Tunnel surveying is a type of underground surveying for the construction of tunnels. Methods and procedure of tunnel surveying is discussed.
Special instructions on the type of survey and the equipment that must be used in the tunnel surveying procedure, for the tunnel construction are initially given by the planners.
The basic procedure of tunnel surveying is to align the center line in the ground and transfer that to the tunnel. This also involves leveling the surface on the ground and the internal of the tunnel.
Methods and Procedure of Tunnel Surveying
The steps that are involved in the tunnel surveying are detailed in the following points.
1. The initial procedure is to carry out a preliminary survey that is later made more precise by surveying the line on the surface of the area under consideration
2. From the start of the excavation, as a part of tunnel construction, it is essential to keep up even minute accuracy with the center line that is already marked. When a new area of excavation is commenced, the center line of the before finished work has to be carried forward over the new face. No shifting of the center line in vertical or horizontal direction is accepted, when the opposite faces meet with adjacent headings.
3. Ordinary engineers transit and properly handled works give satisfactory results for the construction of short tunnels.
4. Tunnel transit that is large and sophisticated, is fitted to a striding level, that helps in keeping the transverse axis horizontal. This is necessary for long tunnel construction.
5. The instruments above mentioned should undergo periodic maintenance, calibration as well as checking.
6. The procedure of leveling is carried out in a normal way. But areas with steep slopes are measured with utmost care. This is ensured by having equal values for backsight and foresight. This would reduce the errors that are caused by the human, like improper or mal -adjustments of the instruments.
7. There are two methods that can be employed to measure the horizontal distances. They are stepping and inclined sights. Any of these methods can be employed based on the area under consideration and convenience.
The steel tapes used for the measurement are checked for errors due to any cause of tension or temperature changes. This would result in cumulative errors in the measures.
The below figure shows the alignment of a curve line, based on which the construction must be proceeded (figure-1).
Fig.1: Alignment of Curve Line in Tunnel Surveying
The center line of the curve in the figure is the alignment through which the center line of the tunnel must proceed. This is the curve that must be determined. The headings consist of short tangents that are drawn to the curved lines as ab, bc, cd etc. To locate the center line of the area, offsets from the above-mentioned tangents are set off.
When the heading has proceeded up to x y, point like ‘n’ on the tangents is aligned as shown in the figure. We then set off the calculated offset, i.e. n-o that will intersect the center line of the heading.
After each blasting of the heading face, the center line recommended being transferred.
How is Transferring of Center Line into the Tunnel Carried Out?
The transferring of the center line is a method that is done with great care, precision and accuracy. The accuracy throughout the execution of the same must be guided, else it can give up many related problems. Carelessness can cause the lines from opposite faces not to meet.
Even small deviation from the centerline can cause, over breaking. This whole undesirable work brings large expenditure than it has to be. The problem because more tiring when the procedure is undergone on hard rock strata. As shown in the figure, well elaborate equipment are made ready for the proceeding of the work.
Fig.2: Arrangement in Transferring the Center Line Points to the Tunnel
Through the shaft constructed, the plumb bob weighing 22 lbs are taken down through the shaft, thus transferring the two center line points to the bottom as shown. It is carried out vertically downwards. The plumb bobs are suspended with the help of piano wires that are passing through the grooves, that is connected to a shaft for easily leveled movement of the plumb bob.
When the plumb bob reaches the bottom, they are replaced by heavy iron so that the weight will keep the wire straight. For higher precaution of the wire to resist any kind of vibration or oscillation, it can be immersed in a container of oil.
Once level, the line joining the two plumb bobs are extended with the help os a theodolite that is placed on the shaft floor, that is designated with respect to one point of the shaft roof, as shown in the figure.
A similar reference point is made to the opposite side of the roof, initially noted. Hence the center line is marked on the shaft floor and continued for further tunnel extension.
How to Transfer the Tunnel Grade?
The same piano wire and the plumb bob combination can be used to transfer the tunnel grade to the shaft bottom. Here at 5 feet below the springing level of the roof, two points are marked on the wire.
Initially, the reduced level of the top mark is determined, with respect to a benchmark i.e. a point near to the mouth of the shaft. And later the reduced level of the second mark is also determined.
With all the above measurements, a benchmark is determined at a point on the tunnel side wall. Based on this benchmark the floor levels are transferred and continued measurement.
Decision of Shape and Size of the Tunnel
The shape of the tunnel is decided based on the nature and the type of the soil or the ground that is penetrated during the surveying and the excavation procedure. After surveying and related construction works, a choice of shape can be made. The size of the tunnel is already measured in the design, as it is based on the scope or requirement of the project.
Fig.3: Shapes for Tunnels
Among different cross sections, the circular cross sections are found to be the best shape to bring huge resistance towards the internal and external pressures. This also has another peculiarity of bringing larger size for a smaller diameter. But in engineers point of view, they demand high lining and covering to protect it from high vibration in railway uses.
So, an optimized shape is a tunnel with circular top and vertical side walls Another shape is the horseshoe type. They are compromised with the problems related to the circular tunnel shape.