Characteristics of Water Ingress in Norwegian Subsea Tunnels

ORIGINAL PAPER

Characteristics of Water Ingress in Norwegian Subsea Tunnels

Bjørn Nilsen

Received: 16 May 2012 / Accepted: 5 August 2012

� Springer-Verlag 2012

Abstract Water ingress represents one of the main

challenges in subsea tunnelling, particularly when this

occurs in sections with poor rock mass quality. This paper

is discussing the main characteristics of water ingress in

subsea hard rock tunnels based on the experience from

almost 50 such tunnels that have been built in Norway.

Following a brief description of the geological conditions

and the basic design of the subsea tunnels, pre-construction

investigations and investigations during excavation are

discussed with particular emphasis on prediction of water

ingress. Two cases with particularly difficult conditions;

the Bjorøy tunnel and the Atlantic Ocean tunnel, are dis-

cussed in detail. In these cases, large water inflow with

pressure of up to 2.4 MPa was encountered at major faults/

weakness zones during excavation, and special procedures

were required to cope with the problems. Based on the

experience from the Norwegian projects, it is concluded

that continuous follow-up by experienced engineering

geologists, probe drilling with the drilling jumbo and pre-

grouting where required are the most important factors for

coping with water ingress and ensuring stability.

Keywords Subsea tunnel � Water ingress �Pre-investigation � Probe drilling � Water sealing

1 Introduction

Since the early 1980s almost 50 subsea tunnels have been

built in Norway (Nilsen and Henning 2009; Nilsen 2011).

The majority of these are road tunnels, but there are also

subsea tunnels for oil and gas pipelines and for water

supply and sewerage. The tunnels are located in a variety

of geological conditions, ranging from hard, very good

quality Precambrian rock to less competent Paleozoic

(Caledonian) rocks such as shale, schist and phyllite.

The locations of the main Norwegian subsea tunnels

are shown in Fig. 1, and key data for the projects are

given in Tables 1 and 2. As can be seen from Table 1, the

7.9 km long Bømlafjord road tunnel is the longest of the

Norwegian subsea tunnels, and the Eiksundet road tunnel

is the deepest, with its lowest section 287 m below sea

level.

In most cases, tunnel excavation has been completed on

time and on schedule without major problems or surprises

during excavation. In some cases, very difficult conditions

have, however, been encountered. The greatest challenges

have been represented by weakness zones/faults, often

containing swelling clay, and by large water ingress (Dahl

Johansen 1995; Karlsson 2008; Nilsen and Palmstrøm

2009; Nilsen 2010). Combinations of very poor rock mass

quality and large water inflow, such as in the major faults/

weakness zones of the Bjorøy tunnel and the Atlantic

Ocean tunnel, have been particularly difficult.

The intention of this paper is, based on participation in

investigations and expert panel evaluations, to discuss the

characteristics of water problems in the Norwegian subsea

tunnels, and how such problems may be minimized through

investigations at the pre-construction stage as well as

during excavation, and through appropriate measures dur-

ing tunnelling.

B. Nilsen (&)

Department of Geology and Mineral Resources Engineering,

Norwegian University of Science and Technology (NTNU),

Sem Sælands vei 1, 7491, Trondheim, Norway

e-mail: [email protected]

123

Rock Mech Rock Eng

DOI 10.1007/s00603-012-0300-8

2 Geological Characteristics and Basic Design

Compared to ‘‘conventional tunnels’’, subsea tunnels have

several distinctive features. Regarding engineering geol-

ogy, the following are the most evident, see Fig. 2:

• Much of the project area is covered by water, and in

most cases there is a considerable soil cover on the sea

floor. Special investigation techniques are thus required

and interpretation of the investigation results is more

uncertain than for most on-shore tunnel projects.

• The locations of fjords and straits are in most cases

defined by major faults or weakness zones in the

bedrock. Even in generally good quality rock mass, the

deepest part of the fjord, and thus the most critical part

of the tunnel, often coincides with significant weakness

zones representing very difficult ground conditions.

• The potential of water inflow is indefinite, and the

hydraulic pressure is often very high (up to 2.5 MPa

and more). In addition, all leakage water has to be

pumped out of the descending tunnel.

• The saline leakage water represents considerable prob-

lems for tunnelling equipment and rock support

materials.

• The down-sloping tunnel from both sides causes high

costs for transportation and water pumping. The

consequences of a cave-in or severe water ingress in

a subsea tunnel may be disastrous.

Most Norwegian subsea tunnels thus will cross faults or

weakness zones consisting of very poor rock quality. The

most unstable conditions are caused by heavily crushed

rock with gouge, often including active swelling clay.

Water seepage in such zones may dramatically reduce the

stand-up time and thus increase the excavation problems.

The length of a subsea tunnel, and therefore the cost, to

a large extent is defined by the depth, the minimum rock

cover at the critical point(s), and the inclination. The

maximum inclination for Norwegian subsea tunnels has

typically been 6–8 %. Inclinations up to 10 % have been

used in some cases.

The typical geometry of subsea road tunnels as shown in

Fig. 2 illustrates the importance of optimising the mini-

mum rock cover. Excessive rock cover makes the tunnel

unnecessarily long, causing extra construction and operat-

ing costs, while too little rock cover may cause risk for

unacceptable stability problems and water ingress, and in

the worst case loss of the entire tunnel. Optimisation of

minimum rock cover, therefore, is a key issue in the

planning of subsea tunnels.

Today, the basic requirement concerning rock cover for

Norwegian subsea road tunnels minimum 50 m (NPRA

2006). A rock cover of less than 50 m may be accepted

when detailed site investigations have demonstrated

favourable rock mass conditions (taking into account the

typical occurrence of fault zones at the deepest point). This

is left much open to interpretation, and rock cover less than

20 m has been used for shallow water and favourable rock

conditions. Minimum rock cover used for some Norwegian

subsea road tunnels is shown in Fig. 3.

The rock cover can be looked upon as representing an

arch of sufficient bearing capacity (considering the water

pressure), a margin for undetected variation (‘‘surprises’’),

and a margin for ‘‘reaction time’’ in case of fallout. The

latter proved useful in the Ellingsøy tunnel, where a cave-

Fig. 1 Location and geology of

Norwegian subsea tunnels

B. Nilsen

123

in developed at a fault zone and progressed upwards at a

rate of 1 m/h before stopping after 10 m, and in a similar

case at the more recent Atlantic Ocean tunnel (see Sect. 5).

Due to the risk of incidents like these, smaller rock cover

than 50 m has to be evaluated by independent review and

approved by the Directorate of Public Roads.

The current requirement regarding minimum rock cover

defined by the Norwegian Public Roads Administration

(NPRA) is conservative and for most tunnel projects

probably much on the safe side since, as illustrated in

Fig. 3, many tunnels built before 2006 (and also some after

that time) have minimum rock cover much less than 50 m.

The main reason for the conservative NPRA-requirement is

that the important decision regarding minimum rock cover

is to be taken at the design stage of the tunnel project, when

information about the rock mass conditions is limited and

uncertain as described in Sect. 3.

If favourable conditions have been proven, and/or the

sea depth is small, less rock cover may be approved by

NPRA (2006). As shown in Fig. 3, a rock cover of only

15 m was accepted for one recent project (No. 29; Nord-

asstraumen) located in good quality rock mass and at a sea

depth of only 4 m. No stability problems were encountered

during excavation of the tunnel. On the other hand, insta-

bility has been experienced in tunnels with high rock cover,

such as in the Ellingsøy tunnel (No. 2 in Fig. 3) where a

cave-in reaching 8–10 m above the tunnel crown occurred

during excavation (Olsen and Blindheim 1989) and in the

Atlantic Ocean tunnel, which is described in detail in

Sect. 5.2.

Table 1 Key data of completed Norwegian subsea road tunnels

No. Project Year

completed

Main rock types Cross

section, m2Total

length, km

Min. rock

cover, m

Max. depth

below sea, m

1 Vardø 1981 Shale, sandstone 53 2.6 28 88

2 Ellingsøy 1987 Gneiss 68 3.5 42 140

3 Valderøy 1987 Gneiss 68 4.2 34 145

4 Kvalsund 1988 Gneiss 43 1.6 23 56

5 Godøy 1989 Gneiss 52 3.8 33 153

6 Hvaler 1989 Gneiss 45 3.8 35 121

7 Flekkerøy 1989 Gneiss 46 2.3 29 101

8 Nappstraumen 1990 Gneiss 55 1.8 27 60

9 Fannefjord 1991 Gneiss 54 2.7 28 100

10 Maursund 1991 Gneiss 43 2.3 20 92

11 Byfjord 1992 Phyllite 70 5.8 34 223

12 Mastrafjord 1992 Gneiss 70 4.4 40 132

13 Freifjord 1992 Gneiss 70 5.2 30 132

14 Hitra 1994 Gneiss 70 5.6 38 264

15 Tromsøysund 1994 Gneiss 60a 3.4 45 101

16 Bjorøy 1996 Gneiss 53 2.0 35 85

17 Sloverfjord 1997 Gneiss 55 3.3 40 100

18 North Cape 1999 Shale, sandstone 50 6.8 49 212

19 Oslofjord 2000 Gneiss 79 7.2 32b 134

20 Frøya 2000 Gneiss 52 5.2 41 164

21 Ibestad 2000 Micaschist, granite 46 3.4 30 125

22 Bømlafjord 2000 Greenstone, gneiss, phyllite 74 7.9 35 260

23 Skatestraumen 2002 Gneiss 52 1.9 40 80

24 Eiksundet 2007 Gneiss, gabbro, limestone 71 7.8 50 287

25 Halsnøy 2008 Gneiss 50 4.1 45 135

26 Nordasstraumen 2008 Gneiss 74a 2.6c 15 19

27 Finnfast 2009 Gneiss, amphibolite 50 5.7 ? 1.5 44 150

28 Atlantic Ocean 2009 Gneiss 71 5.7 45 249

a Two tubesb Assumed rock cover from site investigations, proved to be lacking at deepest pointc Crossing a narrow strait, only 40 m length under sea


123

The few incidents of cave-in at Norwegian subsea

tunnels are all related to crossing of major weakness

zones/faults so far away from shore that they could not

be reached by core drilling during the pre-construction

investigations. The seismic velocities as shown in Fig. 3

are low to very low for all the cases of instability

Fig. 3 Minimum rock cover

under sea versus depth to

bedrock for Norwegian subsea

road tunnels. The numbers for

circles in drawing correspond

with tunnel numbers in Table 1,

and the colour of filling

indicates seismic velocity

Fig. 2 Longitudinal profile of

typical Norwegian subsea

tunnel

Table 2 Some main Norwegian subsea tunnels for oil, gas and water

No Project Year

completed

Main rock

types

Cross

section, m2Total

length, km

Min. rock

cover, m

Lowest level,

m below sea

1 Frierfjorden, gas pipeline 1976 Gneiss and claystone 16 3.6 48 253

2 Karstø, cooling water 1983 Phyllite 20 0.4 15 58

3 Karmsund (Statpipe), gas pipeline 1984 Gneiss and phyllite 27 4.7 56 180

4 Førdesfjord (Statpipe) 1984 Gneiss 27 3.4 46 160

5 Førlandsfjord (Statpipe) 1984 Gneiss and phyllite 27 3.9 55 170

6 Hjartøy, oil pipeline 1986 Gneiss 26 2.3 38 (6 m at piercing) 110

7 Kollsnes (Troll), gas pipeline 1994 Gneiss 45–70 3.8 7 m at piercing 180

8 Karstø, new cooling water 1999 Phyllite 20 3.0, 0.6 a 60, 10

9 Snøhvit, water intake/outlet 2005 Gneiss 22 1.1/3.3 a 111/54

10 Aukra, water intake/outlet 2005 Gneiss 20/25 1.4/1.0 5/8 (5.5 at piercing) 86/57

a Information lacking

B. Nilsen

123

(representing cave-ins that in all cases were stopped

from a few to up to 10 m above the tunnel crown).

However, correlations between seismic velocity and rock

mechanical parameters are very uncertain. Due to the

problem of defining reliable input parameters for ana-

lytical and numerical analyses, such analyses are not

carried out, and the decision regarding minimum rock

cover is rather based on empirical approach as described

above.

3 Information from pre-construction investigations

The Norwegian subsea tunnels, as shown in Fig. 1, are

located mainly in hard, Precambrian rocks (typically gra-

nitic gneiss). As described above, even in generally good

quality rock conditions, the deepest part of the fjord, and

hence the most critical part of the tunnel, however, often

coincides with weak zones or faults, which may cause

difficult excavation conditions. Thus, the geological con-

ditions are often challenging for these tunnels, and exten-

sive site investigations are needed.

In addition to normal geological surveys on both sides of

the fjord, and on any adjacent islands, the site investiga-

tions rely heavily on seismics in the first stage. Acoustic

profiling is first carried out to cover a large area to deter-

mine the most suitable corridor, extensive refraction seis-

mics is then carried out to select the best alignment and to

provide information about soil deposits above the bedrock

and about weakness (low velocity) zones in the bedrock. In

most cases, core drilling to further investigate major

weakness zones is also carried out. An example of soil

accumulations and seismic low velocity zones registered

during pre-construction investigations for the Frøya tunnel,

and weakness zones actually encountered during tunnel-

ling, is shown in Fig. 4.

If possible, directional core drilling as illustrated for the

Bømlafjord tunnel in Fig. 5 is used from shore to critical,

deep points of the alignment, which often represent major

fault zones. In this case, a 900 m long directional core hole

towards a low point in the bedrock (not the deepest) hit

moraine where rock was expected based on seismic

investigation. This was checked by further directional core

drilling and the tunnel alignment was adjusted (from 7.0 to

8.5 % slope) to be located in the bedrock below the mor-

aine deposit as shown in the figure.

In the Bømlafjord case, the inaccurate drilling turned out

to give very valuable, unexpected information resulting in

adjusting of the planned tunnel alignment. The directional

core drilling in this case was of great importance for

ensuring satisfactory rock cover at a critical section of the

tunnel alignment, and thus for the safe location of the

tunnel. In many other cases, i.e. the Frøya tunnel (Nilsen

et al. 1999), the Finnfast tunnel (Gilje 2009) and the

Atlantic Ocean tunnel (Karlsson 2008), core drilling at the

pre-investigation stage has provided valuable information

about the character of faults and weakness zones. However,

incidents such as the one at Bømlafjord and at the

Oslofjord tunnel (Haug 1999) clearly emphasize the need

to carry out supplementary drilling during tunnel excava-

tion. This is discussed in more detail in Sects. 4 and 5.

Core drilling from drilling ships has been applied in a

few cases, when other methods have not been feasible.

Such drilling is seldom cost effective and not always

conclusive; if feasible it may be better to plan for more

directional core drilling.

The total cost of pre-construction investigations typi-

cally amount to 6–10 % of the excavation cost. As always

in tunnelling, much effort is put into avoiding ‘‘surprises’’.

Many so-called ‘‘unexpected geological conditions’’ are in

fact foreseeable, but may be more difficult to identify due

to the subsea location. A certain remaining risk will exist,

even after significant and relevant site investigations. This

is why risk control during planning and construction

becomes important. Continuity in planning and investiga-

tion should always be aimed at to ensure that interpreta-

tions from early phases are brought forward to the detailed

design and construction phases.

Based on the pre-construction site investigations as

described above, an overview of the location of the bed-

rock under sea and the rock mass quality is obtained, and

the correlation between estimated and as-built rock mass

quality often will be quite good as illustrated in Fig. 4 and

described in several reports; i.e. Melby and Øvstedal

(1999), Lien (2000), Melby et al. (2002), Nilsen and

Henning (2009).

Some uncertainties will however still remain, particu-

larly for the deepest sections of the tunnels far from shore.


the 5.2 km long Frøya subsea

road tunnel (from Nilsen and

Palmstrøm 2001)


123

For such sections, interpretation of the rock mass condi-

tions has to rely mainly on the results from seismic

investigation. Experience shows that there is no general,

reliable correlation between seismic velocity and rock

mechanical parameters (although attempts have been made

to correlate seismic velocity with for instance Q value,

Fig. 5 Directional core drilling

for the Bømlafjord tunnel where

the upper, 900 m long borehole

(BH-1) was originally planned

to follow the planned tunnel

alignment, but drilling was

inaccurate and the hole

penetrated into moraine. A

second, deeper hole (BH-1A)

was then drilled, and based on

this, the tunnel was located

30 m deeper than originally

planned to ensure sufficient rock

cover (minimum 35 m). From

Palmstrøm et al. (2003)

Fig. 6 Lugeon values in core drill hole versus water inflow encountered in the Karmsund gaspipe tunnel

B. Nilsen

123

Barton (2002)). The results from the refraction seismic

investigations for the Atlantic Ocean tunnel (Fig. 7) illus-

trate this. Here, as will be further discussed in Sect. 5.2,

similar seismic velocities along the alignment turned out to

represent quite different rock mass conditions.

Concerning estimation of water ingress, the situation is

particularly difficult. The general experience is that none of

the main investigation methods for the undersea section of

the planned tunnel; core drilling and refraction seismic

investigation, give reliable prediction of the risk of large

water ingress.

An example illustrating the shortcoming of Lugeon

testing in this connection is shown in Fig. 6, where Lugeon

values along the core drill hole for the Karmsund gaspipe

tunnel (cfr. Table 2) and sections with large water inflow in

the tunnel are given. As can be seen, the high Lugeon

values do not correspond well with sections of large water

inflow in the tunnel. The section of high Lugeon value

(6 Lugeon) near the start of the 348 m long core drill hole

(upper circle) is possibly corresponding with the major

water inflow on the right hand side of the tunnel section,

while the higher Lugeon value (8 Lugeon) further down in

the hole (lower circle along hole) has not been registered as

major inflow in the tunnel. Sections with moderate Lugeon

values along the hole have neither been registered as sec-

tions with water inflow in the tunnel. This indicates that

open fractures are not necessarily water conducting all the

way from the drill hole and down to the tunnel.

Figure 7 also illustrates the general shortcoming of

refraction seismic investigation for reliably estimating

water ingress. Here, in the Atlantic Ocean tunnel, major

water ingress occurred in a weakness zone with seismic

velocity of 2,800 m/s, while several nearby weakness

zones with identical and very similar seismic velocities

gave no water ingress in the tunnel.

Since reliable prognoses for water ingress are very difficult

to come up with at the pre-construction investigation stage,

continuous investigation focusing on this during tunnel

excavation is crucial to have control with the water conditions.

4 Investigations during excavation

As described in Sect. 2, the most difficult rock mass con-

ditions are often encountered at the deepest part of the

tunnel. Any uncontrolled major water inflow here may

have severe consequences. Systematic percussive probe

drilling by the drilling jumbo as illustrated in Fig. 8 is the

single most important element for safety. By applying

criteria related to inflow per probe hole on when to pre-

grout, the remaining inflow can be controlled and adapted

to preset quantities for economical pumping (a maximum

of 300 l/min km).

The Norwegian subsea tunnels are all drained structures,

mainly designed as illustrated in Fig. 13, and with heavy

rock support such as reinforced shotcrete arches or

Fig. 7 Longitudinal profile of the Atlantic Ocean tunnel. Thick lines are faults/seismic low velocity zones (with seismic velocity in km/s)

representing assumed weakness zones

Fig. 8 Principles of probe

drilling and pre-grouting.

Typical length of probe drilling

holes is 30 m, and the overlap is

typically 10 m


123

concrete lining (also drained) only where very poor rock

conditions are encountered. Pre-grouting is used for water

sealing only when required. This is based on economical

optimization, since pumping out minor volume of leakage

water may be less expensive than to seal it with pre-gro-

uting. The decision whether to grout or not is based on the

result from probe drilling and the experience that a corre-

lation exists between water inflow from probe drill holes

and water ingress into the tunnel. For medium long subsea

tunnels (up to 3–5 km), maximum inflow of 3–5 l/min for

one probe drill hole and a total of approx. 10 l/min for 4–5

probe drill holes have been commonly used limits for pre-

grouting. Since all major leakage sections will be grouted

based on this basic principle, and since many sections will

be dry, this normally will give a final inflow of consider-

ably less than 300 l/min km (as illustrated in Fig. 21).

The final decisions regarding grouting have to be taken

at the face, and to be regularly adjusted according to the

results and experiences from tunnelling. Follow-up at the

tunnel face by well-qualified engineering geologists and

rock engineers is of great importance.

For recent projects (i.e. Finnfast (Gilje 2009) and the

Karmsund tunnel, presently under construction) instru-

mentation of the drilling jumbo; measurement while dril-

ling (MWD) and drill parameter interpretation (DPI) have

been applied for predicting rock mass conditions, including

water, ahead of the tunnel face. An example illustrating the

potential of MWD for estimating water, fractures and rock

strength ahead of the face is shown in Fig. 9. In the upper

part of the figure, red and blue represent hard, competent

rock while yellow and brown represent weak rock, in the

middle red and yellow represent high and medium degree

of fracturing, respectively, and in the lower part blue rep-

resents water inflow (higher inflow the darker blue).

Use of MWD and DPI has a great potential for pre-

dicting rock mass conditions ahead of the tunnel face, but

the method is still at the development stage, and interpre-

tation of data is in some cases uncertain. Further devel-

opment of MWD/DPI based on systematic monitoring at

ongoing tunnel projects, such as at the Løren tunnel

described in another paper in this issue of RMRE (Høien

and Nilsen 2012), indicates that in the future MWD/DPI

most likely will become an important tool also for pre-

dicting water ingress in tunnels. As basis for the decision

on whether to pre-grout or not, measurement of water

inflow in probe drill holes as described above is, however,

still the preferred method.

5 Tunnelling experience

All subsea tunnels in Norway have been excavated by drilling

and blasting as illustrated in Fig. 10. This method provides

great flexibility and adaptability to varying rock mass con-

ditions and is cost effective. The 6.8 km North Cape tunnel

was considered for TBM, but the risk connected to the

potential high water inflow was considered too large.

Fig. 9 Use of MWD for predicting rock mass conditions ahead of the

tunnel face at the Karmsund subsea road tunnel (from Moen 2011)

Fig. 10 Drill and blast excavation with shotcrete and bolts for

immediate rock support in the Karmsund subsea road tunnel

B. Nilsen

123

A combination of fibre reinforced shotcrete and rock

bolting is most commonly used for rock support. In poor

quality rock, spiling bolts are used, and sometimes also

reinforced shotcrete ribs as illustrated in Fig. 11. The trend

is that shotcrete ribs (sometimes supplemented with con-

crete invert) are used in poor rock conditions instead of

concrete lining. An example of concrete ribs used in poor

rock conditions on a recent project is shown in Fig. 12.

All rock support structures are drained whether they are

made of cast-in-place concrete lining, shotcrete ribs or

shotcrete/rock bolting. The most commonly used inner

lining in Norwegian road tunnels is shown in Fig. 13.

Grouting, when required according to probe drilling, is

carried out as pre-grouting in drill holes typically 25–30 m

ahead of the face, and with 1–2 blast rounds of overlap.

This procedure has been successful even in the deepest of

the Norwegian subsea tunnels where grouting against water

pressures up to 2.5 MPa has been efficiently achieved withmodern packers, pumps and grouting materials. Grouting

pressures up to 10 MPa are today quite common with

modern grouting rigs as shown in Figs. 12 and 14.

In some cases, it has however been a great challenge to

deal with the high water pressure and difficult conditions

that may be connected to water ingress in the subsea tun-

nels. To illustrate this, two of the most difficult cases in this

context that have been encountered will be briefly dis-

cussed in the following; the Bjorøy tunnel and the Atlantic

Ocean tunnel.

5.1 The Bjorøy tunnel

The Bjorøy tunnel is relatively short and shallow (1,765 m

long with its lowest section 85 m below sea level) and is

located in traditionally favourable rock conditions (Pre-

cambrian gneiss). The sea floor above the tunnel is covered

with 5–10 m thick soil consisting of heavily consolidated

moraine, as sketched in Fig. 15.

Fig. 11 Principles of excavation through poor stability weakness

zones in the Frøya tunnel applying the spiling technique (based on

NFF 2008)

Fig. 12 Spiling, fibre reinforced shotcrete ribs and grouting prior to

excavation through zone of poor quality rock

Fig. 13 Principle sketch of inner water/frost lining commonly used

in Norwegian road tunnels (modified after Bollingmo et al. 2007)

Fig. 14 High pressure grouting rig in operation


123

No major challenges were expected for this project.

During construction, a fault zone representing very difficult

conditions was however encountered. Due to insufficient

refraction seismic investigations, this was quite unex-

pected. The fault zone, a shown in Fig. 15, was encoun-

tered under the sea at a rock cover of about 35 m and near

the deepest part of the tunnel. It was sub-vertical and

3–4 m wide, but crossed the tunnel axis with an acute angle

of about 25 , and thus gave a crossing length of 20–25 m. It

consisted of crushed rock mixed with clay, sand and coal

fragments. Based on dating of the coal fragments, it was

later interpreted as a Jurassic, tensional fault zone.

Thanks to continuous probe drilling that was also carried

out for this project, the fault zone was identified well

before excavating into it. Sealing and stabilizing measures

therefore could be done in time. To collect information of

the character of the zone, core drilling from the face was

carried out. Pieces of drill core, sand, coal fragments and

sections of core loss from the central part of the zone are

shown in Fig. 16.

The Bjorøy fault zone had very high permeability, and

blow out preventers as shown in Fig. 17 were required for

drilling of investigation holes and later holes for grouting.

Investigation and planning before tunnel excavation was

resumed took 4 months. Excavation through the zone at

70 m below sea level was a very time-consuming operation

involving extensive grouting, drainage, spiling, stepwise

excavation piece by piece, conventional bolting/shotcreting

and application of shotcrete arches, which were all required

in order to stabilize the situation and get through the fault

zone. In order to stop water inflow and to seal the zone, 243

tons of cement and 16 tons of acryl were required, and as

rock support 280 m3 of shotcrete and 160 m3 of concrete

for the invert, in addition to a very high number of rock

bolts were required before the zone was successfully

crossed after 23 weeks (Dahl Johansen 1995).

5.2 The Atlantic Ocean tunnel

The 5.7 km long Atlantic Ocean tunnel has a maximum

depth of 250 m below sea level (see longitudinal profile in

Fig. 7). The bedrock in the area consists of Precambrian

granitic gneisses of mainly good quality. Pre-construction

investigations as common for this type of project were

carried out, including conventional onshore engineering

geological mapping, core drilling, offshore acoustical

profiling and refraction seismic investigations. Based on

the latter, several low velocity zones representing faults/

weakness zones were detected under water, including

zones with seismic velocity as low as 2,500 and 2,800 m/s

near the lowest level of the planned tunnel as shown in

Fig. 7.

Based on overall evaluation of the rock mass conditions,

a minimum rock cover of 45 m was approved, but it was

realized that several of the low velocity zones under sea

might be quite challenging, and this was taken into account

in the planning of excavation and rock support.


the Bjorøy subsea tunnel (after

Dahl Johansen 1995)

Fig. 16 Result from core drilling through the central part of the

Bjorøy fault zone

Fig. 17 Blow out preventers in drill holes at the face of the Bjorøy

tunnel. White colour is chemical grout that has been seeping out of

fractures

B. Nilsen

123

At the end of February 2008, excavation had reached

Profile 6242 (see Fig. 18), located 2,380 m from the wes-

tern tunnel entrance. Mainly good rock conditions had been

encountered and only local, minor leakages requiring

grouting. Several nearby fault zones with seismic velocities

down to 2,800–3,100 m/s had been crossed without major

problems. These zones contained crushed rock and clay

gouge, and had little water leakage. Probe drilling, which

was carried out on a routine basis in the tunnel, indicated

poor quality rock in the 2.8 km/s zone near the bottom of

the tunnel (see Fig. 7), but little water leakage. Thus,

similar rock mass conditions as in the previous faults/

weakness zones were expected. As extra precaution, the

great water depth and limited rock cover taken into con-

sideration, grouting was however carried out in order to

seal the joints and also possibly improve the quality of the

zone material. Excavation was started with reduced round

length (3 m), shotcreting, systematic radial bolting and

installation of 6 m long spiling bolts.

The rock mass quality of the zone was found to be very

poor, and after blasting the reduced round length, there was

a tendency of small rock fragments coming down between

the spiling bolts. Attempts to stop this by applying shot-

crete were unsuccessful, and after a few hours a 5–6 m

high cave-in of the roof had developed, covering the full

tunnel width and the 3 m round length. Based on holes

drilled later, it was found likely that the cave-in progressed

about 10 m above the tunnel roof as illustrated in Fig. 18.

In order to stabilize the tunnel, excavated material was

filled up against the tunnel face. The area between the

tunnel roof and the fill material was sealed with shotcrete,

and concrete was pumped into the slide scar. Additional

rock support behind the unstable section was installed, and

an approximately 10 m long concrete plug was established

to seal the tunnel. This plug was completed 8 days after the

cave-in. Probe drilling through the plug indicated consid-

erable water leakage, and extensive grouting of the backfill

material and the surrounding rock past the slide scar was

required. Based on careful excavation/reduced round

lengths, shotcreting/radial bolting and spiling with self

drilling anchors, the tunnel face was re-established after

5.5 weeks at the same position as it was before the cave-in.

Core drilling through the weakness zone was carried out as

sketched in Fig. 18, and showed that the zone was more

than 25 m wide and had considerable water leakage.

Concrete lining was established past the cave-in area as

shown in Fig. 19, and resumed tunnelling was based on a

procedure including continuous pre-grouting, spiling,

excavation with reduced round lengths/piece by piece,

shotcreting/radial bolting and installation of reinforced

shotcrete arches. This process was very time consuming

due to extensive water leakages (up to 500 l/min in one

single drill hole) at very high pressure (up to 23 bar).

Installation of packers in the crushed rock mass was very

difficult and often required pre-installation of tubes in the

grout holes. In many cases, several round of pre-grouting

were required before a short advance of the tunnel face was

found to be safe. Tunnelling was continued approx. 20 m

ahead of the previous cave-in area from the west side. This

position was reached about 10 months after the date of the

cave-in. The rest of the fault zone was excavated from the

east side based on a similar procedure as described above.

Fig. 18 Cave-in situation approx. 225 m below sea level in the

Atlantic Ocean tunnel

Fig. 19 Excavation through the cave-in area of the Atlantic Ocean

tunnel


123

More than 1,000 tons of grout (mainly micro cement,

but also standard cement and polyurethane) was needed to

seal the leakages of the approximately 25 m wide fault/

weakness zone. After completion of the tunnel in Decem-

ber 2009, the total leakage was only 500 l/min (or 88 l/min

per km tunnel), which can be characterized as quite low for

this type of tunnels (Karlsson 2008).

6 Discussion and conclusions

As illustrated by the Bjorøy and Atlantic Ocean tunnel

cases, large water inflow is in some cases connected to

major fault zones. In other cases, and in fact even more

often, the largest water inflows are however not directly

connected to a major weakness zones, but rather to distinct,

continuous single joints or, in some cases, to the distal parts

of a weakness zone. This is most likely due to the high

content of low permeability gouge (clay) in the central part

of weakness zones.

Somewhat surprisingly, the main water ingress in subsea

tunnels is in many cases not under the sea, but under land

(in the onshore part of the tunnel). An example illustrating

this is the Ellingsøy tunnel, which was all excavated in

Precambrian gneiss. As shown in Fig. 20, the largest

inflows (up to 400 l/min in one single probe drill hole)

were encountered under land on one side, while the inflow

under the sea was less, and the other side under land had

only minor inflow. The bedrock above the tunnel has

mainly no or only very sparse soil cover. Under the sea,

about 25 % of the tunnel alignment is covered by more

than a couple of meters thick soil consisting mainly of

moraine. Maximum soil cover, in the middle of the fjord, is

about 40 m.

The example in Fig. 20 illustrates that a soil cover at the

sea bottom is no guarantee that large water ingress may not

occur below a relatively thick soil cover in a subsea tunnel.

In the Ellingsøy tunnel case, the soil is however moraine

with a relatively high permeability. For cases with a con-

tinuous, more than a few meters thick layer of low per-

meability soil (i.e. clay) at the sea bottom, the general

experience is that this would considerably reduce the water

ingress.

For several of the completed tunnels, there have been

indications that the magnitudes and orientations of rock

stresses have distinct effect on water ingress (i.e. steep

joints oriented perpendicularly to the minor principal stress

give largest ingress). Documentation of the rock stress

situation in the subsea tunnels has however been insuffi-

cient for any final conclusion on this issue.

An interesting experience based on the completed pro-

jects is that the water ingress is in many cases decreasing

with time. As illustrated in Fig. 21, the total water ingress

in some of the Norwegian subsea tunnels is reduced by

more than 50 % in less than 10 years. This is believed to be

caused by sedimentation/sealing of water conducting dis-

continuities by particles that are washed out from soil at the

sea bottom and/or filling in joints. As can be seen in

Fig. 21, there is no unambiguous connection. What will

happen depends on many different factors such as the

Fig. 20 Water inflow registered by probe drilling during excavation of the Ellingsøy subsea tunnel. Black sections indicate inflow in one single

probe drill hole [50 l/min, inflows [100 l/min are indicated separately

Fig. 21 Development of total

water ingress in some

Norwegian subsea road tunnels

(based on Melby et al. 2002)

B. Nilsen

123

character of sediments at the sea bottom and in filled joints,

water pressure, permeability relationships, etc. In some

cases, the site conditions as shown in the figure give no

reduction of water ingress with time. Prediction of the

potential development is thus difficult.

Regarding prediction of water ingress, the experience

from the completed Norwegian subsea tunnel projects is

that standard pre-construction investigations do not offer a

reliable basis for evaluation. The parameters with basically

the highest potential for reasonable prediction, seismic

velocity and Lugeon value, have been found to be of

limited value for predicting water ingress. For the former,

this can be explained by the fact that many different rock

mass parameters, of which the majority are not ground

water related, may affect seismic velocity. For Lugeon

testing, the incapability may be explained by the basic

principle of this test, which is to measure the capacity of

the rock mass to accommodate water rather than measuring

the potential for water ingress. Because many of the frac-

tures and other discontinuities in the rock mass are not

water conveying, and/or are not in contact with the leakage

reservoir, a high Lugeon value does not necessarily mean

that there will be large leakage into the tunnel. Lugeon

testing is still being used as a tool for preliminary inter-

pretation and estimation for core drilling during pre-con-

struction investigations, but during tunnelling, as decision

tool regarding pre-grouting, monitoring of water flow from

probe drill holes is found more reliable.

The main conclusion is therefore that investigation, eval-

uation and decision regarding water ingress and sealing

requirement mainly have to be done during excavation.

Standard probe drilling ahead of the tunnel in this connection

is the most important investigation method. For further

investigation of difficult rock mass conditions, core drilling

may be a valuable supplement. MWD/DPI in this connection

is believed to have a great potential for the future. In any case,

continuous follow-up at the face by experienced engineering

geologists and a high state of readiness are crucial for coping

with the challenges that may be encountered regarding sta-

bility and water ingress in deep subsea tunnelling.

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Documents

Characteristics of Water Ingress in Norwegian Subsea Tunnels