Voussoir Beam Research Articles

Breaking mechanism and control technology of sandstone straight roof in thin bedrock stope

The key problem to be solved urgently is how to avoid the occurrence of support break-off and water inrush in the stoping of sandstone straight roof under the action of load transfer in unconsolidated aquifer. For this reason, taking the thin bedrock 1602 (3) working face of Huainan (the middle part of Anhui Province) Panyi Coal Mine as the engineering background, this study establishes the stope mining model by using the discrete element UDEC software and the mathematics mechanical model of the support load, and analyzes the reason of support crushing and decides to re-mining the working face by using the compulsive roof caving method. It is concluded that when the working face of sandstone straight roof is broken, the “voussoir beam” structure cannot be formed and acts on the support in the form of cantilever beam, but only when it falls to the high key stratum can the “voussoir beam” structure be formed and at this point, at this time, the bracket bears the weight of the rock layer in the range from the fractured sandstone layer to the lower critical layer. The working resistance of the support increases with the increase of the thickness and the breaking length of straight sandstone roof. When the breaking length of the roof reaches a certain extreme value, the support crushing accidents will occur. Managing roof with compulsive roof caving method can reduce the intensity of rock pressure in the stope, and the working face can be safely stoped, which provides a certain reference for similar conditions.

Study of reinforcement support mechanisms for wide-span horse-shoe-shaped openings in horizontally layered jointed rock using the distinct element method

Wide-span openings in horizontally layered jointed rock were analysed using the distinct element method (DEM) to provide additional insight into rock bolt support mechanisms and forces. The rock bolt forces in the DEM calculations exhibited distinct mechanisms, identified here as reinforced rock unit, beam building or suspension mechanisms. Each of these mechanisms was found to be associated with certain characteristics in the jointed rock structure. In jointed rock with thick horizontal beddings, the rock bolt forces at the roof tapered off with distance leading to a reinforced rock unit where each layer exhibited a self-support mechanism akin to a voussoir beam. In jointed rock with thinner horizontal beddings not capable of self-supporting, a beam building mechanism took place. When the rock bolts were extended into a rigid layer above the thinner layers, a suspension mechanism took place. Potential inaccuracies of thrust line predictions and the benefits of rock bolt tensioning were investigated, and their effects were found to be sensitive to the bedding thickness. A case history of the cavern roof of the underground car park for the Sydney Opera House was modelled. The calculated deflections of the cavern roof compared well with field measurements reported in the literature. The findings in this paper highlight the importance to ensure that the reinforcement support for wide-span underground excavations in horizontally layered jointed rock are tailored to the geological conditions being encountered.

Effect of Strata Conditions on Shield Pressure and Surface Subsidence at a Longwall Top Coal Caving Working Face

This paper presents a comprehensive analysis of roof strata conditions, shield pressure, and ground surface subsidence above Panel 42105 of the Buertai Coal Mine in Inner Mongolia, China. A 3D geological model and a 2D numerical simulation model are established to examine the roof strata structures with varying strata conditions. Numerical simulation results demonstrate that the hard rock 1 and 2 roof layers are, on average, likely to break 20 m and 57 m behind the coal face, respectively. It is concluded that when the immediate roof is relatively thin, a cantilever beam is formed, whereas a Voussoir beam is formed when the immediate roof is relatively thick. The patterns of shield pressure and surface subsidence under different roof conditions are also examined. The shield working pressure under the Voussoir beam structure appears to be lower than that under the cantilever beam structure. The average horizontal distance behind the working coal face at which the maximum surface subsidence occurs is 92.1 m. On-site monitoring data are used to explain the causes of the unusual shield pressure and its relation to surface subsidence. Finally, guidelines to predict the shield pressure under similar conditions are provided.

In Situ Studies on the Characteristics of Strata Structures and Behaviors in Mining of a Thick Coal Seam with Hard Roofs

The movements of overburden induced by mining a thick coal seam with a hard roof extend widely. The effects of breakages in the hard strata on the strata behaviors might vary with the overlying strata layers. For this reason, we applied a test method that integrated a borehole TV tester, borehole-based monitoring of strata movement, and monitoring of support resistance for an in situ investigation of a super-thick, 14–20 m coal seam mining in the Datong mining area in China. The results showed that the range of the overburden movement was significantly high, which could reach to more than 300 m. The key strata (KS) in the lower layer main roof were broken into a ‘cantilever beam and voussoir beam’ structure. This structure accounted for the ‘long duration and short duration’ strata behaviors in the working face. On the other hand, the hard KS in the upper layer broke into a ‘high layer structure’. The structural instability induced intensive and wide-ranging strata behaviors that lasted for a long time (two to three days). Support in the working face were over-pressured by large dynamic factors and were widely crushed, while the roadways were violently deformed. Hence, the structure of a thick coal seam with a hard roof after mining will form a ‘cantilever beam and voussoir beam and high layer structure’, which is unique to a large space stope.

The effects of the rotational speed of voussoir beam structures formed by key strata on the ground pressure of stopes

Underground coal mining leads to fracture and movement in the overburden and the occurrence of weighting events in stopes. There are certain key strata that play an important role in controlling overburden movements. After the key stratum breaking, the fractured blocks would form the voussoir beam structure. The rotational speed of the voussoir beam structures directly affects the intensity of the ground pressures in a stope. In this study, a mechanical model was constructed to describe the rotational motions of the voussoir beam structures formed by the key strata based on elastomechanical theories. The angular velocity equations for the rotational motions of the voussoir beam structures in the key strata were obtained. Moreover, the manner in which loads and block lengths affect the angular velocity was investigated. The rotational angular velocity of the fractured blocks was found to increase with greater overlying loads on the key strata and shorter block lengths. The main roof has a major influence on strata behavior in stopes, and increases in the angular velocity of its voussoir beam structures increase roof subsidence rates, the yield of powered supports within each coal mining cycle, and the intensity of weighting events in stopes. If one or more of the voussoir beam structures formed by key strata above the main roof have greater rotational speeds than those of their underlying key strata, additional loads are applied to the latter, causing their rotational speeds to increase. If this application of loads is transmitted to the main roof, there is a strong possibility that highly intense weighting events will occur as a result of increases in the rotational speed of the main roof's voussoir beam structures. The research result provided a rational explanation for the significant differences in the weighting events of the No. 52304 working face in the Daliuta Coal Mine, which was covered by thick versus thin Aeolian sands. The results may be greatly significant in evaluating the intensity of weighting events, predicting the scope in which the strata movement affects the weighting events in stopes and determining a reasonable powered support capacity.

Subsidence Prediction of Overburden Strata and Surface Based on the Voussoir Beam Structure Theory

The hard and stiff strata (key strata) bear the overburden load in the form of a voussoir beam structure (VBS) after break. The VBS affects both the surface subsidence and the stope underground pressure. Therefore, the reasonable method to predict the surface subsidence is based on the whole subsidence formulae of the VBS. This study first establishes the subsidence formulae of the VBS analytically. The influence of the block length on the subsidence curve and the VBS level on the zero-subsidence range are then analyzed based on the subsidence formulae of the VBS. The results show the half-subsidence curve of the VBS is an S-shaped curve. The block length hardly affects the S-shaped subsidence curve determined by the width of the undercompacted zone. Furthermore, a greater undercompacted zone width corresponds to a greater offset distance of the inflection point. The higher the VBS level, the farther the zero-subsidence range, and the flatter the subsidence curve. The subsidence of the highest VBS can approximately represent the surface subsidence when the topsoil is thin enough.

Roof Strata Behavior and Support Resistance Determination for Ultra-Thick Longwall Top Coal Caving Panel: A Case Study of the Tashan Coal Mine

The Longwall Top Coal Caving (LTCC) method has greatly improved the production of ultra-thick underground coal resources. However, face fall and support closure have been becoming highly frequent accidents at the working face, and seriously threaten the safety of miners. The key to avoiding these problems is to reveal the structural evolution of the roof strata and then choose a reasonable working resistance for the hydraulic supports. According to physical modeling, theoretical analysis and field observation of the LTCC panel, four kinds of structural models can be found and defined, in consideration of the coincident movement of key strata (KS) and the mining activities of upper face in overburden strata. The KS are performed as cantilever structures, hinged structures and voussoir beam structures at three different positions in roof strata. The structural characteristics of the KS and its movement laws are shown in the four structural modes. The loads acting on the support in the four typical structural models are also analyzed. The structural instability of the broken roof strata on the upper caving panel caused by the lower ultra-thick coal seam mining is considered to be the main reason for its face’s falls and support failures. Consequently, a method is proposed for calculating the working resistance of the support in the LTCC face, which is verified by the mining pressure monitoring in practice.

Characteristics of the Roof Behaviors and Mine Pressure Manifestations During the Mining of Steep Coal Seam

Abstract A steep seam similar simulation system was developed based on the geological conditions of a steep coal seam in the Xintie Coal Mine. Basing on similar simulation, together with theoretical analysis and field measurement, an in-depth study was conducted to characterize the fracture and stability of the roof of steep working face and calculate the width of the region backfilled with gangue in the goaf. The results showed that, as mining progressed, the immediate roof of the steep face fell upon the goaf and backfilled its lower part due to gravity. As a result, the roof in the lower part had higher stability than the roof in the upper part of the working face. The deformation and fracture of main roof mainly occurred in the upper part of the working face; the fractured main roof then formed a “voussoir beam” structure in the strata’s dip direction, which was subjected to the slip- and deformation-induced instability. The stability analysis indicated that, when the dip angle increased, the rock masses had greater capacity to withstand slip-induced instability but smaller capacity to withstand deformation-induced instability. Finally, the field measurement of the forces exerted on the hydraulic supports proved the characteristics of the roof’s behaviors during the mining of a steep seam.

Voussoir beam model for lower strong roof strata movement in longwall mining – Case study

The paper presents the influence of varying immediate roof thickness on the lower strong roof strata movement and failure pattern in longwall coal mining with large mining height. The investigation is based on 58 geological drill holes and hydraulic shield pressure measurements around the longwall Panel 42105 of the Buertai Mine in Inner Mongolia Autonomous Region, China. The longwall Panel 42105 is characterized by relatively soft immediate roof strata of varying thickness superposed by strong strata, herein defined as lower strong roof. A voussoir beam model is adopted to interpret the structural movement of the lower strong roof strata and shield pressure measurements. It is shown that when the immediate roof is relatively thick, the broken overlying lower strong roof tends to form a stable voussoir beam with previously broken layer, thus not exerting high pressure on the hydraulic shield and working face. When the immediate roof is relatively thin, the broken overlying lower strong roof tends to behave as a cantilever beam, thus exerting higher pressure on the hydraulic shield and working face. Comparison of model predictions with measured time-weighted average shield pressure (TWAP) shows good agreement.

A Case Study of Effective Support Working Resistance and Roof Support Technology in Thick Seam Fully-Mechanized Face Mining with Hard Roof Conditions

This paper presents the engineering geological properties and roof control tecnology for a thick coal seam fully-mechanized face mining with hard roof conditions (THC) at the Jinhuagong Coal Mine (JCM), northwest China. The effective support working resistance and appropriate roof control technology are two critical factors for safe and productive mining in the THC. The load-estimate-method (LOEM) is the effective method to determine the support working resistance for normal working conditions (the mining height less than 3.5 m). In order to prevent support crushing accidents from happening and to ensure the safety and high-efficiency in the THC, the LOEM was modified based on the structure of the overlying strata in the THC. The strata which can form the voussoir beam structure in normal working conditions and will break in the form of cantilever beam in the THC is defined as the key strata in the immediate roof. Therefore, the hanging length of the key strata in the immediate roof was considered in the LOEM. Furthermore, a method for calculating the hanging length of the key strata in the immediate roof and its influencing factors were proposed using cantilever beam theory analysis of the structure of the overlying strata. Moreover, in order to fully fill the goaf area with caving roof to reduce the energy accumulation of main roof movement, it was decided to apply destress blasting technique (DEBT) at the JCM to control the large hanging length of the hard roof, so as to reduce the impact of the hard main roof movement on the working face. The key technique parameters of the roof caving borehole were also proposed. The obtained results demonstrated that the theoretical analysis is reasonable, and the chosen support type and the DEBT could meet the roof control requirements. The THC has achieved safety and high-efficiency mining.

Characteristics of Strata Behavior during Thick Seam Mining by Fully-Mechanized Top Coal Caving in a Loess-Covered Gullied Region

This study systematically investigates the overburden movement and strata behavior at a fully-mechanized top coal caving (FMTCC) face in a thick coal seam in a loess-covered gullied region by field measurement and theoretical analysis. A comparative analysis was performed to examine how the attitude of surface gullies and the structure of overburden affect face support resistance. The characteristics and evolution mechanism of strata behavior during FMTCC under gully slopes in a loess-covered gullied region were discovered through a mechanical analysis of the roof structure and a comparison to the mining of a typical shallow seam under gully slopes. The study found that as the FMTCC face advanced, the strata behavior initiated in the middle part of the face and then continued to both ends, and its intensity was higher in the middle part than in the upper and lower parts. During mining under gullies, the increases in support resistance were generally less than 5% (except for the maximum increases of 7–9%), indicating that the occurrence of gullies on the surface had little influence on the strata behavior at the FMTCC face. Due to the increased burial depth and the number of key strata, the roof at the FMTCC face under gullies formed a stable load-bearing structure called a voussoir beam, which was the key difference from the strata behavior characteristics in the shallow mine under gullies.

Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining

To investigate the abnormal ground pressures and roof control problem in fully mechanized repeated mining of residual coal after room and pillar mining, the roof fracture structural model and mechanical model were developed using numerical simulation and theoretical analysis. The roof fracture characteristics of a repeated mining face were revealed and the ground pressure law and roof supporting conditions of the repeated mining face were obtained. The results indicate that when the repeated mining face passes the residual pillars, the sudden instability causes fracturing in the main roof above the old goaf and forms an extra-large rock block above the mining face. A relatively stable “Voussoir beam” structure is formed after the advance fracturing of the main roof. When the repeated mining face passes the old goaf, as the large rock block revolves and touches gangue, the rock block will break secondarily under overburden rock loads. An example calculation was performed involving an integrated mine in Shanxi province, results showed that minimum working resistance values of support determined to be reasonable were respectively 11,412kN and 10,743kN when repeated mining face passed through residual pillar and goaf. On-site ground pressure monitoring results indicated that the mechanical model and support resistance calculation were reasonable.

Analytical approaches for studying the stability of laminated roof strata

This paper presents two analytical approaches for studying the stability of voussoir beams by taking into account the horizontal loading condition of the beams: with and without horizontal in-situ stresses. For beams without being subjected to horizontal stresses, a bilinear truss represents the compression arch formed in the voussoir beam. The stability of the compression arch is analysed by the energy method. An iteration algorithm was proposed for the solution. The results of the approach are verified with UDEC simulations as well as experimental results. For beams subjected to horizontal in-situ stresses, the classic beam theory was employed to study their behaviour. The assumptions for the analytical approaches are verified by numerical simulations. The study shows that an increase in the Young's modulus of the beam rock improves the stability of the beams against buckling. However, it elevates the stresses in the horizontal direction within the compression arch, which may result in crushing failure in the beam abutments and the midspan. Low horizontal in-situ stress reduces the risk of beam buckling but increases the risk of sliding at the abutments. However, high horizontal in-situ stress may cause either beam buckling or crushing failure in the abutments. The developed analytical approaches would be applicable for roof stability analysis in stratified rocks.

Numerical investigation of arching mechanism to underground excavation in jointed rock mass

In this paper, the discontinuous deformation analysis (DDA) is applied to numerically investigate the formation of pressure arch and the stabilization mechanism of rock bolts for underground excavation in laminated rock mass. A global pressure arch with elevated stress is observed sustaining the weight of the overburden and providing the natural shield over the opening. Below the pressure arch is a loosened zone, where a group of local voussoir beam arches are formed to transfer the weight of each layer to the abutments. Detailed parametric studies regarding the in-situ stress ratio, joint friction angle, overburden depth and dip angle of strata are conducted and their influences on the formation of global pressure arch and local voussoir beam arches are identified and discussed. Based on the observed phenomena, the corresponding reinforcement scheme is proposed and the effectiveness and stabilization mechanism are examined.

Investigation and analysis of the rock burst mechanism induced within fault–pillars

An investigation is made of the characteristic strata movement and mechanism underlying fault–pillar induced rock bursts (FPIRBs) in order to mitigate rock burst damage in fault areas. A mechanical analysis of the fault–pillar model is established and roof rotation criteria is obtained. A formula is derived for the average static stress in the pillar through theoretical analysis, physical simulations, and engineering practice. The results show that when a coalface approaches a fault area, two or more roof strata simultaneously fracture in the fault area, leading to an increase in the dynamic and static stresses in the pillar. The most important factors affecting FPIRB are the static stress in the pillar and the dynamic stress induced by fault slides. The roof block rotates more easily when the pillar width is smaller, the roof thickness larger, and the roof subsidence smaller. The average static stress in the pillar increases with decreasing pillar width and/or increasing roof fracture length. The stress is greater if there is a voussoir beam structure, in which case the stress is directly proportional to the squared length of the fractured roof, and inversely proportional to the squared width of the pillar just before rotation occurs. After rotation, it is directly proportional to the roof fracture length and inversely proportional to the pillar width. Based on the FPIRB mechanism and analysis of the mechanical model, six methods of FPIRB prevention are proposed. Also, we find that FPIRB occurrence can be effectively reduced by the use of de-stress blasting and large diameter drilling.

Approximating the stope stability function using genetic algorithms and neural networks

The stability of stopes in underground mines is an active research area worldwide. Stope failures are the main cause of fatal accidents in the mining industry and the intensity of failures is likely to increase as the scale of underground mining activity increases. This work investigates stope stability using four multidisciplinary random variables. The stope stability function is approximated by training feed forward neural networks with a genetic algorithm and resilient back propagation. The proposed method is used to approximate the stope stability function of three existing bauxite stopes and is compared to results obtained from the empirical stability graph method, the voussoir beam analogue and discontinuous deformation analysis. The neural network model is shown to be superior to the other methods, as, unlike the conventional approaches, it predicts the failure or the likelihood of stope instabilities.

Structural characteristics of key strata and strata behaviour of a fully mechanized longwall face with 7.0 m height chocks

Strata behaviour during the operation of super great mining height (SGMH) longwall face is different from that of normal mining height due to the extra large boundary of caved roof strata. The key to control the strata behaviour is to understand the structural characteristics of key strata (KS) and its movement law. Through field observation, physical model simulation and theoretical analysis on the first longwall face with 7.0m mining height in the world, three kinds of structural model, which are affected by the relative position of KS in the overburden, are found and defined. Model A is defined as when the stable voussoir beam structure can be developed in the overlying KS and the strata behaviour is gentle. Model B is defined as when the cantilever structure is developed in the first KS while voussoir beam structure is developed in other overlying KS, and the continuous distance of periodic weighting (CDPW) is typically longer compared with that of normal mining height face. As to Model C, the structure of KS is the same as that in Model B, but the first KS will be forced to break in advance due to the break of the second adjacent overlying KS, which will present the weighting step and strata behaviour severity with a periodic alternating between long/gentle and short/strong. With respect to the three kinds of structural models, a method which is suitable for the calculation of working resistance for 7.0m height chocks was proposed. The suitable working resistance for the 7.0m height chocks of LW22303 face in Bulianta coal mine was determined to be 17,612kN.

Stress arch bunch and its formation mechanism in blocky stratified rock masses

Stress arch is a common phenomenon occurring in continupous materials and has also been proved to have great influences on the self-stabilization of soils or rock masses after excavation. In this paper, based on UDEC simulation, stress redistribution after excavation is investigated for a kind of special discontinuous material, i.e. blocky stratified rock mass. A layered stress arch system is observed with each stress arch lying over another. This special phenomenon is defined herein as “stress arch bunch”. Effects of dip angle of bedding plane, lateral pressure and joint offset on this stress arch bunch are studied. Its formation mechanism is also discussed based on voussoir beam theory.

Deformation mechanisms and stability analysis of undermined sedimentary rocks in the shallow subsurface

The deformation mechanisms and the stability of an undermined sedimentary rock layers were studied using the numerical model FLAC (Itasca, 2006), specifically validated for such configurations. The lowermost stratum in a sequence of successive layers is modeled as multi-fractured Voussoir beam. Boundary conditions, material properties and geometrical constraints for the numerical modeling were chosen to reflect rock mass properties that are characteristic to sedimentary rock masses. A series of ordered models, spanning over a representative range is presented. The indeterminacy parameters of the Voussoir beam model are determined using non-linear regression of the numerical results over a dimensionless space. The results show that the thickness coefficient of the compressive arch, n, formed within the deflecting beam is non uniform: at the abutment na ranges from 0.3 to 0.4 inversely proportional to beam stiffness, at the mid-span nm ranges from 0.4 to 0.5. Snap-trough failure of the beam occurs for beams of normalized composite stiffness EN<1.25·103, for which na<0.2. Finally, a straightforward procedure for determining the stability of the Voussoir beam and the Factors of Safety against different failure modes is proposed and the associated uncertainties are discussed. The procedure is applied for back analysis of the roof failure in the ancient underground water reservoir of the UNESCO world heritage site of Tel Beer-Sheva, excavated in discontinuous sedimentary rock mass. It is shown that the proposed procedure can be successfully extrapolated to natural rock masses.

Experimental Research on Instability Activation Mechanism of Old Goaf Overburden Rock

Taking the 2204 working face of Xinzhi Colliery in Huozhou as the background, through experiment of analog simulation, analyze the structure type of old goaf overburden rock breakage, instability activation mechanism of overburden rock and instability form and condition for stability of voussoir beam of old goaf. As the result, after mining and breakage, the rock mass structure of old goaf and overburden rock can be divided four types. This secondary rock structure has an important effect on old goaf activation. The existence form will decide the basic characteristic of old goaf activation. Through the mechanics analysis on voussoir beam of old goaf , it is obtained that instability due to sliding mainly depend upon the physical mechanics characteristic of key block and sideward rock, W/T value, and additional load of building on surface and so on. Rotational instability is related to the physical mechanics characteristic of key block and sideward rock and thick and length ratio h/l of the hanging rock