Dynamic Responses And Accumulative Settlement Of Subgrade Under Aircraft Load In High-filled Airport

The runway in mountainous areas usually passes across different geological unit and has the form of cut and fill subgrade.On the one hand,under the long term moving aircraft load,additional dynamic stress is induced within a certain depth,which will cause the lead to dislocation?movement or even broken of the soil particle.Then the accumulated plastic deformation will come into being at the subgrade.On the other hand,the existent of pavement impede the moisture content field of subgrade exchange with the outside environment,which change the balance between the orginal surface and the atmosphere,leading to the phenomenon,the so called "canopy effect",whereby moisture accumulates underneath the pavement.The canopy effect will weaken the engineering mechanics properties of the soil and decrease the strength.The dynamic responses and the uneven settlement of the runway Subjected to the moving aircraft load and canopy effect,the uneven settlement will be induced easily,which severely affects the airworthiness of the runway.To improve the airworthiness of the runway against these problem are the crucial point in the highfill deformation and stability analysis at mountainous airports.The main research contents and results are as follows:1.An aircraft-pavement-subgrade coupling system is established and the dynamic responses of pavement and subgrade under the moving single wheel load and Boeing737 main gear load are studied by a semi-analytic finite element method.The results show that accelerated speed effect influences the reverse stress interval but no obvious effect to the amplitude values of stress components in the pavement.When the lift force was considered,the decrease ratio of amplitude values of stress components decreases with the speed increases and shows parabolic shape.The accelerated speed effect shows no obvious influence to the time history curves of stresses and amplitude values in the shallow soil but an amplification effect to the shear stress in the deep soil.Due to the superimposed effect of main gear,the amplitude value of longitudinal stress in the pavement is 1.5 times than vertical stress.The amplitude value of tensile stress interval can be 0.5MPa.Thus,it should strengthen the crush resistance and tensile design at the upside pavement.The superimposed effect of main gear shows an amplification effect to the amplitude values of subgrade,which increases with the subgrade soil depth increases.2.The condensation and freeze is the main mechanism of canopy effect in cold and dry soils areas.Based on electromagnetic wave theory,an empirical calibration equation is proposed using the technology of time domain reflectometry(TDR).Result shows that the equation can offer an error of ±0.05 g/g for most soils encountered in geotechnical engineering.It shows good accuracy for soils with dry density around 1.3?2.3 g/cm3.The accuracy can meet the demand of engineering under usual range of compaction energy and pore fluid conductivity.The empirical equations can be used to test gravimetric water content by the TDR method conveniently and efficiently without calibration.To overcome time and energy wastage and probe insertion problems resulting from the high compaction level of the rock-soil mixture when using the traditional calibration method,a TDR coaxial test tube calibration arrangement using an upward infiltration method was designed.The arrangement can calibrate the relationship between dielectric constant and water content of rock-soil mixture conveniently and efficiently.3.The influence of CSR(cyclic stress ratio).confining pressure.moisture content and initial static deviatoric stress to the dynamic properties of rock-soil mixture was studied by tri-axial apparatus.A cumulative deformation model of rock-soil mixture and the cumulative settlement calculational method under moving aircraft load was established.In the initial stage,the permanent strain increases quickly.With the increase number of cycles,the cumulative rate of permanent strain decreased till to a steady count.Then the permanent strain reaches a steady value.With the increases of CSR,the resilient modulus decreases and cumulative strain increases obviously.The cumulative strain can reach 0.138%when CSR is 1,which is 11.15 times than CSR equals to 0.2.With the increases of confining pressure,both the resilient modulus and cumulative strain increase.Compared with the saturation sample result,the cumulative strain for sample with optimum moisture content has a 50%decreasing amplitude ratio.With the increases of initial static deviatoric stress,both the resilient modulus and cumulative strain increase.The hyperbola cumulative deformation model of this paper has a good accuracy to predict the development of cumulative strain and the final value.The cumulative settlement value is 0.6-1.2mm.If the canopy effect was considered,the cumulative settlement value can have a 50%rate of increase.The speed of load has an obvious effect to the final settlement value.When the soil depth equals to 5m,the cumulative settlement under moving load at 75m/s is 30%larger than that at 20m/s.4.An airplane taxiing model of six degree of freedom which can consider the effect of aerodynamic lift was established to research the influence of settlement amplitude?wavelength and taxiing velocity to aircraft vibration under typical settlement model of inhomogeneous subgrade.The results show that the impact coefficient for posterior main landing gear tire force and root mean square of weighed centroid acceleration increase linearly with the increase of settlement amplitude.Both values have the increasing tendency with the taxiing velocity increasing or the wavelength decreasing.Based on security and passenger comfort,the settlement control standard for inhomogeneous subgrade was proposed.5.A field monitoring system was established at Chengde airport.The results show that the time history curve of accelerated speed in the pavement presents multimodal phenomenon.The time history curves of instantaneous strain and soil pressure in the subgrade show tension-compression alternating regions,which is coherence with the numerical result.The constant zone of subsurface temperature is around 2.5m depth.The moisture test result indicated that the moisture accumulates underneath the pavement and the canopy effect phenomenon appears.The cumulative settlement value is millimeter level and has a concave settlement line tendcy from cut-fill region to fill-cut region.