Method for excavating a working face

Abstract: The present invention relates to a method for forming a parallel bore hole and a slant bore hole (or V-shaped V bore hole) together within the same area of a working face, and then blasting the working face by using a delay electric detonator. The method of this invention includes the steps of drilling a number of slant hole by a predetermined angle in the horizontal angle cut pattern or in the vertical angle cut pattern about the central zone of a working face, drilling a number of parallel cut hole within a projective area of the above mentioned slant holes; loading an electric detonator in the slant holes and an explosive material by indirect priming in the parallel cut hole up to the bottom of the slant holes; blasting the slant holes to form a slant free surface; blasting a center cut hole out of the parallel cut holes to form two free surfaces having a funnel shape; and sequntially blasting a middle cut hole and an outer cut hole out of the parallel cut holes to form a cubical space, and as a result, the drilling is achieved easily, the drilling time is reduced, the necessary quantity of explosives is low, and the blasting efficiency is high.

Description: The present invention relates to an aggregate blasting method for the excavation of a working face having one free surface in a tunnel and, particularly, to a method of forming a second free surface comprising these steps: drilling a number of slant holes around the central zone of a working face and a number of parallel cut holes within an area surrounded by the slant holes in the working face; loading an electric detonator and an explosive material in the slant holes and the parallel cut holes; and sequentially blasting the slant holes and centre cut holes, middle cut holes and outer cut holes out of the parallel cut holes in order to form a cubical space, whereby a second free surface is easily formed. Generally, with a blasting method for excavating a working face having one free surface, a method for drilling cut holes in order to obtain a second free surface is divided into an angle cut method and a parallel cut method according to a drilling method of cut holes. The V-cut method in angle drilling and the burn cut method in parallel drilling are now generally used in a tunnel blasting. The angle cut method is applied to the short hole blasting of soft rock, and the burn cut method is applied to the long hole blasting of hard rock. However, the burn cut method, represented in FIG.1, has many problems as follows: 1) In order to increase the blasting efficiency based on a theory of blasting, the projective area of a pilot hole against a free surface must be large. However, the projective area of the parallel hole is small, as shown in FIG. 5B, and the blasting efficiency is low because the blasting pressure acts at 40°-60° against the working face 9 as shown in FIG. 17B. 2) The relief hole 1 shown in FIG. 1 is difficult to drill, and the drilling must be done so that the spacing of relief holes 1 may be 5cm to 7 cm. Accordingly, a high level of drilling technology and expensive drilling machines are required. Also, the interpenetration of relief holes may frequently occur. 3) Because the relief hole 1 has a large diameter, its drilling time is long. 4) A rock is difficult to reinforce because rock damaged zone 6 is large, and the scaling time of fragmented rocks is long. 5) A ventilation pipe, an electric panel and a drill's water pipe are damaged because the fragmented rocks badly scatter. 6) A number of pilot holes must be formed because the projective area of the pilot hole is small. Also, since the explosive material must be tightly loaded, a large quantity of explosive material is required. 7) The working time per excavation cycle is long. The cost of reinforcement and excavation is high because the excavation efficiency is low. Also, the angle cut method as shown in FIG. 2 has many of the following problems: 1) The advance of the slant hole is shorter than that of the parallel hole. So, the blasting efficiency is the lowest in the case of a long hole. 2) The spacing of slant holes on the hole bottom section must be about 100mm. If the spacing of slant holes on the hole bottom section is more than 100mm, then the blasting efficiency decreases. If the spacing on the hole bottom section is less than 100mm, then the slant holes might be interperforated. 3) Because the holes are drilled with a dip, the hole bottom section and the drilling depth are not uniform. Thus, the rock damaged zone is large and the blasting pressure is large. Also, the possibility of an accident because of the fragmented rock is increased, and it takes a long time to scale fragmented rock. 4) A number of cut spreader holes 12 (see FIG. 1) must be drilled to load tightly the explosive material in the centre of the working face. 5) The cost of reinforcement and excavation is high because the excavation efficiency is low. Consequently, so far, the previous angle cut technique mainly depends on the procedure of increasing the projective area of a slant hole in order to increase the blasting efficiency by the formation of a second free surface. However, the spacing of the slant holes on the hole bottom section must be about 100mm and the loading density of the explosive material must be large. To obtain two free surfaces, the blasting method by the parallel cut must include the steps of drilling a relief hole 1, the hole diameter of which is ø102-120mm, and cut holes 2, the spacing of which is 5-7cm, loading the explosive material in the cut holes 2, and blasting the cut holes 2. Alternatively, to form a second free surface, an unloaded hole having a large diameter is drilled by a tunnel boring machine. Accordingly, the previous conventional techniques require a high level of technology and machinery. Glückauf, 1984; 120(6): 346-352 shows a drilling pattern used for excavating the working face of a mine in Poland which drilling pattern comprises a plurality of parallel and angled, drilled blasting pits. US-A-4175490 discloses a process for converting subterranean rock into an enclosed, rubble-ized zone that is overlayed with an overburden for use as a storage area, ore body teaching or for the recovery of carbonaceous materials from subterranean deposits. Blast holes are preferably drilled through the overburden at an angle, sloping inwardly and downwardly to provide withdrawn wedge relief of a block of the overburden. Initial lifting of the block is obtained by drilling a series of blast holes arranged substantially along planes having a substantially horizontal component, placing explosive charges in the holes and exploding the charges to provide a gas-filled fracture forming a void space underlying the block. Preferably the blast holes are arranged along opposed, downwardly converging planes so as to produce a wedge-shaped block of raised overburden. To aid in the initial lifting of the block, and also to initiate fragmentation of an underlying deposit, further charges are decked in selected bores which are extended downwardly so as to place the further charges within the underlying deposit and adjacent to the first charges. The primary object of this invention is to provide a blasting method for tunnelling a working face having one free surface, so as to form easily a second free surface. A further object of the present invention is to provide a blasting method for tunnelling a working face having one free surface that is capable of remarkably reducing the damage zone of the working face. Another object of this invention is to obtain a high blasting efficiency without sophisticated machinery and highly skilled technicians. According to the present invention therefore, there is provided a method of excavating a working face for tunnelling, which working face has one free surface, said method comprising drilling a plurality of slant holes and a plurality of parallel-cut holes in the working face and being characterised by: drilling a plurality of converging slant holes around a central zone of the working face to a first, notional bottom line, each slant hole having a bottom end on said first bottom line; drilling a plurality of first parallel-cut holes in said working face within an inner area surrounded by the slant holes on the working face to a second, notional bottom line; and said first bottom line being disposed intermediate said working face and said second bottom line. Following is a description by way of example only with reference to the accompanying drawings in which: FIG. IA is a front view of a working face showing a drilling pattern according to a bum cut method. FIG. IB is a cross-sectional view taken along the A-A' line in FIG. IA. FIG. 2A is a front view of a working face showing a drilling pattern according to an angle cut method. FIG. 2B is a cross-sectional view taken along the A-A' line in FIG. 2A. FIG. 3 is a cross-sectional view which explains the blasting process in accordance with the embodiment of the invention. FIG. 4 is a schematic view of the projective area of a slant borehole. FIG. 5 is a schematic view of the projective area of a parallel borehole. FIG. 6 is a fragmentary cross-sectional view of a slant borehole in accordance with the embodiment of the invention. FIG. 7 is a schematic view of the position and spacing of slant boreholes in accordance with the embodiment of the invention. FIG. 8 is a fragmentary cross-sectional view of a slant borehole in accordance with the previous method. FIG. 9 is a schematic view of the drilling spacing and positions of cut holes over a sectional area of a working face in accordance with the embodiment of the invention. FIG. 10 is a schematic view of the drilling spacing and pattern of boreholes over a sectional area of a working face in accordance with the embodiment of the invention. FIG. 11 is a fragmentary cross-sectional view of a loading pattern of explosive materials in cut holes. FIG. 12 is a schematic view of the loading positions of a primer. FIG. 13 is a fragmentary cross-sectional view of the loading positions of a primer in cut holes. FIG. 14 is a front view showing a blasting sequence of a working face in accordance with the embodiment of the invention. FIG. 15 is a schematic view of a blasting process of cut holes in accordance with the embodiment of the invention. FIG. 16 is a schematic view of a tunnelling process of a working face in accordance with the embodiment of the invention. FIG. 17 is a schematic view of a blasting pattern in one free surface and two free surfaces respectively. Before explaining the present invention in detail, it is to be understood that the invention is not limited in its application to the details illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practised or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Referring to FIG. 3, in order to excavate a working face having one free surface at the 200' line up to a predetermined tunnelling line 210, a number of slant holes or V-holes 21 are drilled around the core of the working face either in a horizontal angle cut pattern or in a vertical angle cut pattern. Thereafter, a number of parallel cut holes 20' are drilled within an area surrounded by said slant holes 21 in the working face. The parallel cut holes 20' consist of a number of centre cut holes in an area 22a, middle cut holes in an area 23a and outer cut holes in an area 24a, as shown in FIG. 16. Also, a number of parallel holes 20 are drilled within the circumferential zone of said cut holes 20' and 21 as shown in FIG. 3. The parallel holes 20 consist of a number of centre spreader holes in an area 25a, stopping holes in an area 26a and contour holes in an area 27a, as shown in FIG. 16. An electric detonator is loaded into the slant holes 21 and the parallel holes 20 and 20'. The explosive material in said parallel cut holes 20' is loaded from the bottom line of said parallel cut holes up to the bottom line of the slant holes with indirect priming. The bore holes are blasted in an initiation sequence according to the numerals indicated in FIG. 3B', and then fragmented rocks are scaled, so that a cycle of excavating a working face is accomplished up to the predetermined tunnelling line 210. Now referring to the drawings for the purpose of illustrating the present invention, a proper quantity of slant holes 21, as shown in FIG. 9, are drilled either in a horizontal angle cut or a vertical angle cut pattern, in variance with the excavation conditions, for example, rock strength, tunnel pattern, the sort of explosive material used, etc. In the invention, slant holes of the V-Hole Type (Double V Type, Baby V Type, Multi V Type), Diamond Type, Prism Type, Pyramid Type, etc. may be employed in accordance with the drilling direction of cut holes, the working conditions, etc. Since the blasting efficiency increases in proportion to the projective area according to the theory of blasting, the projective area of the slant holes must be enlarged to increase the blasting efficiency. In the above descriptions, the projective area is defined as the area which a hole projects onto a working face. As shown in FIGS. 4 and 5, as a slant hole's angle increases, the projective area increases. The projective area of a parallel hole is equal to its sectional area. With the present invention, the spacing and angle of the slant holes are determined by the excavation conditions. For example, the spacing of the slant holes on the hole bottom section is reasonably 30-50cm. Although the drilling angle of the slant holes or the positions of the bottom ends and top ends thereof varies with the slant holes, the blasting efficiency does not decrease because the rocks beneath the bottom of the slant holes are easily blasted by the explosive material in the lower parts of the centre cut holes in area 22a, as shown in FIGS. 15 and 16. However, with previous, conventional techniques, the blasting efficiency decreases when slant holes on the hole bottom section are interpenetrated or when the spacing between the slant holes is increased. To avoid this problem, the slant holes must be precisely drilled at 10cm intervals on the hole bottom section. In the present invention, the blasting efficiency does not decrease, even in the event that the spacing between the slant holes on the hole bottom is 30cm to 50cm. Accordingly, in the present invention, there is no need for the spacing between slant holes on a hole bottom to be about 10cm. A proper quantity of slant holes 21 is drilled around the centre of the working face, and then a number of parallel cut holes 20' is drilled within an area surrounded by said slant holes 21 on the working face. In the present invention, it is preferable to drill many parallel cut holes 20' around the core of the working face, i.e. centre cut holes 22, if possible. The spacing of centre cut holes 22 in area 22a is preferably 200mm to 300mm. The spacings of the parallel cut holes 23 and 24 in areas 23a and 24a, except for said centre cut holes 22 in area 22a, are 100mm to 500mm, preferably 400mm to 500mm (see FIGS. 9, 11, and 16). In the invention, parallel cut holes of the four section type, three section type, spiral type, baby type, double spiral type, etc. may, of course, be employed in accordance with the drilling direction of the cut holes, working conditions, etc. FIGS. 9A, B, C, D and E show the spacings and positions of the parallel cut holes in a great working face, a great-middle working face, a middle-small working face and a small working face respectively. FIGS. 10A, B and C are schematic views representing the drill spacings and patterns of boreholes in a great working face, a middle working face and a small working face, respectively. Referring to FIG. 10, the spacing between parallel holes 20, excepting said parallel cut holes 20', is determined according to the excavation conditions and the spacing between the outer cut hole 32 and a cut spreader hole 31 is preferably 100mm-500rmn, as shown in FIG. 10. In loading, as shown in FIG. 11, the degree of loading in a slant hole 21 is determined by the excavation conditions. However, in the present invention, the slant holes 21 are loaded with explosive material up to 80% of their length and an instantaneous electric detonator to blast the slant holes 21 simultaneously. Referring to FIG. 12, a booster (or a primer) may be loaded in the slant holes 21 with indirect priming or middle priming. In the above descriptions, the electric detonator is a detonator ignited without any time difference at the same time that electric power is supplied. Alternatively, the electric detonator in the slant holes 21 may be a delay electric detonator. Electronic detonator loading types are divided into indirect priming, middle priming and direct priming, according to the position of the primer as shown in FIG. 12. Indirect priming loads the primer on the hole bottom section as shown in FIG. 12A, middle priming loads the primer between the hole bottom section and the hole entrance of the working face as shown in FIG. 12B and direct priming loads the primer around the hole entrance of the working face as shown in FIG. 12C. As shown in FIG. 13, the centre cut holes 22 are loaded with explosive material only from the bottom line of said cut holes 22 up to the bottom line of the slant holes 21 and are also loaded with a delay detonator with indirect priming. The detonator is probably a MS(millisecond) delay electric detonator which has the shortest explosion time from ignition to the blasting to achieve blasting pressure immediately after the blasting of the slant holes 21, thereby easily blasting the other parallel cut holes, excepting said centre cut holes 22. The middle cut holes 23 and the outer cut holes 24 are also loaded with delay electric detonators having indirect priming only from the hole bottom section up to the boundary formed by slant holes 21. The aforementioned delay electric detonators range in type from those featuring short explosion times to those possessing long explosion times as the distance extends from the parallel centre cut holes outwardly. The cut spreader holes in area 25a, the stopping holes in area 26a and the contour holes in area 27a, shown in FIG. 16, are preferably loaded with DS(decisecond) delay electric detonators. In the above descriptions, the MS delay electric detonators range in sequence from those having short explosion times to those having long explosion times as the distance extends from the core of the working face outwardly. Even in the case that the cut spreader holes in area 25a, the stopping holes in area 26a and the contour holes in area 27a are loaded with MS(millisecond) delay electric detonators, there is not a large change in blasting efficiency. Referring to FIG. 14, a roof stopping hole 26', a wall stopping hole 26", a floor hole 26 "', a roof hole 27', a wall hole 27" and a floor spreader hole 27'" are loaded with delay electric detonators having different initiation numbers, as in previous, conventional blasting methods. The blasting steps of this invention will become apparent from the following detailed description based on FIG. 15. First, the slant holes 21 are blasted simultaneously by an instantaneous electric detonator (see FIG. 15A). The slant holes in area 21a are easily blasted since the unloaded zones of the parallel cut holes act as free surfaces. Immediately after the slant holes in area 21a are blasted, the centre cut holes in area 22a are blasted by a delay electric detonator, preferably a MS detonator to form a second free surface having a funnel or crater shape. So, the parallel centre cut holes in area 22a are easily blasted by the blasting pressure of the slant holes i.e. converge (see FIG. 15B). In the above descriptions, the explosion times (from ignition to blasting) of the MS electric detonators loaded in the centre cut holes 22 are between 0.01-0.05 seconds. Accordingly, since the centre cut holes 22 are blasted by MS electric detonators immediately after the blasting of the slant holes 21, with a time difference of milliseconds, the two blasting pressures are multiplied by each other. With this accomplished, even a hole bottom section is efficiently blasted. As a result, a funnel shaped second free surface can be easily obtained as shown in FIG. 15B. Thereafter, referring to FIG. 15C, the middle cut holes in area 23a are sequentially blasted to extend the second free surface. Subsequently, the outer cut holes in area 24a are blasted, so that a second free surface of a cubical shape is formed, as shown in FIG. 15D. FIGS. 15A through D are schematic views representing the blasting process of the cut holes. As described above, this invention is characterised in that two free surfaces are, first of all, obtained by blasting cut holes. Once a second free surface of cubical shape is formed around the core of a working face, blasting from cut spreader holes 25 can be achieved completely by even a small quantity of explosive material, since the blasting pressure acts directly on the free surfaces as shown in FIG. 17 A. However, in the case that the blasting is accomplished on one free surface as shown in FIG. 17B, there are the problems that the bore hole may be blasted in the shape of a crater, a tight loading of explosives must be made and the possibility of a blown out shot is high. Referring to FIGS. 16E through G, the cut spreader holes in area 25a, the stopping holes in area 26a and the contour holes in area 27a are sequentially blasted by MS electric detonators or DS electric detonators. The method for excavating the ,working face according to an embodiment of the invention has the following benefits in contrast with the previous methods. In the following we have tabulated the results of various tests which we have carried out. (1) The drilling is accomplished easily and the drilling time is reduced. (i) In a burn cut method, drilling must be done so that the spacing of the pilot holes in the periphery of the relief holes may be 5 to 7cm. In an angle cut, the spacing of the slant holes must also be about 10cm on the hole bottom section. Accordingly, the above blasting methods need a sophisticated level of drilling techniques and machines. On the other hand, according to the present invention, the spacing of slant holes extends to 30 to 50cm on the hole bottom section. Even in the case that the drilling degrees of the slant holes are different from each other and/or the slant holes are not aligned in a straight line, the result of test blasting reveals that the blasting efficiency is not reduced. (ii) In an angle cut method, cut spreader holes must be tightly packed with explosives. So, in consideration of that, a number of holes are required. With the burn cut method, relief holes having large diameters must be drilled. On the other hand, in the present invention, the drilling time is reduced because relief holes are not required. (2) The specific explosives quantity is low. (i) According to the present invention, blasting is efficiently achieved even with a small quantity of explosives because the unloaded portions of a number of parallel cut holes surrounded by the V-holes function as free surfaces. (ii) The blasting of the parallel cut holes beneath the bottom of the V-holes is boosted by the blasting pressure in the V-holes so that the parallel cut holes beneath the bottom of the V-holes are efficiently blasted. Accordingly, there will be few bootlegs after the blast. (iii) The second free surface is formed by the centre cut holes in the shape of a cube. Therefore, the explosives quantity used in the blasting of peripheral holes is reduced. (iv) The blasting efficiency is improved to the greatest level. Therefore, the explosives quantity used in excavation is less than methods in the prior art. (3) The blasting efficiency is high. (i) The centre cut holes are completely dug out without any bootleg. Accordingly, the blasting efficiency is high. (ii) The drilling work is easy, and the blasting efficiency is unchanged, even in the case that interpenetration of the holes occurs. (iii) Regardless of the drilling conditions, the blast is efficiently achieved. Therefore, the blasting efficiency is unchanged. (4) Reinforcing work is easy, because the damage zone of the working face and the peripheral rock is very small. (i) Owing to the parallel holes, the depth to the hole bottom section is constant and the blasting pressure acts in the direction of gravity. Accordingly, the damage zone of the working face is very small. (ii) The damage zone of the peripheral rock is small because the blast is efficiently achieved under the conditions that the peripheral holes are not tightly loaded. (iii) The blasting pressure is reduced because the centre cut holes are sequentially blasted with each other with a time difference of milliseconds or deciseconds. Accordingly, the vibration of blasting is remarkably reduced. (iv) Secondary deformation is restrained because the damage zone in the peripheral rock is small. Therefore, reinforcing work (rock bolting, shotcreting, concrete lining, etc.) is easy. (5) Accidents are prevented because the quantity of the fragmented rocks is low. (i) The quantity of fragmented rocks is low because the damage zone of the working face and the peripheral rock is small. Accordingly, the danger of a cave-in is reduced. (ii) The scaling time (by human power or machines) is reduced because the quantity of fragmented rocks is low. (iii) The blasting face in the working face is fine because the quantity of fragmented rocks is small and the overbreak is low. (6) The flying distance of the muck is short, so that the damage to equipment is reduced. Also, the muck has a proper size, so that the muck is easy to remove. (i) The flying distance of the muck is so short that a ventilation pipe can be installed near to the working face. Accordingly, the working efficiency is improved because ventilation after blasting is facilitated. (ii) The work is efficiently performed because an airpipe, a drill's water pipe and an electrical panel for the drilling and reinforcing works can be installed near to the working face. Also, the reinforcing work can be performed within a suitable time. (iii) The muck can be easily removed because the flying distance of the muck is short, and the rock is blasted in a suitable size according to the unloaded holes within V-holes. (7) The working time per excavation cycle is reduced, and therefore the cost of excavation can be reduced. (i) The working time per excavation cycle is reduced because the drilling time, muck removing time, charging time, reinforcing time and so on are shortened. (ii) The cost of excavation is lowered because the working time per excavation cycle, explosives cost, reinforcing cost, removing time of fragmented rock and muck removing time are reduced. While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art.

Claims: A method of excavating a working face for tunnelling, which working face has one free surface (200'), said method comprising drilling a plurality of slant holes (21) and a plurality of parallel-cut holes (20') in the working face and comprising the steps of : drilling a plurality of converging slant holes (21) around a central zone of the working face to a first, notional bottom line, each slant hole having a bottom end on said first bottom line; drilling a plurality of first parallel-cut holes (20') in said working face within an inner area (21a) surrounded by the slant holes on the working face to a second, notional bottom line; and being characterised by said first bottom line being disposed intermediate said working face and said second bottom line. A method of excavating a working face as claimed in claim 1, characterised in that the spacing of said slant holes (21) on said first bottom line is 30 - 50 cm. A method of excavating a working face as claimed in claim 1 or claim 2, characterised in that said first parallel-cut holes (20') comprise centre parallel-cut holes (22) in a central region (22a) of said inner area (21a), said centre parallel-cut holes having a spacing of 20 - 30 cm, the remainder of said first parallel-cut holes outside said central region having spacing of 10 - 50 cm. A method of excavating a working face as claimed in claim 1, claim 2 or claim 3, characterised by loading electric detonators and explosive materials into said slant holes and said first parallel-cut holes, each first parallel-cut hole being loaded from its respective bottom end substantially up to the slant holes with indirect priming. A method of excavating a working face as claimed in claim 4, characterised in that each slant hole is loaded with explosive materials with indirect or middle priming. A method of excavating a working face as claimed in claim 4 or claim 5, characterised in that the electric detonators loaded into the slant holes comprise delay electric detonators or instantaneous electric detonators. A method of excavating a working face as claimed in claim 4, claim 5 or claim 6, characterised in that the electric detonators loaded into the first parallel-cut holes comprise delay electric detonators or instantaneous electric detonators. A method of excavating a working face as claimed in any of claims 4 - 7, characterised by drilling a plurality of second parallel-cut spreader holes, stopping holes (26',26", 26"')and contour holes (27',27",27"') in said working face outside said inner area. A method of excavating a working face as claimed in claim 8, characterised by loading electric detonators and explosive materials into said second parallel-cut holes. A method of excavating a working face as claimed in any of claims 4 - 9, characterised by blasting said slant holes (21) to form a slanted free surface and, thereafter, blasting said first parallel-cut holes (20'), said first parallel-cut holes comprising centre parallel-cut holes (22) in a central region (22a) of said inner area (21a), outer parallel-cut holes (24) towards the outer edge of said inner area and middle parallel-cut holes (23) intermediate said centre and outer parallel-cut holes, said centre parallel-cut holes being blasted after blasting of the slant holes to form a funnel-shaped, free surface, and said middle and outer parallel-cut holes being blasted sequentially to form a cuboidal space. A method of excavating a working face as claimed in claim 10, characterised in that said centre, middle and outer parallel-cut holes are blasted sequentially with a time difference ranging from milliseconds to deciseconds. A method of excavating a working face as claimed in claim 8; characterised by blasting said slant holes to form a slanted free surface; blasting said first parallel-cut holes, said first parallel-cut holes comprising centre parallel-cut holes in a central region of said inner area, outer parallel-cut holes towards the outer edge of said inner area and middle parallel-cut holes intermediate said centre and outer parallel-cut holes, said centre parallel-cut holes being blasted after blasting of the slant holes to form a funnel-shaped, free surface, and said middle and outer parallel-cut holes being blasted sequentially to form a cuboidal space; and thereafter sequentially blasting said cut spreader holes, stopping holes and contour holes. A method of excavating a working face as claimed in any preceding claim, characterised in that said slant holes are drilled at a predetermined angle in either an horizontal angle cut pattern or a vertical angle cut pattern. A method of excavating a working face as claimed in any preceding claim, characterised in that said slant holes do not interpenetrate. A method of excavating a working face as claimed in claim 14, characterised in that said first parallel-cut holes comprise centre parallel-cut holes in a central region of said inner area, which centre parallel-cut holes extend between the bottom ends of said slant holes. A method of excavating a working face as claimed in any of claims 4 - 12, characterised in that said slant holes are loaded with explosive materials up to 80% of their length.

Inventors: KIM MYONGKYU, YOON YEONGIAE, KIM, MYONGKYU, YOON, YEONGIAE
Applicants: SUNKYONG ENGINEERING & CONSTRU, SUNKYONG ENGINEERING & CONSTRUCTION LIMITED
Application year: 1995
Main classification: E21C37
IPC codes: F42D3, E21D9, F42D1, E21C37