a. Graduate School of Science and Technology, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan
b. National Institute of AIST
c. Faculty of Engineering, Kumamoto University
d. Tokyu Construction Co., Ltd.

Specific energy of the chipping upon after jet drilling is discussed from a latticed multi-point drilling experiment, to make clear the utility of chipping as an excavation measure in hard rock. The experiment is conducted using the full-size specimen of Amakusa sandstone of the uniaxial compressive strength: Sc= 84 MPa, and Granite of Sc= 190 MPa. The optimum drilling interval is evaluated as well as the specific energy Es, after consideration to the relation between the crater depth: h and the excavation volume: V. It is clarified that the dimension ratio of m=Sc/Es is in a range of from 0.40 to 0.55 in the case of the driving pressure: pI=90 MPa. A model named the repetition model is proposed, to forecast the specific energy of the continuous chipping by the Third Power Method, and its validity is successfully verified by comparing the computation to the experimental results. Subsequently, it is discussed that the continuous chipping results in an excavation of higher efficiency than both of the pure water jet cutting and the abrasive water jet cutting.