Study On State Of Subgrade Engineering In Permafrost Region During Operation Period Of Qinghai-Tibet Railway

The Golmud-Lhasa section of the Qinghai-Tibet railway (QTR) is 1,142 kilometers in length, of which 546.41 kilometers crosses large areas of continuous permafrost region (the talik region of the permafrost area is 101.68 kilometers). The total distance of subgrade situated in permafrost region is 321.706 kilometers, accounting for 72.3% of the total length (about 444.73 kilometers) of subgrade region. Since QTR covers the permafrost section with more than 4,000 meters above sea level and the length of 965 kilometers or so, cold and oxygen lacking, permafrost and fragile ecosystem in the area are three major technical problems in QTR construction, and the permafrost ranks the top of the three in constructing and operating QTR. Therefore, as the saying goes that "The key to success or failure of QTR is subgrade, and the key to subgrade is permafrost". The whole project started on June 29^th, 2001 and opened for operation on July 1^st, 2006.With the purpose of protecting the permafrost effectively and maintaining the stability of subgrade engineering in the permafrost area, , aiming at permafrost subgrade, after positively protective measures of engineering, such as the crushed-rock subgrade, the two-phase closed thermosyphon subgrade, and the subgrade of crushed-rock slope protection, are taken in building the railway, the protective effect on the permafrost is obvious. However, during the completion and operation of the project, some diseases have been found at some sections in permafrost region, which affects train speed and safe operation. Therefore, studying on subgrade state in permafrost during QTR' operation is a premise of assurance of its long-term, safe and reliable operation in permafrost region. The dissertation, by employing the methods of field survey and observation, laboratory test, theoretical analysis and calculation, will make an analysis of the surrounding temperature and permafrost character which affect state of subgrade engineering of QTR, explore the changing mechanism of the state of subgrade engineering in operation period through field investigation and monitoring of the state of subgrade engineering in permafrost region during operation period, examine the changing features of the state of subgrade engineering under different geological environment and in different stages of construction and operation and research on engineering countermeasure which can assure that state of subgrade engineering conform to OTR's operation condition and make an evaluation about long-term effect. After the research on the state of subgrade engineering in the permafrost area during operation period of QTR, the innovative conclusions will be presented as follows:(1)The state of subgrade engineering includes the following manifestations in three aspects:Mechanical state of the subgrade: it means evident vertical deformation (deformation of frost heaving and thaw settlement) and horizontal deformation (crack in subgrade), and phenomena of stability loss in subgrade resulted from the deformations.Thermal state of the subgrade: it means configuration of ground temperature field at different phases after QTR construction (temperature variations at different parts of soils).The hydrothermal environmental changes: it means that changes in ambient suprapermafrost water and surface water, lateral heat erosion have an influence on subgrade deformation and ground temperature field during construction and operation.(2)By field investigation, observation and analysis of the state of subgrade engineering in different stages during QTR's construction and operation , the subgrade mechanical state, are studyed, which includes the development process of subgrade deformation and engineering crack and the influence on the operation of QTR and the thermal state of the subgrade, as the dominant factor resulted in the subgrade mechanical state, are stated. Besides, corresponding countermeasures are put forward, for improving mechanical state of the subgrade should begin with the improvement of thermal state of the subgrade.(3)Changes in state of subgrade in permafrost region are closely related to the ambient hydrothermal environmental conditions. During operation period, ambient hydrothermal environmental conditions play a inducing and driving role in the three stages of crack in subgrade: cracks in earlier stage, fissuring in middle-term stage, and splitting and collapse in late stage.(4)The internal reason of changing mechanism of subgrade engineering state lies in cohesive strength of fill and compressibility of base soils, and the foundamental cause was asymmetric configuration of ground temperature field in fill and base soils. The latter evolving is related to the effect of outside hydrothermal environmental.(5)Engineering measurements of controlling asymmetric configuration of ground temperature field are the key method to prevent and improve the diseased subgrade engineering, with auxiliary protection of hydrothermal environment of surrounding, with cohesive strength of soils enhanced as far as possible at the precondition of deducing frost heaving.(6)Aiming at the most evident manifestation of state of subgrade engineering in permafrost—subgrade deformation and evolving mechanism and developing stages of cracks, engineering countermeasures for different stages are put forth definitely as follows, improving fill grain composition and cohesive strength (layered reinforcement in fill) during construction, strengthening subgrade with engineering measures of cooling base soils and improving ground temperature field configuration (crushed rock slope protection and two-phase closed thermosyphon) and taking the measures of prevention subgrade-toe from puddling and thermokarst phenomenon from forming (clearing out longitudinal drains beside subgrade toes).(7)Numerical calculation results of major engineering countermeasures shows that the key to prevention and improvement of cracks in subgrade is to control seasonal freeze/thaw speed and thickness of freezing/thawing soils, reduce deformation difference of middle and marginal part in subgrade and decrease tensile strength generated by subgrade body during melting season.(8)During QTR operation period, since the improvement of diseased subgrade engineering state in permafrost region is different from protective improvement in construction stage, it was necessary to consider the specific conditions of disease control in subgrade engineering. It is suggested that the sorts of the thermosyphon combined with crushed rock slope protection structure, the crushed rock berm reinforcement structure, and the crushed rock slope protection reinforcement structure will be adopted, according to the differences of regional permafrost, climate conditions and original subgrade structure.(9)Reinforcing inspection of the subgrade engineering state—especially during earlier melting season, dealing with diseased engineering state in earlier stage in time is an effective method of getting twice the result with half the effort.During the process of the present study, phased conclusions have ever been adopted in engineering reinforcement design and disease control during every stages of QTR construction, and have benn also examined during long-term operation of Qinghai-Tibet railway in permafrost.