| More and more highway has been built on soft foundation in parallel with the rapid development of highway. And the problem of soft foundation treatment in loess areas has been focused on for a long time. According to the characteristic features and construction demands the High Pressure Jet Grouting Method (HPJGM), which has obtained ideal results in the practical application in Yingzhong Highway, has been offered in this study. The High Pressure Jet Grouting Method(HPJGM), which is a new kind of soft foundation strengthening method, is firstly provided in this dissertation. HPJGM means sending a grouting pipe with a jet to a preset position in the ground by drilling of a bore machine, then with some high pressure devices jetting grouting or water flow, which is a high pressure flow up to 20 Mpa or more, to attack and destroy soil. And at the same time, the boring bar is lifted gradually, the grout and soil particles are reduced to mix together, and the mixture becomes a solid after induration. This solid is a kind of compound foundation, which can increase the load-carrying capacity, reduce the sedimentation deformation, improve the strength and stability of the soft foundation, make the construction speed faster, and shorten the total construction period.By comparison of the practical results in Yingzhong highway and theoretical research, it is easy to find out with the HPJGM that treatment the maximum sedimentation volume has been changed from 67.67cm to 12.5cm and that the average load-carrying capacity has been improved from 85kPa to 208kPa; that the total cost is saved about 5.5% by comparing to the deep agitating piles treatment; and that the total construction period of HPJGM is shorten more half a year than that of the piling preloading method. The group efficient of the jet grouting piles is obvious. It can not only shorten the induration period but also improve the construction progress effectively. In addition, in this thesis the principle of forming the compound foundation strength has been studied by both inner experiments and mathematical modeling analysis, and the damaging principle of cement piles in compound foundations has also been made clear. Furthermore, on the basis of experimental studies some theoretical puzzles such as the calculating method of the embankment sedimentation in loess areas have been discussed, and some new design and construction techniques such as setting collar-beams between piles, laying cushion at the top of piles during construction have firstly been provided. All these will be able to offer some referable resources and play some guide role for further soft foundation treatment study in loess areas.
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