPa,ws13.18, which proved to1 2

be almost in line with the actual data provided by thestress measurement at the site. The equivalent modulusof elasticity is Es12 900 MPa, which almost matchesthe tangent modulus of elasticityE s12 600 MPa of thet

geologically good zone subject to the jack tests. How-ever, the displacement of the underground cavern endwall measured in the direction perpendicular to theprevalent discontinuity, was greater than that of thesidewall, while the equivalent modulus of elasticity(perpendicular to dominant joint set) computed by theback analysis presented a substantial anisotropy at some1y4;1y7 in the parallel direction. The rock behaviordata obtained from the model cavern and the backanalysis were used to determine the physical propertyfor the predictive analysis and back analysis for theunderground powerhouse cavern excavation to be men-tioned below.

2.3. Cavern behavior caused by the excavation

2.3.1. Overall cavern behaviorDistribution of cavern surface displacement in stage

of full section excavated is indicated in Fig. 5. Themaximum displacement at the sidewall is 56 mm at theF cross-section of the penstock side(d zone) (Kudohet al., 1999a,b).

2.3.2. Resultant behavior of critical management zone(d zone)

Thed zone had been originally estimated as the goodzone, but then was designated as critical managementzone for occurrence of displacement during the excava-tion as a result of the re-evaluation of geology. Theresultant behavior is as follows:

a. The area adjacent to the cross-section F–G wasmudstone dominant, which was relatively evenly dis-tributed over the wide area. During vault excavation,numerous long(maximum length at 6 m) joints strikesouth to north with a steep dip to the east(JNS-hseries) was detected. Additionally, a kink band(densely jointed zone) (N20Wy85E(50)) that crossesat a lower angle with the sidewall was confirmedduring bench excavation, which had not been detectedin the preliminary geological study.

b. Displacement(sidewall displacement at 17 mm)occurred at the spring line on the penstock side duringthe period the second stage expansion work of vaultexcavation to first bench excavation of main body ofthe cavern. It was considered to derive from the


Fig. 4. Study flow in back analysis.

above-mentioned south–north series joints, and con-stituted asymmetrical deformation behavior unlike thetailrace side(sidewall displacement at 3 mm).

c. Displacement occurred at the sidewall of the penstockside as a result of cutting-down the bench. It wasconsidered to derive from the above-mentionedsouth–north series joints and kink band. Displacementof 19 mm and 17 mm was detected at the upperportion of the sidewall(EL.646 m) and middle(EL.636.0 m), respectively, as of the completion ofthe excavation of the 8th bench. These figures exceed-ed the predicted figures(16 mm, 16 mm) based onthe results of the back analysis upon completion ofthe 6th bench excavation, and neighboring prestressedanchor load constituted a breach of the excavationcontrol criteria(294 kN).

Therefore, the excavation of the area concerned wasdiscontinued to conduct the back analysis based on the

anisotropic inhomogeneity model with consideration tothe kink band. The analysis results were used to calcu-lation of displacement coming out upon completion ofthe excavation, and to calculate the required load to beremoved from the prestressed anchors. Then new pres-tressed anchors were installed additionally, and theexisting anchors’ loads were reduced(294–235 kN).The results of the back analysis are indicated in Fig. 6.

As a result, the displacement at the final stage of theexcavation was contained within the predicted rangeupon completion of the 8th bench, while the prestressedanchor load was limited to the yield load(330 kN).

2.4. Conclusions

The observational construction system with an empha-sis on the back analysis produced the following assess-ment results:

a. As a result of the preliminary evaluation of the modelcavern, the back analysis has proved to be an effectivesurvey and review method to determine the rockdeformation property and initial stress for evaluationof the rock behavior during excavation of the under-ground powerhouse cavern, which would not beaffected by local factors(such as geological property)inherent to the in situ rock test.

b. The back analysis proved to be effective as a measureto explore the causes of discontinuous deformationbehavior hardly detected in the preliminary investi-gation for underground cavern excavation, and sub-sequently to make adjustments to the predictionanalysis and rock support design.

c. This underground cavern presented an example ofexcavation of one of the largest underground cavernsin Japan, under the severe conditions such as highlydeveloped jointed rocks, complex geological structureand intensive initial stress, which was made possibleby applying precise geological survey and sophisti-cated observational method system.

3. Okutataragi Power Station (Kansai Electric PowerCo., Inc., 1998)

Kansai Electric Power constructed the OkutataragiPower Station(pumped storage power station) with amaximum output of 1212 MW in Hyogo Prefecture in1974. Twenty-four years later, the project to expand thecapacity (720 MW) was implemented to meet thegrowing demand for electric power in the service area.In June 1998, the power station with a maximum outputof 1932 MW started its operation. The summary ofconstruction of a new underground powerhouse cavernfor the capacity expansion project is discussed in thissection.


Fig. 5. Displacement on the cavern surface(at the completion of excavation).

Fig. 6. Back analysis results and maximum shear strains based on the results.

The underground cavern was excavated above thewater conduit with a length of approximately 3.5 kmconnecting the existing upper and lower regulatingreservoirs. The powerhouse cavern is located approxi-mately 250 m below ground surface and 200 m awayfrom existing powerhouse caverns. The undergroundcavern’s dimensions are 25 m wide, 47 m high, 130 mlong, while its cross-section has a bullet-like shape.Cross-section views of the underground cavern areindicated in Fig. 7. The cross-sectional area for exca-

vation was 1000 m , and the excavation volume was2

94 000 m (Seoka et al., 1997).3


The geology surrounding the power station is mainlycomprised of volcanic rocks of Ikuno Group Chubu Sgsuch as rhyolite and homogeneous volcanic rocks in themiddle of the Cretaceous period of the Mesozoic era.Porphrite and diabase were identified as the intruded


Fig. 7. Cross-section of the powerhouse cavern.

Table 2Rock properties

Test results

Deformation wJack Testx (=10 MPa)3

Tangent elastic modulus Modulus ofLoading Unloading deformation

Average 16.9 17.3 14.5(Range) (7.7–26.0) (7.7–26.8) (3.3–25.7)Very Good Class(Unloading) Ave. 23.5=10 MPa3

Good Class(Unloading) Ave. 15.7=10 MPa3

Fair Class(Unloading) Ave. 4.9=10 MPa3

(existing report)

Strength wRock Specimen TestxUnconfined Compression Test(58.9–147.1 MPa)

Triaxial Compression TestGood Classts15.7 MPaqtan 608Fair Class ts7.3 MPaqtan 498

wIn situ Shear TestxGood Class(Peak) ts2.5 MPaqtan 57.38Good Class(Residual) ts2.0 MPaqtan 42.98

Rock stress wInitial Rock Stress MeasurementxOverburden: 250 ms s7.9 MPa1

s s4.7 MPa3

us53.68

rock, which strikes almost south to north(in the direc-tion of the longitudinal axis of the underground cavern).Dominant join set has a strike in the directions south tonorth and east to west and a dip with large angle of 708

or more. The results of the rock test conducted for thesurvey audit are indicated in the Table 2.

3.2. Cavern stability analysis and rock support design

The rock support design was conducted in accordancewith the procedures described below.

a. Assume that the whole cavern is an isotropic homo-geneous CH-class rock.

b. Non-linear visco-elastic and sequential excavationnumerical analysis with the use of FEM is adoptedas cavern stability analysis model. The analysis resultsare indicated in Fig. 8.

c. Design the prestressed anchor for the supporting areasunder the assumption that shear failure and tensilefailure compose an unstable rock mass. Establishedrock support patterns are indicated in Fig. 9. Shotcreteand rock bolts, and prestressed anchors to be installedin the end constitute the support members.

d. Design of the prestressed anchors was not providedfor the area, which is not subject to(c) above, onlythe rock bolts and shotcrete were applied. Thereforethe significantly asymmetrical design was providedfor the cavern cross-sections as indicated in Fig. 9.

Such mitigated support pattern is a major character-istic of this underground cavern. The only differencefrom the Kazunogawa power station(Section 2) lies inthe difference of the rock in collapse mode, which is


Fig. 8. Results of cavern stability analysis.(Failure zone in stage offull section excavated.)

Fig. 9. Standard rock support patterns.

subject to the assumptions based on the property of therock surrounding the respective underground cavern.The area surrounding the cavern is composed of hardigneous rock(rhyolite), and the rock behavior duringexcavation is assumed to be dominated by significantmovement of keyblocks, judging from the property ofthe discontinuity. Therefore, the rock support pattern forthe initial stage of excavation was limited to a minimum,and an emphasis was put on the supplementary rein-forcement based on observational construction systemincluding the keyblock analysis as described below. The

basic concept behind pursuing optimization of the finalsupport configuration based on the observational con-struction system for both Kazunogawa and Okutataragiis exactly the same.


The observational construction system using the key-block analysis in addition to the general measurementcontrol is described in this section.

3.3.1. Keyblock analysisa. Modeling of the discontinuity

The flow chart of the excavation control system fromthe joint survey to the keyblock analysis and then tothe design and execution is indicated in Fig. 10.

b. Assessment of characteristics of the discontinuityIn order to determine the characteristics of the dis-continuity such as internal friction anglew, cohesionstrength c, the multi-stage three-axial compressiontest using the block sample and boring core, anddirect shear test are conducted. At the same time, inorder to verify the test results, a tilt test and a directshear test of polished rhyolite are conducted forsupplemental purposes.w is distributed in the rangeof the basic friction angle(28.18) to the inclinationangle(46.58). The angle tends to depend heavily onthe interlocking situation. Thus,ws32.48 and cs0kPa were determined as such, as the result of thedirect shear test of the polished rhyolite is assessedto indicate the lowest measure of the internal friction-al anglew based on the fact that the joint roughnesscoefficient (JRC) at the site has proved to be rela-tively small at 1–8.

c. Analysis results and assessmentThe slide modes of the identified unstable block areclassified into three types of mode:


Fig. 10. Study flow for keyblock analysis.

Fig. 11. Rock supports for keyblock failure.

i. wedge sliding mode;ii. plane sliding mode;iii. falling mode.

The basic equation without directions of each workingforce at the extreme equilibrium analysis on the basisof the Fig. 11 is indicated as follows:

F s R c,w qR tc qT yW (1)Ž Ž . Ž . .s j c

where F ssafety factor,R sstrength of discontinuity,s j

R sshear strength of shotcrete,Tssupport force of rockc

bolt or prestressed anchor,Wssliding force.Londe et al.(1969) proposed to give a safety factor

corresponding to the uncertainty of each constituentfactor of the equation of equilibrium. Eq.(1) based onthis proposal is expressed as follows:

µ ∂1s R c,j 1y f qR tc 1yf qT 1yf yW (2)Ž .Ž . Ž .Ž . Ž .j j c e t

where f , f , f : indicates, respectively, safety factors ofj c t

the strength of discontinuity, shear strength of shotcreteand the support force of a rock bolt or prestressedanchor.

Herein Eq.(2) has been introduced, and the safetyfactor of shotcrete and the discontinuity resistance is setat f sf s3.0, taking into account dispersion of strength.c j

Regarding the support force of rock bolts or prestressedanchors, each support member is designed atf s1.0 ast

the safety factor has been already reflected to the designprocedure of tension load to be installed.

An example of the supplementary reinforcementdesign to the unstable block identified at the vault partis indicated in Fig. 12. The unstable block with 10.5 min height, 144 m in volume and 3678 kN in weight3

was detected in the range of approximately 10 m fromthe immediate top of facing to the ending corner of thevault, as a result of the analysis of joint survey at TD120 m of the central heading. The outline of thesupplementary reinforcement design is indicated asfollows:


Fig. 12. Calculation sheet of keyblock analysis.

a. A layer of the primary shotcrete 8 cm thick wasinstalled, and its expected strength was 440 kPa atthe age of 2 h. The shear resistance of the perimeterof 31 m is calculated in the following equation:

440 kPa=8 cm=31 ms1091 kN (3)

b. In this case, the keyblock sliding force is equivalentto the weight of the keyblock at 3678 kN, and thesafety factor is calculated asF s0.3. The resistances

required to meetF s3 is calculated in the equations

below. This is equivalent to support force of rock

bolts or prestressed anchors used for supplementaryreinforcement.

3678 kNy1091 kNy3s3314 kN (4)

c. The number of the required rock bolts in the followingcases:(A) ultimate tensile force between rock boltand injected grouting material;(B) cohesive forcebetween rock bolt and grouting material; and(C)tensile stress of the rock bolt. Among these threecases,(C) would require the largest number of rockbolts, which is calculated as follows:


3314 kNymin (0.6 T , 0.75T )yAsu sy rb

s3314 kNymin (0.6=500 MPa, 0.752=352 MPa)y4.9 cm

s26 (5)

where A is the cross-section of the rock bolt; andrb

T , T is the ultimate tensile strength and yieldsu sy

strength of rock bolt.d. Similarly, the number of prestressed anchors required

is calculated as follows.

3314 kNymin (0.6 P , 0.75 P )su sy

s3314 kNymin (0.6=967 kN, 0.75=823 kN)

s6 (6)

whereP , P are ultimate tensile strength and yieldsu sy

strength of wire strand, respectively.

Therefore, 26 rock bolts or six prestressed anchorsare required as a supplementary reinforcement in addi-tion to the primary shotcrete. Herein prestressed anchorsare adopted as supplementary reinforcement for thereasons that a larger number of rock bolts would berequired, and the length ofLs5 m is insufficient forthe height of keyblock at 10.5 m.

3.4. Conclusions

As in the past, keyblock theory proposed by Goodmanand Shi (1985) has been applied to design the mostsuitable layout of underground cavern that minimize thescale of unstable blocks in the excavated rock surface.There are many advantages in applying the keyblockanalysis to the daily excavation management. Firstly, asit would be possible to detect unstable blocks immedi-ately after blasting and scaling, a safer working environ-ment for workers and on site equipment could beprovided. Secondly, a required amount of rock supportscould be designed effectively. The observational con-struction method including real time processing of key-block analysis would be considered to contribute towardrational excavation management of large undergroundcaverns in the jointed rock mass.

4. Underground oil storage project (Japan Oil Stor-age Co. Ltd., 1995)

History of the use of underground cavern for oilstorage in Japan started with construction of three bases:Kushikino (Kagoshima prefecture, andesite), Kikuma(Ehime prefecture, granite) and Kuji (Iwate prefecture,granite). The Agency of Natural Resources and Energystarted a broad regional survey of site locations in 1975.It was followed by the verification test of the under-ground oil storage technology with water injection meth-

od at Kikuma (started in 1980). Construction oflarge-scale bases started in 1987. Oil-in has been com-pleted at all the bases in 1995.

Currently, the underground oil caverns at three baseshave maintained stable operation. Dimension of cavernsare different from base to base. Each base consists ofsome caverns(up to 10) excavated in parallel. Cavernscale ranges from 22 to 30 m in height, 18 to 20.5 min width, and 230 to 555 m in length. Storage capacityranges from 1.5 million kl to 1.75 million kl and thetotal excavation volume of the underground cavernsranges from 1.6 million m to 2.2 million m(Hoshino,3 3

1993).

4.1. Design

An underground cavern for oil storage is subject tothe regulations of Fire Service Law, and it is necessaryto review the items stipulated in its guideline(datedMay 1987). The following four requirements should bereviewed for designing an oil cavern:

1. ensure enough clearance between rock caverns;2. ensure stable groundwater level surrounding a rock

cavern;3. ensure cavern stability against the load of a rock

cavern; and4. critical groundwater level of a rock cavern(the

minimum groundwater level to be maintained toprevent vapor or oil leakage).

4.1.1. Advanced parallel underground cavernsThe underground oil storage facility comprises some

parallel caverns with a similar size to ensure storagecapacity. Clearance between caverns is determined bydynamic conditions and underground hydraulic condi-tions. As for dynamic conditions, the Fire Service Lawstipulates a requirement to ensure the clearance betweenoil caverns at or greater than the figure(L) as specifiedbelow:

Ls B qH qB qH y4qR qR (7)Ž .1 1 2 2 1 2

Where, L is the distance between the inner walls ofadjacent rock tanks.B , H , B , H , R , R are as1 1 2 2 1 2

indicated in Fig. 13.Analysis is conducted for the following items:

a. analysis method: double linear elastic analysis;b. analysis model: tandem cavern model;c. load conditions: initial stress, interval water pressure

inside rocks, seismic inertial force; andd. loosened zone: local safety factor,F -1.5.s

As a result of the review, the interval between centersof the oil caverns was set at 50 m at the Kuji andKushikino Bases, and 65 m at the Kikuma Base(JSCE,1996).


Fig. 13. Minimum clearance between rock tanks.

4.1.2. Assurance of stability of groundwater levelCrude oil to be stored remains hermetically sealed

under pressure of underground water, which flowsextremely slowly across the space of the surroundingrocks. This technology was developed in the 1950s inScandinavia, and is generally called the ‘water injectionmethod’.

The permeability coefficient of rock and undergroundwater recharge represents the most important parametersfor a water injection type of oil cavern. In case thepermeability coefficient of a rock is extremely smalland the underground water recharge is relatively large,the natural underground water method is applied; whileif the permeability coefficient is small but recharge issmall as well, or the permeability coefficient is large,the artificial water injection method is adopted for forcedwater supply through bore holes. The guidelines of theFire Service Law requires evaluation of the soundnessof the water injection function even during a droughtthat might occur with an interval of 100 years, or duringa change in ground formation. Underground waterrecharge of a dry year at these sites was estimated basedon the precipitation data of the observed dry years andthe hydrological tank model method. As for the changein ground formation, the recharge was estimated underthe assumption that trees have been burned down dueto forest fire, and with consideration to a change in run-off coefficient. Additionally, FEM was applied for thesaturatedyunsaturated seepage flow analysis to ensurethe groundwater level, and the water injection functionwas evaluated through the reproduction model develop-ment and simulation of changes in the groundwaterlevel.

The results of analysis of seepage flow related to theartificial water injection at Kuji Base are indicated in

Fig. 14. The analysis results were used to establish thecritical ground water level at each point, so that theunderground water around the oil cavern(within therange of horizontal distance in five times of the maxi-mum width of an oil cavern from its inner wall) uponits completion would be secured to maintain stable waterlevel to constantly offset the expected change in waterlevel.

Kushikino Base adopted the natural water injectionmethod combined partially with an artificial water injec-tion method, while the Kuji Base and Kikuma Baseadopted the artificial water injection method.

4.1.3. Stability assurance of surrounding rocksAs an oil cavern with the water injection function

remains constantly below the groundwater level, depend-ence on highly corrosive rock support materials shouldbe kept lower. An oil cavern is not basically subject toscheduled overhaul(maintenance) during operation,dependence on the rock supports would be minimized,and the strength inherent to a rock should be utilized tothe fullest extent. Accordingly, no prestressed anchor isused on the concerns of its long-term durability at anybase, and the support design is provided with the use ofrock bolts and shotcrete only. Standard support cross-sectional view of a representative rock at Kuji Base isindicated in Fig. 15. In case ordinary supplementaryreinforcement with rock bolts and shotcrete is found notto ensure sufficient stability during construction work, achange is made to the excavation shape.


One of the major purposes of the observationalmethod is to ensure dynamic stability during excavation


Fig. 14. Seepage analysis with FEM(Kuji Base).

Fig. 15. Standard rock support patterns(Kuji Base).

as in the case of the underground powerhouse. A largerweight of underground water management during con-struction work is a major characteristic of the under-ground oil storage(Fukuhara and Hasegawa, 1995;Sakurai et al., 1998).

4.2.1. Observational construction system concerning theunderground water management

The case of the Kuji Base is introduced below. Therock condition of Kuji Base is harder and finer comparedwith that of the other two bases. Overall, it has devel-oped discontinuity, with great permeability coefficient.As a result of the preliminary survey, developed discon-tinuities perpendicular to longitudinal axis of the oilcavern, with a extension in direction of depth of cavernwere detected, which cause the formation of a so-calleddeveloped ‘water path’ in the surrounding rock.

The flow chart of the observational constructionsystem for the purposes of underground water control isindicated in Fig. 16.

Among individual sub-systems constituting the sys-tem, the permeability test system, the drill loggingsystem, and the groundwater level monitoring networksystem were developed as a tool for actively collectingthe hydraulic geology information and its use. Thegroundwater level monitoring network system allowsmonitoring the surrounding groundwater level as nec-essary by networking groundwater level monitoringbores installed around the oil cavern through a fixed-line telemeter. It enables real-time monitoring of achange in groundwater level during the constructionwork in progress, allowing automatic output of a ground-water level contour chart based on the level inside eachobservation bore.

The observational construction system was employedfor the purposes of such underground water managementto implement cut-off grouting and partial water injectionfrom the beginning of construction of the oil cavern toits completion(partial water injection to the tunnel evenduring the construction work to prevent lowering of thelocal groundwater level). The careful groundwater levelmanagement accompanied this goal. As a result, thegroundwater level upon completion of the oil cavernexcavation exceeded the critical water level, includingthe level after oil-in. The seepage volume after oil-inupon completion of the oil cavern excavation declined


Fig. 16. Observational construction system for underground water control(Kuji Base).

below the design spring water level, which allowednormal start-up of the operation.

4.3. Conclusions

So far, some European and the US experts doubtedthe feasibility of constructing a large-scale undergroundstorage base with a deep cavern with a large cross-section as seen in Europe or the US, due to higherfrequency of earthquakes and volcano eruption in Japan,which belongs to the Pacific rim volcanic zone. In thisregard, it has great significance to have proved thatconstruction of a large-scale underground storage base

by implementing location selection, design and construc-tion based on well-planned geological surveys. There isno doubt that this experience would help improve thefuture construction process of an underground cavernwhich requires highly hermetic conditions such as con-struction of an LPG storage base or disposal of radio-active waste inside ground structure(Hoshino, 1993).

5. Takayama Festival Float Art Museum (TobishimaConstruction Co. Ltd., 1998)

The Takayama Festival Float Art Museum is the firstunderground museum constructed in a large-scale under-


Fig. 17. Takayama Festival Float Art Museum(Gifu Pref.).

Fig. 18. Distribution of discontinuities estimated from in-situ surveys(parallel to the longitudinal axis of the geological survey tunnel).

Table 3Rock discontinuities properties

Friction Cohesion Condition ofangle (MPa) discontinuities(degree)

In-situ shear test 56 2.20 GeneralLaboratory shear testNo. 5 boring core 35 0.107 WeatheredBlocks with joint 20 0.015 Very weak

filled clay

ground cavern(Fig. 17). This underground museumrepresents a product of the underground space technol-ogy developed through construction of conventionalunderground caverns combined with the typical archi-tectural technology. For the construction, expertise ofboth engineers who have experience in undergroundcavern construction, and architects who have beeninvolved in design and construction of a museum. Themuseum to be constructed in an underground cavern forthe first time in Japan required permission by theMinistry of Construction under the Building StandardLaw, and its accident prevention measures and structurewere rated(Nakada et al., 1996; Chikahisa et al., 1999).

5.1. Shape of an underground cavern

The idea behind the shape of an underground cavernand its arrangement is quite interesting. The shape ofthe underground cavern(referred to hereunder as exhi-bition hall) has a height of 20 m and width of 40.5 m(flatness ratio at 0.5), and the overburden at the locationhas a very shallow depth at 30 m. This overburden wasdetermined with due consideration to the cost balancebetween the required rock support to stabilize the exhi-bition hall and the extension of the access tunnel(referred to hereunder as exhibition tunnel).

Considering the access from the ground surface, theaccess tunnel should be shorter in distance, but a lot ofrock supports would be required to ensure stability ofthe underground cavern as the exhibition hall would becloser to the ground surface. Conversely, to extend thelength of the access tunnel would raise the expenses oftunnel excavation higher.


The geology surrounding the underground cavern iscomposed of surface soil, cohesive soil of the colluviums

deposit and the late Cretaceous period, and igneous rockor welded tuffs derived from pyroclastic flow, called asNohi rhyolite of Paleogene, from top. The Atotsukawafault extending north-east to south-west strikes atapproximately 25 km from the construction site, whilethe Adera fault extending from north-west to south-eastis located 40 km south. Discontinuity analysis wasconducted for the support design and as an observationalmethod of the underground cavern based on joint surveyresults. Distribution of discontinuity surrounding theunderground cavern is indicated in Fig. 18, while thecharacteristics of the discontinuity is indicated in Table3 (Chikahisa et al., 1997).

5.3. Stability analysis and support design

Major characteristics of this underground cavern arethe detailed stability study of the underground cavernsubjected to the occurrence of earthquakes(Nakada etal., 1996). In case of the underground powerhouse, thecaverns are excavated below over 100 m and inside arelatively hard rock, which does not require particularattention to the cavern stability against an earthquake.Design of the underground cavern, as discussed above,needs to ensure fire prevention and structural stabilityin compliance with the Building Standard Law.


Table 4Design concept to estimate loosened zones

Conditions Design concept

Stationary The underground opening can resist without damage the static workingloads after construction.(By allowable stress method).

Earthquake The underground opening can respond, without damage, to an earthquake(Level 1) motion that can occur once or more during the service life of the opening.

(By allowable stress method).

Earthquake No fatal damage results even in case of the strongest earthquake(Level 2) motion that has ever occurred near the site.

(By the ultimate strength method).

Table 5Maximum velocity on the ground surface in the dynamic stress-deformation analysis

Level Velocity on Amplification Velocity on the Remarksthe bedrock factor ground surface

Level 1 10 1.9 25 Effects of middle-scalemiddle-distance earthquake

Level 2 25 50 Effects of the Atotsugawa fault

Units: cmys.

Fig. 19. Result of response analysis.Fig. 20. Loosened zone estimated from dynamic analysis using thelevel 2 earthquake ground motion.

5.3.1. Support design based on the dynamic stress-deformation analysis

Stress deformation analysis of the cavern was con-ducted to review the static stress conditions upon com-pletion of excavation and stress conditions uponoccurrence of an earthquake. The analysis conditions

are indicated in Table 4. Level 1 represents an earth-quake recurring in 200 years under the assumption ofthe service life of an underground cavern for 100 years.Level 2 represents the greater in magnitude of the largestearthquake ever occurred at the site or the estimatedearthquake to be generated due to the fault activity inthe neighborhood. Design ground motion used for stressanalysis is indicated in Table 5. The input waveformwas subject to the single-dimensional response analysisusing four waves as indicated in Fig. 19, El Centro N–S wave was adopted to generate maximum accelerationat the depth of the exhibition hall.


Fig. 21. Support members around the exhibition hall.

Fig. 22. DDA block model(C class rock mass).H

Table 6Analysis cases and conditions for DDA

Case Shear strength of discontinuity Condition of stress Time step(s) Number of

Friction angle(8) Cohesion(MPa)and load step

1 56 2.20 Initial stress 0.2 5002 35 0.107 Initial stress 0.2 5003 20 0.015 Initial stress 0.2 5004 56 2.20 Initial stress and 0.05 500

inertia forceSeismic intensity is0.65 (horizontal),0.33 (vertical)

The results of the dynamic analysis using the Level2 ground motion are indicated in Fig. 20. The loosenedarea indicates that it contains the factor with the localsafety rate below 1.1. Fig. 21 indicates the design ofthe support pattern of the exhibition dome used toreinforce the expected loosened area. The primary sup-port member is the prestressed anchor, and the stainlesssteel fiber reinforced shotcrete and rock bolts wereadditionally used for supplementary reinforcement.

5.3.2. Prediction and verification based on the discon-tinuity analysis

Discontinuous deformation analysis(DDA), one ofthe discontinuity analysis methods, was employed topredict the discontinuous behavior of the rock adjacentto the underground cavern(Chikahisa et al., 1997). Fig.22 indicates the two-dimensional analysis model usedfor the DDA. The analysis conditions are indicated inTable 6. The analysis was conducted for the followingfour cases: three cases depending on the level of shearstrength at the discontinuity as the initial stress releasedue to excavation is assumed as load conditions in orderto review the normal conditions, and the fourth casewas intended for review the conditions during an earth-quake by continuously loading the Level 2 seismic

inertial force. The initial vertical stress was set at theself-weight of the rock equivalent to the overburden(svssH), while the horizontal stress was set at 1y2 ofthe vertical stress(shs0.5sv), based on the test resultsof the AE method. The results of the analysis(displace-ment chart) are indicated in Fig. 23. Slippage of a blockat the top of sidewalls and pressing a block out of theleft sidewall were detected. Opening of the discontinuitydeveloped to the distance of approximately 5.7 m fromthe excavation surface. Maximum displacement was 54cm, showing the behavior of significant deformation.The results of the analysis in the case 4 of occurrenceof an earthquake, the tension stress was seen at twoblocks only at the top of left sidewall with the maximumdisplacement of 4.0 cm.

5.4. Conclusions

Generally, an underground cavern intended for publicspace has greater flatness and smaller overburden com-pared with an underground powerhouse or an under-ground oil storage facility. In the case undergroundcavern requiring high safety factor due to large flow of


Fig. 23. Results of DDA(displacement and deformation of the blockin C class).H

indefinite number of people is excavated under unfavor-able conditions, a highly advanced level of technology,as adopted in this cavern, is required.

6. Concluding remarks

Construction of an underground cavern is comprisedof five major factors: geological investigation, stabilityanalysis, support design, excavation method, and rede-sign of rock support based on observational constructionmethods. These components should be integrated intothe total system for the design and construction ofunderground caverns rather than being evaluated indi-vidually. Some major projects, including large-scale rockunderground caverns in Japan were discussed abovefocussing on the observational construction method,respectively. The author is eager to mention that scholarsand engineers in Japan have studied many unique andpractical numerical models for cavern stability analysis,especially for jointed rock mass. These results have beenreported in the international conference so far. A typicalreference showing the status of study in Japan, ‘Com-parison of computational models for jointed rock massthrough analysis of large-scale cavern excavation’ byHorii et al. presented at the ISRM congress in Paris,1999 (Horii et al., 1999), should be recommended tohelp the readers to understand the outline of advancednumerical models for practical use.

Acknowledgments

The author is grateful to Mr H. Yoshikoshi and MrY. Hibino (Tokyo Electric Power Co., Inc.), Mr A.

Okamoto(Japan Underground Oil Storage Co., LTD),Professor N. Shimizu(Yamaguchi University), Mr A.Fukuhara(Electric Power Development Co., Ltd), MrH. Chikahisa and Mr K. Kobayashi(Tobishima Corpo-ration), Mr Y. Tateno (Oya Stone Museum), JapanSociety of Civil Engineers and other persons for provid-ing invaluable material for this paper.

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