Research On Subgrade Settlement Of Qinghai-Tibet Highway At Chumaer River In Permafrost Regions

Due to global climate warming and the effect of thermal aggregation on asphalt pavement, subgrade deformation of Qinghai-Tibet highway in permafrost regions will aggravate. In addation, with sunny-shady slope effect of subgrade, there are often great difference between deformations at the right and left sides of subgrade. Hence, comfort and safety of running vehicles will be disturbed severely.In order to study the deformation characteristics of Qinghai-Tibet highway subgrade at Chumaer River in permafrost region, mechanic tests of clay, which had been subjected to different freeze-thaw conditions, were conducted in laboratory and settlements of subgrade were analyzed according to the in-situ monitored data and the results of finite element calculation. The main conclusions are as follows:(1)When the water content of clay is less than13%, the clay displays brittle failure and its stress-strain curve shows strain-softening characteristic. However, when the water content is more than13%, the clay displays plastic failure and its stress-strain curve shows strain-hardening characteristic. As the water content changes from9%to17%, cohesion and internal friction angle of clay both decrease gradually, but its elasticity modulus reduces at first and then reachs to stability.(2) For the clay whose water content is13%, it displays plastic failure and its stress-strain curve shows strain-hardening characteristic after12freeze-thaw cycles.With the increase of freeze-thaw cycles, cohesion, internal friction angle and elasticity modulus of filling decreased firstly and then increased, they come to the minimum after seven or eight freeze-thaw cycles.(3)When the water content of clay is13%, its stress-strain curve presents strain-softening characteristic at-1℃and-2℃and strain-hardening characteristic at-3℃and-4℃. With the decrease of freezing temperature, cohesion and internal friction angle increase at first and then become stable, however, elasticity modulus increases linearly.(4)With the increase of initial water content from14%to18%, cohesion, internal friction angle and elasticity modulus of the clay decrease gradually at-3℃.(5) In-situ monitoring of ground temperature and deformation at different layers under embankments on Chumaer River indicate that the stability of roadbed in this region is mainly affected by settlement. The settlement on the left shoulder is caused by freeze-thaw cycles in seasonal active layer while on the right shoulder by creep in warm permafrost layer. Thermalprobe has positive function to control the deformation of warm permafrost roadbed.(6) The calculational results of coupled heat-stress fields indicate that the maximum settlements at the center, the shoulder and the slope toe of sub grade are123mm,101mm and54mm after20years respectively.