Study On Settlement Control Of Embankment And Soft Foundation Of High-Speed Railway In Red Rock Area

Along with construction of high speed railways, engineering problems such as settlement control of soft base embankment in red rock area, utilization of red layered weak soil filling and structural type of subgrade bed increasingly become key technical problems emphasized by specialists in engineering circle:Red bed soft base has low strength, high compressibility and weak water permeability, and usually generates great settling volume and differential settlement under the action of embankment load, which cannot meet the requirements of post-construction settlement and can easily have transverse slide along the foundation base. In minor cases, hidden damages in engineering will be caused. In major cases, train operation safety will be endangered. Therefore, safe, economical and reasonable treatment method for red bed soft base must be found out to effectively reinforce the red bed soft base.For railways with a long section through red rock area, Group A and B fillers lack and thus red bed soft rock with weak engineering properties has to be used as filler of the embankment and bottom layer of subgrade bed. Necessary enhancement, reinforcement or modification through special construction technique will be performed on this filler to meet the requirements of high speed railways for subgrade filling.For making up the relatively weak subgrade bed structure due to using Group C filler of red bed soft rock for bottom layer of subgrade bed in red rock area, measures must be taken to properly strengthen surface layer of subgrade bed.At present, construction of high speed railways has just started in China, engineering experience in building high speed railways in red rock area is especially lacking. No corresponding data basis is provided for compliance of post-construction settlement of soft base embankment in red rock area with control standard of Code for Design of Railways in terms of design and construction. Therefore, it is necessary to conduct researches based on engineering construction to instruct engineering practice.In this paper, scientific researches are carried out by combination of theoretical research, soil test, numerical simulation analysis, centrifugal model test, dynamic model test, field test and monitoring, running test and testing etc based on the construction of Suining-Chongqing Railway Line, the first 200km/h mixed passenger and freight railway in red rock area of the southwestern region. "Mode of Structural Design of Soft Base Embankment in Red Rock Area" has been put forward and key technical problems such as settlement control of soft base embankment in red rock area, reasonable utilization of red layered weak soil filling, structural type of subgrade bed and its dynamic characteristics have been solved.This paper presents the following major innovations:(1) "Mode of Structural Design of Soft Base Embankment in Red Rock Area" has been first established based on the goal of systematically controlling subgrade settlement. Such a mode is obtained by considering such characteristics as lots of red bed soft base along Suining-Chongqing Railway Line, lack of Group A and Group B Fillers, difficulty in finding acceptable graded broken stones and strict post-construction settlement control and so on and systematically analyzing subgrade structure and factors of weak foundation, red bed filler, subgrade bed structure, construction parameters etc affecting subgrade settlement. Such mode of structural design is proved to be feasible for soft base embankment in red rock area on the basis of research in two test sections, and settlement of soft base embankment meets the requirement of design specification, therefore, the key problems such as settlement control of soft base embankment in red rock area, rational utilization of red rock filler, structure shape and dynamic character of foundation bed etc are effectively solved.(2) In-depth research is made on physical mechanic property and engineering characters of red bed soft rock(soil) via a number of soil tests. Material composite(mainly by clay material such as illite and montmorillonite) of red rock &soil and its hydrophilicity, characters of low strength and high compressibility of red soft soil, characters such as softening, disintegration and dilatation etc of red rock are elementary revealed. Discrimination standard and comprehensive discrimination method for laboratory test indexes of red bed soft soil have been first established, which provides scientific basis for reasonable selection of composite foundation with dry jet mixing piles and gravel. piles and reasonable utilization of red bed soft rock filler for filling embankment and bottom layer of subgrade bed.(3) Settlement control technology of red bed soft base embankment has been put forward systematically based on researches on settling characteristics of composite foundation with red bed soft base dry jet mixing piles and gravel piles and embankment filled with red bed soft rock. During the process, treatment method and design parameters of red bed soft base provided with composite foundation with dry jet mixing piles and gravel piles are determined through theoretical calculation, centrifugal model test and numerical analysis; the treatment ways with dry jet mixing piles and gravel piles are proved to be feasible through high precision systematical observation on settlement of two soft base embankment test sections; observation data is analyzed by inverse analysis and hyperbolic empirical formula revealing settlement of soft base embankment in red area is established, which can be used for reference for settlement forecast.(4) Comparative analysis for different computation methods is put up and the conclusion of "the results of stress amendment method is basically consistent with that of centrifugal model test and observation, and the results of composite modulus methods and numerical analysis is smaller with that of observation data" is gotten, and suggestion of "stress amendment method should be introduced for settlement computation of soft base in red rock and pile and soil stress ration should be confined from 3 to 6" is by first time put forward.(5) Compaction standard and construction technique for embankment filled with red bed soft rock filler are first determined through centrifugal model test and field compaction test and monitoring. That is, the compaction coefficient of the red layered soft rock filling materials is no lower than 0.93 and K₃₀ no lower than 110kPa; it should be controlled that the size of the coarse particle in the gradation of the filling materials is no larger than 15cm, that the optimal moisture capacity ranges from 11ï¼…to 12ï¼…and that the maximum dry density is about 19kN/m³; construction technologies of loose paving by 30～40 centimeter and static pressing and vibrating pressing by twice and 8 times respectively with a 22 ton vibratory roller is introduced. Such fruit is introduced as the quality control standard in subgrade construction and is brought into the Suining-Chongqing Railway Subgrade Construction Detailed Rules and has been popularized and applied with good effects, which is of great importance to guarantee the quality of roadbed reclamation.(6) Through dynamic model test and comprehensive test and testing of operation of multiple passenger and freight trains on site, dynamic characteristics of bottom layer structure of subgrade bed filled with red bed soft rock and surface layer structure of cement stabilized graded crushed stone subgrade bed are revealed, subgrade bed structure with red bed soft rock filler in bed bottom layer and 3ï¼…cement stabilized graded crushed stone in bed surface layer is proved reliable, which can improve strength and rigidity of surface layer of subgrade bed. Reducing dynamic stress, dynamic displacement and acceleration of bottom layer of lower subgrade bed will significantly improve ride quality and also facilitate long-term stability of embankment filled with red bed soft rock filler.In a word, close combination of theoretical research and engineering practice is followed in the work, namely, using technical method of verifying theoretical findings with engineering practice. Innovative achievements have been made in structural design mode for red bed soft base embankment, settlement control of red bed soft base embankment, reasonable utilization of red bed soft rock filler, structural type of subgrade bed and its dynamic characteristics, which creates tangible benefit in construction of Suining-Chongqing Railway Line. A complete scientific research process of "conceptual model establishment-theoretical research and experimental research-on site test and monitoring-theory advance" has been accomplished. Scientificity and application value of key technique settlement of embankment and soft foundation of high-speed railway in red rock area have been proved. All above have fostered development of railway engineering construction technology in red rock area in China to a certain extent.