| High-fill box culvert upper fill load is large,at the same time,due to the difference of stiffness between box culvert and soil body caused by the stress concentration at the top of the box culvert further increases the culvert top soil pressure,which leads to high fill box culvert construction cost increase,and even causes structural disease;therefore,it is important to take load reduction measures to reduce the culvert top soil pressure.The application of EPS board on the top of high fill box culvert to reduce its load has been applied in engineering,and the long-term creep of EPS board and filling will change the stress state of box culvert with time,and the long-term load transfer law and culvert earth pressure change law is not clear,long-term load reduction effect is difficult to predict,the existing box culvert design theory also fails to clearly reflect the long-term force characteristics of EPS board and load reduction effect.Therefore,the study of long-term stress characteristics and load reduction effect of box culvert under EPS plate load reduction conditions has important theoretical significance and engineering application value.The thesis first carried out model tests on the short-term force characteristics of box culverts under reduced load conditions of EPS boards,which showed that: The vertical earth pressure coefficient in the middle of the top of the culvert decreases with the increase of the filling height during the filling process;Through the frictional effect between the inner and outer soil columns,the reduced load at the top of the culvert is transferred to the outer soil column body,causing the vertical earth pressure coefficient of the outer soil column at the top of the culvert to increase gradually with the increase of the height of the fill,while the increased vertical earth pressure of the outer soil column will cause the increase of the horizontal earth pressure of the fill on the side of the culvert and the pressure of the substrate.Due to the inward deflection of the side wall of the box culvert,the side wall of the box culvert is ’ unloaded ’,and the horizontal earth pressure decreases first and then increases along the depth.Subsequently,a numerical calculation model consistent with the test is established on the basis of the model test,and the correctness of the numerical model is verified by comparing the numerical simulation results with the model test results without opening the creep analysis module;Then the verified numerical model is used to open the creep analysis module to study the stress characteristics of the box culvert under long-term load.On this basis,combined with the creep properties of EPS plate,the mechanical model of particle element of culvert-soil system under long-term conditions is established,and the theoretical calculation formula of vertical earth pressure on culvert top plane in the whole life cycle(50 years)of box culvert considering EPS board creep is deduced according to the theory of equal settlement surface.The accuracy of the theoretical method is verified by comparing the numerical simulation results with the in-situ test results.It is found that: with the creep of EPS board,the vertical earth pressure of the soil column in the culvert top gradually decreases with time and then increases slightly and gradually stabilizes,and the earth pressure coefficient at the time of stabilization is 0.48;The vertical earth pressure of the outer soil column gradually increases with time and tends to be stable.The horizontal earth pressure on the side wall of the box culvert increases nonlinearly with time and gradually stabilizes.Due to the friction between the culvert side and the filling soil,the increase of the base pressure under the side wall of the box culvert is greater than the increase of the vertical earth pressure on the culvert side.The phenomenon of base stress concentration is more obvious with the increase of time.Considering the possibility of long-term creep of the fill in the actual project,the long-term force characteristics of the culm-soil system were further studied based on the above study when the EPS board and the fill were creeping together.The long-term distribution law and the change law with time of the vertical earth pressure at the top of the culvert,the horizontal earth pressure at the side of the culvert and the base pressure were obtained by numerical simulation.And on the basis of the above verified ’ culvert-soil ’system mass-component mechanical model,further considering the creep nature of the fill,the theoretical calculation formula of the vertical soil pressure in the culvert top plane under common creep conditions is derived;The theoretical calculation results of longterm stress characteristics of box culvert under common creep conditions are verified by numerical simulation results.Research shows that: the vertical earth pressure of the inner soil column at the plane of the culvert top shows a change with time,first decreasing,then increasing and finally fluctuating up and down,and the vertical earth pressure of the outer soil column at the culvert top shows an opposite change law due to the frictional effect of the inner and outer soil columns;The difference between the vertical earth pressure of the soil column body inside and outside the culvert top plane at the time of stabilization is about double.The horizontal earth pressure at the measurement points within the side wall shows the characteristics of increasing and then decreasing with time and gradually stabilizing,and the horizontal earth pressure at the time of stabilization is 33.26% higher than that at the time of completion of filling on average.The change characteristics of the base pressure with time are consistent with the change pattern of the inner soil column at the culvert top plane,which is slightly larger than that when the filling is finished,and the growth of the horizontal earth pressure at the box culvert sidewall range under longterm conditions should be considered in the actual project to strengthen the stress monitoring of the box culvert side wall.Finally,the analytic hierarchy process(AHP)is used to stratify the earth pressure of the measuring points in the roof,culvert side,base and culvert side of the box culvert as the evaluation factor,and the fuzzy matter-element method is used to calculate the comprehensive weight of the layered evaluation factor.After that,the combined weights of the evaluation factors were used to calculate the Hemming closeness at different time points.The overall load reduction effect during the whole life cycle of the EPS board with load reduction measures is quantified according to the Hemming approach degree.Using the above evaluation model,the Hamming approach degree is calculated for the longterm stress state of the box culvert and the load-reducing culvert under the condition of only considering the creep of EPS board,the common creep of EPS board and filling soil,and the box culvert without load-reducing measures under the creep of filling soil,and the long-term load-reducing effect under different conditions is evaluated.The study shows that the overall effect of load shedding culvert structure within 5 years after the completion of filling is better than that of box culvert with EPS board under load shedding condition;and with the increase of time the overall effect of load shedding of both will gradually converge.After 10 years of filling,the overall effect of the two load reduction methods will be worse than that without load reduction measures,and the difference will gradually decrease with the increase of time.The vertical earth pressure at the top of the culvert with both EPS board load shedding and structural load shedding measures under long time is smaller than that without load shedding measures. |
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