Study On The Structural Characteristics And Model Of Marsh Turfy Soil In The East Of Jilin Province

Turfy soil is an organic complex soil which usually found in cold regions, such asChangbaishan, Xing'anling, Qingzang plateau, northwest Sichuan and Yunan provinces, mostof Xinjiang province in China. It forms in areas such as moors, muskegs, pocosins, mires, andpeat swamp forests, where the land is waterlogged and subject to reducing atmosphere withlow temperature to slow down the decomposition of plant residues. It characterized by highcompressibility, low stability, high organic content, great void ratio, high compressibility andcomplex orientation of partially decomposed vegetation matters because of the special geneticenvironment and historic environment. These properties are all related to the internal structureof turfy soil. So it is necessary to study the structure which influenced the physics mechanicsproperty of turfy soil. That can shed new light on essence rule of mechanics.Now developing with a large scale of engineering construction in the regions of sectional frostzone of swamp turfy soil has already become a kind of development trend. Among the varioussoft soil groups, turfy soil is the less common-seeing one. Owing to the high speed ofeconomic development, it is necessary to study the mechanical properties of peat soil whichinfluenced by its structure with the combination the peat soil-related projects. It can reveal theinherent mechanism of peat soil engineering problems and establish a reasonable descriptionof peat soil strength and deformation characteristics of the constitutive model. Through theresearch of engineering problems in peat soil areas, a higher demand on the reliability,stability, endurance and security of project can be met up with, which will largely promote thedevelopment and application of the structural theory of soil.For the above reasons and practical applicability of the study, this paper were studied onstructural characteristics and model of marsh turfy soil under the support of the NationalNatural Science Foundation of China 'study on the engineering geological characteristics ofturfy soil considering time effect in seasonal frost region' and 'Study on the structuralproperty and constitutive relations for marsh peat of seasonally frozen region', etc. fourtransportation construction projects in Jilin Province. The research contents as follow: The study began with the field survey of systematic study the formation and the sedimentarycharacteristics of turfy soil. It discusses the different between turfy soil and peat soil aboutmaterial composition, physical mechanics character and spatiotemporal nature. Through thecharacterization study on the organic content of turfy soil as well as its physical andmechanical indexes, the mechanism of mechanical effects resulting from the compression andstrength properties of turfy soil with different organic contents is analyzed. Through the testof decomposition degree analyze the changing regularity of decomposition. Contrastexperiments have been done on undisturbed and remolding turfy soil samples. From the testthe structural characteristics and varying regularity of load can be obtain. Consider buildingBP network constitutive model and RBF network constitutive model which based on theinfluence of strength with decomposition degree and combined with the test data. Simulationresults show that these canbe effectiveness and good generalization ability.1. Study on the distribution, formation conditions of marsh turfy soil in the east of JilinProvince, then systematically discuss the differences between marsh turfy soil in the east ofJilin Province and peat soil at Dianchi region of Yunnan province:1) Compare to the peat soil, carbon and sulphur content of turfy soil is low but oxyhydrogenand nitrogen content is slightly higher. Because of the formation time of the turfy soil isshorter and degree of carbonation is weaker. The mineral content of turfy soil has abundantquartz which obviously different from peat soil. The mineral content of peat soil consists ofmainly illite and chlorite, without kaolinite.2) Turfy soil has higher organic matter and low degree of decomposition than peat soil. Thereare a lot of unrotten plant root and plant fiber in the turfy soil. The reticulate structure offibrous root and stem leat distribute in disorder. So the turfy soil has aerial structure and largepores development. The plants in peat soil are fuzziness which mainly decomposed. Thetypical structures are cellular structure, aerial structure and globular structure.3) Higher the moisture content, porosity, compressibility, permeability and the lower specificweight are due to the lower decomposition degree and higher organic content of turfy soilthan peat soil. Comparison the shear strength between peat soil and turfy soil, the internalcohesion is relative decline and the internal friction angle is relative increase. It is because that there many plant residue in the turfy soil.2. The organic matter effect on physical and mechanical properties of turfy soil are as follows:1) Density and particle density of turfy soil linearly decrease when the organic contentincreases, whereas the moisture content increases linearly. The void ratio increasesexponentially with the organic content. The organic content has significant influence onengineering properties of turfy soil. The compressibility increases with the organic content.The turfy soil is composed of a large amount of partially decomposed plant cellulose, whichincreases the organic content and accordingly the compressibility.2) The cohesion of the turfy soil, mainly associated with colloids and clay particles, ispositively related to the organic content. The partially decomposed plant cellulose providesadditional strength intertwining and constraining the lateral displacement of turfy soil.Secondly, there are many undecomposed plant residue in turfy soil. They are criss-crosswhich produced a part in tractive force.3) The permeability decreases when the organic content increases. With the same organiccontent, the horizontal component of the permeability is less than its vertical component.3. The structure characteristics and the structural strength of turfy soil are studied by the testsof constitutive property, the test results are as follows:1) The lower degree of decompose is, more loose the structure of turfy soil own and the morethe void ratio and moisture content is. With the degree of decomposition increasing, porespace is filled with particle material and produces synthesizes inorganic/organic compositeswhich made the void ratio and moisture content decrease, density and particle densityincrease.2) With the degree of decomposition increasing, compression curve of undisturbed soilsamples and remolded soil samples all gradually change slower. It is because that with thedegree of decomposition increasing, the structure of turfy soil changes from aerial structure toflocculation structure gradually, and the structure gradually become denser, the compressionbecome lower than before. The compression curve of undisturbed soil samples have a tippingpoint basically under the same degree of decomposition but the compression curve ofremolded soil samples don't. With the increasing of the loading, the pore space and compressibility of undisturbed soil samples gradually approach the index of remolded soilsamples.3) The deviatoric stress and pore water pressure of undisturbed soil samples and remolded soilsamples increases as the increasing of the axial strain under the same confining pressure. Thedeviatoric stress and pore water pressure of remolded soil samples are less than the index ofundisturbed soil samples. All of these because the constitutive property of undisturbed soilsamples are superior to the constitutive property of remolded soil samples. The deviatoricstress-axial strain curve of undisturbed soil samples and remolded soil samples are allappeared as strain hardening, some undisturbed soil samples showed strain softening in thedifferent confining pressures. With the degree of the decomposition increasing, the deviatoricstress and pore water pressure decreased.4) The shear strength index is less than other soils. The cohesion general has little changed,but angle of internal friction has obvious changed. Total stress indices of remolded soilsamples are always smaller than the indices of undisturbed soil samples but differ ofcorresponding effective stress indices between remolded soil samples and undisturbed soilsamples are not obvious. Secondly, the shear strength index has the very big connection withthe degree of decomposition such as the lower degree of decomposition made larger the angleof internal friction. It is prompted by the plant residue of turfy soil acts as reinforced duringthe shearing. The destructed form on shearing mainly divides into two modes such as singleshearing zone and protruding in the middle of samples. They are existed in undisturbed soilsamples and remolded soil samples.5) Structure strength of turfy soil is mainly affected by confining pressure, degree ofdecomposition and other factors. The constitutive property of turfy soil increases with thepressure then decreases under different degree of decomposition. The curves of constitutiveproperty all have crest value. Structural strength changes from strain that shows as strainsoftening. With the degree of decomposition increasing, the constitutive property andstructural strength of turfy soil decrease.4. In order to fully show the effects of structure of turfy soil, based on the theory of artificialneural network, a constitutive model of turfy soil is achieved. The results follow as: 1) Based upon artificial intelligence method, adopts turfy soil triaxial test data trainingintelligence network, self-act generating network parameter, and gets reinforced soil BPneural network constitutive model, RBF neural network constitutive model, thus avoiding thedifficulties of mathematical modeling determine function parameter.2) The BP neural network constitutive model of turfy soil has good fitting precision and goodgeneralization ability. Through comparing and analysis the prediction accuracy and the testdata, it can fully describe the influence of the turfy soil.3) Simulation results show that the RBF neural network constitutive model can achievepreferable effect with the different degree of decomposition. Under the training and checkingprocess the curves of model simulating are in accordance with experimental curves which candescribe the law of strength and deformation.4) RBF neural network constitutive model of turfy soil learning is much faster than BP neuralnetwork constitutive model. And the training error of RBF neural network constitutive modeis smaller than BP neural network constitutive model as well. This is mainly up to the RBFneural network using the least square method to get threshold value and weight, but BP neuralnetwork using vice gradient descent method to regulate weight.