Experiment Study On Expansion Constitutive Model Of Expansive Soil In South-To-North Water Transfer Project

In order to provide scientific guiding on disaster management of expansive soil in the project of south-to-north water transfer, firstly, studied the fundamental characteristics of Nanyang weak expansive soil and medium expansive soil and Handan strong expansive soil.The pilot project are:①the basic properties of index test;②Experimental compaction;③expansionExperimental reduction;④mechanical characters;⑤mineral composition analysis;⑥Zeta potential test;⑦cation exchange capacity test.through the basic physical tests, obtained the three kinds of expansive soil's fundamental characteristics:natural moisture content, natural density, specific gravity, natural dry density, void ratio, saturation, liquid limit, plastic limit, plasticity index, silt content, clay content, rubber content and the average range value; through the compaction tests of three types of expansive soils, obtained the maximum dry density and the optimum moisture content of light and heavy compaction test, and the maximum dry density of heavy compaction test is greater than the maximum dry density of light compaction, the optimum moisture content is opposite; through the analysis of relationships between free expansion and boundary water content, particle size and clay content, the conclusions obtained as follows:plastic limit, plasticity index, clay content, colloidal particle content are proportional to free expansion, and powder particle content is inversely proportional to free expansion; under different initial conditions of Nanyang and the weak expansive soil and Handan strong expansive soils,swelling pressure were test. the conclusions obtained as follows:under the same dry density, the greater the initial water content,the smaller expansive force, the the greater initial water content,the smaller the expansive force.under the condition of same initial water content, with density increases also increases the dry swelling pressure; shrinkage test of Nanyang weak expansive soils was conducted under the same initial moisture content but different compactedness, obtained under the same initial water content conditions, with the degree of compaction increase, the contraction of indicators show the following trends:shrinkage limit water content increased, contraction coefficient decreased, linear shrinkage decreases, the volume shrinkage rate decreases. By shrinkage test found that the process of expansive soil was divided into three phases:the first stage is the linear shrinkage, the second stage is slow contraction phase, the third stage is stable phase of contraction; CD triaxial tests were conducted on the original sample of Nanyang weak and medium expansive soil and remodeling Handan strong expansive soil samples,obtained the indicators of effective stress, deviation stress～axial strain curve and volumetric strain～axial strain curve.The differences of intensity parameters and curves arecaused by the differences of expansive soil mineral composition, grain size composition, mechanical conditions and granular soil, non-homogeneity, anisotropy and so on. It's the result of both internal cause and external cause; through X-ray experiment, obtained the mineral composition and content of each expansion soil, The mineral composition of expansive soil is mainly the clay mineral,which is more than 50% of total amount of mineral, the stronger the expansion soil, the greater the total amount of clay and montmorillonite and illite content; Through the zeta potential test,obtained Zeta potential is proportional to the strength of expansion, due to Zeta potential and water film thickness of expansive soil was positively related, so it can be said, with water film becomes thicker and expansion soil becomes stronger; through cation exchange capacity tests,obtained cation exchange capacity is related to expansive strength,the amount of clay minerals (mainly montmorillonite, illite stone) and Zeta potential.Through analysis of the test results above, an initial understanding of the three basic characteristics of expansive soils is obtained, at the same time, noted the swelling strength of expansive soil (where the free expansion as a standard) is related to many physical and chemical indicators of their own. Mainly displays in:expansive soil with strong free expansion, often with a large liquid limit, plastic limit, plasticity index, clay content, rubber content, content of montmorillonite, illite content, cation exchange volume, Zeta potential and water film thickness, these indicators can also be characterized by expansion from the strong and weak from another side, which help understand formation mechanism of expansion characteristics of expansive soil from micro-level.Based on the full knowledge basic characteristics of the three expansive soil, carried out loaded swelling sate test under the conditions of lateral restraint (K0 state), discussed relativity between loaded swelling sate and initial water content, compaction degree, overlying load, derived the empirical formula of constitutive model of expansion soil, preliminaryly discussed the application of empirical model in the project. The conclusion of this experiment are:under the same degree of compaction, with the water content increases, the swelling ratio decreased;under the same initial water content, with the degree of compaction increases, the expansion rate gradually increased; under the same degree of compaction and initial water content, with the load increases, the swelling ratio decreased.when the load changes from 0 to 6.25kPa process, swelling strain of the three types of expansive soil decreases rapidly,then with the load increases, the expansion strain decreases slowly; under the same degree of compaction in the curve, of" loaded swelling rate～load",the initial water content curve and the abscissa intersection hour values rather large, which a numerical point of intersection is obtained by loading the expansion force of expansion, we can see, in the same degree of compaction, the greater the initial water content the smaller the swelling pressure; deformation of Handan stron expansive soil can be divided into three obvious stages, the first stage is rapid expansion, in a very short period of time, the amount of expansive can reach a great value; the second stage is transformation for the expansion phase, in this phase, the amount of swelling rate of increase slowly; the third stages is stable phase, in this stage, the amount of expansive soil swelling is leveled off, basicly no change.Nanyang weak expansive soil and Nanyang medium expansive soil and Handan strong swelling of expansive soils are charged with the fact water content, compaction, the law of the overlying load respectively, obtained constitutive empirical model of each expansive soil, and foreach empirical formula, laboratory tests have been carried out to prove the the rationality of derived formula. Obtained by comparing the three empirical formulas, presented their unified expression: Where:δep-loaded swelling rate (%); P-compaction;ω0-initial moisture contentσ-cover load; P0-equal 1 kPa; A, B, C, D, E, F,G,H-test parameters.The coefficient of in the constitutive model-A, B, C, D, E, F, G, H -is different due to the different external state (the initial water content, pressure solidity, load, etc.) and internal factors (mineral composition, clay content, cation exchange capacity, Zeta potential, free expansion, limit water content, etc.).Based on mathematical calculation, obtained the range of independent variable of each constitutive model variables (initial moisture content, compaction, overlying load).Discussed the application of the formula. Derived thickness formula of expansive soil slope, thickness of concrete lining of canal slope formula, expansive soil swelling potential energy formula, and the expansion formula for estimation of the amount of uplift quantity.Through the triaxial tests of Nanyang medium expansive soil and Handan strong expansive soils, obtained the following conclusions:volume strain increases with the increase of volume stress, which indicates that the volume stress is inversely proportional to volume strain. When degree of compaction is 98%, initial moisture content is 20.4%, empirical formula between volumetric strain and volume stress of Nanyang medium expansive soil is:εv=-4.7179ln (p/P0) +22.801. When the degree of compaction is 95%, initial moisture content is 26.5%, empirical formula between volumetric strain and volume stress of Handan strong expansive soil is:εv=-6.297ln (p/P0)+30.899.εv is the volumetric strain,p is volume stress, P0=1kPa. Swelling volume strain decreased with the decrease of water absorption, which indicates that the water absorption is proportional to volumetric strain. When the degree of compaction is 98%, initial moisture content is 20.4%, empirical formula between volumetric strain and water absorption of Nanyang medium expansive is:εv=1.333Δω-8.1984. When the degree of compaction is 95%, initial moisture content is 26.5%, empirical formula between volumetric strain and water absorption of Handan strong expansive soil is:εv=1.7025Δω-15.115.εv is the volumetric strain,Δωis water absorption. Expansive soil water absorption gradually decreases with the volume increases stress. Finally, through multiple nonlinear least squares regression analysis, obtained the empirical formula between the volume and volume strain stress, water absorption of Nanyang medium expansive soil: Obtained the empirical formula between the volume and volume strain stress, water absorption of Handan strong expansive soil: These two where:εv-volumetric strain,p-volume stress,Δω-water absorption, P0=1kPa.Based on Einstein (1972) and Wittke (1976) proposed hypothesisof three-dimensional expansion of expansive soil, tried to establish the link between K0 constitutive model and triaxial constitutive model, finally, use Visual Basic 6.0 programming language to write software according to the derived formula in order to facilitate the practical application.