TUNNELING IN SOFT SOILS

Characteristics of Soil ideal for Tunnel Excavation

Unaffected when exposed to air

Composed of homogeneous material

Should not have water bearing strata

Capable of easily excavated

Choice of Method

Equipment available

Method of sequence of excavation

Size of tunnel

Type of ground

Characteristics of Soft Soils

Use of explosives not required

Progress of work is slow – presence of heavy strutting and supports

No need for heavy and costly mechanical equipment

Requires timbering to support the section excavated

METHODS OF TUNNELING IN SOFT SOILS

Forepoling MethodNeedle Beam MethodArmy MethodAmerican MethodEnglish methodBelgian MethodGerman MethodAustrian Method

FOREPOLING METHOD

Old, slow and tedious method

Tunnels of small dimensions for laying

sewers , pipes etc can be laid

Sequence of operations to be adhered to in

correct order

Shaft is sunk from surface and protected by timber sheetingA wooden bent is set up covering the first face of attackA line of sheeting above the cap is cut along the top line of holes and soil from behind the sheeting is permitted to come outSmall holes at close intervals are drilled through sheeting Fore poles or spiles – with wedge ends are entered and driven through cut into ground with upward inclination of 170 mm per metre

After all roof and part of side spiles are driven to half their lengths, a timber is laid across the back ends of spiles and wedging them down, the front ends of spiles are cantilevered upFace sheeting is cut across lower line of holes and loose soil is run into tunnel until face assumes a natural slopeHorse head is set up about 600 mm from sheeting and earth beneath the forward end is scooped out for a length of 500 mm and face supported by breast board is placed underneath the forward point of the spile

The next cap supported on a bridge is set

and temporarily supported on a single post

Side spiles are driven for their full lengths

A heavy horizontal beam is pushed forward to

support the forward cap

The breast board is extended to grade level

by adding new sheets

AMERICAN METHOD

Suited for large – sized railway

or highway tunnels

Top drift is driven and

supported by laggings,

segment and two posts

Sides of drift are widened and

supported on shoulder

segment timbers and struts

AMERICAN METHOD

Wall plates of 5m length are introduced at springing supporting arch setWall plates are underpinned by propsTimber arch segment and roof load are transferred to wall plate and posts for supportSides and benching are fully cleared and tunnel is lined.

TUNNELING WITH SHIELD

Metal cylinder made of hard steel plates bent to required curvature and thickness by riveting two or three platesEquipment that acts as a bridge with roof for workers to work on extension of lining and face of attackTravelling support for erecting scaffolding near face of attack

Parts of Shield

Cutting Edge or Leading Edge

Central Part or Trunk Portion

Tail or Rear portion

Steps of Tunneling with Shield

Excavation

Support for face to be attacked

Assembly for primary lining

Shove of shield

Muck removal