Estimation of Size Effects in Rock Strength based on Specific Energy
It is well known that rock strength decreases with increasing of specimen size. Accurate knowledge for the size or scale effects in rock strength is very important to apply the small-scale experimental results to the large-scale underground space design. However, the investigation of the size effects requires tremendous efforts and expenses in most cases. In this study, a new method to estimate the size effects of rock based on the specific energy in excavation was proposed. The method needs minimum efforts without extra equipments. Through the careful examination of eleven excavation methods, it was found that the specific energy decreases with the excavation scale following the equation:
(Specific energy) ∝ (Excavation scale)-2α with α=0.2.
Cutting spacing, penetration or geometrical mean of the two was found to be appropriate as excavation scale. Specific energy is roughly proportional to square of strength. Then, the following equation stands:
(Rock strength) ∝ (Excavation scale)-α .
This result is very conformable to the size effects deduced by the results of the compression tests. The size of cuttings in excavation was examined and the close relation was found between the excavation scale and the size of cuttings. Relation between the specific energy and the work index in comminution was also discussed.
KEY WORDS: Specific Energy，Size Effect，Excavation，Cuttings，Work Index，Particle Distribution