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Publication numberUS2549076 A
Publication typeGrant
Publication dateApr 17, 1951
Filing dateOct 25, 1945
Priority dateOct 25, 1945
Publication numberUS 2549076 A, US 2549076A, US-A-2549076, US2549076 A, US2549076A
InventorsBrennecke Cornelius G, Currie Robert D, Gallagher Robert T
Original AssigneeGen Reinsurance Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Mine roof testing
US 2549076 A
Abstract  available in
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Claims  available in
Description  (OCR text may contain errors)

R. T. GALLAGHER ET Al. 2,549,076

2 Sheets-Sheet 1 MINE ROOF TESTING lleszlf Iii April 17, 1951 Filed oct. 25, 1945 e. l W l,

ATTO RN EYS April 17, 1951 R. T. GALLAGHER ET A1. 2,549,076

MINE ROOF TESTING Filed Oct. 25, 1945 2 Sheets-Sheet 2 @05E/PTD. CUR/WE.

INVENTORS Patented Apr. 17, 1951 MJNE ROOF TESTING Robert T. Gallagher, Bethlehem, Pa., Cornelius G. Brennecke, Raleigh, N. C., and Robert D. Currie, Trucksville, Pa., assignors to General Reinsurance Corporation, New York,'N. Y., a corporation of New York Application October 25, 1945, Serial No. 624,358

3 Claims. (Cl. i3-67) ratus and methods. More specically, the invention comprises apparatus Aand methods for testingl the strength of the rock structure of mine roofs, in order `to identify unsafe rock areas before roof falls occur.

Despite eiorts `taken to reduce mine accidents and fatalities, mine roof falls still account for approximately 60% of the total underground fatalities in coal mines. In other words, over 1,000 miners are killed and more than 15,000 are seriously injured by mine roof falls each year. This results from the fact that the majority of mine workers are employed at the mining face where new roof material of unknown strength is continually being exposed.

The method which fcrgenerations has been employed for testing the rock structure of mine roofs andwhich is still in general use, comprises striking the roof Va sharp blow with a toolv such as a miners bar or a hand hammer and listening to the resulting sound waves emanating from the roof material. The character of the resulting sound is then interpreted as indicating'solid or loose rock. One diiculty with this method is that the sound produced by the ring of the metal bar or hammermay obscure the sound from the roof rock. Although this test is the one most generally employed, it is so unreliable that mine workers frequently disagree as tothe roof condition thus indicated. The only sure test of a roof by this method is possible in the event that the roof material comprises a relatively thin piece of rock which has drawn away from the main roof to form a pocket enclosing air or gas. The sound emitted from the striking of such a formation results in a drummy sound readily identified as'an unsafe roof, and-in this case appropriate props are customarily employed. However, many equally unsafe roofs result from diierent formations which do not produce sounds capable of identification, as where the roof material is irregular in shape or thickness, or where a crack or crevice is at an' angle to the main roof, so that the vibrations are not reflected back to the ears of the tester, or the sounds arenot distinguishable from those emanating from safe rock.'

Another method for determining the condition of the roof rock includes striking the rock to lproduce'vibrations therein and perceiving the vibrations by touching the ngers to the rock.

Occasionally loose rock can be Vthus detected if it is small enough and suiiiciently'loose'tovibrate and, of course, 'the fingertips of the operator are sufliciently sensitive and he is thoroughlyexperienced 'in the technique. Additional methods heretofore employed in determining the strength of mine roofs include visual inspection for slips, aws, inclusions "or change in strata material, and in the interpretation of these observations Aby one highly skilled as the result of long study and experience. Although the records of accidents due to falling mine roofs,'especially in coal mines, have stimulated investigation and research-directed toward lesseningsuch'accident's, no method or'apparatus resulting-in any substantial improvement has been proposed for many years.

The apparatus and method according tothe present invention make possiblevfor the'rst time the testing of the rock structure of mine roofs Withvsucli a high degree of sensitivity and accuracy that unsafe roof conditions can easily be recognized rby evenunskilled workers with great reliability.`r As a resultfof actual experience in coal mines it has been found that unsaferooi conditions can readily be determined'loy`v means of thisv inventiomand.appropriatemeasures-taken to prevent roof falls under circumstances where previous testing-Y' 'techniques 'would' indicate the roof tobesafe'i Briefly, the present invention comprises,l rst, striking the mine roof a single blow, preferably with a constant blow hammer, to generateacompression-Wave train of free oscillations of a certain amplitude vinan area of known rock characteristics, electrically measuring the effective amplitude of this compressionwave train at a detection point in thisrst area, similarly generating another compression wave train of substantially the same amplitude in an adjacent area of unknown rock characteristics, electrically measuring the effective amplitude of the second compression wave train at eiectively the same detection point, and comparing the two measurements. ilf the measurements*arefsubstantially the same it may be assumed" that the rock between the two areas is ksubstantially uniform, whereas if the second measurement is considerably lower thanV the first theindication is that unsafe rock existsr between'the twolocations or test-stations where the compression waves were generated. By' closing in on the low-indication area, vthe boundaries of 'unsafe rock can lquickly beascertained. This procedurecan most effectively be carried out by including. compensation for the attenuation in compression ywave :transmission through the rock resulting fromlthe xvarious distances 4whichfthe compression "waves must travel vfromthe testing stations Awhere the generate the compression waves in the rock, a. pickup device to detect the waves and transform them into electrical vibrations, a suitable electrical amplier energized by the pickup,l andan indicating device actuated by the output of the,

amplifier for indicating measurementsofthe am.- plitudes of the detected waves. The amplifier may include a compensator for the purpose-above mentioned.

A more complete understanding of the invention will be had from the following specification considered in connection with the accompanying y drawings, wherein:

Fig..- 1 illustrates .the-apparatus. in .accordance .with lthe inventionfas employedin testing fa roof in a .coal mine; l

Eig.,2 is a -circuitdiagram of the electrical ap- 1paratus employed inaccordance `with theinvenA Fig. 3 is a plan view of an.assembly ofna constant. blow manner Whichvmay-be used in connection with :the invention;

Fig. 4 isa longitudinal'central cross sectional view of thehammer` of Figf 3;

Fig. `5 is a cross sectional'view-of the hammer taken along the line *5i- 5V of Fig.=3; yand Fig; 6 is-a lcross-sectional view ofthe hammer taken along theline 6-6 of Fig 4. Y g

In Fig. 1 a section of a coal -mine is representedv ashaving a roofv -I,shown partly in section ex.- posing strata orf-layers 29, 30,-3I,v342,.33, 34, supported at intervals .".by'rpillars 2. These pillars divide lthe Vmine .proper 'into chambersf'ZU :and

gangwaysr 2l which may beof Various dimensions depending somewhat' upon .the nature of .the rock and coalnstructures inthe particular mine. LUsually the pillars alone are not-sucient .to support the roofs of' the ychambersyso :that wooden `props are customarily employed at intervals :to holdY the roof from collapsing- In .orderftoisimplify the drawing, such vpropsare notshown inthe illustration. It will evident, however, that the use of `props isundesirable because-they entail .time and expense. to. erect andgreatly` interfere with communications and traflicwithin'the mine. I'f the roof Astructure isV strong .enough to'y bepselfsupporting,f props are noti. required; vwhereasfif theroof is notfstrong andan insuicient number of-props `are used 'there is-the constant'danger of -a roof fall orcave-in, probably resulting in' a fatal accident; Byfmeans ofthe present'inven tion, the safety of theroof structure may be y quickly ascertained? vbyla foreman `or a miner as the-cutting exposesfnew rockfpermitting adequate props to be inserted-before the roof falls.

Method of mmerocfi-testmgr- In accordance withlthe' invention,.a. hole v3 (Fig. 1)` 1s first drilleda footor. adjacent the roof of the mine, preferably. horizonta1ly,but y1n any 'event at aniangle such that the pickup .device will respond satisfactorily to thecompression wave energy tobe detected. Thetype Aof roof falls. Into this .hole islinsertedadetector e or pickup device 4 which is described below more detail. Such devices are sometimes referred to as geophones The geophone 4 is electrically connected by a suitable length of insulated cable 5, say feet or more, to the amplifier box 5 preferably containing the remainder of the electrical apparatus, and which includes the mentioned amplifier, the necessary controls, and the meter or indicating device which is actuated by the output current from the amplifier and which should'be observable on the exterior of the box 5.

The compensator having previously been calibrated to` suitthe characteristics of the rock, as explained vbelow in connection with Fig. 2, it should` beset atthe first or 25 foot setting of the idevice.. The operator may then take up a position at a test station, such as the point marked A in Fig; fl near the location 3 of the geophone. YThe roof rock in the general vicinity of station A should be of known strength. If thena hammer vblowis struck on the roof rock at test station A the indicator ion' apparatus 5 should Ygive a maximum, or yapproximatelymaximum, reading... Iflr it does notfthe'fvolume Y will leave the compensator set at 25 feet aridfwill then strike the roof rock .with a ihammerblow of'a force substantially equal to that ofthe blow struck at station A2, at the sam-e timeobserve ing themeasurement.indicated on the meter. j Ii. the rock inthe area between stationmB and detection point 3 is of.. uniform structureyand substantiallymthe same as that. between pointed and station fA?, the meter reading at'station"Bx .will be Substantially the `same as' at station AL Ori the other hand, if loose rock" or other'str'iitw` tural defect in the roof material.'intervenes,`lf eV tween. point 3 and station .B, the meterread ing will be considerably less than that at station A, in which event suitablepropsjmay be placed to .support the roof .with safety.

The area rof the unsafe; portion of thereof may be more .closely located by proceeding vbackwards from station B toward station A, or toward pickup point 3, bygone or more pathsstrikin'g similar-hammer blows at intervalsiof a fewV feet each; and .observing the meter reading at each new testing station. ABy this means of closing in on the unsafe area, areasonably accurate voutlineof the lunsafe area may`be determined 1y set -thecalibrator to. the 50-foot -point. .Tlj1`eV cable .E here, assumed. to. bey ofsomewha,t. more than 100.feet in length will.. permit .the voperator to, proceed.. atleastas far.. asv station...lii` before removing thegeophone 4 from "holel3 'Land rein- "sertne Aitin a new hele 3'; After inserting-the geophone in hole 3 the operator might start the new series of tests at or near station E as a known basis and then proceed to unknown station F. After adjusting the compensator to suit the distance between station F and point 3', he would proceed as before, always working from a rock area of known strength and safety to an area of unknown strength. In order to insure tests of greatest accuracy the operator should always set the compensator to a point most closely corresponding to the distance be tween the geophone 4 and the test station at which the hammer blow is struck.

The procedure above described can readily be carried out by a single operator. However, it will be clear that the same result may be achieved by employing two men, one of whom remains at the location of the indicating device and the other of whom proceeds to the successive test stations with the hammer. While such procedure is entirely feasible it sometimes is undesirable because it requires communication between the hammer man and the instrument man, which is difficult or inconvenient when the mine is being worked.

Apparatus The detector or pickup device Ll herein termed the geophone is an electro-mechanical transducer which transforms mechanical wave energy into electrical vibrations or Wave energy. Such a device of the piezo-electric or crystal type has been found especially successful in connection with the present invention. This crystal pickup device may be suitably mounted in a closed cartridge of heavy metal, such as brass, shaped to t hole 3, and might be 1% inches in diameter by 8 inches long. Inasm-uch as the vibrations to be detected travel horizontally, in general, the greatest response is attained if the crystal is mounted in the cartridge so as to be effectively horizontal when used.

The amplifier box 5 shown in Fig. l preferably contains the remainder of the electrical apparatus which is connected to the geophone i by a suitable cable 6. It is very desirable that the equipment comprising amplifier box 5 be compact and of light weight because it must usually be carried considerable distances through a mine where walking is diicult and arduous. By the use of vacuum tubes of the hearing aid type, energized by dry batteries of the hearing aid or similar compact type, and by judicious selection of components known in the art, a suitable amplifier may be constructed which will meet the requirements above outlined. The amplier box is preferably arranged to be su.- pended on a strap to be hung around vthe test mans neck or waist so that the necessary controls and indicating meter mounted on the exterior of the box may be accessible for observa tion and manipulation, as required. The circuit arrangement of a suitable amplifier and indicating device is represented in Fig. 2.

The apparatus illustrated in Fig. 2 includes an amplifier comprising three vacuum tubes preferably of the hearing aid type in order to permit the apparatus to be compact and of light weight. In fact, all of the apparatus should preferably be selected with that object in View. Accordingly, vacuum tubes V1 and V2 may be of Raytheon type CK51OAX and tube V3 of type CKBO'YAX. The tubes V1 and lV2 are double tetrodes, resistance-capacity'coupled in conventional manner to provide'subst'antially linear amplification of ve stages. The input to tube V1 is coupled through transformer T1 to the pickup device or geophone 4. Tube V3 is a pentode of the power output type. To the output of this tube is connected output resistor R15 to which is connected, through a coupling condenser Ca, a suitable indicating device or meter 22. The meter here represented is a microammeter having a 0 to 50 microampere scale. The meter being a direct-current instrument is connected to a full-wave rectifier of the copper oxide type so that it may indicate alternating current. Other suitable types of indicating devices may, of course, be used. It is sometimes desirable that the output be checked by an oscilloscope or by a telephone receiver. For this purpose a jack (not shown) may `be connected across resistor R15, or at any other suitable point in the system in a manner well known in the art.

To enable those skilled in the art to construct and use an amplifier in accordance with the invention herein described, values of the various circuit elements as indicated by their reference characters in Fig. 2 are suggested below. It should be understood, however, that the specific circuit arrangements here shown, as well as the values of the elements below listed, are given merely by way of example of one embodiment of the invention.

R1-200,000 ohms R22 megohms Cz-.OOZ mfd. R3-5 megohms Ca-.002 mfd. R4-2 megohms Cir- .05 mfd. R5-3 megohms C5-.002 mfd. R10-200,000 ohms Cri- .002 mfd. R11-2 megohms C'1-.05 mfd. R12-5 megohms Cia-.005 mfd.

R13-2 megohms f R14-5 megohms R15-15,000 ohms In order to simplify the circuit arrangement, a single high voltage or B Abattery 23 is employed by way of compromise. In View of the particular vacuum tubes selected for the amplier here described a battery of 45 volts may be used. ply the filaments or cathode heaters may be connected as shown in series with a control switch 28. The opening of switch 28 in the low voltage circuit effectively disconnects both batteries because when the cathodes are unheated the resistances of the anode circuits `are so high that the anode circuits to which battery 23 is connected, are eifectively on open circuit. Thus, there results no spark sufficient to be dangerous in thepresence of mine gases. A voltage dropping resistor 25 is here employed because it happens that the normal filament voltage of tubes V1 and V2 is less than that of tube V3. If the rated filament voltages be the same for all tu-bes this resistor should be omitted.

One feature of the invention which requires description in some detail is the compensator 27. This comprises, as showna resistor Re-Rv-Ra, variable in steps, the effective resistance of the compensator being connected in series between the slider of potentiometer resistor R5 and the rst control grid of Vacuum tube V2. The resistance values of resistors Rs-Rq-Ra should be chosen in any given instance to suit the charac-J teristics of the rock in the mine in which'the equipment is employed. To provide ,one illustrative example of the order of magnitude which such compensating resistors might have,` values are given below. It should be understood, how- A battery 24 of suitable voltage to sup--` ever, that these values vwere selected to'provide Re-2.25 megohms R7'.25 megohm Riz-.10 megohm f Rsi-.40 megohm Calibration of compensator The calibration of the compensator is effected by giving the resistors R9, Ra, R1 and Re values such that at each point of switch S1 the gain .of the amplifier will be increased an amount sufflcient to compensate for the attenuation in compression wave transmission through the rock between the detection point and each test station as represented respectively by the corresponding contact point on switch S1. The resistance values of these resistors above given were arrived at empirically as a result of measurements of attenuation of compression waves through the rock of a coal mine, and of the resulting drop in signal output from the amplifier, from which the necessary increase in amplifier gain at the different distances could be determined. The values of the mentioned resistors were selected to provide such gain, keeping in mind that the effect of these resistors'on the gain is somewhat interdependent. Itis to be noted that in the circuit of Fig. 2, as ,the switch is .advanced from the 25 foot point toward the 100 foot point, the resistance connected between the control grid and cathode of tube V2l is increased, resulting in a proportionate increase of signal voltage onethe grid.

From the foregoing it Will be evident that the precalibration ofthe compensator as `above described should be made in advance of the actual.

testing of roof material in connection with mining operations. However, once the proper values of resistors R9 to Re are determined, the apparatus may readily be calibrated by the operator,

by adjustment of R5, to suit the local conditions under which the tests are to be made.

Such calibration maybe as follows: First, an area in the mine must bey selectedwherein the rock is known to be solid and safe for a distance of, say, 100 feet. Then the geophone 4 is inserted in the rock at one extreme of this distance, and withthe switch S1 set at the 100-foot (or other appropriate) point, a hammer blow is struck at the other extreme of the 100-foot distance. the exterior of box is then adjusted .until a substantially maximum meter reading is obtained. If further check is desired, successive similar hammer blows may be struck'at 75, 50 and 25 foot Resistor R5 (which is adjustable from f distances, with the switch S1 set at the cor- `as above describedwill. usually hold good for many different locations in the same mine, and, indeed, in different mines, without readjustment of the resistors Rs and R9. However, it is at times necessary to readjust the volume control R5 at the start of thetests so that the indicating meter will read substantially maximum, if the intervening rock is solid.

The compensator having been adjusted and calibrated for approximately specific distances between testing stations, as above described, ,the indications or readings of the meter 22 will be substantially constant when uniformly solid rock is tested at the various predetermined distances from the geophone. If the testing stations are at positions intermediate to those for which `the apparatus is calibrated, viz., say 35 feet whenY switch S1 is set for 25 feet, or at, say, 60 feet when the Switch S11 is set for 50` feet, the meter reading will be slightly less than maximum. Such slightly decreased reading will not confuse the operator because if in an actual test of unknown rock a section of soft or otherwise dangerous rock occurs between the test station and the `location of the geophone, the increased attenuation will be so marked that the reading on meter 22 will be much lower than the slight decrease of reading due merely to attenuation of a few feet of solid rock. Accordingly, it is possible, and frequently desirable, especially in equipment t0 be used by unskilled miners, that the dial of the indicating meter 22 be divided and marked in zones labeled Safe and Unsafe, for example. The safe Zone might cover the area of say the top one-third, or even the top one-half, ofthe meter scale. Actually the rsettings of the calibrating device 2 overlap to some extent, i. e., the 25a-foot point for switch S1 might produce satisfactory indications for distances from 0 to 35 feet from the geophone, the 50-foot point being satisfactory for distances of from, say, 20 feet to 55 feet from the geophone, etc. Thus it will be seen that the compensation is not critical, but it becomes nevertheless, increasingly important as the distance from the geophone to the testing station is increased.

In addition to the electrical apparatus above mentioned, the other apparatus required in practicing the present invention'is a suitable hammer or other means for striking the roof rock in order to generate sound or compression waves which are to be picked up and recorded. Although such waves may be generated, by the impact on the roof` rock of any suitable hard object, as mentioned previously, it is preferable that the device be capable of striking successive single blows of uniform intensity. If blows on the roof at successive positions vary greatly in intensity, the indications of the indicating meter will correspondingly vary, from which it may be diicult to ascertain whether the variations in readings are due to variations in intensity of hammer blows or to variations rock structure between the successive positions. However, it has been found in practice that the hammer blows need not be of exactly uniform intensity because as a rule thevariations in rock structure of such nature as are likely to result in a roof fall wiil result in a considerable decrease in the meter reading, whereas ordinary variations in the intensity of the hammer blows will effect comparatively small changes inthe meter readings. However, inorder to eliminate one possible source of error, it is preferable that a hammer of the constant blow type, as illustrated in Figs. 3 to 6, inclusive, be employed;

A construction of constant blow hammer found suitable vfor presentrpurposes is shown in its assembled-form in Fig. 3A whereina metal casing fl,

which may be constructed from a suitable length of heavy iron pipe open at one end 8 and closed by a pipe cap 9 at the other end, encloses a metal plunger I3 (Figs. 4 and 5). This plunger is urged toward the front end by two tension springs I I, one on each side of the plunger. To the back of the plunger I9 is secured a plunger rod I2 terminating in a hand grip I3 by which the plunger may be pulled back against the tension of springs Il. A dog IG fixed to rod i2 may be engaged with any of the teeth on rack I5 by rotating rod I2 aiter it has been drawn back to any desired position. Such positions may be predetermined by observing the calibration marks I'I, along the outside of casing '1, in their relation to index I5 which is attached. to a bolt IB passing into plunger l5 and extending through a suitable slot 25 on the side of casing I. Another similar bolt and slot (Fig. 5) are en the opposite side of casing 'I.

In use, the hammer is cocked by drawing back handle I3 until index I5 coincides with the calibration line II corresponding to the predetermined blow desired. The handle may then be turned to the left to lock the hammer at that predetermined setting. Then when the operator reaches the desired test station where a blow is to be struck, he presses the forward end 8 of the hammer against the rock surface and turns the knob I3 to the right, grasping it loosely enough so that when the plunger I2 is released the force of the springs II will pull the handle I3 out of the operators fingers. Plunger Il then strikes the rock with a blow of predetermined force, as a result of which a single wave train of free, damped oscillations is generated in the rock. If the hammer is released when the forward end is not against a hard object, such as a rock face, the handle I3 will be drawn against cap 9 with considerable force. A buffer such as spring I9 is therefore interposed on rod I2 between cap 9 and handle I3 to absorb the force of the blow.

What is claimed is:

1. The method of testing a mine roof for safety, which includes creating at a first test station a rst single train of damped compression waves in an area of rock of known strength, detecting said waves at a pickup point in rock within said area, measuring the effective amplitude of the detected waves, advancing to a second test station in an area of rock of unknown strength, creating in rock at said second station a second single train of damped compression waves, detecting said second waves at said pickup point, amplifying said second waves to a degree which compensates for the difference in normal attenuation of wave transmission characteristics between said rst and second stations, and measuring the effective amplitude of the detected and amplied waves created at said second station, whereby the strength of the unknown rock may be determined by comparison of said measurements.

2. The method of testing a mine roof for safety, which includes striking the mine roof at a rst station with a single blow to create a compression wave train of free oscillations in a limited roof area of known strength, converting said Waves into electric waves at a certain point in the rock in vsaid area, amplifying said electric waves and indicating the eiective amplitude of said electric waves, advancing from said known area toward a rock area of unknown strength, striking the mine roof at each of a plurality of successive stations separated by approximately equal increments of distance from said point with a single blow of effectively the same intensity to create a compression wave train of free oscillations at each station in succession, increasing the degree of said amplification to compensate for the effective increase in normal attenuation of compression wave transmission in said rock due to the increase of distance between said point and said successive stations, converting said successive waves into electric waves at said point, amplifying said last named waves, and indicating the eiective amplitudes of said last named waves whereby they may be compared respectively with the rst named indications.

3. The method of testing mine roof rock for safety, which comprises creating a compression wave train in said rock successively at each of a plurality of stations separated increasingly distant from a point in a rock area of known strength, receiving said waves at said point, converting the received waves to form electric waves, amplifying said electric waves, compensating said amplification on the basis of normal attenuation of compression wave transmission in said rock by amplifying the electric waves in proportion to the increase of distance between stations, and indicating said amplied waves, whereby to ascertain the relative strength of the rock between said successive stations.




REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,130,657 Armstrong Mar. 2, 1915 1,923,107 McCollum Aug. 22, 1933 2,340,272 McCarty Jan. 25, 1944 2,345,679 Linse Apr. 4, 1944 2,364,655 Pratley Dec. 12, 1944 2,376,195 Scherbatskoy May 15, 1945 2,378,237 Morris June l2, 1945 2,388,703 Peterson Nov. 13, 1945 2,389,472 Tyzzer Nov. 20, 1945 2,461,543 Gunn Feb. 15, 1949 FOREIGN PATENTS Number Country Date 394,712 Great Britain July 3, 1933

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U.S. Classification73/594, 330/178, 367/191, 73/579
International ClassificationE21F17/00, E21F17/18
Cooperative ClassificationE21F17/185
European ClassificationE21F17/18B