| In recent years, the concrete-filled steel tube support (CFST support) has been successfully applied in more than 20 deep-buried soft-rock mines with good economic effects and promising engineering values. Greatly different from the loading modes and working states of the common CFST structures in ground, the CFST support, applied in the deep-buried soft-rock roadways, would suffered the normal load of surrounding rock under working conditions, and the CFST structures used in the roadway allowed certain plastic deformation. So there is very necessary to make a thorough study on the working mechanisms of the CFST circular arch under the normal even loading condition. Based on the CFST support, the mechanical properties of 30 CFST short columns,127mm in diameter with different wall thickness, and the mechanical properties of 11 CFST circular arches under six-point even loading condition, were designed and tested. Combined with theoretical calculations and numerical simulation, the analysis and appraisal in working mechanisms of the CFST short columns and the CFST circular arches were conducted, and the support design procedures of the CFST support in deep-buried roadways basing on research results was proposed. The main results of tests are as follows:1 The working mechanisms of the CFST short columns under axial compression.By axial compression tests of 30 CFST short columns, combining with theoretical analysis, the main conclusions are as follows:(1) The ultimate bearing capacity of the CFST short columnsIn the light of the measured values of the ultimate bearing capacity of the CFST short columns, through the regression analysis of the relationship between Nu, the ultimate bearing capacity of the CFST short columns,127mm in diameter and t, the steel tube wall thickness, the equation was obtained as Nu=222t+615.28.With the increase of wall thickness of the CFST short columns, the deformation characteristics of plastic stage transformed from plastic softening to plastic hardening. Using the limit equilibrium theory, the ultimate bearing capacity of CFST short columns under the minimum principal stress of steel was deduced. And the theoretical results were closer to the measured values.(2) The working mechanism of the CFST short columns with axial compression was analyzed on the basis of the developing process of axial and circumferential strains of steel tube in the axial compression process of the CFST short columns. According to the results:Before the CFST short columns reached the elastic limit in macroscopic view, the volume deformation of the CFST samples transformed from the compression to the expansion. And the development of the elasto-plastic deformation of the CFST short columns was controlled by the constrain strength of the concrete from the steel tube.2、The working mechanism of the CFST circular arches with different steel line density under even loading condition.Through experiments, the bending mechanical properties of the CFST circular arches,140mm in diameter, under even loading condition, were also tested, where 5mm, 6mm,7mm and 10mm in wall thickness, respectively. Through experiments, the bending mechanical property of the CFST circular arches under the even loading condition with 6mm wall thickness,127mm,140mm,152mm and 168mm in diameter either, were tested. Combined with theoretical calculations and numerical simulation, the main conclusions are as following:(1) The monitoring of axial strain of core concrete in the CFST circular arches.For the study of the working mechanism of the CFST circular arches under even loading condition, the monitoring device and program for axial strain of core concrete in the CFST circular arches were designed, and the axial strain monitoring of core concrete in mid-span location of the CFST circular arches with tube diameter range from 127mm to 168mm was achieved.(2) The calculation of the elastic ultimate bearing capacity of the CFST circular arches.According to the formula of internal force of the CFST circular arches under normal even loading condition in elastic state, by theoretical derivation, the axial force and bending moment in the interior of the CFST with elastic state could be solved. The theoretical derivation results shown that:under the normal even loading condition and fixing in two sides, the yield occurred in the foot of the CFST circular arches was earlier than it occurred in the mid-span of the arches. This was matched with the research results.Combined with the estimating methods of the ultimate bearing capacity of the CFST circular arches under the bending composite condition and the calculation methods of the internal force of the CFST circular arches, the actuator load in mid-span of the CFST circular arches under the elastic limit state was solved. And the solver solution was coincided with the change of the distribution of axial strain in the mid-span of the CFST circular arch under all levels of load.(3) The calculation of the plastic ultimate bearing capacity of the bending deformation of the CFST circular arches.After the comparison of the measured ultimate bearing capacity of the samples with lower sectional steel ratio and the estimated ultimate bearing capacity without the moment, both of them were closed to each other. The theoretical analysis thought that, in the plastic deformation state of the CFST circular arch samples, there was attenuation of the bending stiffness, and the additional force caused by the axial small deformation of the circular arches was also attenuated. The decrease of the internal moment of the circular arches made the ultimate bearing capacity of the CFST circular arches closer to the axial compressive ultimate bearing capacity of the CFST. And the higher the sectional steel ratio, the more unapparent the attenuation of the bending stiffness. So, with the increase of the sectional steel ratio of the samples, the gap between the measured values of the ultimate bearing capacity and the estimated values of the CFST circular arches without the moment was enlarged.(4) The strain distribution law of the steel tube and core concrete in the bending process.Under the normal even loading condition, basing on the distribution of the axial strain of steel tube and concrete in the mid-span location of the CFST circular arch with all load levels, the measured results were the following:① On the elastic stage of the steel tube, the axial strain values were bigger than the measured strain values of the concrete. ② On the plastic stage of the steel tube, the difference of the axial strain between steel tube and concrete was gradually narrowed.③ At the beginning of the deformation of the circular arch samples, there was relative movement between the steel tube and concrete.④ Under different load levels, the distribution law and developing tendency of the axial strain of the concrete were closer to that of the steel tube.⑤ The differences of the loading methods and the load transferring methods were the basic reasons, which led to the differences of the working mechanisms between the CFST circular arches and the CSFT short columns.(5) The numerical analysis results of the bending property of the CFST circular arches.The numerical analysis results of the CFST circular arches were as following: ① For the CFST, the axial force in mid-span position was basically the same as the axial force in the arch feet, while on the elastic deformation stage of the CFST circular arches, the bending moment in the mid-span position obviously smaller than that in the arch feet. ② For the CFST circular arches, there was linear growth of the axial force with the increase of load. And on the elastic deformation stage, the bending moment in the arches increased linearly with the increase of load, while on the plastic deformation stage, there was attenuation of the bending moment.③ The developing procedure of the internal force of support matched with the theoretical analyzing results. The measured values of the ultimate bearing capacity of the CFST were basically the same as the values of the simulation and the values of the theoretical calculation.3> The bending property of the CFST circular arch reinforced by the round steel with different diameter.Under the even loading condition, the bending mechanical property of the CFST circular arch samples, Φ 140 X 6mm in size, with the strengthening round steel,8mm、 16mm、24mm and 32mm in diameter, respectively, were tested by experiment. Combining with the theoretical analysis and numerical analysis, the main results are as following:(1) By the way of layered calculation and accumulation, the elastic ultimate bearing capacity of the CFST and reinforced round steel were calculated. Then the elastic ultimate bearing capacity of the steel tube and the ultimate bearing capacity of the circular arches in the mid-span location of the CFST circular arches with the reinforced round steel,8mm, 16mm,24mm and 32mm in diameter, were estimated. The theoretical calculating values matched well with the measured results.(2) The axial strain distribution of the steel tube and concrete in the mid-span location of the CFST circular arches under even loading condition at all levels of loads were analyzed. The measured results shown that: ① In the elastic stage of the samples, the differences of the axial strain of the steel tube and concrete between the samples reinforced by strengthening round steel,8mm in diameter, and the samples without the strengthening round steel, were increasing, while there was almost the same axial strain of the steel tube and concrete between the samples reinforced by strengthening round steel,16mm in diameter, and that,32mm in diameter.② In the plastic stage of the steel tube, there was almost the same axial strain of the steel tube and concrete of the samples, which were reinforced by strengthening round steel,32mm in diameter. While the other samples, the axial strain of the steel tube was higher than that of the concrete.③ For the samples reinforced by strengthening round steel,8mm in diameter, the axial strain distribution of the core concrete and steel tube were almost adjusted simultaneously. While for the samples reinforced by strengthening round steel,16mm and 32mm in diameter, the adjustment of the strain distribution of the core concrete was later than that of the steel tube.4、The design methods of the CFST support in deep-buried roadways.The discussion of the deformation mechanism and controlling methods of the surrounding rock in the deep-buried roadways with dynamic pressure was made. And combining the experimental results with the theoretical calculation, the design methods of the CFST structures was proposed, and the methods with the effects of the engineering practicing application were verified. And the main conclusions are as following:(1) After the excavation of the roadways, the elasto-plastic deformation was almost not monitored. So the monitored deformation of the roadway surrounding rock was the rheological deformation, which was related to the times. And the main means to control the rheological deformation of the roadway surrounding rock are as following: ① The increases of the supporting reactions of the supports and the stability of the roadway surrounding rock. ② The improvements of the physical and mechanical properties of the roadway surrounding rock.③ By moderate pressure yielding, to move the peak of the support pressure of the roadway to the interior of the roadway surrounding rock.(2) The dynamic pressure of the working faces had an effect on the stability of the deep-buried roadway surrounding rock and a certain magnitude persistent disturbance actions on the surrounding rock. On the light of the experimental results of the rheological and disturbance effects of rock, the high-strengthening supports with certain flexibility were greatly fitted for the deep-buried roadway with dynamic pressure.(3) The smaller moment and giving first place to the axial force were the reasonable working state of the CFST supports. Steel tube wall thickness of CFST support which possess the characteristic of no decline of the bearing capacity within a certain deformation range should not be too small. When the lateral coefficient of the surrounding rock pressure of the roadways was certain, the optima shape of CFST support is analyzed theoretically. The rise-span ratio of the roof-arc of the equal strengthening supports was 2, and the rise-span ratio of the two sides of the circular arches was(4) By analyzing the engineering practice of the CFST support used in Xingdong coal mine second level belt dip roadway and the numerical simulation, the support method, based on the CFST support, applied in the Xingdong coal mine second level belt dip roadway combined the following features:① The great supporting strength, and a large surplus amount of bearing capacity. ② The supports dominated by the axial force, while there was smaller bending moment.③ A certain axial compressing deformation was generated in the supports, while the bearing capacity was not declined. So the CFST support method was fitted for the deep-buried roadway with dynamic pressure, and the design methods of the CFST support in the deep-buried roadway with dynamic pressure were verified. |
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