Abstract：The crushed rock embankment can provide cool energy for the permafrost beneath it and guarantee the stability of permafrost without thawing. The cooling effects of the crushed rock embankment are already seen to be verified by in-situ data. However，the mechanism is still to be explored and the design parameters also need to be optimized. The heat transfer in crushed rock railway embankment could be treated as the air convection problem in porous media. Here，the local thermal equilibrium is assumed to exist between the air and the grains；and the air is expressed by the Boussinesque approximation. The heat transfer equations，the motion equation，and the conservation equation are achieved with finite element formulas of Galerkin¢s method；and then，a program 3G2003 to simulate the heat transfer in crushed rock embankment is developed. Simulations are performed on the crushed rock roadbeds composed of crushed rock with diameters of 3，6，9 and 15 cm，respectively. The results shows that the 9 cm-crushed rock embankment has the best cooling effect，which will cause a decrease of 1.18 ℃ in the temperature at the base of embankment；and air convection occurs in entire interior embankment. The temperature field shows prominent nonlinear characteristics，and the average air speed at the bottom of embankment is 0.73 m/h. The embankment composed of crushed rock of 6 cm in diameter will induce a decrease of 0.78 ℃ in temperature，accompanied by weak air convection with air speed of 0.21 m/h at the base of embankment. The embankments consisting of 3 cm and 15 cm diameter crushed rock have poor cooling effect，which will have 0.21 ℃ and 0.28 ℃ decrease in temperature，respectively，where air speed at the bottom approximate zero. Therefore，the 9 cm diameter crushed rock is recommended for the construction of Qinghai—Tibet railway so that permafrost embankment can be protected.
