Research On Diseases Of Highway Subgrade And Treatment Countermesure Recommends In Permafrost Regions

The Qinghai-Tibet Highway (QTH) crosses more than550kilometers of permafrost zonewith harsh environment and complex geological condition. Along the QTH, specific diseaseslike thawing settlement、waves and longitudinal cracks were commonly developed, whichexerted threat on the highway safety and comfortable operation. To solve the diseasesmentioned above based on the roadbed disease investigation along the QTH in permafrostregions, numerical simulation, theoretical analysis, laboratory and field test, the roadbeddiseases were studied systematically, and corresponding treatment measures were put forward.The main studies focused on the aspects as follows:(1) Based on the roadbed disease survey data along the QTH in recent years, the roadbeddiseases in permafrost regions were statistically analyzed. The survey result showed that thethickness of residual thawed layer increased with mean annual ground temperature ofpermafrost. The changing rate of the thickness of residual thawed layer with mean annualground temperature increased with ground temperature. The relation between the thickness ofresidual thawed layer and embankment height was a in a concave parabola law. The sink andwave increased first and decreased later with the increase of mean annual ground temperature.The degree of compactness of the embankment filling near the shoulder and slope got lostseverely. The moisture content of the embankment filling was increased. Overall, the sink andwave increased with ice content of the underlying permafrost. Longitudinal crack increasedwith ice content of the underlying permafrost. Frost boiling mainly happened in deepseasonally frozen soil regions, and occasionally happened in permafrost regions.(2) The temperature and deformation fields of embankment in permafrost regions werenumerically studied through a coupled thermal and elaso-plastic model. The conclusions wereas follows. The embankment deformation under the same embankment height increased withmean annual ground temperature. The embankment deformation under the same mean annualground temperature decreased with embankment height. The analysis of temperature fieldshowed that the main reason for the embankment deformation was the degradation ofunderlying permafrost induced by its warming. It was found by comparing the position andform of0℃and-1.5℃isotherms that, ensuring the long-term stability of the underlyingembankment by increasing embankment height was not effective. When the embankmentheight was over4m, the thermal elasto-plastic deformation induced by freezing-thawing cyclewas about1.5cm. The strength reduction of embankment filling after experiencing freezing-thawing cycles could not be neglected.（3）Based on the freezing-thawing loose of embankment filling in permafrost regions,freezing-thawing cycle tests for fillings with different silt clay content and different moisturecontent were carried out. The influence of freezing-thawing cycle and moisture content on theresilient modulus of embankment filling was studied. The test results showed that when thesilt clay content was over15%, the reduction law of resilient modulus kept generallyunchanged. Under higher moisture content, the resilient modulus reduced greatly after1timeand2times of freezing-thawing cycles. After this, with the increase of freezing-thawingcycles, the reduction got slower until steady. The reduction of the samples with lowermoisture content was relatively uniform. The reduction law of the sample with a silt claycontent of12%and higher moisture content was similar with that with a silt clay content of15%and lower moisture content. The reduction after numbers of freezing-thawing cycles wasnot obvious for the sample with a silt clay content of12%and lower moisture content and thesample with a silt clay content of9%. After the same times of freezing-thawing cycles, theresilient modulus of the samples with silt clay contents of20%,15%and12%decreased withmoisture content. The resilient modulus of the sample with a silt clay content of9%changedlittle with moisture content. It was conclude that the influences of silt clay content, moisturecontent and freezing-thawing cycles on the resilient modulus of coarse-particle soil should notbe neglected.(4) The test result from field test of crushed-rock interlayer was analyzed. It was foundthat the crushed-rock interlayer could effectively eliminate the residual thawing layer underthe embankment. At the same depth, the ground temperature under crushed-rock interlayerembankment was obviously lower than that of ordinary embankment. The crushed-rockinterlayer well protected underlying permafrost. Based on the analysis on5years’ observedground temperature data, the thawing of underlying permafrost under crushed-rock interlayerembankment was about a month later, and the freezing was a month earlier than that underordinary embankment. The thawing rate and ground temperature increasing rate undercrushed-rock interlayer embankment were obviously smaller than that of ordinaryembankment. The crushed-interlayer embankment effectively restricted the heat income ofblack pavement and global warming.(5) Based on the analysis above, the corresponding treatment measures were put forwardon improving embankment filling and the application of crushed-rock interlayer. In order todecrease the deformation of embankment filling induced by freezing-thawing cycles, it wassuggested that the silt clay content of embankment filling should be no more than15%. At the same time, embankment drainage should also be ensured. Especially for warm and ice-richpermafrost regions, the controlling standard should be decreased to12%. To decrease thethawing settlement induced by permafrost warming up, crushed-rock interlayer was suggestedto protect underlying permafrost. The11years operation of the QTH validated the coolingeffect of crushed-rock interlayer.