| In the northeast seasonally frozen area,highway tunnels usually develop shallowly buried bias sections near the tunnel entrances and exits due to alignment selection and other reasons.The tunnel in the biased section of the entrance is not only subjected to asymmetric loading,but also faces the threat of frost damage.In this paper,the evaluation of the degree of topographic bias tunnel bias in the seasonally frozen area the analysis of the influencing factors and the characteristics of the temperature field distribution are studied by using a combination of field monitoring,theoretical analysis and numerical simulation,relying on the Huitougou Tunnel.The results have maken up for the lack of cross-sectional temperature field studies in topographic bias tunnels in monsoon frozen areas,and provided a scientific basis for stability evaluation,designing and construction of topographic bias tunnels.The main research results of the paper are as follows.(1)The on-site monitoring was carried out on the typical topographic unsymmetrical-loaded tunnel cross section and a tectonic unsymmetrical-loaded tunnel cross section.The monitoring data shown that the stress and internal displacement of the surrounding rock in the typical topographic bias tunnel section were obviously asymmetrically distributed,with the small degree of asymmetry.The stresses in the tectonic bias tunnel section were significantly asymmetrical due to the presence of weak interlayer in the surrounding rock on the right side of the tunnel,and there is a risk of tension cracks in the second lining surface of this section.(2)The deficiencies of the existing bias coefficients were analyzed by combining the stress and displacement characteristics of typical unsymmetrical-loaded tunnels.According to the distribution characteristics and magnitude of stresses in tunnel monitoring data,the principle of construction monitoring data feedback design was followed.The maximum internal force or stress N_d of the lining on the deeply buried side and the maximum internal force or stress N_s of the lining on the shallow buried side are taken as the measurement objects.The ratio of the maximum value to the minimum value of the two is defined as the unsymmetrical coefficient(UC).(3)Based on the empirical values of overburden thickness for topographic bias tunnels given in the current specifications,numerical simulation methods are used to calculate single-line and two-line railroad tunnels under different slopes and different surrounding rock levels,respectively.The distribution range of unsymmetrical coefficient for double-line railway tunnel is from 1.01 to 1.55,and the average value is1.22.The distribution range of unsymmetrical coefficient for single-line tunnel is from1.05 to 1.58,and the average value is 1.15.The final value of unsymmetrical coefficient for terrain-biased tunnel is 1.2.(4)A two-dimensional plane strain model was developed using a typical deviated tunnel section in the Huitougou Tunnel.Single-factor analysis was carried out by the control variables method to investigate the effects of changes in topographic factors(slope angle and overburden thickness),rock properties(weight,Poisson’s ratio,modulus of elasticity,cohesion,angle of internal friction and rock grade),construction method and depth-span ratio on the UC.The calculation results show that the bias coefficient gradually increased with the increase of slope angle,decreased with the increase of overburden thickness,decreased and then increased with the increase of rock level,and increased with the increase of depth-span ratio.The priority of excavating the deeply buried side will significantly reduce the tunnel bias coefficient,while the twin-side heading method will reduce the bias coefficient by excavating both sides at the same time.The upper and lower divisional excavations have little effect on the UC.(5)Multi-factor analysis of each influencing factor of the bias tunnel was carried out using the orthogonal design method.The L25(5~6)orthogonal table was selected for the experimental design,and the bias coefficient was used as the assessment object.The sensitivity of the bias coefficient to each factor was determined.(6)Temperature monitoring was carried out for the topography-biased tunnel section and the structure-biased tunnel section,respectively.The monitoring data shown that the temperature of the surrounding rock on both sides of the tunnel varied sinusoidally.As the distance from the tunnel wall increased,the annual maximum temperature of the surrounding rock decreased,the annual minimum temperature increased,the annual average temperature increased,the amplitude decreased,and the phase difference increased.Meanwhile,according to the analysis of the fitted parameter characteristics,the annual average temperature T_m and annual temperature variation T_aof the deep buried side of the typical topographic bias tunnel were basically larger than those of the shallow buried side,while t_c values were smaller than those of the shallow buried side,The fundamental reason for the asymmetric temperature distribution is in the surface inclination.The annual mean temperature T_m and phase difference t_c of the surrounding rocks on both sides of the tectonically bias tunnel section were approximately the same,while the annual temperature variation T_a was more different.The unsymmetric distribution of temperature was due to the existence of soft structural surface on the right side of the tunnel which leads to the inconsistent thermal conductivity of surrounding rocks on both sides.(7)According to the basic laws of aerodynamics,the concept of Boussinesq vortex viscosity coefficient is adopted,and the turbulent kinetic energy k and turbulent dissipation rateεare used to represent the vortex viscosity coefficient,and the wind field near the wall surface is solved using the wall function method.A two-dimensional numerical model was developed for the longitudinal section of the Huitougou tunnel.The initial temperature field of the mountain body and the temperature field of the surrounding rock under different wind speed and wind temperature conditions were calculated.The results show that the temperature of the surrounding rocks in the shallow 15 m range of the initial temperature field changed with the outside air temperature,and the highest temperature in the tunnel penetration area is 13°C.The calculation of the longitudinal temperature field of the tunnel after ventilation shown that the change of air flow rate and temperature in the tunnel could significantly affect the temperature distribution of the surrounding rocks in the tunnel.(8)A three-dimensional numerical model was established to calculate the temperature field distribution of typical topographic unsymmetrical-loaded tunnel section.The numerical simulation results revealed that the temperature distribution characteristics of each measurement line were basically consistent with the monitoring results.On this basis,the temperature field changes of the tunnel when the slope angle and the overburden thickness change were analyzed.The results shown that when the slope angle changed,the temperature field of the tunnel envelope changed.The surface temperature field connected with the tunnel changed from the tunnel vault to the shallow buried side,and the temperature of the shallow buried side changed more,while the temperature of the deep buried side was less affected.As the thickness of the overburden gradually increased,the temperature field of the surface surrounding rock and the temperature field of the tunnel surrounding rock gradually changed from mutual influence to mutual non-influence.And when the thickness of overburden exceeded15m,the temperature field of tunnel surrounding rock is basically not affected by the surface temperature.(9)Based on the distribution of tunnel freezing areas under different working conditions,the equivalent average freezing depth was calculated based on the overall lithospheric freezing and swelling theory.The variation of the freeze swelling force and unsymmetrical coefficient were studied.It was found that when the thickness of the overburden was greater than 10m,the change of topographic factors on the tunnel freezing condition was negligible.And when the cover layer was less than 10m,the tunnel freezing depth should be analyzed specifically according to the actual situation. |
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