Research On Settlement Behavior Of Highway Widening On Soft Soils And Improvement Methods

Due to the requirement of increasing traffic, it is very urgent to widen the existing highways in China. Since many highways in the east-southern coastal region of China were built on the soft soils, highway-widening projects on soft soils will be very important in this area in the future days. The China Highway Construction code (JTJ017-96) requires that the post construction settlement of new highways should be less than 30cm, but higher requirement of the controlled settlement is needed for the widening highways, especially for the embankment near bridges. Therefore, different ground treatment methods should be used to deal with different segments of highways.Based on the Biot's consolidation theory, this project presents the solution of the initial excess pore water pressure; and effective stress in the subsoil under the embankment, as well as some brief formulas of the immediate settlement, the consolidation settlement and the total settlement. From the analysis, it is found that the changing of the effective stress in the ground results in the immediate settlement. The initial effective stress of the ground subjected to the strip load increases firstly and then decreases along depth, its maximal value, p/Ï€, is at the depth of half time of the width of the strip load. The initial excess pore water pressure much decrease with the depth. The initial excess pore water pressure is less than 0.2p at the depth of two times of the width of the strip load. Immediate settlement is rather large for the soft soil ground.With the aid of a commercial elastic-plastic finite element package, the settlement of the ground, the deformation of the embankment, associated with their stress distribution are analyzed. Additionally, the computed solutions are compared with those of in-site measurement. The time of pavement construction should be determined by the difference of the post settlements between the new and the old embankment.It is suggested that the vertical drain consolidation method be applied to normal segments of the highway. Introducing the concept of sensitive soil, a method considering stress dependent coefficient of consolidation is proposed to calculate the consolidation of the sensitive soil with vertical drains. With the dissipation of porepressure, the soil structure of the upper subsoil gradually destroys downwards, the coefficient of consolidation of the upper subsoil becomes small, at the same time, the coefficient of permeability of the vertical drains drops down, because of the kinking effect. The combination of these two effects delays the consolidation process; The settlement and the excess pore water pressure calculated by this method agree well with in-site measurement. The consolidation character of sensitive soil with vertical drains is presented.In order to minimize the difference of settlement between the embankment and the bridge, the pile-supported and geosynthetic-reinforced embankment is recommended. Based on the assumption of the gravity balance of embankment filling above the equivalent area for the single pile, the present project improves the conventional Hewlett's three-dimensional soil-arching analysis method. The computational analysis shows that the pile space, the size of pile cap, the height of embankment and the internal friction angle of the embankment filling mainly influence the pile efficacy. With the same coverage of the pile cap, the pile efficacy linearly increases with the decreasing of the pile space for the case that the soil arch is of elastic. While for the case that the soil arch is in plastic state, the pile efficacy is almost a constant value. The inclusion of geosynthetic(s) in the filling will enhances the pile efficacy and minimizes the settlement of the embankment.The subsoil and the embankment treated by a single pile is simplified as a circular column. The distribution and dissipation of the initial excess pore water pressure are analyzed using a elastic-plastic model, as well as the Geosynthetics tension and settlement of the embankment. Through the analysis of t...