| NO. OF PART. [5] | NAME OF PART. | NO. OF HOLES. | ||
|---|---|---|---|---|
| 1. | Heading | 6 | to | 9 |
| 2. | Right wing of heading | 3 | to | 5 |
| 3. | Left wing of heading | 3 | to | 5 |
| 4. | Shallow trench with core | 2 | ||
| 5. | Deepening of trench to floor | 6 | to | 9 |
| 6. | Narrow mass of core to left | 3 | ||
| 7. | Greater mass of core to left | 6 | to | 9 |
| 8. | Culvert | 1 | ||
| Total section | 30 | to | 43 | |
[5] The location of the parts numbered is shown by [Fig. 14], [p. 36].
The quantity of explosives required for blasting depends upon the quality of the rock, since the force of the explosives must overcome the cohesion of the rock, which varies with its nature, and often differs greatly in rocks of the same kind and composition. The quantity of explosives required to secure the greatest efficiency in blasting any particular rock may be determined experimentally, but in practice it is usually deduced by the following rules: (1) The blasting force is directly proportional to the weight of the explosives used, and (2) the bulk of the blasted rock is proportional to the cube of the depth of the holes. It is usually assumed, also, that the explosive should fill at least one-fourth the depth of the hole.
The following table gives the depth of holes and amount of dynamite used at each advance in the [Fort George Tunnel] illustrated on [page 135].
| Order of Firing. | Kinds of Holes. | Depth. | Charge. | Kind of Dynamite. | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Bench Holes | - | 1st round 2nd round | 4 | grading | 3′ to 5′ | 50 | lbs. | 40% | climax | |
| 5 | bench | 9′ 6″ | 45 | „ | 40% | „ | ||||
| 6 | trimming | 3′ to 9′ | 42 | „ | 40% | „ | ||||
| Heading Holes | - | 3d round | 8 | center cut | 9′ | 56 | „ | 60% | „ | |
| 4th round | 8 | side | 8′ | 48 | „ | 40% | „ | |||
| 5th round | 6 | dry | 8′ | 36 | „ | 40% | „ | |||
CHAPTER IV.
GENERAL METHODS OF EXCAVATION: SHAFTS: CLASSIFICATION OF TUNNELS.
A number of different modes of procedure are followed in excavating tunnels, and each of the more important of these will be considered in a separate chapter. There are, however, certain characteristics common to all of these methods, and these will be noted briefly here.
