| Nong’an county of Jilin province locates in the central region of Songliao plain. Becauseof low topography, poor drainage and higher mineralization degree of shallow groundwater, itis one of counties with much serious soil salinization and wide range. The amount of sodiumbicarbonate is so high that the saline soil in this area belongs to carbonate saline soil. Nong’ancounty is the typical season frozen area where cold in winter and hot in summer. Under thecondition of such climate, all kinds of structures built in this region are often affected by thefrost heaving and intensity attenuation due to repeated freezing-thawing of saline soil here.The frost-heaving diseases are common plaguing the engineering works in this region.Inthis region, the soil at the embankment slope toe or dam toe of a water diversion and irrigationproject often heaves and cracks under the action of carbonate and freezing, then the soilsubsides and damages under the action of water and salt after melt next year.The problems ofexcessive deformation and insufficient stability occur at the foundations of bridges andculverts under the action of salt-frost heaving. The retaining wall against subgrade slope ofthe high-filling and deep-digging road partly fractures due to the frost heaving effect of thecarbonic acid saline soil behind the wall. The subgrade pavement of carbonic acid salinesoil frosts heaving and boiling. These above problems cause a great deal of impact in use anddurability on the project in this region. The saline soil engineering in the area suffers from theaction of repeated freezing-thawing due to the climate feature in the seasonal frozen region.The freezing-thawing intensity of saline soil attenuates with the increasing offreezing-thawing times and directly impacts on the stability of structures. For example, theproblems, such as the freezing-thawing slump of roadbed slopes, the thawing settlement ofsubgrade body,insufficient bearing capacity of the saline soil base under freezing-thawing,ect.. The frost heaving and freezing-thawing damages of saline soil engineering add moremaintenance costs and the secondary input of construction funds. These diseases impact onthe development of engineering construction and have become a key of success or failure in saline soil engineering. Therefore the research on the frost heaving and repeatedfreezing-thawing intensity of carbonic acid saline soil has become the urgent problem underdifferent states and conditions.At present, scholars have been studying the frost heaving and freezing-thawing intensityof both chlorine saline soil and sulphate saline soil in the coastal region, Xinjiang, Qinghai,Gansu, and so on. Furthermore, they have obtained a lot of valuable achievements. While fewstudies are engaged in the carbonic acid saline soil in the western region of Jilin province,especially research achievements on the intensity attenuation characteristics under repeatedfreezing-thawing are rarely reported on journals.In this paper, the carbonic acid saline soil samples collected from Nong’an county of Jilinprovince are taken as the research object. The frost heaving characteristics of saline soil arestudied by frost heaving force and frost heaving rate test indoor. The intensity attenuationcharacteristics are studied by the ultrasonic tests and single axis compressing intensity testsunder repeated freezing-thawing. Firstly,the basic physical and chemical properties of soilsamples are analyzed, test conditions are determined combining with the average content ofsoluble salts,the moisture content,the compactness,and the practical engineering requirement.Then the tests of the indoor frost heaving rate and frost heaving force are carried out on thesoil samples with different compactness, water content and salt amount in order to analyze thefrost heaving characteristics of carbonic acid saline soil.Secondly,combining with the climatecharacteristics of seasonal frozen region,the ultrasonic tests and single axis compressingintensity tests under repeated freezing-thawing are conducted on the soil samples withdifferent compactness,moisture content and salt content.The attenuation rules of wavevelocity and intensity are analyzed, as well as the correlation between them.Finally, theinfluencing factors on the freezing-thawing intensity attenuation of carbonic acid saline soilare evaluated by using the grey correlation method and the rough set theory. On this basis, theprediction models, which can be used to predict the freezing-thawing intensity attenuation ofcarbonic acid saline soil, are established by the BP neural network.On the one hand, the effectfactors and frost heaving mechanisms of the carbonic acid saline soil are determined throughthe research. On the other hand, the main factors influencing the repeated freezing-thawingintensity attenuation are made clearly by means of the rough set theory, and the BP neuralnetwork prediction models of the freezing-thawing intensity attenuation of carbonic acid saline soil are established.The main researches in this paper are as follows:1. The physical and chemical properties of saline soil in Nong’an are analyzed. The grainsize analysis determines the soil is silty clay with strong frost heaving sensitivity because theparticle size is above75%which the particle size is of0.005mm to0.075mm. The soil meetsthe requirements of non-uniform coefficient for Cu>5and curvature coefficient for Cc=1～3,so the size distribution of soil samples is fine.The average content of total soluble salts in soilsamples is up to0.95%, mainly carbonate. The PH value of soil ranges from7.5to8.5takingon weak alkaline. The maximum of organic matter is less than1.5%and it is non-organic soil.The content of sodium ion,14-20mmol/100g, results in the strong dispersion of soil samples,thicker combined water film tends to appear surrounding the soil particles, causes thecapillary water rising difficultly.But, the not frozen water content promote the frost heavingeffect due to a high content of sodium ion. In conclusion, the saline soil in Nong’an isidentified as carbonate powder clay with high content of soluble salt, weak alkaline,non-organic matter, fine grading, and high concentration of sodium.2.Research on the single frost heaving property indoor is made, especially on the frostheaving rate and frost heaving force. The initial temperatures are below0℃regardless offrost heaving rate or frost heaving force arising. The frost heaving changes sharply attemperatures ranging from0℃to-7℃, because the not frozen soil water content is the mostabundant in this phase of temperature. Then gradually tends to be stable after-10℃. Theeffect of salt expansion abates with the temperature falling and the frost heaving becomesdominant in the later stage.The minimum moisture content of saline soil with frost heaving isover16%.The not frozen water content increases with the moisture content increasing,so thefrost heaving is proportional to the moisture content. The frost heaving of low salinity soil isproportional to the compaction degree, while it is inversely proportional to the compactiondegree in high salinity soil. The frost heaving is proportional to the salinity content when lessthan1.5%, on the other hand is inversely proportional to the salinity content. The frostheaving rate and frost heaving force of saline soil show the same performance in many cases.The greater frost heaving rate, the greater frost heaving force. Both complement each other.But the condition generating frost heaving force is much higher than the frost heaving rate.Overall, the frost heaving of saline soil reaches the peak when the moisture content is high, salt content is low, and the compaction degree is large.3. Under the action of freezing-thawing when cycling of50times, the attenuationcharacteristics of indoor ultrasonic wave velocity and single axis compressing intensity ofsaline soil are studied. The results show that all kinds of ultrasonic wave velocity of soildeclines with the freezing-thawing times. The wave velocity drops sharply atfreezing-thawing of0-5times, because the soil particles are in a state of flux under theinflation and thawing settlement.The ultrasonic transmission channel be severely weakened,sothe wave velocity have plunged.The values of wave velocity tend to be smoothwhen thefreezing-thawing states of soil samples become stable after10times.The wave velocity of soilwithout salt is obviously inversely proportional to the moisture content. While the wavevelocity of saline soil is proportional to the moisture content after freezing-thawing of5times.Such proportional relationship is much clear along with increasing salinity. In generalspeaking, the wave velocity is proportional to the compaction degree.The single axiscompressing intensity of saline soil falls sharply at the stage of freezing-thawing cycling of0-3times, because the coupling strength between the soil particles and solid state are severelyweakened in the early stages,10cycling times later it becomes basically stable. Thefreezing-thawing intensity is proportional to the compaction degree and is inverselyproportional to the moisture content and salinity. This relationship of freezing-thawingintensity between the three factors will be weakened with the increase of freezing–thawingtimes. The opposite regularities appeare even when the moisture content and salinity arelarger.Through study on the correlation between wave velocity and freezing-thawing intensity,results show that two of attenuation characteristics becoming increasingly similar whenhigher compaction degree, lower moisture content and less salt content. Conversely, thecorrelation between them is worse.4. The grey relational degree and rough set weight of factors influencing the repeatedfreezing-thawing intensity attenuation on saline soil are analyzed and evaluated.By using thegrey relational degree and rough set theory, the main factors influencing the repeatedfreezing-thawing intensity attenuation are determined, which are freezing-thawing times,moisture content, compaction degree, and salt content. Among them, the wave velocity isreduced as the repeated influencing factor. The weights are sorted as freezing-thawing times（0.2944）> moisture content（0.2757）> compaction degree（0.2290）> salt content （0.2009）. The freezing-thawing times is the primary influencing factor, while the influenceand function rule of other factors are weakened under the action of freezing-thawing.5. Based on BP neural network, the prediction models of freezing-thawing intensityattenuation are established. According to the grey relational degree and rough set theory,determine the main influencing factors of freezing-thawing intensity, set several parameters—the compaction degree, moisture content, salt content and freezing-thawing times—as theinput layer, set freezing-thawing intensity as the output layer to establish three-layer BPneural network models.65samples selected from1134samples are used for neural networktraining, training results show that the model has good convergence. Then20samples selectedfrom the remaining samples are used for test, the average error of test is9%. The model canbasically meet the requirements for forecasting and describing the freezing-thawing intensityaccording to the standard of the civil engineering safety coefficient (1.2-1.3). Theestablishment of model also proves conclusions of gray correlation degree and rough set arecorrect. In order to reduce the prediction error of model, in the future these aspects need to beimproved such as normalizing data, selecting representative samples, increasing the numberof samples, and so on. |
