TUNNELING IN SOFT SOILS
Feb 10, 2016
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