Heavy metal contaminated soils not only destroy environmental health

but also threaten the safety of geotechnical engineering. The control variable method was used to study the compaction characteristics evolution and active mechanisms of arsenic (As) and cadmium (Cd) single and composite contaminated soils with different concentrations through soil chemistry

microstructure and compaction tests. The results showed that in single contaminated soil

the As could enhance optimal moisture content by 5.90%

but decrease the maximum dry density by 1.02% through thickening diffusion double electric layer

promoting small pores and fine aggregates in flocculated structure. The effect of Cd on the soil particles is opposite. Cd compressed the diffused- double layer so that the number of small pores and fine aggregates were reduced. As a result

the optimal water content was declined by 8.03% and the maximum dry density is increased by 1.00% of Cd contaminated soil. Furthermore

the influence of Cd on the compaction characteristics of soil is greater than that of As. For the As and Cd co-contaminated soil

As and Cd had synergistic interaction on the optimal moisture content

while no interaction on maximum dry density. The correlation between macro- and micro-characteristics of contaminated soil was established by fitting the relationship between fractal dimension and the maximum dry density with a quadratic function. The results of this study can provide key parameters and theoretical support for mitigating the environmental and engineering disasters of heavy metal contaminated soils.