Tunnel Surveying
Specialized underground surveying for tunnel projects. El-Eman Office provides survey services for major projects in Egypt.
Linking the surface to the tunnel
The biggest challenge in tunnel surveying is carrying coordinates and direction from the surface to underground accurately. Surface control points are set by GNSS and total station measurements, then carried through a shaft or the tunnel portal to the inside. In vertical shafts, a plumb line or vertical projection instruments transfer position, while a gyroscope carries direction (azimuth) accurately where there is no outside line of sight. The transfer is repeated more than once to verify it.
The control network inside the tunnel
- A network of points is run along the tunnel walls at spacings that grow as excavation advances.
- The network is usually a traverse whose angles and distances are measured with high accuracy.
- Any small angular error accumulates over long distances, so repeated observations and periodic direction checks are used.
- Visibility is affected by dust, humidity and atmospheric refraction in the warm and cold air inside the tunnel.
Guidance and monitoring of excavation
During excavation, especially with tunnel boring machines (TBM), guidance systems measure the position and heading of the machine relative to the design alignment, and deviations are displayed to the operator to correct the course. The surveyor independently checks these systems with periodic manual measurements. The positions of installed lining rings are also measured to find the actual deviation and document the executed alignment.
Monitoring deformation and convergence
- Fix monitoring targets on the tunnel walls at selected sections.
- Measure the distances between targets at regular intervals.
- Compare readings with the initial reading to detect convergence or settlement.
- Monitor buildings and utilities above the tunnel when tunnelling in cities.
- Notify engineers immediately when agreed alarm limits are exceeded.
For safety, work inside tunnels follows strict ventilation, lighting and entry-permit procedures, and the survey team is trained in them.
Documenting the finished tunnel
After excavation and lining are complete, a final survey of the tunnel documents its alignment and actual sections. The built dimensions are compared with the design to make sure the clear section is not undersized, and a file is delivered showing coordinates and levels along the tunnel. 3D laser scanning is now used to document the whole inner surface quickly and with good accuracy.
- Record the positions of manholes, openings and fittings.
- Keep the data for future maintenance.
- Review deviations and assess their effect on operation.
Frequently asked questions
How do two tunnels driven from opposite ends meet?
Through precise shared control on both sides that keeps coordinates and direction consistent, checked before the expected meeting point with independent measurements to estimate the likely error.
Why doesn't GPS work inside a tunnel?
Because satellite signals do not penetrate the ground and concrete, so direct measurement methods and gyroscopic instruments are used.
Why is convergence monitoring important in tunnels?
Because it reveals movement of the walls and surrounding ground early, allowing support measures before a problem develops.
