Tunnel is an Engineering Marvel that basically enables transportation to pass through a Mountain as well as underground formations. They are of paramount importance for the infrastructure development in hilly mountainous areas and serve as connecting link to inaccessible terrains and regions.

Introduction to Tunnels

This section is a brief introduction to Tunnels. It gives details about shapes of tunnel section and construction process of tunnels.

Shape of Tunnel Section

(a) Circular – The Circular tunnel is economical as it reduces earthworks.

It is suitable for non cohesive soil. It is used in tube railway system and sewers.

Mostly, we consider it unfit for roads and railway as they require more flat bottom formation.

(b) Horse Shoe –

• External pressure is resisted by arch top and curved side
• Best for road (no filling)
• Used for soft rocks.

(c) Rectangular –

• Hard Rocks
• RCC
• Pedestrian traffic

(d) Elliptical – Soft material

(e) Egg shaped –

• Provide self cleaning velocity
• resist external and internal pressure
• RCC

(f) Segmented roof

• Subways, navigation tunnel
• Hard rock

Sequence of Tunneling

Tunneling in Soft Soils

This section deals with methods of Tunneling in Soft soils.

Methods of Tunneling in Soft Soil

American Method

First we drive drift at the top, then widen it to get springing level.

English Method

Same as above but final load is taken by underpinning.

Support by shoulder, Lagging, Plates, and crown bars.

Forepoling Method

We drill holes into timber sectioning.

Then, We drive Forepoles into cut in sheets.

Needle Beam

• Needle beam in central drift.
• Economical
• roof supported by trench jacks.

Linear Plate Method

• Absolute control of running ground water.
• Minimize ground subsidence
• We use Pressed steel liner plates to support soil.
• No timber plank, light weight and economical.

Shield Method

• Circular machine with cutting edge
• Inter structures are made by tiny girder which are braced.
• Rotary excavators are used.

Compressed Air

• For water bearing stratum
• Compressed air is forced into enclosed space, which prevent collapse.
• Used in conjunction of Shield
• Air pressure force back percolating water.

Army method, Austrian method, Belgian method and Italian method are some other tunneling methods used for soft stratum.

Tunneling in Hard Soils

This section deals with methods of Tunneling in Hard soils.

Methods of Tunneling in Hard Soil

Heading and Benching

• Hard rock
• First, it goes by excavating heading at top.
• Then, we dig up bottom level, called benching.
• Heading is always ahead by 4-5 m.

Full Face

• Hard rocky soil
• we excavate full face from top to bottom simultaneously.

Drift

• Hard rock
• We excavate drift at center then widen it in all sides.
• Good ventilation.

Perimeter

• German method
• Hard rocks
• Excavated from perimeter to center.

Pilot Tunnel Method

• Hard Rock
• We construct pilot tunnel parallel to main tunnel and connect them by short tunnels.

Thickness of Lining of Tunnel

\large \textcolor{blue}{T = 82 D}

D – diameter of tunnel in meters