Explain the method of tunnelling in soft rock by "Needle Beam Method".

Mumbai University > Civil Engineering > Sem 8 > Construction Engineering

Marks: 10Marks

1 Answer

The needle beam consists of a stout timber beam or a composite flinched beam and form the main temporary support during the excavation. This method is suitable for soils where roof can withstand for a few minutes.

Sequence of operations:

A drift(monkey drift) of about 1 metre is driven on the working face.

The roof of this drift is lagged with sheeting carried on wooden segments.

These segments in turn are carried on two trench jackes,set in hitches in the side of monkey drift as shown in figure.

The needle beam which is usually 5 to 6 m long is skidded ahead into the monkey drift.

The front end of the needle beam rests on planks on the floor of the drift, while the rear end is carried on stout post resting on the tunnel floor

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A trench jack is now placed on the centre line of the needle beam to support the segment, the other trench jacks are removed and the drift is widened sideways, and supported by lagging, segments and trench jacks resting on the sides of the needle beam as shown in figure


i. suitable for soils where roof can stand for few minutes

ii. economical

iii. This method works all right on brick lining.


i. Heavy beam has to be pushed forward by hand.

ii. No of trench jacks required are large and they interfere with the efficient working of the gang

iii. Use of beams and bracing gives difficulties in mechanical methods of concrete lining.

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