Tunnel Alignment: What is its Importance in Tunnel Construction?

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Tunnel alignment is simply the position or layout of the tunnel on the ground; the alignment can be either horizontal or vertical. While a horizontal alignment runs along the tunnel center, a vertical alignment defines the changes in elevations of the tunnel.

Tunnel alignment is an essential design consideration one should perform before undertaking tunnel construction.

This article explains the importance of tunnel alignment in tunnel design and construction.

Objective of Tunnel Alignment

The objectives of tunnel alignment are:

1. To increase the quality and reliability of the tunnel service.
2. To reduce the impact of surface transit operations faced in sensitive locations.
3. To reduce the number of transit trips.

The selection of tunnel alignment is dependent on:

1. Topography of the area
2.  Points of entrance and exit of the area
3.  Alignment restraints
4.  Environmental conditions

Alignment and Gradient of Tunnel

A detailed tunnel surveying with precise measurements is performed to set the alignment of a newly constructed tunnel. If the setting of alignment goes wrong, the ends of the tunnel won't meet.

When the work starts, the centerline is marked on the ground with high precision, so that it gives the correct length to the tunnel. After this, an accurate survey is performed to ensure that the correct centerline or horizontal alignment of the tunnel and the other levels are transferred to the underground positions.

Some of the important points to remember while deciding the alignment and gradient of the tunnel are:

1. The tunnel must be provided with a minimum gradient value as per the standards and recommendations.
2. The alignment of the tunnel must stay straight as far as possible so that the shortest and most economical route is achieved.
3. Always check for provisions for proper ventilation and adequate lighting.
4. The side drains in a tunnel must possess at least a gradient of 1 in 500 to have effective drainage. The gradient is provided from the center towards the end, especially for longer tunnels.

Classification of Tunnels Based on Alignment

Based on the tunnel alignment, tunnels can be classified as:

1. Off-spur tunnels
2. Slope tunnels
3. Spiral tunnels
4. Saddle or base tunnels

The tunnels with short lengths that are used to negotiate minor obstacles are called off-spur tunnels. When tunnels are aligned along the natural slope in valleys, it is called as saddle or base tunnels. Slope tunnels are tunnels that are constructed in steep hills to facilitate economy and safety in operation.

When tunnels are constructed in the form of loops along the interiors of mountains, they are categorized as spiral tunnels. Saddle or base tunnels are constructed in narrow valleys along natural slopes. This method of alignment increases the length of the tunnel to avoid steep slopes.

Surface Alignment and Measurements

Tunnels can be aligned either straight or in an inclined manner. The check for accuracy of alignment and levels is achieved using vertical shafts. The determination of surface alignment and measurements is performed during tunnel surveying. This is called setting out of the tunnel, which involves the following steps:

1. Surface survey to fine centerline or tunnel alignment on the surface: In this method, a preliminary survey is performed by precise traversing using theodolite to join the two ends of the proposed tunnel. With the help of a standard plan and a detailed survey, a final alignment is selected.
2. Transfer of alignment from surface to underground: This step involves transferring the centerline or alignment on the surface to the underground shafts.
3. Setting out underground and final leveling: The leveling of the underground surface is performed in the usual way. Nearest benchmarks are used to transfer the levels underground.

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