Study On Static Mechanical Behavior And Load Transfer Mechanism Of Soil Arch

Urban rail transit not only saves the urban ground space,but also eases the ground traffic pressure.At present,it has been regarded as the main measure to solve urban traffic problems such as urban traffic congestion and air pollution.However,while people enjoy the convenience of rail transit,they also encounter many safety problems,tunnel stratum deformation and differential change,deformation of surface pavement,inclination and even collapse of surrounding buildings,lining cracking,longitudinal cracking.Meantime,large areas of loess cave-dwellings are distributed in the loess areas of North and Northwest China,it has the advantages of adjusting measures to local conditions,integrating with nature,green environmental protection,saving farmland and so on.It has great practical significance in solving the housing problems of rural residents,saving building resources,reducing building pollution and so on.In the past hundred years,the loess cave-dwelling has been affected by natural environment weathering and erosion,and the water seepage on the top of the kiln has been affected by moisture and drying repeatedly for a long time.In addition,the man-made damage and insufficient protection have made the loess cave-dwelling in a complex state of soil loosening,local collapse,joints all over the place,water seepage and rain leakage,interface cracking,insufficient bearing capacity and other diseases,and even some caves are in a dangerous state of impending collapse.As the bearing structure of tunnel,culvert and loess cave-dwelling,soil arch determines the stability of their structure.The study on the static mechanical behavior and load transfer mechanism of the soil arch can provide theoretical basis and technical support for the accurate design and construction of the loess tunnel,the scientific evaluation of its safety performance and the formulation of scientific and reasonable reinforcement scheme.At the same time,the effective use of soil arching effect will help to reduce the cost of tunnel construction.Based on the material dispersion and structure particularity of loess tunnel,the mechanical behavior and deformation law of the loess tunnel are less studied,and the relevant research on the structural stress failure has not been carried out.In this paper,four models of soil arch structure are designed for vertical static loading test based on displacement control.The static mechanical behavior and load transfer mechanism of soil arch structure is analyzed and studied.The weak position and "arching effect" area of soil arch structure under stress state are found out,which provides theoretical support for the design,construction,safety assessment and reinforcement protection of loess tunnel.The study work of this paper is as follows:(1)Four soil arch models with different thickness of soil cover were designed.The thickness of soil cover is 0.6m,0.8m,1.0m and 1.2m respectively.Through the vertical static load test based on displacement control,the test phenomenon was analyzed and the conclusion was drawn: with the gradual increase of the load,the shear inclined crack at the edge of the loading area is the main part of the soil arch structure and extends downward,the arch foot and the side wall bottom are the weakest part of the soil arch structure.(2)Through sorting out the test data,the bearing capacity and deformation,the distribution and change rule of the vertical earth pressure,the influence rule of the soil cover thickness on the vertical earth pressure,the change rule of the soil arching effect and the influence of the soil cover thickness on the soil arching effect of the soil arch structure are studied.The results show that: the bearing capacity of the soil arch structure increases with the increase of the soil cover thickness,the settlement value of the arch crown decreases,and the arch is symmetrically deformed.The stress concentration is the most obvious at the arch foot and the mid span with the buried depth less than 0.3m,and the vertical earth pressure at each measuring point decreases with the increase of the soil cover thickness during the experimental process.The soil arch effect decreases gradually from the vault to the loading surface,there is no soil arch effect at the shallower part of the buried depth,and there is a soil arch effect within the height range of 0.6m from the vault.The soil arching effect in the height range of 0.3m ～ 0.5m from the arch crown increases with the increase of the thickness of the overburden.(3)The static elasto-plastic analysis of the soil arch under monotonic load was carried out by Abaqus,and the calculation results of bearing capacity-displacement curve,the vertical earth pressure at each measuring point and displacement inside the arch were compared with the test results,which were in good agreement.According to the calculation results,the static characteristics of the soil arch structure were analyzed,the stress-strain distribution and failure mode of the soil arch structure model under the vertical static load were defined.(4)By changing the geometric parameters of the soil arch structure,the finite element modeling was carried out,and the influence of arch rise span ratio,side wall and span number on the bearing capacity and failure mode of the soil arch structure was analyzed and studied.The results show that: with the increase of the rise span ratio,the area of soil peeling off at the arch foot of the arch is larger when the failure occurs;when the side wall exists,the bearing capacity of the soil arch structure model will be reduced,but the failure mode will not be affected;the multi span weakens the middle side wall,so that the bearing capacity of the single span of the model will be reduced,and the middle side wall will be damaged first when the failure occurs.(5)According to the calculation results,the effect area of soil arch was analyzed,and it was clear that the minimum thickness of soil cover required for maintaining the stability of soil arch structure was one half of the arch span.The boundary line function of the effect area of soil arch was deduced,and the method of arch dead load value was obtained.