Dynamic movement within strata overlying coal mines and the distribution of the movement boundary are keenly investigated topics but are hampered by the difficulties of obtaining reliable monitoring data of movement within rock masses. The work presented in this paper combines physical experiments with a digital photogrammetry method to investigate these two topics. Two similar material model experiments were conducted, and a high-precision, close-range digital photogrammetry method was employed to observe movement in the model. The results, including dynamic movement tracking of targeted points, indicate the presence of six different movement areas in the overlying strata in the model. Based on motion vector directions determined for the model, the overlying strata were divided into three zones: (1) a zone with the movement vector oriented vertically downward, (2) a zone with the movement vector oriented towards the goaf center, and (3) a zone with the movement vector oriented towards the coal pillar. Dynamic movement tracking shows that all zones experienced an initial, active, and decline stage, but the timing, duration, and movement tracks are different for each zone. The overall analysis reveals that the movement boundary in the strata overlying the rock mass is not a straight line, but an S-shaped curve, in contrast to the traditional findings with respect to the movement boundary. These findings may prove significance in guiding better prediction of movement and deformation inside a given rock mass and related improvements in protecting engineered infrastructure.

Experimental research on dynamic movement in strata overlying coal mines using similar material modeling

Intensive and massive coal mining causes a series of geological hazards and environmental problems, especially surface subsidence. In recent years, backfill-strip mining has been applied to control mining subsidence in order to realize sustainable development of the mining environment. To accurately predict the surface subsidence of backfill-strip mining, a prediction method of subsidence superposition of backfill-strip mining is proposed on the basis of the traditional probability integral method prediction model. In analyzing the distribution of the actual subsidence space, the surface subsidence problem of backfill-strip mining can be regarded as the superposition of surface subsidence caused by backfill mining and strip mining. Then, the appropriate prediction parameters will be chosen, and the surface subsidence caused by the backfill mining and strip mining will be predicted separately. The surface subsidence values of the backfill-strip mining are equal to the superposition subsidence values predicted by the backfill mining and strip mining prediction method at the same surface location. A similar material model and a numerical simulation model have been built to verify the feasibility and accuracy of the superposition prediction method. The comparison results of the surface subsidence values show that the superposition surface subsidence prediction method is reasonable. The average relative error of this superposition prediction method is less than 6.7%, and its accuracy is 3.9%~11.4% higher than that of the conventional prediction method. The superposition prediction method can satisfy the precision requirement of engineering applications. This study provides a scientific technical reference for safe mining engineering design and surface disaster protection for backfill-strip mining.

Surface subsidence prediction method of backfill-strip mining in coal mining

A four-camera videogrammetric system for 3-D motion measurement of deformable object

In consideration of the high filling costs and backfill material shortage at present, backfill–strip mining, which combines the advantages of strip mining and backfill mining, has gradually been adopted to control surface subsidence. In this study, similar material modeling is established to simulate strata movement characteristics of backfill–strip mining. The displacement and deformation values of this similar material modeling are precisely acquired through close-range photogrammetry and optical image methods, respectively. On this basis, structural and movement characteristics of the overlying strata are investigated in different stages to reveal the strata subsidence control mechanism of backfill–strip mining. The dynamic deformation characteristics of the overlying strata in mining are also explored. This study provides a scientific technical reference for safe mining engineering design and surface disaster protection for backfill–strip mining.

Experimental research on strata movement characteristics of backfill–strip mining using similar material modeling

Field examples of fast exploitation from underground coal seams in Bulianta mines, China, show that unexpected developmental rules of settlement and cracks can occur. In situ observation and a physical model consisting of sand, plaster, mica, and calcium carbonate were jointly employed to study the movement of strata, as well as the developmental characteristics of surface cracks. The physical model was observed with a high-precision industrial photogrammetric system. The results indicate that ground cracks are caused by strata deformation, but the formation of ground cracks can, in turn, promote the deformation of strata. Moreover, by contrast with coal mining at a speed of about 2 m/day, we found that the ground does not achieve full subsidence until the advancing distance exceeds 2.2 times the mining depth under rapid excavation (approximately 12 m/day), which would cause large errors for surface settlement prediction. OCF (opening and closing fractures) above gobs are self-closing, but the duration of the closing phase is 3.6 times that of the opening phase, different from the symmetric distribution caused by mining with slower speed. The whole developmental cycle is inversely proportional to mining speed, and fast excavation can shorten development time of OCF. However, the horizontal tension deformation is the most stable factor for predicting ground cracks regardless of excavation speed. The research results can provide theoretical basis for dynamic prediction of ground subsidence and cracks development caused by underground coal mining.

Ground cracks development and characteristics of strata movement under fast excavation: a case study at Bulianta coal mine, China

A digital photogrammetry measurement system (XJTUDP) is developed in this work, based on close range industry. Studies are carried out on key technologies of a photogrammetry measurement system, such as the high accuracy measurement method of a marker point center based on a fitting subpixel edge, coded point design and coded point autodetection, calibration of a digital camera, and automatic image point matching algorithms. The 3-D coordinates of object points are reconstructed using colinear equations, image orientation based on coplanarity equations, direct linear transformation solution, outer polar-line constraints, 3-D reconstruction, and a bundle adjustment solution. Through the use of circular coded points, the newly developed measurement system first locates the positions of the camera automatically. Matching and reconstruction of the uncoded points are resolved using the outer polar-line geometry of multiple positions of the camera. The normal vector of the marker points is used to eliminate the error caused by the thickness of the marker points. XJTUDP and TRITOP systems are tested on the basis of VDI/VDE2634 guidelines, respectively. Results show that their precision is less than 0.1 mm/m. The measurement results of a large-scale waterwheel blade by XJTUDP show that this photogrammetry system can be applied to industrial measurements.

Exploitation of photogrammetry measurement system

In order to obtain more comprehensive three-dimensional (3-D) deformation data, improve the flexibility and precision of 3-D deformation measurement, and describe the real-time displacement of various points in the measurement scope, we researched speckle methods in this field. First, we expounded basic theories of speckle technique and elaborated on several important aspects of 3-D measurement, including camera calibration, images matching, and 3-D coordinate and displacement calculation. Then, we established the relevant mathematical models and developed a corresponding measurement and analysis software, XJTUDIC. Aided with the appropriate hardware devices, this software recorded and analyzed the whole process of tensile deformation. When compared with the results of the simulation analysis using ANSYS, this method achieved a relative error within ±0.75%, demonstrating the reliability, feasibility, and advantage of this method and its corresponding applied software and hardware.

Application of the speckle technique for three-dimensional deformation measurement

In order to meet the requirement of obtaining the object figure quickly and accurately, XJTUDP software has been
developed successfully by oneself based on photogrammetry theory. The contents of composing and explored of this
system are introduced in this paper. The VDI/VDE2634 testing program is taken as referenced project, the self-designed
framework of cube is taken as tested target, and then the conclusion that precision of XJTUDP may meet the standard of
industrial measurement has been reached out. Finally, large-scale waterwheel leave is taken as example to conduct
measuring, it is proved that photogrammetry system explored by ourselves may be imposed applying on the field of
industrial measuring successfully.

Application of photogrammetry technology to industrial inspection

The strain measurement is very important in engineering, there are many traditional methods, and have a lot of new methods. In this paper, introduced how to use the speckle method for accurate measurement of strain, and compared with traditional methods. It not only described its basic principle, but also developed the corresponding software. And carry out specific tensile experiment, showing the method have the advantages of non - contact, high precision, simple and flexible operation, overall measuring and so on.

Speckle technique applied to precise measurement of strain

The traditional deformation measurement have many limitations, According to actual measurement need, it is requested not only comprehensive, flexible, high-precision, but also can describe the various points displacement within the measurement scope at any time, get the most comprehensive data. In this paper, used speckle methods to carry on deformation accurate and comprehensive measuring. First, compared with traditional methods, showed its many advantages, including: non-contact, high precision, operation simple and flexibility, etc. Then specified the main basic theory of the speckle method, established relevant mathematical models, developed the appropriate measurement and analysis software XJTUDIC, and used the software with the appropriate hardware equipment to test a specific tensile deformation process, and compared the results with using traditional extensometer measurement methods, through calculation got the error is within ± 0.2%. This fully validated the method and corresponding software and hardware were reliable, feasible and excellent.

Zhi-Xin Chen

Papers

Exploitation of photogrammetry measurement system

Application of the speckle technique for three-dimensional deformation measurement

Application of photogrammetry technology to industrial inspection

Speckle technique applied to precise measurement of strain

The Research of Speckle Technique in Deformation Measurement Fields