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GENERAL ASPECTS OF TUNNELGENERAL ASPECTS OF TUNNEL

CONSTRUCTION CONSTRUCTION

ITA/AITESITA/AITES – – Training CourseTraining CourseTUNNEL ENGINEERINGTUNNEL ENGINEERING

ITA/AITESITA/AITES

5th of May 2005 11/51/51

PreparedPrepared by by ““S. PelizzaS. Pelizza””

IstanbulIstanbul -- 20052005



General aspects of tunnel construction General aspects of tunnel construction

The instability conditions that can occur in and around a tunnel

depend on the type of soil or rock in which the excavation is being

carried out and on the natural (preexisting) state of stress in the

rock mass.

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Examples of collapsesExamples of collapses

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The place where it is necessary to act to obtain the stability of the

void is always the last section of the tunnel that has just been

excavated and where neither the sides nor the face have still been

supported.

The just excavated length must remain stable for the time

necessary to carry out all the removal and support installationworks:

the tunnel is bu i l t at the face

This length is usually known as “unsupported span” and the

corresponding stability time is known as “characteristic time” or

“self-bearing time”

The unsupported span depends on the geotechnical quality of the

rock mass and on the stress state around the void.

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Bieniawski, 1987Bieniawski, 1987

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Serena Tunnel,Serena Tunnel, ItalyItaly

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General aspects of tunnel construction General aspects of tunnel construction 77/51/51







Installation of bolts in different rock typesInstallation of bolts in different rock types ShotcreteShotcrete

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Ground geotechnical properties

Natural state of stress

Shape of the tunnel section

Size of the tunnel section

Construction methods

Underground water

The stability conditions of a tunnel are controlled by :

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Ground geotechnical propertiesThe stability conditions are defined both by the strength of the

rock in comparison with the natural and induced tension around

the void and by the material deformability, that can show a

elastic, brittle, plastic, soft strengthening, etc. behaviour;

Natural state of stressThe in-situ tensions locked in the rock mass can depend from

the geostatic and gravity forces (nature of the material anddepth of the excavation, i.e. overburden); tectonic;

geostructural-morphological conditions.

The opposition to the rock mass tension undertaken by the soil

determines the stability conditions for the tunnel;

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Shape of the tunnel sectionThis influences the rock mass tension concentration around the

void (edges are points where the tension concentration is

maximum). The excavation section shape can vary from

rectangular (best use of the free section in road and railwaytunnels) to circular (more homogeneous induced tensions), up to

polycentric (in order to get the best advantages from both the

approaches);

Size of the tunnel sectionThis influences the ask for resistance by the supports, both short-

term and long-term ones;




Construction methodIt’s not a joke! This influences the scheduling for the support

installation or reinforcement execution (that can be prior or in

concomitance of the excavation), that hence influence the

convergence (that has to be mastered) and therefore the askfor resistance by the supports (short-term or long-term ones);

Underground water

It is a fundamental parameter for the tunnel construction, thathas to be studied in a deep detail. The interference of the

excavations with the underground water shows many aspects:

natural and induced tension field modification, soil alteration,

nasty interferences on the working operations andenvironmental impact.





Ground geotechnical quality

(Hoek and Brown,1980)

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Loose ground

The tunnel must be stabilized using supports

generally stiff and continuous

Often presupports and/or ground

improvements techniques and are necessary

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ClayClay SandSand

collapse shapecollapse shape

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Hard rock mass with few discontinuities

The tunnel is stable.

Local supports may be necessary.

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F t d k ith k bl k t t bl




Fractured rock mass with rock blocks not stable.

The instability is linked to key blocks.

Systematic supports are necessary.

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Rock mass with a high stress condition which can

plasticize and fracture the rock mass

Continuous supports to control the displacements

of the tunnel are necessary.

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Apart from surface tunnels in loose soils whose behavioris determined by cohesion and internal friction o the

ground, the most frequent and complex case that

presents a great variety of aspects is that of tunnels inmaterials which undergo a plastic conditions due to the

stress re-distribution after the opening of the void.The

opening of the tunnel determines the movement of the

rock mass towards the void and the excavation andstabilization methods are to be designed in such a way as

to put the deformations under control .

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LunardiLunardi, 2000, 2000

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The displacement and the stability of a tunnel is always




p y y

a tri-dimensional problem

and as such it should always be studied and analyzed

Hoek, 2002

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The excavation of a tunnel is made up of a set of operations

(excavation, mucking and stabilization of the void with

supports and/or reinforcement) that are organized in such a

way as to produce the greatest length of completed tunnel in

the shortest possible time, using the smallest possiblenumber of operations and of stabilization structural elements,

while guaranteeing the permanent stability of the work,

without creating dangerous conditions for the workers andwithout creating irreparable damage to the objects (both

natural and man-made) that exist around the work area.

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Conventional cyclic method using Drill and Blast The excavationConventional cyclic method using Drill and Blast The excavation can becan be

carried out full face orcarried out full face or parzializedparzialized faceface

Conventional cyclic method using punctual excavating machinesConventional cyclic method using punctual excavating machines

((roadheader roadheader , high impact hammer, mechanical excavator, etc.). The, high impact hammer, mechanical excavator, etc.). Theexcavation can be carried out full face orexcavation can be carried out full face or parzializedparzialized excavation, with orexcavation, with or

without preventive reinforcement of the groundwithout preventive reinforcement of the ground

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Full face mechanized excavation, using TBM for the excavation of Full face mechanized excavation, using TBM for the excavation of tunnels in rocktunnels in rock

Example of an EPB machine: 1. Cutting wheel , 2. Drive unit , 3. Push cylinder, 4. Air lock , 5. Screw

conveyor, 6. Erector , 7. Screw conveyor gate , 8. Segment handler , 9. Segment crane – 10. belt

conveyor

Full face mechanized excavation method, using mechanized shieldFull face mechanized excavation method, using mechanized shield

s ands and

with pressure at the face for the excavation of tunnels in soilwith pressure at the face for the excavation of tunnels in soil

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C ti l t lli th d Full face mechanized tunnelling




Waste rock

extraction

Tunnel already supported L

L’

Cutting head

Continuous wste

rock exctraction

Head thrust

(approaching force)

Self supporting or already lined tunnel

Conventional tunnelling method Full face mechanized tunnelling

L’ = advance per excavation cycle or pull. It has to be at least

as long as the free span, if natural stability for a given time(characteristic time or self-sustaining time), needed for the

cycle operations, is present

Operations for the excavation cycle:

-Rock destruction for the advance L’ with explosive or with

hammer/roadheader -Spoil extraction, that has to be completed after each advance;

-(smoke clearing, in case of use of explosive);

-scaling;

-spoil extraction (excavated material has to be brought out

from the tunnel);

-void lining (supports installation).

The advance is performed in two steps:

-Active stroke of the cutting head for the excavation (and

the spoil extraction) usually1,4 ÷ 1,8 (2,0) m;

-Advance of the whole machine when the cutting head has

been stopped.

The free span coincides with the last stretch of the tunnel

where the rock is being excavated and the supports cannot

be installed yet (very variable length, depending on the

TBM type: open, shielded, with or without face

counterpressure)

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Installation of bolts in different rock typesInstallation of bolts in different rock types ShotcreteShotcrete

3030/51/51




When the unsupported length is too short it can beWhen the unsupported length is too short it can be

changed through various methods:changed through various methods:

to reduce of the excavation section into smaller portionsto reduce of the excavation section into smaller portions

forfor parzialisedparzialised face. In smaller face the control of theface. In smaller face the control of the

stability is easier, the required characteristic time isstability is easier, the required characteristic time is

shorter, smaller amount of waste rock is to be removedshorter, smaller amount of waste rock is to be removed

and fewer supports have to be installed;and fewer supports have to be installed;

to use preventive reinforcement of the rock mass aheadto use preventive reinforcement of the rock mass aheadof the face;of the face;

to improve the rock mass properties;to improve the rock mass properties;

to apply a counter pressure at the face.to apply a counter pressure at the face.

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to reduce of the excavation section into smaller portionsto reduce of the excavation section into smaller portions

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to improve the rock mass propertiesto improve the rock mass properties

The main problem when tunneling through difficult geotechnicalThe main problem when tunneling through difficult geotechnicalconditions with conventional methods is to control of conditions with conventional methods is to control of deformation:deformation:

without support or reinforcement the ground plasticizes and tendwithout support or reinforcement the ground plasticizes and tends to sinks to sink

into the opening (fall of ground from the upper part of the tunninto the opening (fall of ground from the upper part of the tunnel face,el face,

tunnel face extrusion, tunnel face failure). This phenomenon istunnel face extrusion, tunnel face failure). This phenomenon is calledcalled““decompressiondecompression””..

To stop this decompression is necessary to useTo stop this decompression is necessary to use ““pre confinementpre confinement

techniquetechnique””(defined as any action that favours the formation of an arch(defined as any action that favours the formation of an arch

effect in the ground ahead the tunnel face).effect in the ground ahead the tunnel face).TThe following types of reinforcement can be identified:he following types of reinforcement can be identified:

-- improvementsimprovements;;

-- reinforcementsreinforcements;;

-- prepre--supportssupports;;-- drainage systemsdrainage systems

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t i th k ti




to improve the rock mass propertiesto improve the rock mass properties

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Serena Tunnel,Serena Tunnel, ItalyItaly

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t ti i f t h d f th ft ti i f t h d f th f




to use preventive reinforcement ahead of the faceto use preventive reinforcement ahead of the face

Courtesy GEODATA S.p.A.,Courtesy GEODATA S.p.A., TorinoTorino

3737/51/51

t ti i f t h d f th fto use preventive reinforcement ahead of the face





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4040/51/51

to apply a counter pressure at the faceto apply a counter pressure at the face





BOBO--FI High speed railway line (Italy)FI High speed railway line (Italy) -- CavetCavet

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MaidlMaidl et al., 1994et al., 1994

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Ground and water pressure actingon the face Transducer to control the pressure of

backfilling injection

Pressure transducers inside the

bulk chamber

Scale

Counter-pressure inside the bulk chamber=

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to apply a counterto apply a counter

h ft th f




pressure at the facepressure at the face

MaidlMaidl et al., 1994et al., 1994

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MONITORINGMONITORING

Monitoring has an essential role in tunneling. It is theMonitoring has an essential role in tunneling. It is the

only mean, in a context of great variability andonly mean, in a context of great variability and

uncertainty of allowing the excavation project of a tunneluncertainty of allowing the excavation project of a tunnel

to be adjusted in an objective way during workto be adjusted in an objective way during work

procedures: the tunnel design and the tunnelprocedures: the tunnel design and the tunnel

construction cannot exist without monitoring. Monitoringconstruction cannot exist without monitoring. Monitoring

is the mirror that reflects the soul of a tunnel.is the mirror that reflects the soul of a tunnel.

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General aspects of tunnel construction General aspects of tunnel construction 5050//****







ITA/AITESITA/AITES



5353//5151

Clause de non-responsabilité pour les rapports des groupes de travail de l'AITESL’Association Internationale des Travaux en Souterrain (AITES) publie ce rapport, conformément à ses Statuts, pour faciliter les échanges d’informations afin :

d’encourager l’utilisation du sous-sol au profit du grand public, de l’environnement et du développement durable;

de promouvoir les progrès dans la planification, le projet, la construction, l’entretien, la réhabilitation et la sécurité des tunnels et de l’espace souterrain en

rassemblant et confrontant les informations, ainsi qu’en étudiant les questions qui s’y rapportent.

Cependant, l’AITES décline toute responsabilité en ce qui concerne les informations publiées dans ce rapport.

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ne constituent pas un avis professionnel or juridique (si vous avez besoin d’avis spécifiques, consultez toujours un professionnel dûment qualifié).

Disclaimer for the reports of ITA working groups

The International Tunnelling Association (ITA) publishes this report to, in accordance with its statutes, facilitate the exchange of information, in order:

to encourage planning of the subsurface for the benefit of the public, environment and sustainable development

to promote advances in planning, design, construction, maintenance and safety of tunnels and underground space, by bringing together information thereon and

by studying questions related thereto.

However ITA accepts no responsibility or liability whatsoever with regard to the material published in this report.

This material is:

information of a general nature only, which is not intended to address the specific circumstances of any particular individual or entity;

not necessarily comprehensive, complete, accurate or up to date;

sometimes collected from external sources over which ITA services have no control and for which ITA assumes no responsibility;

not professional or legal advice (if you need specific advice, you should always consult a suitably qualified professional).

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