Patent Publication Number: US-3876251-A

Title: Mining and tunneling apparatus involving alternated application of thermal and mechanical energy in response to sensed rock condition

Description:
United States Patent Boyd et al.  
 MINING AND TUNNELING APPARATUS INVOLVING ALTERNATED APPLICATION OF THERMAL AND MECHANICAL ENERGY IN RESPONSE TO SENSED ROCK CONDITION Inventors: James Boyd, 630 Fifth Ave. New  
 York, N.Y. 10020; Lawrence A. Garfield, 1505 Tary Ct. Colorado Springs, Colo. 08908; Clifford Hanninen, 14 Elm St. {P.O. Box 832). White Pine, Wis. 49971; Eugene Maki, White Pine Copper Company, White Pine, Mich. 49971 Filed: Dec. 17, 1973 Appl. No: 425,263  
 Related U.S. Application Data Continuation of Ser. No. 332.552. Feb. 15, 1973, abandoned.  
 U.S. Cl 299/l; 299/14 Int. Cl. EZIc 37/16 Field of Search .1 299/1, 14; 239/4, 102  
 Apr. 8, 1975 [56] References Cited UNITED STATES PATENTS 1.284398 11/1918 McKinlay ttttttttttttttttttttt 299/14 X 3.015.477 H1962 Persson et al 299/1 3.700.169 10/1972 Nayden et al 239/102 X Primary E.\&#39;aminerErnest R. Purser Attorney, Agent. or FirmMorse. Altman. Oates &amp;  
 Belle 1 1 ABSTRACT A mining or tunneling apparatus is provided for continuous underground excavation through rock of relatively high to relatively low hardness. The apparatus comprises impactors and heaters having alternately operative and inoperative modes which are selected in reference to the sensed and indicated homogeneity of the rock condition of the excavation face. Functional relationships between the applied thermal and mechanical energy are given. Optional separation of ore and gangue at the mining face is outlined.  
 6 Claims, l3 Drawing Figures PLIENIEBAPR 81975 (3.876.251  
 SHEET 1 OF 4 FIG.8  
 PATENTED 81975 3.876.251  
 sum 2 OF 4 FIG. IB  
 INCHES l l l l 3 6 9 l2 PATENTEDAPR 81975 sum u BF 4 FIG.5  
 FlG.4  
 FIG.7  
  m h H M S D N A s R E m VALUES-ONLY MINING FIG. 9  
  MINING AND TUNNELING APPARATUS INVOLVING ALTERNATEI) APPLICATION OF THERMAL AND MECHANICAL ENERGY IN RESPONSE TO SENSEI) ROCK CONDITION This is a continuation. of application Ser. No. 332.552. filed Feb. 15. I973 and now abandoned.  
 BACKGROUND The present invention relates to underground excavation and. more particularly. to excavation in rock for the purposes of mining and tunneling. Various prior excavation techniques have included: drilling and blasting; mechanical cutting; and thermal spalling. In the case of the drilling and blasting. holes are drilled mechanically into the rock face. are filled with an explosive charge such as dynamite. and the region in the vicinity ofthe holes is fragmented by the explosive power of the charge. In the case of the mechanical cutting. pneumatically. hydraulically or electrically actuated percussion moils or rotary cutting tools are applied to the excavation face. by which fragmentation of the rock at the face is effected. In the case of thermal spalling. heat from an intense source is transferred to the rock face in such a way as to cause differential expansion of the region underlying the face and to cause ten sile and/or shear strain separation of portions of the rock therefrom. Drilling and blasting, relatively inefficient from the standpoint of conversion of energy into loose rock and inadequately controllable in the amounts and sizes of loose rock produced, inherently is a tortuous and interrupted process that concomitantly may require costly and time consuming installation of supports in portions of the excavation region. which may be weakened by shock at considerable distances from the excavation face. Mechanical cutting. although capable of more or less uninterrupted prodedures. has been found to be undesirably slow. particularly at deep levels. because of the strength limitations inherent in steel percussion and cutting tools. no matter how sturdily designed. And thermal spalling has been used only occasionally because ofthe large amounts of thermal energy needed. the low efficiency of heat transfer generally achievable and the hazards and cost involved in removing the products when combustion heating is used. At present. excavation through rock having a Compressive strength of 25,000 psi. (pounds per square inch) can be achieved at a rate of approximately to feet per day. In view of the increasing modern need for rapid excavation in hard rock. existing excavation rates have become inadequate.  
 SUMMARY OF THE INVENTION The primary object of the present invention is to provide an apparatus and process for continuous underground excavation through rock of relatively high to relatively low hardness in relation to a reference hardness that is relevant to the operation of the apparatus. The term hardness herein refers to rock condition in situ. The apparatus comprises an impacting assembly and a heating assembly having alternately operative and inoperative modes. which are selected on the basis of the detected hardness of any excavation face. The impacting assembly has an operative mode. in which it rips successively exposed excavation faces, and an inoperative mode. which is selected when the impacting assembly reaches an excavation face ofa hardness that would subject elements of the impacting assembly to LII LII  
 uneconomic wear if the operative mode were to continue uninterruptedly. The heating assembly has an operative mode. in which it forms a kerf in any excavation face of relatively high hardness. and an inoperative mode in which the impacting assembly can resume its ripping function with acceptable wear as a result of the presence of the kerf. It has been found that applying the heating assembly to any face having relatively high hardness in order to form a kerf and applying the impacting assembly at other times results in an overall minimization of consumed energy per ton of fragmented rock and an overall lengthening of the mean time between failure of equipment. The arrangement is well adapted to programmed automation of mining cycles. permitting closer supervisory control and reduced manpower requirements.  
  Other objects of the present invention will in part be obvious and will in part appear hereinafter.  
  The invention accordingly comprises the processes and devices, together with their steps and elements. which are exemplified in the following detailed description. the scope of which will be indicated in the up pended claims.  
 BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of the nature and objects of the present invention. reference is made to the following detailed description. taken in connection with the accompanying drawings. wherein:  
  FIG. IA is an exaggerated perspective illustrating a mine face undergoing a process of the present invention;  
  FIG. IB is a vertical section. intersecting the mine face. illustrating the kerf formation step in accordance with the present invention;  
  FIG. IC is a vertical section. analogous to that of FIG. 1B. illustrating the ripping step in accordance with the present invention;  
  FIG. 1D is a horizontal section corresponding to that of FIG. 1C.  
  FIGS. 2A and 2B illustrate certain principles of the present invention;  
  FIG. 3 is a perspective view of a thermo-mechanical mining apparatus embodying the present invention;  
  FIG. 4 is a top plan view of an infrared heating subassembly. constituting a component of the device of FIG.  
  FIG. 5 is a side elevation view of the subassembly of FIG. 4;  
  FIG. 6 is a detail view of an infrared heating lamp. constituting an element of the subassembly of FIG. 4;  
  FIG. 7 is a side view of the infrared heating lamp of FIG. 6; and  
  FIG. 8 is an electrical schematic diagram part of the control circuit of the device of FIG. 3.  
  FIG. 9 is a cross-sectional view of a mining site typically mined in accordance with the present invention.  
 DETAILED DESCRIPTION Rock Formation Toward Which Present Invention is Directed Generally. each of the processes of the present invention described below involves the rapid excavation of a hard rock mass. ranging in compressive strength from 5.000 to 80.000 p.s.i. (pounds per square inch). Typi cally, the hard rock mass is one of the common sedimentary, igneous or metamorphic types. The specific :xamples below relate to rock formations from which :opper ore is extracted. In one such rock formation to )e mined is a 6 foot thick shale that lies. at a depth &#39;anging downwardly to 3400 feet. between upper and ower layers of sandstone. The shale includes intereaved strata of copper rich ore and barren rock. Varinis mining techniques previously have been employed. Dne technique, called room-and-pillar mining. involves excavating and transporting ore in such a way as to relain spaced pillars of rock. which continue to support the roof above the floor. Another technique. called longwall mining. involves removing ore without retaining pillars of rock. while the roof is adequately but temporarily supported by a system of steel posts or props. which are alternately advanced toward the receding excavation face in such a way as to permit caving into the already excavated region. In either case. the ore is loaded into trucks or conveyors for transport.  
 Thermomechanical Excavation in Accordance with FIGS. IA. 18. 1C. and 1D The thermo-mechanical apparatus and process of FIG. IA is intended for longwall excavation of an open ing. particularly a mine opening, having a floor 32. a roof 34 and an excavation face 35. Typically the excavation face is several hundred feet wide and approximately five feet high. In the illustrated embodiment. the heating assembly (FIG. 18) includes a plurality of heating elements 38 enclosed in a plurality of mounts. one of which is shown at 39. These mounts are disposed along excavation face 35, each being individually constrained for reciprocal motion between an operative position contiguous with the face and an inoperative position remote from the face. The impacting assembly (FIGS. 1C and 1D) includes a plurality ofimpacting elements 4| on a single mount 40 that passes between the face and the heater mounts when the latter are in their inoperative positions. In the operative mode of the impacting assembly. a single pass of the impacting mount causes impacting elements 41 to contact the entire ex cavation face so as to rip out an underlying solid increment of rock. i.e., to remove the underlying rock to a predetermined depth. for transfer by a loader 42 to a conveyor. In this operative mode of the heating assembly. heating mounts 3&#39;) are juxtaposed. either in serial or in parallel. along a hozizontal line 36 between the roof and the floor. in order to cause kerf spalling and [one Conditioning. as will be explained in detail below. As shown in FIG. 1A. a control 20 transmits synchronizing signals to mechanical and electrical power units 21. 22 via links 23. 24 and receives feedback signals from sensors. for example microseismic sensors. which are in contact with the rock. via links 26. 28.  
  In a first form of the impacting assembly the impacting elements are fixed to their mount so that an eccentric vibratory motion imparted to the mount is trans mitted to the impacting elements. In one specific example of this first form of impact or. the assembly weights about 22.000 pounds and vibrates 25 times per second over about a 0.2 inch t avel. In a second form ofthe impacting assembly. the impacting elements are reciprocable along their axes w ith respect to their mount as it traverses the face linearly. The second form is illus trated as including a plurality of tool steel impact moils 4| which are reciprocable. with respect to their mount 40. in a plurality of cylinders 43. actuated by a power source 45. for example. an electric hydraulic percussion device. As in FIG. 1D, the direction 44 of recipro cation of impact moils 41 is oblique with respect to the excavation face and the direction 47 of traversal of mount 40 is parallel with respect to the excavation face. In one specific example of this second impacting assembly each moil delivers L000 foot pounds per blow at a frequency of one to ten blows per second.  
  Preferably the heating units are combustion free. for example. are characterized by an electromagnetic source of infrared or coherent radiation or a charged particle source of electrons or ions. In one such heating unit. as shown in FIG. 15. heating elements 38 are in the form of infrared emitting lamps which are backed by associated reflectors 45 that concentrate the infrared energy at the kerf now to be described. Preferably the temperature at the surface of the kerf ranges between l.000 and ].400F and the distance between the heating lamps. specifically the tungsten filaments. and  
 the excavation face is less than l2 inches. preferably about 4 inches.  
  In order to facilitate comprehension of the process of the present invention. the following definitions are presented.  
 I. Kerf Space is defined as the empty space of a groove formed by any means in a rock face.  
 2. Kerf Rock is defined as the region of rock immediately adjacent to and so profiling the kerf space.  
 3. Heat Affected Zone is defined as the region of rock whose resistance to mechanical ripping has been reduced by heating. which generates solid state discontinuities at which applied mechanical energy tends to concentrate. Under this definition. rock in a heat affected zone exhibits lower resis&#39; tance to mechanical ripping than rock in a zone that is not heat affected. i.e.. in a zone that has been unaffected by applied heat or that has not been subjected to applied heat.  
 4. Thermal Response Class refers to the tendency or lack of tendency of an in situ rock mass to become heat affected when subjected to heat treatment. It has been found that. in general. the greater the homogeneity of an in situ rock mass. the greater its tendency to become heat affected when subjected to heat treatment. It is useful to categorize in situ rock mass into the following three classes. it being understood that the boundaries between the classes are only approximate:  
 a. Class I Rock is defined as the strongest and soundest. most homogeneous rock. with major discontinuities or joints on the average at least two feet apart.  
 b. Class II Rock is defined as intermediately strong and sound. intermediately homogeneous rock. with occasional specimens as strong as Class I specimens. but with major discontinuities or joints on the average less than two feet apart and more than 6 inches apart.  
 c. Class III Rock is defined as heavily faulted. bedded. and/or jointed. inhomogeneous rock. with discontinuities on the average less than 6 inches apart.  
 . Solid state discontinuities fall into two classes:  
 a. Natural discontinuities of the type referred to above in connection with definition 4. above; and  
 h. Synthetic discontinuities of the type referred to above in connection with definition 3.  
  In the case of all three classes of rock. it is believed that the efficiency of heat transfer into the mining face is improved by operation of the impacting assembly which interacts with the heating assembly by removing fractured rock that would tend to insulate. from the heating assembly. the rock underlying the excavation face.  
  It is further believed that the efficiency of forward penetration of the impacting assembly into the excavation face is maximized by the heating assembly in the following manner.  
 EXAMPLE I In reference to FIG. IB. typically in the case of Class I rock. 4.(l(l() F. tungsten filament electric lamps. 4 inches away from the excavation face. heat a l4 inch wide band along the longitudinal center of the excavation face for 12 minutes to produce an initial kerf space 46 that is l to 3 inches deep and a heat affected zone. extending approximately to limit 49A in the kerf rock. that is l to 3 inches deep. The heat affected zone then is ripped mechanically to produce a total kerf space that is 2 to 6 inches deep. It then is feasible mechanically to rip the heat nonaffected overkerf rock 48 and underkerf rock 50 to about the limit 498 to establish again a flat excavation face and to repeat the heating and ripping cycle.  
  There are interrelated reasons why overkerf and underkerf rock not affected by heat can be mechanically ripped economically in the presence of kerf space but not in the absence of kerf space. First. the kerf space relieves the compressive overburden stresses which hold even heavily jointed rock together. Second. stresses applied to the overkerf and underkerf rock build up fracturing tensile strenses much more quickly than in the absence of the kerf. After the overkerf and underkerf rock have been ripped off. it again is not feasible economically to rip the resulting flat Class I rock face only mechanically. Preferably. during each thermo-mechanical cycle. the thermal spalling at the kerf removes from 3 to 30 percent of the rock totally separated during this thcrmo-mechanical cycle.  
  In certain cases. the cracking is aided by chemical agents. which are present naturally or may be provided artificially A natural chemical agent is present in mine rock in the form of considerable amounts of free water. i.e.. water which can be removed at temperatures of above ZtlllF to 225F. This in situ water is induced by the applied heat to diffuse through the network of cracks formed as above. The effect of this moisture transfer may be to reduce the strength of the rock by lowering the specific surface energy of the material. An artificial chemical agent can be applied. for example. in the form of a surfactant such as an aqueous detergent sprayed on the excavation face. The effect ofthis surfactant is considered. from one viewpoint at least. to reduce the strength of the surface region ofthe rock at and near the mining face.  
  The additional natural discontinuities in Class II rock and. particularly. in class III rock profoundly affect their characteristics relative to those of Class I rock. Specifically. the high concentration ofnatural discontinuities in Classes II and III. in comparison with Class I. cause: l reduced strength of the same order as that caused by the snythetic discontinuities of the heat affected zone discussed above; (2) reduced speed of sound. a phenomenon which is useful in detecting the presence ofdiscontinuities (for example. by microseismic sensors); and (3) reduced heat transmissivity. by reason of which thermal spallation may be partially or wholly precluded. One one hand. the above described heating assembly l spalls Class I rock effectively and (2) spalls Class II rock to a limited extent but (3) does not spall Class III roc k. On the other hand. the impacting assembly l may not routinely rip Class I rock that is unaffected by heat. (2) can rip Class II rock that is unaffected by heat but preferably is not applied to Class II rock that is unaffected by heat. and (3) can rip Class III rock routinely. Thus. the modes ofthe impacting and heating assemblies are interrelated in such a way as to remove rock from an excavation face efficiently no matter what its thermal response class.  
  The production of kerf space 36 and heat affected zone 49A. as in FIG. IB. involves two phenomena: l spallation and (2) thermal cracking.  
 SPALLATION ANALYSIS Spallation occurs when a conchoidal fragment of rock spontaneously bursts free from the surface of a rock mass that has been subjected to thermal shock. Spallation also is considered to occur when a fragment loosens but does not burst free of the surface. The prcsent understanding of the mechanism of spallation is as follows:  
  FIG. 2A shows an intense heat flux (BTU/hr/ft&#34;) 51 impinging on a rock surface 53. As the surface is sub jected to thermal shock. a thermal gradient 55 along an axis 57 is developed in the rock mass. The shape of the gradient. depth .r versus temperature I is a function of heat flux intensity. i.e.. rate of heat transfer. The induced thermal stress is a direct function of the thermal gradient. The stress most important to spallation is the tensile stress 0&#39;, parallel to the axis and the compres sive stress tr, perpendicular to the X axis. These stresses are illustrated at 59 and 61. Rock. being a brittle material. tends to break in tension. Therefore. if a discontinuity ofcritical size and orientation exists in the region of maximum 0,. as shown in FIG. 2A. a crack will initiate as at 63 and will propagate in the direction indicated by the arrows. In a real situation. the crack propagation path will depend on the shape of the heat flux distribution and on the compressive stress state (I... The tensile stresses initiate fracture but the stored strain. due primarily to compressive stress 0... provides the kinetic energy to extend the crack and to cause the fragment to spall. It is believed that. in Class I and more often in Class II rock. even a negligible compressive stress 0,. often propagates a tensile crack with the result that a flat plate-shaped fragment loosens from the rock mass but lacks the strain energy to burst free.  
 THERMAL CRACKING ANALYSIS In connection with thermal cracking. a thin layer of rock. I to 5 inches deep. which is to become or has become kerf rock. is heated to a temperature ranging upwardly to approximately 500F.  
  Stresses developed by the thermal gradient are upproximated by:  
 0&#39; EaA T/l-v Where:  
 0&#39; confined thermal stress E modulus of elasticity u Poissons ratio AT t I0) I, final temperature T initial temperature It can be shown that stresses can be developed within the rock mass. of the magnitude of the rocks tensile strength, by a temperature increase of 70F above i with E X a 5 X [0. v= 0.3. and 1 80F. Laboratory tests have shown that the tensile strength as a function of temperature, for so-called Red Massive Rock, is a minimum of 1000 pounds per square inch in the range of 200 400F. Below 200F to room temperature the strength continually increases. These data indicate that cracks can be initiated at very low temperature gradients and that a crack. once started. propagates with ease. even through cold rock is permitted by boundary displacements. Such cracks are suggested at 67 in FIG. 2B.  
  As indicated previously, in situ rock often naturally contains from a to 2 gallons of free water. It is believed that this water is forced into motion by pressure due to any thermal gradient. by vacuum generated at any crack apex. and by wetting (spreading) ofdry rock surfaces. eg a new crack surface. The water then reacts with the rock surface to reduce its specific surface en ergy The strength of brittle materials is given as: 0&#39;  
  V-iEy/rrC Where:  
 0&#39; applied stress E modulus of elasticity y specific surface energy C inherent crack length The above relationship indicates that as y is reduced. rock strength is reduced. and that as crack length increases. rock strength is reduced. Further weakening occurs as a result of steam formation and condensation EXAMPLE ll Thermal Alone 228.000 BTU/ton X 200 ton/hr X 1/047 97 X 10 BTU/hr (for 200 tons/hr) Thermomechanical Thermal Energy 50 tons/hr X 228.000/0.47 BTU/ton 24.3 X l0 BTU/hr (for 50 tons) Mechanical Energy 300 hp 2544 BTU/hr 0.764 X l0 BTU/hr (for 150 tons) Total 25.06 X 10 BTU/hr. (for 200 tons) Ratio /97 =0.258 25.8 percent EXAMPLE lll It was found that spallation rates. in grams per minate, at a depth of 1.l [0 feet underground and at a depth of feet underground, were in the ratio of approximately 375 to one. This observation illustrates that the final stress state is a superposition of various stresses which are induced by tectonic forces. excavation con figuration, heat distribution etc. Thus, stresses in rock due to natural causes can be used advantageously, in combination with stresses due to artificially applied energy, for fragmenting rock. Specifically. the deeper the excavation the greater the natural stress state and the greater the spallation effect for a given amount of applied heat flux.  
 The Thermo-mechanical Longwall Mining System of FIG. 3  
  ln longwall copper ore mining. typically. two parallel service tunnels are driven from 200 to 800 feet apart and are connected by a third tunnel or gallery. Ore then is separated from the retreating wall (mining face) of this gallery while the roof of this gallery is supported above steel caps that are carried by props. These props and caps are designed to support the weight of the overlying rock, with an ample factor of safety. As each incremental depth of slice of ore is mined. the props together with the caps. in a sequence of steps. are released, moved forward toward the mining face and reinstalled. In the newly unsupported regions remote from the mining face caving occurs into the underlying excavated region.  
  FIG. 3 illustrates a longwall face 50 of a gallery 52 that extends across and beyond a pair of service tunnels at 54 and 56. A plurality of props 58 extend from the mine floor 60 upwardly to a plurality of caps 62, on which the roof of the gallery is supported. Adjacent to the longwall face is a series ofinfrared heaters 64, each of which is constrained for reciprocal motion at right angles to the longwall face by a drive linkage 66 and a pair of parallel rails 67. Adjacent to the longwall face also is a mechanical ripping and plowing unit 68, which is movable along the longwall face under the control of a chain 69 on suitable wear and back plates that are parallel to and adjacent to the longwall face. Mechanical ripping and plowing unit 68 includes a plow 71 (details not shown) and two series impact moils 73a. 73b at opposite vertical edges of the plow. When unit 68 is moving in one direction. impactors 730 are oriented obliquely in mechanical contact with the longwall and when unit 68 is moving in the other direction. impactors 7311 are oriented obliquely in mechanical contact with the longwall. the arrangement being such that ripping of the longwall occurs with each pass of unit 68 in either direction. Plow 71 includes an upwardly and outwardly directed inner face by which rock is scooped from the vicinity of the longwall and channeled onto a panzer conveyor 75. Unit 68 and conveyor 75 are con trolled by drive motor 77 and gearing 79, 81. The arrangement is such that heaters 64 focus infrared radia tion along a zone between the floor and the roof. se quentially producing a region of weakness having greater than the original discontinuity density. As unit 68 is moved along the longwall. each of heaters 64 in sequence is retracted to permit unit 68 to pass between it and the longwall face and is returned to proximity with the longwall face after unit 68 has cleared. This reciprocal movement of each heater is under the control of the position of unit 68, which automatically initiates dimming in order to maximize equipment life and to economize electrical power consumption. The  
 heater. ripper and plow device and the conveyor are advanced as a unit toward the longwall face, as the mining process continues, by the action of hydraulic cylinders anchored at one end to the roof support props and at the other end to the panzer conveyor. Props 58 which normally are pressured between the floor and the roof of the mine. are advanced by: I sequentially releasing the caps 62 on one set of props. (2) retracting the horizontal cylinder attached between the props and the panzer conveyor, and (3) repressurizing the freed caps 62 against the roof in a new advanced position. The process is repeated for adjacent props. down the length of the longwall until all have been repressurized at advanced positions.  
 The Infrared Heating Unit of FIGS. 4, 5 and 6 As shown in FIGS. 4, 5 and 6, each heating unit comprises a housing 80 within which the operating components are mounted and enclosed. Within the housing, infrared radiation is generated by a bank 82 of six groups of six electrically energized lamps. which are arranged to present a perpendicular face emitting infrared radiation. Behind each lamp is positioned a reflector 84. by which infrared radiation from the lamps is focused on the mining face for the purpose of producing a zone of discontinuities. Mounted at the front face of housing 80 in order to protect the lamps from fragmented rock are a coarse outer screen 88 and optionally a fine inner screen 90. Between the screens and the lamps is a quartz plate 91 which shields the lamps drom dust while passing infrared radiation. A blower 92 blows cooling air through the housing. via a filter 94 at the rear, past the lamps and through openings at the front.  
  Details of one of the lamps are shown at 96 in FIGS. 6 and 7. which is one example of a suitable lamp. This lamp includes a U-shaped quartz envelope having a bight 98 and a pair of legs I00, I02. Within bight 98 is a tungsten filament 104, the opposed ends of which are integral with tungsten leads 106, 108 to metal terminals I10, I12 (FIG. 6.). Thus the glass to metal seals at the free extremities of legs I00. 102 are spaced from the heat emitting bight in order to reduce seal temperature and to increase lamp life. Also the glass to metal seals, when the lamp is in operation. are located behind reflectors 84 so as to be air cooled efficiently. In one case lamp has a inch radiating length of filament consum ing 2.000 watts of electricity at a tungsten filament temperature of 4000F and 86 percent of the total elec trical input or I720 watts of infrared radiation is emitted from the lamp in the wavelength range of 0.6 microns (yellow) to 4 microns (infrared). Non radiating space between the lamps is about W2 inches.  
  The function of reflectors 84 is to collect and redirect onto the minewall face as great as possible a fraction of the backwardly directed energy. Preferably the reflector is composed of material. having a high coefficient of reflectivity for the band of wavelengths emitted by the infrared lamps and yet being mechanically strong and capable of operation at elevated skin temperatures with minimum cooling. The energy that is absorbed by the reflector causes a surface layer several microns thick to become hot and to re-radiate. Examples of materials which are suitable for reflector construction are magnesium oxide, magnesium carbonate and aluminum oxide. These compounds can be molded and fired into hard briquettes having high reflecting pure white surfaces. They are self-cleaning and can operate at high surface temperature without damage.  
  In one case, blower 92 has a capacity of 400 to L000 c.f.m. to provide cooling air for the lamp leads. the quartz lamp body and the reflector surfaces. A large portion of the air is directed to pass between the lamps and the reflectors so as to lower the surface temperature of quartz lamps to about l,600F. In addition, the thermal energy absorbed by the outgoing air is directed through the front of the lamp enclosure toward the minewall. whereby additional preheating of the wall is effected.  
 Operation of the Thermo-mechanical Longwall System of FIG. 3 in Reference to FIG. 8  
  Each of heaters 64 of the device of FIG. 3 contains a control circuit of the type shown in FIG. 8. This circuit, which incorporates six lamps. is energized by a transformer 120. A master switch I22. when closed. applies the output of the secondary of transformer I20 across six parallel paths each of which includes a power relay 124 (for an associated lamp). a current relay 126 and a power level toggle switch 128. The arrangement is such to establish across the lamps, a normal current which is the maximum current maintained across the lamps when the device of FIG. 3 is in operation with all switches 122. 126. 128 closed. When any heater 64 is retracted to its position remote from the excavation face, its associated switch 128 opens to economize power consumption while preventing undue lamp cooling, which might give rise to thermal shock. When any heater 64 is extended to its position adjacent to the excavation face, switch 122 closes.  
 Optional Separation of Ore and Gangue at the Mining Face In Reference To FIG. 9  
  FIG. 9 is a cross-sectional view of a geological formation in which the system of the present invention typically is employed. This cross-section specifically de picts a geological formation in the copper mine of the White Pine Copper Company. White Pine. Michigan. The broad designation of parting shale and upper shale ore strata actually are composed of alternating substrata of rich and lean copper bearing material as shown at 132 for copper-rich and at 134 for copper lean strata. In one form. impactor 40 of FIG. 1C is designed so that on one pass it gouges out lean strata 134, separating and discarding the fragments of such strata from the rich strata 132. which latter is the only material that is further processed through the crushing. grinding. milling and flotation circuits. Ripping of the entire parting shale is known as values-only mining because. in contrast to room-and-pillar&#34; mining. essentially none of the barren upper and lower sandstone is mined with the copper bearing shale. The foregoing technique of optional separation of ore and gangue at the mining face might be called truly values only since it not only avoids dilution of the shale ore with barren sandstone. but it also minimizes further processing of barren strata of the parting shale itself. FIG. 9 also illustrates longwall mining and caving with the use of movable posts I36.  
 CONCLUSION The above described embodiment of the present invention involves the synergistic combination of thermal energy and mechanical energy for efficiently fragmenting hard rock from a mining or tunneling face. In its broadest sense. the heating assembly is the preferred one of many types of thermal. hydraulic. abrasive and laser units that can act to cut kerfs in an excavation face. Since certain changes may be made in the above disclosure without departing from the scope of the present invention. it is intended that all matter contained in the foregoing description and shown in the accompanying drawing be interpreted in an illustrative and not in a limiting sense.  
 What is claimed is:  
  1. An excavation apparatus for continuously excavating rock from successively exposed excavation faces. said excavation apparatus comprising a heating means constrained for movement between an operative mode and an inoperative mode. an impacting means constrained for movement between an operative mode and an inoperative mode. said heating means when in its operative mode being contiguous with one of said successively exposed excavation faces in order to apply thermal energy to selected portions thereof. said heating means when in its inoperative mode being remote from said one of said successively exposed excavation faces. said impacting means when in its operative mode passing between said one of said excavation faces and said heating means in order to apply mechanical energy to all portions thereof. means for sensing and indicating the homogeneity of said rock underlying said one of said successively exposed excavation faces. and control means for selecting the mode of said heating means and the mode of said impacting means in response to deter minations of said means for sensing and indicating.  
  2. The excavation apparatus of claim 1 wherein said heating means includes a plurality of infrared elements.  
  3. The excavation apparatus of claim 1 wherein said impacting means includes a plurality of impacting elements.  
  4. The excavation apparatus of claim 3 wherein impacting elements are carried by a single mount.  
  5. The excavation apparatus of claim I wherein said heating means includes a bank of infrared emitting tubes. reflectors rearwardly of and respectively associated with said tubes for focusing radiation therefrom toward said excavation face. an infrared transparent sheet forwardly of said bank. and screen means forwardly of said infrared transparent sheet for protection against rock fragmented from said excavation face.  
  6. The excavation apparatus of claim 1 wherein said heating means includes a plurality of infrared generat&#39; ing elements disposed along a line approximately midway between the floor and the roof bounding said one of said excavation faces is i said