What is Tunnel Surveying?
Methods and Procedure of Tunnel Surveying
Instructions for Tunnel Construction
What is Tunnel Surveying?
Tunnel surveying is a type of underground surveying used in the construction of tunnels. It is employed to measure and map the internal area of a tunnel and align the center line of the tunnel with the ground.
Methods and Procedure of Tunnel Surveying
The procedure of tunnel surveying involves aligning the center line in the ground and transferring that to the tunnel. It also involves leveling the surface both on the ground and the inside of the tunnel.
Instructions for Tunnel Construction
Special instructions on the type of survey and the equipment that must be used in the tunnel surveying procedure for tunnel construction are initially given by the planners. This ensures that the survey and construction are carried out correctly and safely.
Methods and Procedure of Tunnel Surveying
Introduction to Tunnel Surveying
Tunnel surveying is the process of carrying out a survey of an area that is to be excavated for the construction of a tunnel. The accuracy of the tunnel is maintained by surveying the line on the surface of the area, and any deviations from the center line are not accepted. This article explains the steps involved in tunnel surveying, from the initial survey to the maintenance of instruments and the methods of measuring horizontal distances.
Steps of Tunnel Surveying
- Preliminary Survey: The first step in tunnel surveying is to carry out a preliminary survey of the area to be excavated. This survey is later refined to more accuracy by surveying the line on the surface of the area in question.
- Keeping Up Accuracy: 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. Any shifting of the center line in vertical or horizontal direction is not accepted when the opposite faces meet with adjacent headings.
- Instruments Used: Ordinary engineers transit and properly handled works give satisfactory results for the construction of short tunnels. For long tunnel construction, a large and sophisticated tunnel transit is fitted to a striding level, to keep the transverse axis horizontal.
- Maintenance and Calibration of Instruments: The tunnel transit instruments have to be periodically maintained and calibrated for accuracy.
- Leveling: 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.
- Measuring Horizontal Distances: There are two methods that can be employed to measure the horizontal distances: stepping and inclined sights. The steel tapes used for the measurement are checked for any errors due to tension or temperature changes.
- Alignment of Curve Lines: The below figure shows the alignment of a curve line, based on which the construction must be proceeded (figure-1).
Heading Alignment:
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.
Offset Determination:
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.
Transferring the Center Line:
After each blasting of the heading face, the center line recommended being transferred.
How is Transferring of Center Line into the Tunnel Carried Out?
Transferring the Center Line: A Precise and Accurate Process
The process of transferring the center line requires a great deal of precision and accuracy to ensure that the lines from opposite faces meet. If there is any carelessness during the procedure, it can result in overbreaking or large expenditure. This is even more difficult to achieve in hard rock strata, which requires well-elaborated equipment for the job.
Avoiding Overbreaking with Careful Execution
In order to ensure that the center line is accurately transferred, it is important to take great care during the process. Any deviation from the center line can lead to overbreaking, which can result in large expenses. Therefore, it is important to pay close attention to the procedure in order to avoid this issue.
Using Elaborate Equipment for Transfering the Center Line
The process of transferring the center line in hard rock strata requires well-elaborated equipment in order to ensure accuracy. This equipment is designed to facilitate the task and ensure that the lines from opposite faces meet. Without the use of this equipment, the process would be much more difficult and would require much more effort and expense.
- Introduction to Plumb Bob and Piano Wire:
The plumb bob is a 22-pound device suspended from piano wires, which are connected to a shaft for level movement. This allows the plumb bob to travel down the shaft and transfer two center line points to the bottom. - Leveling the Piano Wire:
For extra precaution, the piano wire can be immersed in an oil container to resist any vibrations or oscillations. Once level, the line joining the two plumb bobs is extended with the help of a theodolite placed on the shaft floor. - Marking the Center Line:
A reference point is made to the opposite side of the roof, initially noted. This allows the center line to be marked on the shaft floor and continued for further tunnel extension.
How to Transfer the Tunnel Grade?
Transferring the Tunnel Grade to the Shaft Bottom
To transfer the tunnel grade to the shaft bottom, a combination of piano wire and a plumb bob can be used. The process begins by determining the reduced level of the top mark, which is located 5 feet below the springing level of the roof. This is done with respect to a benchmark near the mouth of the shaft. The reduced level of the second mark is also determined in order to establish a benchmark on the tunnel side wall. This benchmark is then used to transfer the floor levels, with further measurements taken to ensure accuracy.
Decision of Shape and Size of the Tunnel
The Process of Deciding Tunnel Shape
The shape of a tunnel is determined based on the geological characteristics of the soil or ground it will penetrate. A survey and related construction works must be completed before a suitable shape can be decided upon. The size of the tunnel is predetermined by the requirements of the project.
Factors That Influence Tunnel Shape
The type of soil or ground that will be penetrated has a significant influence on the shape of a tunnel. Surveying and related construction works are necessary to determine the most suitable shape for the tunnel. Furthermore, the size of the tunnel is predetermined depending on the requirements of the project.
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. Read More: Tunnel Engineering -Features, Advantages and Methods of Tunneling in Construction Ventilation in Tunnels -Types of Ventilation Systems in Tunnel Construction Underground Surveying–Methods And Applications