| Set in the support problems of deep soft rock roadway, this paper takes the CFST circular arches as research object. Firstly, some mix proportion tests of ordinary concrete and steel fiber reinforced concrete(SFRC), were carried out. Then a proportioning program of concrete with different strength classes and dosage of steel fiber was obtained, and the uniaxial compressive strength and splitting tensile strength of concrete cube were tested. Secondly, a mechanical test of CFST circular arch under six-point even loading condition was conducted. Combined with the theoretical calculations and numerical simulation,the influences of three factors on the mechanical behavior of the CFST circular arches were analyzed thoroughly, rise-span ratio, strength grades of the core concrete and dosage of steel fiber concluded. Finally,basing on the research results of the mechanical tests of the CFST circular arches, and taking the extremely soft rock roadway with large section on +786m level of temporary sump in Qingshuiying Mine’s secondary shaft as an example, this paper gives a detailed introduction of the practical application of enclosed composite support of the CFST support by comprehensive means of field researches, laboratory tests, theoretical analysis, numerical simulations and insitu engineering tests. The main findings and innovative results of this paper are as follows:1 The proportion tests of the strength grade of concrete and the dosage of steel fiber.In order to study the influence of the core concrete strength grade on the CFST circular arches, the proportion tests of the concrete were conducted, firstly. Based on the Code of Design of Concrete Structures(GB 50010-2010), the dosage of the configuring material of the concrete were determined by setting up five kinds of w/c ratio. Then through the tests on the flowing properties of concrete, the dosage of the water reducing agent was also determined, which could satisfied the flowing properties and the casting requirement. Finally, by testing the compressive strength and splitting tensile strength of concrete cubes and statistically analyzing the tested data, the concrete strength grades with the five kinds of w/c ratio were determined, and four kinds of water cement ratio(0.3, 0.35, 0.4, 0.45) were selected to conduct the bending performance tests of the circular arches.On the basis of the common concrete proportion with four kinds of the strength grades, the steel fiber reinforced concrete was configured. Firstly, by testing the SFRC flowability, the influence of the dosage of the steel fiber on the concrete flowability was studied. Then the influence of the volume quantity of the steel fiber on the mechanical properties of the different concrete strength grades was also studied through the test on the compressive strength and splitting tensile strength of the SFRC samples. By the comparative analysis on the influence how the steel fiber would affect the compressive strength and splitting tensile strength, the reasonable steel fiber dosage was determined finally.2 The tests of the mechanical properties of the CFST circular arched under sixpoints even loading condition.By conducting the tests of the mechanical properties of the CFST circular arches under six-points even loading condition, the mechanical properties of 10 CFST circular arches with different rise-span ratio(0.154, 0.207, 0.26), different strength grade of the concrete(C50, C45, C40, C35) and different volume quantities of the steel fiber(1.0%, 1.5%, 2.0%,2.5%), were studied. The results are as following:(1) Through the monitor of the deformation progress of the circular arch samples, the relative load-displacement curve was obtained.The deformation progress of the circular arch samples with different risen-span ration, different strength grade of the concrete and different volume quantities of the steel fiber, could all divide into four stages: the compression stage, the elastic deformation stage, elastic-plastic phase and the plastic deformation stage. And in the plastic deformation stage, there were good ductility and bearing capacity presented by the circular arches.(2) By developing the strain monitoring device of the pre-buried concrete, the simultaneous monitoring of the strain of the steel tube ektexine and the interior strain of the core concrete in the mid-span of samples were carried out.The strain of the steel tube ektexine and the interior strain of the core concrete in the mid-span of 10 CFST circular arch samples were monitored. And the studies concluded that:(1) Throughout the loaded process, the steel tube and core concrete, in the mid-span of the CFST circular arches, always were in pressured state, and there were not phenomena of neutral layer offset occurred in the pure bending tests of the straight beam.(2) With the increase of the load, there were a good synchronicity between the developing tendency of the strain of steel tube and core concrete.(3) In the stage of compacting deformation, owing to the pore, void and the lower continuity of the concrete inner, the compression strain of the steel tube was slightly larger than that of the concrete collectively.(4) Under the same loading condition, the smaller the rise-span ratio, the bigger the strain value of the steel tube and core concrete. This also indicated that the smaller the rise-span ratio, the bigger the axial compression of the circular arches inner, leading to the larger compress variation of the CFST members along the axial direction. And when producing the same strain, the bigger the rise-span ratio, the bigger the value of the applied load.(5) Under the same level of load, the lower the strength grade of the concrete, the bigger the strain value of the steel tube and core concrete. When reaching the ultimate bearing capacity of the circular arches, the final axial crushing deformation of the steel tube and core concrete became more serious with the lower strength level, and the value of compressive strain of the concrete was bigger than that of the steel tube ektexine integrally.(6) The compactness of the concrete was affected by the dosage of the steel fiber. For 4 circular arch members with different dosage of the steel fiber, the value of compressive strain of the steel tube were all bigger than that of the core concrete. Especially, when the dosage of the steel fiber has reached 2.0% and 2.5%, the strain value of the concrete was far less than that of the steel tube.(3) The obtainment of the yield load and ultimate bearing capacity of the circular arch members.The yield load and ultimate bearing capacity of the circular arch members and the corresponding displacement of the mid-span location were obtained. The experimental results are as the following table:Compared with the samples, 0.154 in rise-span ratio, the yield load of the samples, 0.207 and 0.26 in rise-span ratio, improved 50.8% and 61.5%, respectively. And meanwhile, there were 42.7% and 68.3% improvement of the ultimate bearing capacity, separately. Thus, the bigger the rise-span ratio, the bigger the yield load and ultimate bearing capacity of the samples, and the stronger the ability to resist deformation of the circular arches. And the in-plane bearing capacity of the circular arches was subject to the rise-span ratio significantly.Relative to the C35 circular arch sample, the yield load of the C40, C45 and C50 samples improved 2.08%, 7.29% and 10.42%, and the ultimate bearing capacity improved 2.97%, 4.7% and 6.76%, respectively.After the using of the steel fiber in the core concrete, the yield load and the ultimate bearing capacity were all reduced in different degrees,particularly when the dosage of the steel fiber over 2.0%, there were more obvious decline of the yield load and the ultimate bearing capacity of the circular arches.Dosage of steel fiber is not conducive to the improvement of the bearing performance of the CFST circular arch.In summary, the influencing degree of the three factors on the bearing property of the CFST circular arches were finally obtained, which shown that the influencing degree brought by the rise-span ratio was bigger than that brought by the strength grade of the concrete, and the influencing degree brought by the strength grade of the concrete was bigger than that brought by the steel fiber.(4) The analysis of the deformation and failure patterns of the CFST circular arches.Under six-points even loading condition, the yield deformation of 10 circular arch samples firstly occurred at the inside arch feet location. When reaching the ultimate bearing capacity, there were obvious expansion and thicken at arch feet location of the both ends, and the curvity of different positions of the samples were no longer consistent.(5) The obtainment of the expression of the yield load of the CFST circular arches under the even loading condition.Based on the theory of linear elastic yield, the expression of the yield load of the CFST circular arches with the even loading condition were obtained. This expression could be applied to solve the approximate solution of the yield load of the deep circular arches, 0.26 in rise-span ratio. But for the shallow arches, 0.154 and 0.207 in rise-span ratio, with significant axial compressing deformation before yield, the expression was not fitted for the solutions.(6) By using the finite element simulation, the emphasize analysis that the influences of the rise-span ratio and the concrete strength grade on the axial force and the bending moment of the circular arches was conducted.(1) Under the same loading level, the smaller the rise-span ratio, the bigger the axial force of the circular arches. When they reached the ultimate bearing capacity respectively, there were not obvious difference between the axial force of the circular arches. And when reaching the ultimate bearing capacity, the higher the concrete strength grade, the bigger the axial force of the circular arches.(2) On the elastic deformation stage, the smaller the rise-span ratio, the bigger the inner bending moment of the circular arches, and the worse the flexural resistance, so to change the concrete strength grade had no obvious influence on the moment. And the maximum inner bending moment of the circular arches with different rise-span ratio and different concrete strength grade all occurred on the arch feet location, while the bending moment and axial force of the arch feet were biggest. So the circular arches bended firstly on the arch position.(3) When the circular arches reached the ultimate bearing capacity, for the circular arches with different rise-span ratio and different concrete strength grade, the moments of the arch feet were all decreased, and the maximum moments were appeared near the mid-span position.3 The application research of the CFST support in extremely soft rock roadway with large section.On the CFST support section of the Qingshuiying Mine’s temporary sump roadway on +786m level, the original composited supporting method was useless, which integrated bolt-mesh-shotcreting, concrete wall and U36 steel support, and the roadway deformation was too serious to remain stable.In order to solve the supporting problem of the serious roadway deformation, the characteristics of roadway deformation and failure were investigated, and then physical and mechanical parameters were tested. The test shown that there was obvious swelling and water softening of the temporary sump roadway, and it belonged to high swelling and extremely soft rock. Combined with the roadway geology and the test results of the physical and mechanical parameters, roadway deformation and failure reasons were analyzed. The analysis shows that the main causes of the temporary sump roadway deformation and failure were the physical and mechanical properties of surrounding rock, the water effect, and the support method. Based on the research above, enclosed composite supporting scheme was designed specially, which integrated erect elliptic concrete-filled steel tubular support, bolt-mesh-shotcreting, strengthening anchor cable and steel fiber reinforced concrete arch wall.Mechanical model of strengthening pressure ring mainly based on secondly support resistance, bolting parameters(strength, length, diameter, pretension, row and line space), and roadway mechanical properties. Through the mechanical model, the pressure ring structure was elaborated, the pressure ring geometric thickness was determined, the mechanical boundary conditions of the pressure ring were given out, and the expression of the pressure ring ultimate bearing capacity was deduced. Thus the calculation indicated that the maximum support resistance of the compositing support structure could reach 1.99 MPa. And this method could satisfy the roadway support requirement. Then, there were three key factors which affect the ultimate bearing capacity of pressure ring, including secondary support resistance, cohesion and internal friction angle of anchor. The research showed that: as for the extremely soft-rock with high clay mineral content and remarkable swelling and softening, increasing the secondary support resistance played a critical role on improving the capacity of pressure ring.Finally, by using FLAC3 D, the simulation analysis on the support effect of the composite support scheme was conducted, and the rationality of the composite support scheme was verified further. Then the composited support technology based on the CFST support was successfully applied to the project site, and the monitor of the surrounding rock deformation was performed also. The results showed that: the converging deformation of roadway has been controlled effectively, and there were good mechanical properties of the supporting body, so it could fulfill the roadway supporting requirement. |
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