| Expansive soil is a kind of high plastic clay, it was classified as special soil for itscharacteristics of strong swell-shrink, developing fissures and over consolidation. Thecharacteristic of water swelling and drying shrinkage has caused great difficulty and loss toexpansive soil area. For half a century, different researchers have explained itsswelling-shrinkage mechanism, and also came up with kinds of physical and chemicalmethods to reinforcement the expansive soil foundation, but non of them is very perfect in apractical application.The chemical modification of expansive soil was studied, based on theswelling-shrinkage mechanism of expansive soil and the theory of adsorbability andmodifying effects of some organics. They study choosed four common inorganic matterssodium silicate(Na2SiO39H2O), calcium chloride(CaCl2), aluminium chloride(AlCl36H2O）and calcium oxide(CaO) for the chemical experimental, and there were two organicmatters:cationic-type polyacrylamide(CPAM) and hexadecyl trimethyl ammoniumbromide(HTAB). The experimental mixed up the inorganic matter with the organic matter totreat the water swelling and drying shrinkage characteristic of expansive soil. Before thesubject experiment, the test method of free swelling ratio was improved first, referring to theGeotechnical test specification. Then the free swelling ratio was used to measure themodification effect of modified soils by different kind and different quantity of the chosenchemical materials. The best modifier was chosen to modify the expansive soil samples, andthe deformation and strength characteristics of modified soils was tested. Finally, the studycompared the soils treated by the chosen best modifier with the soils treated by5%calciumoxide. Study results are as follows:1) On the test method, the study of the improvement of free swelling ratio testshows that: the preparation the soil sample and experiment environment both can make a bigdifference to the test result of free swelling ratio. The grain-size composition, environmenttemperature and the degree of drying all have an effect on the final test result, especially theeffect caused by grain-size composition. Some suggestions was proposed according to theresearch results: first, control the grain-size composition of soil samples by classifying the soilto three grain grades,such as0~0.1mm,0.1mm~0.25mm and0.25~0.5mm, make the soilsamples consisted of the three grain grades by the ratio of0%~20%：20%~50%： 20%~50%.Second, make the contrastive test under the same temperature. Third, cool thedrying sample in a drying oven to make sure its dryness. Finally, the results indicates that thecomparability of the test by improved free swelling ratio method was greatly increased.2) On the sifting of modifiers, the experiment treated the expansive soil by different kindof inorganic materials, and it shows that the modification effect of Na2SiO39H2O is betterthan CaCl2and AlCl36H2O. The free swelling ratio test results show that the bestmodification combination of inorganic material and organic material is Na2SiO39H2O andCPAM. The modification effect of HTAB alone is also good. Adding Na2SiO39H2O of2.5%and CPAM of0.5%or HTAB of0.5%alone to the expansive soil from Ankang can make itsatisfy the engineering need.3) The deformation and strength experiment of modified soils shows that themodification effect was good. First, on the deformation, the test result of linear shrinkage ratio,volume shrinkage ratio and free load swelling ratio of expansive soils modified by Na2SiO3 9H2O of2.5%and CPAM of0.5%was greatly improved compared with the originalexpansive soils. Second, on the strength, the cohesion of modified soil showed a markedimprovement, and the modification changed the strain softening characteristic of the originalexpansive soils. Although the cohesion of expansive soils modified by Na2SiO39H2O andCPAM is smaller than the soils modified by CaO of5%, the later soil has the tendency ofstrain softening, so the former still has a great advantage.4) On the study of modification mechanism, the metathesis of inorganic cations withhigh valence do have the ability of reducing the hydrophilia of expansive, but this kind ofeffect is very weak compared with the condensation of some inorganic materials and theadsorptive property and surface modification of some organic materials. Moreover, theexistence of inorganic cations can make an influence on the clay adsorbing of CPAM, also,the adsorbing work of organic polymer can interact with the work of Lattice replacement andagglomeration of inorganic ions. In addition, the Surfactant’s modification to clay particle isvery effective to the improvement of the expansive soils’ hydrophilia, and, the study showsthat the modification effect of HTAB is remarkable. |
