Assertion (A): Faces for attacking excavation and construction of tunnels are opened by constructing pilot tunnels. Reasoning (R): Pilot tunnels are suitable at locations when horizontal approach to the centre line of tunnel is shorter than deep vertical shafts.

Assertion (A): Faces for attacking excavation and construction of tunnels are opened by constructing pilot tunnels. Reasoning (R): Pilot tunnels are suitable at locations when horizontal approach to the centre line of tunnel is shorter than deep vertical shafts. Correct Answer Both (A) and (R) are true but (R) is the correct explanation of A

Concept:

Pilot Tunnels:

• These tunnels are driven parallel to the main tunnel and connected to the centre line of the main tunnel with cross cuts at many points.
• These are used in locations, where horizontal approach to the centreline of the tunnel is shorter than deep vertical shaft, which will obtain additional faces for attack.
• The cross-section of the pilot tunnel usually measures about 2.4 m × 2.4 m.
• The pilot tunnel offers the following advantages:

1. It helps in removing the muck from the main tunnel quickly.
2. It helps in providing proper ventilation and lighting in the main tunnel.

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Explanation:

From the above figure,

It is clear that the cross cuts are provided to connect pilot tunnel with main tunnel faces. Hence faces for attacking excavation and construction of tunnels are opened by constructing pilot tunnels.

Hence, Assertion (A) is correct.

As these tunnels are provided at locations when horizontal approach to the centre line of tunnel is shorter than deep vertical shafts.

Hence, Reason (R) is correct.

Since,

(R) is correct explanation of (A). Option (a) is correct.