Experimental Study On Salt Heaving Properties Of Inland Sulphate Saline Soil In Cold And Arid Regions

The salt heaving of sulphate saline soil is a complicated academic topic which is highly concerned by geotechnical engineers and researchers. There are many important factors affecting the salt heaving properties, including the macroscopical ones such as salinity composition, soil type, initial dry density, water content, temperature and overlying loads, and the microcosmic ones such as soil structure, inner frictional resistance, cohesion and the soil particle interactions. This thesis carried out the salt heaving tests and theoretical prediction for sulphate saline soil, which was expected to provide theoretical guidance for the evaluation and prevention of salt heaving disaster.As one of the problematic soils in the geotechnical engineering, the saline soil distributes widely over the world, covering nearly100countries and regions from tropical to frigid zones, especially in arid and semi-arid areas. There are many types and largely-distributed inland saline soils in Northwest of China, which characterized with arid and semi-arid environment, high evaporation-precipitation ratio and remarkable temperature variations. These inland saline soils contain some sodium sulphate and possess great potential of salt heaving hazards. Salt heaving seriously affects the stability of highways, railways, industrial and civil buildings on saline soil foundations, restricting the engineering practice and resulting in enormous economic losses. The studies of salt heaving can not only understand the generant mechanism, developmental trend and influencing factors of salt heaving, but also provide a theoretical guidance for evaluation and prevention of the salt heaving hazards.The Gobi saline soil in Guazhou, Gansu province was selected as the typical representative of inland saline soil in cold and arid regions in Northwest of China. Through field investigation, theoretical analysis and laboratorial experiment, this thesis intended to investigate the characteristics of saline soil, calculate the crystal amount of sodium sulfate in soil, and finally explore the impacts of soil type, initial dry density, salt content and water content on salt heaving. Based on the results of test and calculation, this thesis aimed to establish a predictive relationship between salt heaving and soil properties for sulphate saline soil, which could be used for evaluating and preventing the salt heaving deformation. The major conclusions of this study can be summarized as the followings.(1) By field investigation. the inland saline soil in cold and arid regions was characterized, such as the grain size composition, salinity content and micro-structural features. Results showed that there were a large number of coarse particles in the saline soils, and that the major component of soluble salt was sodium sulfate and some sodium chloride. It was found that most of the soil in research area could be divided into sulphate saline soil or sulfurous saline soil; the salinization degree was middle or weak, only a few soil was classified to strong or super salinization; and the salts were mainly distributed within the depth ranging from50cm to260cm. Controlled by the local topography and geomorphology, the inland saline soil formation in cold and arid regions was created by the original hydrogeology and climatic condition, with obvious discontinuity and inhomogeneous in horizontal and vertical profiles.(2) The temperature and humidity of the stratum, the most important factors influencing salt heaving, were measured by thermo-sensitive probe and Time Domain Reflectometry (TDR) at a saline soil site during winter and summer in Guazhou, Gansu province. Based on the monitoring data, a mathematical model was established to describe the variation and hysteresis of the soil temperature and humidity with response to the air. It was found that shallow soil was highly influenced by the air, with an obvious variation and a marginal hysteresis in temperature and humidity. At a certain depth, on the other hand, soil temperature and humidity came to almost constant.(3) The salt heaving is mainly generated by sodium sulphate crystallization in soil. For example, when thenardite (Na2SO4) absorbs water, mirabilite (Na2SO4·10H2O) is precipitated directly from a saturated sodium sulfate solution at a certain temperature, finally the solid volume of salt crystal increases4.18times, resulting in salt heaving. By collecting data from experiments and manuals, a static model was established to describe the soluble properties of sodium sulphate in soil, which could analyze the crystal amount of sodium sulphate. Using this model, the critical temperature, critical salt content and critical water content for crystallization of sodium sulphate were calculated, which help determined the initiative temperature, initiative salt content and initiative water content of salt heaving in sulphate saline soil. Taking the theoretical decrease in temperature as a case, the crystallization of sodium sulphate in saline soil was analyzed as effected by salt content, water content and sodium chloride. It was found that critical values for salt content and water content were existed for crystallization of sodium sulphate. That is, the crystallization was increased when salt content or water content was higher than the critical values; otherwise, the crystallization was decreased. The effect of sodium chloride on crystallization of sodium sulphate was depended on the combination of salt content and water content. Under a favorable condition, sulphate crystallization in soil could be restrained by addition of sodium chloride. Contrarily, sodium chloride could accelerate the sulphate crystallization and, therefore, the salt heaving. A concept, the salt-water ratio, was introduced in this study, which normalized salt content and water content at the same time and help simplified the analysis of sodium sulphate crystallizations in soils.(4) The natural saline soil collected from Guazhou, Gansu province was selected as experimental material, which was compacted in laboratory into specimens with various soil type, dry density, salt content and water content. Subsequently, the prepared specimens were put into the plant growth chamber in which the environmental temperature and humidity were strictly controlled artificially to investigate the salt heaving with influencing factors. Test results supported the following conclusions:1) The more the coarse-grain size content is, the weaker the salt heaving of sulphate saline soil exhibits.2) There are two initial critical dry densities influencing salt heaving with the increase in dry density, the salt heaving decreased within the lower initial dry density, then increased within the higher ones, and slightly decreased above the higher one.3) When the water content was invariable, the salt heaving is increased with the increasing of salt content under a critical value; otherwise salt heaving decreased when the salt content exceeded the critical value.4) Similarly to the salt content, the water content also exists a critical value influencing the salt heaving of sulphate saline soil. In detail, the salt heaving increased when water content was lower than a critical value, and then decreased when the water content is higher than the critical value at a certain salt content.(5) Based on the analysis and calculation on crystallization of sodium sulphate, an equation for predicting salt heaving of sulphate saline soil under ideal conditions was established. Referenced to this study and existing research results, the equation was modified in consideration of water that couldn't participate in the dissolution and crystallization, the soil pore that plays an important role in absorbing salt crystal, the soil structure and interactions that restrained the salt heaving. The predicted results by the modified equation were proved to be consistent well with the experimental value.(6) Through the field investigation and laboratorial test results on saline soil in Guazhou, Gansu province, some suggestions which prevent salt heaving by controlling salt and water were given to improve the saline soil foundations in cold and arid regions of China. These advices are expected to provide theoretical guidance for the evaluation and prevention from a geotechnical salt heaving.