Expressways to be constructed in permafrost regions on the Qinghai-Tibet Plateau may adopt wide embankment which can absorb much more heat than the existing narrow embankment. Therefore, the thaw settlement of wide embankment will be more severe and the lifespan of pavement structure will be much shortened. In order to solve the problem of pavement structure cracking caused by thaw settlement of wide embankment in permafrost regions, the finite element method was used to simulate the temperature field and thaw settlement of wide embankment. Then the damage behavior and damage evolution process of pavement exposed to thaw settlement were analyzed from the perspective of integration of embankment and pavement. And the pavements of expressways in permafrost regions were optimized in terms of material and structure respectively. The main contents of this thesis are listed as follows.
(1) Considering the characteristics of wide embankment in permafrost regions, the basic equations, calculating parameters, boundary conditions and initial conditions of temperature field were put forward. According to the finite element theory, the temperature field model with the second type and third type of temperature boundary conditions was established using ABAQUS and its secondary development platform. The model was used to study the temperature field of wide embankment and the cooling effect of existing engineering measures on wide embankment. The applicability of existing cooling measures of narrow embankment to wide embankment was also evaluated. The results show that the wide embankment of high-grade highways in permafrost region has faster heating rate and poorer thermal stability compared with the narrow embankment of low-grade highways. It also has greater thermal disturbance to the underlying natural frozen soil. When the highway in permafrost regions utilizes wide embankment instead of narrow embankment, the permafrost table will descend and the heterogeneity of frozen soil thawing will be intensified. Compared with other three types of wide embankment, the composite embankment with EPS insulation layer and crushed stone layer has the lowest temperature, the smallest temperature rise and the best thermal stability.
(2) After studying the constitutive model of frozen soil, EP model was selected to simulate the mechanical behaviour of permafrost, which was programmed in ABAQUS through the UMAT subroutine. Then a moisture-heat-stress coupling model was constructed to calculate the thaw settlement of wide embankment using the sequential coupling method based on the temperature data obtained by the temperature field model. The results show that with time increasing, the thaw settlements both at the centerline of the embankment and the left side of the slope increase linearly, but the thaw settlement at the centreline increases faster. When the width of embankment increases from 10m to 18m, 26m, 34m, 42m and 50m, the transverse thaw settlement curve changes gradually from quadratic curve to basin shaped curve. The thaw settlement on the top of embankment increases on average by 7.1cm when the embankment height rises 1m. For embankments with different heights, when the embankment width increases, the differential thaw settlement first increases and then decreases. The maximum differential thaw settlement happens when the width is between 18m to 26m. The existing embankment cooling measures in permafrost regions can improve the thermal stability of embankment, but it will also increase the uneven settlement of embankment. Therefore, it is necessary to develop new permafrost cooling measures, which can not only lower the temperature of embankment (especially wide embankment), but also reduce the adverse effects of uneven temperature distribution on the lifespan of pavement structures.
(3) Based on the finite element models of temperature field and thaw settlement, the fatigue damage model of pavement structure considering the thaw settlement effect was established according to the basic theory of damage mechanics. Then the damage behavior and damage evolution process of pavement structures subjected to vehicle loads and thaw settlement are analyzed, and the influence of thaw settlement on the design indexes of the asphalt pavement is also studied. The results show that the coupling effect of vehicle load and settlement additional stress leads to sharp increase of the damage of pavement structure, which is much larger than the damage cause by vehicle load or thaw settlement alone. The influence of vehicle load on pavement structure in permafrost regions is greater than the influence of thaw settlement in the early stage, which accounts for the main part of pavement structure damage. With the increase of time, the effect of thaw settlement on pavement structure damage keeps increasing, which plays a decisive role in the pavement structure life. The pavement structure will still crack under the effect of thaw settlement even without vehicle load.
(4) The pavement of high-grade highway in permafrost regions is optimized in terms of pavement material and structure respectively. The asphalt pavement with low thermal conductivity was studied first and its road performance and heat resistance effect were evaluated. Then a reflective-resistant-ventilated coupling structure was proposed and its cooling effect for wide embankment was evaluated using the temperature field model built previously. After that, high-performance pavement structure was proposed from the perspective of pavement structure optimization and the pavement structure damage model considering the thaw settlement was used to compare the lifespan of different pavement structures. The results show that the vermiculite powder added into the low thermal conductivity pavement should not exceed 12%. The addition of vermiculite has little effect on road performances of asphalt mixtures, which meet all the requirements of technical specifications. Compared with the samples without vermiculite powder, the internal temperature of asphalt mixture with vermiculite powder decreased obviously. Therefore, the low thermal conductivity layer has good thermal resistance effect. The reflective-resistant-ventilated coupling structure, which consists of re?ective coating, heat-resistant pavement, and crushed-stone embankment, provides a promising way to protect the wide embankment of expressway in permafrost zones. The high-performance pavement structure proposed in this paper (4cmAC-13C+5cm AC-20C + 12 cm ATB-25 + 6cm AC-13C + 18cm unbounded aggregate+20cm cement stabilized macadam) has a strong adaptability to thaw settlement deformation, which can be used in the construction of the Qinghai-Tibet Expressway.
In conclusion, the finite element model proposed in this paper can predict the temperature field, thaw settlement deformation and pavement structure damage of high grade highway in permafrost regions. The optimized pavement measures can effectively improve the life of pavement structure. The study provides a theoretical and scientific basis for the construction of high-grade highways in permafrost regions.