What is Underground Surveying?
Peculiarities of Underground Surveying
Underground surveying is distinct from surveying on the surface, due to the unique challenges and conditions that present themselves. These include the need for artificial illumination to view instrument crosshairs, read verniers, and sight targets due to low lighting, cramped working spaces, and difficulty setting instrument stations due to having to drive plugs into drill holes in rock. Additionally, many underground workings are wet, with water dripping from the roofs of passageways and running along the floors.
Challenges of Poor Lighting
One of the most pressing challenges of underground surveying is the poor lighting, which necessitates the use of artificial illumination to properly view instrument crosshairs, read verniers, and sight targets. This is an essential part of the surveying process, and without this artificial light, the accuracy of the survey would be compromised.
Cramped Working Spaces
Due to the confined nature of underground passageways, the working space for surveying is often quite cramped. This can make it difficult to maneuver the instruments and can lead to challenges in setting up the equipment.
Difficulty of Setting Instrument Stations
Instrument stations and benchmarks for levelling must often be set into the roof of a passageway in order to minimize disturbance from the operations being carried out. This can be difficult due to the need to drive plugs into drill holes in rock.
Wet and Damp Underground Workings
Many underground workings are wet, with water dripping from the roofs of passageways and running along the floors. This can make it difficult to carry out the survey accurately and can lead to problems with the instruments and other equipment.
Applications of Underground Surveys
Tunnel Construction Using Underground Surveys
Underground surveys are used to construct tunnels and other underground utilities when open excavation is uneconomical, typically when the excavation is more than 20 m. Such surveys reduce the grade and shorten the distance between two points separated by a mountain or ridge. This is essential for modern rapid transit in a city. The engineering operations that need to be performed include establishing an exact alignment, proper gradient, and permanent stations marking the proposed route.
The survey work in connection with tunneling can be divided into:
- Surface survey
- Transferring the alignment underground
- Levels in tunnels
Surface survey:
Survey for Tunneling
A surface survey is conducted to connect the points representing each portal of a tunnel. Through a traverse, the azimuth, distance, and differences in elevation of each end of the proposed tunnel are measured. Depending on the local conditions and the proposed length of the tunnel, the most suitable method of working can be adopted. The survey should always be based on a suitable local coordinate system. To permanently reference the alignment, a system of monuments is placed within an area outside each tunnel portal.
Tunneling Work Process
Once the survey is complete, the tunneling work can begin. The construction process involves a series of steps, beginning with the excavation and removal of soil and rocks. Next, the tunnel is lined with concrete and reinforced with steel. Other components, such as ventilation and drainage systems, are then installed, and the tunnel is inspected for safety. Finally, the tunnel is opened for use.
Line and Grade Stakes in Tunnels
Line and grade stakes are an essential part of the construction process in tunnels, as they are used to ensure accuracy when constructing the tunnel. These stakes are typically set in the roof of the tunnel to prevent displacement and destruction from regular traffic and machinery. Additionally, if the stakes are placed on the floor, they should be offset into an area along the tunnel’s edge.
Transferring the alignment underground
Plumb Bob for Vertical Alignment
Plumb bob is a traditional tool used for vertical alignment during long tunnel excavation. It is typically suspended from a wire or twine and controlled by suspending it in a pot with high viscosity oil. This ensures that the bob can oscillate slowly and precisely, allowing for an accurate vertical line to be established.
Optical Collimator for Vertical Alignment
Optical collimators are becoming increasingly popular for vertical alignment during long tunnel excavation. This type of plummet is manufactured so that it can be interchangeable with theodolites on their tripods. As the line of sight of a theodolite in adjustment will transit in a vertical plane, it can also be used to check perpendicularity. Additionally, optical collimators are more convenient than plumb bobs, as they can be used to set marks directly on the floor of a completed shaft without the need for wires.
Laser for Vertical Alignment
Laser equipment is also used to provide a vertical line of sight during long tunnel excavation. A laser generates a light beam of high intensity and of low angular divergence, which can be projected over long distances. This allows for a visible line of reference that can be used to ensure accuracy and precision during excavation.
Levels in tunnels:
Transferring Levels Underground
Transferring levels underground can be relatively straightforward, with little difficulty encountered at the ends of the tunnel. However, use of specialized equipment is required for measuring the height of the shafts. The most commonly used equipment includes steel tape, chain, constructed rods, and steel wires. More recently, EDMI (electronic distance measurement instruments) have also been used to measure the height of the shafts. The main objective when using any of these items is to accurately measure the height of the shaft relative to a benchmark of known value.
Maintaining a Vertical Line:
Ensuring the EDMI unit and reflectors are in the same vertical line is an important feature of EDM. This is essential to ensure accurate measurement and to make sure that the readings are reliable.
Stable Support:
It is important for EDM to have a stable support for both the EDMI unit and the reflectors. This ensures that the readings obtained are not affected by external factors such as vibration or movement.
Visibility:
For EDM to operate properly, good visibility between the EDMI unit and the reflectors is essential. This will ensure that the readings obtained are accurate and reliable.