Study On The Settlement Rules And Treatment Methods Of Soft Soil Embankment On Ningxia Alluvial-Lacustrine Plain

Owing to the special geological property of alluvial-lacustrine plain along the Yellow River, there are often soft soil layers of different thickness under the roadbeds along the expressways, which exerts a great effect on the highway's long-term stability and safety usage. Thus, the study on the treatment and settlement problem of soft soil embankment on alluvial-lacustrine plain is significant to the road construction in the area along the Yellow River.On the basis of geological, hydrological survey and indoor soil experiment on the soft foundation along the Yinchuan Ring Expressway, the paper summarizes the structure property and engineering property of alluvial - lacustrine plain's soft soil. The soft soil in this area is formed because, since the Quaternary, the Yellow River channel in this area continued moving from the west to the east. In this moving process, lots of oxbow lakes were formed, which made the lacustrine bog on second terrace develop and then form current-formed and soft flowing silt and silt soil. Soft soil has high clay particle and clay powder contents, while the sand grain content is very low. Soft mucky soil contains lots of humus and plant roots, which makes soil a little alkalescent. Compared with soft soil in the southern littoral area, the physical indexes of soft soil in this area are non-typical, with 28.33% average moisture content, and 0.81 void ratio. However, referring to mechanical index, it presents medium, high compressibility and low bearing capacity, which brings a serious damage to project construction. So, the soft soil layer has to be treated.Resorting to the geotechnical centrifuge model tests, this paper conducts researches on the effect of such three consolidating soft foundation methods as Replacement of Sand-Gravel Cushion, Sand-Wick and Gravel Compaction Pile, simulates the process of road embankment filling by stages and the gradual consolidation process of soft base after construction, arrives at the embankment settlement curve according to time, and makes a preliminary forecast on the final settlements and settlement after construction. The outcome reveals that settlements in these three soft foundation treatments mainly happen during the process of road embankment filling by stages, the settlement related with time shows a good linear relationship, and the settlements during construction account for 70% of the gross after construction. After completion of the construction, the settling speed would greatly slow down, and the sedimentation increasing rate raises up in form of logarithmic curve; as to the amount of settlements after construction and the final, Replacement of Sand-Gravel Cushion is smallest, Gravel Compaction Pile takes the second place, Sand-Wick is the greatest; whether the bedding course of Replacement of Sand-Gravel Cushion basement can form skeletal structure is critical to the settlements in Replacement of Sand-Gravel Cushion; with the same drain radius and depth, the settlements of sand-wick composite basement increase along with the increase of spaces between drains. However, when the spaces between drains decrease to a certain degree, the increase extent of basement concretion is not obvious. Sand drain basements may result in uneven basement settlement in process of drainage consolidation; under the condition of unchanged radius and depth of the cross section, if the length of gravel compound foundation pile reduces properly from the center to both sides, the settlement of cross section is even, the embankment is entirely stable, and at the same time, the project cost can be reduced.On the basis of the study on the affecting factors of the stability of soft soil embankment, through large-scale soil engineering numerical analysis software Geostudio, the paper makes numerical calculation of internal structural characteristic under force and slope stability of the embankment, selects K39 +900 and K40 +160 section to make analysis. The numerical results show that soft ground has better stability. The numerical analysis of the two centrifugal models of natural soft soil ground and replacement sand-gravel cushion shows that safety factor of replacement sand-gravel cushion doubles, and the shear strength parameters are also greatly improved.Combining the engineering practice, the paper makes an in-depth analysis and research on the reinforcement mechanism, application scope, design method and construction technique of such three treatment methods as Replacement of Sand-Gravel Cushion, Sand-Wick and Gravel Compaction Pile, and determinates the settlement monitoring program of embankment construction. With the analysis on the monitoring results, it summarizes the settlement rules of the foundation in the filling construction process treated by the three treatment methods. The results show that the maximum section sedimentation does not lie in the center-line, but differs with different geological conditions of the survey section. Compared with the vertical settlement, the horizontal displacement of the ground after treated by these three techniques is extremely small. Therefore, the main control indexes should lie in the vertical sedimentation and deformation in the filling construction process. The treated foundation in the filling construction process remains very stable all the time and the treatment effect is quite sound. With the materials such as field monitoring settlement, soil horizontal displacement, pore water pressure after construction and so on, the paper makes analysis of economy on the treatment effect of the Yinchuan Ring Expressway's soft foundation.The results show that such three treatment methods as Replacement of Sand-Gravel Cushion, Sand-Wick, and Gravel Compaction Pile, can effectively reinforce the soft soil embankment on alluvial-lacustrine plain, and reduce settlement, which are sound and effective methods to deal with foundation. The research results of and suggestions on the treatment of soft soil embankment on alluvial-lacustrine plain put forward in this paper accumulate experiences for the later expressway's construction in this are.