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 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 long- term 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.

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.

1. Introduction

T he Foundation for Scientific and Industrial Research at the Nor- wegian Institute of Technology

(SINTEF) has studied subsea rock tunnels since 1986. Initially, a major state- of-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. De- tails of tunnels completed since 1988 have been added to the table.

Present address: Tore S. Dahl~, SINTEF Rock and Mineral Engineering, N-7034 Trondheim, Norway; Bj$rn Nilsen, Nor- wegian 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 of the 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.

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-and- blast techniques have been used for all of the tunnels.

The Norwegian subsea tunnels are situated in a variety of geological struc- tures, 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 envi- ronments on the condition of rock support materials.

A l though the tota 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

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 deteriora- tion of rock 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-

Tunnellin~,~ and Underground Space Techn.logy, Vol. 9, No. 2. pp. 151 158, 1994 Elsevier Science Ltd Printed in ( ; real Britain 0886-7798/94 $6.00 + .00

~ ) Pergamon 151

Table 1. List of subsea rock tunnels in Norway.

Project Tunnel Type*

Year Completed

Cross- Section (rn =) Main Rock Types

Total Length Lowest (km) Level (m)

9.4 -80 1. Vollesfjord W 1977 16 Gneiss

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 4.8 -180

O 1983 (all) 27 (all) Gneiss, phyllite 3.4 - 170 5.2 ° Ferdesfjord 5.3 ° Forlandsfjord 3.9 -160

6. Hjartey O 1986 26 Gneiss 2.3 -105

7. Alesund 7.1 • Etlingsoy R 1987 (both) 68 (both) Gneiss 3.5 -140 7.2 ° Valderoy 4.2 -137

8. Kvalsund R 1988 43 Gneiss 1.6 -56

9. God ly 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 R 1992 (both) 70 (both) Phyilite 5.8 -223 19. ° Mastrafjord Gneiss 4.4 - 132

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

search act ivi t ies on subsea rock tunnels. Pa r t i cu la r emphas i s was placed on eva lua t ing s tab i l i ty and op t imum rock cover. The following sect ions dis- cusses some resu l t s of this research .

2. Design Pract ice Subsea rock tunnels have become

increasingly popular in Norway, par- ticularlyforroadpurposes. Since 1988,

12 tunne l s have been comple ted (see Table 1), and severa l a re unde r con- s t ruc t ion or in the p l ann ing s tage. The p l ans include some very chal leng- ing projects. The ex t reme case is rep- r e sen t ed by the H a r e i d Tunnel , which has a p l anned l eng th of 132 k m and a m a x i m u m dep th of 630 m below sea level.

As a resul t of na t iona l regula t ions , the max imum g rad i en t for a road tun-

nel in Norway is l imi ted in most cases to 1:12.5. Hence, the p a r a m e t e r t h a t will ma in ly define the length of a subsea tunne l is the m i n i m u m rock cover. I f the rock cover decision is too conser- vat ive , the resu l t will be considerable ex t r a costs due to ex t r a tunnel length. For a typical Norwegian subsea road tunne l (cross-sectional a rea 50 m2), a reduc t ion of only 1 me te r in the mini- m u m rock cover m a y rep resen t a cost

152 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 9, Number 2, 1994

~ 75

o ~ L

°50

O"

ROAD.

PIPELINE ~ 05.2 TUNNELS ~

"~'~- 4 eS.3 "~

WATER TUNNELS

.~. o 7

o5.1 6o

e2

B

/.SO0 SO00 5500 Average seismic velocity (m/s)

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

e

._on

e- 8

e-

~.\ n - - f r o m fresh water section 0.~0 I ' \ +.-- . from sea water section

e - - - f r o m sea water section

0.30 \ ~,~

0,20 ~ \ \ ~ "\,+~ Limit for high r isk of rebar corroslon.~

0,1 0 '~ \ .

0 ~0 80 120 160 200 240 260

Penetration depth (mm)

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

75

E 50

J=

O

O 25

Oe

(D s-2 @2

05.3

O $ 5.30 O5.1

• 10 --S.IR

~)5.1 20 i~ 0 5.2

5.3 7.1R ~ ) 7 . 2

10 • m 4 •7.1 7.2 0 % 6 05.2 . ~ 0 ~

7.2~ 02 X7.2 4 13 / 8 1 0 12_1,_~ ,~_' . . _ I • % _ .

V 'x'x"~ ot'.e" d oTJ ./ ~-

72 "

SO

LEGEND

• v = 2 0 0 0 - 2 9 0 0 m / s

• v = 3 0 0 0 - 3 9 0 0 m / s

o v = 4 0 0 0 - 4 9 0 0 m / s

o v = 5 0 0 0 - 6 0 0 0 m / s

x v = Unknown

R Case of Instab i l i ty

20 24)

02

/ /

/

SOIL •

~ - - - - - - - -~ -~ - - R-- O c K / ~ - TUNNEL

J 100 150 2 0 0

BEDROCK DEPTH ( h w + hs) , m

Figure 3. Minimum rock cover under sea as a function of the bedrock depth.

reduction in the order ofNOK i million (approx. $US135,000). However, if the rock conditions are poor and the rock cover too smA11, severe stability problems and large water inflow may result, as discussed in Section 4, below.

An important basis for the 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-

eral trend for new projects has been a gradual but slow reduction of minimum 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 "critical rock 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 represent- ing the minimum rock cover as a function of the bedrock depth may be de-

Volume 9, Number 2, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 153

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. How- ever, 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. Compar ison with Projects in Other Countr ies

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 Chan- nel 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

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

one-third (Seabrook, Forsmark, Saltsj¢, and a part of the Kanmon road tunnel) are in granitic or gneissic rocks. Un- like 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 pub- lications, 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 unambigu- ous (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 compa- rable 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-

Year Length Deepest Point Project Type* Completed (kin) (m) Cross-Section Reference

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

Blindheim and Seabrook, U.S.A. W 1980 5.0 -70 2 x ~6.7m Helgebostad 1980

Forsmark 1 & 2, W 1985 2.3 -75 80 sq. m Carlsson 1985 Sweden

1 x 90 sq. m Inoue 1986; Matsuo Seikan, Japan RR 1985 53.9 ~ -250 2 x 18 sq. m 1986

Saltsj~, Sweden W 1989 7.5 ~ -60 Q3.5 m Smith 1989

Channel Tunnel, RR Under 49.2 --100 2 x 1~8.5 m Eurotunne11987; U.K./France construction 1 x Q5.7 m Smith 1988

Under 7.9 -68 2 x (~8.5 Ostenfeld et al. 1989 Storbaelt, Denmark RR construction

Sydney Harbour, W 1990 3.1 -130 (D4.1 m Wallis 1987 Australia

Boston Harbor, W Under ~ 11.0 -125 Q7.6 m Williamson 1989 U.S.A. construction

Under Gibraltar, RR planning/ ~ 50.0 -400 2 x ~7.3 m Serrano et al. 1988 Spain/Morocco proposed

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

154 TUNNELLn~G AND UNDERGROUND SPACE TECHNOLOGY Volume 9, Number 2, 1994

100

75'

.=, S0 ¸

¢J

O O iv-

25

0

FOR~MARK

SEIKAN I

LEOENO • Completed project

0 Under construction X Proposed /planned

.! CHA ..,,

GIBR, -300

i

SALTS3G -" "" J K A . . o C . . I F sopo.

" "- s .-KAN.O.

CHANNEL T. • STOREBAELT O

KANMON, RR.

sol, ~ ~

0 SO 100 iS0 200

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 minimum rock covers of Norwegian projects.

ably small reck covers. There is good reason to believe, however, that in both these cases the rock conditions are better than those in the Kanmon case.

Figure 4 also shows that in comparison with all other completed projects, the Seikan tunnel has a verylarge rock cover. However, such a comparison must take into consideration the ex- traordinary dimensions of the Seikan Tunnel project, and the very difficult geological conditions under which it was built.

4. Stabi l i ty Prob lems SINTEF research has clearly shown

that stability problems resulting from 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

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 represent- ing probably the most difficult situa- tions 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-

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

The Vard~ Tunnel Cave-ins 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- bear- ing rock was recorded. A water leak- age 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 to- wards 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 drop- ping 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) Comments

Voilsfjord Vardo Varde Ellingsey Karmsund Slemmestad

16 53 53 68 27 10

14 10 20 70 80 50

40 35 45 45 55 35

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.

Volume 9, Number 2, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 155

iiiiii)iiiiiiiiiiiiiii

V-- BUSSESUND

LEGEND: - - " ========================= " ~-'f~11~ ~ SILTSTONE

, : : . : : : : : : : : . : : : : : : : : : . : : : : . : : : : ~ "

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,--.:.~. ~ O N {.R E 1 E LINING KFILL

I F - - - - 2.5OO 2 550

Figure 5. Cave-in at Vard¢, station 2,500. (Based mainly on Grcnhaug and Lynneberg 1984).

rock ahead the tunne l is considered. At Karmsund, this sys tem was es tabl ished by 8 to 10 dril lholes, from 15 to 20 m long.

4. Spiling. Spi l ing is done with a bol t spacing of 0.3 to 0.5 m and length of 6 to 8 m. The boreholes are dr i l led from some mete r s behind the working face, wi th an angle of about 15 ° to 20 ° to the tunnel axis. The bolts are grouted i f possible.

5. Short blast round. Blas t round lengths down to 0.8 m have been used. The tunnel cross-section m a y be di- v ided into s epa ra t e blasts .

6. Rapid reinforcement. All exposed rock surface a t the working face may be shotcre ted immedia te ly af te r blast- ing (before the mucking opera t ion is s tar ted) . Steel-f ibre re inforcement is used.

7. Mucking out.

main faul t area . In th is area , several weakness zones were ident i f ied dur ing the preinvest igat ions .

The working face was located a t s ta t ion 9,900, about 700 m from the shore of EllingsCy. The rock cover was about 45 m, and the sea dep th was about 70 m.

A 2.5-m b las t round was done to shape up the tunnel face before casting. Wi th in a shor t t ime, however, rock fall f rom the working face developed. Shotcre t ing was not successful due to seeping wate r in combinat ion with clay. The weakness zone was beyond reach for spiling. Wi th in six hours, a cave-in developed to about 7 m above the tunne l roof. I t was then decided to seal the working face wi th a concrete p lug from the i nne r p a r t of the cast section. The resu l t ing p lug was approx imate ly 7 m long and contained about 700 m 3 of concrete.

Excavat ion th rough the concrete plug and the weakness z o n e - - a total of 20 m - - w a s careful ly done wi th in 5 weeks.

5. Excavation and Reinforcement Methods

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 associa ted wi th subsea tunnelling activi t ies , the procedures for tunnel l ing th rough ext remely poor rock conditions a re cons tant ly improving. A stepwise procedure of the following charac te r is as follows:

1. Exploratory drilling program. The dr i l l ing p rog ramme inc ludes both core dr i l l ing and percuss ive dri l l ing.

2. Grouting. For l eakages over a cer tain level, grout ing is performed.

3. Drainage. For cases charac ter - ized by a combinat ion of poor rock conditions and water , dr i l l ing to d ra in the

~ 7 ~ ELLINGSOYFJORD -_._--

..... ,================================================================== == === :: :

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:::::::::::::::::::::::::::: \

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VvvVVvvyevYvVVVVVVVVVV '

v ,~ :~ :W~ ,~W~WW,~$~ ,~W~ : : : CRUSHED ROCK

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


Table 4. Data from preinvestigations.

Tunnel Station No.

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

RQD Minerals

Vollsfjorden 775

Gneiss 2,700

2O 0

0-8 +

1,2,4,5,7

Varde 2,500

Shales _> 3,300

10 0

1-1.4

0-20 2,6,7

Vard¢ 2,100

Sandstone 2,500

3O

Elllngsey 9,900

Gneiss 2,800

5

Karmsund 1,600

Meta.ssndst. 3,500

80 0

0 -3 25-80

1,2,3,5,6,7

SRV 650

Gneiss > 2,200 +

-< 10 +

+ Uncertain data - Core dril l ing not performed. Minerals: 1---swelling clay. 2---chlorite. 3- - ta lc . 4 - -m ica . 5 - -a l te reds iderock . 6- -quar tz . 7--calc i te.

8. Additional rapid reinforcement. When necessary, the rock surface that is exposed after the mucking operation is shotcreted. This may 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 par t of the working face may 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 tha t is encountered.

6. D i s c u s s i o n a n d P l a n s for F u r t h e r R e s e a r c h

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. Part lybecause 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 may be un- derestimated. If instability occurs, it may take several days before the situ-

ation is stabilized, although it is a mat ter of hours to establish a proper sealing of the cave-in area by piling up rock and]or by using a mobile steel shielding tha t is specially constructed for the purpose.

A diagram such as that shown in Figure 3 may be useful as input to a general cost analysis study for subsea tunnels. For the actual design however, when optimum rock cover is ad- dressed, 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 Nor- wegian subsea tunnels that have been studied, the main facts about the ac-

Table 5. Data from Norwegian subsea tunnelling projects.

Tunnel Station No.

Vollsfjorden Varde Varde 775 2,500 2,100

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

EIIingsey 9,900

Karmsund 1,600

* 9 10 2 * * * -< 10 * -< 30 * *

SRV 650

DESCRIPTION OF WEAKNESS ZONE: • Thickness of zone (m) <_ 10 _<15 15 -< 10 _> 100 -< 10 + • Dip (o) 60 35 60 70 75 (?) 30 + • Q-value * >0.015 _> 0.015 * * >0.05 + • Minerals 1,2,4,5,7 1,2,6,7 1,2,6,7 1,2,3 2,5,6 1,2,5 + • Stand up t ime (hours) * -< 0.5 < 0.5 < 2 * -< 60+

CAVE-IN DESCRIPTION: • Max. height above roof (m) * 7 * -< 10

TUNNELLING THROUGH THE ZONE: • Length of blast round (m) * > 0.8 * _> 2 > 1 * • Excavation rate (m/week) * 5 2.5 3.5 10-15 *

• Information not known. + Uncertain data. - No cave-in developed. Minerals: 1---swelling clay. 2---chlorite. 3---talc. 4~rn ica. 5~a l te red siderock. 6---cluartz. 7--calci te.

Volume 9, Number 2, 1994 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 1 5 7

tual problem zones which were known before tunne l l ing are listed in Table 4. As shown, the weakness zones in ques- tion normally are less t han 15 m thick, and have a seismic velocity lower t han 3,500 m/s. Core dri l l ing has indicated zones of relatively moderate perme- ability, as well as RQD values down to 0.

P e r t i n e n t da ta 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 tand-up time is very short, according to Norwegian s tandards.

As indicated by Figure 4, the rock cover for subsea t u n n e l s genera l ly seems to be conservative. There have been several hundred lake taps in Nor- way over the years. Large water leakages have occasionally represented a problem for drill ing and charging a lake tap blast round when the distance to the water reservoir has been small (10 m or less). To our knowledge, however, severe s tabi l i ty problems due to rock cover have never occurred. Apparently, minimiz ing rock cover is not so much of a technical problem for solid rock, but ra the r a quest ion of economics. [ ]

7. References Blindheim, O. T. and Helgebostad, J. 1980.

TBM tunnelling at Seabrook: Bore- ability 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, 323- 331.

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,

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 state- of-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.


Documents

Stability and rock cover of hard rock subsea tunnels