Abstract：The permafrost experimental segment is located in the region of Qingshui River along Qinghai—Tibet Railway，with an average elevation of 4 470 m，mean annual ground temperatures between －1.40 ℃ and －0.46 ℃ and permafrost table in the range of 1.5 to 3.5 m. It was begun to construct in September，2001，finished in November，2001. The many longitudinal cracks of roadbed emerged in succession in October，2002 or so. for the sake of the research on the reason of cracking of roadbed and the larger settlement of rubble stone ventilation embankment，the cross-sections of DK1026+630 and DK1025+583 were chosen to dissect in May，2003. The basic conditions of roadbed after excavation were really recorded，at the same time the field and laboratory tests were conducted. The settlement of roadbed and the regular pattern of cracking were also analyzed. Lastly the settlement of roadbed was theoretically calculated. The above results indicate that the settlement of roadbed is composed of both the compressive settlement of roadbed body and the thaw and compressive settlements for frozen subgrade under the external loads. Because the compaction degree is an even and good in two cross- section of roadbed，the settlement of roadbed primarily consists of thawing and compressing deformation in the base strata. The calculating values of settlement of roadbed are basically in accordance with real measuring ones of that. The crack in cross-section DK1026+630 pierces through the roadbed body and belongs to thaw-settlement fissure，which is caused by differential cross settlement of roadbed that is formed on the basis of thawing and compressing settlement in weathered marl under base surface in the sun side of roadbed . Since the rubble can adjust the shape of artificial permafrost table and control the differential cross settlement of roadbed，the rubble ventilation embankment shows a larger settlement in the initial period of completion it without thaw-settlement fissure.
