Rock Burst in the Kan'etsu Tunnel

Abstract

In excavation of the Kan'etsu tunnel, rock burst occurred expectedly. Especially for about 1 year since July 1980, many broken pieces of rock had sprung out frequently. Rock burst should be classified into sometypes, such as springing of rocks, cracks on face without springing of rocks, and only breakdown noises onface or in bed rock. Those types seem to be a series of forms of rock burst. Some examples of occurenceprocess of rock burst are shown in figure-3, -4, -5, -6.From our experiences of rock burst, it may be pointed out that those forms of rock burst have somecharacters. They are as follows:(1) It occurred even under small overburden.(2) It occurred in the formation of quartz-diorite, but not of hornfels.(3) It occurred mostly at face, and few at backward and side wall.(4) The existence of specific areas where rock burst occurred was recognized in the tunnel.(5) At the areas with inflow of ground water, it did not occurred.(6) It was related greatly to joint, fractured zone and so forth.(7) The sizes of brocken rock pieces were various, but the shapes of them were all generally flat.(8) It was closely related with working cycle of tunnel excavation.Some investigations including measuring of initial stress in bed rock and its change during tunnel excavation, core discing in the horizontal bored and sampled core, acoustic emission survey, rock properties tests, andtemperature measuring of face and sidewall, were carried out at the rock burst areas.As a result, significant informations on the behavior of bed rock related with tunnel excavation and onthe relation between rock burst and working cycle of excavation, were obtained.The rock mass at the rock burst areas was elastic and high brittleness. And rock burst seemed to beclosely related to geological weak zone of bed rock.For a countermeasure against rock burst, it was adopted as a keystone of tunnel excavation that many rockbolts were installed in working face. The tunneling method that we adopted by utilizing the above mentionedinvestigations of bed rock and analysis of our experiences of rock burst, may be appraised proper.It is useful that the system of acoustic emission survey is composed completely in order to control tunnelexcavation work at the rock burst areas.