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Introduction to Bridge and Tunnel Engineering

10/23/2013 1Prem N. Bastola

introduction

• A structure constructed over an obstacle to provide the passage 

• Road bridge‐movement of traffic

• See NRS 2045

• Cross drainage structure span greater than 6m

• In worst cases culverts can go up to 8m

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Characteristics of ideal bridge

• Line of bridge should be centered to the line of approach as far as possible

• It should be leveled

• Sufficient width 

• Safe for standard load

• Adequate underway width and height

• Strong foundation 

• Economical 

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Ideal location 

• Straight reach of river 

• Steady regime of river

• Narrow and well defined channel

• Rocky and non erodible foundation 

• no sharp curves in the approaches 

• Easy construction‐ no excess works

• Least drainage works

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Factors affecting  bridge type

• Strong enough• Sufficient • Economical• Aesthetic• Economy in overall construction • Span • Topography and soil condition • Funds available• Traffic types and intensity

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Classification 

• According to span• Minor bridge‐more than 6m span and upto 20m in length

• Medium‐ Span less than 20m and total length above 20m

• Major – span length greater than 20m• According to loading• Major bridge‐ IRC class AA loading• Medium‐ IRC class A• Temporary‐ IRC class B eg timber bridge

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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• According to structure• RCC T or simply supported• Cantilever• Double cantilever• Arch • Suspension • Cable stayed• Steel• movable

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• Type of material 

• Timber

• RCC

• Masonry

• Steel

• Floating eg boat

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Components parts

• Foundation for the abatements and piers or towers

• Abutment and piers

• River training works like revetment for slopes, aprons for bed u/s and d/s guide bunds

• Approaches to the bridge to connect the road

• Decking‐ girders or trusses or slabs

• Bearing for girders

• Handrails guard stones etc

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Hydraulic analysis

• Detailed study of characteristics of river beneath

• Information such as channel stability

• Sediment discharge

• Scour

• Sediment deposition

• Hydrodynamic forces

• Hydraulic data from survey

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Importance of hydraulic factors

• Geotechnical, hydraulic and structural combination

• The hydraulic parameters affecting bridge design are 

• Site reconnaissance

• Review and analysis of available water

• Hydraulic survey

• Maximum flood level

• Minimum flood level

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• Design flood

• Bed and bank characteristics

• Approach velocity and direction

• River meandering characteristics

• Normal scour depth 

• Backwater effect

• Flow velocity 

• Structural loading 

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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• Soil characteristics

• Economy of construction 

• Availability of resources‐manpower, machine and money 

• Proper freeboard

• Vertical clearance and height

• Location and geometry of piers

• The cost of alternative scheme etc

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River training works

• River Bank and protection structures

• Provision of spur, guide bunds

• For detail‐ ‐see IRC guideline 89‐1997

• Further reading‐ any text book on Irrigation and hydraulic structures 

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Tunnel

• Road tunnels as defined (AASHTO) TechnicalCommittee for Tunnels

• are enclosed roadways with vehicle access

• further defines road tunnels not to includeenclosed roadway created by highway bridges,railroad bridges or other bridges.

• Road tunnels are feasible alternatives to crossa water body or traverse through physicalbarriers

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• such as mountains, existing roadways,railroads, or facilities;

• or to satisfy environmental or ecologicalrequirements.

• In addition, road tunnels are viable means tominimize potential environmental impact suchas traffic congestion, pedestrian movement,air quality, noise pollution, or visual intrusion;

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• to protect areas of special cultural or historicalvalue such as conservation of districts,buildings or private properties; or for othersustainability reasons such as to avoid theimpact on natural habit or reduce disturbanceto surface land.

• Tunnels may be ventilated by shafts leading tothe surface or by exhaust fans at the ends.

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• Planning for a road tunnel requires multi‐disciplinary involvement and assessments

• should generally adopt the same standards as forsurface roads and bridge options, with someexceptions.

• Certain considerations, such as lighting,ventilation, life safety, operation andmaintenance, etc should be addressed specificallyfor tunnels.

•

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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Tunnel cross section

• There are three main shapes of highway tunnels 

• – circular, rectangular, and horseshoe or curvilinear. 

• Circular tunnels are generally constructed byusing either tunnel boring machine (TBM)

or by drill and blast in rock.

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• Horseshoe configuration tunnels are generallyconstructed using drill and blast in rock or

• by following the Sequential ExcavationMethod (SEM), also as known as New AustrianTunneling Method (NATM)

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Elements

• A road tunnel cross section must be able toaccommodate the horizontal and vertical trafficclearances, as well as the other required elements. Thetypical cross section elements include:

• Travel lanes

• Shoulders

• Sidewalks/Curbs

• Tunnel drainage

• Tunnel ventilation

• Tunnel lighting

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• Tunnel utilities and power• Water supply pipes for firefighting• Cabinets for fire extinguishers• Signals and signs above roadway lanes• CCTV surveillance cameras• Emergency telephones• Communication antennae/equipment• Monitoring equipment of noxious emissions andvisibility

• Emergency egress illuminated signs at low level(so that they are visible in case of a fire or smokecondition+)

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• The tunnels are usually equipped with various systems such as

• ventilation, 

• lighting, 

• communication, 

• fire‐life safety, 

• traffic operation and control including messaging,

• operation and control of the various systems in the tunnel. 

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Classes of Roads and Vehicle Sizes

• A tunnel can be designed to accommodate any class of roadsand any size of vehicles.

• Road tunnel A86 in Paris, for instance, is designed toaccommodate two levels of passenger vehicles only andspecial low height emergency vehicles are provided

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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Protection and safety

• Standard for Road Tunnels, Bridges, and OtherLimited Access Highways provides thefollowing fire protection and life safetyrequirements for road tunnels:

• Protection of Structural Elements

• Fire Detection

• Communication Systems

• Traffic Control

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• Fire Protection (i.e., standpipe, fire hydrants, water supply, portable fire extinguisher, fixed water‐base fire‐fighting systems, etc.)

• Tunnel Drainage System

• Emergency Egress

• Electric, and

• Emergency response plan.

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Tunnel Drainage

• Good design anticipates drainage needs.

• Although tunnels are approximately horizontal,they must be built with sufficient gradient forproper drainage.

• Usually sump‐pump systems are provided atthe portals and at low points.

• Roadway drainage throughout the tunnel usingdrain inlets and drainage pipes should beprovided.

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• The drainage system should be designed todeal with surface drainage as well as anygroundwater infiltration into the tunnel.

• Other areas of the tunnels, such as ventilationducts and potential locations for leakage,should have provision for drainage.

• Accumulation of ice due to inadequatedrainage provisions must be avoided for safepassage.

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Lighting Requirements

• Lighting in tunnels assists the driver inidentifying hazards

• or disabled vehicles within the tunnel while ata sufficient distance to safely react or stop.

• High light levels (Portal light zone) are usuallyrequired at the beginning of the tunnel duringthe daytime to compensate for the "BlackHole Effect" that occurs by the tunnelstructure shadowing the roadway

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• These high light levels will be used only duringdaytime.

• Tunnel light fixtures are usually located in theceiling, or mounted on the walls near theceiling.

• the location, size, type, and number of lightfixtures impact the geometrical requirementsof the tunnel and should be taken intoconsideration

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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Hole" (Left) and Proper Lighting (Right)

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Ventilation Requirements

• The ventilation system of a tunnel operates tomaintain acceptable air quality levels forshort‐term exposure within the tunnel.

• The design may be driven either by fire/safetyconsiderations or by air quality

• which one governs depends upon manyfactors including traffic, size and length of thetunnel, and any special features such asunderground interchanges.

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• Ventilation requirements in a highway tunnelare determined using two primary criteria

• the handling of noxious emissions fromvehicles using the tunnel and

• the handling of smoke during a fire.

• Computational fluid dynamics (CFD) analysesare often used to establish an appropriatedesign for the ventilation under fireconditions.

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• An air quality analysis should also be conductedto determine whether air quality might governthe design.

• Air quality monitoring points in the tunnel shouldbe provided and the ventilation should beadjusted based on the traffic volume toaccommodate the required air quality.

• The two main ventilation system options used for tunnels 

• longitudinal ventilation and transverse ventilation. 

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• A longitudinal ventilation system introduces airinto, or removes air from a road tunnel, with thelongitudinal flow of traffic, at a limited number ofpoints such as a ventilation shaft or a portal.

• It can be sub‐classified as either using a jet fansystem or a central fan system with a high‐velocity nozzle.

• it includes a series of axial, high‐velocity jet fansmounted at the ceiling level of the road tunnel toinduce a longitudinal air‐flow through the lengthof the tunnel

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• A transverse ventilation system can be either afull or semi‐full transverse type.

• With full transverse ventilation, air supplyducts are located above, below or to the sideof the traffic tube and inject fresh air into thetunnel at regular intervals.

• Exhaust ducts are located above or to the sideof the traffic tube and remove air andcontaminants.

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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• With semi‐transverse ventilation, the supplyduct is eliminated with its "duties" taken overby the traffic opening.

• When supply or exhaust ducts are used, theflow is generated by fans grouped together inventilation buildings.

• Local noise standards generally would requirenoise attenuators at the fans or nozzles.

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Tunnel linings

• Tunnel linings are structural systems installedafter excavation

• to provide ground support,

• to maintain the tunnel opening,

• to limit the inflow of ground water,

• to support appurtenances and

• to provide a base for the final finishedexposed surface of the tunnel.

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• Tunnel linings can be used for initialstabilization of the excavation

• permanent ground support

• or a combination of both.

• The materials for tunnel linings covered arecast‐in‐place concrete lining

• precast segmental concrete lining

• steel plate linings and shotcrete lining

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• Road tunnels are often lined with concrete andinternal finish surfaces.

• Some rock tunnels are unlined except at theportals and in certain areas where the rock is lesscompetent.

• rock reinforcement is often needed.

• Rock reinforcement for initial support includesthe use of rock bolts with internal metal strapsand mine ties, un‐tensioned steel dowels, ortensioned steel bolts.

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• To prevent small fragments of rock fromspalling, wire mesh, shotcrete, or a thinconcrete lining may be used.

• Shotcrete, or sprayed concrete, is often usedas initial lining prior to installation of a finallining, or as a local solution to instabilities in arock tunnel.

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• Shotcrete can also be used as a final lining. 

• It is typically placed in layers with welded wire fabric and/or with steel fibers as reinforcement. 

• The inside surface can be finished smooth and often without the fibers.

• Precast segmental lining is primarily used in conjunction with a TBM in soft ground and sometimes in rock.

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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• The segments are usually erected within the tail shield of the TBM. 

• Segmental linings have been made of cast iron, steel and concrete. 

• Presently however, all segmental linings are made of concrete. 

• They are usually gasketed and bolted to prevent water penetration. 

• Precast segmental linings are sometimes used as a temporary lining within which a cast in place final lining is placed, or as the final lining

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Methods of tunneling

• Tunnel constructed in soft material requiretemporary support by means of

• Suitably spaced bents of wood

• Steel with lagging

• Liner plates

• Forepoling/ spiling [see mine engineering]

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Tunneling in firm ground

• Traditional methods as drilling and blasting• Excavation by tunneling

• In traditional method‐ full face method• Suitable for comparatively firm soils where excavationportion can hold itself for sufficient time to permitmucking

• The area to be excavated is divided into 3 sections.• The top section is cut and removed followed by similaroperations to section below

• It is suitable for small size tunnel

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• Top heading and benching

• When excavated portion can not hold itself bythe time mucking and supporting operationsare carried out

• Heading is excavated and supported to the fulllength or part before benching is commenced

• The heading is always ahead of benching by aconvenient length

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• Drift method

• Suited for large size tunnel

• A pilot tunnel / drift is made in the side orcentre of the tunnel

• The drift is then widened by drilling holes onits faces

• Drift provides suitable arrangement forsupporting the excavation

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Tunneling in soft ground

• Tunneling with liner plate

• Suitable for medium soft ground

• Also for small drifts for running ground

• Liner plate is placed at the crown segment in apre‐excavated cavity at top and two adjacentliner plates bolted on each side

• Plates are supported by props

• The arch section is gradually widened down

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

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• Needle beam method

• Full section of the tunnel is broken out

• The plates are set one by one

• Plates supported by radially set props/jacksfrom a centrally placed longitudinal girdercalled needle beam

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Shield method 

• Suitable for tunneling tube railway in clay• Consists for circular shield of thick steel plates with stiffeners

• Operated manually or by machine• The shield is pushed forward into the excavation by hydraulic jacks

• The linings are circular in form• The cycle is‐ excavating and supporting the face, advancing of shield and adding another ring to the permanent lining

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Tunneling in rock

• Tunnels are driven in rock by repeating in sequence the operation of drilling holes in the rock face

• Loading the holes with explosive, blasting

• Removing and disposing off the muck

• Full cross section may be excavated or one or more drift may be excavated

• Methods‐ full face, top heading and benching, drift method

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• Sequence of operation for construction of tunnel in rocky strata

• Marking tunnel profile• Setting up and drilling• Loading explosive and blasting• Removing foul gas• Checking misfire• Scaling• Mucking• Erecting support and lining

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Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

Reference Material- introduction to Bridge and Tunnel [TE-II]/2015

Reference Material- introduction to Bridge and Tunnel [TE-II]/2015