The survey of the final lining intrados is typically carried out either on 50-m long stretches while the final lining is being poured, or after the entire final lining has been poured.
The following information can be obtained:
- The thickness of the final lining at any point (if the geo-referenced surface of the primary lining is available).
- The geo-referenced as-built surface of the lining intrados textured with high resolution pictures, which may be used to:
- Compare the built lining intrados against the design intrados; this may be done at discrete stationing or continuously in three dimensions. The comparison may include dynamic envelopes (e.g., rail, subway, road and highway tunnels), and hydraulic profiles (e.g., water and wastewater tunnels).
- Baseline existing defects by documenting the existing cracks and spalls right after construction (i.e. at tunnel commissioning). As for cracks, the following is provided: crack length, location, and orientation (very useful to ensure that the grouting holes actually intersect the crack). As for spalls, the following is provided: area, depth, volume, and location of each spall.
- Provide a reference for future inspections of the tunnel in order to determine any displacement (e.g., convergence or settlement of the ceiling), and any changes in the lining defects: cracks (length, width, number), spalls (area and number), and water ingress.
The accuracy may be better than 1 mm, and may be adjusted per the client’s needs.