Mohave County miner., 1883-05-06
Swelling and BOttSm-Break
 ing Ground.

breaking ground. In some campl there is more houble from this thar
 in others, of course, according to the
 nature of the formation. When it
 occurs, however, it requires very
 skiIlful and careful timbering to pre-
 vent accid mt. The cause of bottom
 breaking ground is given by Mir
 Henry $. Drinker, Tn his elaborate
 work on "Tunneling," when occur.
 ring downward, as being iH the ma- jcriiy of cases due to hidden caves
 or sink holes as in limestone forma- ton. Breaks at bottom occurring upwards, he says are the cases where
 the ground is so heavy, that though
 the sides and roof may be firmly
 held, the material is so running as to
 be semi-hquid, and therefore trans
 mits the side pressure around, and so forces up the tunnel or drift bot
 tom where # is not properly secured
 by an Invert.
 In one of the tunnels of the Cln-
 cinnati Southern R. R., the curious
 fact was noted by the resident engi
 neer, that 1t would not only fall
 from the roof, but would not stay
 down in the bottom. The bottom
 was hard tough sandstone in lay CTS of four to eight inches thick These layers would, with rumbling sound, spring up and break into pieces with a report like 8 small blast. This would happen sometimes after no work had been done in the tunnel for several days, so that 1t could not be attributed to the effects of powder. One or two layers would generally come up, breaking off at the wall and then the sprlnging cease. These lodges came up with ch force, that men sitting on them, would be thrown over In one in- stance a sill. on which posts for tim bering were set, was put in on the bottom; the sill extended seven and q half feet beyond the last post, which was Jagged and packed overhead, so that at the post. the sill was lmmov- able. One of these lodges springine up, forced this sill-a piece of 8x1z | of white oak--up out of its position s;;x inches in seven and half feet.
 Similar phenomena have been ob served in the granites at Munson. Mass, aud in the lower carbonifer ous sandstone of Ohio. It is prob able that the cause of the break is owing to a state of tension existing | in the rock or place; then, when the strata are cut, tendency to wrong resnlts.
 In cutting the Sutro tunnel much trouble was occasionally experienced from bottom breaks, more, however, from swelling ground. This was nsually met in the roof by extra tim bering. while the bottom was simply allowed to rise until it reached q ,er+ain nOint where if is cut down again repeatedly,
 At point 17.890 feet from the tunnel mouth they entered g belt o3 swelling ground composed of DOT phyry and clay, which was very diff cult to penetrate. The rock sWell6o 70 such degree that timbers 14x16 nches thick would be broken three lours after they were placed, and, al ,hough gang of men were kept a X ork easing up behind the timbers. hey were continually breaking, anc t was exceedingly difficult to make lny progress. Not only did the tim ,ers break but at times the posts would press through the caps. Fol | long time they could suggest DC means to withstand the pressure, but inaliy new plan of timbering was devised which was successful.
 They first placed the ordinary F .1G iuch square set of timbers, and hen made an excavation above the zap. in which they placed two angle braces meeting at an apex. By this means the great pressure from above gas prevented from coming directly ,n the caps. 1t beng taken up by hese angle braces, upon which it vas exercised first. They were thus pressed gradually into the sides of he tunnel until the apex at which hey met was finally forced down Jose to the caps. When that time rrived they were dug out and placed gain. In this manner the main imbers were saved, and since they placed two inch tagging on top of hese braces and tkree-inch tagging ,n top of the main caps, the men were protected from falling rocks, md the work could progress without lelay. The tagging above the. up per set was purposely lighter than