It has long been recognised that cracked and fractured rock has an effect on the performance and penetration rate of tunnel boring machines.
The performance of mechanical excavation machines in intact rock is relatively well understood, however, little research work has been undertaken to assess the effects, on these machines, of cracked and fractured rock.
This paper describes a series of carefully controlled laboratory experiments that detail the behaviour of disc cutters in composite blocks of rock(simulated jointed rock). The results of these tests are detailed and a discussion on the effects of cracked and fissured rock on the performance of tunnel boring machines is presented.
One Of the major factors affecting the performance of tunnel boring machines is the degree of fracturing of the rock to be excavated For the purposes of this paper three broad categories of fractured rock are proposed to simplify discussion and comparison. The three categories are defined in Table 1.
In order to investigate the performance of TBN's in fractured rock a series of carefully Controlled laboratory experiments were undertaken using single pristine disc cutters in simulated jointer rock specimens. Details of the rock cutting rig and instrumentation have been published previously(Howarth 1978,1980a).
The rock cutting rig and layout of the simulated jointed blocks of rock are shown in Figure la and lb.
(Table in full paper)
The performance of mechanical excavating machines in intact rock has been extensively researched and is well documented. Laboratory investigation of the performance of rock cutting tools in category C type fractured rock would be extremely difficult due to the large number of variables and technical difficulties associated with preparing suitable rock samples. This study, therefore, was designed to elucidate fundamental and practical aspects of excavating category type B fractured rock.
In order to simplify experimentation and interpretation of results the following system was used to define the geometry and dimensions of the simulated jointed blocks and joint patterns.
It should be noted that for the purposes of this investigation jointed rock is defined as a series of regular blocks of rock, artificially separated to give open joints or fissures of known regular dimensions.
The experimental programme involved measuring cutting forces(thrust and d) rolling),yield(quantity of rock excavated and energy expended(see Appendix for in definitions of rock cutting parameters) cutting for the following range of joint specifications J(20–l00,0–3,30–90).
Three experiments were undertaken. The first investigated the relationship between joint width(T),penetration(P), disc-edge angle(ф)and confining force(C-see Appendix). This was a partial factorial experiment, the design of which is based on a method of developing experimental plans proposed by Protodyakanov and Teder(1970a,1970b)and involves a systemic selection of combinations of levels of variable from a full factorial matrix. The variables in this case being those defined above. The method involves the systematic Sampling of a full factorial matrix so that the effects of one variable (at all levels) can be compared with mean values of the levels at which all of the other Variables are tested.
