Overbreak and its importance

Overbreak is the excavation of rock beyond the designed (neat) tunnel profile. It is the volume or thickness of rock removed outside the theoretical excavation line, usually expressed as a percentage of the design area or as extra thickness in cm.

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

Importance

• Increases excavation, mucking and disposal volume.
• Needs extra shotcrete/concrete lining to fill the void, so cost increases.
• Reduces stability: loosened rock above the crown can fall.
• Delays the construction schedule and endangers workers.
• Shows the quality of the rock mass and of blasting practice, so it is a performance indicator in the contract (payment is normally only for the neat profile plus a permitted tolerance).

Factors of overbreak

Geological:

1. Joint spacing and orientation (joints dipping to the crown or parallel to the axis).
2. Bedding, foliation, faults and shear zones.
3. Weak or weathered rock, low rock strength.
4. Groundwater flow and weathering.
5. High in-situ stress (spalling).

Construction:

1. Excavation method (drilling and blasting vs. TBM).
2. Blast design: drilling accuracy (look-out angle), hole spacing, explosive type and charge per hole.
3. Time lag before support is installed.
4. Tunnel size and shape.

Good practice: controlled blasting (smooth blasting, presplitting), systematic mapping and prompt support.