Geological documentation in tunnelling

Geological documentation in tunnelling is the systematic recording of the geology encountered in the excavated face, walls and roof, during construction, in the form of maps, logs, photos and reports.

What is documented (face and tunnel mapping)

1. Rock type, colour, grain size and mineral content.
2. Rock strength and weathering grade.
3. Discontinuities: bedding, joints, foliation, faults and shear zones, with strike, dip, spacing, persistence, roughness, aperture and filling.
4. Groundwater: inflow rate, location, seepage, water quality.
5. Rock mass classification (RMR, Q) of each round.
6. Overbreak and instability: falls, squeezing, rock burst, gas.
7. Support installed (bolts, shotcrete, steel ribs) and any problems.
8. Photographs, sketches and plan/section of the tunnel (usually at 1:100-1:200), drawn after each blasting round, with chainage.

Procedure

• Mapping is done at the face right after excavation and before shotcrete covers it.
• Data are plotted on the tunnel log (development map of roof and walls) and longitudinal geological section.
• Prediction of the geology ahead using probe drilling and seismic methods.

Importance

1. Checks design assumptions and the pre-construction geology.
2. Choice and modification of support based on the actual rock class.
3. Provides basis for payment, claims and contract disputes.
4. Safety: warns of unstable zones, water and gas.
5. Provides a permanent record for maintenance, future repair and for similar projects.
6. Helps to predict conditions ahead and plan construction.