Stress Arch Research Articles

Study on the bearing structure and stability of overlying strata: an interval gob in shallow buried coal mining of Northwest China

The bearing structural characteristics of overlying strata in overlying gobs are the main factor in inducing dynamic disasters in roof in close-distance coal seam group mining. In the present work, the bearing structure and stability characteristics of overlying strata in the shallow-buried interval gob (SBIG) of Yushenfu mining area in north Shaanxi Province, China, have been studied by numerical and physical similarity simulations and theoretical analyses. Research results showed that the instability of gob temporary coal pillars (TCP) led to the rotary instability of basic roof, increasing the stress arch in overlying strata with the expansion of the failure range of overlying rocks giving rise to increases of arch height and superimposition of adjacent stress arches. This way, “trapezoid-semicircular arch” caving of overlying strata was performed. “W-shaped voussoir beam” bearing structure was formed by the fracture of the basic roof of gob and “double arch bridge” bearing structure was created by interval coal pillar (ICP) and fracture rock. In this work, the bearing structure model of interval gob overlying rock “W-shaped voussoir beam + double arch bridge” was established. On the basis of “voussoir beam structure,” the stability of “double arch bridge” type bearing structure formed by the rotation of broken rock block on the top of the basic roof of the interval gob was analyzed. When the support capacity of coal pillar was weakened or the overlying strata were changed, the stability of asymmetrical stress inclined arch equilibrium structure was lost. The stress distribution of “double arch bridge” structure overlying the interval type gob was analyzed by numerical simulation, and the stress transfer law of pier column in floor was obtained. It was found that with the decrease of the width of pier pillar, the “double arch bridge” structure became asymmetric, and the change characteristics of stress inclined arch height were decreased. The arch roof transferring to pier side was divided into prevention and control areas of roof dynamic disaster in the mining of lower coal seams. The purpose of this paper was to provide theoretical guidance for dynamic pressure prevention and control of mining under shallow interval gob.

Characteristics of evolution of mining-induced stress field in the longwall panel: insights from physical modeling

The evolution of mining-induced stress field in longwall panel is closely related to the fracture field and the breaking characteristics of strata. Few laboratory experiments have been conducted to investigate the stress field. This study investigated its evolution by constructing a large-scale physical model according to the in situ conditions of the longwall panel. Theoretical analysis was used to reveal the mechanism of stress distribution in the overburden. The modelling results showed that: (1) The major principal stress field is arch-shaped, and the strata overlying both the solid zones and gob constitute a series of coordinated load-bearing structures. The stress increasing zone is like a macro stress arch. High stress is especially concentrated on both shoulders of the arch-shaped structure. The stress concentration of the solid zone in front of the gob is higher than the rear solid zone. (2) The characteristics of the vertical stress field in different regions are significantly different. Stress decreases in the zone above the gob and increases in solid zones on both sides of it. The mechanical analysis show that for a given stratum, the trajectories of principal stress are arch-shaped or inversely-arched, referred to as the “principal stress arch”, irrespective of its initial breaking or periodic breaking, and determines the fracture morphology. That is, the trajectories of tensile principal stress are inversely arched before the first breaking of the strata, and cause the breaking lines to resemble an inverted funnel. In case of periodic breaking, the breaking line forms an obtuse angle with the advancing direction of the panel. Good agreement was obtained between the results of physical modeling and the theoretical analysis.

Breaking and mining-induced stress evolution of overlying strata in the working face of a steeply dipping coal seam

The breaking features and stress distribution of overlying strata in a steeply dipping coal seam (SDCS) differ significantly from those in a near-horizontal one. In this study, the laws governing the evolution of vertical stress release and shear stress concentration in the overlying strata of coal seams with different dip angles are derived via numerical simulation, rock mechanics tests, acoustic emissions, and field measurements. Thus, the stress-driven dynamic evolution of the overlying strata structure, in which a shear stress arch forms, is determined. Upon breaking the lower part of the overlying strata, the shear stress transfers rapidly to the upper part of the working face. The damaged zone of the overlying strata migrates upward along the dip direction of the working face. The gangue in the lower part of the working face is compacted, leading to an increase in vertical stress. As the dip angle of the coal seam increases, the overlying strata fail suddenly under the action of shear stresses. Finally, the behavioral response of the overlying strata driven by shear stresses in the longwall working face of an SDCS is identified and analyzed in detail. The present research findings reveal the laws governing the behavior of mine pressure in the working face of an SDCS, which in turn can be used to establish the respective on-site guidance.

Risk assessment of water inrushes from bed separations in Cretaceous strata corresponding to different excavation lengths during mining in the Ordos Basin

Evaluation of the risk of bed-separation water inrush (BWI) hazards in Cretaceous strata is required for safe mining in the Ordos Basin, China. In this paper, three coal mines in the Ordos Basin were selected for detailed evaluation of the water inrush risk. The multifactor superposition method and stress balance arch theory were used to obtain the water inrush risk in the mines from bed separations at different excavation lengths. First, the related BWI factors were analyzed, and the BWI risk index at any coordinate point in the study area was calculated using multifactor superposition. The weights of relevant factors were obtained using the entropy weight method. Then, based on stress arch theory, the overall BWI risk index in the bed-separation zone corresponding to any excavation length was calculated. Finally, the boundary values between different risk levels were obtained by comparing the overall risk in the mining area with and without BWIs. With the help of these boundary values, the overall BWI risk in bed-separation zones corresponding to any excavation length in some mining areas was evaluated. The evaluation results were verified in practice, which demonstrated the scientific basis for and practicality of this BWI risk assessment method.

Study on bearing characteristic of rock mass with different structures: Physical modeling

In this paper, to study the stability of surrounding rock during roadway excavation in different rock mass structures, the physical model test for roadway excavation process in three types of intact rock mass, layered rock mass and massive rock mass were carried out by using the self-developed two-dimensional simulation testing system of complex underground engineering. Firstly, based on the engineering background of a deep mine in eastern China, the similar materials of the most appropriate ratio in line with the similarity theory were tested, compared and determined. Then, the physical models of four different schemes with 1000 mm (height) x1000 mm (length)x 250 mm (width) were constructed. Finally, the roadway excavation was carried out after applying boundary conditions to the physical model by the simulation testing system. The results indicate that the supporting effect of rockbolts has a great influence on the shallow surrounding rock, and the rock mass structure can affect the overall stability of the surrounding rock. Furthermore, the failure mechanism and bearing capacity of surrounding rock were further discussed from the comparison of stress evolution characteristics, distribution of stress arch, and failure modes in different schemes.

Study on the Fracture Law of Inclined Hard Roof and Surrounding Rock Control of Mining Roadway in Longwall Mining Face

In the inclination direction, the fracture law of a longwall face roof is very important for roadway control. Based on the W1123 working face mining of Kuangou coal mine, the roof structure, stress and energy characteristics of W1123 were studied by using mechanical analysis, model testing and engineering practice. The results show that when the width of W1123 is less than 162 m, the roof forms a rock beam structure in the inclined direction, the floor pressure is lower, the energy and frequency of microseismic (MS) events are at a low level, and the stability of the section coal pillar is better. When the width of W1123 increases to 172 m, the roof breaks along the inclined direction, forming a double-hinged structure, the floor pressure is increased, and the frequency and energy of MS events also increases. The roof gathers elastic energy release, and combined with the MS energy release speed it can be considered that the stability of the section coal pillar is better. As the width of W1123 increases to 184 m, the roof in the inclined direction breaks again, forming a multi-hinged stress arch structure, and the floor pressure increases again. MS high-energy events occur frequently, and are not conducive to the stability of the section coal pillar. Finally, through engineering practice we verified the stability of the section coal pillar when the width of W1123 was 172 m, which provides a basis for determining the width of the working face and section coal pillar under similar conditions.

Stress Analysis of Extra-high Arch Dam with Closed Arch Grouting during Summer Constructions

In this paper, the stress state of the summer arch grouting area of the extra-high arch dam is studied to determine how the summer arch grouting will affect the dam body, considering the dead weight, water pressure, temperature and other loads. The results show that the most unfavourable working case after considering the dead weight and water pressure of the arch dam is that no water storage in the winter. The upstream arch direction should consider a certain amount of tensile stress superimposed by the dead weight and water pressure. The high stress of the surface concrete in the closed arch grouting area in summer may cause the strength of the transverse joint to reopen after grouting. After the transverse joint pulled apart, the arch stress in other parts is released.

Development and Research on the Vertical Center Diaphragm Method Applied in Shallow Tunnel Construction

As a common method applied in the construction of tunnels with Grade IV and Grade V surrounding rock, the center diaphragm (CD) method has the advantage of resisting the inward horizontal convergence of the tunnel. However, due to the small lateral earth pressure of the shallow tunnel, the curved center diaphragm would have an unstable stress state and cannot provide sufficient support to the surrounding rock. Based on the CD method, this study presents a vertical center diaphragm (VCD) method with an axisymmetric structure. The application condition of the VCD method relies on the comparison of the surface settlement and tunnel deformation with the two methods in different surrounding rock grades and buried depths by using a three-dimensional finite-difference code. Based on the Shenzhen Eastern Transit Expressway Connection Line Tunnel, which has six lanes of double lines, the deformation regularities and mechanical characteristics of the VCD method, including the surface settlement, tunnel deformation, internal force of the center diaphragm, surrounding rock pressure, and steel arch stress, are investigated by numerical calculations and a field comparative test. The results obtained in this study provide several suggestions for constructing shallow tunnels. Furthermore, the construction efficiency and economy of the VCD method are evaluated.

The effect of rise-span ratio on the rigid-framed arch bridges

The rigid-framed arch structure is a popular bridge structure on medium and small span bridge such as flyover. Rise-span ratio affects the rigid-framed arch structure performance, in order to analyse the influence on the rigid-framed arch structure of different rise-span ratio, a calculation model of the rigid-framed arch structure on the basis of the project established using the large general finite element software, by adjusting the rise value to change the rise-span ratio, analysing and comparing the structural internal force and deformation condition of the rigid-framed arch in different rise-span ratio under the action of constant live load, temperature change, and foundation settlement. The influence of different rise-span ratio on the rigid-framed arch stress summarized.

Arching effect analysis of granular media based on force chain visualization

The arching effect significantly affects the mechanical properties such as deformation and loading transmission of the granular media. To visually observe the arching process and force chain distribution, with two steel columns set underneath in the device to cause localized deformation, two-dimensional loading model experiments of photoelastic particles were conducted. By exploring the arching mechanism, the following main conclusions were drawn: (1) Two kinds of arches were found; one is called “stress arch”, which can block the loading transmission path and shield the passing stress, while the other is an intrinsic arch generated under appropriate compressive state that can cause mesoscopic inhomogeneity of deformation. (2) Both the geometric parameters and the stress state affect the formation of the stress arch; for instance, the accumulating space S of the steel columns will increase the influence range of the stress arch until its disintegration happens, which is consistent with the trap-door experiments.

Roof-breaking mechanism and stress-evolution characteristics in partial backfill mining of steeply inclined seams

Steeply inclined seams buried at dip angles of 35–55° are considered hard-excavated seams in the mining industry. A partial backfilling method based on sub-regional features of surrounding rock deformations and failure and support stability was proposed to realize safe, efficient production, to promote sustainable development of coal mining and disaster prevention in western China. Through experiments, calculations, and theoretical analysis, a mechanical model was established, and governing equations for the deflection curve, bending moment, and rotation were deduced. Results reveal the filling percentage, filling ratio, and working-face dip angle as major factors causing roof failure. The degree of roof deformation decreases with increasing filling percentage, filling ratio, and working-face dip angle. With an increase in filling percentage and filling ratio, the range and magnitude of the stress-concentration zone at a T-junction increase, whereas those pertaining to the stress-release zone in the roof decrease. The stress-arch profile of the roof changes from a single peak to double peaks with reduction arch height. Additionally, the maximum arch height shifts towards the centre with increasing moderation of asymmetric features of stress arches. With an increase in working-face dip angle, stress concentration at the coal-wall head T-junction decreases, and a double-peak stress arch develops. Therefore, in order to obtain the best backfill mining effect, a higher filling percentage is preferred; with a maximum filling value of 0.93, the reasonable range for the filling ratio is 1/3 to 1/2 and the reasonable dip angle of the working face is approximately 35°.

Performance of Super‐Large‐Span Tunnel Portal Excavated by Upper Bench CD Method Based on Field Monitoring and Numerical Modeling

The number of super‐large‐span tunnels is increasing in both new construction and reconstruction projects in China recently. In super‐large‐span tunneling engineering, the deformation properties and mechanical behaviors of tunnel portal structure are more complex than those of common tunnel due to the flatter shape and larger construction span. The mechanical behaviors of rock mass change in response to different sequential excavation methods and supporting parameters. The upper bench CD method has been gradually applied in the construction of super‐large‐span tunnels in China. In this paper, the design parameters for the supporting structure of super‐large‐span tunnel were studied by the field monitoring and numerical modeling in a case study of Laohushan tunnel. It was found that the crown settlement was larger than the clearance convergence, and the stress of arch was greater than that of the side wall in tunnel portal section. The invert structure was flat with small curvature. Therefore, the shotcrete was mainly subjected to tensile stress. The use of H200 × 200 steel rib with spacing of 60 cm and C25 shotcrete with thickness of 30 cm is recommended. The results of this paper provide basis for the development of design specifications and construction standards for super‐large‐span tunnels and provide reference for similar projects in the future.

Simulation of recovery of upper remnant coal pillar while mining the ultra-close lower panel using longwall top coal caving

With the depletion of easily minable coal seams, less favorable reserves under adverse conditions have to be mined out to meet the market demand. Due to some historical reasons, large amount of remnant coal was left unrecovered. One such case history occurred with the remnant rectangular stripe coal pillars using partial extraction method at Guandi Mine, Shanxi Province, China. The challenge that the coal mine was facing was that there is an ultra-close coal seam right under it with an only 0.8–1.5 m sandstone dirt band in between. The simulation study was carried out to investigate the simultaneous recovery of upper remnant coal pillars while mining the ultra-close lower panel using longwall top coal caving (LTCC). The remnant coal pillar was induced to cave in as top coal in LTCC system. Physical modelling shows that the coal pillars are the abutments of the stress arch structure formed within the overburden strata. The stability of overhanging roof strata highly depends on the stability of the remnant coal pillars. And the gob development (roof strata cave-in) is intermittent with the cave-in of these coal pillars and the sandstone dirt band. FLAC3D numerical modelling shows that the multi-seam interaction has a significant influence on mining-induced stress environment for mining of lower panels. The pattern of the stress evolution on the coal pillars with the advance of the lower working face was found. It is demonstrated that the stress relief of a remnant coal pillar enhances the caveability of the pillars and sandstone dirt band below.

Study on early-warning System of Karst Tunnel Collapse

During the construction of karst Tunnel, tunnel collapse often occurs. The establishment of early-warning system of tunnel collapse can effectively ensure the construction safety, but the research is insufficient. Based on the combination of theoretical analysis and engineering practice, this paper puts forward to early-warning system for karst tunnel collapse. (1) The evaluation index system of karst tunnel collapse is composed of anchor axial force, displacement, anchor pull-out force and steel arch stress. (2) The relationship between evaluation index and warning level is determined, and the early-warning system of tunnel collapse is established. (3) The model applied to Chong’anjiang tunnel collapse warning, and achieved good results.

Sensitivity of Tidewater Glaciers to Submarine Melting Governed by Plume Locations

AbstractThe response of tidewater glaciers to ocean warming remains a key uncertainty in sea level rise predictions. Here we use a 3‐D numerical model to examine the response of an idealized tidewater glacier to spatial variations in submarine melt rate. While melting toward the center of the terminus causes only a localized increase in mass loss, melting near the lateral margins triggers increased calving across the width of the glacier, causing the terminus to retreat at several times the width‐averaged melt rate. This occurs because melting near the margins has a greater disruptive impact on the compressive stress arch that transfers resistance from the side walls to the body of the glacier. We suggest that the rate of terminus advance or retreat may thus be governed by the difference between ice velocity and submarine melting in the slow‐flowing zones away from the glacier center.

Practices, experience, and lessons learned based on field observations of support failures in some Chinese coal mines

Modelling, experimental study, and field observation are prevailing research methods in coal mine support application. Many criteria and rationales have been established to evaluate the stability of coal mine excavations from both qualitative and quantitative perspectives. Nevertheless, most coal mine support designs are still highly dependent on empiricism or semi-empiricism, even with the existence of the aforementioned theories. Oftentimes, most lessons are learned via the trial and error approach. A more critical fact is that further in-depth improvement of or research into coal mine support appears to be largely bottlenecked. Under such circumstances, support failures in four representative coal mines in China are selected as research objects, through which dominating implied principles are investigated and re-clarified by field observations. In the Datong coal mine, an extremely fractured roadway is successfully maintained by simply changing the end-encapsulated bolting system to a fully bonded bolting system, with the results indicating the formation of an artificial compressive stress arch. In the Kongzhuang coal mine, a deep-buried shaft with a super-large section is effectively stabilized by tieback cable bolts, and a 561-day-long load-monitoring procedure verifies the effectiveness. In the Donghuantuo coal mine, the support ideology of retaining, reinforcing, and holding functions is presented to interpret bolt support failure under an aqueous environment. In the Hulusu coal mine, 5400-mm-span entries are stabilized by simply installing four tendons on the roof; the effectiveness is further proven, and an enhanced beam building mechanism is interpreted. At the end of this study, the effectiveness of a pretension force, a fully bonded bolting system, longer tendon support, and mesh/shotcrete is discussed. All of the examples given in this study are proposed to meet the principles of high efficiency, low cost, and operational friendliness. Additionally, the authors hope that through this paper, more comprehensive research/analytical methodologies will be able to bridge support theories and applications, which is the main purpose of this work.

Experimental Study and Estimation of Groundwater Fluctuation and Ground Settlement due to Dewatering in a Coastal Shallow Confined Aquifer

The coastal micro-confined aquifer (MCA) in Shanghai is characterized by shallow burial depth, high artesian head, and discontinuous distribution. It has a significant influence on underground space development, especially where the MCA is directly connected with deep confined aquifers. In this paper, a series of pumping well tests were conducted in the MCA located in such area to investigate the dewatering-induced groundwater fluctuations and stratum deformation. In addition, a numerical method is proposed for the estimation of hydraulic parameter, and an empirical prediction method is developed for dewatering-induced ground settlement. Test results show that groundwater drawdowns and soil settlement can be observed not only in MCA but also in the aquifers underneath it. This indicates that there is a close hydraulic connection among each aquifer. Moreover, the distributions and development of soil settlement at various depths are parallel to those of groundwater drawdowns in most areas of the test site except the vicinity of pumping wells, where collapse-induced subsidence due to high-speed flow may occur. Furthermore, the largest deformation usually occurs at the top of the pumping aquifer instead of the ground surface, because the top layer is expanded due to the stress arch formed in it. Finally, the proposed methods are validated to be feasible according to the pumping well test results and can be employed to investigate the responses of groundwater fluctuations and stratum deformations due to dewatering in MCA.

Impact Analysis of Hard Roof on the Morphological Evolution of Stress Arch

Impact Analysis of Hard Roof on the Morphological Evolution of Stress Arch

Rational placement of measuring systems in high dams

To solve the problem of rational placement of measuring systems in concrete dams, the experience of full-scale observations of horizontal displacements and stresses in the body of the arch dam of Inguri HPP (Georgia) was used with the use of statistical processing of the results of observations and the Barlow-Hunter-Proshan mathematical model. The type of the law of distribution of horizontal displacements and arch stresses was established by analyzing the histograms of these measured indicators of the static state of the structure. It is established that the change of horizontal displacements along the dam height is well described by the one-sided normal distribution law, and the arch stresses in the controlled alignment - by the normal distribution law. The laws of distribution of random variables are established. Using the Barlow-Hunter-Proshan mathematical model enabled the use of economic criteria on the basis of the frequency characteristics of the indicators of trouble-free operation of the construction and obtain the recurrence relation for the sequence of coordinate points of the measuring systems. The results of calculations of the coordinates of the recommended location of straight plumb lines in the center console and tensometric rosettes in the measuring range of the Ingur dam at the level of 210 m for a rational placement scheme are presented. The calculation showed that the number of measuring systems installed in the body of the Ingur dam exceeds the optimum value.

Simple innovative comparison of costs between tied-arch bridge and cable-stayed bridge

The proposed paper compares tied-arch bridge alternatives and cable-stayed bridge alternatives based on needed load-bearing construction material amounts in the superstructure. The comparisons are prepared between four tied arch bridge solutions and four cable-stayed bridge solutions of the same span lengths. The sum of the span lengths is 300 m. The rise of arch as well as the height of pylon and cable arrangements follow optimal dimensions. The theoretic optimum rise of tied-arch for minimum material amount is higher than traditionally used for aesthetic reason. The optimum rise for minimum material amount parabolic arch is shown in the paper. The mathematical solution uses axial force index method presented in the paper. For the tied-arches the span-rise-ration of 3 is used. The hangers of the tied-arches are vertical-The tied-arches are calculated by numeric iteration method in order to get moment-less arch. The arches are designed as constant stress arch. The area and the weight of the cross section follow the compression force in the arch. In addition the self-weight of the suspender cables are included in the calculation. The influence of traffic loads are calculated by using a separate FEM program. It is concluded that tied-arch is a competitive alternative to cable-stayed bridge especially when asymmetric bridge spans are considered.