Hard Rock Strata Research Articles

Study on the mechanism of “separation stratum water-inrush” induced by impact energy of the key stratum sudden breaking

Subsidence deformation of the bottom rock stratum of the bending belt was different due to deflection difference; the separation stratum space was easy to form between the soft and hard rock stratum, and the separation stratum water formed under the water supplies. Based on catastrophe theory and the mechanics theory, the pointed catastrophic model of key stratum (the rock beam structure overlying the separation stratum space) was established. The impact energy released by the key stratum breaking was calculated. The “crushing” modes and “cleavage” modes of separation stratum water-inrush were proposed under the impact energy of key stratum. The periodic step distance of two kinds modes of separation stratum water-inrush was calculated. At last, separation stratum water-inrush of extra-thick coal of Guo Jiahe mine was taken as an example to study, and the aim was to provide technical reference for the prevention and treatment of separation stratum water-inrush.

Investigation of Deep Mine Shaft Stability in Alternating Hard and Soft Rock Strata Using Three-Dimensional Numerical Modeling

The problem of shaft instability has always been a major difficulty in deep mining practices. The shaft fracture has a high probability of being located near the aquifers and the soft–hard rock contact zone. This paper describes the deformation and stress characteristics of surrounding rock and the shaft lining under the interactive geological conditions under soft and hard rock strata in Anju coal mine, Shandong Province, China. Using the Method of Geological Strength Index (GSI ) and considering the rock-softening characteristics of water, the parameters of rock mass are calibrated. By means of the 3DEC-trigon method, the variation characteristics of surrounding rock and the shaft lining are simulated. After shaft excavation, under the condition of no support, shear failure and tensile failure occur in shallow surrounding rock shafts, and a pressure relief zone is formed. Shear failure is the main destruction mode in deep surrounding rock. Because of the different strengths of the surrounding rock, the deformation of the surrounding rock is significantly different. After the surrounding rock is softened by water absorption, the difference is magnified. The maximum shear stress and plastic zone appear near the interface between soft and hard rock. Under the condition of shaft lining support, uneven deformation of surrounding rock surely leads to nonlinear variation of pressure on the shaft lining. Under the action of an inhomogeneous pressure field, partial shear failure occurs in the shaft lining, and the shear failure area increases after the surrounding rock is softened by water. Because of the nonlinear deformation of the shaft lining, it is easy to produce stress concentration and bending moment near the interface between hard and soft strata. The control methods of advance grouting and pressure relief excavation are proposed to improve the stability of the shaft, and a good effect is gained.

The Mechanism Study of Cracks Propagation of Different Floor Strata Combinations Under Mining

To obtain the water resistance performance of floor strata under different rock layers and fracture combination, the two cracks’ propagation path under different situations was given by using minimum plastic zone theory according to the change of the fracture factor and the plastic strain of different strata combination cracks under the influence of fault, water pressure and mining. The shortest distance of the rock bridge between the two cracks after bursting in different strata combination was obtained. And the water resistance performance of every strata combination was analyzed. The conclusion was that the water resistance performance of the floor strata under the 3214 combination of soft–soft–hard–hard was the worst. While the 1243 combination of soft rock, hard rock, soft rock, hard rock was the best. The fracture factor combination and the plastic strain of the crack would reach maximum under mining condition. And the crack was mostly easy to destroy. The distance of the rock bridge between the two cracks after bursting was the shortest under the 3214 combination of soft–soft–hard–hard strata. The distance of the rock bridge between the two cracks after bursting was the longest under the 1243 combination of soft rock, hard rock, soft rock, hard rock strata. And the distance of the rock bridge between the two cracks after bursting would be shorten when considering the water pressure inside the crack and fault, which resulted in the decrease of rock water resistance performance.

Overburden movement characteristics of top-coal caving mining in multi-seam areas

The fully mechanized top-coal caving mining method was used to mine the 15 m thick No. 3–5 seam at a depth of 450 m in Tongxin mine, located in the Datong area, China. The seam was overlain with strong strata that are hard to cave and four partially mined coal seams. Excessive mine roadway closure and severe ground conditions at the longwall face and the tailgate were induced by periodic weighting and remnant coal pillars left in the four mined coal seams above. These ground conditions were studied using physical modelling and microseismic monitoring. The results indicated that hard rock strata above the coal seam did not cave readily, causing minor and major face loading events. The ground conditions deteriorated when mining was undertaken below the remnant pillars left in the four upper Jurassic seams. In addition, the physical model showed overlying longwall goaf collapse reaching the upper goafs and causing global movement in the strata. The data from the physical model and the underground microseismic survey presented here agreed closely with the visual observations and reported events that occurred at the underground longwall face in Tongxin coal mine. Overall, this study demonstrated the mechanisms of the overburden strata movement, longwall weighting occurrences and deteriorating ground conditions along the longwall face and the tailgate roadway when mining was undertaken under the overlapped Jurassic remnant coal pillars. Methods to minimize these bad ground conditions were suggested, using stress relief hydraulic fracturing methods and blasting, such as pre-splitting and softening of coal pillars and roof strata in key places. A design of small coal pillars <3–5 m in size were recommended for overlying seams to minimize stress concentrations in seams below.

A STUDY ON THE LAW OF OVERLYING STRATA MIGRATION AND SEPARATION SPACE EVOLUTION UNDER HARD AND THICK STRATA IN UNDERGROUND COAL MINING BY SIMILAR SIMULATION

The breaking of hard and thick key layers can easily cause dynamic disasters, such as rock burst and mine seismicity, and seriously threaten safe mining. To analyze the characteristics of overburden breakage and law of evolution of separation cracks under mining of hard and thick rock stratum, and to further reveal the process of the occurrence of dynamic disaster, the fracture characteristics of overlying strata during mining were analyzed via similar simulation in this study. The characteristics of surface movement and deformation before and after the breaking of hard and thick strata were then discussed based on the law of overlying rock displacement. Finally, the development law of separation fissure in hard and thick rock stratum was revealed. Results show that the key stages of overlying strata movement during mining are immediate main roof breaking; main roof cycle breaking; and hard and thick rock breaking. The hard and thick strata, which are the key strata, bear the weight of the overlying strata with small subsidence before breakage. Development of the separated stratum stops at the bottom of the hard and thick strata and maintains an unclosed state for a long time, providing the formation space for the accumulation of gas and water in the separated bed. When the hard and thick rock is broken, the subsidence of the overlying strata increases dramatically, and the separated stratum is closed rapidly, inducing gas outburst, water inrush, dramatic surface subsidence, and other disasters in the working face easily. The outcomes of this study are significant to the safe mining of working face under similar geological conditions.

Predicting Underground Strata Movements Model with Considering Key Strata Effects

Longwall coal mining causes a large scale of subsidence in both overlying strata and ground surface. No matter where subsidence or deformation happened, its damaging effects to underground mining work or surface infrastructures are not anticipated. To accurately predict surface subsidence over the mining area, surface subsidence prediction models based on influence function method are developed. Based on the surface subsidence model, in this paper, a prediction model for overlying strata movements is developed with innovatively optimizing three key parameters used for setting the boundary condition. A load is derived using the beam theory with the simple overlying strata model, which is important for getting the radius of major influence, one of the three key parameters. Many features of progress subsidence are reproduced in the model and finally found to fit reasonably well with numerical simulation results using the same geological parameters. The modelling study reveals that the newly proposed subsurface model is flexible and versatile to represent the subsurface subsidence profile under various possible conditions. The key strata, especially the hard rock strata, act like an elastic beam and can overhang the load from above strata so that the displacement of rock mass or stress distribution in strata will be drastically changed when close to key strata. The proposed subsurface subsidence prediction model discloses some basic laws of overlying strata subsidence with the mining of coal seam.

Square-form structure failure model of mining-affected hard rock strata: theoretical derivation, application and verification

The coal-bearing strata consist of rock and coal strata with diverse thickness or mechanical strength, and thus they, especially the thick–hard rock strata, are distinguishing in movement and failure laws during coal mining. The failure of the thick–hard rock strata may affect the strata stability and cause serious coupled dynamic hazards and environmental problems. In the paper, the failure model of thick–hard rock strata was derived theoretically, which was applied to calculate the failure laws of the thick–hard igneous sill, and then, the model was verified by the methods of numerical simulations and surface subsidence measurements. The results indicate that: (1) the failure laws of the hard roof strata were analyzed based on the four-edge-clamped thin-plate assumption, and thus, the square-form structure failure of the hard roof stratum, which case is defined as the SSF model, occurs most possibly with the square gob for larger normal stress components; (2) the SSF model was applied to calculate the failure laws of the thick–hard igneous sill, and it would be broken with the approximate square-form gob of 310 m long and 350 m wide in Yangliu Coal Mine, Huaibei Coalfield, China; and (3) both the simulation results and the surface subsidence measurements indicate that the initial failure length of the thick–hard igneous sill was about 320 m with the square-form gob, verifying the correctness of the SSF model. The results could provide the theoretical guide for mining rock strata control and dynamic hazards prevention.

Deformation analysis of a soft–hard rock contact zone surrounding a tunnel

The Mounigou tunnel crosses complex geological strata, mostly with surrounding rock of poor quality with an IV or V classification. Failure in the form of collapses or squeezing deformation occurred many times during the tunnel construction. Especially, in the soft and hard rock contact zone of phyllite and meta-sandstone, the deformation failure was most obvious. This paper uses the FLAC-3D modeling analytical software to establish a 3D-numerical model, on the basis of a field tracing survey and laboratory tests to analyze the stress and strain changes before and after excavation of the tunnel. After excavation there are obvious stress concentrations and differentiations near the interface of the soft-and-hard rock stratum. Stress transfers and concentrates towards the hard rock stratum while in the soft rock zone stress release could be observed to some degree. The range of plastic deformation is obviously larger in the soft rock zone than in the hard rock zone. Both differences in principal stress and principal stress ratio increase after excavation. The amplification of the difference in principal stress is larger than the augmentation of the principal stress ratio. Moreover these principal stress amplifications are smaller in the soft rock zone than in the hard rock zone. The difference in principal stress amplifications between the soft and hard rock zones, together with the fact that soft rock strength is far lower than that of hard rock, causes local “relatively high geostress” effects in the soft rock. It is the main controlling factor of the large deformation of soft rock in the tunnel. It is concluded that the large deformation in the soft and hard rock contact zone in combination with the surrounding rock deformation failure in the Mounigou tunnel and the damage of the supporting structure, belongs to the type of soft rock plastic-squeezing failure.

Stonecutters Bridge: design of foundations

For Stonecutters Bridge, Hong Kong, the foundation support is most efficiently provided by bored piles utilising end bearing and friction in the hard rock stratum, which is typically between 45 m and 67 m below ground level on the east side and between 48 m and 75 m below ground level on the west side. Local areas of deeper weathering are observed beneath the west tower and the adjacent back-span pier. The foundations for each pier and tower comprise a thick reinforced concrete cap supported on a grid of large-diameter, bored piles. The bored piles are socketted in rock and typically enlarged at the base up to 1·5 times the pile shaft diameter to provide resistance through both shaft friction and end bearing with no contribution from the overlying superficial deposits. The design used an iterative process to achieve compatibility between the bridge superstructure and substructure. Additional considerations for seismic loads and ship impact were also taken into account.

The Major Problems and Countermeasures on the Shield Machine Tunneling in the Hard Rock Stratum

With the rapid development of urban construction, the shield construction technology has been greatly developed. Shield tunneling method generally adapts to relatively uniform soil, soft rock and sand rock strata, and the shield tunnel construction has been more mature for the homogeneous strata. However, for the soft and hard interbedded strata, how to solve the problems of breaking the hard rock, increasing the tunneling efficiency and reducing the construction costs, are difficult problems in the shield tunnel construction. In this paper, based on a great deal of theoretical analyses and engineering practices, the effective methods of improving the efficiency of tunneling, reducing cutter wearing and decreasing the construction costs in the process of the shield tunneling construction in hard rock and gravel layer are proposed.

Hybrid intelligent method optimization of a soft rock replacement scheme for a large cavern excavated in alternate hard and soft rock strata

Optimization of a soft rock replacement scheme for a large cavern excavated in alternating hard and soft rock strata is a complicated non-linear mechanical problem having a large parameter search space. Obtaining a global optimum solution is the key to the problem. A hybrid intelligent method is proposed for this purpose. It is an integration of an evolutionary neural network and finite element analysis using a genetic algorithm. The non-linear relation of the soft rock replacement scheme with the displacement and damage zone of the cavern due to excavation in the given geological conditions is learnt and represented by a forward neural network whose structure and connection weights are global optimally recognized by using the genetic algorithm. The learning samples are obtained from finite element calculations. The optimal soft rock replacement scheme, having the minimal displacement and damage volume induced by cavern excavation, is searched in a global space using the genetic algorithm. The new methodology is used to evaluate soft rock replacement schemes for the Shuibuya cavern in China excavated in strata consisting of alternating soft and hard rocks. The results indicate that the new methodology can recognize the optimal soft rock replacement scheme for a large cavern in such complicated geological conditions and the neural network model can provide a solution which is close to the finite element analysis for the same geological and construction conditions.

The influence of a thick hard rock stratum on underground mining subsidence

The influence of a thick hard rock stratum on underground mining subsidence