Bei Jiang

Mechanical behaviors analysis on a square-steel-confined-concrete arch centering and its engineering application in a mining project

Aiming at the support difficulties of mine galleries under the complex geological conditions of high buried depth, high in situ stress and tectonic fractured zones, a novel square-steel-confined-concrete (SSCC) arch centering supporting system is proposed in this paper. To validate the feasibility of this supporting system, a uniaxial compression test on an SSCC short column is performed and the results illustrate that it has better ductility and higher bearing capacity than the U-shaped column. Numerical analysis on the mechanical behaviors of SSCC arch centering is conducted considering three factors of core concrete strength, steel tube thickness and confinable effect coefficient. Two influence indexes of steel tube thickness (μt) and core concrete strength (μc) influencing the ultimate bearing capacity are put forward and μt has more significant influence. Therefore, an SSCC arch centering (side length 150 mm, tube thickness 8 mm and filled C40 concrete) is determined to perform laboratory and in situ experiments. Combined with numerical simulations, the deformation failure behaviours of the SSCC arch centering are eventually obtained. The laboratory experimental results show that the maximum deformation appears near the spandrel of the arch centering and the ultimate bearing capacity is 1286.9kN. An SSCC and a U-shaped arch centerings have been simultaneously used in the supporting system of the gallery in Zhaolou coal mine project, and the monitoring results (after 157 days) show that their average displacement of the SSCC arch centering is 10.33 mm, which is only 18.3% of that obtained in the U36 arch centering. The above research results indicate that this kind of SSCC arch centering supporting system could efficiently control the deformation of the surrounding rock mass in mining galleries.

Mechanical properties of square-steel confined-concrete quantitative pressure-relief arch and its application in a deep mine

Abstract Providing support in deep and complex roadways is one of the major challenges in the mining industry; thus, a square-steel confined-concrete (SQCC) support system has been proposed and an SQCC quantitative pressure-relief arch (SQCC arch) has been developed for roadways in complex environments that are subject to high stress or are affected by faults or mining operations. A self-developed full-size large-scale test system for underground confined-concrete arches has been used to perform indoor comparative tests on a vertical-wall semicircle SQCC arch vs. a U29 arch, which is commonly used in mining. A combination of an indoor test and numerical analyses has shown that the SQCC arch deformation and failure pattern is ‘bulged arch top, retracted arch leg’. The most severe damage position occurs in the area between the middle of the arch leg and the spandrel, and the quantitative pressure-relief device can be used effectively. Compared with the U29 arch’s ‘Z’-shaped bending failure pattern at the arch leg, overall out-of-plane instability and rapid decline in bearing capability, the SQCC arch’s ultimate bearing capability is 2.15 times that of the U29 arch. In addition, the SQCC arch has an excellent bearing capability in the late stage. The SQCC arch support system is implemented in the fault-affected Zhaolou coal mine roadway to reinforce control over the surrounding rock. After 157 d, the maximum deformation at the arch measurement point is 33.1 mm, and the surrounding rock deformation is well controlled.

Research on reasonable coal pillar width of roadway driven along goaf in deep mine

Driving roadway along a goaf is commonly adopted for mining face of thick seam in a deep mine. Determining a reasonable width of coal pillar is a key scientific problem for driving roadway along a goaf in a deep mine. The paper took a roadway driven along a goaf at Zhaolou coal mine which is a typical kilometer-deep mine in China as engineering background. Field monitoring, model test, and numerical experiment are conducted. Stress and displacement evolution mechanism are analyzed with different pillar widths. The test results show that with the increase of coal pillar width, the peak stress value at the coal pillar working slope and integrated coal beside the roadway increases firstly and then tends to be stable, its position is transferred to the side of the roadway, and the deformation of coal pillar decreases gradually during roadway excavation. The coal pillar deformation and roadway vertical displacement increased as the coal pillar width increases under high abutment pressure. In order to reduce the waste of non-renewable resources and meet the requirements of bearing capacity and stability of coal pillars, a method is proposed for setting a reasonable width of coal pillars and the specific width of coal pillars is designed and applied in engineering practices based on the above research. All the tests are significant in the study of driving roadway along a goaf in a deep mine.

Method for Measuring Rock Mass Characteristics and Evaluating the Grouting-Reinforced Effect Based on Digital Drilling

Various types of broken rock masses, such as those in fault-fracture zones and fracture zones, which form as a result of disturbance from tunnelling, are often encountered during underground engineering construction. These rock masses have low self-supporting capacity and poor stability, which can easily cause damage to surrounding rock, such as large deformation features, collapse and falling blocks, etc., posing a threat to construction safety. During a field project, reinforcement by grouting is a primary means for addressing the aforementioned problems. The effective measurement of rock mass characteristics (e.g., rock layer interfaces and the broken area of surrounding rock) provides a basis for the reasonable design of a grouting scheme. The quantitative evaluation of the effect of rock mass grouting is essential for optimizing the grouting scheme. In view of this, in this study, a multi-functional rock mass digital drilling test system and a special polycrystalline diamond compact drill bit for digital testing were developed and were applied to conduct digital drilling tests on intact, broken and grouted rock masses. In addition, a digital drilling test (DDT) technique-based method for measuring rock mass characteristics in real time and rapidly evaluating the grouting effect was proposed. The proposed method is capable of identifying rock layer interfaces, determining the broken area and obtaining the equivalent strength of grouted rock masses. This method is advantageous for obtaining quantitative and rapid test results, which can provide a theoretical basis and technical means for optimizing the grouting parameters and designing support schemes for underground engineering construction.

Comparative study on the mechanical mechanism of confined concrete supporting arches in underground engineering

In order to solve the supporting problem in underground engineering with high stress, square steel confined concrete (SQCC) supporting method is adopted to enhance the control on surrounding rocks, and the control effect is remarkable. The commonly used cross section shapes of confined concrete arch are square and circular. At present, designers have no consensus on which kind is more proper. To search for the answer, this paper makes an analysis on the mechanical properties of the two shapes of the cross-sections. A full-scale indoor comparative test was carried out on the commonly used straight-wall semi-circular SQCC arch and circular steel confined concrete arch (CCC arch). This test is based on self-developed full-scale test system for confined concrete arch. Our research, combining with the numerical analysis, shows: (1) SQCC arch is consistent with CCC arch in the deformation and failure mode. The largest damages parts are at the legs of both of them. (2) The SQCC arch’s bearing capability is 1286.9 kN, and the CCC arch’s ultimate bearing capability is 1072.4kN. Thus, the SQCC arch’s bearing capability is 1.2 times that of the CCC arch. (3) The arches are subjected to combined compression and bending, bending moment is the main reason for the arch failure. The section moment of inertia of SQCC arch is 1.26 times of that of CCC arch, and the former is better than the latter in bending performance. The ultimate bearing capacity is positively correlated with the size of the moment of inertia. Based on the above research, the engineering suggestions are as follows: (1) To improve the bearing capacity of the arch, the cross-sectional shape of the chamber should be optimized and the arch bearing mode changed accordingly. (2) The key damaged positions, such as the arch leg, should be reinforced, optimizing the state of force on the arch. SQCC arches should be used for supporting in underground engineering, which is under stronger influence of the bending moment and non-uniform load on the supporting arches. The research results could provide a theoretical basis for the design of confined concrete support in underground engineering.

Mechanical Effect Analysis and Comparative Site Tests on Surrounding Rock with Different Bolt Anchoring Lengths and Pre-tightening Forces

Different bolt anchoring lengths and pre-tightening forces produce different mechanical and control effects on the surrounding rock. For clarification of the effects, a numerical comparative test is conducted to analyze the stress field distribution features of surrounding rock with different bolt anchoring lengths and pre-tightening forces. A high pre-tightening force quantitative exerting device is developed for high strength bolts with a function of automatic anchor retreat. With the device, a test on site is conducted at a typical roadway of Zhaolou Coal Mine in China; this test is on the comparative plans designed with different bolt anchoring lengths and pre-tightening forces; and the controlling effect is obtained on the surrounding rock in each supporting plan through the analysis of the test. This study shows it is conducive for the bolt pre-tightening force diffusing in surrounding rocks to exert high pre-tightening force and arrange a certain length of non-anchoring section; and it is conducive to form an effective bearing structure of anchored rocks. The new device is self-developed with independent intellectual property rights; and it can meet the requirement of roadway bolt support under high stress with many advantages such as large scale quantitative of pre-tightening force exerting and being easy to operate. Increasing pre-tightening force is an effective way to improve the control effect of surrounding rock; and an appropriate reduction of the bolt anchoring length will not significantly weaken the effect of the surrounding rock control, under certain conditions. The above conclusion can provide reference and experience for the anchoring support design and construction of the underground engineering.

Research on an evaluation method for the strength of broken coal mass reinforced by cement slurry based on digital drilling test technology

Fault fracture zones, high stress, extremely soft rock, and other complex conditions of underground engineering cause the surrounding rocks to become friable and fragmented after chamber excavation. It is common to observe phenomena such as bolt body slips, fractures, severe support buckling and breaking, resulting in safety issues. The broken rock mass is often reinforced by cement slurry, and the relative strength increase in the surrounding rock is an important index used to evaluate the reinforcement effect. However, it is currently challenging to quantify the strength of surrounding rock after reinforcement by cement slurry. A digital drilling test technique provides an effective method for quantitative evaluation of the strength of a broken coal mass reinforced by cement slurry. The key is to establish a quantitative relation between the drilling parameters and the strength of the rock mass after reinforcement. Therefore, this study used the 1000-km-deep Zhaolou coal mine in China as an engineering background. Based on a multi-function rock drilling test system developed by the authors, digital drilling tests and uniaxial compression tests of broken coal mass reinforced by cement slurry were performed, and the variation law of drilling parameters with uniaxial compressive strength was analyzed. Additionally, a quantitative relational model between the uniaxial compressive strength and drilling parameters was built by means of an energy analysis method. A method to quantitatively evaluate the reinforcement strength of a broken coal mass based on a digital drilling test technique was developed. A study on the impact laws of different water:cement ratios and coal particle sizes with respect to the strength of broken coal masses after reinforcement was also conducted. The study results provide an effective method for the quantitative evaluation of the reinforcement effect of broken surrounding rocks and optimization of the cement slurry parameter.

Relationship model for the drilling parameters from a digital drilling rig versus the rock mechanical parameters and its application

Measuring rock mechanical parameters is an essential step for support design in an underground project. To quickly obtain the surrounding rock mechanical parameters at a construction site in real time using a digital drilling rig, a quantitative relationship between the drilling parameters and rock mechanical parameters should be defined. In this paper, based on the fracture characteristics of rock cutting, a relationship model for the drilling parameters of the digital drilling rig versus the rock mechanical parameters (DP-RMP model) is created. Based on the multi-function rock drilling test system developed by the authors, rock drilling tests for different drilling parameters and theoretical results are compared and analysed to prove the validity and accuracy of the DP-RMP model. Additionally, the influence laws of the cohesion and internal friction angle on the rock cutting drilling torque are investigated. Drilling parameters from the digital drilling rig-based rock mechanical parameter inversion method is proposed; the feasibility of this method is verified by rock drilling test results. The study provides a theoretical basis for quickly obtaining the rock mechanical parameters using drilling parameters on site in real time.

Limit Analysis of Roof Falling in Roadway with Water-Rich Surrounding Rock

In this paper, a roof falling and failure mechanism of roadway with water-rich surrounding rock is deduced based on upper bound method of plastic mechanics and variational principle. For the derivation, a roof falling structure of the roadway is constructed according to the characteristics of the roadway; and it is also based on Hoek–Brown strength criterion and its associated flow rules. The structure is made with the consideration of the influence from stress level on roof rock, formation water pressure and supporting load. A typical roadway with water-rich surrounding rock is taken as an example of field practice to analyze the influence of different mechanical parameters on roof failure mechanism. The roadway is located in Guotun Coal Mine, which is over a 1000xa0m deep. The calculation results of this study show the size of falling piece from roof represents the safety performance of roof; the size will be enlarged along with the increase of empirical parameter A, tensile strength, compressive strength and supporting load of rock; and meanwhile the size will be reduced along with the increase of empirical parameter B, stress and weight of rock. Among all the factors, tensile strength of rock and stress level of roof rock have a significant effect on roof failure mechanism; parameter B determines the fracture shape of the falling piece from roof; and the fracture curvature of the falling piece decreases with the increase of the parameter B.