In order to analyze the deformation characteristics and mechanisms caused by high tectonic stress in gentle dip strata, a physical modeling with circular tunnel was designed. The model was constructed by the so-called “Physically Finite Elemental Slab Assemblage (PFESA)” to bring about the structural effect of the deep strata. The gravity was fixed while the lateral pressure increased step by step to mimic high tectonic stress. In order to observe the displacement in different area, the sketches of monitoring points and frame in the model surface were drawn down through video pictures in different periods and to be compared. For the sake of analyzing the deformation and failure mechanism of layers, rock structural mechanics models were set up for the left side and right side in the same stratum, respectively. For verifying the experimental phenomenon and its mechanism, infrared images were utilized based on the temperature variation mechanisms of material. Through systematic study, this paper enriches the research methods of model test and can provide a certain reference for practical engineering of similar conditions.
With the decreasing shallow resources, mining industry starts to focus on extracting deep underground resources. The environment makes the deep rock mechanical characteristics and engineering responses very distinctive [
The most widely used method is numerical simulation when analyzing the formation of deformation. References [
In this paper, the PFESA technique is used to build a large-scale geomechanical model to mimic deep gentle inclined stratified rock mass. A video camera and an infrared thermal imager are set up to observe the deformation and temperature variation of the model before and after excavation of the circular roadway under symmetrical loading. The measurements can help us understand the deformation mechanism in mechanical characteristic and energy-dissipation regimes.
Deep rock masses in China coal mine can be generally divided into three categories, that is, sandstones, mudstones, and coal rocks [
Material properties of the real rocks.
Rock types | Volumetric weight |
Compressive strength (MPa) | Elastic moduli |
Poisson’s ratio | Friction angle |
---|---|---|---|---|---|
Sandstone | 26.55 | 63.98 | 25.77 | 0.151 | 33.71 |
Mudstone | 25.78 | 43.78 | 21.01 | 0.127 | 36.35 |
Coal seam | 13.50 | 26.15 | 4.51 | 0.358 | 40.07 |
The dimensions of the physical model are
Material properties of the elementary slabs.
Dimensions |
Simulated rock types | Ratio of water-gypsum | Volumetric weight |
Compressive strength (MPa) | Elastic moduli |
---|---|---|---|---|---|
40 × 40 × 3 | Sandstone | 0.8 : 1 | 14.68 | 6.748 | 1.28 |
40 × 40 × 2 | Mudstone | 1 : 1 | 11.25 | 4.663 | 0.94 |
40 × 40 × 1 | Coal seam | 1.2 : 1 | 8.40 | 3.382 | 0.81 |
Pictures of elementary slabs and the physical model; (a) picture of elementary slabs made of gypsum; (b) picture of the physical model constructed completely.
The whole loading scheme of the test includes three processes: (1) process to establish the in situ stress conditions (i.e., initial condition before mining activities start); (2) simulated roadway excavation process; (3) process of adjusting lateral pressure until the model is damaged if the model had not failed after process (2).
In situ stress condition was established by applying horizontal load (to mimic the tectonic stress) and vertical load (to mimic the gravity of the overburden) on the model, in a plane stress state. The “Geological Disaster Simulation Testing Machine whose model type is YDMC-C” was equipped with a host machine, specimen mold, truck, hydraulic control system, and data collection system. By using YDMC-C, the top and two side boundaries were imposed uniform load, respectively; the bottom of the model was fixed on the basement of the machine. The model imposed load gradually until vertical load
The loading plan of the physical model experiment.
Load level | Simulated depth | Lateral pressure coefficient | Oil pressure | Duration | The cumulative recording time | |
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Top pressure/MPa | Lateral pressure/MPa | |||||
Preloading | 0.4 | 0.4 | 12 hours | 0 min | ||
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1 | 200 m | 1 | 0.6 | 0.6 | Loading: 15 min |
15 min |
30 min | ||||||
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2 | 400 m | 1 | 1.0 | 1.0 | Loading: 15 min |
45 min |
60 min | ||||||
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3 | 600 m | 1 | 1.6 | 1.6 | Loading: 15 min |
75 min |
90 min | ||||||
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4 | 800 m | 1 | 2.0 | 2.0 | Loading: 15 min |
105 min |
120 min | ||||||
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Excavation of the roadway | The first step | Excavation: 14 min |
134 min | |||
151 min | ||||||
The second step | Excavation: 22 min |
173 min | ||||
190 min | ||||||
The third step | Excavation: 37 min |
227 min | ||||
248 min | ||||||
The fourth step | Excavation: 46 min |
294 min | ||||
311 min | ||||||
The fifth step | Excavation: 24 min |
335 min | ||||
351 min | ||||||
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5 | 800 | 1.2 | 2 | 2.4 | Loading: 17 min |
368 min |
381 min | ||||||
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6 | 800 | 1.4 | 2 | 2.8 | Loading: 17 min |
398 min |
411 min | ||||||
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7 | 800 | 1.6 | 2 | 3.2 | Loading: 17 min |
428 min |
441 min | ||||||
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8 | 800 | 1.8 | 2 | 3.6 | Loading: 4 min | 445 min |
Simulated boundary stress conditions during the roadway excavation.
After excavation and the subsequent stabilization finished, the following fifth-eighth steps of lateral loading were started in turn. The deformation of model occurred and the destruction finally appeared during the process. As shown in Figures
Screenshots of test video according to the loading scheme and phenomenon of deformation; (a)–(d) screenshot of the model in stabilization period after the fifth-seventh loading, respectively; (e)-(f) screenshot of large deformation in lower right area and lower left area which occurred after the eighth loading began, respectively; (g) screenshot of the model when it was destroyed.
350 min
380 min
410 min
440 min
443 min
444 min
445 min
During the whole test process, the quiet period accounted for the most of the time (0–440 min), and then obvious deformation popped up in about 3 minutes and divided into two parts as mentioned above. Ultimately, deformation of the tunnel was heavy and asymmetric.
In order to analyze the deformation and the movement of the model, a concentric circle with three times the radius of the tunnel together with its external square and the whole model outline are painted as the observation frame. Every 45° four equal diversion points are also painted on the segment between the tunnel and the concentric circle along the radius direction and are regarded as displacement observation points, which can be seen in Figure
Through the video picture, as Figure
Stacking sketches of the images of Figure
By observing the locus of points in Figure
Taking advantage of rock structural mechanics [
Diagrams of rock structural mechanics models; (a) diagram of rock structural mechanics model for the physical model; (b) diagram of rock structural mechanics model for stratum in the left of physical model; and (c) diagram of rock structural mechanics model for stratum in the right of physical model.
For the same layer, as shown in Figure
As
The actuating range of these two mechanical analysis models, as
After excavation, it turned into bidirectional compression stress state along the tangent direction of the roadway from triaxial stress state in a certain range of surrounding rock. Because there is no supporting measure for the tunnel,
With the growth of the lateral pressure coefficient
Infrared (IR) thermography, as a nondestructive, remote sensing technique, has been widely used in detection of the onset of unstable crack propagation and/or flaw coalescence for concrete and rock, based on the fact that the heat generation is caused by the intrinsic dissipation due to elasticity and inelasticity of the material under external loading [
During deformation and failure process of rocks, thermographic imaging records temperature variations on the surface in view and displays it as infrared images with false colors, where a high temperature (in warm color) denotes shear fracturing from the frictional effects; low temperature (in cool color) represents the tensile fractures indicating permanent plastic damage [
The infrared camera CX320 (by Korea) shown in Figure
Picture of infrared imager and infrared images; (a) picture of infrared thermal imager; (b) infrared image taken after the first loading; (c)–(f) the infrared images taken around the time point of Figures
Figure
As shown in Figure
Diagrams of arrangement of the temperature ROIs and temperature distribution curves; (a) diagram of arrangement of the temperature ROIs on the infrared image before excavation; (b) diagram of temperature distributions from bottom to top of the model; and (c) diagram of temperature distributions from left to right of the model along the rock tendency.
As shown in Figure
The actuating ranges of the two analysis models as discussed in Section
After excavation, a circle of red color appeared, as marked in Figure
Infrared images of the destroyed process; (a)–(e) are infrared images taken after the eighth loading, coming along with the destroyed process.
Area 1 was pointed in the lower part below the tunnel where the displacement was obvious in the first destroyed process. Area 2 was set in the lower left side of the tunnel where the strata were compacted and moved as a whole in the second destroyed process. The temperature-time curves of two areas were as shown in Figure
Diagrams of the arrangement of the temperature ROIs and temperature variation curves; (a) diagram of the arrangement of the temperature ROIs on the infrared image before the destroyed processes; (b) diagram of temperature variation curves of the ROIs in destroyed process.
The second destroyed process was as short as about 40 s, and the curves had the same trend as the first destroyed process: the curve of area 2 kept increasing basically; the trend of area 1 started to drop after the temperature increasing in the beginning period. The impact of the extrusion from the left strata which have integrated closely as a whole made the broken strata around area 1 become compact and the friction between the fractures and joints was obvious, the temperature rose rapidly, and then the strata became loose gradually and its temperature kept dropping and finally peeled off into pieces, just as shown in Figure
This paper analyzed the characteristics and mechanisms of deformation based on the physical model test on deep circular roadway in layered rock. The main conclusions were summarized as follows.
The model was finally destroyed by floor heave and shrinkage of two sides. The direction of displacement which caused shrinkage was along the direction of the strata tendency, indicating that the occurrence of shrinkage should only conquer the friction between the layers. There was a certain angle between the direction of displacement which caused floor heave and the layers which manifested the formation of floor heave.
The inclined angle made the stress state of the left side and right side of the model different and affected the trends of movements. The strata in the right side were easy to slide, especially the lower coal seam and mudstone; the strata in right side were hard to slide and become compact. After excavation, combining the increasing lateral pressure coefficient, the movement trends were more obvious; the sliding of the strata in the right side of tunnel triggered the shrinkage; the shearing slide in lower side of tunnel gave rise to floor heave.
Low temperature state indicated tensile fractures generated in lower right area because of bending of layers which was under small interlayer pressure and large trust force. The color of the left area was warm because of result of the frictional influence. The width of the cool color was broader than that of the warm color along the dip orientation of the strata in the lower part of the model, and the mechanics analysis model of the left side had a broader actuating range.
The temperature trend of the strata in the shearing slide area increased rapidly first and then dropped slowly both in two destroyed processes. In the first process, the increasing indicated the strata became more compact than before along the shearing slide; the dropping manifested the formation of bending and breakage. In the second process, the increasing manifested the frictional function of the fractures and joints under the impact from the left compact layers; the decrement meant the strata became loose which corresponded to the final destroyed phenomenon. The temperature of the lower left area where strata moved as whole kept increasing basically and the increment mainly came from the friction effect.
The authors declare that they have no conflicts of interest.
This work was supported by the National Key Research and Development Plan of China (2016YFC0600901), the National Natural Science Foundation of China (Grant nos. 51374214, 51134005, and 51574248), the Special Fund of Basic Research and Operating of China University of Mining & Technology, Beijing (Grant no. 2009QL03), and the State Scholarship Fund of China.