Tunnel surveying is a specialized form of underground surveying essential for the construction of tunnels. It involves detailed procedures and the use of specific equipment to ensure accuracy in aligning the tunnel’s center line and maintaining its level. This article explores the methods and procedures of tunnel surveying, highlighting the steps, tools, and considerations involved.
Overview of Tunnel Surveying Process
The core process of tunnel surveying involves transferring the alignment of the center line from the surface to the tunnel and maintaining that alignment throughout construction. This includes leveling the surface ground and the interior of the tunnel to ensure precise execution. Any deviation can lead to significant issues, including misalignment of opposite faces and increased costs.
Steps in Tunnel Surveying
1. Preliminary Survey
The initial step is to conduct a preliminary survey to analyze the area and establish the alignment on the surface. This survey is critical to ensure the precision of subsequent steps.
2. Center Line Accuracy
From the start of excavation, strict adherence to the center line is required. Each new excavation area must align perfectly with the previous section. Any vertical or horizontal shifts can lead to misalignment when opposing faces meet, causing costly and time-consuming corrections.
3. Use of Engineering Tools
For short tunnels, ordinary engineers’ transit can provide satisfactory results. However, for longer tunnels, sophisticated tunnel transits fitted with striding levels are essential to maintain horizontal alignment.
4. Instrument Maintenance
The accuracy of measurements depends heavily on the condition of the instruments. Regular maintenance, calibration, and checks are crucial to prevent errors.
5. Leveling Process
Standard leveling methods are used, but extra care is taken in steep slope areas. Equal backsight and foresight measurements are ensured to minimize human errors or instrument misalignments.
6. Measuring Horizontal Distances
Horizontal distances can be measured using either the stepping method or inclined sights, depending on the terrain and convenience. Steel tapes used for these measurements must be checked for tension and temperature-induced errors to avoid cumulative inaccuracies.
Curve Line Alignment in Tunnel Surveying
The alignment of curved paths in tunnels requires precision. The center line of the curve is determined and aligned with the tunnel’s center line. Short tangents are drawn to the curve, and offsets from these tangents help locate the center line. Each section of the tunnel is aligned incrementally, and the center line is transferred after every blasting of the tunnel face to ensure consistency.
Transferring the Center Line into the Tunnel
1. Procedure
Transferring the center line from the surface to the tunnel is a critical process requiring extreme precision. Small deviations can cause significant issues such as overbreaking or failure to align opposing tunnel faces.
2. Equipment and Methodology
This process involves plumb bobs and piano wires suspended through a shaft. The plumb bobs, weighing around 22 lbs, are used to transfer points vertically. Heavy weights or oil immersion help minimize wire vibrations. Theodolites are then used to extend the center line accurately.
3. Marking and Extension
Once the center line points are transferred to the shaft floor, they are marked and extended using theodolites to guide further construction.
Transferring the Tunnel Grade
The same plumb bob and piano wire combination is used to transfer the tunnel grade. Reduced levels are determined at specific points, and benchmarks are established. These benchmarks are then used to transfer floor levels accurately, ensuring a consistent grade throughout the tunnel.
Decision on Tunnel Shape and Size
1. Factors Influencing Tunnel Shape
The shape of a tunnel is determined by the type of soil or ground and the structural requirements. The goal is to ensure stability and resistance to internal and external pressures.
2. Common Shapes
- Circular Tunnels: These are highly resistant to pressure and maximize size efficiency for a smaller diameter. However, they require substantial lining to withstand vibrations, particularly for railway tunnels.
- Horseshoe-Shaped Tunnels: This shape balances the benefits and challenges of circular tunnels, offering a practical alternative.
Conclusion
Tunnel surveying is a meticulous process requiring precision, proper equipment, and careful execution. From preliminary surveys to center line alignment and shape decisions, every step is critical to the success of tunnel construction. By adhering to these methods and procedures, engineers can ensure efficient, cost-effective, and safe tunnel construction.