The Development Laws And Mechanisms Of Damage In Pavement Structures With Subgrade Modulus Decay And Corresponding Prevention And Control Measures

As the major structures,asphalt pavements and cement concrete pavements have been used in China and abroad widely.However,the distress including permanent deformation and fatigue cracking in pavements always appeared under the repeated wheel loading.The decay of subgrade modulus which is an important index of anti-deformability has the important and direct influence on the damage modes and the ability of pavement structure to resist damage in pavement structures under the wheel loading.In this dissertation,through the literature review and field investigation,the latest research progress of pavement damage was comprehensively reviewed and commented.Through sophisticated numerical simulation technology,the laws and mechanism of pavement damage with the subgradc modulus decay were researched systematically.relying on the results of field tests and indoor fullscale dynamic model test.The responding control measures of pavement damage for some areas where the subgrade modulus decay is common were put forward.The main research results obtained are as follows:(1)The characteristic indexes,including resilient modulus,foundation reaction modulus,California bearing ratio(CBR)and so on,of subgrade modulus were summarized and classified.The inner relationships between these indexes were analysed.The test philosophy,method,instrument of the indexes in the current specifications were discussed,and the change laws of subgrade modulus varying with stress condition,packing type,moisture content,compaction state and gradation composition were known.(2)Based on finite element software KENSLABS,the calculation model analyzing damage of cement concrete pavement under traffic load was established.Introducing foundation seasonal adjustment factor and zero-maintenance fatigue criterion,the influencc of the overall decrease of subgrade modulus on fatigue cracking index of pavement was analyzed,the dependence of equivalent axle load factor and number of repetitions for one passage of multiple axles on subgrade modulus was discussed,the relationship between cracking index of pavement and core parameters of pavement design including slab thickness,modultus of rupture of cement concrete,single-axle weight,repeated times of single-axle load in one day was knowin.The influence of Cement stabilized gravel base on fatigue cracking damage was got.The results indicates that,with the decreasing of foundation seasonal adjustment factor,the fatigue cracking index of cement concrete pavement increased,and the smaller foundation seasonal adjustment factor is,the faster fatigue cracking index increases;equivalent axle load factor and number of repetitions for one passage of multiple axles are effected by slab thickness and subgrade modulus;there is a direct relationship between the change ranges of fatigue cracking index of cement concrete pavement varying with slab thickness,modulus of rupture of cement concrete,single-axle weight,repeated tmes of single-axle load in one day and subgrade modulus;while improving subgrade modulus,some measures including adding slab thickness appropriately,forbidding overloading,setting cement stabilized base should be taken into account preferentially to control the fatigue cracking of cement concrete pavement under traffic load.(3)Based on the asphalt pavement test section constructed in Chengdu-Deyang-Nanchong expressway,mechanical behaviours of three typical asphalt pavements with same subgrade modulus under traffic load are analyzed and compared,including semi-rigid base,inverted and composite asphalt pavement from two different perspectives of static analysis based on elastic multi-layer theory and dynamic in-situ test with moving vehicle load.The research results show that horizontal fatigue cracking is more likely appear in asphalt pavement relatively.In the pavement structures with same thickness,semi-rigid base asphalt pavement has great advantage in decreasing fatigue cracking of asphalt surface itself and permanent deformation,the force bearing status of composite asphalt pavement is close with semi-rigid base asphalt pavement which can be replaced by composite asphalt pavement partially;the dynamic strain at the bottom of asphalt surface of inverted asphalt pavement isn't very sensitive to axle load,but the thickness of asphalt surface or asphalt treated base of which should be increased appropriately to improve force bearing status;the tensile stress of asphalt treated base should be controlled strictly to decrease the fatigue damage of inverted and composite asphalt pavement.(4)Based on the asphalt pavement test section constructed in Chengdu-Deyang-Nanchong expressway,through the elastic multi-layer theory software KENLAYER,the differences between the influence of subgrade modulus on the damage of three typical asphalt pavements including semi-rigid base,inverted and composite asphalt pavement were discussed,considering the viscoelasticity of asphalt surface,nonlinearity of graded broken stone,movability of traffic load.And the strain at the bottom of the surface of asphalt pavement test section under the moving vehicle at a constant speed are analyzed further.The research results show that,the influence of subgrade modulus decay on permanent deformation damage is bigger overall than fatigue cracking damage in asphalt pavement.The damage ratio of asphalt pavement under the triaxial load is bigger than that under the biaxial load,then than that under the uniaxial load,and the slower the vehicle speed,the worse the stress state of pavement structure is.The sensitivity of fatigue cracking damage ratio on subgrade modulus decay under the uniaxial load is stronger than the biaxial load,then the triaxial load,but the order is inverse for permanent deformation damage.The differences between the sensitivity of damage ratio of pavement structures on subgrade modulus decay under different axle weight are not evident,but the faster the vehicle speed,the more obvious the sensitivity of fatigue cracking damage ratio on subgrade modulus is.According to the descending line of the sensitivity of fatigue cracking damage of pavement structures on subgrade modulus decay,it is inverted,composite and semi-rigid base asphalt pavement,orderly.According to the descending line of the sensitivity of Permanent deformation damage of pavement structures on subgrade modulus decay,it is inverted,semi-rigid base and composite asphalt pavement,orderly.The inverted asphalt pavement has a certain advantage on the prevention of fatigue cracking in winter,but its seasonal dependence on the ability to resist fatigue cracking damage is larger,and the impact of season on the ability to resist fatigue cracking damage of semi-rigid base asphalt pavement is smaller relatively.The combination of load and seasonal conditions can significantly influence the fatigue cracking damage resulting from the action of wheel load,especially in the inverted asphalt pavement.More attention should be pay to the control of overload in summer to avoid the rapid accumulation of fatigue cracking of asphalt pavement.(5)Considering the universality of use in our country,the semi-rigid base asphalt pavement with the condition of subgrade modulus decay was discussed,as a special case.Based on the indoor full-scale dynamic model test,the influence of the moisture content and degree of compaction of subgrade on the permanent deformation and the stress at base bottom of semi-rigid base asphalt pavement were analyzed.Through the elastic multi-layer theory software KENLAYER,the influence of subgrade modulus on the structure damage of semi-rigid base asphalt pavement was discussed,with the consideration of the viscoelasticity of asphalt surface,the nonlinearity of graded gravel subbase and subgrade,and the movability of traffic load.The research results show that,the bigger the subgrade modulus,the more significant the effect of increasing the thickness of the surface,base and subbase on improving the design life of asphalt pavement controlled by the permanent deformation damage is.The smaller the subgrade modulus,the more obvious the effect of adding the thickness of subbase on restraining the fatigue cracking damage is.For the design life of asphalt pavement,when the subgrade modulus is larger relatively,the thickness of surface and base has a reasonable value and there is no obvious effect of adding subbase thickness on improving design life of asphalt pavement when subbase thickness has increased to a certain critical value.When the subgrade modulus is smaller relatively,the increase of thickness of each layer can improve the design life of asphalt pavement relatively.With the decrcase of the subgrade modulus,the damage mode which determines the design life of asphalt pavement may transform.According to the descending line of the influence degree of core parameters on design life controlled by Permanent deformation damage,it is subgrade modulus,surface thickness,base thickness and subbase thickness,orderly.According to the descending line of the influence degree of core parameters on design life controlled by fatigue cracking damage,it is surface thickness,base thickness,subgrade modulus and subbasc thickness,orderly.(6)The basic technical thought about prevention and control of pavement structural damage in areas where modulus decay is common had been put forward initially.The road engineering can be divided into different periods such as survey,design,construction,operation.The main engineering countermeasures of pavement damage from the aspect of the transportation,subgrade,pavement and waterproof and drainage system during these periods were been summed up,respectively.And the suitable period,condition and scope of the application of control measures were also presented.