Research On Treatment Technology For Subgrade Settlement In Segregated Permafrost Region Of Boketu-Yakeshi Expressway

The construction of highway in permafrost region undermined the original existence condition of permafrost. Combining with global warming, they caused the melt of permafrost under subgrade. The permafrost itself is an unstable soil that is very sensitive to temperature. Cyclical change of temperature leads to permafrost heaving in cold season and thawing settlement in warm season. In the influence of climate change, subgrade height, route and so on, under the alternate effect of frost heaving and thawing settlement, differential settlement of subgrade emerges, then it causes occurrence of road diseases. Therefore, the subgrade settlement problem is a critical problem for highway construction in permafrost regions to be resolved.Aiming at this problem, subgrade temperature field in segregated permafrost region, permafrost table recurrence interval, control standard for subgrade settlement, replacement gravel+XPS insulation board combination type subgrade, quality control of subgrade construction and treatment effect in subgrade settlement are studied in this paper.(1) According to the actual weather changes along Boketu-Yakeshi expressway, the finite element models for subgrade temperature field taking into phase transition account are established. The influence law on subgrade temperature, permafrost table and permafrost degradation rate is studied by factors such as subgrade structure, pavement structure, subgrade height, grade of side slope, route toward and water content.(2) Based on temperature field calculation results, the relationship among permafrost table and temperature, subgrade height is established, and the permafrost table recurrence interval model base on Gumbel curves is settled up. The permafrost table recurrence interval is fixed. The extremum values of permafrost table recurrence interval have been obtained in various road sections, and accordingly subgrade settlement control measures and pavement maintenance-repair methods are recommended.(3) According to tensile properties of pavement material, material fatigue properties, cross slope, longitudinal slope, synthetic slope and ride comfort, control standards of subgrade differential settlement are proposed.(4) According to the maximum security value of permafrost in20years, taking zero temperature depth as permafrost melting depth, the reasonable thickness of XPS board is recommended. Based on zero temperature point locating in the replacement gravel layer, replacement gravel depth is recommended.(5) Taking compressive strength and allowable compressive stress of XPS board as indicators, according to stress analysis of XPS board under different levels of subgrade settlement and under weight and vehicle loads, taking into account the results stress dispersion principle compute results and the influence of rollers in construction, the reasonable placing depth of XPS board is recommended.(6) Aiming at permafrost distribution characteristics in BoYa expressway, replacement gravel+XPS insulation board combination type subgrade is recommended to use. With reference to pavement mechanical analysis, the pavement structures for different levels settlement suitable for permafrost region are recommended. According to the settlement under weight and temperature, the recommended subgrade structures are proved to be reasonable.(7) With reference to relevant subgrade standards, according to field compaction test results, the gravel subgrade compaction quality control standards are proposed as the index of settlement difference and settlement rate, and key technical points during permafrost excavation and gravel subgrade construction are systematically studied.(8) The key issues of XPS board subgrade construction quality control are in-depth studied. Engineering performance requirements of XPS board, construction season, jointing type of board, reasonable compaction thickness of subgrade fillers above XPS board are obtained.(9) Replacement gravel+XPS insulation board subgrade and vertical grogrid dealing with permafrost transition road section test road are paved. Through temperature, moisture content and settlement observations, the treatment effect for subgrade settlement of various schemes is verified.