Experimental Study On Salt Expansion And Melt Sinking Behavior Of Coarse-grained Sulphate Soil

Usually,the rock and soil with soluble salt content greater than 0.3% is called salty soil.At present,there is no clear definition for coarse grained salty soil.Generally speaking,the coarse grain mass is 50% larger than the total mass and the soil with salt content of more than0.3% is called coarse grained salty soil.Salt swelling in saline soil is the phenomenon that the volume of the soluble salt in the soil increases with water absorption and crystallization at low temperature,and then causes the swelling and deformation of the soil.The solution of saline soil is the subsidence phenomenon caused by the load of the immersed soil,which is mainly caused by the dissolution of the soluble salt and the erosion and deformation of the soil.At present,the study of salt swelling and collapsibility of saline soil is mainly based on fine particles.The research on coarse grained salty soil is not enough,and lacks systematic and mature theoretical support.Coarse grained salinized soil is mainly studied around the availability of coarse particle filler,and coarse grained salty soil has obvious difference in salt expansion and collapsibility due to its own engineering structure.Therefore,it is necessary to carry out a systematic test and Study on the salt swelling and dissolution characteristics of coarse grained saline soil.Therefore,based on the construction project of Tehran Isfahan high-speed railway,the characteristics of salt expansion and collapsibility of coarse coarse sulphate soil along the line are studied.The following conclusions are drawn:(1)Through the experimental study on salt expansion of crude coarse grain of sulphate salty soil,found in the outside temperature periodic change,salt heaving has accumulated well,with decreasing test temperature of the whole soil,salt heaving sensitivity will be greatly improved,but the maximum amount of salt expansion and salt expansion the sensitivity is independent.Compared with low water content,coarse sulphate saline soil showed strong salt expansion under optimum water content,and its salt sensitivity increased greatly.(2)The natural coarse particles of sulphate saline soil collapsibility test showed that water and soil erosion effect of dissolved salt crystal particles is the main reason in the dissolution of saline soil,coarse grain coarse saline soil to a certain extent,will inhibit the dissolution occurred depression phenomenon,approximation of the sodium sulfate content under the condition that the skeleton is more coarse particles of collapsibility phenomenon is not obvious.(3)The particle composition of the granular saline soil is very different,and its salt swelling characteristics are also different.The smaller the composition of the coarse-grained saline soil,the more obvious the salt expansion effect under the same conditions.Salt contentand water content are the important factors that affect the salt expansion characteristics of coarse saline soil.For coarse grained salty soil,when the content of sodium sulfate is below a certain range,coarse grained salty soil may not show salt expansion.Under the influence of soil porosity,the swelling temperature of the coarse-grained sulphate soil in different grain groups is different,but it is approximately 5?.(4)Particle size index is a key index to influence the collapsibility of coarse grained salinized soil.Fine granular coarse sulphate salty soil shows obvious collapsibility under low and middle salt content,and it decreases with the increase of coarse grained salty soil particle group.Under the same sodium sulfate content,there is a great difference between the collapsibility and the coefficient of settlement of coarse granular saline soil from different grains.The evaluation of collapsibility of coarse sulphate saline soil should be considered from the perspective of particle size and sodium sulfate content.(5)Through the salt expansion simulation analysis,found that the coarse particle sulfate saline soil and salt expansion mainly occurred in shallow surface,shallow soil salt expansion accumulation area is the need to focus on the construction of the stratum in shallow soil;salt heaving soil weakening,the outside temperature changes affect relatively small,gradually weakened with soil the increase of the depth of salt expansion trend.The ground temperature of soil tends to be constant.The change of external temperature has no significant influence on its temperature field.At the same time,the phenomenon of salt expansion does not appear at the same time,which is affected by gravity load.Under the condition of a great change in the temperature boundary of the external environment,the salt expansion of the ground presents gradual incremental change within the finite temperature cycle period,and eventually becomes stable.(6)For the determination of the depth of coarse grained saline soil foundation,there is no uniform method.Most of the depth of immersion is the largest depth of subsidence.Aiming at the shortcomings of the existing methods,this paper puts forward the "fitting method" and "equivalent method" of collapsible depth,based on which,we study the threshold of settlement coefficient of ballastless track.It is found that the regulations for the expansion and settlement coefficient of building foundation threshold can not be applied to the construction requirements of high-speed railway in saline soil area,and it is pointed out that threshold coefficient varies within a certain range instead of fixed value.