Study On The Great And Important Engineering Geological Problems For High-speed Railway Construction In Loess Area

China's loess area approximately composes 6.63% of the total national land area, in which the collapsible loess accounts for about 3/4.The earth's surfaces of Shaanxi,Gansu,Qinghai, Shanxi and Henan province et al. in the middle reaches of the Yellow River are almost entirely covered by loess and form continuous distribution area of 440,000 square kilometers,which compose the Loess Plateau landscape with a continuous stratum,large thickness and broad area. Because of the loess with special engineering properties of water sensitivity, big pore and structure, the various engineering diseases usually appear in the railroad engineering construction of loess region, such as the differential settlement of embankment, the tunnel collapse in high humidity loess, the arch ring crack of loess tunnel in current railroad and deformation or failure of pile foundation caused by negative friction, which give bigger harm to transportation and maintenance of railroad.China's high speed railway (passenger dedicated line) construction starts late but develops very quickly. The design speed of Zhengzhou-Xi'an passenger dedicated line is 350km/h, it is the first high speed passenger dedicated line constructed in the collapsible loess region both in China and the world. The loess distributes widely along Zhengzhou-Xi'an passenger dedicated line. It accounts for about 80% of the total length of line, in which the lengths of collapsible and self-weight collapse loess are respectively 65% and 27%.The collapsible classification from the slight( I class) to the very serious(IV class) is distributed in the whole line, which covers all types of collapsible loesses in China.Because of the complex engineering properties of collapsible loess along Zhengzhou-Xi'an passenger dedicated line, the special requirements of the loess roadbed, bridge and the tunnel in the line, as well as the shortage of provided research achievements and engineering experiences, the study of major engineering geological problems of high-speed railway construction in loess region illustrated with Zhengzhou-Xi'an passenger dedicated line have extremely important practice value and theory significance. The overall research ideas, main research contents and achievements of the thesis are as follows:(1) The technical requirements of passenger dedicated line are discussed from the survey, design, construction and inspection. the research progress of loess engineering geology is analyzed. The great and important engineering geological problems to need be solved are also proposed in Zhengzhou-xi'an passenger dedicated line through loess area.(2) The regional engineering geological environment of loess related to Zhengzhou— xi'an passenger dedicated line is researched from several main aspects of physical geography overview (geographical location, terrain and landform, climate and hydrology, earthquake), formation lithology, geologic structure and hydrology geological feature,etc.(3) The thorough analysis of engineering geological characteristics and the exact evaluation of collapsibility of loess related to Zhengzhou—Xi'an passenger dedicated line. The engineering geology properties are analyzed and summarized such as distribution, material composition, structure, physical and mechanical characteristics of loess along the line. These problems are also researched through the field pit test with water immersion, which include the self-weight collapse deformation characteristics of loess (i.e.the development and change law, self-weight collapse settlement and its' change with depth, the lower limit depth of collapsible soil layer and the collapsible area induced by water immersion), the collapsible types and grades of loess sites, the correction coefficientβ₀ varying with region and soil, the collapsible sensitivity and permeability of loess. On this basis of it the collapsibility of loess is accurately evaluated in Zhengzhou-Xi'an passenger dedicated line.(4) The suitability and effect for the treatment measures on collapsible loess foundation, technology of improved loess fill and construction method are testified through the quality test and settlement observation of expiremental roadbed. The settlement of roadbed after acceptance is analyzed and predicted according to 15mm standard of passenger dedicated line. Therefore it provides the fundamental basis for the ballastless track application in Zhengzhou-Xi'an passenger dedicated line.(5) The construction method, suitability of supporting structure and control indexes of deformation are studied combining with the overall engineering geological characteristics, the engineering geological conditions and support parameters designed for important loess tunnels with double passenger dedicated lines.(6) These contents are experimentally studied and analyzed that the influencing factors,development laws for negative friction,neutral point and settlement of piles in collapsible loess, the relationship between pile additional settlement induced by water immersion, pile-soil relative settlement and the negative friction,depth of the neutral point. The key problems needed to pay attention. are proposed about pile foundation construction in collapsible loess region.(7) The influences of roadbeds, tunnels, bridges and culverts on geological engineering environment of loess are discussed in the construction of Zhengzhou-Xi'an passenger dedicated line. On the basis of it the corresponding countermeasures and measures are gived for the environmental protection.