煤矿开采中的软岩优化工程毕业论文外文翻译.doc

Optimization of soft rock engineering with particular reference to coal miningAbstract Soft rock engineering is a difficult topic which has received much attention in the field of rock mechanics and engineering. Research and practical work have been carried out, but much of the work has been limited to solving problems from the surface. For overcoming the difficulties of large deformations, long duration time-dependent effects, and difficulties in stabilizing the soft rock, the problem should be tackled more radically, leading to a more effective method of achieving optimization of the engineering system in soft rock. A summary of the optimization procedure is made based on engineering practice. 1. Introduction There are many soft rock engineering problems around the world, involving engineering for mines, highways, railways, bridges, tunnels, civil subways, buildings, etc. Engineering losses have occurred because of volumetric expansion, loss of stability of the soft rock, etc. This has been an important question to which much attention has been paid in engineering circles, and in the field of academic rock mechanics. Since the 1970s, considerable research and practical efforts have been made in the field of soft rock engineering in various countries, but the major efforts were concentrated on such aspects as the method of construction, the design and reinforcing of the supporting structures, measurement and analysis of the rocks physical and mechanical properties, its constitutive relations and engineering measurement. It has been found that the soft rock engineering problem involves complex systematic engineering including such subsystems as classification of soft rocks, judgement concerning the properties of soft rock, project design and construction. Only by considering the integral optimization of the system can we obtain an improved solution to the problem. Hopefully, a radical approach can lead to engineering feasibility, lower costs and engineering stability in order to achieve the engineering objectives. 1.1. Mechanical properties of soft rock and associated engineering Soft rock is an uneven and discontinuous medium. Its strength is low, with a uniaxial compressive strength usually lower than 30 MPa. Some soft rocks expand when they are wet. Cracks in some soft rocks will propagate easily which makes them exhibit volumetric expansion. Large deformation and creep can occur in soft rocks. Many soft rocks are compound ones which have composite properties formed from two or more sets of constituent properties. Soft rock can be graded into divisions according to its properties. After engineering has occurred, soft rock can deform rapidly and by time-dependent deformation, owing to its low strength and sensitivity to the stress field. With the effect of water, the expansive minerals in soft rocks volumetrically expand, which causes large convergent deformations, which leads to damage of the surrounding rock. The mechanical properties of soft rocks appear so various and different that it is difficult to express them with mathematical formula, which is the technological challenge for soft rock engineering. 1.2. Engineering in soft rock and its optimization Because soft rock engineering can induce large deformations, the maintenance of the engineering can be difficult. Moreover, volumetric expansion and loss of stabilization of the surrounding rock often causes damage to supporting structures. If we use strong supports to control the deformation of the surrounding rock, the engineering cost will be high, and the construction time will be increased by repeated installation of support, sometimes the support itself has to be repaired. In order to obtain the benefits of easier construction and lower cost, the integral optimization of the system must be carried out and managed in a systematic and comprehensive way. Design and construction are the two important steps in soft rock engineering. These must begin by understanding the physical and mechanical properties of soft rock, in the context of the stress field, hydrogeology and engineering geology. The engineering design plan is conceived as a whole according to the theory of rock mechanics and combining practical data from adjacent or similar projects, including integrating the many factors. The establishment of the correct soft rock engineering system should come from practice, basing on a full mastery of the factors. The scheme is shown in Fig. 1. Fig. 1. Engineering system for soft rock. Optimization of soft rock engineering is achieved by making the surrounding rock interface with the supporting structure such that the engineering will be stable. The key technological strategy is to avoid a high stress field and enhance the supporting ability of the surrounding rock. Feasible measures are as follows: reducing the external load; optimizing the engineering structures size and shape, improving planar and cubic layouts of engineering; choosing better strata, and structure orientation, etc., as shown in Fig. 2. Fig. 2. The principle of the optimization process. According to these ideas, take the development of a coal mine in soft rock as an example. Integrated optimization of the development system of the mine should take the relevant factors into account: existing information; an overall arrangement for optimal development and production; eliminate adverse factors; and deal with the problems of soft rock by a simple construction method. The content of the first part of the optimization includes: choosing the mine development method; deciding on the mining level; and determining layers in which the main roadways are to be located. Also important is arranging a reasonable layout of the pit bottom and chamber groups and seeking to reduce the deviator stress caused by mutual interference of the openings. Openings perpendicular to the direction of horizontal principal stress should be avoided when choosing the driving direction of roadways. Optimizing the layout of the mining roadways reduces the damage to support caused by moving loads introduced by mining. Further optimization is related to the geometry and size of the roadway sections, the supporting structure, and the method and technology of construction. Finally, by measuring and monitoring during construction, feedback information can be obtained to ensure that the engineering is running on the expected track and, if there is any deviation, corrective action can be implemented. The system is shown in Fig. 3. Fig. 3. Systematic optimization of coal mining in soft rock. 2. Engineering examples 2.1. Mine No. 5 in Youjiang coal mine, China The mine is situated to the east of Baise Coalfield, in the West of Guangxi Zhuang Autonomous Region. It belongs to the new third Period. The mine area is located at the edge of the south synclinal basin. There are three coal layers; the average thickness of each seam is 12 m; above and below the coal layers are mudstone, whose colours are grey, greyish white, and green. There are minerals of mixed illite and montmorillonite in the rock, montmorillonite 58%, and illite 720%. The rocks uniaxial compressive strength is 45 MPa, the average being 4.8 MPa. There are irregular joints in the rock, but distributed irregularly, and the rocks integral coefficient index is 0.55. Most of the cracks are discontinuous, without filling matter in them. The surrounding rock is a soft rock subject to swelling, with low strength, and is quite broken. The strike of the coalfield is NEE, the dip angle of the coal layers is 1015°. The mine area is 6 km long along the strike, and 1 km long along its inclination, its area is 6 km2, the recoverable reserves are 4,430,000 tons. In the adjacent mine No. 4, the maximum principal stress is NNESSW, approximately along the seams inclined direction. A roadway perpendicular to this direction has convergence values of 70100 mm, the damage of roadway supports is 51%. A roadway parallel to the direction of maximum principal stress has convergence values of 2040 mm, the damage rate of supports is 12%, and the average damage rate of the mine is 40%. In the design of the mine, a pair of inclined shafts were included. The level of the shaft-top is +110 m, the elevation of the main mining level is located at 120 m. Strike longwall mining is planned, arranging with uphill and downhill stope areas, as shown in Fig. 4. Fig. 4. Development plans for Mine No. 5 in Youjiang. The first optimization measure is to weaken the strain effect of the surrounding rock in the mine roadway caused by the stress field. Roadways are arranged as far as possible to be parallel with the maximum principal stress (that is, approximately along the inclined direction) so as to reduce the angle between them. The strike longwall mining is changed into inclined longwall mining, the mine is mined upward by using the downhill stope area, the main mining level is elevated by 20 m, 1131 m of roadways are saved and the cost of the roadway construction and facilities is saved ¥2,760,000 ($336,600). The new system is shown in Fig. 5. Fig. 5. Development system plans after optimization for Mine No. 5 in Youjiang. The second optimization measure is to change the layout of the pit bottom and openings to be parallel with the maximum principal stress as far as possible. The total length of roadways initially designed was 1481 m, and 30.11% of them were arranged to be perpendicular to the maximum principal stress. After amendment, the total length of roadways is 1191 m, which is a decrease of 290 m, and with only 24.69% of roadways that are perpendicular to the principal horizontal stress, roadways are easier to maintain. As shown in Fig. 6 and Fig. 7. Fig. 6. Layout of the pit bottom and chamber initially designed for Mine No. 5 in Youjiang. Fig. 7. Layout of the pit bottom and chamber after the optimization for Mine No. 5 in Youjiang. The third optimization measure is to excavate the section of the roadway in a circular arch shape to reduce the stress concentrations. In order to increase the supporting ability of the surrounding rock itself, after the roadway has been excavated, rockbolts are installed as the first support. Considering the expansivity of the surrounding rock, guniting is not suitable. The secondary support is the use of precast concrete blocks. Between the support and the surrounding rock, the gaps should be filled with a pliable layer of mixed lime-powder with sand. This produces the effect of distributing the stress and has a cushioning effect when the soft rock is deforming; also, it inhibits the soft rock from absorbing water and expanding. This scheme is shown in Fig. 8 Fig. 8. Optimization design for the supporting structure of the main roadway for Mine No. 5 in Youjiang. The fourth optimization measure is to simplify the chamber layout so as to reduce the number of roadways. For example, in order to decrease the stress concentrations by the roadway, the number of passageways in the pumproom and the sub-station can be reduced from three to one, and the roadway intersections connecting at right-angles can be reduced from 14 to nine. The fifth optimization measure is in accordance with the different stratigraphical lithologies which the roadways pass through, and for harder rock regions to change the roadway section into a structure with straight-sided semicircular top arch and arc bottom arch, and another structure with a straight-sided horse-shoe arch, so that the investment of supporting structure can be saved when there are better rock masses with comparatively few fractures. In construction, waterproofing and draining off the water should be implemented, and the catchment in the roadway bottom should be strictly prevented because it may cause the bottom rock to expand. When the opening groups are excavated, the construction sequence must be considered, enough rock pillar must be reserved, and the construction method of short-digging and short-building must not be used, so that the interactions can be avoided. By the optimization described above, after the roadways have been constructed, the serviceable roadway is 95.5% of the total, 55.5% more than that of the adjacent mine No. 4. The length of the roadway was reduced, and ¥3,700,000 ($450,000) saved. In addition, ¥3,000,000 ($360,000) was saved in the maintenance costs of the roadways before the mine was put into production, so, the cost saving totals ¥6,700,000 ($810,000) in all. After the mine has been turned over to production, the main designed capacity was reached in that year, and added to the saved money for the maintenance cost of roadways in production, there was ¥8,700,000 ($1,050,000) saved. 2.2. The coal mine at Renziping, China The mine lies to the south of Qinzhou coalfield in Guangxi Zhuang Autonomous Region. It belongs to the new third Period and synclinal coal basin tectonics. There are two coal layers in it, the main seam thickness is 1215 m. The roof and floor of the coal layers are arenaceousargillaceous rocks, whose colour is greyish white, and whose essential minerals are quartz and kaolinite. The uniaxial compressive strength of the rock is from 10 to 15 MPa. Rock masses are quite integral with fractures only in it occasionally. It belongs to the class of soft rock that has weak expansion, lower strength, and is quite broken. There are faults around the coalfield basin which are 8 km long and 1.5 km or so wide. Slopes are inconsistent, the edge angles are 2540°, and the bottom of the coalfield is gentle. Affected by tectonic stress in the NWSE direction, there is an inverse fault in the south. After the mine had been developed and put into production, a main roadway at the 250 m level was excavated along the strike, and the mine was mined by the uphill and downhill stope area. Affected by the rock stress, many parts of the main roadway have ruptured, parts have been pressed out, and supports have been broken; the serviceable rate of roadway supports was less than 40%, which seriously affected the haulage and ventilation of the mine road. In the following 10 years of production, the rated production output was not achieved and losses occurred leading to economic disbenefit. Through on-the-spot observations, it is apparent that the coalfield is affected by the tectonic stress field, that the deformation in the soft rock is serious, and is larger than that caused only by the vertical stress component. The technological reformation measures for the mine are proposed as follows. The first measure is to extend the depth of the shaft and abandon the main roadway excavated along the strike, and transform it into a bottom panel stonedoor along the synclinal basin minor axis to make it parallel with the main principal horizontal stress. The mining face can be laid on top of i