technical development for the seikan tunnel

Technical Development for the Seikan Tunnel

Akira Kitamura

Abstmct-In the construction of the Seikan Tunnel, because of both the great length of the tunnel and the condition of the undersea portion of it, dt@culties were expected, such as the impossibilip of confinning the geology of the tunnel route using reconnaissance. In order to overcome these d@culties, many techniques were develo#ed during the construction of the Seikan Tunnel. This pa@r desm’bes several examples of such techniques, including a horizontal advance boring to confirm the prior geology; the developmntt of grouting methods oriented toward tightening the ground and sealing off the sea water; and each of the excavating techniques that were tried and failed in completing the umiersea tunnel.

Advance Boring

B efore excavating the pilot tunnel and service tunnel, (1) dredging, ‘(2) sonic, magnetic and seismic

prospectings, (3) boring, and (4) seabed observation using a midget submarine were conducted at sea in order to investi- gate the geological conditions of the proposed route. Because of the consider- able water depth, i.e. 140 m, and the strong tidal current of the Straits, i.e. maximum of 7.8 knots, these surveys had to overcome various difficulties and only approximate geological conditions could be revealed.

The pilot tunnel and service tunnel will be dug in the unknown section of the undersea. In order to excavate these tunnels safely, it is necessary that geological conditions of the planned route and volume of water inflows be accurately ascertained prior to excavation. The advance borings are to move forward from the transverse chambers which are excavated on both sides of the tunnel.

Since the greater part of the undersea consists of sedimentary rock, it is prefer- able that three bore holes be drilled, even though it has been said that two bore holes would be sufficient. this consideration is based on the fact that it is possible to assume the strike and dip of strata from the past tunneling experience, and a plane can be formed from three points. the length of one bore hole is approx. 60GlOOOm.

It is necessary that the exploratory formings be performed as quickly as possible so as to provide the information

Akira Kitamura is Director of the &ikan Undersea Tunnel Construction Bureau, Japan Railway Construction Public Corpora- non. Present address: Takugin Bldg, 1%~ Wakamatsu-cho, Japan.

Hakodate, Hokkaido,

Re%m6-L.ors aJe la construction du tunnel de Seikan, on s’attenda i de nombreuses d$iculris telles que 1Tmpossibiti a? contrmer la gologie du trace du tunnel do&par l’itapc de reconnaissance; ces dtf&-ultis Ctaient dues ci la fois a la grande longueur du tunnel et aux conditions de la pati’e sous la mer. Ajin de surmonter ces d$cultis, plusieurs techniques furent aVveb#ees #endant la construction du tunnel de Seikan. Cet article dinit plusicurs exernples de ces techniques telles que des son&ages horizontaux i l’avance pour con&nzcr la giologie, ie aVvelo@ement des mithodes de cimentation visant i cornpresser la terre et i bow&r 1s infiltrations d’eau a% mer et, toutes 1s techniques d’excavation qui

furent essayits et qui ichoutrent jusqu’ci l’aboutissement du tunnel.

necessary for excavation of the tunnel. At the same time, however, the technology required for boring at the desired position is necessary, as the borings move forward simultaneously in a horizontal direction and, therefore, tend to bend downward.

Change of Boring Methodr (Fig. 1) Wire line boring method. The advance

boring is to be performed by the wire line boring method, which is frequently used for vertical boring because the core can be obtained not by pulling out a rod with an excellent workability, but rather by using it horizontally.

The results, however, revealed the following defects:

(1) In areas of friable ground, jamming accidents in the drill pipe operation occur due to the cuttings and degraded muck because of the narrow annular area (the gap between wall of the bore hole and the rod). (2) The wall of the bore tends to collapse because the bore hole pressure fluctuates widely between the time when the inner tube is extracted and inserted. (3) Because the core barrel of the wire line is heavy, the bend tends to be large. (4) If water inflows increase, insertion of

1. Wire line borinr method

3. Double-pipe roverso circulation boring method

Legend

Rod

Core

Rod (innerpipe)

Core barrel

Casing pipe at the mouth

later swivel

@ Driller chuck

@I Hoist

0 Water pumping

4. Hydraulic prossore circulation boring rethod 8 Slime and deraterine

0 Preventer

0 Valve

Q Yud fluid tank

8 Submerged PUMP

Figure 1. Schema of tunnel boring mcthodr.

TnawAIing and U&ground Spocr T&mIa~, Vol. I, No. 314, pp. 341-349, 1986. 0866-7798166 t3.00+.00 Priotcd in Crur Britain. 0 1986 Pergamon Journals Ltd. 341

the inner tube becomes difficult, requiring grouting to be performed in order to seal off water inflow

Auxiliary methods on the wire line method. As boring is performed in the undersea section, the ground becomes soft and water inflow increases. Therefore, in order to retain the wall of the bore hole, the grouting method, casing tube method, mud fluid method and hydraulic pressure method were combined with the boring by using the wire line method as follows:

(1) The grouting method is used to grout the bored section in order to seal off water inflow and prevent the natural ground from collapsing, and boring is performed ahead of the grouting. However, grouting is very time- consuming.

(2) By using the casing method, the bored section is widened, a casing is inserted, and the boring is performed ahead thereof in order to retain the wall of the bore hole mainly in the friable natural ground. However, this method was not successful because the wall of the bore hole collapsed before the casing could be inserted after widening the bore hole at the section where the natural ground suffered major collapse.

(3) In the mud fluid method, mud fluid is used for circulating water. This method takes advantage of the interface effect of the mud fluid and the difference in weight between the mud fluid and seepage water. In the advance boring, however, the mud fluid method was nothing more than an experiment because of the difficulty of adjusting the concentration due to the mud fluid's dilution by seepage water, and making no use of the difference in weight due to horizontal bore hole.

(4) In the hydraulic pressure method, the bore hole is maintained under pressure and the core boring is performed by using the wire line boring method. This method was adopted in order to prevent the natural ground from collapsing due to flushing of seepage water and to omit water seal-off. However, because the fundamental defects of the wire line boring method had not been overcome, i.e. difficulties in hauling out cuttings and friable muck, this method was used in only two cases.

Reverse circulation boring method. Several of the methods described above were experimentally tried, but ended in fail ure. Finally, therefore, the reverse circulation boring method (so-called because the flow of circulation water is reversed) was adopted, in which circulation water is pumped outside the rod, and then the core and cuttings are passed inside the rod and sent outside the bore hole, together with circulation water. This method proved successful.

Advantages of this method are as follows:

(1) Because it uses the continuous core

recovery system, tile rate of advance is fast.

(2) The removal of core, cuttings and degraded muck is easy, and drill- jamming accidents rarely occur.

(3) The structure is simple because a bit is attached to the top of the road; thus, there are fewer accidents.

In spite of these advantages, however, there are certain disadvantages to the use of this method, such as lower core recovery ratio, particularly in the case of soft rock at greater depths (the core is sometimes broken into many slices with work surfaces, etc.)

In the case of bad geology, the double- pipe reverse circulation boring method is used to break through this area because the wall of the bore hole collapses during the pulling out of the drill pipe and the insertion of the casing. This method consists of the outer pipe serving as casing and inner pipe serving as drill pipe, arranged in combination.

The drilling is so programmed that when it becomes impossible to rotate and advance the rod in the reverse circulation method and the outer rod in the double- pipe reverse circulation method, a rod of diameter one size smaller is used, and the boring is then continued.

Making the Boring Longer In vertical boring, the frictional resist-

ance of the rod is low unless there is an excessive bend in the bore, and the effective use of mud fluid is made possible. In horizontal boring, however, not only is frictional resistance high, but there also is no effective auxiliary means. Therefore, if longer advance boring is intended, it is necessary that a machine possess great rotating force and thrust, and that the tools corresponding to the strong machine be used.

For this reason, a drilling machine was developed that possessed: (1) a low center of gravity; (2) great rotating force (1400 kgf/m) and thrust (16 000 kg0; and (3) a large inside-diameter spindle that can handle large-diameter casing and rods (165 ram) for exclusive use in horizontal boring. Also, some improve- ments were made on rod, such as increasing the thickness and increasing the design strength of the materials.

As a result, a maximum record boring depth of 2150 m has been achieved in the construction of the Seikan Tunnel.

Preparation of the Suppositional Geological Map (Fig. 2)

As a result of advance boring, the location and volume of seepage water can be established; and, from boring cores, the type of rock and values of physical properties, e.g. specific gravity, compressive strength, Rock Quality Designation, can be identified. By com- bining this information with the record of the excavated section, geological and seepage water conditions that would

appear in the tunnel and surroundings area can be assumed.

The suppositional geological map is, thus, prepared monthly, based on which the grouting plan (zone and length) and the excavation plan (pattern of temporary support and thickness of lining) are determined.

In assuming geology, a combination of the following method is used:

(1) A plane is formed by three points from three advance borings.

(2) A plane is formed by one boring, and the strike and dip of ground strata of the excavated section.

Grouting Grouting performed prior to the

excavation transforms the surrounding natural ground of the tunnel into a grouting zone having low permeability. The tunnel is constructed within this zone, using similar design and construction methods to those used for construction of mountain tunnels.

As a result of grouting, the compressive strength of the ground is improved and the water inflow is sealed off, making it possible to excavate the tunnel safely. Because the inflow of seepage water into the tunnel is reduced after the comple- tion of grouting, drainage costs may be expected to be reduced.

Design Zone of grouting. The zone of grouting

relates not only to the safety of tunnel excavation but also to the quantity of seepage water. Therefore, it is important to make a clear correlation of these so as to perform economical grouting.

Initially the grouting zone was assumed to be an elastic body, and the grouting area was specified to be three to four times the diameter of the excavation in consideration of the relativity of water pressure acting upon the grouting zone and the reaction of support.

Thereafter, Prof. Toshihisa Adachi of Kyoto University undertook an analysis in which the grouted area is assumed to be an elasto-plastic body, satisfying the Mohr-Coulomb-type plastic yield condition. The purpose of this analysis was to establish how the deformation of the tunnel and the volume of seepage water depend on the size of the grouted area.

This grouting model aims at resisting water pressure that corresponds to the depth below sea level of the grouted zone, and at receiving the earth pressure of the zone loosened due to excavation with support and lining (see Fig. 3).

The analysis concluded that to reduce volume of seepage water and to perform the excavation in a stable condition, the grouting area must be approximately three times the diameter of the tunnel does not necessarily attain better results (see Fig. 4).

Based on the results of the theoretical analysis previously described and the

342 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume l, Number 3/4, 1986

Facing reported iu March | I Vicing reported in February 9,1051,90 i s~ om I 9,016~90

P r o f i l e 26kaTO7m39 261m79~t39

I I n,., " r t

= . . . . . . . .

405 6 0 9 601 ~-~---~-"

P lan

, = , , , , , _ e - 3 o - - - " " ' " - : - _ _ _ : , . ~ , ' , ' , v , ~ , , , , , , ~ ~ , 7 - -

- - _ _ _

-=~_~ . . . . . , : : - : _ ~,,~,,:- - = ~,~;'~,!:;'~,'J , , ~

~ , ~ - - - : - - - - - - _ - - ~ : _ _ £ ~ ~ ~ : ~ _ 2 7 . ' - . . . ( : - ~ . . . . :~-'~,~:,.::':~(~,..'X,"Y';,%

/ F

A

56.7

@

1 C-'q

Legend

Sandy mudstone Sandy tuff Fine tuff Tuffaceous sandy mudstone Strike and dip of fault Strike and dip of stratum Columnar section by boring

Slaking test A (good)~O (bad)

Unconfined compressive strength (kgf/cd)

Seepage volume ( £ / a i n ) RQD 1 ~ 2 (bad) 3 ~ 7 8 ~I0 (good)

9 , 2 0 0 m 9 O0 m I I

9,

26K~OOm

Figure 2. Suppositional geological map of the Yoshioka Pilot Tunnel (As of March 1980).

Water p r e s s u r e

Rearh

Figure 3. Model diagram of the grouting zone (a=radius of excavation; T=thickness of lining; R=radius of grouting; Rp=radius of the loosened zones).

Therefore, if a void still remains, the grout can reach the far area from the zone required.

Because the cement milk is a liquid with suspended solids and permeation into small cracks is impossible, LW grout was used to improve the permeability. LW grout is composed of water glass (Japanese Industrial Standards No. 3) and Portland cement milk. Several attempts were made with grouting materials thereafter, and finally the standard grouting was performed using the col- lidal Portland blast furnace slag cement and lower molar ratio water glass.

experience of grouting, the grouting area was generally designed to be three times the diameter of excavation and approximately five times the diameter of excavation, in case bad geology was encountered in the Seikan Tunnel.

An example of grouting design for this tunnel is shown in Fig. 5.

Improvement of Grouting Materials The initial grouting was started by

using cement milk of Portland cement. Although the time required for the mix- ture of cement and water to gel through hydration differs depending on the temperature, the time required for the process is approximately several hours.

Kilometer of Pilot tunnel

Kilometer of Main tunnel

For the construction of the Seikan Tunnel, grouting materials were required to meet the following conditions:

(1) Because the materials are used to seal off the high-pressure seepage water and are also used to improve the soft rock, the materials are required to have high strength.

(2) To form the required cut-off zone in the natural ground, the materials excelling in permeability and the geling time can be adjusted.

(3) The cut-offzone is not a temporary

5.5

kg : ko /i00 kgp = i0 kg

~ k g f / c ~ P ~ Legend

~ P'(a) ' Support ing react ion 0.0394 ~ 5 "Qo -~ ~ Coefficient of permeability

ko " Natural ground .~ ~,,,. kg ' Grouting zone

t o - - . . . . .~ . . . . 1;0 kgp: Grout ing zone , ~ - " - --' P las t i c zone

0.0197 "~ 4.5 - - . , , ' . , F'(a) : 0 k g f / c ~ . . . . ,, Qo ~ ~ ..o~. 1.0

Permeated water

2.0 '

17.6 Radius of grout ing (m)

Figure 4. Relationship among radius of grouting, tunnel radius change and quantiO~ of permeated water.

Volume 1, Number 3/4, 1986 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 343

P i l o t tunnel

o e 7 A-7

Plan sL .X~, C o n ~

a

1 7 ]

Main tunnel

Honshu s i d e

3A

C--

A-A Section (1A)

:) B-B S e c t i o n (2A) C-C Sect ion (3A-C)

)

No. of g r 0 u t i n g s Grout ing f a c i n g ( km m )

~ax. g r o u t i n g p r e s s u r e

( k g f / c ~ )

1 25,483.91 80

2 25,444.00 80

3 25,404.00 80

4 25,364.00 80

I Volume of grout f i l l i n g vo ids

( d )

4 ,031 .5

2 ,658 .8

1 ,820 .3

1 ,306.3

Stage No. of bo r ings

1 A 17 2 h 18 3 h 18 3 B !7 3 C i0

C 6 Tota l 86

Figure 5. Design of the main tunnel grouting (for

structure for excavation but, rather is required to be maintained permanently. It must also have durability, i.e. long- term strength.

Therefore, the development of such materials was completed specifically through a durability test aiming at obtaining material with an unconfined compressive strength of 40 kgf/cm ~ or more at the age of three days, and a geling time of more than three minutes. Although a grouting material meeting the compressive strength and the geling time was found, there still was weakness in durability and workability.

Therefore, it was decided that the following material and mix (chemical composition) were to be used:

As for the cement, granulated blast furnace slag with latent hydraulicity was mixed in, polar liquid organic compound was added as a grinding agent; and durability was increased against aera- tion. Thus, the cement was determined to be 55% of blast furnace slag, specific

the No. 14 main tunnel at Yoshioka Section).

gravity of 3.02, and specific surface of 6170 cm2/g. Water glass used had a lower molar ratio (SiO2/Na20) of 2.2. The mix (chemical composition) was as shown in Table 1.

Forecast and Results of Volume of Seepage Water

Before the commencement of excavation, the volume of seepage water into the tunnel was forecast at the seabed section.

Dr Yoshimasa Kobayashi of Kyoto University assumed the following:

At the depth of h below the seabed, when an infinitely long cylindrical tunnel having sufficiently small diameter r0 against h is dug, the following equation will be introduced considering the seepage flow, which will be governed by Darcy's law:

Q = 2*rk P b - P0

ln(2h/r0) where, Q--volume of seepage water;

k=coefficient of permeability;

PG=water head (water depth) at the seabed; P0=water head at the wall surface of the tunnel (0 in case the tunnel is maintained at the atmospheric pressure).

Assuming that the coefficient of permeability becomes lower by about two figures as a result of grouting, i.e. in the general section 10-Gcm/s in sedimentary rock, 10-Scm/s in igneous rock; and assuming conservatively that in the fault section the coefficient of permeability will become lower by about one figure, 10 -4 cm/s, the volume of seepage water was forecast as shown in Table 2.

As seen from the comparison between forecast and result in the table, it is evident that the initial forecast turned out to be adequate. In 1975 during the initial stage of construction, however, when reforecast was made based on the actual experience of the seepage water encountered, that of the undersea section was quite large, i.e. 105 m3/min, which

344 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 1, Number 3/4, 1986

Table 1. Standard grout mix,

Water glass (Sodium Silicate)

Molar ratio

2.2

Type of cement

Colloidal portland

blast furnace slag

cement (slag) 55%

Cement milk

Water/cement ratio (%)

Mix ratio

Water glass

cement milk

Geling-time

(Min-s)

Compressive strength

~3(kgf/cm 2)

100 1.0 1-31 77.6

150 1.0 2-27 21.6

200 1.0 3-40 13.3

was reduced to a quarter, thanks to the improved grouting technique described above.

To reduce the cost of pumping seepage water, it was, therefore, necessary to reduce the volume of seepage water as much as possible. The grouting was performed after lining had been completed at the location where seepage water was quite large in quantity.

Excavation

Pilot Tunnel and Service Tunnel Because the cross-sectional areas of

the pilot tunnel and service tunnel are small, i.e. about 20 m 2, full-face excavation by blasting was employed, except for a small portion of the section where squeezing ground was encountered. For this section, the short bench cut method of excavation was employed.

In order to eliminate the effects of blasting on the surrounding ground and to achieve a speedy and labor-saving excavation, tunnel boring machines (Wallmeyer drilling and breaking type)

were used at the initial stages of excavation in the area of good geology. The construction length was a total of 4.1 km.

Excavation was accomplished by using free cross-sectional excavators, such as the AM-50 Minor manufactured by Alpine Ltd, and the MRH-S40K by Mitsui-Miike Works. The total length of excavation was 3.3 km.

Main Tunnel The main tunnel is the double-track

Shinkansen type with a cross-sectional area of approx. 90 m 2 (see Fig. 6).

At the Tappi Section in the initial stage of excavation, geology was comparatively sound, though accompanied by seepage water. Therefore, the excavation was started by using the upper half- heading method, which allowed an easier grouting operation. On the other hand, at the Yoshioka Section, the bottom drift method--which at that time was the standard method for excavating mountain tunnels in Japan--was used, because there was no seepage of water.

As the excavation advanced into the

middle of the undersea section, soft and weak rock was encountered, requiring grouting operation. Therefore, the side drift method became a standard method of excavation thereafter.

In the section characterized by power- ful ground pressure accompanied by squeezing, the circular short bench method with spring line drifts was employed, using newly developed, high- strength steel pipe support (steel pipe packed with spiral reinforcing steel and mortar); the excavation was completed without accident.

Effectiveness of the Support Member The design of the members of support

in the excavation is an interesting issue not only from the viewpoint of supporting the ground safely, but also from that of excavation economy. It may be said that the design of support members mainly depends upon empirical rules prepared on the basis of past construction experience.

In the Seikan Tunnel, the pattern of support members mainly depends upon

Arch - sho tc r e t e ck (Th i ckne s s =lOcm) Archconcrete

bolt @.:Y \ t.=4.o0 Dz~ .--N

Sidewall concrete~~!~..._ ~- ~o.~ ~ ® ^~J.~z~ ! ] " ~ ~ 1

/ ' / / Invert concrete " ~

Legend

(~ Excavation of d r i f t (~) Steel support (15OH) (~) Sidewall sho tc re t e (~) Rockbolt 0 Sidewall sho tc re t e ® Excavation of upper hal f (~) Steel support (20OH) (~) Arch-sho tc re te (~) Excavation of lower half

Archconcrete (~ Excavation of inver t

Invert concrete

Figure 6. Working drawing of the main tunnel.


Table 2. Forecast and results of seepage water (unit ." m¢/min).

Forecast Resul ts (1964) (1985)

H o n s h u s ide Underground section 13.2 Undersea section 17.7 18.5

H o k k a i d o s ide Undersea section 5.0 5.9 Underground section 1.4

Total 39.0

empirical rules prepared on the basis of past construction experience.

In the Seikan Tunnel, the pattern of support was determined by considering examples of other tunnels constructed in the past, as well as experience gained during construction of the Seikan Tun- nel. The support member used consists of steel support, shotcrete and rockbolt. the effectiveness of each support member was investigated by using the following analyses;

(1) Analysis using Rabcewicz's equation.

(2) Finite element analysis. (3) Analysis of measurement.

Analysis using Rabcewicz's equation.Rab- cewicz's equation calculates the bearing capacity of support members based on the configuration of the tunnel when it collapses.

The effectiveness of support members was calculated in this case using the amount of increase in each support

member that was required to raise the gross bearing capacity by 10% and, therefore, the cost.

Results of the analysis are shown in Table 3, from which the following con- clusions may be drawn:

(1) If the standard support members are used in the Seikan Tunnel, the bearing capacities of steel support and shotcrete are about 20% of the gross bearing capacity, respectively, having sufficient ground supporting effectivess. In terms of construction costs, the shotcrete is considered the best means of support.

(2) An examination of the relationship between the physical properties of the ground and the gross bearing capacity in order to determine the corresponding characteristics of Rabcewicz's equation with the physical property of the ground showed that the ground bearing capacity exceeds 83% of the gross bearing capacity in the ground, with the internal angle of friction of 42 ° and unconfined

Table 3. Results of analysis by Rabcewicz's theory.

compressive strength of 77 kgt/cm:. Therefore, the application of die equation will be limited to ground with the soft rock.

Finite Element Analysis. H steel support, shotcrete and rockboh were considered for support. In judging their efl;ective- ness, deformation of the tunnel, degree of safety of the ground (area of plastic zone), and degree of safety of the support member were used.

In this case the analysis model assumed excavation of the main tunnel using the side drift upper half section method in the soft rock, with the tbllow- ing premises:

(1) In the structural model, plane elements were made up of the ground and side wall concrete; beam elements were of H steel support and shotcrete; and rod elements were of rockbohs (Fig. 7).

(2) Considering the initial earth pressure, successive analysis was made according to the sequence of construction. Because the three-dimensional effect due to the progress of working face is introduced using the plane analysis technique, the stress-relief method was used. Also, the technique was made as simple as possible, e.g. the ground was assumed to be a perfect elasto-plastic body aiming at qualitative analysis.

(3) Using the experimental design, the analysis--based on four factors (pitch of excavation; thickness of shotcrete; number of side draft rockbohs; number of upper half rockbolts) and two levels-- was performed (Table 4). Also included were calculations for the case where no support was used, and for the case where

Initial design

Design condition

Bearing

kgf/cm (%)

Design condition of each support meml~ in the calm of the Initial gross bearing capacity being Increased by 10%

Tunnel Radius of excavation 5,70 m - - - -

Ground Unconfined compressive strength 15 kgf/cm e 7.47 (54.8) - - Internal angle of friction32 degrees

Pitch of excavation 0.9 m - - 0.67 rn

i Steel support Cross sectional area 64 cm 2 (200 H) 2.40 (17.6) 92 cm ~ (250 H)

Shotcrete Thickness 15 cm 3.11 (22.8) 21 cm

t~ Rockbolt Number (pitch) 14 (2.02 m) 0.66 (4.8) 34 (0.76 m) Length 3.8 m

Total - - 13.64 (100.0) - -

Note: 1. Rockbolt is prescribed to insert in the only slip surface of shearing. 2. Rockbolt length is the numerical value in the case of maximum gross bearing capacity.

Construction cost (1000 Yen/m)

Initial Addition

B AB

Consttu~ioncostpel unltbearlngeapacity

1000Yen/m k~/cm 2

I B/A

B+AB

A+AA

193

136

262

4920

527

389

218

6054

470

198 220

220 125

311 330


Table 4. Factors and levels used in the finite element analysis.

Level

1 2

Factor A

Pitch of excavation

(m)

0.9 1.2

B

Thickness of shotcrete

(m)

0.1 0.2

C

Number of side drift rockbolt

D

Number of upper half

rockbolt

Table 5. Effectiveness of support member.

Item Factor

Convergence I During excavation of fourth of side drift

enlargement

Ultimate value of side drift

Pitch of excavation

3 (2.8%)

Shotcrete

1 (11.3%)

Side drift rockbolt

2 (8.5%)

Upper half rockbolt

i

2 3 1 (8.5%) (7.1%) (18.4%)

Convergence of upper half 2 1 4 3 (5.5%) (10,6%) (0,9%) (2.3%)

Settling displacement of upper 2 1 - - half crown (5.0%) (11.7%)

Area of plastic zone 2 2 1 2 (1.0%) (t.0%) (5.3%) (1.0%)

Note: Percentage was calculated by the rate from the mean value due to the variation of changing the levels of factors.

Table 6. Factors and categories considered in analysis of ground behavior.

Factor Category

Support pattern

Physical property of the ground

Steel support (Side drift) Stool support (Upper half) Shotcrete (Side drift) Shotcrete (Upper half) Rockbolt (Side drift) Rockbolt (Upper half)

Strata Seepage water (ground water) Cracks Compressive strengt h Grading

Size of support and placing interval Thickness and timing of construction Number and timing of construction

Single or alternate Quantity Number of vertical cracks Division of compressive strength Texture below 741~m

Ground grouting Position of grouting Distance of excavation point from position of grouting

upper half rockbolts were extended by 6 m .

Results of the analysis are as follows:

(1) Figure 8 shows the changes in convergence of the fourth enlargement of the side drift by each step of construction. Case 0 shows where no support was used. Each pattern of support varies within the slat area for cases 1 through 8;

the difference between those two figures is the effectiveness of the support member.

(2) With regard to the effectiveness of each support member, when only the main effectiveness was considered--i.e. omitting the interactive effectiveness-- the results were the same as those shown in Table 5.

(3) Where the ground consists of soft

rock, as in the case of the middle of the undersea section in the Seikan Tunnel, suitable effectiveness is displayed as a whole support. Although effectiveness of each support member is not considered remarkable, it may be said that the shotcrete works most effectively. Since the rockbohs in the side drift greatly reduce the plastic zone occurring near the invert, they are considered effective.

(4) Examining the case where the upper half rockbohs were extended from 4 to 6 m, we see effectiveness from the viewpoints neither of displacement nor of the plastic zone.

Analysis of results of measurement Analys i s by quant i f ica t ion theory.

Behavior of the ground as affected by the excavation of the tunnel varies depending upon various factors, e.g. geology, method of excavation, support pattern. Here, the statistical analysis by the type 1 quantification of qualitative data from the mathematico-statistical point of view was conducted. The deformation of the tunnel (displacement due to upper half excavation, i.e. convergence and settling displacement of side wall concrete) makes the objective variable. The factors and categories adopted in this analysis are shown in Table 6.

Results of analysis. Following are the results of analysis from the data obtained at Nos. 13, 14 and 15 of the Yoshioka Section of the main tunnel in the middle of the undersea portion (for which soft rock ground of approx. 2330 m was excavated using the side drift upper half section method):

(1)Steel support. Because most of the support was constructed using a combination of 150 H in the side drift and 200 H in the upper half of the main tunnel, no merit was seen from the size of support. Nor was the effectiveness of the spacing of the steel support clear (0.9 and 1.0 m, respectively).

(2) Shotcrete. The effectiveness of shotcrete was found to be higher in the upper half of the main tunnel and lower in the side drift. The thickness of shotcrete would be more effective if the entire thickness were applied at the facing, i.e. without dividing it into two applications immediately after the excavation.

(3) Rockbolts. The effectiveness of rockbolts was very high, particularly for roekbohs constructed in the side drift. The timing of the construction of upper half rockbohs is better when they are installed immediately after the construction.

(4) Physical property of the ground and grouting. Because the analysis conducted this time was within the area where the difference in the physical property of the ground was comparatively small, the influence due to the physical properties was less. No influence due to the distance from the excavating position to the grouted facing was observed. Table 7 shows the results of measure-

Volume l, Number 3/4, 1986 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 347

Table 7. Results of measurement (Nos.13-15 main tunnel at Yoshioka Section).

Location of measurement (length constructed)

Convergence (mm)

Ground displacement

Upper half max-mean-min

Sidewall concrete max-mean-min

mean

No. 13 (L=700 m)

59-21-10 (26)

49-26-13 (43)

Yoshioka

No. 14 (L=830 m)

No. 15 (L=800 m)

17-10-4 (38)

26-12-4 (70)

/1 : No. 13,15

Lz : No. 14

Main tunnel 40

22-16-11 (34)

25-9-3 (52)

17

Remarks

Pilot tunnel (A Point) (mm)

Elasto-plastic boundary (m)

Max. axial force of steel support (tf)

Max. axial force of rockbolt (tf)

Distance from wall surface concrete

Side drift mean

Upper half mean

Side drift mean

Upper half mean

(3)

0-2.3 (3)

(3)

2-3.3 (1)

H V H V H V

60 40 33 30 78 61 (4) (5) (2) (4) (6) (7)

138 108 (3) (4)

112 91 (1) (2)

18 (3)

5 (4)

20 (8)

4 (6)

164 169 (2) (4)

19 (15)

8 (10)

H : Horizontal axial force V : Vertical axial force

Note: ( ) indicates number of data.

Steel Support (2OOH) Shotcrete Roekbolt

\ Upper half ~=25m =4.0m

Rockbolt \ / ~$idewall concrete ÷=25a \ / S t e e l support (150H) Lf4.0m \ / Shoterete

Figure 7. Structural model of the main tunnel.

ment at Nos. 13----15 in the main tunnel of the Yoshioka Section. In addition, the load-sharing ratio between steel support and shotcrete was tested and measured; the results obtained are shown in Table 8.

Conclusion Summarized below are the results of

analysis using Rabcewicz's theory, finite element analysis, and analysis of the results of measurement, respectively:

(1) The support pattern designed for and constructed in the Seikan Tunnel was adequate.

(2) In cases where steel supports (150 H in the side drift, 200 H i~ the upper half of the main tunnel) were installed at spacings of between 0.9 and 1.2 m, large axial force occurred, and the supports proved effective in the area where no large deformation could be allowed, e.g. in the undersea section.

(3) Shotcrete of design thickness 10-20 cm was effective and would be even more so if the entire thickness were applied all at once, immediately after the excavation. When considering the cost of construction, shotcrete is a most economical and effective member of support•

(4) Rockbohs of 0.5 per m r with a length of approx. 4 m will suffice. When the excavation is carried out using the side drift method, the rockbolts driven in the side drift have proven to be effective and must be included in the design of rockbolts.

Poatsor~t Excavation of the Seikan Tunnel con-

tinued for 20 yr, overcoming difficulties


Table 8. Load-sharing ratio between supports.

Sharing of support reaction

Sharing of axial force of support 7 5 2 5

Sharing retio (%)

Steel support Shotcrete

36 64

2~

20

E

~ ,o

.-

0

I

Case 0

J

e 1-8

J ' ' ]" 3. 5 2 5 m m

J

Sd Sd Oh Oh Uh Te Te l l 66 I00 0 55 i00 0 I00 0 i00 ( ~ )

Excavation progress

Note " Sd=Side drift , Oh=Upper half , Te=Third enlargement , l=Invert Figures indicate the releasing rate of ground pressure.

Figu~ 8. Convergence of the fourth enlargement of the side drift, for each construction step.

that included as many as four submerg- ences by sea water, and enormous earth pressure. At the end of August 1985 the Seiken Tunnel finally was completed.

From the experiences in the undersea coal mine, the desirable tunnel depth was determined to be 100 m below the sea bed. Based on the hardships caused by inrushing of sea water, the grouting zone was reconsidered and the grouting materials were improved.

We are fully convinced that the success of the Seikan Tunnel owes much to those experiences, hardships, and the resulting technical developments which turned misfortune into a blessing. Thus, failure teaches success. []


technical development for the seikan tunnel

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