Show PALLING ON ROOFS NI IN COAL MINES ROBBING LINE TIMBE RINGS PLAY SEE by J f W M paul some roof jac mim material jui such sl as shale has a tendency to 01 bend puaa before breaking pule and some mos material such sl as limestone and sandstone tends to 01 bend TOA very little before breaking where the qa coal sa s1 is removed UT in pillar extraction the dill roof breaks or it 4 depresses either through a bending action or 10 through the al ld pillars sel crushing or io if j the dill bottom si is soft jos material majos the al floor may heave unless the dill roof jow eata breaks al S all cju these forces bring biam weight uc on the roof jow supports pue and ultimately result in itt breaking or io crushing them it II would appear under these conditions that the types of jo supports used should oct be so os designed that they will not jou break under cipun the preliminary load col p resulting from the first movement of 10 the roof jool this still desirable result ui in part em xem may X oct be accomplished by the dill use of lo caps on the props by the sn use of jo tapered props or io by specially designed props such liong su as the sarre the proper dzis size of jo caps to ol use can oct be determined luo only by experience the duo cap should oct be the full width of jo ill the top dol of jo the prop oct be not less than ZI 12 inches long and thick enough to 01 admit of jo the prop doid becoming embedded imbedded im ui in the duz cap before f the prop doid will break juais props that have been tapered it at the dill top do or jo it at the aj foot have been found to 01 give good service in ul lingwall long wall work since the tapered pua end will burr cinq under loads that op do not break the aill prop doid A I 1 type dd of jo metal ludw id prop doid do that raj si is being used ul in some european mines PUP and ST is known as the sarre ortus prop cload consists of jo two tj ito or I 1 steel members held together by iron links and wooden wedges these props will yield IL at the joint u and pu decrease ut in length 1261 sl as the weight increases by the depression 30 or the roof this type of jo prop cload on account of jo its sl isoo cost sr is expected to 01 Iq be recovered PIAO after it 41 has suta served its sl dind purpose aso roof jIs stresses Sassa stresses Sas ui in the dill roof lutu material Jula pue and the coal may oct be ui in equilibrium before the coo coal 1 ST is mined but the removal of the dill coal ap destroys spoils this equilibrium and UT in the dill effort made to 01 reestablish reto a e state of jo equilibrium there will oct be movement which results ui in cracks put and edict breaks si ui in the dill roof jow strata some alu atu btu material I almia such su as flexible qs shale DIL and coal teo will yield before tailing this phenomenon si is very boid pronounced with some ale or slate and coal 1100 roofs which throw off or io small pieces S while alm this still movement is s ul in progress pub and ample time given for the dill retreat 1 of JG the dill miner 10 ak who may observe the dill tailing of 10 the gill or ao small pieces the dill harder rocks 1001 itans h 0 as S sandstone pule and limestone being of jo we an inflexible may amu break with little or 10 ou no yielding or 10 warning to ol ill the miner the action of JO the dill roof along the robbing ia line ou si is different ju ajol that of 10 the roof 1001 UT 0 in solid work in solid work in u callju trie sand pue rooms su as le a rule only the dill immediate roof 1001 etu maal ST is the D source of jo falls the weight of jo the MUM main roof 1001 cg carried 1 q by the pillars but ui in robbing work lioa and in u long juo C short wall work the stability of lo the immediate roof is s acted aci Y ill the D action of the main roo 1 which ordinarily ULD can aou t be held ui in place dould by the otil use of jo timber awl i supports sv As the oai is S removed along the gill robbing line out pul and at the face 0 of buol g or short wall work the main roof low begins to 01 subside aa 10 I 1 ten U with a crushing effect on the iwo coal pillars and its move lt uses cracks to ol appear ui in the immediately axial roof thus laal paw the immediate roof ul in the working zone ducz ST is likely to 01 fi pue cause accidents ezzu slug unless it si is maid properly xi pue and apu ade jam nn bouat i yield an P M with till timber which yields without breaking 0 the preliminary load 1 l he direction and length 0 pul and the regularity jait X of jo the al Iq break NL zie re contributing factors to 01 I roof movement and have alq luS senior JD mining engineer bureau o of 0 mines their influence on falls which ew may X occur at places where men OIE are engaged at work eliere the main roof ST is mostly strong shale sudden collapse pa osal of jo the dili immediate roof is s not so os probable su as where strong sandstone or 10 limestone compose the aai material so of the main roof with iak 11 a roof that ST is irregular ia in ui coming mop down si as the dill robbing quoi line retreats there ST is greater danger of jo falls of ju the immediate roof ui in the zone Ak where jq jaak men uw may oct be engaged U ea pdt ut in the dill robbing work methods of JO mining for S safety the dill wooi room and pillar method p that gives most ISOM promise of jo increased houi paavo safety against falls sl is the use of jo narrow Irm MOI rooms pup and wide pillars where wide rooms pula and narrow ellla pillars si daie are used the main lupow roof jow si is often disturbed q by the olli first or lo advance capi DU work and jj if the uius main roof jow roq begins 0 sui to 01 move UT in the gill first mining it 41 makes auw so pillar recovery increasingly dangerous ep e pue and pu much loss of jo pillar coal results I 1 it therefore may ew X oct be seen that the dili question of jo protection e against suver falls of jo roof put and aps sides s si is materially influenced by the method of jo mining practiced timbering along the robbing line should always consist of jo posts with duo cap pieces and when the roof jow si is duzs scaly to or ino cut by cracks or io slips sails crossbars should be used pul and under some daos roof jow conditions fore poling bui or use of jo timber cribs will be act found necessary to 01 t dingui insure the dill CS safety of JO the workmen caution nu telltale or talking props should always emal ax oct be set jos near ill the pud end of jo the pillar which will give OAT warning of JO roof movement the dill immediate Dut roof jow along kuolt the robbing owl line UT in many A mines may aw oct be caused to ol break at au prearranged places by placing oe 0 1 props ut in oi rows ism two om or three j rows being used for boj this still purpose PUL and having substantial cupit eo caps acl placed axed parallel lall to 01 the dill owl line so of fracture or io timber cribs may oct be used to ol good purpose ui in this work ul in pillar work where the dill same amount of jo thought si is given to 04 the dill safety of JO alie workmen SL as ST is given to 01 the dill recovery of 10 the ill coal some S system of jo roof support can we usually be found that will give the maximum protection to 04 the alij workmen pue and such gons a 6 system s si is possible of jo development by a zhuo careful study of jo the aj D local 1 conditions guided by a knowledge of jo engineering principles and practical experience in ui mining |