US3163010A - Mine roof stabilization detection - Google Patents

Description

Dec- 29, 1964 J. E. CARPENTER MINE RooF sTABILIzATIoN DETECTION Filed April 12. 1963 INVENTOR. JOSEPH EMMETT CARPENTER ATTORNEY United States Patent Gftice glhl lfatented Dec. 29, ld@

3,163,010 MINE RGS? STAESIHZATN DESEEN .loseph Emmett Qarpenter, Thatcher, Ariz., assigner to American Cyanamid Qompany, Stamford, Conn., a corporation of Maine Filed Apr. 12, i963, Ser. No. 295,258 l Claim. (Cl. dll-36) This application is a continuation-in-part of application Serial No. 698,516, filed November 25, 1957, Mine Roof Stabilization, now abandoned.

'lfhis invention relates to a method of stabilizing rock formations, particularly mine roofs, as in coal mining, by permeating a rock formation with an ambient temperature setting, liquid, polyester resin, and confirming penetration of the polymer by using a fluorescent tracer.

In subterranean operations, particularly in mining, it is necessary to stabilize rock formations so that the rock formations will remain in place during subsequent operations. For example, in the coal mining industry it now costs around 40 to 55 cents per ton to roof bolt a mine. ln this operation as the coal is mined out, there is left over the tunnel, the mineral formation which rested on the coal. This frequency consists of a thin layer of roof coal, a thicker layer of shale, and above this layer, sandstone or other firm material. Shale is notoriously weak and apt to break loose in chunks or slabs which fall on men working in the mine with serious or fatal results. At present, about 75% of the deaths in mining bituminous coal in the United States are caused by roof falls. Such rock falls are normally not given the publicity given to mine disasters in which several men are trapped at a single time in a mine, but over the years the number of men lost through rock falls in mines exceeds losses from other sources. Such accidents occur most frequently within to 50 feet of the working face of the coal seam, from which the coal is being mined.

To prevent these rock falls, that is to prevent the roof from falling in, it has been customary to either timber a mine, in which method heavy timber supports are introduced to hold up the roof, or to use roof bolts, in which method holes are drilled through the rock formation into a more solid rock formation and metal bolts inserted, with a metal plate on the end of the bolt, thus retaining the roof in position. Many underground mines have the entire roof studed with these roof bolts. Because both timbers and roof bolts must be placed in the roof of the mine after the roof is accessible, that is after the coal has been removed, there is a gap between the portion of the roof which is supported by the roof bolts or timbers, and the working face of the mine. in many instances by tapping on the rocks, or inspecting for looseness, it is possible to anticipate rock falls and then either specially timber or specially linsert. roof bolts to support loose formations close to the working face. ln common with many other human activities it is normal for miners to become careless of commonly occurring dangers, and from time to time a miner gets careless and in an endeavor to increase his production and get out a large quantity of coal, forgets or postpones strengthening the roof and an accident occurs.

Usually after a mining operation the roof of a coal mine is a layer of low grade coal known as roof coal, overlain by a layer of shale of varying thickness. This shale is overlain in turn by a strata of sandstone, usually Well consolidated. The roof coal layer is not removed in mining operations because of its high content of noncombustile material. This roof coal is frequently not firmly bonded to the shaly material above it. Shale is thin-bedded mudstone of varying degrees of compaction and strength. lt tends to part along its main bedding planes and as the underlying coal is removed in the mining operations, it cracks and falls in chunks or slabs. Such falls are known as rock falls or roof falls and cause the accidents and deaths to coal miners as described above. These conditions are aggravated by modern continuous mechanical mining methods.

If the individual -beds or blocks of shale can be united to each other, then the roof will tend to remain in position after removal of the underlying coal and the mine is much safer.

Additionally, it is desirable that the shale be solidified or stabilized ahead of the working face in the mine. While this has been long known to be a theoretically desirable objective, it has been regarded as impractical. The injection of cement into underground formations has been used to `stabilize he formations, ybut usually such cementing is of greatest value in stabilizing against compression or shear stresses. The injected cement is weak under tension itself, and also takes too long a time to set to be satisfactory for stabilizing underground rock formations.

A stabilizer is desired which has good adhesion to rock. The adhesive is preferably a liquid at room temperature, or at the temperatures of the underground rock formation, so that it will flow readily when injected into the rock formation; it must cure at that same temperature to set up and hold the rock together. The underground formation may vary from a temperature slightly above freezing on up to in the neighborhood of 139 F. which is about as hot as a miner can stand. It is desirable that the adhesive cure rather rapidly so that formations can be stabilized withn a short period of time. The adhesive should cure in the presence of water.

lt has now been found that polyester resins meet the requirements and may be used to bind together and stabilize underground rock formations. The polyester resins have good :adhesion to rocks, cure in the required temperature range, in a reasonable time, and in the presence of water. These polyester resins may be cured at the temperatures found in mines from slightly above freezing or below up to the temperatures as high as those at which a man can work or even higher. Polyester resins may be prepared so as to cure within a period of within 15 minutes to several days. Polyester resins that eure in less than 10 to l5 minutes do not remain liquid long enough to be placed in rock formations if prepared in a pot, and pumped into the rock formation. However, very fast curing resins may be used if the components are mixed in the pump or injecting nozzle as they are being injected into the underground formations. lf the resins cure in a period of longer than -about 72 hours, too long a time is required for them to be as effective as desired.

Many of the polyester resins are essentially non-toxic and some of them are sufficiently low in price so as to be presently economically practicable.

The polyester resins are storage stable in pre-injected form and are readily activatable for cure. The amount of catalyst added or the amount of promoter added may may be adjusted so that the rate of cure is within desired time limits at mine temperature. Further, the catalyst may be added just before use so that the pot life is satisfactory for injection, or the catalyst may be added in a mixing nozzle as the material is injected into a rock formation so that only a matter of a few moments is required between the time the catalyst is mixed with theresin and the time the resin is in position in the underground formations.

To inject the resin, holes are drilled into the rock formation which is to be stabilized. Under pressures of 20 to 2,00() pounds per square inch the adhesive resin will flow from 5 to 15 feet from the hole in which it is injected into the rock formation.

Coloring materials are added to locate and identify the resin. A iluorescent dye, such as the 7-dialkylaminocoumarins, such as for example 4-methyl-7-dimethylaminocoumarin, 4methyl7diethylarninocoumarin, 4- ethyl-7-diethylaminocoumarin, or 6-diethylamino-9-(2- carboxyphenyl)-isoxanthene-3-diethylammonium chloride or 4-(2-hydroxyethylamino)-N-butylnaphthalamide such as disclosed in United States Patent 2,415,373, February 4, 1947, M. Scalera and A. W. Joyce, Substituted 4-Hydroxyalkylarnino-l,S-Naphthalic Acid Imides, may be used. Anthracene and other fluorescent compounds may also be used in larger concentrations. The use of a liuorescent dye in the polyester resin permits the use of ultraviolet light to inspect a rock formation as new surfaces are formed to determine whether or not it has been permeated by the adhesive resin. Obviously in a coal mine where the light may be easily extinguished and an ultraviolet light used as the sole light source, small quantities of a fluorescent tracer in the adhesive resin show up so that the degree of penetration is easily ascertained. The dye does not interfere with the cure of the resin. Prom 0.01% to 0.5% dye gives good results. For the use as a tracer, the tracer may be either a dye or a pigment. The dyes are molecularly dispersed. The pigments are dispersed in polymolecular particles, usually in the submicron particle size.

The adhesive resin may be injected straight up above the shaft or tunnel into the rock formation but conveniently is injected through sloping drill holes which are drilled above or through the working face and back into the rock formation over the unpenetrated areas. Such treatment provides that even as coal is removed from the mine in subsequent mining operations, the roof over the area in which the-miner is working has been prestabilized by the use of the polyester resin. Thus the roof is kept in a stable, safe condition ahead of the rapidly advancing mechanical mining equipment. Side walls can be equally readily reinforced where desired, with or without bolts or reinforcing rods.

The exact formulation of adhesive resin is chosen to suit the particular conditions involved. Polyester resins which are esters of alpha, beta unsaturated dicarboxylic acids and dihydric alcohols may be dissolved in solvents such as styrene, or other solvents containing unsaturated linkages. Such solutions are stable, over a long period of time in the absence of a promoter or catalyst. soluble cobalt salt such as cobalt naphthenate or cobalt tallate or any other fatty acid soap of cobalt is added as a. promoter. The quantity of promoter used influences the rate of cure. A catalyst is added just prior to the time of use. A peroxide type catalyst such as benzoyl peroxide, methyl ethyl ketone peroxide or other organic type peroxide is added to the styrene solution of the polyester and causes the resin to solidify. The cure begins when the promoter and catalyst are both present. Such a resin solidiiies at room temperature and does not require pressure. The various pieces of rock are caused to adhere to each other to form a solid formation. The exact quantity of the promoter and catalyst are chosen to give the desired rate of set at the temperature existing in the mine. Also depending upon the injection equipment and the direction of injection, it may be desired that the formation stabilizer set as rapidly as possible consistent with leaving time for injection., while in other cases slower cures, up to several days, may be desired. Usually periods of longer than 72 hours are not desired. Inasmuch as the setting does not occur instantaneously, it is desirable that the initial set or initial thickening allow suiiicient time for the liquid adhesive resin to penetrate the formation and yet the final set occurs sufficiently rapidly that the roof will be retained in position before the miners work thereunder.

The resins which dan ibe used as adhesive are broadly those set forth by Ellis in United States Patent 2,255,313, Ethylenic-Alpha-Beta Synthetic Resins and Process of Making Same, September 9, 1941, and con-sits of a solution of a linear polyester in an alpha-beta unsaturated monomer. The linear polyester is a condensate of fumarie and/or maleic acid or anhydride with a dihydric alcohol. ln the presence of a peroxide catalyst, cross linking occurs between the linear polyester and the alphabeta unsaturated monomer to give a hard, strong solid. For use underground, it is important that the monomer not be excessively volatile or toxic, but otherwise any of the monomers used by Ellis are satisfactory, provided, as he points out, they are miscible with the polyester. it is desirable to use rather large proportions of monomer So as to reduce the viscosity of the solution to be injected as much as possible, so that compatibility is of prime importance.

For economic reason, styrene is preferred as the monomer, but methylstyrene has the advantage of a higher dash point. To increase the solubility of the linear polyester in styrene it is desirable to incorporate a moderate proportion of phthalic anhydride into the polyester at the time of its preparation. The use of propylene glycol rather than e-hylene glycol as the dihydric alcohol is also advantageous for this purpose.

The use of the present invention is shown diagrammatically in the drawings in which:

FlGURE l shows a hole for resin injection drilled in a rock formation.

FIGURE 2 shows the undisturbed rock formation.

FIGURE 3 shows a miner injecting resin into holes drilled at various angles.

The accompanying gure shows illustratively an underground mine in which a coa-l seam 1 is overlain by a shale seam 2 which is overlaid by solid rock 3. A tunnel 4 is drilled through a major part of the coal seam and above which is a roof which is subject to failure. In this roof are drilled holes which may be vertical 6 or inclined '7 and in these holes is injected through an injection nozzle 3 a polyester resin dissolved in styrene containing a promoter and catalyst.

ln an underground mine a vertical hole is drilled through approximately 5 feet of soft shale and inclined holes are drilled through this same shale for distances of approximately 30 to 40 feet in front of the working face of the mine.

A typical polyester is prepared as follows:

6.2 mols propylene glycol 2.5 mois maleic anhydride 3.5 mols phthalic anhydrideare heated at 200 C. until an acid number in the range of 25 to 45 is obtained. Two parts of this polyester and 1 part of styrene are dissolved together to give the polymerizable composition. To this is added 0.003% cobalt as cobalt naphthenate. The fluorescent tracer is conveniently incorporated at this time.

lust prior to injection in the rock formation methyl ethyl ketone peroxide is added to the composition. The conventional range of 0.5% to 2% is preferred, depending on the setting rate preferred.

The liquid resin described above, with the catalyst freshly added, is injected into these holes with a pressure of 1,000 pounds per square inch. After the holes have received all of the resin which may be readily injected at this pressure, the resin is allowed a period of 24 hours to set and at the end of this period the roof is found to be stabilized and not to require roof bolts to hold the shale in position. Additionally, as the tunnel is pushed further under the stabilized roof, the roof remains in position and there is a much reduced danger to the miner, until he gets to the end of the area penetrated by the resin. With a fluorescent dye as above mentioned, an ultraviolet light is used for inspection to ascertain the degree of penetration, or when bonded areas are mined, the degree of penetration can be ascertained. A pilot hole can lbe drilled and an injection periscope, such as a stratnscope, used for inspection.

Similar results can be obtained with solutions of polyesters in styrene in which the ratio of polyester to styrene varies lfrom 4:1 to 1:4. Methyl styrene can be used in place of styrene with satisfactory results.

Inasmuch as it is difficult to determine the effectiveness of such resins underground because of the lack of control of variables, a laboratory test is to determine the effectiveness may `be made by laminating a piece of slate between two pieces of tile each approximately 9 inches by 2 inches by 1A inch in which the adhesive resin is spread on the piece of slate and the two pieces of tile pressed together and allowed to cure Without pressure at room temperature. For test purposes either 70 F. or the temperature in a particular mine may be used. The resulting laminate is broken on a testing machine by supporting the ends of the lower piece of tile and pressing against the center of the upper piece of tile. Such a laminate, Itested before the resin has time to set, or without a resin Abeing in place, stands a pressure of between about 70 and about 165 pounds.

When the above polyester resin in the example is tested, it is found that after a one hour curing time the breaking strength is about 340 to 420 pounds. After curing for 16 hours this breaking strength has increased to 575 to 70() pounds.

The resins used herein are operative in wet rock formations, and ,in fact can be cured when completely immersed under water. Water may reduce the strength of the bond, and increase curing time. In one test, a laminate prep-ared from slate and tile which had been soaked in Water overnight before applying the resin gave a breaking strength of 695 pounds after 16 hours.

The resin can be used to fasten 'bolts into drilled holes, wit-hout expansion sleeves; and the introduction of a bolt into a resin-containing hole helps to both bond the bolt in the hole, and force resin into the adjacent formation to give better bonding than if the resin all remained in the hole. The degree of desired penetration into adjacent strata varies with the strength of the Strata. Usually the weaker strata which need reinforcing are the most readily penetrated by the resin ybeing squeezed into the strata as the bolt is inserted. Fluorescent inspection of typical holes, wit-h further mining or test bores, gives a confirmation as to the degree of penetration and rock conditions.

I claim:

A method of stabilizing underground rock formations which form mine roofs and confirming the stabilization of said rock formation by fluorescent inspection which comprises: drilling a hole into a rock formation having zones of weakness and which is subject to failure in tension as well as shear, injecting under pressure into said hole and through said hole into the rock formation a formation adhesive resin which is liquid and self-curing at ambient temperatures, and which is selected from the group consisting of solutions of linear polyesters in liquid monomeric unsaturated polymerizable compounds containing an alpha-beta ethylenic linkage, said solutions also containing a peroxide type catalyst, a cobalt promoter and a uorescent tracer, whereby said resin flows into and fills said zones of weakness and cures in position, thereby ad'hesively uniting said formation at the zones of weakness, and stabilizing said rock formation against roof falls, cutting through the stabilized underground formation, and illuminating the new formation surfaces with ultraviolet light and inspecting for the fluorescent dyed polyester resin, thereby confirming the present of the polyester resin in the formation.

References Cited by the Examiner UNITED STATES PATENTS 2,636,127 4/53 De Forest 252-30l.2 2,920,203 1/60 SwitZer m. 252-3012 3,030,870 4/62 Gill 252 301.2 3,091,935 6/63 Brown et al. 61-45 EARL I. WIIMR, Primary Examiner.

JACOB L. NACKENOFF, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OE CORRECTION Patent No. 3,163,010 December Z9 1960f Joseph Emmett Carpenter It s hereby certified that error appears n the above numbered patent reqlrng correction and that the said Letters Patent should read as corrected below.

line 27, for "present" read presence 20th day of July 1965.

Column 6 Signed and sealed this (SEAL) Attest:

EDWARD J BRENNER

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