Spatial variation of shallow and deep soil moisture in the semi-arid loess hilly area, China
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Abstract. Soil moisture in deep soil layers is the only relatively stable water resource for introduced vegetation in the semi-arid Loess Plateau of China. Characterizing the spatial variation of deep soil moisture is significant for vegetation restoration with respect to the topographic conditions. In this study, we focused on analyzing the spatial variations and influencing factors of soil moisture content (SMC) in shallow (0–2 m) and deep (2–8 m) soil layers based on soil moisture observation in the Longtan watershed. The vegetation type of each sampling site for each comparison is same, while varies with slope position, slope gradient, or slope aspect. The following results are found: (1) compared with shallow SMC, slope position and slope aspect may affect shallow soil moisture more, rather than deep layers. Slope gradient however, affect both shallow and deep soil moisture significantly. It indicates that high difference of deep soil hydrological processes between shallow and deep soil moisture remains, which can be attributed to the introduced vegetation and topography. (2) The vegetation growth condition has significant negative relation with deep soil moisture. This result indicates that plants under different growth conditions may consume soil moisture differently, thus causing higher spatial variation of deep soil moisture. (3) The dynamic role of slope position and slope aspect on deep SMC has been changed by introduced vegetation in semi-arid environment. Consequently, vegetation growth condition and slope gradient may be the major factor contributing to the spatial variation of deep soil moisture.Ⅰ. RESEARCHED PROFILEIn China, the most detailed study on loess was carried out in the Heimugou loess profile located at Luochuan County, Shaanxi Province, where the loess has a thickness of 135m. According to the lithologic character, loess of this section can be divided into Pouto loess, Malan loess, Lishi loess and Wucheng loess. The layer of red clay, 15m
Lithology
Loess plateau
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Yili area is located in the west of Xinjiang and is an intermountane basin of Tianshan, in which the loess is widespread. Only a few studies were carried out during past decades. For this reason, there are different opinions on the origin of loess. The results of grain size analysis of loess and reworked loess show that the textural features of both are same on one side and different on the other side. Firstly, the composition of grain size of loess is similar to that of the reworked loess, but the sand particles of loess is higher and the clay particles is lower than that of reworked loess. Secondly, the frequency curve of loess and reworked loess occures to be positive normal distribution. The mode of both locates in the range of 15-30μm, but the percentages are different. The amount of loess is less than 20 %, while the maxmum percentage of reworked loess reaches 50 %. Thirdly, the sorting of loess is better than that of the reworked loess, even though both are poor sorting deposits. The skewness of loess ranges from 0. 13 to 0. 43 and most of them is lower than 0. 3. Comparied with the loess, the skewness of reworked loess is higher and the mediun size moves toward coarse side. The kurtosis of loess is medium, while peak of reworked loess is steep. In summary, the loess and the reworked loess were transported and sorted by different dynamics before the dust deposited. In addition, the depositing environments were also different. The evidences from the grain size show the loess in Yili area is a typical eolian deposit.
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Abstract The disintegration property of loess is the wetting and subsequent disintegration of loess in water, which is generally an important index for resistance to erosion and disintegration of wet loess slopes and foundations. In this study, a disintegration instrument is developed in this laboratory and used to study the disintegration properties of fly ash-modified loess in foundations and Roadyes-modified loess in subgrades. Disintegration tests are used to compare samples of loess modified with different amounts of fly ash and Roadyes, different water contents and different dry densities; the influence of fly ash and Roadyes content on the disintegration of modified loess is analyzed. The differences in disintegration properties between the pure loess and modified loess are compared to explore the evolution of disintegration properties of modified loess and the optimal incorporation levels of fly ash and Roadyes. The experimental results show that the incorporation of fly ash reduces the disintegration of loess, while the incorporation of Roadyes likewise decreases the disintegration of loess. The disintegration of the loess modified with the two curing agents is better than that of the pure loess and loess mixed with a single curing agent; the optimal incorporation levels are 15% fly ash and 0.5‰ Roadyes. Comparing the evolution of the disintegration curves of samples of loess with different modifications shows is a linear relationship between time and amount of disintegration for pure loess and Roadyes-modified loess. Thus, a linear disintegration model is established in which the parameter P is the disintegration rate. According to the exponential relationship between time and amount of disintegration of fly ash-modified loess and loess modified with both fly ash and Roadyes, an exponential disintegration model is established in which the water stability parameter Q affects the strong and weak disintegration of the modified loess. The relationship between the water stability of the loess (modified with added fly ash and Roadyes) in water and the initial water content and dry density is analyzed. The water stability of the loess first increases and then decreases with increasing initial water content and gradually increases with increasing dry density. When the sample density is the maximum dry density, the sample has the best water stability. These research results provide a basis for the application of loess modified with added fly ash and Roadyes.
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According to the property of water and soil loss on loess slope surface. The affecting factors of erosion on loess slope surface and the relationship of the rainfall with runoff on loess slope surface is analyzed in this paper. It can be used for reference in engineering practice on soil and water conservation of loess slope surface.
Slope failure
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Most researches related to loess mechanical properties highlight loess foundations and slopes,while little attention has been paid to loess tunnels.Several typical characteristics of loess both as a special soil and surrounding rock of loess tunnels,such as sensitivity to water content,failure at small strain,anisotropy,loess joints and its impact on stability of loess,are studied from the perspective of stability of tunnel and underground engineering.It is concluded that the existing methods and results associated with mechanical behaviours of loess can't take the actual characteristics of loess into account when the loess behaves as surrounding rock of deep-buried tunnels.The necessary tests and corresponding methods are suggested mainly aimed at the surrounding rock classification of loess tunnels.
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In this paper,collapse characteristics of intact loess from three typical loess areas under different moisture condition are studied,and the principles of loess collapsibility influenced by water content,dry density as well as pressure and behaviors of these in different loess are approached respectively.Base on the collapsibility difference existing in different loess area,a concept of rate of potential collapse is presented,and the test results show that it can be used to analyses the loess collapsibility and reflect the structure characteristics for different kinds of loess preferably.
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This paper reports the analytical results of major chemical components of 94 loess samples from the Xiashu loess profile in the middle and lower reaches of the Changjiang River, the major component contents and the weathering intensity of Xiashu loess were compared with those of Luochuan loess from Northwest China. The results show no obvious difference in the contents of major components in the different Xiashu loess profiles, but some difference in the contents of readily leaching elements of loess and the chemical weathering intensity of loess among the different Xiashu loess profiles.
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Loess is a typically structured soil. The study on loess soil structure is an important aspect of soil structure research, contributing to the broader understanding of soil structural properties. This paper draws on primary data collected from confined compression tests of unsaturated loess, including intact loess and anthropogenic modified loess, and discusses the key influences on changes in the structural properties of unsaturated loess. Research results indicate that increasing vertical pressure, increasing water content and increasing soil density promote gradual structural weakening of unsaturated loess; and the structural integrity of the loess is reinforced in some instances by the increasing of cementing agents. Other important findings are that the key influences on loess structure are not uniform across loess regions, and that achievement of higher loess structural integrity is difficult under very high and very low clay content.
Soil structure
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