Overbreak and tunnel geological survey

Overbreak is the excavation of rock beyond the designed tunnel profile (the volume or thickness outside the neat line), caused by geological conditions and blasting. It is expressed as a percentage of the design area:

$$\text{Overbreak}(\%) = \frac{A_{actual}-A_{design}}{A_{design}}\times 100$$

It increases excavation, filling and support cost and decreases safety.

Outline of the geological survey of a tunnel

1. Desk study: topographic and geological maps, air photos and satellite images, earlier reports, to fix the corridor.
2. Surface mapping: along the alignment at 1:5,000-1:1,000; map rock types, joints, bedding, foliation, faults, folds, shear zones, springs, and slope stability at portals.
3. Geophysical survey: seismic refraction and resistivity profiles along the axis to find weak zones, water, and depth of weathering and overburden.
4. Drilling: core boreholes at portals, below valleys, and at suspected faults; RQD, packer tests, groundwater level.
5. Rock testing and classification: RMR, Q; lab strength and swelling tests.
6. Geological longitudinal section along the axis with predicted rock classes, water inflow and support types, and hazards such as squeezing, rock burst, gas.
7. During construction: face mapping, probe drilling ahead, monitoring, and updating the geological model.