Chip shape and secondary fragmentation through TBM excavation

The chips through TBM tunneling are well-known for one of useful indices to reflect the geological conditions. The flat and elongated chips whose width are equal to the spacing of cutter trace indicate the cutting face with less joints and good practice of TBM excavation with less secondary fragmentation rate. Through a case history in granitic rock, the authors proposed the new index, which is the ratio of length of major axis to thickness. Also the authors studied the relationship between the index and the excavation efficiencies. In conclusion, it was clarified that chips with the new index over 3.5 were generally observed when a TBM drove with less than 30% the secondary fragmentation rate. (author)