Study On Culvert-Soil Interaction And Subgrade Treatment Of Expressway In Mountain Area

High fill culverts have been widely used in expressway in mountain area. Due to the improper ground treatment and the stresses states of culverts cannot be accurately reflected by linear earth pressure theory in current Chinese General Code for Design of Highway Bridges and Culverts, many problems of culverts frequently take place during the construction process or in service time. Moreover, when the road crosses the soft ground in mountain area, the depth and the composition of the soft soil are not uniform. The constructions of high embankments over soft ground face several challenges, such as potential bearing failure, global or local slope instability, intolerable total or differential settlements, and large lateral movement during both construction and post-construction periods. The objective of this dissertation focus on these problems in the construction of expressway in mountain area, and the main contents are as follows:(1) The full scale experiment, theoretical method and numerical simulation are conducted to investigate the interaction among the embankment fill-culvert-ground soil system. The influencing factors, such as the topographic condition, the geological condition, the characteristics of embankment fill, the dimensions of culvert, the eccentric load effect, and so on, on the stresses states and displacements of culvert-soil system are systemically analyzed by FEM simulation. The influence of embedding effect and the wide effect of the culvert foundation, as well as the consolidation of soft ground on the bearing capacity of ground are also discussed. The compatible deformation-based foundation design methods of high fill culverts are proposed.(2) Horizontal geosynthetic reinforcement combined with vertical reinforcement using piles or pile walls (Biaxial Reinforced Composite Foundation) can provide an economical and effective solution to soft ground improvement to support high embankments. The mechanism and the efficacy of biaxial reinforced composite foundation are studied with theoretical analysis and numerical simulation. The influencing factors on the performance of biaxial reinforced composite foundation are discussed in detail. The distribution characteristics and the dissipation process of excess pore water pressure in the construction process and post-construction period are also discussed under four different conditions. Furthermore, the time-dependent behavior of biaxial reinforced composite foundation is studied in this dissertation.Based on the researches mentioned above, the following conclusions can be drawn:(1) The earth pressure on the top of culvert nonlinearly increases with the height of backfill over the culvert. The results show that the current Chinese General Code Method is unsafe in the calculation of earth pressure on the culvert top for the wide trench installation (B≥3b) and embankment installation culverts.(2) The existences of equal settlement plane and soil arch effect in embankment fill are verified by the full scale experiment and numerical simulation. The soil arch can release the earth pressure concentration on the top of culvert, but it is instable.(3) The interaction among embankment fill-culvert-ground soil is influenced by the topographic conditions, the geological conditions, the characteristics of embankment fill, the dimensions of culvert, the eccentric load effect, the installation conditions, the embedding and the wide effect of culvert foundation, as well as the consolidation of soft ground. The design of the culvert and the foundation should consider these influencing factors comprehensively.(4) The biaxial reinforced composite foundation can obviously reduce the total and differential settlements, minimize the lateral displacement, and enhance the stability of embankment. It can be a preferred method in the soft ground improvement under high embankment.(5) The tension along the geosynthetic layer is not uniform. In general, the tension in the geosynthetic layer decreases with the distance from the centerline of the embankment. In addition, there is an obvious increase in the tension over the pile walls. Moreover, the maximum tension in the geosynthetic layer occurs below the shoulder of the embankment due to the lateral movement of embankment fill.(6) At the cross section of embankment, the maximum horizontal displacement at the level of pile wall head occurs near the most right (or the left) side pile wall and decreases towards centerline of the embankment. The maximum axial force among all pile walls is near the centerline and decreases with the distance from centerline, but it has a little increase below the embankment shoulder, whereas the maximum bending moment occurs below the slope of the embankment.(7) The behavior of biaxial reinforced composite foundation is influenced by the tensile stiffness of geosynthetic, the layers of geosynthetic, the pile wall modulous, the ratio of pile wall distance to the width, the strength of interface, the characteristics of embankment fill and ground soil, the depth of pile wall, the consolidation time and load schemes. The best performance of biaxial reinforced composite foundation can be obtained by optimizing these influencing factors.(8) When the pile walls do not penetrate the soft clay of the ground, the locations of neutral point fluctuate with the increase of the consolidation time, and they are stable finally, the depth of neutral point are closely between one-step load scheme and step by step load scheme.