| Red soil is one kind of the soil which widely distributes and almost covers the southeast areas where have developed economy in our country. With the rapid development of economy, red soil ---the main soil in the South is widely used as construction material and foundation. In fact, the bearing capacity of red soil foundation is often acquired as common clay soil,which ignores the potentiality of red soil foundation and causes great waste in engineering construction. So it's necessary and urgent to study red soil profoundly.It is well known that there is a high strength in red soil because of the free ferric oxide which results in a strong structural connection. Therefore, to understand the special engineering geology feature of red soil, the studies on formation condition, existing form and content of free ferric oxide are important parts.At present, though many studies on red soil have been done, there still are some problems. First, emphasis was put on the comparison of properties between red soil and red soil from which free ferric oxide was removed, and no study was done on the properties of red soil with different ferric content. Secondly, not some single but synthetic environmental factor was the main study direction in influence factor of free ferric oxide. Additionally, red soil is one sort of structural soil, however, the study on its structural effect still remained in the stage of description about the engineering geology feature, which makes there is a blank of constitutive model of regional special soil such as red soil.So based on granite residual red soil in Guangdong, the study on the effect of free ferric oxide and environmental influencing factor to red soil is carried on in this paper on the basis of indoors mechanical tests, chemical tests, quantitative analysis of microstructure parameters and the extracting of microstructure fractal parameters, in order to search the function of free ferric oxide in the progress of red soil strength formation and increase and find the cementation mechanism of free ferric oxide. At the same time, micro structural feature of samples with different free ferric oxide content is studied, and the relationship between free ferric oxide content and microstructure fractal dimension is established. Finally, disturbed state model considering free ferric oxide content and damage model based statistic theory of red soil are build and testified through indoor triaxial shearing test.Following study is done in the paper:First of all, the function of free ferric oxide in red soil is studied and the properties of red soil samples with different free ferric oxide content are determined. Results show that:○1 with the increase of free ferric oxide content, there is an enhancement in aggregate with larger particle diameter and reduction in the content of clay particle and in the dimension of grain size distribution.○2 the higher the free ferric oxide content in soil is, the lower pH value is, the larger specific area is, and the lower the total content of cation exchange is. What's more, there is an elevation in plastic limit of samples, while a decrease in plastic index I p. Additionally, the cohesion force and the angle of internal friction present an increasing tendency, which leads the enhancement of shearing strength and a better mechanical property.○3 macro mechanical property of red soil with different free ferric oxide content are shown as the followings: there is structural yield stress in original sample, which increases with the content of free ferric oxide. And there is an obvious inflection point in e ? logp curve because of the existence of structural yield stress. On the other hand, there is great difference in consolidation coefficient of samples before and after structural yield stress, causing the strength enveloping curve present a fold line type. Whereas the e ? logp curve of remolding soil basically is a line, and its consolidation coefficient is a constant.Secondly, environmental influencing factors of free ferric oxide in red soil are studied, and the influencing mechanism of free ferric oxide is investigated from the aspects of environmental temperature and pH value. From the research, conclusions can be drawn as followings:○1 with the increase of temperature, the silt content in soil increases, cation exchange capacity decreases and mechanical property turns better under some pH.○2with the increase of pH, the particle diameter turns smaller, the dimension of grain size distribution becomes larger, and there is a decrease in aggregating degree of soil grains as well as an increase in cation exchange capacity and specific surface area under some temperature.○3 environmental temperature and change of pH result in the change of free ferric oxide content in soil, leading to the alteration of red soil engineering geology property.Thirdly, qualitative, quantitative and fractal features analysis of microstructure of red soil is done. Results show that:○1 free ferric oxide exits in red soil in terms of"integument"and"bridge", with the increase of free ferric oxide content and temperature and the decrease of pH value, the microstructure of samples turns compact.○2 with the increase of free ferric oxide content, there is a reduction in shape fractal dimension of grain in microstructure unit and a decrease in direction fractal dimension of structure unit, on the other hand, there is an increase in shape fractal dimension of microstructure pore, but a reduction in distribution fractal dimension of pore.○3 red soil is mainly composed of sand and silt which constitute the skeleton of soil grains in which structural connection formed of free ferric exits. Physical model of microstructure can be obtained by abstracting the microstructure of red soil. The model can be described as followings: red soil with higher structural strength can be taken as a multi-layer frame structure which is restrained in side direction. In this structure, some grains with larger size such as gravel, sand and silt form vertical pillar through cementation, while some single or few silts inlaying vertical pillars form beam. Stiff connection caused by cementation of ferric oxide exits in vertical pillars and beams. This physical model can be used to explain why red soil in nature shrink and large scale crack occurs with the lost of water.From the analysis mentioned above, the main mechanism of free ferric oxide influencing red soil strength can be drawn as followings:○1 the function mechanism of free ferric oxide in red soil can be attributed to physicochemical function and structural connection function.○2 the physicochemical function of free ferric oxide mainly reflected from the aspects of grain size, pH value, specific surface area, cation exchange capacity and plasticity of red soil. The structural connection function of free ferric oxide is demonstrated in micro and macro aspects. Free ferric oxide connects smaller particles into larger aggregates in terms of"integument"and"bridge". The more free ferric oxide soil contains, the stronger of cementation effect is. From the aspect of macro shearing strength indexes, with the increase of cementation effect of free ferric oxide, the shearing strength of soil becomes larger.○3 influence of temperature and pH value on the property of red soil can be attributed to the change in content and form of free ferric oxide which caused by change of temperature and pH and leads to the change in red soil property.Finally, disturbed state conception and damage theory are introduced into the establishment of stress-strain model of red soil. During the progress of constructing model, free ferric oxide content is taken into account in the first time, based on which disturbed state model and damage model on statistic theory of red soil are build. What's more, the built models are testified by triaxial shearing test. Study shows that:○1 in disturbed state model, there is a positive correlation between K,G, Z d and free ferric oxide content, and a negative correlation between Ad and free ferric oxide content.○2 in statistic damage model, there is a positive correlation between E,qu,F0 and free ferric oxide content, and a negative correlation betweenν,m and free ferric oxide content.○3 the construction of disturbed state model and damage model puts forward a feasible method in stress-strain description of structural soil. These two models both have merits and drawbacks: in disturbed model, the interaction between micro cracks and micro mechanical model needn't be taken into account in analysis progress, but there are more parameters need to be solve. Damage model based on statistic theory with fewer parameters isn't restrained by micro-mechanism, and mathematic tool are sufficiently used in solving problems. However, parameters in Weibull function are obtained through tests fitting, which weaken the theory value of model in some degree.○4 comparison between theory calculation and test results shows that theory calculation curve got from two kinds of models both can reflect the factual stress-strain character of red soil. The constitutive model of red soil taken account of free ferric oxide content is more convenience in modeling the deformation features of red soil with different free ferric oxide content and similar other properties.
|
PDF Full Text Request