a new method for determining hazardous roof conditions

7/29/2019 A New Method for Determining Hazardous Roof Conditions

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AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERSTechnical Publication No. a088Class F, Coal Technology, november 1946

A New Method for ~ e t e r & n i n ~azardous Roof ConditionsB Y C. G . BRENNECKE*ND R . T. GAL LAG HER,^ MEMBERA . I . M. E .

WITH DISCUSSION(Chicago Meeting. February 1946)

Tms paper is the result of a researchprogram entered into cooperatively bythe General Reinsurance Corporation andLehigh University in January 1944. Theresearch program was instituted to finda more positive method to determine thepresence of hazardous roof conditionsin mining operations. The co-authors,together with Mr. R. D. Currie, Engineerfor the General Reinsurance Corporation,started working on the project in January1944, first undertaking a library search;second, a preliminary study based onsome existing equipment; third, the designof new and more suitable equipment;and finally, the field testing of equipmentand methods.

PURPOSEAny study of the accident statisticsof mining operations will disclose therather startling fact that approximately

50 to 60 per cent of all fatal undergroundaccidents in the anthracite and bituminousmines are caused by falls of roof andcoal, the metal and nonmetal mines show-ing only a slightly smaller percentage.Advancements in safety engineering haveproduced definite results in the decreaseof accidents due to blasting, haulage,electricity, and explosions but no suchdecrease is shown in those caused by falls

TABLE .-Fatdities from Falls of Cod,Rock, or Ore as Contpared with TotalUndernround FatalitiesEPeriod

Fall ofPeriod Ore or1 Lock

'G Under-groundTotal Per Cent

U.S. Bur. Mines Bull:. Coal Mine Accidents inthe U.S. for Various Years. Metal and Non-metalMine Accidents in U.S. for Jarious Years.

Under-groundTotal

Manuscript received at the office of the Of roof, coal, or ore. Table I l . 2 is a corn-Institute Peb. zo . 1946.* Head of the Department of Electrical Engi- pilation of statistics as published by theneering. North Carolina State College, Raleigh, united states B~~~~~ of ~ i ~ ~howingNorth Carolina.t Associate Professor, Department of Mining the percentages of fatal accidents causedEngineering. Lehigh University, Bethlehem,Pennsylvania. References are at the end of the paper.

Copyright. 1946, by the American Institute of Mining and Metallurgical Engineers. Inc.Printed in U. S.A.

Per Cent


2/17

7 A NEW METHOD FOR DETERMINING HAZARDOUS RO0,F CONDITIONSby roof falls in the various years. Thecoal mines show a gradual increase in thepercentage of fatal accidents of this typefrom 1906 to the present. The percentageincrease is th e resu lt of t he decrea sein the other types of fatal accidents andno t necessarily a n increase in th e acc identsfrom falls of roof.

Th ere is a gre at dea rth of inform ationon roof-testing methods in the miningliterature. Most of the published dataon mine roof deal with the control ofmine roof to aid in the mining operationor to prevent surface subsidence. Somefew articles have appeared within recentyears on the testing and control of mineroofs fmm a sa fety ~ t andp o in t .~ -27

Three independent rcsearch projectswere undertaken during the 1930's bythe Safety in Alines Research Board inEngland and the Mining and Coal Divi-sions of th e United St ate s Bure au ofMines. These projects originally wereno t directed toward detect ing hazardousroof conditions but ccrtain of the resultsof these te sts are applicable to it. A sho r trCsumC of thes e findings is pe rti ne nt her e,for they have lcd to the present methodan d equipm ent design.

The Safety in Mines Research Boardin England and the Coal Division of theUnited S tates Bureau of Mines starte dinvestigations in t h e coal fields of En glan dand the bituminous fields of the UnitedStates, respectively. Most of this researchwork w as in th e form of convergencestudies ; tha t is, studies of the distanc eth a t the back or roof of a n undergr oundopening tends to sag or drop down as theam oun t of ma terial removed increases.The floor also tends to heave or rise upinto th e opening as removal of ma terialtakes place. T h e floor mov em ent is smallin relation to the roof m ovement. I nEng land m ost of the w ork was done inlong-wall mining operation^^.^^-^^ while

in the United States the project wasapplied in general to room-and-pillarThese studies were directedtowa rd roof mo vem ents as affected bymining methods, rate of removal, per-centag e of recovery, an d surface subsi-dence. Of the data obtained, the followinginformation was found of value to ou rresearch program in the detection ofhaza rdous roof conditions:

I . Convergence studies indicate whenthe main roof might fail (the distancebetween t he free en d of the m ain roof a ndthe pillar line).

2. Convergence data indicate whenbending of the main roof is sufficient tocause buckling in the immediate roof(i.e., the immediate roof changes fromsafe to hazardo us).

One of th e prog ram s in England6cbrought ou t the in teres t ing fac t tha twith certain types of roofs the positionof the tension cr ack th a t appears parallelto th e long dimension of th e opening couldbe controlled; that is, i ts position relativeto th e width of th e opening could be pre-determined. This tension crack could becaused to appear along one rib simplyby h avi ng th at corner of the working facenear the rib in a more advanced positionthan the remainder of the working face.

The Mining Division of the UnitedStates Bureau of Mines started an in-vestigation on pillar 1 0 a d i n g . ~ ~ ~ ~ ~n t h i sprogram the material forming the pillarwas studied in the laboratory. Pi l larspecimens were vibrated under differentloads and a curve was obtained showingth e relation of acoustic velocities to load.The actual mine pi l lar was then vibratedand its acoustic velocity determined.An inspe ction of th e curve would givethe load on the pillar. These tests haveno direct bearing on the present projectbu t the y did lead to the succeeding Bur eauof Min es studies on rock bur sts, whichhave a direct bearing on this researchprogram.


3/17

C. G. BRENNECKE AND R. T. GALLAGHER 3T he rock-burst program was under- PRELIMINARYW O R K

taken by D r . Leonard Ober t , Senior For the f ir s t fie ld tes t made in thePhysicist , and M r. W ilbur Duv all , under locat ion chosen in the Do rrance coll iery,th e general direction of D r. R. S. D ea n, two sets of microseismic recording equ ip-

FIG. I.-BLOCK DIAGRAM OF U.S. UREAUF MINES'MICROSEISHIC RECORDING EQUIPMENT.Assis tant Director , Uni ted Sta tes Bureauof Ob ert an d Du vall designedequipment to detect and record the micro-seisms that were the result of minuterock movements. This equipment is de-scr ibed part ial ly later in this report andmore ful ly in the United States Bureau ofM ines p ~ b l i c a t i o n s . ~ ~ ~ ~ ~he rock-burstprogram was directed toward determiningwhen and where a rock burst would occur .The equ ipment fo r th i s s tudy has beenused suc cessfully in a nu m be r of localities.

The l ibrary search indicated two l inesof a tt a c k on the problem of haz ardo usroof conditions :

I . T he use of convergence me ters.2. The use of a device to amplify andrecord natural rock noises or controlled

blows.The la t ter method was f inal ly adoptedas the more pract ical and on i t i s basedour present method and equipment .

T he Lehigh Valley C oal Co. madeavai lable i t s mining operat ions for thenecessary f ield studies. The Dorrancecolliery was chosen because mining opera-t ions were being carr ied on in manydifferent coal seams. This immediatelygave access to different roof conditionsfor test ing various procedures and equip-ment. A si te was chosen in the Five-Footseam where the roof rock was a sandysla te wi th num erous ver t ica l cracks runningalong both th e str ike an d the d ip of thecoal seam. T he coal seam a t th is locali tyhad a dip of ab ou t 30". This par t icularsection of th e Five -Fo ot seam had beenfirst mined about three years previous toth e beginn ing of t h e field studies.

ment were borrowed from the UnitedSta tes Bure au of M ines through th ecoo peratio n of Dr . R . S . D e a n a n d D r .Leonard Oher t . Dr . Ober t was k indenough to contribute his time in helpingto insta l l an d adj ust the equipm eot .T he pa rts of th e microseismic equip-ment are shown in the block diagram inFig. I. The geophone used as a vibrat ionpickup consists of a piezoelectric crystalclamped in canti lever style and mountedtogether wi th a matching t ransformerin a section of iron pipe approximately thedim ensio ns of a stic k of p ow der. Th elinear amplifier receives the output ofthe geophone and amplif ies i t by anad jus t ab le amount tha t may be as mucha s ~oo,ooo imes. The output of the l inearhigh-gain amplifier is then fed to alogarithmic amplifier, which produces anoutput propor t ional to the logar i thmof i ts inpu t signal , and thu s reduces therange of signal variation until differencesof more th a n ~o,ooo o I can be recordedclearly on one inch of tap e width . Th epower amplifier, which follows the loga-rithmic amplifier, amplifies the signalby an amount suf f ic ient to actuate thetape recorder shown as the f inal unit .The ent i re equipment i s dual in nature;that is , two separate geophones areprovided with the amplifiers described,and record simultaneously on a singletape.At the recording stat ion a hole wasdril led in the roof ma ter ial a t a n angleof about 5" to the horizontal . The holewas made deep enough to accommodatea s m a n y a s th r ee g e op ho ne s s i m u l t a r l e ~ u s l ~ .


4/17

4 A NEW METHOD FOR DETERMINING HAZARDOUS ROOF CONDITIONSA spot on the roof in the en t ry opposi tethe dri l l hole was designated as tests t a t i o n N o . I (0 i t . ) . O the r s t a t ions werechosen on the roof a t intervals of 2 5 f t .a d v a n c i n g u p t h e e n t ry t o w a rd t h e c o a lface for a total distance of 2 50 fee t .

At each of the tes t s ta t ion s t rad i t iona ltapping methods were used to locate aspot where the roof was t ight and onewhere i t was loose. The roof was struckw it h a mi ne r' s b a ~ n d mi ne o fficialsand engineers of experience were askedto judge the t ightnes s of the roof from theres ul ta nt soun d. Vibrat ion of t he roofresul t ing from the tapping and fe l t byfinger t ips was also used a s a cr i terion.On th e basis of the opinions form ed, sp otswere ma rked " t i gh t J ' o r " loose" and soreco rded in the da t a .

A t t he end of the prel iminary fieldmeasurements , dr i l l ho les were made a teach of these spo ts to check whe ther theorig inal judgment as to t ight or loosecondi t ion was correc t . It is interest ingto note tha t in severa l ins tances wherethe t rad i t i ona l me thods gave a fau l tyindicat io n of th e condit ion of the roof,a nearly correct indicat ion was givenby t he equ ipmen t be ing t e s t ed . Th i scomparison i s shown in Table 2 .

The f i rs t t es t s us ing the microse ismiceq uip m en t consisted of ta ppi ng th e roofw ith a sm a ll h an d h a m m e r a t th e v a ~ i o u st ight and loose posi t ions for a l l s ta t ionsfrom o to 2 5 0 f t . N o t m u c h h a d b e enexpec ted from this series of tes ts , bu t asurprising degree of correlat ion was foundbetween the condit ion of the roof and thetape record produced. In genera l , a b lowmade a t a t i gh t posi ti on p roduced arecorded s ignal of a s mu ch a s 1 0 t imes( 2 to I on the logari thmic sca le) the s ignalproduced by an ident ica l b low on looserock.

A specimen recording from the f i rs t'k tests is shown in Fig. 2 . T h e d a t a r e p r o -? '&. A , . * . ' d u c e d w e re t a k e n d u r i n g t h e e a r l y m o rn i n g

. . .' ' . , hours , t o avo id , a s fa r a s poss ib l e , t he


5/17

C . G . B R E N N E C K E A N D R . T. GALLAGHER 5

reco rding of noises inciden tal to theope ration of th e mine. E ven so, the resultsobta ined f rom tests a t the posi tions far thestremoved from the geophone hole wereobscured b y the registering of vibra tionscaused by mine cars in motion, cablesstr iking the rock, manway doors closingforcibly. and the like.

T he figures a t the to p of the record ofFig . 2 give the t ime. S tar t ing a t j :15 a .m. ,seven successive blows were struck on"t ig ht" roof a t s ta t ion No. I , followed

with a rooo-cycle low-pass filter. Thesmall degree of difference between thetwo records indicates that , in the slaterock under test , the energy spectrum ofvibrat ions result ing from shock is fair lyuniform from zero to several thousandcycles, even af ter the vibrat ions have beenat tenuated by several hundred feet oftransmission.

A comparison of Table 2 and Fig . 2is of interest . W hen, as a t the 25-f t, 75-f t ,and 100-f t stat ions, the "t ig ht " rock is

T A B L E .-Sample Duto Ohtailzed at Dorrance Colliery wi th C7.S. Bu re au of Mi ne sMicroseismic Equipm ent

Roof Condit ion Found a f t e r Comple-tion of Testa(Roof Band Drilled a t Each TestPoeition for U-inch Depth)

- Ra sm c e of Crackmpth - seal.-idth - No Scale

af ter a shor t pause by seven blows on"loose" roof a t the same location. Oneminu te late r , a t 5:16 a.m., the processwas repeated a t s ta t ion No. 2 (25 f t f romgeophones) and, af ter similar intervals,a t a l l the remaining s ta t ions. Dis tancesa n d " t i g h t " (T) and "loose" (L) designa-t ions are given a t the bottom of therecord. A t four of the sta t ion s the testwas repeated.I t is in t e res t ing to no te tha t t he upperrec ord ing s of Fig. 2 were made using anamplifier con tainin g a 1000-cycle high-passfilter, while the lower record is that ofimpulses passing through an amplif ier

truly t ight and the "loose" cracked, therecord sh ows a co nsistently higher responsefor a blow on the " t igh t" than for oneon the "loose" (this is n ot so well show na t the 25-ft st at ion , where the receivedampli tude of the blows on "t ight" causedthe recording pen to str ike i ts stop) . Onthe o ther hand, when, as a t the 50- f tand 125-ft s ta t ions , both " t igh t" and"loose" proved to be sol id to a dep th ofone foot , reversals of the expected dataappear .Results obtained in these f irst testswere sufficiently promising to make itseem worth while to continue this l ine of


6/17

6 A NEW METHOD FOR DETERMINING H A Z A R D O U S R O O F CONDITIONSinves t iga t ion. Seve ral d i ff icult ies were the ou tp u t s ignal of the l inear ampli f ierap pa ren t , however . Obv ious ly , it was i rn - was moni to red by a ca th ode - ray osc il lo -poss ib le to im pa r t exac t ly the s am e scope. Th is \ \$as done in a rl a t t e m p t tomo me ntum to the ha nd ham me r each de te rmine whe ther an y d if fe rence couldt ime i t was used. I t app ear ed des i rable , be observed in the \vave sha pcs of s ignals

F I G .3. FIG. 4FIG. 3 -ASSEXBLED COXSTANT-BLOW HA.\LhIElI.F I G .4.-DISASSE~IBLEDONSTANT-BLO\\' HAAI>IER.for th is reason, to develop some type ofa u t o m a t i c h a m m e r t h a t w o u l d g i v e areproducible b low. Secondly , the va r iousgeophones on loan f rom the Uni ted S ta tesBureau of Mines were found to differgreat ly among themselves in response,an d th is ha d to be considered as a possib levary ing f ac to r . F ina l ly , the bu lk andweigh t o f the equ ipment made i t qu i teunsu i tab le fo r an y so r t of por tab le use ,and i t required 60-cycle a l tern at in g cur ren tat 115 volts , a type of electr ical servicenot general ly avai lable in mines .

I t s h o u l d b e m e nt io n ed t h a t a s t h eprel iminary tes t proceeded in th is locat ion

received a s a resu l t of b lows on t i gh t andloose rock. The osci l loscope indicat ions ,whi le they ver i f ied the previous observa-t ion tha t h igher ampl i tudc s igna l s wererece ived f rom the t igh t rock than f ro m theloose: d id no t ind ica te w i th a n y r egu la r i tya pronouncecl difference in the frequencycom pone nls . While a nu m be r of te s tsshowecl a rather greater content of highfrequencies in the response f rom t ightrock, the indication dicl not seem reliableto an y degree . Ro ugh penc il ske tc hesof the osci l lograms \ i rere m ad e b u t noa t t e m p t w a s m a d e t o p h o t o g r a p h t h eoscil locope screen.


7/17

C . G . B R E N N E C K E A N D R . T . G A L L A G H E R 7

It was dec ided to cons t ruc t a smal lpo r tab le l inea r ampl i f i e r , the ou tpu t o f

'1'0 overcom e the hu m an erro r i~ iv ol ve d which \vould be rectif ied an d read on ain t e s t ing by means o f a hand hammer , m i c r o a m m e t e r . A nu rnb er of t rial arnplificrs

4, ,J s,LEVER

FIG. . FIG. .);I(:. 5.-~\\IPLIFIER-RE\'ERSESIDE.FIG. .-AUPLIFIER-TL-UE SIDE.

the cons tan t -b low hammer shonrn inF igs . 3 a n d 4 was deve loped . F ig . 3 s h o w sthe as sembled hammer as i t \vou ld appearwhen held again s t thc roof re ad y too p e r a t e . A heavy s tee l head i s d rawnback wi th in the ou te r i ron p ipe , ex tend inga pair of springs, and is held in posit ionb y t u r n i n g t h e h a n d l e so t h a t a d o g o n t h ehan dle rod engages in the tee th of therack . Wi th the hand le in pos i t ion , a sshown in F ig . 3, a s l ight counterc lock\v iseturn of the knob re leases the head, whichis then car r ied up to s t r ike the rooF.T h e force of the b low is determ ined by t hepos i t ion in which t he dog h as been engaged.T h e v a r i o u s p a r t s o f t h e h a m m e r a r eshown and l abeled in F ig . 4.

were cons t ructed ant1 tes ted wi th varyingdegrees of success in the s am e loc atio nwhere thc p re l iminary t e s t s had been runan d a l so in o th e r pa r t s of the mine . F ro mthe exp erience g ained in these tes t-s , i twas decided to bui ld a pocket-s ize ampl i f ier ,u s ing hear ing -a id vacuum tubes . F igs .j , 6, a n d 7 shot \ th is ampl i f ier , bothopened ancl assembled . F ig . 8 is a wiringd iag ram fo r the ampl i f i e r . The c i r cu i tshow n h as an over-a ll gain of approx i-m a t e l y j0 ,000. Hear ing -a id ba t t e r i e s a reused as power supply . A geo pho ne of thesam e type used in the Burea u of M inesm i c r o s e i s m i c e q u i p m e n t a n d a s m a l lm i c r o a m m e t e r c o m p l e t e t h e e q u i p m e n t .F ig . g i s a pho tograph of t h e e n t i r e e q u i p -


8/17

8 A N E W M E T H O D F O R D ET ER bIIIIN IN G H A Z A R D O U S R O O F C O N D I T I O N Sm e n t , a n d F i g. ro sh ow s a n o pe ra to r th e ~ s - \ ~ o l tla te-supply l ine whi le theequ ip ped fo r te s t ing . T h e reduc t ion in s ize s e t was in opera t ion migh t cause a spa rk ofac he ve d by th i s design made the equ ip - su fhc ien t in tens i ty to ign it e me than en ~ e n tpract ica ble for use even in the leas t ill . the conc entra t io i l found a t t he work-

ing face . Fo r th is reason the sw i tching

I

FIG ~ . - ~ - \A I PL I F I ~ ~RASEacces s~b le a r t s of the m ine , wh lle ad eq ua teper fo rmance w as r e tam ed

I t \vas neces sa ry a l so to g lve con-s idera t lon , whi le de s ig n~ ng Lhe ampl i f ier ,

a r r a n g e m e n t s h o w n i n F i g . 8 was used .I t is t o be n o t e d t h a t o n l y t h e 1 4 6- v ol tf i lament c i rcui t i s ac tual ly swi tched onand off by switch S, . T h e q j - v o l t b a t te r yi s p e r m a n e n t l y c o n n e c t e d , b u t s u p p l i c scu r ren t on ly wh i le the f i l amen ts a reh e a t e d .

F ie ld work w i th the po r tab le equ ip -ment- gave r esu l t s th a t were eve n moreencourag ing than those ob ta ined w i th theBu rea u of RiIines eq uip m en t . A cal ibra t ionprocedure was adop ted to t ake ca re o fvar ia t ion s in the t ra~;sm iss ion proper t iesof th e variou s types of rock te s ted . T h iscal ibra t ion was efiected as fo l lows:

In F igs . 5 to 8, R, is a h igh-res is tancepo ten t iomete r , ac ros s wh ich the ou tpu ts ignal of amplif ier s tage 2 a p p e a r s . I tmap be ad jus ted by a screwdr iver . S iv i tchSZ i s a three-pos i t ion swi tch , operatedby a lever , which connects the input ofs t a g e 3 to va r ious po in t s on the ne tworkRf , Rs , R4 . The se r esi s to r s a re so chosen

to the ever -p resen t exp losion hazard in a s to compensa te fo r the no rmal a t t en ua -coal mines . A ny swi tchin g ar ra nge m ent t ion of the acous t ic s ignal in the rock .th a t a l lowed the pos sib il i ty of b reak ing T hu s , the e lect r ica l a t t en ua t io n ach i eved


9/17

C . G . B R E N N E C K E A N D R. T. G A L L A G H E R 9by moving S p f rom the roo-i t . posit ion method bu t provides a more posi tiveto the do-f t. posit ion corresponds to the me ans of indicat ing whether the roof isacous ti c a t t en ua t i on o i 40 f t . of rock . t i gh t o r l oose . I n the new metho d , a s i n

An are a of kn ow n t igh t roof is used for the old , the roof is st ru ck a blow wit h

FIG. 9.-C O>~ PLETE OOF-TESTIXG EQUIPNENT.cal ibra t ion purposes . With swi tch S2 s e t ' t s o me i n s t ru me n t . T h i s h l o w s e t s u p a na t the 100-ft . posit ion , and the geophone acoust ic wave, which travels in al l direc-ins ta l led in the roof roo f t . f rom the t ions a t a ve loci ty depending on the char -poin t of hammer b low, potent iometer R I acter i s t ics of the roof ma ter ia l . T h eis adj us ted unti l a hlow on the roof registers acous t ic \ \lave t ravels by the sho rte stfu ll sca le on the mic roa mm ete r . T he t ime pa th to t he geophone , where i t isin s t r um en t i s t hen ca l ib ra t ed for t he conver ted in to an e l ect romot ive force.typ e of rock in the ca l ibra t ion area . W he n T h e charac ter i s t ics of the e lec t romot ivein use for ac tu al meas urem ents , swi tch Sq force deperid on the characterist ics of theis se t to a posi t ion corresponding to the acous t i c wave , and the ene rgy ou tpu t o fd is tance between hammer b low and geo- the geoph one is a funct ion of the ener gyp h o n e . R , is not readjus ted except to of the acoust ic waye. T h e genera tedcheck ca l ibra t ion or to reca l ibra te for a e lec t romot ive force is then ampl i fieddifferent t v ~ ef rock. sufficientlv to ac tu at e the needle of an. i nd ica t ing in s t rumen t . The read ing g iven

T H E NEWROOF-TESTINGE T H O D by th is ins t ru lne nt is a measure of theT he new roof tes ting* m ethod re ta ins o ut p ut of the amplifier, which is , in tur n,

a po r t ion of t he o ld sound an d v ib rat ion ~ ' op or t io na l t o t he amp l i tude o f t h eorig inal acoust ic wave. The grea ter t h e*Pa te nt appl ica tion made b y The General energy in the acoust ic wave, the grea terReinsurance Corporation on method andequipment. the sw ing of the needle.


10/17

I 0 A NEIV M E T H O D F O R D E T E R M I N I N G H A ZA R D O U S R O O F C O N D I T IO N ST h e energy ava i lab le in the acous t ic

wave when i t reaches the geophone is afunct ion of t he or ig inal energy impa r tedt o the roof when t he blow is s t ru ck. th e

FIG. 10.-OPERATOR EQUIPPED FOR ROOFTESTING.

l eng th and con t inu i ty of the acous t icpa th anc! the phys ical charac ter is t icsof the acous t ic medium .If a cons tant-b low ham me r is used for a l l

roof tes ts , the impar ted blow wil l a lwaysbe the same. T t was found dur ing the t es twork tha t the b low f rom a miner ' s barproduced energies that were near ly a lnraysles s than those f rom the cons tan t -b lowhammer , a l though the d i f fe rence was no tg rea t . The leng th o f the acous t ic pa thgeneral ly is a measure of the shor tes td i s tance be tween the b low po in t and thegeophone . Th is d i s tance i s no t ve ry c r i ti ca l

when tbe equ ipm en t i s used fo r d i s ta~ lce sno t exceed ing loo f t . , an d can b e ad j u s t edfor on the amplif ier . The phys ical proper-t ies of th e acous t ic medium a re imp or t an t ,for the wave veloci t ies wil l vary con-s iderably in d if ferent rocks . Th is d if ferencein velocit ies is accounted for in the calibra-t ion of the equ ipm ent a t the s t - a l t of an ysel ies of tes ts . T h e cont inu i ty of theacous t ic medium i s by fa r the mos t im-por tan t - f ac to r . If the medium i s con-t inuou s , on ly a cer ta in f ract ion of theenergy will be clissipated in the trans-miss ion f rom blow point to the geophone;bu t if the med ium is disc ont inuo us, agre ate r f ract ion of the energy is los t a t thed iscon t inu i ty . If the cliscontinuity is ana i r ga p (c rac k) , the energy loss i s g rea t .Th us , the acous t ic wave will ind ica te thepresence of a n a i r ga p , which , in t u n ,is a means of determining the t ightness orloosen ess of th e roof mat eri al." "If a tes t area is chosen so that the

dis tance f rom the geophone to the blowpoin t i s equa l fo r bo th a t igh t and looseroof ant1 the same blow is imparted toeac h ty pe of roof in t ur n, the blow on th etight roof \ \ r i l l ca use t he neeclle of th eindicat ing meter to regis ter a fu l l swing,\\;bile the blow on the loose roof will causethe needle to move only a smal l par t of aFull swing.

At the conclus ion of the ba s ic researchan d eq u i p m en t d e s i g n , r z comple te s e t sof th e newly developed equi pm en t wereco n s tr u c ted a t L eh i g h U n i v e r s i t y , s othat f ie ld tes ts could be carr ied out s imul-taneous ly in a number of different loca-t ions . Cons iderab le ca re \vas t aken toensure th at each of these sets had ident icalcharac te r i s t i cs . The ampl i f i e r s and thegeophones were tes ted for response andread jus ted o r r ebu i l t un t i l the des i reddegree of uniformity was obtained.

Fig. r r shows the method used to t es tand ca l ib ra te the geophones . By means


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C . G . B R E N N E C K E A N D R . T. G A L L A G H E R I I

of a n audio-oscillator an d a loud-speak er, this hole in such a manner that the outputaud ible tones of u nd istorte d form a nd of the piezoelectric crys tal was fav ored .cons tant volume were produced at numer- The hole was then plugged with cottonou s frequen cies u p to 5000 cycles. T he waste, to seal it from air blasts.

FIG. I.-ACOUSTICTESTS FOR G E O P H O N E CALIDRATION.geophone response was measured by meansof a calibrated amplifier and indicatingmeter . The geophones as f inal ly used hadessentially flat response over severalthousand cycles except a t a resona nt peakwhich usually occurred between 1600and 1700 cycles. The authors did notconsider this peak of eno ugh impo rtanc ein the projected tests to warrant furtherredesign.The new equipment was placed in theha nd s of th e variou s op era tin g officialsin a num ber of the anth rac ite miningcompanies for thorough tr ial and test ingunder various operat ing condit ions. Theequipment was cal ibrated in the f ieldbefore the beginning of each test and thefollowing procedure was then adopted:

I . 4 holC was drilled approximatelyhorizontal ly in the roof s trata to betested and the geophone was inserted in

2. The geophone was connected to theanlpl i i ier and the bat tery supply turnedon. The amplif ier was then adjusted forthe proper distance between blow pointand geophone.3. The rotf rock was struck a blowwith the cons tant -blow hammer and

the needle swing was noted. A full swingwas taken to indicate a t ight roof and asmall swing or no swing to indicate aloose or hazardous roof.

4. The roof was then tested in a similarmanner in var ious other areas , al l wi thina radius of IOO f t from the geophone hole.Wh en the 103-ft dis tance had to beexceeded, it was necessary to place thegeophone in a new hole.

The da ta accumula ted f rom the f i e ldtests indica te t he following:


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I 2 A NEW METHOD FOR DETERMINING HAZARDOUS ROOF CONDITIONS

I . Excellent results have been obtainedwhere the roof rock has a fast velocityand where the cracks, if present, arenearly vertical. Rocks with fast velocityare the slates and some limestones.

2 . Reliable results have been obtainedin roof rocks having a medium fast velocitywith vertical cracks. This type of roofrock requires a higher over-all gain on theamplifier than does the type listed underitem I . The thick-bedded or massivetypes of roof rocks falling in this classifica-tion have not given results as consistentas those of the thin-bedded types. Rocks ofmedium fast velocity are the limestonesand shales.

3. In roof rocks having a slow velocitythe results have not all been good, pri-marily because an amplifier with too smallan overall gain has been used. A fewroofs of this type with vertical cracksrespond well to test procedures. Rockswith slow velocity are the sandstones andconglomerates.

The problem of detecting hazardousroof conditions in mining operations hasbeen with us since the first mine wasstarted. The testing method in presentuse is one that was handed down fromearly mining history. Its judicious usehas saved many lives from rpof falls, butit has never been entirely satisfactorybecause it depends upon a human inter-pretation of the sound heard or the vibra-tion felt. The new method eliminates thehuman interpretation and makes a positiveindication of the roof conditions.

The new method will not give satis-factory results when used with a coal roof.

When the mine roof is very bad, allmen connected with the operations knowof its condition and are particularly carefulto watch the roof. In these mines all rooftests wcru!d indicate hazardous conditionsand the use of the new equipment wouldnot .be warranted.

When the roof strata being tested arenearly vertical, the results are reliable.Bu t completely to test such a roof wouldrequire numerous geophone holes, forthe bedding planes act as vertical cracks,and all tests, except those in the stratain which the geophone is located, wouldappear as loose roof.

Under normal operating conditions,where the roof rock may be tight orloose, or may be tight when first exposedbut gradually turn into a loose roof afterexposure, the new method and equipmentwill give reliable results.

The new testing method should be anaid in deteimining the necessity of timberand just where it should be placed. Sys-tematic timbering is in vogue todaymainly because we have had no definitemethod of determining just where timbershould be placed. I t is better to have toomuch timber than not enough. We allknow of many places where timber in-stalled many years ago has graduallydry rotted and is now worthless, but theroof is still standing as strong as ever.This timber was wasted.

When a mine roof is tested by the newmethod and found to be hazardous, it ispossible to map out the hazardous areavery quickly.

Regardless of what method is used todetect hazardous roof conditions, theremedy is still dependent on humanunderstanding. No detecting method initself will correct the condition. Thehazardous condition must be removed orproperly timbered in order to render thearea safe.

The sponsoring company, The GeneralReinsurance Corporation, and i ts Engineer,Mr. R. D. Currie, are responsible for theresearch program. Mr. R. D. Currie hadthe original idea and workea with theauthors, rendering considerable aid andmany useful suggestions and criticisms.


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C. G. BRENNECKE AND R. T. GALLAGHER 13The Lehigh Valley Coal Co. and itsofficials gave every cooperation in theoriginal studies carried on in the Dorrancecolliery. Thanks are due the men of thevarious anthracite companies who havewillingly given their time and energy tothe field testing of the equipment. TheHearing Aid Division of the RaytheonManufacturing Co., through Mr. F. B.Simmons, was most helpful in providingnew types of vacuum tubes for use in theamplifiers. The authors are obligatedto Mr. Charles Hemmerly, Mr. EarlRodgers, and Mr. Edward Manning foraid and assistance in the constructionof the equipment, and are deeply indebtedto Prof. A. C. Callen for his general direc-tion and criticism of the research program.

BIBLIOGRAPHYI . Coal Mine Accidents in the United States(for various years). U.S. Bur. MinesBull.2. Metal and Non-Metal Mine Accidents int h e United States (for various years).U.S. Bur. Mines Bul!.3. J. E. Jones: Mine Roof Face and Ribs.Illinois Mining Institute, 1940.4. D. W. Phillips: Researches on Falls ofGround. Iron and Coal T r . Rev. (1041)- . .142, 147-149.5. R . Yates: Th e Occurrence of SeriousAccidents from Falls of Ground inDerbyshire , Leichestershire, and Not ting-hamshire (1933) Inst. Min. Eng. Trans.86. 201.6. Nor th of England Ins tit ute of Miningand Mechanical Engineers.a . Firs t Rept. : Suppor t of Workings inMines Committee. Arthur Walker.Progress Reports of a n Investigation

into the Causes of Falls an d Acci-dents Due to Fal!s in Bord-and-PillarWhole Workings: Trans. Inst. Min.Engrs. (1930) 81, 427.b. Second Rept.: Support of Working inMines Committee. Arthur Walker.Zbid (1933) 86, 278-294.c. Third Re pt.: Support of Working inMines Committee. Arthur Walker.Ibid. (1934) 89, 220-240.d . Fou rth Rept.: Suppor t of Working inMines Committee. Arthur Walker.Embodying a Report by H. M. Huds-~ e t h . . W. Phil l i~ s, nd A. Walker.?bid.-(1935) 91, 349-367.t-. Fifth Rept.: Application of Roof Sup-port. Zbid. (19371 94, 324.f . Sixth Rept. : Broken Workings. Zbid.(1940) 9% 97.7. A. Nelson: Roof Testing Mine an d QuarryE n s . (1941) 6. 261-264.8. A. Winstanley: Mechanization and Fallsof Ground Accidents. Trans. Inst. Min.Engrs. (1941) 101, 328.

J. W. Pau l: What a Coal Miner Can Do t oPrevent In ju ry from Falls of Roof.U.S. Bur. Mines Inf . Circ. 6315 (1930).J . W. Paul: What a Mine Foreman CanDo to Prevent Injury from Falls ofRoof in Coal Mines. U.S. Bur. MinesInf . Chc. 6344 (1930).J . W. Paul: W ha t the Superintendent of aCoal Mine Can Do t o Prevent Inju ryfrom Falls of Roof. U.S. Bur. MinesZnf. Circ. 6345 (1930).J. W. Paul: How to Prevent Death andIn iu ry from Fa!ls of Roof in Coal Mines.u.S. Bur. Mines Znf. Circ. 6570 (1932).J. W. Paul and J. G. Calverly: A Studyof Falls of Roof in Pennsylvania MinesCo nt i~ uo us o th e Monongahela River.U.S. Bur. Mines Tech. Paper 550 (1933).J . W. Paul and J. N. Geyer: A Study ofFalls of Roof and Coal in Mines ofHarrison County. West Virginia. U.S.Bur. Mines R. I . 3110 (1931).J . W. Paul and J. N. Geyer: A Study ofFalls of Root an d Coal in Mines Oper-ating in the Sewickley Coal Bed inMonongalia County, West Virginia.U.S. Bur. Mines Tech. Paper 520 (1932).J . W. Paul and J. N. Geyer: A Study ofFalls of Roof and Coal in Mines Oper-ating in the Pittsburgh Coal Bed inMarion and Monongalia Counties, WestVirginia. U.S. Bur. Mines Tech. Pager522 (1932).J. W. Paul and J. N. Geyer: A Study ofFalls of Roof a nd Coal in Mines Oper-ating in the Pittsburgh Coal Bed, Pan-

handle District. West Virginia. U.S.Bur. Mines Tech. Paper 534 (1932).J . W. Paul and J. N. Geyer: A Study ofFalls of Roof an d Coal in the PittsburghCoal Bed. West Virginia U.S. Bur.Mines Tech. Paper 547 (1933).J. W. Paul an d L. N. Plein: A Study ofFalls of Roof and Coal in Mines in theNo. 8 Field of Eastern Ohio. U.S. Bur.Mines R. 1. 3070 (1931).J . W. Paul a nd L. N. Plein: A Study of theMine Roof of the Pittsburgh Coal Bedin the Pittsburgh Mining District.U.S. Bur. Mines Tech. Paper 541 (1932).J . W. Pau l and L. N. Plein: A Study of th eMine Roof of the Coking District ofWest Virginia. U.S. Bur. Mines Tech.Paper 5 6 i (1935).J. W. Paul and D. L. Sibray: Places ofOccurrence of Injur ies from Falls ofRoof. U.S. Bur. Mines R. I . 3203 (1933).J . W. Paul. T. Tomlinson, C. W. Owings:Reducing Accidents fro m Fall s of Roof inCoal Mines. U.S. Bur. Mines Inf. Circ.6225 revised.H. Tomlinson: A Stud y of Falls of Roofand Coal in Mines of Lincoln County ,Wyomina. U.S. Bur. Mines R. I . 7188. - -(1932): -I-I. Tomllnson: A Stu dy of Falls of Roofand Coal in the Brook Cliffs andWasatch Pla teau Coal Fields of Utah.U.S. Bur. hlines R. 1. 3189 (1932).H. Tomlinson: A Study of Falls of Roofand Coal in Northern Colorado. U.S.-.Bur. hlines R. 1. 3199 (1933).H. Tomlinson: A Stu dy of Falls of Roofand Coal in the Rock Springs Field.


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Sweetwater County. Wyoming. U.S.Bur. Mines R. I . 3207 (1933).Midland Counties Ins titution of Engineers:o. Research Committee: First ProgressRep ort , The Influence of Variation inthe Nether Roof on the Incidence ofFalls (Top Hard Seam). Tran s . Inst.Min. Engrs. (1932) 84. 93-1 10.b . Research Committee: Second ProgressReport. T he Influence of Var iation inthe Nether Roof on the Incidence ofFalls-Roadways. I b i d . (1933) 87. 36-

f . The Beeston Seam of West Yorkshire-11. I b i d . (1934) 88, 31 9~ 33 6.g. The Nature a nd B ehav~ our f t he Floorof t he Parkgate and Beeston Seams.

I b i d . (1935) 90, 328-347.h . The Supp ort of Underground Roads.I b i d . (1936) 92, 158.

i . Falls of Ground on Mechanized Facesin the Barnsley Seam. Zbid. (1937) 94,770.i . goof Behaviour in Machine Cut Faces.- I b i d . (1939) 98, 114:Lancashire and Cheshlre Safety in MinesCommittee:a . Report on the Trials with ConcreteP r o ~ s t the Coal Face. Tran s . Inst.

47.c. Research Committee: Third ProgressRepo rt, Th e Influence of the Nature ofth e Nether Roof on Fal!s of Side at th eFace. Zbid . (1936) g r, 93.d . Fif th Progress Report. Th e Effect ofVariation in Rates of Advance in

&fin. Engr. (1941) 101. 196.b . R. Faulkner: Roof Control in the ArleySeam. Zbid. (1930) 81, 507-524.South Staffordshire and WarwickshireIn st itut e of Mining Engineers:a . Safe Workings in Mines Committee:First Proaress R e ~ o r t . r an s . Inst. Min.

Workings Hav ing Different Ty pes ofRoof. Ib i d . (1938) 95, 124.e . Seventh Progress Report, The Supportof Various Types of Roof a t the CoalFace, Part I. I b i d . (1940) 99, 246.Nor th Staffordshire In st it ute of MiningEngineers :a. Safety in Mines Research Committee:The Set ting of Face Props in InclinedSeams, Par t I. Tran s . Inst. Min. Ennrs.

Engrs. (1 9~ 2) 5,-116.b . Safe Workings in Mines Committee:Second Proaress Report . S u ~ ~ o r ttRoadheads. f id . (19j4) 88, 46.-c. Safe Workings in Mines Committee:Third Progress Report, R outine ofWorking Machine Cut Faces. Zbid.-(1932) 85, 2.b . Safety In Mines Research Committee:The Se tting of Face Props in InclinedSeams, Part 11. I b i d . (1936) 91, 324.

c. E. H. Vallis: Use of Steel Props a t CoalFaces. I b i d . (1939) 97, 325-338.d . The Cockshead Seam Part I. GeneralWorking Practice. Ib i d . (1940) loo, 232.e . Th e Working of the Bowling AlleySeam in North Staffordshire. I b i d .(1941) 101, 135.Mining Ins tit ute of Scot land :a . A. Winstanley: Longwall Roof ControlTran s . Inst. Min. Engrs. (1930) 81, 373.b . Report by Safety Committee: TheResistance of P r o ~ s . b i d . (19.12) 8s.

(1935) 89, 241.d . Some Aspects of Roof Control in Rela-tion to Its Effect on Safety and theEconomy of Roof Supports . I b i d . (1940)99, 181.H. P. Greenwald. E. R. Maize. I. Hart-mann and G. S. Rice: Studies of Roof~ o % m e n t n Coal Mines-No. I . Mon-tou r Ten Mine of the Pittsburg h CoalCo. U.S. Bur. Mines R. I . 3355 (1937).H. P. Greenwald and E. R. Maize: Studiesof Roof Movement in Coal Mines-No. 2. Crucible Mine of t he CrucibleFuel Co. U.S. Bur. Mines R. I . 34-52.-(1939).H. P. Greenwald. E. R. Maize and E.Tho mas : Studies of Roof Movement inCoal Mines-No. 3. Gibson Mine of theHillman Coal and Coke Co. U.S. Bur.

. - - -.377-388.c. J. Finlay and A. Winstanley: TheInteraction of Longwall Workings. I b i d .(19.33) 87, 172-189d . Report by Safety Committee : Th eEffec t and Interaction of LongwallWorkings. I b i d . (1936) 91, 180.e . R e ~ 0 r t bv Safetv Committee: pro^

-- -Mines R. I. 3506 (1940).. P. Greenwald, E. R. Maize and E.Thom as: Stud ies of Roof Movement inCoal Mines-No. 4. Studies of Sub-sidence of a Highway Caused by MiningCoal Beneath. U.S. Bur. Mines R. I .3562 (1941).Obe rt: Measurement of Pressures onRock Pillars in Underground Mines. I.U.S. Bur. Mines R. I . 3444 (1939).Obert : Measurement of Pressures onRock Pillars in Underground Mines. 11.U.S. Bur. Mines R. I : 3521 (1940).Obert : Use of Subaudlhle Nolses for th ePrediction of Rock Bursts. I. U.S. Bur.Mines R. I . 3555 (1941).Obert and W. Duvall: Use of SubaudibleNoises for the Prediction of RockBursts. 11. U.S. Bur. Mines R. 1 . 3654

~esistanc;. I b i d . (;936) 93, 93.f . Report by Safety Committee: Supportat Longwall Roadways. Zbid . (1936)93, 391.g. Report by Safety Committee: Methodof Suppor t a t Machine Cut LongwallFaces. Zbid: (1939) 97, P41.Midland Institute of Mlnlnn Engineers,-Progress Reports:a. Th e Beeston Seam of West Yorkshire.Tran s . Inst. Min. Engrs. (1929) 79, 324.b . Metho ds of Working and Roof Supportin the Barnsley Seam. Ib i d . (1930) 81,

3 5c. The Support of Underground Roads bySteel Arches. Ib i d . (1931) 82, 242.d . Supp ort of Roof a t th e Coal Face byMeans of Steel Props. Ib i d . (1932) 84,

(1942).L. Obert and W. Duvall: MicroseismicMethod of Predicting Rock Failure inUnderground Mining. I. U.S. Bur.130-153.e . Methods of Working an d Roof Supportin the Parkgate Seam. Ib i d . (1933) 86.231-263.Mines-R. I . 3797 (1945).L. Obert and W. Duvall: MicroseismicMethod of Predicting Rock Failure in


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C. G. BBEN NEC KE AND R. T. GALLAGHER I sUnderground Mining. 11. U.S. Bur.Mines R. I . 3803 (1945).

44. P. B. Bucky: EBect of ApproximatelyVertical Cracks on the Behavior ofHorizontally Lying Roof Strata. Trans.A.I.M.E. (1934) 09, 12-227.DISCUSSION

H. P. GREENWALD.*-T~~erson who isinterested in the general subject of improve-ments in methods of determining the soundnessof the immediate roof in mines will find allparts of this paper interesting and instructive;whereas the mine official looking for aid insolving his own immediate problems is likelyto limit his attention mainly to the sectionbeginning with the design of new equipmentand ending with the conclusions. The firstof these two viewpoints needs little discussion,as the paper speaks for itself sufficiently wellexcept for the section dealing with field results.I t is worth while to emphasize tha t the scienceof electronics made development of thisdevice possible. The writer of this discussionhas had access to unpublished data coveringa similar investigation attempted a numberof years ago, before present-day electronicdevices were available. That investigationdid not go far and finally died; that it did sois now nothing to excite wonder. The one ideathat both groups of investigators had incommon was the need for a blow on the roofof fixed and reproducible intensity.The section on field results gives one asense of anticlimax. A summary of less than ascork of lines leaves questions on details t hatthe paper dces not answer. The informationgiven in the summary is valuable, but moredetailed data doubtless would be interestingand possibly instructive.An operating o5cial might be expected toconclude that this method needs furtherdevelopment and test before he adopts it.Unfortunately, the authors report that themethod is not satisfactory for coal roof; thisstatement would eliminate its use in manybituminous-coal mines. If the method canbe applied it seems likely that it might beuseful mainly in passageways that standopen for some time and in which there a re no

Superintendent, Central Experiment Sta-tion: Bureau of Mines, Pittsburgh. Pa. Dis-cussion published by permission of the Direc-tor, Bureau of Mines. U. S. Department ofthe Interior.

wide or sudden changes in character of theroof strata. How well the device would fitinto the operating cycle of a rapidly movingface, mined mechanically, remains to bedetermined.It is understood that with the work re-ported here the investigation was stopped.There is a definite need for additional workalong this or parallel lines to obtain a moreuniversal device.RICHARD~l uZ ~. *- Af te r a careful studyand analysis of this paper, I feel that theauthors are to be commended for the workthey have done in tryifig to develop an instru-ment or device which, if only to a small degree,

arouses interest in further research towardways and means of reducing accidents fromfall in mines.From reports of inspectors who investigateall fatal accidents, and from records publishedby various State Departments, as well as bythe United States Bureau of Mines, it isrevealed that more than 50 per cent of allaccidents in coal mines are caused by falls.It must not be assumed, however, that allfall accidents are caused by fall of rock, slate,fire clay, limestone or shale.A large number of accidents occur fromfall of coal, the coal falling from the face of theworking place, the rib of the working place,along haulage roads and traveling ways,or where there is a coal roof. The authorsadmit the instrument is of no value in prevent-ing accidents caused by the fall of this mater ial.There have always been three conventionalmethods of testing roof, and it would stillbe necessary to employ these methods if thedevice conceived by the authors were toreach a stage of perfecticn. The methodsreferred to are thcse of hearing, sight andtouch. To elucidate:

B y Ea r: The roof is tested by hitting i t witha heavy bar, or, with a pick or axe. The twolatter tools were used almost exclusively, someyears ago, and in some cases are still beingused.B y E y e : A visual examination of the roofis made to detect cracks and slips.B y Touch: The miner places the tips ofhis fingers against the roof while it is being* Secretary of Mines. Commonwealth ofPennsylvania. Harrisburg, Pennsylvania.


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tapped with a bar or pick. By the vibrationshe can detect the looseness or tightness of theroof.There is another factor to be consideredin trying to develop an instrument for testinga roof, and that is, in 8s per cent of the casesthe fall of roof t ha t kills or injures the work-men was not exposed 30 min. prior to the timeof the accident; in other words, 30 min. pr~orto the time the roof fell the conditions hadchanged to such an extent that the roofbecame unsupported as a result of the cut tingor blasting of coal.

The majority of acridents are caused byfalling rock which has been jarred loose byblasting, or roof being exposed. In inanycases the piece of fallin:: rock or slate thatcauses an accident is comparatively small.

When one considers this fact it is veryevident that to reduce accidents very muchfrom falls at the working faces it would bealmost necessary to hsve some person stand-ing there all the time with an instrumenttesting the roof to protect the men who aredoing the actual work a t the working faces.

From our investigation it is obvious to usth at three fourths of the men who ar e killedby falls were struck by rock or pieces of roofso small that if the workmen had examinedthe roof a few minutes prior to that time byusing the conventional method of testing roofthey would have disccvered that the roof orparticular piece of s late was loose.

Kearly all the falls occur a t th e working facesmhere there is frequently a draw slate orbone coal immediately above the coal thatthe miner tries to support by posts untilhe can remove all the coal from underneaththe slate or bone coal. After the coal hasbeen recovered he removes the posts andpermits the slate or bone coal to fall. Incases such as this, there is no need for a roof-testing device of the character outlined by th eauthors because the slate or bone is usuallybroken or cracked into small pieces, and oftenthe slate has separated from the roof aboveit and is resting on the posts supporting it,the miner being aware that it is loose. Someof the fall accidents are caused by men knockingthe posts out from under the slate so that itcan fall.In certain methods of mining, and especiallywhere ribs and pillars are drawn, falls are

induced to relieve the pressure on the coal byextracting the timber that is suppcrting theroof. Frequently, in this method of mining,cribs are built to help support the roof untilthe coal has been recovered, after which menare sent into such places to remove the cribsand the timber to induce the fall. In thiskind of work men are sometimes killed orinjured, and a testing device would be of novalue because the roof is badly shatteredand broken just prior to the extraction of thetimber.I be!ieve, however, tha t there is a place for

this instrument, and that is for examiningroadways through which men have to travelto and from their work, or haulagewaysthrough which they are transported to andf ron~ heir working places. Th e roof a t theworlring faces can be tested while the manis working, but in miles of haulage roads itis impossible to test the roof very often.Th e Bituminous Mining 1,au.s of Pennsyl-vania require that where men are transportedto and from their working faces the haulageroad must be examined by a competent personwithin two hours prior to the time for themantrip to enter the mine, and a report isrequired of such an examination.

The law does not, however, require that acertified mine official examine the roof. Insome places I fear this examination is madeby the examiner riding on an electric loco-motive while looking for possible cracks orplaces where the roof has commenced to 'sag,and it seems to me that under such circum-stances the roof-testing device would be mostvaluable.Once drilled, the holes In the rib betweenthe coal and the roof would be used as long asthe haulage road lasted, provided a squeeze didnot take place, which caused the coal under-neath the hole to slough off and destroy thehole.

If it were possible to develop an instrumentin the form of a sounding bar by which thedegree of t ightness or looseness of th e roofcould be registered by means of a n indicator,so that the man testing the roof would nothave to depend entirely upon his physicalsenses, bu t would have the benefit of a mechani-cal device, I believe it would be of some prac-tical value in reducing accidents from fallsat the working faces.


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C . G . B R E N N E C K E A N D R . T. GALLAGHER I 7

T he auth ors have done a great deal of workin developing the instrument for test ing roofand are to be commended for the researchthey have done. The device has cons iderabletechnical and scientific value, b u t is of dou btfulva lue as fa r as application to falls of roof a ndcoal a t the working faces is concerned.

C . G. BRENNECKE n d R. T. GALLAGHER(authors ' reply) .-The auth ors wish to tha nkM r. Greenwald an d M r. Maize for thei r cr it ica ldiscussion of th e paper. T h e conclusionsreached in this research were presented togive as complete and impa rtial a picture of th eresults as possible. Bo th posit ive and negativeresults were noted.Mr. Greenwald notes the failure to includeactual f ie ld data in the paper . The f ie ld datawere not included because i t would haveincreased the length of the paper considerably,mitl iout, in the opinion of th e authors , mate-rial ly incressing the value of the results . Topicture the results i t would have been neces-sary to include sketches of every mining areatested along with the test figures accumulated.The test figures themselves are meaninglessunless accompanied by the field condit ions.Mr. Greenwald's discussion is particularlypert inent when he sugges ts " the need foraddit ional work along this or s imilar l inesto obtain a more universal device." Suchadditional test ing and research is nov beingconducted by the Mine Safety Appl ianceCo mp any of Pit tsburg h, Pennsylvania.Mr. Maize r ight ly points out the threeconventional methcds of testing roof in usetoday, and s ta tes that more than hal f thenumber of accidents are causcd by roof falls .

Evident ly present-day methods e i ther areno t followed or a re of li ttle value in testingfor the presence of hazardous roof conditions.The method and equipment proposed by theauthors would not do away wi th the con-ventional meth ods of test ing b u t would su p-plement the m. O ne of the premises stated bythe au thors was tha t the method and equip-ment proposed would not in i tself remove thehazardous roof falls , but would indicate ina positive m an ner th e portion of th e roofth a t was hazardous . If th e miner knows of t h ehazardous roof condition and still persists inexposing himself to i t without ad eq uat eprotection, no testing method or equipmentwill be of aid in reducin g th e num ber ofaccidents caused by roof falls. Where thissi tuation occurs, a better safety educationprogram is indicated.

In the design of the proposed equipmentand method for testing hazardous roof con-dit ions, the authors had in mind a devicethat could ul t imately be used by the minerhimself. The device was intended to be asau tom atic as possible a nd to give resultsthat did not depend on any of the humansenses , such as the ear , eye , or touch. Theinstrument finally designed was not as uni-versally useful a s ha d been ho ped; b ut i tcomes as close to the "sounding ba r" sug-ges ted by Mr . Maize as the au thors foundpossible.

While this part icular research programhas been completed, it is the hope of theauthors that this or a s imi lar program wjll becontinued by other agencies, in order toreduce the great number of undergroundaccidents caused by roof falls.

a new method for determining hazardous roof conditions

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