Ventilation Performance Of Riprap Embankment In The Qinghai-Tibet Railway

Riprap embankment is widely used in Qinghai-Tibetan railway as a measure for cooling the permafrost. The cooling effect of riprap is achieved by convective heat transfer in embankment. Because of the particular weather conditions, forced convection usually plays the leading role during convective heat transfer in embankment in the Qinghai-Tibet Plateau. So, it is meaningful to make clear the relationship between atmospheric wind field and velocity and temperature fields in embankment, which will provide a useful basis for the riprap embankment engineering design in Qinghai-Tibet railway engineering.The ventilation performance of riprap embankment will be investigated in this study through multiple method including laboratory tests, in-situ tests, numerical simulation and theoretical analysis. And more attention was paid to clarify that the convection mechanism and its affect on cooling effect of riprap embankment.(1) Laboratory tests of different grain size including5-8cm,8-15cm and15-20cm were performed to study the formation mechanism of free convection and the characteristics and distributional pattern of temperature and velocity in riprap layer. The relation equation of natural convection velocity with temperature difference was proposed based the laboratory results.(2) In-situ tests of riprap cooling embankment and riprap slope embankment were also performed and compared to study the ventilation characteristics of riprap layer. The focus of this study is to make clear the ventilation characteristics of layer under forced convection condition and achieve the change rule of temperature and velocity with external wind condition of riprap layer. Wind speed and forced convection contribution on cooling process was studied based on the in-situ tests results.(3) Based on the continuity, non-Darcy momentum and energy equations for convection of incompressible fluid in porous media, and considering outside wind condition, the mathematical model of convective heat transfer for open riprap embankment in permafrost regions was established, and verified by in-situ monitoring data. Then, the velocity and temperature fields of open riprap embankments were studied numerically in permafrost regions along the Qinghai-Tibet railway.(4) According to the convection, conduction and fluid-solid coupled heat transfer theories, the important relationship between flow resistance and velocity was achieved based the theoretical analyses to in-situ monitoring data and test results. Besides, the influence depths of atmospheric wind field to riprap layer were calculated and analyzed through a practical project, and the calculation results show that the influence depths of atmospheric wind field to riprap layer was47cm. Those conclusions provide a basis for the solution of the technical problems in roadway construction in cold regions, such as choosing the reasonable embankment style, design parameters and so on.