| Residual red soil is one kind of special soil, it is developed throughout the tropical zone and subtropical belt. With the durative and rapid development of our country economy, large quantities of industrial civil buildings and various kinds of grade highway, railway, port, are being built. As the main kind of soil, residual red soil has been widely used to be constructing material and foundation. But during the course of the practice engineering residual red soil often be regarded as the common clay, and the index of ground bearing capacity is always bellow its real bearing capacity. So it forms the waste of the high real bearing capacity. The real higher bearing capacity is buried, at the same time, money is wasted. Then, deeply study on the residual red soil of the special geological engineering properties is demanded thirstily. Then, why has red soil higher ground bearing capacity? The reason is residual red soil has higher structural strength. So, for study the red soil deeply, must set on its structural strength. Previous study had proved that the special structural strength of red soil came from the firm agglutination of free ferric oxide. Also for the beneficiation of the free ferric oxide, red color appeared in soil, so call this kind of soil "red soil". So, for study the special engineering properties, on the basic is to study the formation, the content and the inversion of free ferric oxide and to analysis the effect on its mechanical property of red soil body. Then what on earth effect on the formation, the content and the inversion? The main factors include the temperature and the value of pH. When the value of pH is the same, temperature occupies the main portion. So, here main emphasis on the factor of temperature, through a series of tests of all kinds of index, analysis the mechanic of the effect of temperature. And then achieve the objects of improving the practice of residual red soil and supply the basis for deeply study. This paper adopts the method of comparison analysis. In all, made 12 kinds soil samples, with the previous soil, is 13 kinds of soil samples. a series of tests are done on these 13 soil samples, from the results, can analysis the difference, and find out the mechanics the way that temperature effecting on the engineering properties of residual red soil. Fist analysis the results of the basic properties of the original soil, then test the content of free ferric oxide, the content of the amorphous free ferric oxide, the content of soluble salt, content of cation exchange and done triaxial test for getting the mechanical index. Then analysis results of the 13 kinds of soil. Following these is the qualitative and quantifying analysis of the microstructure picture. At late, analysis all these difference, draw a conclude on its mechanics. For analysis the inverting forms and inverting quantity of the free ferric oxide, make the 6 kinds of soil samples be mixed with amorphous free ferric oxide man-induced from outside. Then contrast the difference of the results of the mixed soil samples and the not, coming out the inverting course and analysis the quantity that inverted from amorphous free ferric oxide. From all these analysis, drawing the following conclusions: 1. The engineering geological properties is effected by temperature, with the increase of temperature, the particle size compared bigger, the aggregation extent higher, the plasticity index higher, the content of free iron oxide increased, and also the internal frictional angle in the triaxial test bigger; 2. The engineering geological properties is effected by temperature, but when the temperature is 30℃, the effect is not apparent, when temperature achieved 40℃, some deference can be found, while it becomes 50℃, can find apparent deference; 3. When temperature is the same, the number of days that the soil samples is heated act. The longer the sample being heated, the bigger particle size, the bigger plasticity index, the higher content of free iron oxide, the lower cation exchange content, and the smaller internal frictional angle in the triaxial test. That is to say the sample being heated for 15 days has better engineering geological properties; 4. With the scanning electronic microscope (SEM), study on the microstructure of residual red soil. From the results, can find that with the increase of thetemperature and with the time the samples being heated longer, the structural unit grew bigger, its abundance ratio increased also. So, can see with the temperature increased, the cementation iron oxide became more and more, then formed more granule, and led the particle size bigger, and finally improved the structural strength of residual red soil; 5. The content of amorphous iron oxide increase with the increase of temperature, and then grow the aging coefficient, and faster its inversion velocity, that is to say that more amorphous iron oxide invert to crystal iron oxide; while on the same temperature, the longer the samples being heated the more amorphous iron oxide ages. So, can say the aging coefficient of amorphous iron oxide is related with temperature and the days the samples are heaved; 6. Analysis the mechanics temperature act on the red soil, then find the reason for why the effect on the temperature of 30℃isn't apparent, that is on the temperature of 30℃the velocity of aging is very slow; while on the temperature of 40℃, 7 Days the aging coefficient reach 24.33%, because in Fe(OH)3 system, on this temperature, Ferrihydrite will age within 10~14 days, so, on the temperature of 40℃, can find apparent deference; so when the temperature reach 50℃, the aging coefficient grows to 51.77% within 15 days; 7. Test the total mineral with the method of X diffraction, find after heating, the free iron oxide content increase, and the Goethite content increment apparently higher than Hematite. So, the new crystal iron oxide majority is Goethite, and the Hematite is the second. And the new crystal iron oxide mainly form from two way. One is from the lower valence iron ion, and the other is from the higher valence iron ion, and the two inversion courses list in the follows: 2Fe + ??→Fe(OH)3 ??→α-FeOOH ??→γ-FeOOH ??→Fe2 O3 3Fe + ??→FeHO 2·H 2 O ??→γ-FeOOH ??→Fe2 O3 8. The mechanics of the temperature effect is that the temperature controlled the content and the inversion type, and then effect on the physical and chemical properties. Temperature increase, then the content of free iron oxide increase following, and at the same time the activation ratio decreases, so more crystal iron oxide is formed, then the better cementation appears, lead to bigger particle...
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