Stability and rock cover of hard rock subsea tunnels

Stability and rock cover of hard rock subsea tunnels

0886-7798(94)E0004-3 Stability and Rock Cover of Hard Rock Subsea Tunnels T. S. Dahlo and B. Nilsen A b s t r a c t Initially this paper summarizes t...

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0886-7798(94)E0004-3

Stability and Rock Cover of Hard Rock Subsea Tunnels T. S. Dahlo and B. Nilsen A b s t r a c t Initially this paper summarizes the major results of a recent state-of the-art review of Norwegian subsea tunnels. The review primarily concentrated on assessing the reliability of preinvestigations in predicting tunnelling conditions, and the longterm behaviour of rock support materials. Key data from completed Norwegian projects are used as a basis for discussing the optimum rock cover for subsea tunnels. A comparison with some major subsea projects in other countries is included. Cases of instability and cave-in are documented. For most Norwegian subsea tunnel projects, the rock cover appears to be relatively conservative; however, experience from some of the projects indicates that this apparent conservatism may be a good investment.

1. Introduction he Foundation for Scientific and Industrial Research at the Norwegian Institute of Technology (SINTEF) has studied subsea rock tunnels since 1986. Initially, a major stateof-the-art review was conducted in order to evaluate experience from completed Norwegian subsea tunnels. Owners, contractors and consultants contributed actively so that a broad overview was possible. The state-of-the-art review included all Norwegian subsea rock tunnels completed as of 1988. General details of the tunnels are given in Table 1. Details of tunnels completed since 1988 have been added to the table.

T

Present address: Tore S. Dahl~, SINTEF Rock and Mineral Engineering, N-7034 Trondheim, Norway; Bj$rn Nilsen, Norwegian Institute of Technology/SINTEF, Hcgskoleringen 6, N-7034 Trondheim, Norway. This paper has been adapted mainly from a paper that originally appeared in Proceedings of the 2nd Symposium o n Strait Crossings (Tapir Publishers, Trondheim, Norway, 1990). It is reprinted herein with

the permission ofthe Norwegian Tunnelling Association and the authors. Some new information has been added, based on a paper presented at the ITA Congress "Options for Tunnelling 1993", held in Amsterdam in April 1993.

Tunnellin~,~ and Underground Space Techn.logy, Elsevier Science Ltd Printed in (;real Britain 0886-7798/94 $6.00 + .00

Rdsumd---L'article rdsume d'abord les principaux rdsultats d'une synth~se faite r~cemment sur les tunnels sous-marins norvdgiens. L 'attention est portde en premier lieu sur l'estimation de la fiabilitd des investigations pr~liminaires pour la prdvision des conditions de creusement et sur le comportement "a long terme des mahJriaux de sout~nement. Dans donn~es-cl~s, recueillies clans les ouvrages norv~giens terminus, sont utilisdes comrne base de discussion pour la couverture rocheuse optimale clans les tunnels sous-marins. On donne ~galement une comparaison avec certains projete sous-marins majeurs. Des cns d'instabilit~ et d' affaissement sont cites. La plupart des tunnels norv#.giens sous-marinspr~sentent une couverture rocheuse relativement importante; l'expdrience montre cependant que cette hauteur apparemment importante peur $tre un bon investissement.

Most of the existing subsea tunnels are i~ord crossings on the west coast of Norway, used as road tunnels or tunnels for oil and gas pipelines. The greatest depth below sea level thus far is about 250 m. Ordinary drill-andblast techniques have been used for all of the tunnels. The Norwegian subsea tunnels are situated in a variety of geological structures, ranging from typical hard rock such as Precambrian gneiss to less competent phyllite and poor quality schists and shales. All tunnels cross significant zones of weakness under the sea. The state-of-the-art study concentrated mainly on the following topics: • Preinvestigations. • Tunnelling results. • Behaviour of rock support materials. Particular emphasis was placed on the usefulness of the preinvestigations in predicting tunnelling conditions, and on studying the effect of saline environments on the condition of rock support materials. A l t h o u g h the t o t a l costs of preinvestigations for conventional tunnels under land in Norway often amount to less than 1% of the cost of excavation and rock support, the preinvestigation costs for subsea tunnels normally are between 5% and 10%. Refraction seismic profiling normally

Vol. 9, No. 2. pp. 151 158, 1994

~ ) Pergamon

represents the most expensive part of the investigations, and is also the key investigation method for predicting subsea rock quality. The study found a relatively good correlation between tunnelling conditions and rock quality prognoses based on the preinvestigation results. It also concluded that all major discrepancies have geological explanations. Figure i illustrates the correlation between average seismic velocity and the relative cost of rock support and grouting (in percent of total tunnelling cost). As canbe seen from the table, the average seismic velocity gives a good indication of the degree of complexity and cost level of the project; the type and the cross-sectional area of the tunnel also have a major impact. The typical total cost per meter of two-lane (approximately 50 m 2) subsea road tunnel in Norway today ranges from NOK 30,000-NOK40,000 (approx. $US4,000 -$5,000). No alarming corrosion or deterioration ofrock support materials was identified in the review. In sections with seeping seawater on the concrete surface, however, the chloride content was found to be higher on the exposed surface than the generally accepted limit for a high risk of rebar corrosion (see Fig. 2). In 1989-91, the Royal Norwegian Council for Scientific and Industrial Research (NTNF) supported the re-

151

Table 1. List of subsea rock tunnels in Norway. Tunnel

Year Completed

Total Length

Lowest

Type*

Section (rn=)

Main Rock Types

(km)

Level (m)

1. Vollesfjord

W

1977

16

Gneiss

9.4

-80

2. Frierfjord

O

1977

16

Limestone, gneiss

3.6

-252

3. Vardo

R

1981

53

Shale, sandstone

2.6

-88

4. Slemmestad

W

1982

10

Shale, limestone

0.9

-95

5. Karmoy-K&rsto 5.1 ° Karmsund 5.2 ° Ferdesfjord 5.3 ° Forlandsfjord

O

1983 (all)

27 (all)

Gneiss, phyllite

4.8 3.4

-180 - 170

3.9

-160

6. Hjartey

O

1986

26

Gneiss

2.3

-105

7. Alesund 7.1 • Etlingsoy 7.2 ° Valderoy

R

1987 (both)

68 (both)

Gneiss

3.5 4.2

-140 -137

8. Kvalsund

R

1988

43

Gneiss

1.6

-56

9. G o d l y

R

1989

52

Gneiss

3.8

-153

10. Hvaler

R

1989

45

Gneiss

3.8

- 121

11. Flekkeroy

R

1989

46

Gneiss

2.3

-102

12. Nappstraumen

R

1989

55

Gneiss

1.8

-63

13. Fannefjord

R

1990

54

Gneiss

2.7

- 100

14. Maursundet

R

1990

43

Gneiss

2.3

-93

15. IVAR, J~eren

W

1991

20

Phyllite

1.9

-80

16. Kalst~

O

1991

38

Greenstone

1.2

-80

17. Freifjord

R

1992

70

Gneiss

5.2

- 100

Rennfast 18. ° Byfjord 19. ° Mastrafjord

R

1992 (both)

70 (both)

Phyilite Gneiss

5.8 4.4

-223 - 132

Project

Cross-

• R = road tunnel; W = Water tunnel; O = Tunnel for oil/gas pipeline.

s e a r c h activities on s u b s e a rock tunnels. P a r t i c u l a r e m p h a s i s w a s placed on e v a l u a t i n g s t a b i l i t y a n d o p t i m u m rock cover. The following sections discusses some r e s u l t s of this r e s e a r c h .

2. D e s i g n Practice

Subsea rock tunnels have become increasingly popular in Norway, particularlyforroadpurposes. Since 1988,

12 t u n n e l s h a v e b e e n c o m p l e t e d (see Table 1), a n d s e v e r a l a r e u n d e r cons t r u c t i o n or in t h e p l a n n i n g stage. The p l a n s include some v e r y challenging projects. T h e e x t r e m e case is repr e s e n t e d by t h e H a r e i d Tunnel, w h i c h h a s a p l a n n e d l e n g t h of 132 k m a n d a m a x i m u m d e p t h of 630 m below s e a level. As a r e s u l t of n a t i o n a l regulations, the m a x i m u m g r a d i e n t for a r o a d t u n -

152 TUNNELLINGAND UNDERGROUNDSPACE TECHNOLOGY

nel in N o r w a y is l i m i t e d in most cases to 1:12.5. Hence, t h e p a r a m e t e r t h a t will m a i n l y define t h e l e n g t h of a subs e a t u n n e l is the m i n i m u m rock cover. If t h e rock cover decision is too conservative, t h e r e s u l t will be considerable e x t r a costs due to e x t r a t u n n e l length. F o r a typical N o r w e g i a n s u b s e a r o a d t u n n e l (cross-sectional a r e a 50 m2), a r e d u c t i o n of only 1 m e t e r in t h e minim u m rock cover m a y r e p r e s e n t a cost

Volume 9, N u m b e r 2, 1994

ROAD.

e

n - - f r o m fresh water section

0.~0 ~.\

~ 75

PIPELINE ~ TUNNELS

Lo ~

.~.

05.2

I'

o7

~

o5.1

0.30

6o

°50

"~'~- 4

eS.3 "~

+.--. from sea water section e - - - f r o m sea water section

\

~,~

._on e2

\

0,20 ~ \ \

~ "\,+~

WATER TUNNELS

e-

8

0,1 0

Limit for high risk of rebar corroslon.~

'~ \.

B

O"

/.SO0 SO00 Average seismic velocity (m/s)

e-

5500

0

~0

80

120

160

200

240

260

Penetration depth (mm)

Figure 1. Relative cost of rock support and grouting as a function of average seismic velocity.

Figure 2. Chloride penetration profiles in cast concrete. Examples from the Vard¢ tunnel.

LEGEND

75

Oe

@2



v = 2000

- 2900 m/s



v = 3000

- 3900 m/s

o v = 4000

(Ds-2

05.3

- 4900

m/s

o v = 5000 - 6000

m/s

x v = Unknown

O$

5.30

O5.1

R C a s e of I n s t a b i l i t y

• 10 20 i~0

E

--S.IR 5.2 7.1R~ ) 7 . 2

5.3

20 0 2 24)

~)5.1

50

10 • m

4 7.20

J=

•7.1

% 6 05.2

.

~0~

/

7.2~ 02 X7.2 4

12_1,_~ ,~_'.

O O

'x'x"~ ot'.e" d 25

72

_. ~-

V

SOIL •

~ - - - - - - - - ~ - ~ - - R-OcK/~-

SO

/

13 / 8 1 0

._ I • % oTJ. /

"

/

J

100

150

TUNNEL

200

BEDROCK DEPTH ( h w + hs) , m Figure 3. Minimum rock cover under sea as a function of the bedrock depth. reduction in the order o f N O K i million (approx. $US135,000). However, ifthe rock conditions are poor and the rock cover too smA11, severe stability problems and large water inflow m a y result, as discussed in Section 4, below. A n important basis forthe planning of new tunnels is represented by the results and experience from completed projects. Because of the good results from subsea tunnelling so far,the gen-

Volume 9, Number 2, 1994

eral trend for new projects has been a gradual but slow reduction of minim u m rock cover, as indicated by the diagram in Figure 3. In Figure 3, each point represents a section of critical rock cover for the respective tunnel. The numbering refers to the list of tunnels in Table 1. The term "criticalrock cover" refers in most cases to tunnelling underneath clefts or other depressions in the sub-

sea bedrock, and does not necessarily coincide with the deepest parts of the fjord. The "bedrock depth" represents the sum of water depth and soil thickness on the sea floor. The plots are all based on reports from geophysical and geologicalinvestigations for the respective projects. Based on the plots, lines representing the minimum rock cover as a function of the bedrock depth may be de-

TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY153

fined as shown in Figure 3. It can be seen that the minimum rock cover for completed road tunnels is generally 6 to 7 m greater than it is for the smaller pipeline and sewer tunnels. Figure 3 clearly demonstrates that very often the location of critical rock cover coincides with zones of poor quality rock (seismic low-velocity zones). This is a logical result of the geological prehistory of the fjords and straits, and is certainly a fact which should be kept in mind in planning future projects. As can also be seen from the diagram, the rock cover in cases of instability has been relatively large thus far. An empirical diagram such as the one in Figure 3 may be useful during the planning of new projects. However, the diagram does not give any indication of the level of safety (safety factor) for the respective tunnels. There is reason to believe that the "critical lines" in Figure 3 represent a relatively high level of safety. Thus, the outer end of the Hj art~y oil pipe tunnel, which takes ashore the pipeline from the sea floor, was excavated with no signifi-

cant problems at a rock cover of only 8 to 9 m and a water depth of more than 60 m. Similar experience has been documented for numerous lake taps in Norway for hydropower exploitation. 3. C o m p a r i s o n with Projects in Other C o u n t r i e s Although major differences exist in geological conditions, as well as tunnelling techniques, a comparison of Norwegian projects with subsea tunnels from other countries may be of considerable interest. Key data for some important subsea tunnel projects around the world are summarized in Table 2. Many of these tunnels have a subsea length that far exceeds the lengths of the Norwegian projects; often the cross-section also is considerably greater. The two longest tunnels in the world--the Seikan and the Channel Tunnel--are both included in the table. The majority of the projects in Table 2 (about two-thirds) are in relatively young, sedimentary rocks. Only about

one-third (Seabrook, Forsmark, Saltsj¢, and a part of the Kanmon road tunnel) are in granitic or gneissic rocks. Unlike the Norwegian projects, TBMs have been used, or are planned to be used, for several of the projects in Table 2. Based on available reports and publications, as indicated in the table, the sea depths, soil covers, and minimum rock covers of the respective projects have been identified as described above for the Norwegian projects. Although the data are relatively sparse in some of the cases, the general conclusions based on a comparison are unambiguous (see Fig. 4). As can be seen, several of the plots based on Table 2 represent rock covers that are considerably smaller than for Norwegian subsea tunnels at comparable depth. The extreme case here is represented by the Kanmon railroad tunnel, which in very poor quality rock conditions has a rock cover of only about 10 m. Even more remarkable is the fact that this tunnel was completed in 1944. The Seabrook and Storebmlt Tunnels (TBM projects) have remark-

Table 2. Some major subsea rock tunnels throughout the world. Type*

Year Completed

Length (kin)

Deepest Point (m)

Cross-Section

Kanmon, Japan

RR

1944

3.6

-40

(2 tubes)

Miyaguchi 1986

Kanmon, Japan

R

1958

3.5

-49

-95 sq. m

Wada 1986

Shin-Kanmon, Japan

RR

1974

18.7

-50

-90 sq. m

Miyaguchi 1986

Seabrook, U.S.A.

W

1980

5.0

-70

2 x ~6.7m

Blindheim and Helgebostad 1980

Forsmark 1 & 2, Sweden

W

1985

2.3

-75

80 sq. m

Seikan, Japan

RR

1985

53.9

~ -250

1 x 90 sq. m 2 x 18 sq. m

Saltsj~, Sweden

W

1989

7.5

~ -60

Q3.5 m

Channel Tunnel, U.K./France

RR

Under construction

49.2

--100

2 x 1~8.5 m 1 x Q5.7 m

Storbaelt, Denmark

RR

Under construction

7.9

-68

2 x (~8.5

Ostenfeld et al. 1989

Sydney Harbour, Australia

W

1990

3.1

-130

(D4.1 m

Wallis 1987

Boston Harbor, U.S.A.

W

Under construction

~ 11.0

-125

Q7.6 m

Williamson 1989

Gibraltar, Spain/Morocco

RR

Under planning/ proposed

~ 50.0

-400

2 x ~7.3 m

Project

Reference

Carlsson 1985

Inoue 1986; Matsuo 1986 Smith 1989 Eurotunne11987; Smith 1988

Serrano et al. 1988

*R = Road tunnel; RR = Railroad tunnel; W = Water tunnel.

154 TUNNELLn~GANDUNDERGROUNDSPACETECHNOLOGY

Volume 9, Number 2, 1994

100

GIBR,

SEIKAN I

-300

LEOENO • Completed project 0 Under construction X Proposed /planned

The Vard~ Tunnel Cave-ins

75'

.!

FOR~MARK

.=, S0 ¸

¢J CHA

..,,

O

O iv-

i SALTS3G-" "" 25

J

K A . . o C . . I F sopo. " "- s .-KAN.O. CHANNEL T.



~

sol,

~

STOREBAELT O

KANMON, RR. 0 0

SO

100

200

iS0

BEDROCK DEPTH (hw+ hs), m

Figure 4. Minimum rock covers of the tunnels in Table 2, plotted versus bedrock depths. The two curves indicate the m i n i m u m rock covers of Norwegian projects. ably small reck covers. There is good reason tobelieve,however, thatinboth these cases the rock conditionsare better than those in the Kanmon case. Figure 4 also shows t h a t i n comparison with all other completed projects, the Seikan tunnel has a verylarge rock cover. However, such a comparison must take into consideration the extraordinary dimensions of the Seikan Tunnel project, and the very difficult geological conditions under which it was built. 4.

Stability P r o b l e m s S I N T E F researchhas clearlyshown that stabilityproblems resultingfrom faulted and crushed rock represent a threat to hard rock subsea tunnel projects. As shown in Table 3, two cases of cave-in at the working face

scription by Grcnhaug and Lynneberg (1984). The discussion of the Ellingscy case is based on a comprehensive docum e n t a t i o n r e p o r t by Olsen and Blindheim (1989).

occurred in the Vard¢ tunnel, and one case occurred in one of the/~lesund tunnels (EllingsCy). One case of minor instability occurred in the Slemmestad tunnel (SRV); and in the Karmsund tunnel, stability problems were encountered which might have led to severe problems. The Vollsi~ord tunnel, which is the water supply tunnel to a petrochemical plant, collapsed in several locations shortly after it was taken into use. In all of these cases, the maj or problems have been caused by faulted rock carrying clay minerals and water leakages of relatively high pressure. In this section, the two cases representing probably the most difficult situations at the working face Vard¢ and Ellingscy Tunnels--will be discussed in some detail. The discussion of the Vard¢ case is mainly based on a de-

In the Vard¢ tunnel, two cases of cave-in occurred in the 1,660-m subsea section. In both cases, the working face had to be sealed with concrete in order to establish stable conditions. The first cave-in situation was located at station 2,500, which is about 200 m from the shore and has a rock cover of about 35 m. The tunnel here encountered a complexly tectonized area. A 10 m core loss from the exploratory drilling due to crushed and clay- bearing rock was recorded. A water leakage of about 10 l/rain, was noticed from the core drillhole. Because of poor rock conditions, short blast rounds were used. One blast round gave an overbreak at the upper part of the working face, and leakages from the roof minimized the attachment between the shotcrete and the rock. After mucking out some of the rock debris from the blast, the casting steel shield was shoveled towards the tunnel face. A pile of rock was placed in the inner part of the tunnel close to the tunnel face. This was done to reduce the time and effort necessary to seal up the cave-in area with a concrete plug. The cave-in developed before the casting could start, and crushed rock was constantly dropping onto the steel shield during the operation. The problem was even worse during excavation through the main fault zone (station 2,090 to 2,115). The tunnel was about 1.1 km from Vard¢ island, which implies a rock cover of about 45 m and a sea depth of 20 m. Because of cave-in, the inner part of the steel casting shield was lost, as it became necessary to seal up the tunnel about 1 m from the inner end of the shield. In this zone, the rock conditions were so extreme that the loader got stuck during loading of the trucks.

The Ellings~y Tunnel Cave-in At the EllingsCy Tunnel, a cave-in at the working face occurred in the

Table 3. Summary of cases of instabilit ~for Norwegian subsea tunnels. Tunnel

Cross-Section (m =)

Water Depth (m)

Rock Cover (m)

16

68

14 10 20 70

27

80

10

50

40 35 45 45 55 35

Voilsfjord Vardo Varde Ellingsey Karmsund

Slemmestad

Volume 9, Number 2, 1994

53 53

Comments Collapse after water filling. Cave-in at working face. Cave-in at working face. Cave-in at working face. Cave-in avoided. Rock fall/cave-in tendency.

TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 1 5 5

rock a h e a d t h e t u n n e l is considered. At K a r m s u n d , this system was established by 8 to 10 drillholes, from 15 to 20 m long. 4. Spiling. Spiling is done with a bolt spacing of 0.3 to 0.5 m a n d length of 6 to 8 m. The boreholes are drilled from some m e t e r s behind the working face, with a n angle of about 15 ° to 20 ° to the tunnel axis. The bolts are grouted if possible. 5. Short blast round. B l a s t r o u n d l e n g t h s down to 0.8 m have been used. The tunnel cross-section m a y be divided into s e p a r a t e blasts.

V-- BUSSESUND

LEGEND: --

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,

=========================

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~

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6. Rapid reinforcement. All exposed rock surface a t t h e working face m a y be shotcreted i m m e d i a t e l y a f t e r blasting (before t h e mucking operation is started). Steel-fibre reinforcement is used.

F - - - 2 550

7. Mucking out. Figure 5. Cave-in at Vard¢, station 2,500. (Based mainly on Grcnhaug and Lynneberg 1984). m a i n fault area. In this area, several w e a k n e s s zones were identified during the preinvestigations. The w o r k i n g face was located a t station 9,900, about 700 m from the shore of EllingsCy. The rock cover was about 45 m, a n d the sea d e p t h was about 70 m. A 2.5-m b l a s t r o u n d w a s done to shape up t h e t u n n e l face before casting. W i t h i n a short time, however, rock fall from t h e w o r k i n g face developed. S h o t c r e t i n g was not successful due to s e e p i n g w a t e r in combination with clay. The w e a k n e s s zone was beyond r e a c h for spiling. W i t h i n six hours, a cave-in developed to a b o u t 7 m above the t u n n e l roof. It was t h e n decided to seal t h e w o r k i n g face with a concrete p l u g from the i n n e r p a r t of the cast section. The r e s u l t i n g p l u g was a p p r o x i m a t e l y 7 m long a n d contained about 700 m 3 of concrete. E x c a v a t i o n t h r o u g h t h e concrete plug a n d t h e w e a k n e s s z o n e - - a total of 20 m - - w a s carefully done w i t h i n 5 weeks.

5. Excavation and Reinforcement Methods

~7~

ELLINGSOYFJORD

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As a c o n s e q u e n c e o f t h e p o o r r o c k

conditions associated w i t h s u b s e a tunnelling activities, the procedures for tunnelling t h r o u g h extremely poor rock conditions a r e c o n s t a n t l y improving. A stepwise procedure of t h e following c h a r a c t e r is as follows:

9, BSO

9,900

9,950

1. Exploratory drilling program. The d r i l l i n g p r o g r a m m e i n c l u d e s both core d r i l l i n g a n d percussive drilling. 2. Grouting. F o r l e a k a g e s over a certain level, grouting is performed. 3. Drainage. For cases c h a r a c t e r ized by a combination of poor rock conditions a n d w a t e r , drilling to d r a i n the

Figure 6. Cave-in at EllingsCy tunnel (based mainly on Olsen and Blindheim 1989).

156 TUNNELLING AND UNDERGROUNDSPACE TECHNOLOGY

Volume 9, N u m b e r 2, 1994

Table 4. Data from preinvestigations. Tunnel Station No.

Vollsfjorden 775

Varde 2,500

Vard¢ 2,100

Elllngsey 9,900

Karmsund 1,600

SRV 650

Rock Seismic velocity (m/s) Thickness of zone (m) Core loss (m) Water loss (Lugeon) RQD Minerals

Gneiss 2,700 2O 0 0-8

Shales _>3,300 10 0 1-1.4 0-20 2,6,7

Sandstone 2,500 3O

Gneiss 2,800 5

Meta.ssndst. 3,500 80 0 0-3 25-80 1,2,3,5,6,7

Gneiss > 2,200 + -< 10 +

+

1,2,4,5,7

+ Uncertain data - Core drilling not performed. Minerals: 1---swelling clay. 2---chlorite. 3--talc. 4 - - m i c a . 5 - - a l t e r e d s i d e r o c k . 6 - - q u a r t z . 7--calcite.

8. Additional rapid reinforcement. When necessary, the rock surface that is exposed after the mucking operation is shotcreted. This m a y be supplemented by radial rock-bolting of the roof. At Vard¢ tunnel, spoil was piled up against the working face. 9. Cast concrete lining. The last operation is reinforcement by means of concrete lining, in which the upper part of the working face m a y be included (as in the Vard¢ Tunnel case). The philosophy is typically "design as you go ~, which means continuously adjusting from one meter of tunnel to the next, according to the rock quality t h a t is encountered.

6. D i s c u s s i o n a n d P l a n s f o r Further Research

The most unstable rock conditions are represented by the major faults which often are located approximately in the middle of the t~ord. Therefore, the most severe problems are generally encountered in a late stage of the tunnelling and shortly before tunnel breakthrough. Partlybecause the tunnelling conditions encountered thus far may have been relatively good, the problems may come as a surprise. Even though problem areas are expected from the geological preinvestigations and core drilling ahead of the tunnel is carried out, the problems m a y be underestimated. If instability occurs, it may take several days before the situ-

ation is stabilized, although it is a matter of hours to establish a proper sealing of the cave-in area by piling up rock and]or by using a mobile steel shielding t h a t is specially constructed for the purpose. A diagram such as that shown in Figure 3 m a y be useful as input to a general cost analysis study for subsea tunnels. For the actual design however, when optimum rock cover is addressed, this should be supplemented by more advanced rock mechanics analyses, which implement knowledge of the rock conditions in the critical area. For the cases of instability in Norwegian subsea tunnels that have been studied, the main facts about the ac-

Table 5. Data from Norwegian subsea tunnelling projects. Vollsfjorden 775

Varde 2,500

Varde 2,100

EIIingsey 9,900

Karmsund 1,600

SRV 650

CORE DRILLING AHEAD OF TUNNEL: • Core loss (m) • Water inflow (l/min./borehole)

* *

9 -< 10

10 *

2 -< 30

* *

* *

DESCRIPTION OF WEAKNESS ZONE: • Thickness of zone (m) • Dip (o) • Q-value • Minerals • Stand up time (hours)

<_ 10 60 * 1,2,4,5,7 *

_<15 35 >0.015 1,2,6,7 -< 0.5

15 60 _>0.015 1,2,6,7 < 0.5

-< 10 70 * 1,2,3 <2

_> 100 75 (?) * 2,5,6 *

CAVE-IN DESCRIPTION: • Max. height above roof (m)

*

7

*

-< 10

TUNNELLING THROUGH THE ZONE: • Length of blast round (m) • Excavation rate (m/week)

* *

> 0.8 5

* 2.5

_>2 3.5

Tunnel Station No.

>1 10-15

-< 10 + 30 + >0.05 + 1,2,5 + -< 6 0 +

* *

• Information not known. + Uncertain data. - No cave-in developed. Minerals: 1---swelling clay. 2---chlorite. 3---talc. 4~rnica. 5 ~ a l t e r e d siderock. 6---cluartz. 7--calcite.

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TUNNELLINGANDUNDERGROUNDSPACETECHNOLOGY1 5 7

tual problem zones which were k n o w n before t u n n e l l i n g are listed in Table 4. As shown, the weakness zones i n question normally are less t h a n 15 m thick, and have a seismic velocity lower t h a n 3,500 m/s. Core drilling has indicated zones of relatively moderate permeability, as well as RQD values down to 0. P e r t i n e n t d a t a from t u n n e l l i n g through cave-in areas are listed in Table 5. As can be seen, the Q-values indicate "extremely poor" rock mass. The s t a n d - u p time is very short, according to Norwegian standards. As indicated by Figure 4, the rock cover for s u b s e a t u n n e l s g e n e r a l l y seems to be conservative. There have been several h u n d r e d lake taps i n Norway over the years. Large water leakages have occasionally r e p r e s e n t e d a problem for drilling a n d charging a lake tap blast r o u n d w h e n the distance to the water reservoir has been small (10 m or less). To our knowledge, however, severe stability problems due to rock cover have n e v e r occurred. Apparently, m i n i m i z i n g rock cover is not so much of a technical problem for solid rock, b u t r a t h e r a question of economics. []

7. References Blindheim, O. T. and Helgebostad, J. 1980. TBM tunnelling at Seabrook: Boreability and rock support. NTH, Dept. of Geology, Report No. 14. Trondheim: Norwegian Institute of Technology. Carlsson, A. 1985. Submarine tunnelling in poor rock. Tunnels & Tunnelling (December), 21-25. Eurotunnel. 1987. "The ChannelTunnel-A technical description." Eurotunnel report, August 1987. Grcnhaug, A. and Lynneberg, T. E. 1984. The Vard¢ undersea tunnel--a low-cost project? Proc. Int. Syrup. on Low Cost Road Tunnels, 185-203. Trondheim: Tapir Publishers. Inoue, T. 1986. Survey of the Seikan Tunnel. Tunnelling and Underground Space Technology, 333-340. Matsuo, S. 1986. An overview of the Seikan Tunnel project. Tunnelling and Under-ground Space Technology, 323331. Miyaguchi, K. 1986. Maintenance of the Kanmon Railway Tunnels. Tunnelling and Underground Space Technology, 307-314. Nilsen, B. 1993. Empirical analysis of minimum rock cover for subsea rock tunnels. Options for Tunnelling 1993 (H. Burger, Ed.), 677-687. Amsterdam: Elsevier. Nilsen, B. 1989. The utility of preinvestigations in predicting tunnelling conditions--a study of 10 Norwegian subsea tunnels. Proc. Int. Congress on Progress and Innovation in Tunnelling,

158 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY

Sept. 9-14, 1989, Toronto, Canada (K. Y. Lo, ed), 727-736. Toronto: Tunnelling Association of Canada. Nilsen, B.; Maage, M.; Dahlo, T. S.; Hammer, T. A.; and Smeplass, S. 1989. Undersea tunnels in Norway--a stateof-the-art study. Tunnels & Tunnelling (20:9), 18-22. Olsen, A. B. and Blindehim, O.T. 1989. Prevention is better than cure. Tunnels & Tunnelling (20:9), 41-44. Ostenfeld, K. H.; Larsen, O. C.; Elliot, I. H.; and Hartley, I. F. 1989. Bored railway tunnel under the eastern channel of the Great Belt, Denmark. In Tunnels and Water--Proc. Int. Congress on Tunnels and Water, Vol. III (J. M. Serrano, ed.), 1449-1458. PalmstrSm, A. 1984. Geo-investigations and advanced t u n n e l excavation technique important for the Vard¢ subsea road tunnel. Proc. Int. Syrup. on Low-Cost Road Tunnels, 657-572. Trondheim: Tapir Publishers. Serrano, J. M.; Gonzales, G. G.; and Cornejo, L. 1988. Underground works in Spain. Tunnels & Tunnelling (June), 70-71. Smith, M. 1988. Channel rail tunnel gets under way. World Tunnelling, 10-25. Wada, K. 1986. Maintenance and control of the Kanmon Highway Tunnel. Tunnelling and Underground Space Technology, 315-322. Wallis, S. 1987. Sydney's ocean outfalls keep Bondy Beach clear. Tunnels & Tunnelling (September), 27-30. Williamson, L. A. 1989. The Boston harbour project. World Tunnelling (March), 200-208.

Volume 9, N u m b e r

2, 1994