A Study On The Behavior Of Organic Matter In The Progress Of Solidifying Soft Soil By Cement

Recently, with the development of economy and construction in ourcountry, more and more large scale projects are built on soft soil foundation,in which soft soil engineering problem can occur because of the specialcharacters of soft soil. The technique of cement-soil composite foundation hasbeen widely used in the consolidation of soft soil foundation as a method ofsoft soil engineering technique. Since origin and constituent of soft soil arevarious, there is different in the consolidation effect of cement soil, evenfailure stabilization exited in some projects. According to the study ondeformation and strength, organic matter in soft soil plays a role in theprogress of consolidation;it can impede the hydration of cement, whichinfluences the formation of cement soil strength. Therefore, studying on thebehavior of organic matter in the progress of consolidating soft soil withcement and analyzing all the functions of organic matter will greatly benefitfor engineering practice and improving consolidation method.At present, though many studies on organic matter factors influencingstabilization effect and damage model of cement-soil have been done, therestill are some problems. First, there are lots of factors which influence theeffect of consolidation using cement, however, the conclusion that it isdifficult to consolidating soft soil with high organic content using cement wasdraw from projects, in which other influencing factors weren't taken intoaccount. Secondly, in the study on improvement of consolidation effect withextra additives, much emphasis is put on the hardening result, little on themechanism, which results in the blindness of construction. On the other hand,as to the study on mechanical effect of consolidated soft soil, the mechanismof organic matter is still in investigation, although preliminary understandingsabout hardening mechanism of soft soil foundation have been achieved.So the properties of cement soft soil in Lian Yungang and Guangzhou arestudied in this paper associated with the project of "study on the influence oforganic matter in the progress of marine soft soil foundation consolidation"sponsored by the national natural scientific fund. The behavior mechanism oforganic matter in the progress of strength formation of cement soil is studiedon the basis of indoors mechanical tests, chemical tests, quantitative analysisof microstructure parameters and the extraction of microstructure fractalparameters. The fractal features of microstructures of samples are discussed,the damage variable evolution equation and elastic-plastic damage constitutivemodel containing organic matter factor are established, and the model is testedand verified according to laboratory tests.Firstly, soft soil in different areas is consolidated by cement. Theproperties of consolidated samples show that: ① soft soil in different areaspresents different strength increase characters after hardened with cement. Thestrength formation of cement soil is mainly influenced by granularitycomponent and the content of organic matter. The more clay particle andorganic matter in soil there are, the more difficult it is to form strength ofcement soil. ② microstructure features of cement soil in different areasmake an additional remarks to the strength characters. The more clay particleand organic matter in soil there are, the more harmful to the formation ofcrystal materials, the smaller and fewer crystal materials are, and the looserthe structure of cement soil is.Secondly, soft soil with different organic content is stabilized withcement. In this study, soil sample is from one area, and its organic content ischanged by adding some organic materials into it, in order to exclude theinfluence of other factors. Results show that: ① with the increase of organiccontent, the cohesions of cement soil increase slightly, internal friction anglesreduce evidently, whereas shear strength and unconfined compressive strengthpresent fall down tendency. That is, the higher organic content is the worse themechanical property of cement soil is. ② organic content influence themicrostructure features of samples. The higher organic content is the lowercrystallization degree is, the smaller particle is and the looser structure is.Thirdly, soft soil with high organic content is consolidated with cement.According to the mechanism of organic matter, different extra additives arechosen to harden high organic content soft soil together with cement. And themechanical indexes, chemical property, microstructure features andgranularity component are determined, in order to prove the effect of extraadditives in improving cement soil strength and prove the different influenceof different extra additives. It is observed that: ① it is difficult using cementonly to stabilizing soft soil with high organic content, using proper extraadditive can enhance the cement soil strength, and different extra additive playdifferent role. ② microstructures of samples make supplementaryspecification on the consolidation effect of different program. In sample withbetter consolidation effect, there is more compact structure, more small andmiddle pores and fewer large pores. ③ chemical property and granularitycomponent make a additional remarks on the consolidation effect of differentprogram. In sample with better consolidation effect, there is a more throughhydration of cement, a larger pH value and more commentated grains.At the same time, the fractal features of structural unit and pore inmicrostructure of cement soil with different organic content and cement soilconsolidated by different extra additives are studied, and the macromechanical properties of samples are combined with the microstructures.Results show that: ① according to the analysis of microstructures of sampleswith different organic content, the fractal counting box dimension of grain andpore distribution have close relative to the organic content. The higher organiccontent there is in soil, the lager the fractal dimension of grains distribution is,the lower the granulation degree is and the worse the strength of cementstabilized soil is. ② according to the study on the microstructures of samplestreated with different extra additives, fractal dimensions of structural units andpores distribution both are smaller than that of samples without extra additives,which indicate that the addition of extra additive improve the consolidationeffect of cement, enhance the strength and granulation degree of treated soil,make the structure more compact and lead to a better mechanical property.What's more, different extra additives play different role, there is the bestmechanical property and the smallest fractal dimension in sample with crystalcalcium sulfate. ③ there is self-similar fractal feature in the microstructureof cement treated soil. Studing on the shape distribution characters ofstructural unit and pore by the means of fractal theory can supply favorablegrounds to the analysis of mechanical property of soil mass.On the basis of analysis mentioned above, the mechanism of organicmatter influence the strength formation of cement treated soil can beconcluded as follows: ① when there is a high content of organic matter,because of the more strong cementation of organic matter, the cohesion islarger in the cement soil;On the other hand, because of the impediment tohydration and decomposition effect to hydration production of organic matter,fine grains can't be aggregated into larger particles, causing smaller internalfriction angle in sample with high organic content. ② when there is a highcontent of organic matter in sample, because of the impediment to hydrationand decomposition effect to hydration production of fulvic acid andsedimentation to calcium of humic acid, the unconfined compressive strengthof cement treated soil is low. ③ the component and content of soluble saltand cation exchange capacity of samples change with the content of organicmatter. The higher organic content is the less thoroughly hydration proceedand the larger cation exchange capacity is. ④ the main substance react withcement hydration productions and extra admixtures is loosely combinedhumus in organic matter, so the content of loosely combined humus in soilplay an important role in the stabilization of soft soil foundation. Additionally,the content of loosely combined humus compositions influences thestabilization effect of extra admixtures.At last, on the basis of elastic-plastic damage model of cement treatedsoil established by Tong Xiaodong, choosing reasonable damage valuableevolution equation, develop the elastic-plastic damage model of cementtreated soil taking the organic matter factor into account, and the model istested by laboratory test. Study show that: ① the deformation progress ofcement consolidated soil can be divided into four stages: a. closing stage ofcracks and pores;b. linear elastic response stage;c. stable expansion stage ofmicro deficiency;d. unstable expansion and crack through stage. During thestage of linear elastic response stage, the stress in cement treated soil isn'tlarge enough to make the micro deficiency in materials evolutes, that is,damage doesn't occurs;during the stable expansion stage of micro deficiency,the stress-strain curve of cement soil begin go bend, with the appearance ofplastic deformation, when damage occurs and evolutes. Therefore, elasticproportional limit strain is the strain threshold value of cement treated soil. ②model parameters of different samples have close relative to the organiccontent. There is a negative correlation between K,n,α and organic content,and a positive correlation between εd and organic content. The establishedelastic and plastic damage model of cement soil which takes organic contentinto account is more convenience in model the deformation features of cementsoils with different organic content and similar other properties, so we canhave an more clearly understanding to its mechanical properties.