Water-retention And Mechanical Behaviour Of Unsaturated Loess Over A Wide Suction Range

Unsaturated soils are widespread in nature,and suction has a great influence on the engineering properties of unsaturated soils.As a typical unsaturated soil,loess is widely distributed and deposited in China,especially in northwest China.Loess in northwest China is located in arid and semi-arid areas,and its suction can reach tens or hundreds of MPa in the relatively low humidity environment for a long time.Therefore,it is necessary to study the hydro-mechanical behavior of unsaturated loess in the range of high suction or wide suction.On the basis of reading the relevant literatures of loess at home and abroad,this paper takes Q₃ Malan Loess in Xianyang area as the research soil,and the suction was imposed using the axis translation technique and vapor equilibrium technique with saturated salt solution.The water-retention and mechanical behaviour of unsaturated intact soil and compacted loess with the same dry density over a wide suction range are studied.The micro-pore structure of intact loess and compacted loess is studied by the mercury intrusion porosimentry?MIP?test,and the relationship between the micro-pore structure and its water-retention and mechanical behaviour is also studied.The main research content and conclusions are drawn out as follows:?1?The soid-water retention curves?SWRCs?of intact loess and compacted loess were studied by the pressure plate method and vapor equilibrium technique with saturated salt solution.The SWRCs of both intact and compacted loess showed a weak bimodal curve.From the relationship between water content and suction,it can be seen that the SWRC of intact loess is higher than that of compacted loess with the same dry density in the range of low suction.In the range of intermediate suction,the curve of compacted loess are higher.In the high suction range,the curves coincide with each other.From the relationship between suction and void ratio,it can be seen that the influence of suction on void ratio of compacted loess is slightly greater than that of intact loess.?2?The triaxial test with controlling suction was carried out and the suction was imposed using the axis translation technique and vapor equilibrium technique with saturated salt solution.The strength and deformation characteristics of intact and compacted loess under different suctions and net confining pressures were studied.The results show that the strength increases with the increases of suction and net confining pressure.When the suction is high,the strain hardening or strain softening will occur in the triaxial shear process,which is related to the development of shear zone.When the suction increases to a critical suction value,the strain hardening and then strain softening certainly occur in the specimens,and the greater the net confining pressure,the greater the critical suction value.?3?From results of the triaxial shear test under different net confining pressures,the shear strength parameters under different suctions can be obtained,so that the variation law of the shear strength parameters of the intact and compacted loess with the suction can be studied.The results show that the cohesion of intact and compacted loess increases significantly with the increase of suction.The angle of internal friction of intact loess does not change obviously with suction in the low suction range,but increases significantly with suction in the high suction range.The angle of internal friction of compacted loess increases first then decreases and then increases with the change of suction.The internal friction angle of both increases obviously in the range of high suction.?4?The characteristics of pore-size distribution and its variation under high suction were studied by the mercury intrusion porosimetry tests,and the water-retention and mechanical behaviour of loess were analyzed from the pore size distribution characteristics of the sample.The results show that the pore-size distribution of intact and compacted samples is bimodal.After the high suction,the pores size of the sample become smaller,and the distribution of the pore sizes of the compacted sample changes more obviously.The pore-size distribution and variation of the specimen are obviously related to its water-retention and mechanical behaviour.