Experimental Investigation Of The Influence Of Unloading Rate On The Mechanical Properties Of Saturated Clay Under High Confining Pressure And Its Microscopic Mechanism

This paper presents an experimental investigation of consolidated undrained test for remolded and undisturbed saturated clay by using high-pressure triaxial tests to investigate the influence of unloading rate, confining pressure and soil properties on the mechanical properties of saturated clay, such as stress-strain and pore water pressure. The main contents and understanding are as follows:(1) Different unloading rates have influence on the stress-strain behavior of soil. The stress-strain curves show that: for the remolded soil samples, the stress-strain curve presents strain softening when the unloading rate is 5k Pa/min and 50 k Pa/min; and the smaller unloading rate is, the greater axial strain corresponding to the peak point is. The maximum deviator stress increases with the increasing of unloading rate in the unloading rate within the range of 10 k Pa/min~50k Pa/min; and the greater unloading rate is, the steeper initial slope of the curve is. When the unloading rate is reduced to 5k Pa/min, the trend of stress-strain curve has changed: the maximum deviatoric stress is not reduced but increased; the initial slope of the curve becomes steepened. Namely there is a "critical unloading rate" between 5k Pa/min~10k Pa/min. It has a similar variation of stress-strain curve with remolded soil samples for undisturbed soil samples, but "critical unloading rate" value is different. For undisturbed soil samples, "the critical unloading rate" is in the range of 20 k Pa/min~40k Pa/min.(2) Different unloading rates have influence on the characteristics soil pore water pressure. On the whole, the peak value of pore water pressure of the undisturbed soil samples is larger than that of the remolded soil samples. However, at the same unloading rate the development pore water pressure along with the deviatoric stress curve of undisturbed soil samples and remolded soil samples are not the same. For remolded samples, there exists a "turning point" on pore water pressure with the development of axial strain; the increase rate of pore water pressure with the axial strain before and after the "turning point" is different. Before the "turning point" pore water pressure with axial strain increased rapidly, while the growth rate slows down after the "turning point". The larger the unload rate is, the greater the axial strain value corresponding to "the turning point" is. Analysis of pore water pressure and axial strain curve shows that there exists a "critical unloading rate" in the range of 20 k Pa/min~50k Pa/min, making the characteristic of soil pore water pressure changed near this unloading rate.(3) The influence of different consolidation confining pressures on the stress-strain behavior of the soil samples is investigated by the tests. For undisturbed soil samples, at the same unloading rate, when the confining pressure is 2MPa and 3MPa, the kind of stress-strain curve is strain hardening. The greater the confining pressure is, the greater the deviator stress peak value will be. When confining pressure is 6MPa, the kind of stress-strain curve is strain softening, and the deviator stress peak value becomes smaller. At the same consolidation confining pressure, with the same value of the deviator stress, the smaller the consolidation confining pressure is, the greater the soil deformation will be.(4) According to the experiment with environmental scanning electron microscope, the structure of both remolded soil sample and the undisturbed soil sample becomes much loose, while the number of pores in soil is evidently increased after the triaxial unloading test. It is notable that several tension cracks, most of which are located around the bigger particles, appear on the shear surface after this experiment. Meanwhile, many slight secondary cracks are also derived beside the main cracks.(5) Liquid nitrogen adsorption test is used to study the types changing of soils ample pores before and after unloading test. It is found that the average diameter of the pores is reduced while the rate of unloading increases. After this experiment, the average diameter of the original soil sample is reduced from 5.77 nm to 4.76 nm, while the average diameter of the remolded soil sample is also reduced from 8.38 nm to 6.73 nm. And at the same confining pressure and with unloading rate increasing absorption and desorption curve types vary from L2 to L3 type curve, i.e. the connection of pores becomes better after unloading test.