Failure Mechanism Of Deep Soft Roadway Bearing Structure And Quantitative Yieldabal Confined Concrete Arch Support System

Coal is the main energy in China, with the exhaustion of shallow resources, the coal mining depth is growing and some coal mines is deeper than1000meters. By2014, there are more than40coal mines whose mining depth is much more than1000m in China. Because of the complexity of rock mechanics and the influence of special conditions such as deep mining and soft rock, the deformation and failure mechanism of roadway is still unclear, the roadway failure phenomenons such as large deformation and roof caving are common. The support methods commonly used such as anchor net spray support cannot meet the requirements, and the stability control theory and technology still needs to be improved and perfected. Against this background, the following study work is carried on and some conclusion has beem got.(1) Field detection and test study on the failure mechanism of surrounding rock bearing structureThe typical deformational and destructive law and supporting structure failure law of soft rock roadway at great depth are analysed according to detecting and monitoring. Numerical experimental study is carried out based on the detection results, and the influence of the support strength, crustal stress grade and surrounding rock strength grade to roadway stability is discussed. The failure mechanism of surrounding rock bearing structure of soft rock roadway at great depth is analysed in terms of the limitation of anchor net spray support and the necessity of high rigidity and high strength support. Surrounding rock bearing structure is hard to form or is easy to failure while the radial supporting strength is not enough; arch center with certain rigidity and strength is very useful and necessary, and the strength has a reasonable value. A supporting idea for the soft rock roadway at great depth is presented based on the above analysis.(2) Study on the quantitative yieldable confined concrete supporting systemOn the basis of the field research and status analysis, aimed at the typical issues of softrock roadway supporting at great depth, and according to the presented supporting idea, a quantitative yieldable confined concrete arch support system is invented. The core member of this system is confined concrete arch center with quantitative yieldable jionts. This paper mainly introduced the rectangular section confined concrete arch center and U-steel confined concrete arch center. This technology system has the characteristics of high strength, high rigidity and quantitative yielding, and it can be used for the roadway supporting under difficult geological conditions.(3) Theoretical study on the calculation of confined concrete arch center①The calculation model of confined concrete arch casing jiont is built, and the mechanical behavior of casing jiont are analysed. The formula of hinge-rigid critical angle ω0is deduced theoretically, and the influence law of different parameters is studied and the criteria is obtained. The force and rigidity of casing is calculated and the equivalent inertia moment (equivalent rigidity) of yieldable jiont on UCC arch is obtained.②The inner force expressions of circle and straight wall semicircle arch is deduced with the force method, and the node rotation angle calculation formula under the node articulated model is deduced too. The influence law of arch inner force by different parameters such as load, lateral pressure coefficient, location of jiont, equivalent rigidity of node, ratio of height to diameter.③The ultimate bearing capacity calculation formula of SQCC member, CCC member, UCC member, I-steel member and U-steel member under compression-bending is deduced. Comparative analysis of arch ultimate bearing capacity on strenth is carried out:when side pressure coefficient is1, the ultimate strength bearing capacity of SQCC150×8and UCC29arch center is about1.0MPa, which is1.81times of U36arch center.④Based on the calculation formula of circular arch instability critical load, the instability critical load of different section arch is analysed. Instability critical load of UCC29is0.45MPa, which is1.88times of U29arch; the number of SQCC150*8arch is1.62times of U36arch (the steel consumption is same).⑤The stability of four casing jionts arch center is analysed with the Step convert method, and the influence law of different rigidity and length of the casing to the arch stability are discussed.(4) Mechanical property tests on basic members of confined concrete arch①The axial compression experiments of U-steel, UCC and SQCC short column are carried out. The UCC and SQCC short columns present holistic plastic instability, avoided the bending instability failure morphology of U-steel short column. And the bearing capacity are improved greatly. Unstable failure occurs when the pour hole is installed, and the bearing capacity decreased. At the same time the ultimate bearing capacity recovered when the pour hole is reinforced. The relational expression between the ultimate bearing capacity of UCC29and the shrouding thickness t and the concrete strength grade fcu,k is obtained. And the response characteristics of inner concrete is studied with acoustic emission, which is very useful to the study of inner concrete bearing and failure characteristics. The ABAQUS software is used for numerical experiments of short column members, and the calculation results have good consistency with the laboratory test.②A pure bending numerical model of casing jiont is built, length lo, critical curvature increment△φ0and critical angle ω0of the casing jiont are defined. And the typical4-step M-△φ curve of confined concrete casing jiont is obtained. The mechanical behavior patterns and mechanical parameters are analized. The stress analysis shows that the reverse bending is a bad state to core concrete in curved member. The influence laws of different parameters to different mechanical parameters such as ultimate bending moment, critical curvature increment and equivalent bending rigidity are discussed. The above results provide basis for the design of casing parameters.③The four-point pure bending tests of UCC29members under positive bending and negtive bending are carried out and the typicalM-△φcurve of confined concrete casing jiont is obtained. The bending direction do has effect on mechanical properties of UCC member, and the positive bending is a bad state to core concrete in UCC member.④The eccentric compression of UCC long columns is studied and the axial force-lateral displacement curves with different parameters are obtained. The ultimate axial force of UCC29is1.4-1.6times of U29with positive biasing, and the number is2.2-2.6with negative biasing. The compression bending capacity formula for UCC29members is matched and has been used in the calculation theory of arch.⑤The experiments of eccentric loading columns, eccentric loading columns with pour hole and strengthen eccentric loading columns are studied, and the alternate form, ultimate bearing capacity and stress distribution are analyed contrastively, the N-M curve is consistent with the theoretical calculation results, and the correctness of the theoretical calculation is verified.The mechanical bearing characteristics of confined concrete members is mastered according to the above studies, the reference data for arch center type selection and parameter design is provided.(5) Experimental study on the bearing characteristics of confined concrete arch centersThe mechanics experiment of UCC and SQCC arch center are carried out by using designed mechanics test system of confined concrete arch center underground. Based on the collection and analysis of arch load, deformation and stress, the instability deformation, local failure characteristics and deformation characteristics of arch centering nodes are studied. Numerical experimental study is applied to verify the conclusion based on the laboratory experiment. The main conclusions are summarized as follow:①The deformation of straight wall semicircle arch under uniform pressure commanlly is’rise at top of arch, convergence at leg, slimer and taller on the whole’. The position with maximum deformation is always spring or nearby, and along with strength failure sometimes. The deformation of arch under pressure which lateral pressure coefficient is0.5is’settling at top of arch, non-deformation mainly at leg, lower on the whole’.②The ultrmate bearing capacity of SQCC150×8and UCC29arch centering is much higher than U36and U29arch centering, is1.4-1.7times of U36arch. The ultimate bearing capacity of SQCC150×8-C50is7.4%and12.6%higher than the one with concrete strength C40and C30. The trend is not so significant in UCC29arch.③The ultramate bearing capacity is drop along with the increase of γ (γ=0.5,1or2). The ultimate bearing capacity of UCC29-C40arch with γ,=1.0and2.0is80%and45%of the one with γ=0.5respectivty, and the number of SQCC150x8arch is59%and39%.④The overall yield of the arches in this experiment is caiused by strenth failure, and the overall bulkling happened afterwards. The circular arch under uniform pressure demands higher bending rigidity for bearing, and the straight wall semicircle arch demands higher eccentric compression bearing capacity instead.⑤The quantitative yielding devices at the casing jionts of SQCC150x8-C40arch center have good effect, and consistent with the laboratory test results. The yieldable devices can strengthen the capacity of arch agianst surrounding rock pressure in deep roadway.⑥Field experiments on support systemThe typical mines of Juye mining area is selected to study on SQCC and UCC arch center system. Based on the analysis of surveillance results of deformation, radial force, strain of arch center steels and stress of filled concrete, the SQCC and UCC arch center have remarkable effect on surrounding rock control under the conditions of soft rock roadway in the km deep mine and fault fracture zone:Both of the amount of roof subsidence and two sides convergence are less than20mm, supporting effect is much improved compared with U-steel arch center.According to the comprehensive studies above, this paper defined the deformation failure mechanism of surrounding rock under the typical conditions of deep and soft rock, mastered the mechanical properties and bearing capacity of confined concrete arch center. The remarkable effect on surrounding rock is proved by field experiment study. Finally, a complete surrounding rock control system of soft roadway in deep mine with matching calculation theory is built and improved.