Study On Characteristic Of Foundation Stress Of Large-span Flat Loess Tunnel

With increasing excavation section and span of loess tunnel, loess collapsibility of tunnel deformation and force are more and more prominent, so the study on characteristic of foundation stress of large-span flat loess tunnel is particularly necessary. In most regions in the Northwest, loess structure characteristic expression is good, Northwest is most authoritative region of loess in C hina. In General mountains tunnel excavation, to develop the surrounding rock’s stability performance, maximize the reduction of stress on the supporting structure, excavation can allow a small amount of deformation of surrounding rock convergence, will be part of the stress can be released. But that in the loess areas should not, because of their low strength, their poor carrying capacity, to conduct a study on the stress characteristics of loess is to makes the most sense. In this paper, based on Fu Jia 1# tunnel of "Lan Q in Expressway", with large-span flat loess tunnel as background, properties, foundation stress and deformation of surrounding rock of this tunnel with the construction process was analyzed through combining laboratory test, stress test and numerical simulation. The main results of this paper are as follows:Through the systematic laboratory testing of field sample, relationship between pressure and collapsibility coefficient, compressive deformation behavior of the humidification process and shear strength index of collapsible loess were analyzed. Along with changes in humidity and pressure, the whole process of deformation of collapsible loess curve consists of three segments with two turning points in the composition. Prior to the first turning point, representative structure intensity phase is compaction stages, structure damaged by wet phase between the two turning points, after the second turning point in the consolidat ion of the structure of the new stage, factors of development of three-stage is the effect of pressure and wet conditions.This analysis lay the groundwork for the deformation and stress analysis of loess tunnel.By monitoring the contact pressure of basal and stress of foundation soil in flat and large span loess tunnel and analyzing monitoring data, the results showed that the data of combination between tunnel invert and side wall is maximum. The maximum data of contact pressure in basal reached 197 kPa and the most of stress in foundation soil run up to 79.21 kPa. The above conclusion show that the key stress parts in flat and large span loess tunnel is located in combination between tunnel invert and side wall.The moisture test of tunnel base results showed that the maximum water rate in tunnel base runs up to 6%, loess surrounding rock water rate is low in the test section basal and the rate is conducive to the stability of tunnel foundation. Then because of the water of support structure surrounding rock in tunnel may be changed during construction process, so it is necessary to monitor the moisture of tunnel base over a long period of time.Related mechanical parameters of surrounding rock has been obtained by laboratory test of physico- mechanical parameters, based on this results the whole process of tunnel excavation has been simulated by use finite element method. Through this method can effectively analysis the deformation and stress characteristics of surrounding rock. Besides, the ultimate maximum vertical stress, horizontal compressive stress and horizontal tensile stress values are 0.957 MPa, 0.556 MPa and 0.018 MPa, and there are no tensile stress on the vertical direction.Study of the surrounding rock stress change with the construction process shows that the tunnel excavation section was separated by the supporting structures of C RD met hod, and this structure has well stability, so the vertical and horizontal stress of surrounding rock are change slightly with the construction process. According to field test section when a sensor planted situation, both ends of the test section is actually reinforced composite Foundation, its stiffness relative stiffness of the test section of the original Foundation, eventually lead to the basement contact pressure tunnel, actual stresses in the basement rock is relatively low, their ultimate impact remains to be analysed further, summarizes the effects.