Analysis of cooling effect of crushed rock-based embankment of the Qinghai-Xizang Railway

Crushed rock-based embankment (CRBE) is one of the techniques for protecting permafrost from warming and thawing along the newly constructed Qinghai-Xizang Railway, and has been widely used over warm and ice-rich permafrost. Since 2002, six sites along the Qinghai-Xizang railway have been monitored for changes in soil temperatures and position of the permafrost table under the original natural ground surface and the CRBE. The results show that the CRBE can effectively cool the roadbed, resulting in the decrease of permafrost temperatures and rise of the permafrost table. However, the cooling effect is significantly different over warm and relatively cold permafrost areas. In relatively cold permafrost areas, defined as permafrost with the mean annual ground temperature at depth of zero annual amplitude lower than -1.0 degrees C, the CRBE is much more effective for cooling permafrost and raising the permafrost table. The cooling effect during winter months is significantly greater than the warming effect during summer months, indicating that the CRBE has an overall impact in decreasing permafrost temperatures. In warm permafrost areas, defined as permafrost with the mean annual ground temperature at the depth of zero annual amplitude higher than -1.0 degrees C, the CRBE is effective in raising the permafrost table but not quite effective in lowering permafrost temperatures. However, the soil temperatures near the new permafrost table are very close to 0 degrees C and extremely sensitive to change, Any minimum surface disturbance or climate change impact could potentially increase permafrost surface temperature and result in deepening in the permafrost table. The thermal offset between the top and bottom of the crushed rock layer shows that the decreasing trend of soil temperatures indicates a weak cooling effectiveness for the CRBE. Overall, the CRBE technique can be widely applied in relatively cold permafrost regions, but its application in warm permafrost areas needs further investigations and monitoring. (C) 2007 Elsevier B.V. All rights reserved.