KERF (EN: cut, kerf; DE: Einbruch, Schram; FR: havee, saignee, bouchon; ES: roza; RU: вруб) is the artificial cavity within the coal or rock massif, which is created for the facilitation of its subsequent destruction during the excavation of the useful minerals, or during the conducting of the mine workings. Depending on the machinery, which is used, and on the technology according to the method of the kerf creation, they distinguish the machined, hydraulic, and blasting kerfs.
The machined kerf is performed using the bar of the cutting machine, of the cutting-loading machine, or of the combined mining cutting-loading machine, from the stope of the mine working inwards along the seam of the useful minerals, or along one of its interlayers. According to the place of the situation relatively to the soil of the mine working, they distinguish the following kerfs: lower, middle, upper, vertical, and oblique. Depending on the types of the bars, which are used, the shape of the machined kerfs may be flat, curved, circular, and so-called of the contour type. The parameters of the machined kerfs (depth, height) depend on the technical parameters of the bars.
The hydraulic kerf is created, using the jet of the hydromonitor within the coal or rock massif. The further destruction (breaking) of the pack, which has been undercut, is performed along the layers, which are parallel to the kerf. The hydraulic kerf is performed using the two methods. In case of the first method, they move uniformly the barrel of the hydromonitor from one side of the stope or of the mine working to another side. The second method includes the initial excavation near the sides of the stope or of the mine working, and later within the central part of its cross-section. Usually, they penetrate the kerfs near the soil of the stope or of the mine working, throughout the height of 0.5-0.8 metres, and throughout the entire width. In case of the existence of the strong pack of the coal near the soil of the mine working, they perform the hydraulic kerfs within the soft pack immediately along the boundary with the strong pack. In case of the penetration of the preparatory mine workings at the steep thick seams, it is sometimes expedient to perform the inclined kerf along the division between the soft and strong packs of the coal.
The blasting kerf is formed as the result of the priority detonation of the charges within the group of the blastholes or boreholes, which are situated according to the adopted scheme; the walls of the kerf cavity serve as the additional exposed surfaces during the initiating of the remaining charges (which are situated within the blastholes around the periphery of the stope). Depending on the strength and structure of the rocks, on the cross-section of the mine workings, on the means for the drilling, which are used, on the depth of the destruction behind the explosion, which is required, they use the following kerfs: oblique - with the blastholes, which have been drilled obliquely to the surface of the stope; straight - with the blastholes, which have been drilled perpendicularly to the surface of the stope; combined - which are comprising the blastholes, which are oblique and perpendicular to the surface of the stope. In their turn, the oblique kerfs are subdivided into unilateral (lateral, upper, lower, of the fan type) and multilateral (of the horizontal wedge type, of the vertical wedge type, pyramidal, funnel-shaped, "scissors").
The lateral unilateral kerf (see the Figure # 1, a) is directed to one of the sides of the mine working; they use this kerf for the conducting of the mine working near the boundaries of the deposition with the clearly expressed contact or vertical stratification of the rocks. The upper kerf (see the Figure # 1, b) is directed to the roof of the mine working; they use this kerf for the conducting of the horizontal mine workings along the layered or cracked rocks with the dip of the layers (cracks) from the stope. The lower kerf is directed to the soil of the mine working; they use this kerf under the same conditions, but in case of the dip of the layers (cracks) to the stope. The kerf of the fan type (see the Figure # 1, v) is formed using the group of the blastholes, which are situated within one plane, and which have been drilled along the soft interlayer; they use this kerf for the conducting of the horizontal or inclined mine workings with the ripping of the rocks of the soil or of the roof.
The multilateral kerf of the horizontal wedge type (see the Figure # 2, a) is characterized by the horizontal situation of the wedge (several pairs of the converging blastholes at the distance of 0.2-0.8 metres one from another); they use this kerf for the conducting of the mine workings with the small cross-section. They use the kerf of the vertical wedge type (the wedge is situated vertically; see the Figure # 2, b) in case of the vertical stratification of the rocks. The "scissors" kerf (see the Figure # 2, v) is the variety of the kerf of the horizontal wedge type; this kerf is formed only using the two opposing blastholes, which are situated within the parallel planes, while the projections of the blastholes intersect among themselves; they use this kerf within the mine workings, which are conducted along the coal. The pyramidal kerf is formed using the 3-4 inclined blastholes, which are converging into the common centre (see the Figure # 2, g); they use this kerf within the strong monolithic rocks.
The funnel-shaped kerf differs from the pyramidal kerf by the high quantity of the blastholes, and by their circular situation; they use this kerf within the rocks with the high strength, and mainly for the penetration of the underground mine shafts. Among the straight kerfs, which ensure the compact collapse of the broken mining mass, they distinguish prismatic, barrel-shaped, spiral-shaped, and slot-shaped. The prismatic kerf is straight symmetrical, is formed using the central non-charged and several charged blastholes, which are situated at the corners of the triangle (see the Figure # 3, a), of the quadrilateral (see the Figure # 3, b), or of the hexagon (see the Figure # 3, v). Within the rocks with the high toughness, they increase the quantity of the non-charged blastholes.
The barrel-shaped kerf is straight symmetrical, and is formed using the central kerf blasthole, with the group of the kerf blastholes, which are parallel to the central blasthole, and which are situated around the circle at the same distance from the central blasthole; the latter blasthole is left uncharged, and its walls serve as the additional exposed surface for the the blastholes, which are charged; they use this kerf within the mine workings with any cross-section. The spiral-shaped kerf (see the Figure # 3, g) is straight non-symmetrical, and is formed using the central blasthole, which is not charged, with several blastholes, which are charged, are detonated sequentially, and are situated along the spiral from the centre. The slot-shaped kerf (see the Figure # 3, d) is straight, and is formed using several blastholes, which are situated along one line; they leave the part of the blastholes uncharged, thus their walls serve as the additional exposed surfaces, towards which t here is directed the action of the explosion from the charges within the remaining kerf blastholes; the kerf cavity, which is formed using the explosion, has the appearance of the slot.
In the series of the cases, there assists to the better ejection of the broken rock from the blasting strip the usage of the combined kerfs.
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