The Structured Model Of Soft Soil And The Application In The Settlement Of The Vacuum Combined With Surcharge Preloading

There are much soft soil in the Zhujiang Delta ,which is comprehensive material composed of fine-grained soil ,organic content and mineral substance deposited in the saline water or fresh water. Muck and mucky soil on the top which represent the soft soil are high water content ,high void ratio ,low strength ,underconsolidated ,high compressibility and easy thixotropy, whose engineering properties are very poor. Mechanical tests indicate that soft soil have high strength .When high-way is constructed on the weak foundation, the bearing capacity of soft soil foundation is low, the deformation and the non -homogeneous settlement of the ground are great, and the settlement stabilize lasts long for several years and several decades .So how to predict the settlement of the roadbed is a problem and one of the hot of the geotechnical engineering. Structure and the deformation of natural soil are very complex and it causes different form the reality if the settlement is calculated by linearity and elastane with the assumption that soil are isotropy and semi-infinite body .With the development of the computer ,it is possible that men can calculate and analyze the model with complexity and precision and create model that considering the properties such as elasto -plasticity structure and particulate to predict the settlement ,so the theory is near to the practice .The way of constitutive model is to look for the mechanical model which is simple and can declare the most important problems with according to the special soil special project and special problems.First in the paper ,the author did some laboratory tests on the marine soft soil in Nansha of Guangzhou ,then analyzed the structured properties of the soft soil as follow :which indicate that marine soft soil belongs to the structured soil with natural structural strength. In the compression test ,the feature of the compression curve of the undisturbed sample and the remolded sample is different obviously ,and the undisturbed sample possesses structural yield stress. In the triaxial shear test ,when the confining pressure is lower than the structural yielding pressure ,nearby the structural yielding pressure ,the curve of the stress-strain stress platform of slightly stress decrease ,which is different from the curve of strain hardening of the remolded soil. It is blamed on the counterweight of the damage of natural structural strength in the course of the compression and the strength increase arose from that the structure is more dense when the soil particle closing up to each other. In the sensitivity test ,the test result indicates that soft soil is sensitivity soil.Modified Cambridge model is a elasto-plasticity model ,which is established early and perfectly. It's assumption is based on seme tests ,basic conception is definite such as critical state line ,state surface and elastic wall with definite geometric and physical sense. So it is taken into account by geotechnical mechanical scholars and geotechnical engineering .Structural Cam Clay Model is improved from the modified cam clay model .Structural parameter b (the index of destructive ) and pyi (virgin structure yielding stress) are added in ,which reflect the structure of the soft soil .The stress-strain relation is as follow: +bJ\+k )ps {M*-Tj)(l + eo)ps (l + eo)pdï¿¡ - f K** 9(l-2v*)ll + e0U M*2-tj2(A*-k*) + bAe\-M*dp.Structure cam clay model has been evaluated by the test in the simple stress path and behaves good result In the paper structure cam clay model is evaluated in the practical project with south high-way vacuum preloading road bed as a example.The effect of vacuum preloading and vacuum combined with surcharge preloading is different .Vacuum preloading is under total stress no changing ,pore water pressure is decreased to increase effective stress and soil deformation presents compression. But effective stress is increased by increasing total stress in surcharge preloading method. Because vacuum preloading make the soil create isotropic consolidation stress, not shear stress ,vacuum load can be applied one time and soil is not destabilization .When vacuum combined with surcharge is employed to strength the foundation shrinkage deformation occurs in the strengthening zone presents, so stabilization problem can be settled in fill-up roadbed.It can be seen that calculation results reflect the tendency of the settlement fundamentally by means of the settlement program programmed by Matlab6.5.But the results have inaccuracy with the monitoring value, in which the maximum is 22% and from analyzing we know that parameters of the sample are inaccurate . From the settlement curve ,we can see that in the period of vacuum preloading , speed rate of the settlement is fast and with the weaken of the vacuum along the deepness and right and left edge ,speed rate of the settlement is reducing .In the period of the vacuum combined with surcharge preloading settlement lasting display in the combined action of the vacuum and surcharge preloading , effective strengthening deepness increase ,so soil can be consolidated well.Settlement on site back-analysis can cover the shortage and deficiency of the test data and also from the parameters back-analyzed ,we can obtain relative accurate results. In back-analysis ,it is essential that appropriate...