Experimental Study On Freezing Temperature And Suction Characteristics Of Saline Loess

Freeze-thaw action is a critical inducement of salt weathering spalling of loess slopes.In order to identify the salt weathering range of loess slopes under freeze-thaw environment,it is necessary to determine a reasonable freezing point of loess and well consider the influence of water-salt contents,freeze-thaw and other factors.Previous studies show that suction is a key indicator to control the presence pattern of water and its distribution,and the dynamic change of water-salt contents directly changes the matric suction and osmotic suction of soil,which further affects the phase change of water in soil.However,the relationship between freezing point and suction of saline loess is still lacking in the targeted mechanism analysis and theoretical model.Therefore,it is necessary to carry out freezing point and suction tests on loess with different water and salt contents,analyze the dependence of freezing point on matric suction and osmotic suction,establish a reasonable theoretical model,and provide theoretical support for the prevention and control of salt weathering problems on loess slopes.In this study,the following studies were carried out on loess in Xi’an.For four types of chloride loess（CaCl₂,MgCl₂,NaCl and KCl）,freezing point tests were conducted to investigate the relationship between freezing point and water-salt contents and cation type of chloride loess.Results show that the freezing point of samples varies nonlinearly with increasing water content,and gradually approached that of pure water.It overall declines at higher salt contents,and roughly linear at the salt content less than 1.0%.The freezing point of CaCl₂ and MgCl₂ samples vary significantly compared with NaCl and KCl loess.The influence of cation on the freezing point of loess containing chloride salt was analyzed based on ion hydration theory.For Na₂SO₄ loess,freezing point tests and scanning electron microscope（SEM）tests were conducted on samples after freeze-thaw cycles,and its mechanism on freezing point was analyzed by combining image quantification techniques.Results show that the freezing point of samples decreases after more freeze-thaw cycles and then stabilizes.Freeze-thaw changes the pore structure and particle distribution of samples,which leads to the change of hydrothermal state in the soil and thus indirectly affects the freezing point.The effect of freeze-thaw on matric suction and osmotic suction was analyzed by conducting suction tests based on the filter paper method.The results show that the matric suction tends to decay with freeze-thaw while the osmotic suction is mainly determined by the pore solution concentration,which is not significantly related to the freeze-thaw action.Based on van’t Hoff’s formula and Sposito’s formula,the osmotic suction coefficients of four chloride salt solutions and sodium sulfate solutions were calculated,and the linear relationship between the osmotic suction of salt solutions and the osmotic suction of loess was established（OSS model）.Based on the freezing point prediction model considering suction proposed by Mizoguchi（1993）,the OSS model and v G model were introduced on the basis of the basic structure of the original model,and an improved model was established for the four chloride loesses.The freezing point prediction model applicable to sodium sulfate loess was established considering the freeze-thaw effect.