Definition

Tunnels are artificial underground passages opened at both ends constructed for different purposes.

Required for highways, railways, sewerage, water supply, public utilities and canals.

Necessitated when open excavation of strata becomes uneconomical, not only construction cost but for maintenance as well.

Tunneling

History

The oldest tunnel was constructed 4000 years ago in ancient Babylon to underpass the bed of river Euphrates and to establish an underground connection between the royal palace and Temple of Jove.

The length of the tunnel was 1 km and it was built with the considerable cross-section dimensions of 3.6 m by 4.5 m.

In the second half of the 20th century, with the progress in development of both explosives and equipment, underground construction became feasible.

Geological Considerations or ground properties before constructing any tunnel

project:

The engineering properties of a rockgenerally depend not only on the matrix(structure formed by the minerals) butalso imperfections in the structure suchas voids (pore space ),cracks ,inclusions,grain boundaries and weak particles.

Pore spaces are largely made up ofcontinuous irregular capillary cracksseparating the mineral grains.

Physical discontinuities are present in allrock masses as a result of geologicalactivities.

Introduction of defects into the rock massdue to human activities that alter theproperties of the rock material.

The mechanical breaks in the rock havezero or low tensile strengths, increase rockdeformability and provide pathways forwater to flow.

Terminology:

Circular Horseshoe Vertical walls arch roof

Transportation tunnel cross-sections

Wall

Crown

SpringlineCore (strozze)

Invert

Top heading

Bench

Bottom

Crown: The uppermost part of the tunnel

Drift : A horizontal excavation

Heading : The excavated face of the tunnelInvert : The bottom (floor) of the tunnel

•Springline : The line at which the tunnel wall breaks from sloping outward to sloping inward toward the crown

Station : The distance measured from the portal (chainage)

Portal : The tunnel entranceWall : The side of the tunnel

The choice of tunnelling method may be dictated by:

geological and hydrological conditions Weak rock under high stresses

leads to squeezing ground conditions.

Brittle strong rock high stress conditions may lead to rockbursting.

cross-section and length of continuous tunnel

local experience and time/cost considerations (what is the value of time in the project)

limits of surface disturbance, and many others factors

Choice of method depends upon nature of strata and geometry of tunnel section

Methods divided into three categories Firm ground- reasonable time available for

installing conventional support.

Soft ground- Conventional support can not be installed.

Running ground- Special treatment required before starting excavation

Classification of Tunnelling Methods based on type of strata:

Rough classification of methods

Excavation: An excavation process without removing the overlying rock or soil

• Steps: excavating-remove muck-supporting-lining-ventilation-draining

• Drill & Blast, hammer, roadheader, ....

Cut and cover: trenching to excavate and construct a tunnel, and then backfilling earth over it.

Immersed tunnel: lowering prefabricated tunnel elements into a dredged channel and joining them up under water

• Concrete

• Steel

Tunnelling Methods

Bottom-up method: The main site is excavated, with ground support as necessary, and the tunnel is constructed within. The tunnel may be of in situ concrete, precast concrete, precast arches, or corrugated steel arches. The trench is then carefully back-filled above the tunnel roof and the surface is restored.

Coffer Dam and

hydroelectric Tunnel

Top-down method: Here side support walls and cap beams are constructed from ground level typically with slurry walls, or secant piling. Then a shallow excavation allows making the tunnel roof of precast beams or in situ concrete. The surface is then restored except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed.

Tunnelling in Firm ground Traditional methods. Involving drilling and blasting

Full Face Method. In firm soils where full face excavation can hold it self for sufficient time to allow mucking and supporting operations.

In the full-face method, workers excavate the entire diameter of the tunnel at the same time.

This is most suitable for tunnels passing through strong ground or for building smaller tunnels.

Top Heading and benching. When full face excavation is not possible. Heading should be sufficient distance ahead of benching.

In this technique, workers dig a smaller tunnel known as a heading. Once the top heading has advanced some distance into the rock, workers begin excavating immediately below the floor of the top heading; this is a bench.

One advantage of the top-heading-and-bench method is that engineers can use the heading tunnel to gauge the stability of the rock before moving forward with the project.

Excavation by tunnelling Methods Drift Method. In case of large or pilot tunnel , Drift is made

and then expanded. There can be more than one drifts. Different ‘drift methods’ depending on location of drift can

be used.

Tunnelling in Soft Strata

Tunnels constructed in soft materials require temporary support immediately or shortly after excavation. Way to provide support are: Suitably spaced bents of wood Suitably spaced bents of steel for lagging

(covering) Liner plates Fore poling placed to retain material

between adjacent bents Temporary supports must be designed for

higher working stress compared to those in permanent designs.

Tunnelling in Soft Ground

Instantaneous support is required

No drilling and blasting.

Forepoling is done.

Boards are driven ahead of the last ‘rib’, around periphery

Forepoles act as cantilevers beyond breasting

Soil is excavated after removing the breast board and new rib is erected

Because stand-up time is generally short when tunneling through soft ground, cave-ins are a constant threat. To prevent this from happening, engineers use a special piece of equipment called a shield.

A shield is an iron or steel cylinder literally pushed into the soft soil. It carves a perfectly round hole and supports the surrounding earth while workers remove debris and install a permanent lining made of cast iron or precast concrete.

When the workers complete a section, jacks push the shield forward and they repeat the process.

Tunnelling in Running Ground

Many methods

Principle is, Linear plates are pushed in starting from crown

Cavity is excavated

Additional linear plates are pushed one by one and bolted

Arch section gradually widened down to springing line.

Tunnelling in Rocks Tunnels are driven in rocks by drilling holes in the

rock face, loading the holes with explosives, blasting and removing the broken rock

Each sequence full cross section of the tunnel may be excavated, or one or more drifts may be there.

Drilling pattern for holes of explosives may differ depending upon rock type, cross section, experience expertise and type of explosive suggested.

Methods are similar in principle like Full Face Method

Top Heading and benching

Drift Method

Tunneling through hard rock almost always involves blasting. Workers use a scaffold, called a jumbo, to place explosives quickly and safely.

The jumbo moves to the face of the tunnel, and drills mounted to the jumbo make several holes in the rock. The depth of the holes can vary depending on the type of rock, but a typical hole is about 10 feet deep and only a few inches in diameter.

Next, workers pack explosives into the holes, evacuate the tunnel and detonate the charges. After vacuuming out the noxious fumes created during the explosion, workers can enter and begin carrying out the debris, known as muck, using carts.

Then they repeat the process, which advances the tunnel slowly through the rock.

Fire-setting is an alternative to blasting. In this technique, the tunnel wall is heated with fire, and then cooled with water. The rapid expansion and contraction caused by the sudden temperature change causes large chunks of rock to break off. The Cloaca Maxima, one of Rome's oldest sewer tunnels, was built using this technique.

The stand-up time for solid, very hard rock may measure in centuries. In this environment, extra support for the tunnel roof and walls may not be required. However, most tunnels pass through rock that contains breaks or pockets of fractured rock, so engineers must add additional support in the form of bolts, sprayed concrete or rings of steel beams. In most cases, they add a permanent concrete lining

Sequence of operation in Rocky Strata (Drill and Blast) Marking tunnel profile.

Setting up and drilling

Loading explosive and blasting

Removing the foul gases

Checking

Scaling

Mucking

Rock bolting and lining

Modern Tunnel Construction Methods:

Drill and blast

Mechanical drilling/cutting

Cut-and-cover

Immersed tunnels

Tunnel boring machines (TBMs)

New Austrian Tunnelling Method (NATM)

Road headers

Types of support

Steel arches Steel ribs are used for reinforcement of weaker tunnel

sections, and give rigid to semi-rigid support. The ribs are made from I-beam or H-beam structural steel bent to conform to the requirements of a particular tunnel cross-section.

Timber may be used for packing between the beams and the rock. However, providing continuous bedding against the rock may considerably increase the load-bearing capacity of the arches.

Rock bolts Steel bolts are frequently set in holes drilled into the rock

to assist in supporting the entire roof or individual rock slabs that tend to fall into a tunnel. Rock bolts maintain the stability of an opening by suspending the dead weight of a slab from the rock above

Shotcrete Pneumatically applied mortar and concrete are

increasingly being used for the support of underground excavations.

A combination of rock bolts and shotcrete has proved an excellent temporary support for all qualities of rock.

Shotcrete is best known in tunnelling as an integral component of the NATM (New Austrian Tunnelling Method).

Quick-setting concrete is sprayed onto the bare rock surface immediately after excavation, and rapidly hardens to form a preliminary support until the final lining of conventional poured concrete can be installed.

Wire mesh Wire mesh is used to support small pieces of loose rock

or as reinforcement for shotcrete.

Two types of wire mesh

chain link mesh commonly used for fencing and it consists of a woven fabric of wire, tends to be flexible and strong

weld mesh. commonly used for reinforcing shotcrete and it consists of a square grid of steel wires, welded at their intersection points.

Ventilation during construction Why required?

To furnish fresh air for the workers

To remove the dust caused by drilling, blasting, mucking, diesel engines, and other operations

To remove obnoxious gases and fumes of explosives

How done?

Mechanical ventilation is usually supplied by electric fans, as for example axial flow pressure fans. If air is blown into a tunnel, it may be forced through a

lightweight pipe or fabric duct.

If air is exhausted, it is necessary to use a rigid duct that will not collapse.

The exhaust method has the advantage of more quickly removing objectionable air from spaces occupied by the workers.

Ventilation of tunnels Mechanical ventilation systems provide the temperature,

humidity and air velocity conditions necessary to give tunnel users a reasonable degree of comfort during normal operation.

When a fire occurs in a tunnel, the system must also provide a safe evacuation route for tunnel users and access for fire fighting services.

The choice and design of a ventilation system depends on these main factors: tunnel length and volume admissible air pollution around tunnel portals fire safety considerations. Key pollutants include carbon dioxide, nitrogen oxides,

nitrogen dioxide, hydrocarbons PM10 and lead. Better understanding of ventilation techniques Awareness of new safety and environmental legislation.

Geological Survey For Tunneling

A geological survey is the systematic investigation of the subsurface and surface of a given piece of ground for the purpose of creating a geological map ,model and feasibility studies.

A geological survey employs techniques from the traditional walk-over survey, studying outcrops and landforms, to intrusive methods, such as hand auguring and machine driven boreholes, use of geophysical techniques and remote sensing methods, such as aerial photography and satellite imagery etc.

Types Of Geological Survey Mainly geological surveys are classified into two

types:

Surface Geological Survey

Subsurface Geological Survey

Surface Geological Survey: includes on land geology and geological structures,landforms,hydrology,outcrop pattern ,engineering properties etc

Subsurface Geological Survey: : includes on underground geology and geological structures,Geo-hydrogeology,subsurface rock or soil pattern etc

Surface Geological Survey

Geological profile is prepared along line of tunnel.

Geological observations are done along this

profile like engineering properties of rock/soil,

geological structures like

fold,faults,joints,spring,stream,river alignment and

any seepage etc

Trial boring plan is prepared along the tunnel line.

Subsurface Geological Survey

Trial boring is done along the tunnel line.

Different rock and soil samples are collected from subsurface and their physical and chemical properties are observed.

Some samples are sent to laboratory for determination of engineering properties like shearing,strenght,permeability,porosity,compressive strenght,and other test are performed.

GWT(Ground Water Table) is noted and water bearing strata is marked in geological profile.

Different geophysical surveys like refraction, resistivity, GPR(Ground Penetrating Radar) and bor hole logging etc are also done.

In situ testing like Packer-testing, hydro fracturing, load-testing etc are performed.

A geological/feasibility report comprising all information ,photographs, profile and others necessary data are presented for final tunnel design and construction.

Subsurface

Profiling/3 D

modelling