| Ningxia Hui Autonomous Region is located in the northwest inland,especially in the central arid zone,where there is little rainfall,seasonal and diurnal temperature difference,and high evaporation.Under such climatic conditions,a large number of saline soils are produced in the region,and they are generally(sub)sulfate saline soils.Due to the susceptibility of sulfate saline soils to salt-freeze swelling under temperature change conditions,the resulting soil deformation or salt-freeze swelling force generated when the soil deformation is restrained can cause different degrees of damage to building foundations,posing a greater threat to the safety and stability of infrastructure.In this paper,we used a self-developed salt-freeze swelling test apparatus to carry out the salt-freeze swelling rate test with different salt content,chlorine-sulfur ratio,water content,compaction degree,top loading and fines content,and the salt-freeze swelling force test with different salt content,cooling rate and chlorine-sulfur ratio,and investigated the variation law of salt-freeze swelling rate and salt-freeze swelling force of sulfate saline soils and their influencing factors,and established the prediction model of salt-freeze swelling rate and salt-freeze swelling force.The prediction model of salt-freeze swelling rate and salt-freeze swelling force was established.The analytic hierarchy process and entropy method were used to calculate the subjective and objective weights,and the game theory was used to calculate the combination weights,and a grading model was proposed to judge the degree of salt-freeze swelling of sulfate saline soil in Ningxia region based on fuzzy mathematical theory.The study provides important theoretical support to improve the theory of sulfate saline soil and its foundation treatment.The main research results achieved in this paper are as follows.(1)The salt-freeze swelling rate increased with the increase of salt content in a folded linear manner and was bounded by 2%salt content,and the growth rate of salt-freeze swelling rate was faster when it was less than this value and slower when it was greater than this value.The salt content and the salt-freeze swelling force show a positive correlation,and the higher the salt content,the faster the growth rate.The salt content also affects the temperature at which the salt-freeze swelling force reaches its maximum value,and the higher the salt content,the larger the value.(2)The increase of chloride ion content can effectively inhibit the generation of salt-freeze swelling in a certain range,and the effect is weakened or even disappeared when the chlorine-sulfur ratio is greater than 0.7.When the chlorine-sulfur ratio is in the range of 0.1～0.3,it can significantly reduce the saltfreeze swelling force with the increase of chlorine-sulfur ratio,and the inhibiting effect is weakened beyond this range.In addition,the presence of chlorine-sulfur ratio can eliminate the deviation saltfreezing swelling force,which is found to be related to the concentration of sodium chloride,and the deviation salt-freeze swelling force can be eliminated when the concentration reaches a certain level,independent of the size of chlorine-sulfur ratio.(3)At the same cooling rate,the salt-freeze swelling force shows a trend of increasing and then decreasing with the decrease of temperature.According to the different temperature intervals,the saltfreeze swelling force can be divided into three stages:rapid growth,slow growth and fall back,and the temperature intervals are different for different degrees of salt content.The effect of cooling rate on the salt-freeze swelling force is related to the size of the salt content,with low salt content(S≤1%),decreases with the increase of the cooling rate;higher salt content(S=2,4%),with the increase of the cooling rate,first decreases and then increases;high salt content(S=6%),with the increase of the cooling rate and increases.The cooling rate has a significant effect on the deviated salt-freeze swelling force,and the larger the cooling rate is,the larger the deviated salt-freeze swelling force is.(4)Based on quadratic regression orthogonal analysis,a prediction model of salt-freeze swelling rate was established and the model was simplified by analyzing the significance of each factor.The sensitivity of the effects of the factors was analyzed using SPSS software,and the rankings were obtained as follows:salt content(0.512),chlorine-sulfur ratio(0.075),water content(0.047),interaction of salt content and water content(0.02),interaction of water content and compaction(0.014),interaction of chlorine-sulfur ratio and water content(0.012)and interaction of salt content and fines content(0.008).(5)The final salt-freeze swelling force prediction models for sulfate saline soils and chlorinated sulfate saline soils were developed based on the Levenberg-Marquardt(LM)method and Universal Global Optimization(UGO)method,and the models were verified to have good applicability by literature data.(6)The evaluation index system of sulfate saline soils in Ningxia region was constructed,and the subjective weights of sodium sulfate content,chlorine-sulfur ratio,water content,compaction,upper attachment load and fines content were calculated by Analytic Hierarchy Process,which were 0.434,0.242,0.178,0.043,0.031 and 0.072 in order.the objective weights were calculated by entropy weight method,which were 0.328,0.208,0.122,0.171,0.081 and 0.090 in order.The optimal combination weights were calculated based on game theoretical ideas and were 0.398,0.23,0.159,0.087,0.048,0.078 in order.(7)The affiliation functions of the six influencing factors were analyzed and determined.The reference range of factor benchmarks for salt swelling level of saline soils was given with reference to norms and local standards.By transforming the fuzzy operator,the degree of salt-freeze swelling of the test soil was determined to be between medium salt-freeze swelling and strong salt-freeze swelling.The applicability of the fuzzy comprehensive evaluation model was verified through comparative analysis. |
PDF Full Text Request