Experimental Study Of Permeability Characteristics Of Remolded Loess

With the rapid development of urbanization, the problem that there is not adequate space for population expansion is quite serious in mountainous cities. In Northwest China, especially in the Loess Hilly Regions, the development of local towns is severely restricted by the limitation of land resources. Hence, Yan’an located in the Loess Plateau carried out the land creation projects by cutting hills and filling valleys to expand plain space for city construction. However, this kind of geotechnical engineering projects will change regional hydrogeological conditions greatly, including the infiltration characteristics and the conditions of groundwater recharge, runoff as well as discharge. The saturate hydraulic conductivity of loess is a vital parameter to evaluate the permeability of study area, and has an important impact on the groundwater environment and stability of construction site after the project. How the saturate hydraulic conductivity of loess changes after the project and in different compaction conditions is an important content of engineering reliability evaluation.This thesis, taking the land creation project in Yan’an as a case study, investigated the change rule of saturate hydraulic conductivity after the project and in different compaction conditions systematically by laboratory permeability experiments and theoretical analysis. The microstructure of loess with different stratum and engineering conditions in study area was observed by scanning electron microscopy(SEM), and the results were used to explain the change rule of saturate hydraulic conductivity. The following achievements were obtained:(1) According to the loess physical properties obtained by routine soil tests, the compaction water content and initial water content designed in laboratory were equal to the optimum moisture content used in project. Then the range of dry density and loess lime ratio were designed for soil treatment by combining engineering practice with economic efficiency. After conducting constant-head and varying-head permeability test theoretically using TST-55 type of penetration, we raise the point that the data of constant-head test is more reliable. Moreover,it is verified that our permeability test is fit to the condition of Darcy law by setting different hydraulic slopes to test.(2) The mathematical formulas of saturate hydraulic conductivity and dry density as well as porosity have been obtained by fitting analysis. The results show that, saturated hydraulic conductivity of remolded loess has significant positive index correlation with porosity, and significant positive index correlation with dry density. Besides, by comparing the hydraulic conductivity of intact loess and remolded loess, we find that the saturate hydraulic conductivity of remolded loess is higher than that of intact loess.(3) Through analyzing the permeability of lime treated loess from dry density and porosity, we find the two results have some similarities as well as a bit differences. The similarities: when the dry density or the degree of compaction is same, the saturate hydraulic conductivity of lime treated remolded loess decreases firstly and then increases with the increase of lime-adding ratio. Besides, when the lime-adding ratio is 9%, the saturate hydraulic conductivity of lime treated loess can be minimum. The differences: when the dry density is same, the saturate hydraulic conductivity of the lime treated loess with any lime-adding ratio is smaller than that of untreated loess. However, when the degree of compaction is same, only the saturate hydraulic conductivity of lime treated loess with 9% lime-adding ratio is smaller than that of untreated loess. Hence, we suggest using 9% lime-adding ratio to treat the base of building or roads which are demanded low permeability, at the same time, the degree of compaction should reach 90%.(4) Through combining the Electron Scanning Microscope test with Image-Pro Plus(IPP), we analysis the microstructure of remolded loess from both qualitative and quantitative point of view. The analysis demonstrates that the tendency of the ratio of the large and medium pores in number and area is consistent with the saturate hydraulic conductivity of remolded loess. The lager pores volume is greatly related to the saturate hydraulic conductivity.Overall, this thesis provides a number of basic data for the loess permeability study, and lays a solid foundation for studying the change of groundwater environment of Yan’an in the future.