| The newly-built Zhongwei-Lanzhou Railway is introduced into the existing Zhongchuan Railway through Shuping Town in Yongdeng County,wnich is a new line project.It is necessary to carry out foundation treatment and subgrade filling for the widening section of the existing Zhongchuan Railway.However,the foundation of the widening section is saturated loess,with high water content and high compressibility,and the effect of foundation treatment is not easy to control.In the existing-widening subgrade project,the existing subgrade has stabilized,while in the filling and service stages of the widening section,the foundation will produce certain post-construction deformation,which will affect whether the overall deformation of the existing-widening subgrade is coordinated or the overall structure is safe.In the on-site widening project,factors such as the excavation of the existing subgrade,the filling of the new subgrade,and the disturbance of the construction machinery on the upper part of the subgrade will also have a certain impact on the stability of the existing subgrade,bringing potential risks to the safe operation of the existing lines.Based on these,indoor and outdoor tests,theoretical calculations,field monitoring,numerical simulation and other methods were carried out in this paper to study and analyze the bearing characteristics of the composite foundation of the widening section of the Zhongchuan Railway and the influence laws of the widing subgrade construction on the deformation of the existing Zhongchuan Railway subgrade.It provides a guarantee for the overall safety and stability of the on-site existing-widing subgrade,and the research results can also provide some references for related widening engineering.The main contents and conclusions are as follows:(1)Based on the field investigation,the field soil samples were selected to carry out the indoor physical and mechanical properties test.Accroding to relevant data,the project profile and the geographical environment and geological characteristics of the widening section of the Zhongchuan Railway were mastered in detail.The process test of high pressure jet grouting pile was carried out on the work site,the existing problems were found and solved,then the construction parameters of high pressure jet grouting pile suitable for the work site were obtained.The mechanical evaluation of its core drilling sampling was carried out in the formal application of high-pressure rotary jet grouting pile,which found that the unconfined compressive strength of the core samples taken at different depths of the same pile was different,showing that in the process of pile formation,the strength of the pile body can be changed due to factors such as different soil layer positions,formation conditions changes,and construction parameters fluctuations.(2)The field load test of single pile composite foundation was carried out.The proportional limit load and ultimate load of each test point load-settlement curve were not obvious,and the curve did not show a steep drop,basically showing a smooth curve.When the maximum pressure load was 310 k Pa,the compaction and local shear failure of the foundation were not obvious,reflecting that the bearing capacity of the composite foundation did not reach the limit state during the test.The characteristic value of bearing capacity of single pile composite foundation obtained by load test is 155 k Pa while it obtained by theoretical calculation is 269.2k Pa,the reason is that the field test value needs to be determined according to the relative deformation value,so the test value is less than the theoretical value.(3)The study on the load distribution law experiment of piles and soil between piles is carried out,the conclusion is that the critical height of the on-site widening subgrade in the filling process is about 1.5～1.8 m.When the subgrade filling height is higher than this height,the stress at the top of the high-pressure jet grouting pile will increase rapidly,while the increase of the stress of the soil between piles will slows down obviously,and this critical height can be understood as the height when the soil arch is formed.Compared with the more common trapezoidal subgrade load,the vertical stress generated by the parallelogram load formed by the widening subgrade is different along the transverse direction of the foundation,which is mainly reflected in the high-pressure jet grouting pile.The load distribution between piles and soil between piles is in a process of continuous dynamic adjustment.With the increase of subgrade filling height,there is obvious soil arching effect in subgrade structure.Soil arching completes the transfer of soil load between piles to pile top under the “assistance” of reinforced cushion.When the subgrade filling is completed,the pile-soil stress ratios at three mileage sections are3.1,4.2 and 3.9 respectively.(4)The vertical and lateral earth pressures at the connection position of the new and old subgrades are monitored on site,the stress transfer characteristics between the new and old subgrades during the construction of the widening subgrade are analyzed.It is mainly concluded that the closer the vertical distance is to the step position,the more obvious the influence of the filler layer on the stress change at the step position.If the step position is not handled properly during construction,the stress at the horizontal connection position of the new and old subgrade will continue to attenuate and gradually weaken the connection between the new and old subgrades when the subgrade is filled.This phenomenon is called the “weak zone” effect.(5)Based on the level gauge monitoring system laid on the existing subgrade,the deformation law of the existing subgrade during the construction of the widening subgrade is measured.It is mainly concluded that the deformation of the existing subgrade during the filling stage of the widening subgrade is caused by two aspects: on the one hand,it is the additional deformation caused by the filling load of the widening subgrade,on the other hand,it is the additional deformation caused by the disturbance of the construction machinery.The cumulative additional deformation of the existing subgrade will further increase during the track laying stage of the widening subgrade.After the track laying is completed,the maximum cumulative additional deformation values of the four measuring points selected on the uplink and downlink sides are 21.17 mm and 19.22 mm respectively.The closer the widening subgrade construction is to the upper position,the faster the growth rate of the additional deformation of the existing subgrade is.Compared with the monitoring data of the level gauge at different design heights,the higher the design height of the widening subgrade is,the greater the cumulative additional deformation value of the existing subgrade is.(6)Based on the typical section of the widening section of the Zhongchuan Railway,the finite element model is established to study the deformation law of the existing subgrade when the symmetrical and asymmetrical filling of the widening subgrade on both sides,and the reduction degree of the deformation of the existing subgrade by the foam lightweight soil as the filler is analyzed compared with the ordinary filler.The main conclusions are as follows: when filling symmetrically,the vertical and horizontal deformation laws of the top surface of the existing subgrade are “arch form” distribution and “like straight line” distribution respectively.When filling asymmetrically(first left and then right),the vertical and horizontal deformation laws of the existing subgrade top surface are “first straight and then arch,first left and then right” distribution and “first left pull,then right pull,left main control” distribution respectively.The maximum horizontal deformation value of 4.83 mm on the top surface of the existing subgrade during asymmetric filling is 133.33 % higher than that of 2.07 mm during symmetrical filling.The symmetrical filling method should be adopted as much as possible to reduce the influence of the widening project on the deformation of the existing subgrade.The use of foamed lightweight soil can significantly reduce the influence of widening subgrade filling on the deformation of existing subgrade,it can be selected for application when high filling or large section widening projects are involved. |
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