EXrAVATlXi; MArHlNtls AND 1CUCK DKlLLs 



19 



CHAPTER III. 



EXCAVATING MACHINES AND ROCK DRILLS: 

 EXPLOSIVES AND BLASTING. 



Earth-Excavating Machines. Comparatively few of the labor- 

 saving machines employed for breaking up and removing loose 

 soil in ordinary surface excavation are used in tunnel excava- 

 tion through the same material. Several forms of tunnel 

 excavating machines have been tried at various times, but only 

 a few of them have attained any measure of success, ami these 

 have seldom been employed in more than a single work. In 

 the Central London underground railway work through clay a 

 continuous bucket excavator (Fig. 12) was employed with 



Pit;, r.'. Soft Ground liurket Kxrav 



iin.- : 'Vntrnl lx>n<1n rii.lorjrrotiml 



considerable saving in time and labor over band urk. ami in 

 some recent tunnel work in America tin- contractors made 

 quite successful use of a modified form of strain shovel. These 

 are the most recent attempts to use excavatini; machines in 

 soft ground, and they, like all previous attempt*, must ho 

 classed as experiments rather than as examples of common 

 practice. The shovel, the spade, and the pick, wielded by 



