Contacting the iron ore Xishimen account skarn magnetite bed, anticline ore, the shaft portion gently dipping, dip wings ore generally 10 ° ~ 30 °, a thickness of 1.2 ~ 30m, an average of 15.13m. The ore body is unstable to stable, and the surrounding rock of the roof is Ordovician limestone, which is moderately stable to unstable; the surrounding rock of the floor is Yanshanian diorite , stable to moderate stability. Between the ore body and the surrounding rock of the floor, a layer of skarn with a thickness of 0 to 10 m is extremely unstable. The mine is divided into the mining areas of the South, North and Central China. The mining method has been optimized for several times without sub-column sublevel caving. After more than 20 years of mining, the mining stage has been reduced from 230m to 40m. During the mining process, the top pillars, the bottom pillars, the inter-columns and some sporadic small ore bodies with the mining and destruction of the ore body and the pillared pit are left as residual minerals in the stope, and the amount of minerals is considerable. At first, this type of residual mineral resources was supplemented as reserve reserves during production stress. As mining progressed to the deep, conventional orebody resources were consumed too quickly, and today's residual ore recovery has become a major component of mine production. The mine adopts the bottom structure of the flat bottom trench to induce the caving method to carry out the recovery of the residual ore. It is necessary to arrange the mining, mining and connecting lanes in the lower plate. The stability of the mining project directly determines the success or failure of the recovery of the residual ore. Due to the weak lithology and the destruction of the empty area in the mining stope, the U-shaped steel arch frame and the anchor spray net support can not meet the requirements in the past. Some parts have distortion of the arch frame, the roadway falls, and the tunneling encounters Problems such as the surrounding rock of the bulk make the mining and maintenance of the residual mining and mining project very difficult. In order to solve the problem of excavation in the residual process, this study explores the corresponding excavation support measures on the basis of analyzing the damage characteristics of the residual roadway.
1 problems and causes of excavation support
1.1 There is a problem
With the increase of mining depth of mining and mining bodies and the wide-scale development of shallow residual mining sites, due to the influence of large ground pressure and poor rock mass stability, the previous forms of support have been unable to meet the requirements of mine production. In addition, the impact of shallow mining activities has led to the following problems in production: 1 The ore body contains more empty areas, and the ground pressure is higher in some areas. At the same time, the skarn close to the lower part of the ore body is soft and deformable. The pressure bearing capacity is weak. In this case, U-shaped arch yieldable metal arch forward portion often appear curved, offset phenomenon, a serious distortion of the leg portion arch (FIG. 1), even after repeated repair does not improve. Using the part supported by the spray anchor net, the spray anchor net structure is completely separated from the road surface, and the spray layer is cracked off (Fig. 2). 2 In the mining area, the residual mining area often encounters large caving areas or civil mining landslides. Most of them are filled by falling bodies. Although the sedimentation effect takes a certain time, the road surface can stand. Live, but after the excavation, the roof is easy to collapse, and many roadways are blocked and discarded. 3 In the process of tunneling, there is often a phenomenon that the roof suddenly falls, especially at the junction of the skarnized magnetite and the skarn layer of the floor. A large cavity is formed at the top of the roadway (Fig. 3), and the height can reach 5~. 7m, width 3~5m, working under this condition, there is a safety hazard of falling off the top plate. In addition, when the cavity is too large, the bypass construction is often carried out, and the engineering quantity is large.

1.2 Reason analysis

(1) The support method and the support environment are not suitable. The U-shaped shrinkable metal arch has a high initial support force and can provide a large support resistance. The shrinkability of the arch can be self-adapted to the deformation of the surrounding rock for stress adjustment, and can remain sufficient after large deformation. The bearing capacity, but in the early stage of support, the arch frame and the side wall of the roadway are not tightly attached, and the point support state is often formed. The arch frame is partially stressed and easily deformed [1]. In the multi-space ore body, there are problems such as the obvious increase of ground pressure in the local ore section and the complicated change of the direction of the ground pressure. It is more likely to cause uneven stress on the arch, which causes the supporting arch to be dumped and bent.
(2) The state of the long and short zone of the mine causes the ground pressure to increase. From the perspective of rock mechanics, the ground pressure activities of only a few mines in China's metal mines are caused by high original rock stresses. The root cause of most mine ground pressure activities is the goafs existing in the rock masses [2], empty The high ground pressure caused by the area is mainly manifested in: 1 The roof of the goaf is exposed to the surface, and the stress is transferred after the top support is lost, causing the mining pressure of the local mine section to increase sharply [3]. 2 The existence of the goaf in the ore body of the multi-space area causes the stress distribution around the empty area to be redistributed, and the stress is concentrated on the irregular ore column between the empty areas, resulting in an increase in ground pressure.
(3) Poor stability of surrounding rock. The poor stability of the surrounding rock of the ore body is mainly affected by lithology. The surrounding rock of the ore body is mainly skarn and altered diorite, which belongs to soft rock. Its lithology is soft and easy to weather, and it is seriously cemented. After the support is not timely, the air will expand and deform at a faster speed to form a surface fracture layer, no longer have the support force, and the spray layer will fall off easily after spraying. In addition, the surrounding rock pressure capacity is weak, affected by blasting vibration, ground pressure and other factors, cracks and fracture zones are more likely to develop into the rock mass, forming a broken loose ring, resulting in deformation and fragmentation of the roof, resulting in caving, gangs and anchors. The tray is loose and so on.
(4) Support construction is not timely. The surrounding rock is broken, the weathering and hydrolysis are rapid, and it needs timely support. Spray anchor net support has the advantages of providing continuous support force quickly, maintaining and improving the shear strength of rock mass, and giving full play to the self-supporting ability of surrounding rock. However, there are many construction procedures and timeliness is not easy to guarantee, which directly affects the support of spray anchor net. The long-term nature of quality and its effects.
2 roadway driving support technology
2.1 physical soft broken surrounding rock roadway
In the Xishimen residual mining stope, the weak surrounding rock and the increase of local ground pressure due to the existence of the empty area are the two major factors affecting the stability of the roadway. For this reason, the spray anchor network +U is used for the simultaneous action of the two parts. Combined shrinkable arch combined support technology. When installing the arch frame, it is required to be close to the roadway section, the back is filled with semi-round wood, and the connecting rod is welded between the arches to improve the integrity of the arch frame. During construction, the construction of the spray anchor net should be carried out in time, and then the U-shaped shrinkable metal arch frame can be constructed. This can ensure high support strength and allow a large amount of deformation to meet the support of the roadway service. Sexual requirements. Features of spray anchor net + U-shaped collapsible arch combined support technology: 1 It has the advantages of giving full play to U-shaped steel arches and high support strength of anchors, and can realize combined support of primary and passive [4]. 2 Increase or make full use of the scalability of the U-shaped steel arch, and use the high pressure-retaining external anchor structure of the anchor to achieve the same large deformation of the U-shaped steel arch under high support resistance. 3Using the combined support structure of U-shaped steel arch and anchor to realize the coordinated support between U-shaped steel bracket and anchor [5]. At present, this support measure has been applied to the stope where the ground pressure appears more, and the phenomenon of multiple support in the past has been significantly reduced (Fig. 4).

The bolting and shotcreting support in the joint support of spray anchor net + U type shrinkable metal arch frame is supported by double rib + pipe anchor bolt. The length of the bolt is 1.8m, the diameter is 43mm, and the row is 6-8. The spacing is 0.8 to 1 m and the row spacing is 1 m. After the anchor and double ribs are installed, the spray support is carried out in three times. The spray thickness is 30~50mm, the arch is 20~40mm, and the final spray thickness is 80~100mm. The intermittent time of the spray layer is generally 10 to 15 minutes. When the interval is more than 2 hours, the water should be sprayed first and then sprayed. The nozzle should be perpendicular to the rock face to ensure a distance of 800 to 1200 mm between the nozzle and the rock face. After 2 hours of spraying, water spray curing is carried out, and the curing time is not less than 10d. Adopting 20Mn-25 U-shaped steel, the spacing of the arches is 600-800mm in principle. The spacing of the poor stability should reduce the spacing of the arches. The welding specifications of the column legs are 200mm×150mm×100mm steel plates to prevent the arch from sinking. stable. The upper gap of the arch frame is filled with the split wood or the round wood. The filling of the gap of more than 1 m should be in close contact with the surrounding rock, and the overlap of the beam legs is not less than 300 mm.
2.2 Shenshen scattered surrounding rock
In the excavation process of each mining area, it is easy to expose a large caving area or a civilian mining landslide area, and the area is filled with loose bodies. In the past, when the excavation project encountered a loose body, it was stopped, and it was not suitable for the conditions of the ore body in the multi-mining area. It is necessary to solve the technical problems of the support of the roadway. When the loose body in the vacant area is loose, it has better fluidity, and it is more difficult for the roadway to pass through the loose body; when the loose body is pressed, the face of the roadway can stand up, but the top plate is easy to collapse after excavation. The scattered bodies exposed in the Xishimen Iron Mine Empty Area are mostly compacted bodies (Fig. 5). Based on the particularity of the support of the loose layer and the characteristics of the U-shaped arch support, U-shaped arches and advances are adopted. A combination of anchors. In this scheme, the advanced anchor is pre-arranged at the front end of the roadway face, and is driven into the bulk pile along the eyebrow line with a certain upward inclination angle, which plays a role of pre-supporting, enhances the integrity of the front-end bulk, and prevents the bulk pile from appearing large. Harmful deformation or slumping provides conditions for arch support operations (Figure 6). As a supporting body, the U-shaped arch has the characteristics of high strength, large deformation, and high pressure, so as to ensure that the roadway does not break during the service period. At the same time, the arch structure helps to maintain the overall shape of the roadway, providing a safe space for the workers in the bulk roadway.

The specific length of the leading anchor depends on the fracture of the surrounding rock. Under normal circumstances, it takes 2.5m, and when the surrounding rock is particularly broken, it takes 4m. The leading anchor is exposed to 0.1m, the end is reserved for 1m, the inclination angle of the anchor is 10° to 25° [6], and 10 to 20 are arranged in each row. Generally, the portion where the anchor reserve is perpendicularly intersected with the entrance top plate is used as the arrangement position of the lower row of leading anchors. When the compaction is tight, the spacing of the leading anchors may be appropriately larger; conversely, the spacing of the anchors should be reduced. The distance between the tunnels of the tunnels is 0.6-0.8m, but it varies from case to case, with a maximum of 1m and a minimum of about 0.2m. Under normal circumstances, each row of 2~3 rows of arches is arranged ahead of the queue. The anchor rod and the back of the arch frame are filled with semi-round wood to ensure that the arch frame is in close contact with the top plate, and the utility model also has the function of blocking the gravel falling and injuring people.
Before the construction support, the slurry should be sprayed in time to close the rock and reduce the weathering of the surrounding rock. At present, the tunnel over-discrete layer technology has been successfully applied in the mining area of ​​the multi-air area in the north and south area of ​​Xishimen Iron Mine. This technology safely and effectively solves the support problem of the bulk roadway and enables the residual area of ​​the caving area. Effective recycling.
2.3 over-shooting area
In addition to the soft-breaking rock mass and the loose-body roadway formed by the caving of the empty area, the phenomenon of local large-scale roofing of the skarn interlayer often occurs in the mining roadway in the north and south area of ​​Xishimen. The height of the roof is often 5-7 m and the width is 3~. 5m.
When a roof collapse occurs, the roof is sprayed first, then the pile is cleaned, and a metal arch is placed in the lower part of the landing area. The upper part of the arch is filled with a round top. Because the skarn is broken, the fall is not easy to completely control, and it is easy to fall off the block when spraying. In addition, when the shovel and the erecting bracket are cleaned in the lower part of the roof area, the spray layer is liable to be partially peeled off, thereby causing shot blasting. The safety conditions of working with the erecting bracket are poor, and it is easy to cause a blockage and injury accident, which often causes the roadway to fall off and often fails to be disposed of in time and often scrapped.
To this end, it is proposed to use the overall arch shed over the local roof area scheme (Figure 7) for processing. The metal arch frame used in this scheme is 3m high and 3m wide, and the arch spacing is 1m. One welding and four joints are integrated into one body to form an overall arch shed with a length of 3m. The staff does not enter the roof-topping area, and the roof is removed from the roof area in front of the roof area to prevent the top plate from falling off. The scraper is used to weld the retractable metal arch frame with the rebar into the overall roof area. On the side of the roof area, the person stands on the scraper bucket, inserts the semi-round wood above the bracket to seal the operation, and forms an overall scaffolding to make the laneway of the landing zone unblocked. The scheme welds the shrinkable metal arch into one body, which not only greatly enhances the support strength, but also makes the integral bracket still scalable, which is favorable for pressure release and has strong impact resistance. In addition, the dense round wood at the top of the arch frame forms an effective sealing space, which protects the safety of the workers and equipment in the arch frame, and at the same time makes the arch frame evenly stressed and avoids stress concentration. Generally speaking, the program satisfies the requirement of quickly dealing with local roofing accidents, and has been applied to the 72m level residual mining stope in the South Second District, with remarkable results.

3 Conclusion

The Xishimen iron ore mine is a difficult-to-mine body with unstable rock layers. The large amount of residual minerals left by the recovered areas in the 20th century is suitable for mining by the induced caving method of the bottom structure of the flat bottom trench. The mining engineering is mainly arranged under the ore body. The plate and the shrinkable metal arch support form used in the mine can not meet the stability requirements of the surrounding rock which is softly broken and affected by the goaf. For this reason, according to the characteristics of surrounding rock failure, the support form has been improved and achieved. Good results can be used as a reference for similar mines.
references
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Author: Dou Meilin; Minmetals Mining Co., Ltd. HANXING Xishimen iron ore;
Article source: "Modern Mines"; 2016.4;
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