To reveal the principle and mechanism of grain size distribution on compaction property of cohesionless soil ,a series of surface vibration compaction tests are carried out for cohesionless soils with various grain size distributions ,and the structure types of cohesionless soil are analyzed .With similar grading method ,the grain size distributions applied for tests are determined by scaling the o‐ver‐coarse‐grained soils used as mountain highway subgrade materials .The original grain size distri‐butions of over‐coarse‐grained soils reflect the differences among proportional relationships of grain size groups composed of block stones ,crushed stones ,coarse gravels and medium‐to‐fine gravels .The test results indicate that the inhomogeneous over‐coarse‐grained soils in which the content of over coarse grains is less than 75% are generally of favorable compaction property .The structures of cohe‐sionless soil mainly depend on the continuity of grain size distribution ,and can be divided into three types such as skeleton‐void structure , skeleton‐filling structure and suspending‐skeleton structure . The mechanism of grain size distribution on compaction property of cohesionless soil can be summa‐rized as the mutually‐filling effect of soil grains with various sizes , the suspending effect of large grains and the filling effect of small grains .These effects all can raise the maximum dry density of co‐hesionless soil . The compaction control criterion of cohesionless soil subgrade should consider the effect of grain size distribution on compaction property and should not adopt a uniform porosity .The research findings deepen the knowledge on cohesionless soil structure ,and provide the technical foun‐dation for compaction control of cohesionless soil subgrade .%为了揭示颗粒级配对无粘性土压实性的影响规律和机制，开展了不同颗粒级配无粘性土的表面振动压实试验，并对无粘性土的结构类型进行了分析。试验颗粒级配的确定以山区公路巨粒土填料为原型，考虑了块石、碎石、粗砾、中细砾等粒组的不同相对含量，并采用相似级配法进行缩尺。试验结果表明，颗粒级配不均匀的巨粒夹土或巨粒质土的压实性总体较好。无粘性土结构主要决定于颗粒级配的连续性，并可分为骨架孔隙结构、骨架虚填结构和悬浮骨架结构3种类型。颗粒级配对无粘性土压实性的影响机制可以概括为不同大小颗粒的相互填充效应，大颗粒的悬浮效应和小颗粒的虚填效应，上述效应均有利于提高无粘性土的最大干密度。无粘性土路基压实质量控制标准应考虑颗粒级配对压实性的影响，不宜采用统一的孔隙率。研究成果深化了对无粘性土结构的认识，并为无粘性土路基压实质量控制提供了技术依据。
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