Engineering Rock Mechanics

460

6)

Questions 75.7-75.70: excavation principles

For the case where the TBM has four 500-V electrical motors

providing the torque to drive the cutting head. Each motor uses

10 A when rotating freely, and 110 A when the head is cutting.

Neglect the thrust for this case and take the cutting rate as 3 m/h.

(ii) For-the same cutting information in part (i) but with an additional

continuous thrust of 2.7 MN and an increased cutting rate of

3.6 m/h.

(b) Does the work associated with the thrust contribute greatly to the

specific energy required?

415.4 Comment on the magnitudes of the specific energy values ob-

tained in Q15.1, Q15.2 and Q15.3 for the different circumstances of a

laboratory compression test, blasting, and using a tunnel boring ma-

chine?

Q15.5 The objective of 'pre-splitting' when blasting is to create a con-

tinuous new fracture plane through the rock mass (which will become

the final rock surface) before bulk blasting removes the rock up to the

pre-split plane.

This rock removed up

to the pre-split plane

(a) List the most important factors that need to be controlled to ensure

that the pre-split blasting techniques will indeed create a continuous

pre-split fracture.

(b) The three photographs opposite show different rock slopes that

have been pre-split. The pre-split plane has been created first; then bulk

blasting has removed the rock up to the pre-split plane, as indicated in

the sketch above. Thus, the photographs show one side of the pre-split

'plane' which is the final slope.

an acceptable pre-split rock face;

a pre-split face where the fractures have adversely affected the pre-

a pre-split rock face where one or more of the necessary factors

Which photograph is which case?

Each of the photographs illustrates one of the following:

splitting; and

intimated in (a) above has not been properly controlled.

415.6 In order to be able to estimate the blasting specific energy for

different rock masses (and hence the different amounts of explosive

required for blasting) a Blastability Index (BI) has been proposed (Ji-

meno et al., 1995) using five rock properties. The BI is shown in the

table below, with the various ratings related to the specific rock mass

properties.