The term lining is used to indicate the finishing touch given to the cross section of tunnel.The lining should be such that to prevent the collapse of the strata above and around the tunnel.
Timing for placement of lining:
The correct time for providing lining is governed by the condition of the soil bored through.
Due to technical forces,rock is likely to dilate or the soil is likely to heave,it is advisable to allow sufficient time for such dilations to reduce to reasonable depth.
Sequence of operations in lining:
Placing of concrete to form the kerbs first, followed by side walls and arch and finally the invert .SUITED FOR: horse shoe, D shaped
Placing of concrete to form the invert first followed by sides and arch.SUITED FOR:circular section
Placing of concrete to form the invert,side walls and arch at one time.SUITED FOR:smaller circular section
Formwork:
Generally use collapsible steel forms made up in standard size/unit lengths in the interest of speed and economy.
types: Non telescopic and telescopic
Placing of concrete
The modern practise is to place the concrete by the help of pneumatic concrete placers which force concrete through a pipe into the space above the centering.
The discharge pipe leads to the far end of the crown, feeding the concrete,over the crown.
The concrete slides down the sides of the form.The pipe is gradually withdrawn as concreting proceeds.
The near end is blocked by a bulk head of steel,through which the pipe is passed.
Compaction
Flexible shaft immersion type vibrator with a vibrating needle of 50 mm diameter and 3000 vibrations/minute frequency should be used for vibration of the concrete.
Curing
Curing should be done by spraying water at short intervals or bituminous paints.
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