Study on the technology for highway construction and engineering practices in permafrost regions

In order to systemically summarize and integrate technology of highway construction in permafrost regions on the Qinghai-Tibet Plateau,we studied disciplines and mechanisms of formation and development of problems and challenges in highway engineer-ing in high-altitude permafrost regions based on construction experiences for 50 years and on 30 years of research regarding the Qinghai-Tibet and Xikang highways.Embankments,pavements,bridges,and countermeasures for ecosystem protection in per-mafrost regions were the main study objects.We combined extensive analysis of key problems of engineering stability with inter-action between permafrost and highways,theoretical models,indoor tests,and field tests to reveal the interaction between em-bankments,pavements,bridge piles,and permafrost.In this paper,we propose a series of engineering measures for highway sta-bility,design parameters,key indexes for pavement durability in low temperature,relationships between re-freezing time of bridge-pile foundation and strength development as well as ecosystem recovery in high-latitude cold regions;these eventually form a construction technology package for permafrost regions.

Evaluation of creep models for frozen soils

To model the creep behavior of frozen soils, three creep stages have to be reasonably described （i.e., primary, secondary and tertiary stages）. Based on a series of uniaxial creep test results, three creep models were evaluated. It was shown that hypoplastic creep model has high prediction accuracy for both creep strain and strain rate in a wide stress range. The elementary rheological creep model can only be used for creep strains at low stress levels, because of the restraints of its mathematical construction. For the soft soil creep model, the progressive change from the primary to secondary and tertiary stages cannot be captured at high stress levels. Therefore, the elementary rheological and soft soil creep models can only be used for low stress levels without a tertiary stage; while the hypoplastic creep model is applicable at high stress levels with the three creep stages.

Stress relaxation of warm frozen soil under drained or undrained conditions

To investigate the influence of drainage conditions on stress relaxation characteristics of warm frozen soil, a series of laboratory tests were carried out under drained and undrained conditions. The results indicate that confining pressure obviously influences the relaxation process of warm frozen soil. Under undrained condition, with increase in confining pressure, the critical relaxation du- ration tends to grow as well as instantaneous relaxation. But the relaxation rate is sensitive to confining pressure in the initial stage, and with further development, the effect tends to diminish. Under drained condition, the relaxation rate is greater than that under tmdrained condition in the initial stage but with the development of relaxation, the difference decreases. The volumetric defor- mation of warm frozen clay under drained condition is much larger than that under undrained condition.

Analysis on the Settlement of Roadway Embankments in Permafrost Regions

In permafrost regions, the embankment settlement results from several physical and mechanical processes taking place in the different soil layer, making its mechanism very complicated. Under the thermal influence of embankment, climate warming as well as geothermal source, permafrost may degrade downwards or upwards, which continuously changes the thicknesses of the permafrost layer and the also unfrozen soil layers. Thus, any attempt to analyze the embankment settlement seems to be very troublesome and difficult. In this paper, based on ground temperatures of embankment sections in five typical regions along the Qinghai-Tibet Highway （QTH）, the trends of variation of different soil layers and the thermal gradients are investigated. On this basis, four types of sections, which can represent most of the sections of QTH, are deduced to clarify the possible settlement sources in different soil layers. In order to further confirm and investigate of the settlement sources, an embankment model test was carried out. Combing the in-situ monitoring data with the results from the model test, the main settlement sources in embankment settlement are discussed for the four types of sections respectively. The results will facilitate the analysis of embankment settlement in permafrost regions.