Caso 1 - Tunel Sismico Bolu

5/26/2018 Caso 1 - Tunel Sismico Bolu

1/4

History

The Bolu Mountain Crossing is midway

between Ankara and Istanbul, and repre-

sents the most challenging section of the

motorway construction. Along this 20 km-

long stretch, four important viaducts and a

long tunnel are under construction.

The Bolu tunnel is a twin-tube motor-

way tunnel of about 3 km length, accom-

modating three lanes per tube, linking the

western Asarsuyu valley to the eastern

Elmalik village, on the Ankara side. The

original design featured five support class-es in the tunnel, and two at the portals,

with an excavation area ranging between

190 sq m and 260 sq m. The original static

design was by Geoconsult GmbH of

Saltzburg, Austria, and, for the worst rock

condition, involved preliminary excavation

and backfill of bench pilot tunnels, a three-

layer lining, and a deep monolithic invert.

Excavation of the tunnel started in

1993, and, almost immediately, problems

were encountered with clays. When the

Duzce earthquake occurred in 1999, a

stretch of about 350 m of tunnel collapsed

behind the eastern faces, and major

damage was done to the lining and invert

of both tunnels. Consultants Lombardi SA

were brought in to analyze the seismic

loads induced by the earthquake, which

originated at the North Anatolian Fault.

These analyses examined the depth, direc-tional effects, soil amplifications and dis-

tance from the seismic source, and a panel

of experts was set up to study the results.

Active Faults

Two active faults were recognized

along the tunnel alignment: the Zekidagi

and Bakacak faults (Barka-W Lettis &Associates).

The Zekidagi fault dips at almost 90

degrees, is approximately 6 to 8 km-long,and possibly intersects with the tunnel

alignment at nearly right angles, around

chainage 62+430 in the left tube and

BOLU, TURKEY

ROCK & SOIL REINFORCEMENT 115

Atlas Copco Boomer drilling over the

face for forepoling.

Plan of Astaldi section of the Istanbul-

Ankara highway.

Seismic Tunnelling at Bolu

Overcoming NaturalDisasterThe attempt in the mid-nineties attunnelling through the Bakacak Faultnear the Turkish town of Bolu wasaborted following the massive earth-quake in November, 1999. Thiscaused the collapse of a section ofmined tunnel, which had been exca-vated with preliminary primary sup-port of soil nails and shotcrete.

The overall design has beenrethought, and the tunnel is nowagain under construction. Seismic

principles have been applied to thisproject, which is crucial to comple-tion of the Gumusova-Gerede sectionof the important North AnatolianMotorway linking Ankara and Istan-bul. The design criteria have definedthe fault crossing strategy, and thepractical solutions involve the exten-sive use of Atlas Copco MAI SelfDrilling Anchors (SDA) as primarysupport.


2/4

52+350 in the right tube, over a length of25 m to 30 m. It has a potential for small

future displacement in the range of 0.15-

0.25 in an earthquake of magnitude 6 to

6.25. This section of tunnel was lined

according to the original design, and no

particular problems were experienced

crossing the fault, although high deforma-

tions were recorded.

The Bakacak Fault has been identified

as a secondary fault in the step-over region

between the two major North Anatolian

Fault (NAF) branches in the Bolu region.This clay fault exhibits low potential for

right lateral strike-slip displacements. It is

some 10-45 km-long, composed of several

segments ranging from 3 to 5 km-long, and

rupture displacements of up to 50 cm can

be expected in an earthquake of magnitude

6.25 to 6.5.

Two likely traces of the Bakacak fault,

which dips at 40 degrees, were identified

crossing the Bolu Tunnel between

chainage 62+800 and 62+900 at the left

tube, and 52+730 to 52+800 at the right

tube, over a distance of about 100 m. This

is precisely the zone where excavation was

proceeding at the time of the earthquake.

Crossing Active Faults

Basically, two strategies are feasible to

mitigate the seismic risk induced to tunnels

by ruptures of active faults across the

alignment. These are commonly referred to

as over-excavation, and articulated design.

In the first case, the tunnel is driven

through the fault with an enlarged cross

section. A double lining is installed, and

filled by a porous material, such as foamconcrete. If there is a fault rupture, the

clearance profile is guaranteed by the gap

between the outer and inner linings. This

manner of protection, commonly used for

metro projects, is limited by the width of

the cross section that must be excavated,

and will be most effective when a fault

rupture is concentrated within a few

metres.

The articulated design strategy, on the

other hand, reduces the width of the lining

segments, leaving independent sectionsacross the fault, and for a distance

beside the fault. In a fault rupture, the

movement is concentrated at the joints

linking the segments, containing any

damage in a few elements, without uncon-

trolled propagation.

The maximum length of any single ele-

ment depends on several factors, such as

width of the cross section, expected move-

ment of the fault, compressibility of the

surrounding soil, and element kinematics.

Articulated design was selected as themost appropriate for the large cross section

of the Bolu tunnel, and for the excavation

geometry that had already been defined.

Design Philosophy

When the Bakacak fault was recognized as

active, almost one year after the Duzce

event, the restoration of the original tunnel

was almost complete, and the shape and

type of the cross section adopted was

already defined. The bench pilot tunnels of

the original excavation had already been

backfilled.

BOLU, TURKEY

116 ROCK & SOIL REINFORCEMENT

Standard cross-section of Bolu tunnelshowing massive support.

Shotcrete operations underway in the

top heading.


3/4

The segments geometry was defined by

considering a ratio between length and

width of the tunnel segment equal to one

third, resulting in an element length of

about 5 m. This geometry kept the load on

the single crown segment below an accept-

able threshold value.

For practical reasons, the length of the

segments was reduced to 4.4 m, with a

50 cm joint gap at invert. This facilitated

retention of the original modular reinforce-

ment cage.

Following a fault rupture, the tunnel

will act longitudinally as an embedded

beam, whose extremities are displaced by

the lateral offset of the fault. The assump-

tion made, justified by the geologists, is

that a rupture will be uniformly distributed

across the fault boundaries, with horizontal

displacement. Therefore the shear strain inthe fault soil can be reasonably assumed as

the ratio between expected offset and

width of the fault at tunnel level.

Up to rupture of the joints, the tunnel

will be sheared and bent by the soil as an

embedded beam. Once the joints shear

resistance is attained, each segment will be

free to move independently, according to

external loads.

The maximum acceptable shear resis-

tance of the joint has been defined on an

equivalent elastic model, with soil mod-elled as springs acting in compression. A

displacement is gradually applied to the

extremities, and the shear stiffness of the

joints is designed so as to reach the shear

failure of the joint before lateral overload

of the element cross section, or bending

failure at extremities.

Reinforcement and Joints

Across the fault zone, different support

measures have been adopted. Of these, themost important is an 80 cm-thick concrete

40 N/sq mm prefabricated concrete slab

intermediate lining to be installed between

the primary lining and the inner lining. The

reinforcement bars have been placed only

in the inner (final) lining and at invert,

while the shotcrete and intermediate lin-

ings have been fibre-reinforced.

The primary aim of the reinforcement

design is to provide a high ductility to the

lining. The allowable rotation has been

estimated, and compared to the estimated

rotation for the load conditions. This was

achieved by introducing stirrups at shear,

keeping the spacing below 30 cm, and also

by introducing a light dosage of steel fibres

in the concrete mix, or applying an equiva-

lent double mesh layer. These measures

were installed within the fault, and up to a

distance of 30-40 m from the fault borders.

The joints, at 4.2 m spacing, have been

detailed to prevent soil squeezing between

the segments, and to bridge the static soil

pressure to the surrounding elements, but

opposing a sufficiently low shear resis-

tance in the event of fault rupture.To provide ring closure of the joint at

the invert, a 0.4 m-thick fibre reinforced

shotcrete beam is applied to bridge the

gap. At the crown, the regular 40 cm-thick

shotcrete preliminary lining has been

assessed as sufficient.

The 50 cm-wide joint is filled by two

layers of concrete blocks, with a 10 cm

low density PS layer in between. A water-

proofing membrane is installed below the

concrete block slabs and the invert.

In general, at the crown, three levels oflinings are installed: a shotcrete lining, an

intermediary lining of poured concrete,

and a reinforced final lining. The water-

proofing membrane bridges the seismic

joint gap between intermediary and final

lining. The joint opening in the final lining

has been enlarged to 70 cm, and the gap

will be covered by a steel plate, for the

purposes of ventilation and fire resistance.

The backfilled bench pilot tunnels were

heavily reinforced to provide sufficient

abutment to the crown loads during the

excavation. These beams cannot be inter-

rupted while excavating, so the cutting of

BOLU, TURKEY

ROCK & SOIL REINFORCEMENT 117

Installing prefabricated concrete slab

intermediate lining.


4/4

the joint in the section can only be executed

once the invert is in place.

Excavation and Support

The Bolu tunnel has been advanced on a

new alignment, which diverts around the

collapsed section. It is being driven from

newly established faces within the aban-

doned tunnel on the Istanbul side. A

150 m-long cut-and-cover section was

completed at the Ankara portals before

excavation work could commence from

this end.

The weathered, faulted amphiboliterock, with up to 140 m cover, is broken up

by a Krupp hydraulic hammer mounted on

a Cat 235 excavator, then loaded into road

tipper trucks. The 7 m-high top heading is

opened using 30 x 6 m-long forepoles over

the crown, under which three pieces of the

5-piece steel arches are set at 1.1 m

intervals. Then 20 off, 12 m-long anchors,

each comprising 3 x 4 m lengths of Atlas

Copco MAI SDA, are drilled in and grouted

using an Atlas Copco Boomer drillrig. The

roof and sides are given a 40 cm-thick

application of steel fibre reinforced shot-

crete, and a 50 cm-thick steel bar reinforced

shotcrete temporary invert is installed.

The bench is then advanced 2.2 m at

each side, and the legs of the steel arches

are installed, together with bolts and shot-

crete. Two incremental advances of 4.4 m

allow the invert to be excavated 5 m-deep

over the full width of the heading, and this

is filled with mass concrete with two pre-

fabricated steel reinforcement cages. A pur-

pose-built, self-propelled stage conveyor is

used to transfer the concrete from the fleet

of 8 cu m mixer trucks. The invert concret-ing is maintained within 25 m of the face.

The total excavated area of the tunnel is

160-200 sq m. Where the rock is particu-

larly poor, a 60 cm-thick concrete slab

intermediate lining is installed, and the

annulus backfilled with concrete. This is

followed by a mass concrete in-situ lining,using 150 sq m x 13.5 m-long self pro-

pelled formworks. The final lining opera-

tion is kept within 75-85 m of the face, to

ensure permanent support as early as pos-

sible. Concrete is supplied from two plantson site with 80 cu m/h output capacity,

backed by a 350 t cement storage silo.

Where necessary, very-heavy lattice

girders are placed as temporary support,

and these are cut away as soon as suffi-

cient permanent support is in place.

The first tube breakthrough is scheduled

for August, 2005, with the second follow-

ing before the end of the year.

The finished twin-tube tunnel will

accommodate three lanes of traffic in each

direction, with vehicle cross passages at500 m intervals.

Acknowledgements

Atlas Copco is grateful to the managementof the Bolu project for permission to visitthe site, and to Olivio Angelini, GaetanoGermani and Aziz zdemir of Astaldi fortheir help and assistance in preparation ofthis article. Reference is made to Designand Construction of Large Tunnel Through

Active Faults: a Recent Application byM Russo and W Amberg (LombardiEngineering), and G Germani (Astaldi).

BOLU, TURKEY

118 ROCK & SOIL REINFORCEMENT

General view of the Bolu tunnel face

with invert pouring underway.

Heavy steel reinforcement of the 5 m-deep concrete

invert.

Documents

Caso 1 - Tunel Sismico Bolu