Dr Manoj VermanTunnelling & Rock Engineering ConsultantChallenges of TunnellingA Peep Into The EXCITING WORLD OF TUNNELLINGChallenges of Tunnelling A Peep Into The Exciting World of Tunnelling

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Challenges of TunnellingA Peep Into The Exciting World of TunnellingDr Manoj VermanPresident, Indian National Group of ISRMPresident, International Commission on Hard Rock ExcavationVice President, Indian Society of Engineering Geology

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Let the journey begin.

www.sammode.com

Sardar Sarovar Project, Gujarat

Avtar Kaul (panoramio.com)Getty ImagesAccess tunnel to underground powerhouseUnderground powerhouse

The Himalayas, The Andes & The Alps The most difficult tunneLling media in the world

HimalayasAlpsAndes

Tunnel BehaviourTunnel behaviour is influenced by:Insitu and induced stressGeological structure/jointing, Rock mass strength, andGroundwater conditions.

Different behaviours,different design solutions

Generally Favourable Tunnelling Conditions

Increasing Structural Instability Due to Lack of Confining Stress

Structurally Controlled ConditionStructurally controlled failure in strong rock gravity driven blocks can be controlled by rock bolting

Structurally Controlled ConditionFailure of a wedge in the roof of a 12m span tailrace tunnel for the Rio Grande project in Argentina

Caving in A tunnel in J&K

Increasing Spalling with Higher Stress Level

Brittle conditionHighly stressed massive rock Spalling and slabbing can be controlled by combinations of rockbolts and mesh

Rock burst in gotthard base tunnel, Switzerland (stress driven failure)

Rock burst at tunnel face (stress driven failure)

Brittle conditionMaximum boundary stress / Uniaxial Compressive strength Spalling descriptionsmax/sc 0.9Minor to moderate spallingsmax/sc 1.2Severe spalling occurssmax/sc 1.6Spalling takes the form of rockbursts in hard brittle rock

Increasing Squeezing Problem with Increasing Stress Level

Squeezing conditionSqueezing conditions in weak rock masses failure zone around a tunnel generally requires the installation of passive support (i.e. concrete lining)

> 2m closure

Squeezing in st martin la porte adit to turin-lyon tunnel

Squeezing in head race tunnel of chameliya hydropower project, nepal

Squeezing in head race tunnel of chameliya hydropower project, nepal

Squeezing Condition

Yacambu-Quibor water transfer tunnel, VenezuelaNathpa Jhakri headrace tunnel, IndiaMucha highway tunnel, TaiwanOld mine tunnel, AfricaDriskos highway tunnel, GreecePlot from Hoek and Marinos (2000)

Tunnel Instability ModesTunnel ConditionAnalysis MethodsStress analysis (RocSupport, Phase2, RS3)Rock support interaction analysis Installation sequence Tunnel strain evaluationStereonet methods for joint projections (DIPS)Wedge analysis (Unwedge)Stress analysis to determine depth of failure (Phase2, RS3)Stress analysis that can predict complex instability modes (Phase2, RS3)

Typical Challenges(a) Cover up to 2000m (>50MPa confining stresses)

Maintaining Access

Avalanches, bad roads, landslides, rockfalls, supply problems, payment problems etc

Water under Pressure

Often also hot 50-60C

Ingress of Silt Laden Water at Tunnel Face

Rock Temperature

Faults & rock strength & the two ends of the rock competence scale Weak Ground Squeezing

Competent Ground BurstingGraphite SchistQuartzite

faultsFaults are fractures in crustal strata along which rocks have been displacedThe amount of displacement may vary from only a few tens of millimetres to several hundred kilometresIn many faults, the fracture is a clean break; in others, the displacement is not restricted to a simple fracture, but is developed throughout a fault zone

Fault in strata of the Limestone Group, Lower Carboniferous, near Howick, Northumberland, England

Kaliasaur the famous landslide (on road from Srinagar to Rudraprayag, UttarakhandKaliasaur - Crushed and broken quartzite rock mass associated with the ENE-WSW sub-vertical fault system. Rock mass appears tectonically disturbed and completely broken.

Kaliasaur the famous landslide

faults

Completely crushed material in a fault zone near Devprayag, Uttarakhand

Himalayan fault zonesIn general, the fault zones consist of a highly deformed fault core (core zone) and of a surrounding and very fractured damage zoneIn these fault zones, water circulation can also trigger weathering processes due highly fractured rocks, generating gouge material (clayey sub-products)The expected thickness of the core zone generally varies from 1 to 10m.

Toothpaste Fault(mylonitized & granulated

Mud flow Conditions50m back from face100 m mudflow along tunnelSevere Disruption to already installed supportConditions much easier WITHOUT WATER

Graphitic Schist Fault Zone > 9 months delay ~ severe water

Umbrella Forepoling in progress through 1km wide Daj Khad Fault Zone (nathpa-jhakri project)

THE HIMALAYASComplex Faults

Toothpaste Fault(mylonitized & granulated gneiss)100 m mudflow along tunnelSevere Disruption to already installed supportConditions much easier WITHOUT WATER

Closure in excess of 2mVarious Multi-Drift Methods proposedSome utilized

One of the Faults

Frequently changing geology and other conditions Sectorise the tunnelSeptember 20, 201546

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Wedge instability RockfallSupport class A B C1Main design actionMitigationsConfinement of rock wedgesBolts and fibre-reinforced shotcreteControl of water pressure/inflowDrainages in advanceLong term stabilityConcrete final lining

Examples of Support Types

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48Rockfall/caving Severe caving (faults,..)Support class C2 F

Main design actionMitigationsIncrease self-supporting capacity Forepoling umbrella pipesStabilisation of tunnel face (F)Pre-consolidation by fibreglass elementsControl of water pressure/inflowDrainages in advanceRadial reinforce and confinamentLattice girders/Steel ribs, fbr shotcrete, boltsLong term stabilityConcrete final lining with invert (F)

Examples of Support Types

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HazardWedge instabilityDescriptionGravitational fall of rock blocks isolated by the discontinuity network (weight force>shear strength)InstabilityGenerally sudden, short term instabilityEvolutionFrequently an equilibrium condition is reached, but this may be after fall of other wedges MitigationPreventive bolting, reduction of round length,..

V-Unstable wedges (2/6)Unstable Wedges

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K2

K1

K3Unstable Wedges

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Potential wedge instability from tunnel face

Limit equilibrium analysis

Unstable Wedges

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Technical solution at the face:Inclined Swellex MN16 dowels, L=6mUnstable Wedges

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HazardCavingDescriptionGravitational fall of portions of fractured rock mass or weakly cohesive soils, eventually alimented by the presence of waterInstabilityGenerally sudden, short term instabilityEvolutionThe complete collapse of the face may be reached, with concurrent caving of tunnel crown contour (chimney formation)MitigationDrainages, preconsolidation of the core, prearmature/preconfinement of contour excavation..

Caving

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CavingA tunnel in USBRL Project, J&K

Technical solution for face instability:

d

- Drainages (d)fb

- cemented fibreglass (fb) u

u

- pipe umbrella arch (u)Caving

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Collapse of the face intercepting suddenly unforeseen water bearing, very fractured and weathered rock mass (H=30m)Vispa tunnel (Italy)

Caving

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Technical solution 1 (>silty sands):Menaggio tunnel (Italy)VI-Caving (10/15)- Drainages (d)d

- Protective canopy: jg+ pipe umbrella (u)jg+u

- Jet grouting (jg) at the facejg

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Menaggio tunnel (Italy)

Technical solution 2 (>mix condition):VI-Caving (11/15)- Drainages (d)d

- Protective canopy: jg+ pipe umbrella (u)jg+u

jg+u

- face reinforcement by jg/ cemented fibreglass fb (lower section in silty clay) jg/fb

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VI-Caving

Prevalent clayey - siltPrevalent silty - sands

Jet-groutingFibreglass

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D&B vs. Machines

NOTHING IS A PANACEA

Thank you