Research On Balanced Design Of Drainage And Mechanical Properties Of Graded Crushed Stones

Graded gravel is a kind of granular material without added binder.It has a good stress dissipation effect as a loose particle system which can be used in the base layer of fully flexible asphalt pavement and inverted pavement structure to eliminate or reduce reflection crack.Due to the large porosity,the graded gravel base layer also has a good drainage function of the pavement structure,which can reduce the water damage and prolong the service life.However,current Chinese specifications only give a range of graded crushed stone,and the performance of graded crushed stone in this range is quite different,which causes great troubles for designers to choose.Therefore,in accordance with the application fields and performance requirements of high-grade highways,this paper studies the mechanical properties and drainage performance of graded crushed rock under different grading conditions,and proposes a gradation of graded crushed rock that takes into account both mechanical performance and drainage performance.Firstly,based on the discrete element software PFC 5.0,the model of the graded crushed stone specimen is established,and the experimental parameters,termination conditions,and index calculation of the CBR virtual simulation process are established to realize the rapid CBR calculation of the graded crushed stone under different grading conditions.CBR laboratory tests of five grades of 4.75 mm,9.5mm,13.2mm,16 mm and 19 mm were carried out to obtain the CBR value,which was applied to calibrate the model’s micro-parameters and verify the reliability of the optimized discrete element micro-parameters.Secondly,9 groups of graded crushed stone grading that meet the requirements of expressway and first-grade highway base level use were designed,and virtual CBR test was conducted to study the influence of the pass rate of sieve above 4.75 mm on CBR value.microscale evaluation indicators based on the number of contact points,average contact force and particle displacement was developed to study the change characteristics of the above parameters during the CBR test,and the influence of the sieve pass rate and the maximum particle size particle content on the microscopic parameters was analyzed.The results show that the increase in the maximum particle size particle content contributes to increase the CBR value.Therefore,if the CBR value is found to be unable to meet the specification requirements during the pavement construction process,the maximum particle size content of the graded gravel can be appropriately increased as a solution to increase the CBR value.one.Thirdly,the finite element model of two asphalt pavement structures containing graded gravel base and flexible base pavement was established to study the influence of graded gravel modulus on the mechanical response of pavement structure.It was found that the increase in the modulus of the graded crushed stone base layer contributes to the increase in the fatigue life of the asphalt pavement of the two structures.Therefore,it is recommended to apply large moduli of resilience in the design of the pavement structure.The virtual dynamic triaxial test was performed with the discrete element method to obtain the moduli of resilience under different confining pressure,deviator stress,and gradation,and obtain the constitutive model fitting equation.It shows that the modulus of resilience increases with the increase of confining pressure and deviator stress.Therefore,it is recommended that the graded crushed stone base should be fully compacted during actual construction to ensure a larger confining pressure.Finally,based on the discrete element three-dimensional particle library forming graded gravel,the classification,extraction and visualization of the void structure of the graded gravel are realized.Combined with the FLOW3 D simulation test,it is found that the water permeability coefficient of the graded crushed stone has a good linear relationship with the effective void volume.The laboratory water seepage test was carried out.It shows that the water permeability coefficient of graded crushed stone decreases with the increase of the proportion of fine aggregate,the pass rate of 0.6mm sieve and the pass rate of 0.075 mm sieve.So for the sieve less than 4.75 mm to pass,it is recommended to take a low value within the specification range.Considering mechanical properties and drainage performance,it is recommended to select Gradation 8 as the gradation of the graded crushed stone base layer.