Study On Strength Behavior Of Remolded Unsaturated Cohesive Soil

Theoretic frame and experimental technology of mechanics for unsaturated soil is at basic research period and has long way to reach the maturity of practice. By triaxial experiment, suction measuring and SEM the dissertation is aim to recognize the strength behavior of remolded unsaturated cohesive soil with different moisture and different density state, and to discuss rules of suction variation corresponding to state change of moisture and density, and to analyze influence of microstructure upon suction and soil strength. So it is meaningful to accumulate experimental information and to offer viewpoints for unsaturated soil studying.Experiment soil is a cohesive soil .It is Remolded to form a triaxial experiment specimen by compacting the soil in five layers. According to a design program triaxial experimental specimens of 24 groups within six moisture states and four density states would be made up, however totally 138 cylindrical specimens of 23 groups were completed eventually, 115 of them for triaxial experimental and the others as parallel specimens for SEM.Then UU triaxial compression tests in 23 groups were conducted according to five classes of surrounding pressure as 100kPa, 200kPa, 300kPa, 450kPa and 600kPa. Before triaxail experiment, temperature and humidity of every single triaxal experimental specimen was tested through thermo-hygrometer, and then the matric suction was computed. After triaxail experiment 12 specimens were selected for SEM and each one was taken SEM photos in 4 scales.There are 2 basic Failure forms of remolded unsaturated cohesive soil: shear failure and expansion failure. Failure mechanism includes softening caused by water and destructuring controlled by structure. Selected factor of failure form is surrounding pressure and selected factor of failure mechanism is moisture state.According to the experimental results, failure Mohr circles can be enveloped by one strait line although the remolded cohesive soil has different failure form under different surrounding pressures. So the Columb criterion is appropriate yet.Strength of unsaturated soil can be expressed by the function of physical state. In the moisture and density state, strength flex surface possess certain shape and is regular but not available to be presented by a simple function. Distribution of cohesion looks like an irregular plunging anticline while distribution of internal friction angle looks like a metamorphosed W- type flex surface. Considering single factor impact, there is complexity about soil water upon the shear strength of unsaturated soil and so is the limit strength and deformation modulus.Distribution of matric suction looks like a transformed W- type flex surface in the moisture and density state space, whose shape transformation is controlled by moisture state mostly.Comparison of the strong nonlinear relations of cohesion, internal friction angle, limit strength and deformation modulus, especially matric suction with water content shows that along water axis existing three characteristic water content: w\=\6.34%, w2=n.l6%,w3=20.l3%.Divided by characteristic water content w\, wi, W3, unsaturated rang is of three region: lower moisture state, middle moisture state and high moisture state, corresponding to three kinds of unsaturated soil: heavy unsaturated soil, typical unsaturated soil and light unsaturated soil.To describe f=f (w) by subsection , c=c(w) and 4>= 2 is very important to the strength and deformation nonlinear behavior of unsaturated soil.