| The big thickness dead weight collapsible loess refers to loess worksite with thicknessover15m and heavy dead weight collapsibility. In recent years, the capital constructions oncollapsible loess area are developing and lots of large engineering construction projects haveto be built on thick dead weight collapsible loess worksite, which makes the foundationtreatment getting more and more difficult. The mature foundation treatment of collapsibleloess can only eliminate the collapsibility of foundation soil under foundation bottom with10-15m in depth. The new foundation treatment method of thick dead weight collapsible loessshould be studied so as to get technically advanced and cost-effective foundation treatment.The collapsible loess that has been treated is typical unsaturated soil. Compared to thesaturated soil, the unsaturated soil has more complicated engineering properties. However, thetheoretical system is not complete that limits the engineering application in practice. Thestudy on unsaturated soil is on initial stage due to restriction on test conditions. The study onunsaturated soil consists of consolidation theory and strength theory that relate to deformationand stability of rock respectively.In the design and construction of passenger-dedicated railway line, control of settlementdeformation is the dominate factor and it should meet strength and stability requirements. Inorder to ensure safety and smoothness of line, post-construction settlement is strictlyprescribed in the specification, so it is important to establish rational consolidation theory andapplicable calculation method for collapsible loess foundation after being treated.The Down-hole Dynamic Compaction is a new foundation treatment method based onpile compaction method and dynamic compaction technology. This method is appliedsuccessfully in construction foundation and large oil tank foundation. However, the theorystudy of the down-hole dynamic compaction is behind of engineering application, especiallyin the application of big thickness dead weight collapsible loess.Based on existing theory of unsaturated soil, this paper cites the Zhengzhou-Xi’anpassenger dedicated line as the engineering example to conduct the following studies bymeans of theory analysis, numerical simulation, site test and mathematical statistics:(1) Study on consolidation mechanism of down-hole dynamic compaction. The cylinderexpansion theory is used to analyze compaction effect of down-hole dynamic compaction.The design parameters are defined and construction of test pile and inspection of soil betweenpiles showed that the technology can eliminate effectively the collapsibility of big thickness dead weight collapsible loess with the depth of22m.(2) Study on load bearing and deformation properties of down-hole dynamic compactionand analyze the possibility to treat passenger-dedicated line on big thickness dead weightcollapsible loess by down-hole dynamic compaction. The test results turned out that the loadbearing of big thickness dead weight collapsible loess foundation treated by down-holedynamic compaction is improved substantially and meets the design requirement of loadbearing of foundation for passenger-dedicated line. When basic load bearing of singlecomposite foundation is reached, the average value and maximum value of foundationsettlement is smaller than the requirement of “post-construction settlement ofpassenger-dedicated line subgrade not exceeding15mm”.(3) Analyze the foreign compaction theory of typical unsaturated soil. Considering of airdiffusion and difficulty in measuring, this paper put forward the simplified unsaturatedcompaction formula.(4) Calculate the unsaturated soil foundation settlement by simplified unsaturated soilcompaction formula. In consideration of characteristics and problems in subgrade engineeringof passenger-dedicated line in China, the physical mechanics of unsaturated soil is studied,especially in unsaturated soil and water characteristics and deformation properties. TheABAQUS software is used to make numerical analysis on down-hole dynamic compactioncomposite foundation settlement deformation and soaking in subgrade slope so as to obtainedsimilar rule of long term settlement and soaking observation. This paper summarizes theexisting unsaturated soil study achievements and put forward the foundation settlementcalculation method and new analysis method of big thickness dead weight collapsible loess onpassenger-dedicated line so as to provide the theoretical basis and calculation method foroptimizing design parameters.(5) Long term supervision and subgrade lateral immersion test have been made so as toanalyze the deformation properties of big thickness dead weight collapsible loess compositefoundation treated by down-hole dynamic compaction method, verify the settlementcalculation method of unsaturated soil foundation and post-construction settlementpredication method and exam the foundation consolidation effect.The long term settlement observation test turned out that embankment settlement ismainly reflected by subgrade settlement which is reflected by underlying stratum settlement.The total settlement and settlement in depth of foundation treated tend to be stable after258days constant preloading. The post-construction settlement is0.2mm that satisfied therequirement of pos-construction settlement not exceeding15mm for passenger-dedicated line. Three-points method reflect the development tendency of settlement deformation withhigh correlation coefficient of curve regression, small prediction error and strong sensitivityon abnormal data variation so that the cross-section with the settlement deformation tendencycan be found timely. The aforesaid method is proposed as the optimized method of subgradesettlement deformation predication for passenger-dedicated line. Based on this method, it isproposed take three-point method as priority and use hyperbolic contrast analysis and Asoakaas the auxiliary method to conduct passenger-dedicated line settlement analysis.The immersion test shows: after being treated by down-hole dynamic compaction, thecollapsibility of collapsible soil foundation at the consolidated area is eliminated. Theimmersion has minor effect on the foundation. The deformation of foundation immersed inlong term is within the consolidation depth. The immersed angle outside the subgrade withoutconsolidation is38~42. Therefore, it should not build permanent water-logged facilities suchas fish pond and water pool within the area20m away from subgrade slope. The time of waterponding at the subgrade slope caused by rain fall or other reasons should not be longer than20days.(6) Study on consolidation mechanism of cement improved loess. The study shows: thelonger of compaction delay, the maximum dry density of cement improved loess, the more ofoptimum water content ratio is increasing. It is suggested that the construction efficiencyshould be improved and complete the rolling compaction before initial setting of cement. Theindoor compaction delay time is defined according to rolling compaction delay time onworksite. It is rational to control compaction standard through the maximum dry density. Thestudy results will provide test and theoretical basis for complete “Geotechnique Test Code ofRailway Engineering”(TB10102-2004). |
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