| With the rapid development of cities in China,the utilization rate of suburban space is gradually improved,among which the development of open-cut highway tunnels is more prominent.Sand-pebble soil(SPS)is a kind of backfilling material with strong compression resistance,but the pressure of backfill can not be calculated according to the traditional calculation method because of the existence of coarse-grained pebbles,so it is of outstanding engineering application value to study the physical and mechanical properties of sand-pebble soil and the distribution law of backfill pressure.In this paper,based on the node interchange reconstruction project of Jinzhou Avenue and Xingguang Avenue in Chongqing,according to its various tunnel structural types and the engineering characteristics of sand-pebble soil backfilling in the backfill area,the physical and mechanical properties of sand-gravel soil are analyzed by indoor geotechnical test and large-scale direct shear test,and a continuous-discrete coupling numerical analysis method is established.The displacement field and earth pressure distribution law of SPS backfilling of open-cut tunnel under different tunnel structure and backfill height are studied from macro and meso point of view,which provides theoretical reference for the design and construction of open-cut tunnel backfilling.The main research contents and results are as follows:(1)the natural SPS backfill material is obtained from the construction site,and the gradation scale,indoor water content test and screening test are carried out on the SPS backfill,respectively,and the basic physical and mechanical parameters of SPS backfill are obtained.(2)the large-scale direct shear test of SPS backfill with 300 mm × 300 mm × 400 mm was carried out by using ZJ50-2G direct shear test system.The results show that when the coarse grain content is low,the stress-strain curve shows strain softening,while when the coarse grain content is high,there is no obvious peak value,and continues to rise slowly.There is an approximate linear relationship between SPS shear strength and vertical stress,and the cohesion increases by 68.45%,28.79% and 39.60% respectively with the increase of coarse pebble content.When the coarse grain content increases in the medium range,the internal friction angle also increases,and the overall shear strength increases.The shear surface of SPS specimen shows the slope effect of "high stone and low soil",and the pebbles show three failure forms of crushing,splitting and friction.The main failure mode of contact shear plane of SPS-concrete is surface friction.(3)A method of building discrete element model of backfill reflecting real pebble shape based on 3D scanning technique is proposed.Based on continuous-discrete coupling technique,the displacement field and earth pressure distribution of open-cut tunnel under SPS backfilling are simulated.The following conclusions are drawn: With the increase of backfilling height,the vertical and horizontal displacement groove of soil rises upward from the bottom of the slope,and the central axis gradually falls to the tunnel structure.And related to the grading angle,the displacement growth rate begins to decrease when the backfill height exceeds the middle section of the slope.At the initial stage of backfilling,the backfill is easy to move with the soil displacement,the backfill is gradually stable,and then the backfill sinks under the action of dead weight and compressive strength,but the horizontal displacement is relatively small.The particles in the upper triangular area of the backfill and outside the side wall of the tunnel have a certain falling trend,and the particles at the top of the arch tunnel "slip" to both sides of the tunnel.The overall distribution of contact between particles is strong in the lower part and weak in the upper part,which is due to the poor horizontal bonding effect caused by the lack of restraint between slope and tunnel structure in the upper backfill.(4)through the on-site monitoring of structural strain and earth pressure,it is found that after the completion of structural pouring,the strain of arch shoulder and arch waist increases rapidly,and the main stress area of the structure is in the middle of the structure,which is stable for about 30 days.Earth pressure increases linearly with height and decreases in the later stage.By comparing the numerical simulation,the field measurement and the theoretical calculation,it is found that the backfill pressure growth law of the numerical simulation is close to that of the field measurement,and the simulation results are the same as the calculated values,showing a linear growth.The measured value increases linearly when the backfill height is low,and then the growth rate begins to attenuate,and the growth rate of horizontal earth pressure decreases by 13.8%compared with the calculated value at 10 m backfill height.It decreased by 23.87% at 18 m,and the growth rate of vertical earth pressure and abutment earth pressure of arch tunnel decreased by 17.6% and 25.8% respectively when the backfill height was 11 m. |
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