Active cooling roadbed employed block-stone layer in permafrost regions is an important engineering measure. Thereinto

it is one of the important contents of frozen-soils engineering applied research to study the convection heat transfer process of block-stone layer

and the actual determination of micro wind speed in block-stone layer resulting from low pressure or temperature difference has always been being one of the problems to be solved. This study has got air flow characteristics in block-stone layer for the first time by high-precision micro wind-speed detector. The results indicate that the convection process is closely related to the temperature change; the rapid cooling is completed in accelerating and stable stage

thereafter slowly cooling process; when the temperature difference in crushed-rock layer is higher than the start temperature difference

the correlation between natural convective velocity and temperature difference is linear. With the cooling process the convective characteristics is different in the same condition of temperature difference. The influence of surface temperature fluctuations on the bottom temperature along the depth direction weakened; and there is a linear correlation between retardancy of temperature and depth. The asymmetry of temperature change is caused by different heat transfer ways and heat transfer efficiencies. At the same time

there is the scope of heat conduction in block-stone layer

in order to ensuring the cooling effect thickness of block-stone layer must be greater than the scope of heat conduction. The experimental research results provide strong support for theoretical analysis and simulation and have a great significance for further understanding of cooling mechanism and improvement.