In igniting a safety fuse an open flame such as that furnished by a match or candle is the most satisfactory. For electric fuses the current is generated by a magneto shown in Fig. 120. Pressing vigorously down on the handle closes the circuit and generates an electric current which heats the platinum bridges and explodes the charges. For the small number of charges used in ordinary construction they are connected in series so that if there is a broken connection anywhere no charge will be exploded. If many charges are to be fired and a line circuit is to be used, the final connection should not be made until just before the charge is to be fired in order to obviate the danger of stray currents firing the charge prematurely. Care should be taken to see that all connections are good and that there are no broken wires on the line.
178. Quantity of Explosive.—The quantity of explosive to be used can be determined satisfactorily only by experience on the job in question, as the factors affecting the necessary quantity are so diverse. The figures in Table 64 indicate the relative amounts needed under different conditions.
| TABLE 64 | ||||||
|---|---|---|---|---|---|---|
| Quantities of Explosives | ||||||
| Kind of Rock | Drift in Feet | Feet[[99]] of Hole | Black[[99]] Powder, Pounds | Dynamite[[99]], Pounds | Grade of Dynamite, Per Cent | Remarks |
| Limestone, Chicago Drainage Canal | 12 | 0.40 | 0.75 | 40 | Gillette | |
| Limestone for crushing | 6 | 1.00 | 0.70 | 40 | Gillette | |
| Limestone for cement | 20 | 0.37 | 50 | Gillette | ||
| Limestone, holes sprung | 15 | 0.40 | 0.26 | 50 | Gillette | |
| Sandstone, side cut | 20 | 0.10 | 1.0 | 0.10 | 40 | Gillette |
| Sandstone, thorough cut | 20 | 0.20 | 2.0 | 0.20 | 40 | Gillette |
| Shale, soft side cut | 24 | 0.08 | 0.7 | 0.03 | 40 | Gillette. Open cut |
| Shale, hard thorough cut | 24 | 0.20 | 1.5 | 0.10 | 40 | Gillette |
| Granite for rubble | 16 | 1.36 | 0.20 | 60 | Gillette | |
| Gneiss, New York City | 12 | 1.33 | 0.60 | 40 | Gillette | |
| Gneiss, New York City | 14 | 0.63 | 0.50 | 40 | Gillette | |
| Syenite, Treadwell Mine | 12 | 1.70 | 0.67 | 40 | Gillette | |
| Magnetic iron ore | 12½ | 0.32 | 0.44 | 52 | Gillette | |
| Trap, seamy | 14 | 0.35 | 0.20 | 75 | Gillette | |
| Trap, massive | 17 | 1.00 | 0.70 | 40 | Gillette | |
| Granite, Grand Trunk | 25 | 0.10 | 0.80 | 50 | 50% dynamite used to spring holes | |
| Clay, rock and Gypsum | Tunnel | 1.00 | ||||
| Hard shale | Tunnel | 2.07 | Grade varied ⅗ at 45%, ⅕ at 60%, some at 100% | |||
| Hard rocky slate | Tunnel | 1.60 | 3.57 | |||
| Hard rocky slate | Tunnel | 1.46 | 3.57 | |||
| Mill Creek sewer, St. Louis | Tunnel | 4.00 | 60 | Mun. Eng’g. Vol. 52, p. 14 | ||
Pipe Sewers
179. The Trench Bottom.—It is customary to dig the bottom of the trench to conform to the shape of the lower 45 degrees to 90 degrees of the sewer if the character of the material will allow such construction. In soft material which will not hold its shape the sewer may be encased in concrete or a concrete cradle may be prepared for the pipe. In rock the trench is excavated to about 6 inches below grade and refilled with well-tamped earth so as to form a cradle giving bearing to 60 to 90 degrees of the pipe circumference. For large sewers to be constructed in the trench special foundations are sometimes built.
180. Laying Pipe.—Before the pipe is lowered into the trench the sections which are to be adjacent should be fitted together on the surface and the relative positions marked by chalk so that the same position can be obtained in the trench.
Small pipes are lowered into the trench and swung into position on a hook as shown in Fig. 124. Pipes up to 15 or 18 inches in diameter can be handled by the pipe layer and helper in the trench without assistance. Heavier pipes may be lowered into the trench by passing ropes around each end of the pipe. One end of the rope is fastened at the surface and the ropes are paid out by the men at the surface as the pipe is lowered. If the pipes have been fitted together and marked at the surface it is undesirable to use this method of lowering as the position in which the pipes arrive in the bottom of the trench can not be easily predicted. A cradle may be used for shoving the pipe into position as is shown in Fig. 125.
Fig. 124.—Hook for Lowering and Placing Sewer Pipe.
