Study On Compression And Strength Properties Of Compacted Loess-like Backfill

With the continuous development of reform and opening and the modernization dive, China’s national economy has grown greatly, and the industrialization has expanded rapidly. However, the expanding construction scale and the growing urbanization are in conflict with the natural environment. Problems of the natural environment deterioration and the shortage of construction land become increasingly prominent. Filling the ditch to make the artificial land is a good way to solve the shortage problem of construction land. Northwest China and North China mostly located in the Loess Plateau Region, and the special landforms lead to the emergence of a large number of engineered fills during the economic development process. High-fill projects are plenty. In addition, loess is widely distributed in Northwest China, North China, and other places. The engineering properties of loess are special on the water sensitivity. Therefore, problems of compacted loess-like soils are increasing in geotechnical engineering. For a long time, the engineering of natural loess has been studied deeply, but the systematic research on the strength, deformation and structure of compacted loess is still very few.The problems of uneven or large settlement often appear in untreated or weak filled foundations. These problems usually cause the cracking and sinking of the upper buildings and other engineering accidents. Dynamic compaction is the most common method to treat the filled foundation recently. The compaction quality of soil directly affects the engineering quality of foundation or subgrades. So, the compaction quality is very important to the engineering quality and normal use of the building. Based on the background of National Natural Science Fund Project ’Research on influence factors of strength and deformation of compacted loess and its microstructure (51178287)’, the engineering mechanics of two types of compacted loess were researched. The soil samples were taken form TaiYuan and LvLiang in Shanxi province. The compaction properties of loess were discussed. The strength and deformation of compacted loess were affected by some factors such as water content and compaction energy or immersion, and the impacts of these factors were analyzed. Combined with the microstructure analysis, a reasonable explanation for the mechanical properties of compacted loess and its performance after immersion in water were given from the internal and external aspects. This research helps understand the mechanical properties of compacted loess more deeply. Some physical properties and indexes were given as the references for the compaction quality evaluation of compacted loess and for the design or construction quality control of backfill engineering in loess areas. It has certain practical significance. The main research results are summarized as follows:(1) For un-soaking compacted loess samples, each strength index decreased and the compression strain increased when the water content increased. All of these indexes changed non-monotonically with the increasing compaction energy. The strength and deformation of compacted loess changed differently with the compaction energy when the water content was in different range. When the soil was compacted under the optimal water content on the dry side, it would have the higher intensity index and the lower compressibility. Because the structure of compacted loess was different from the original loess under the compaction energy effect, soil compacted under smaller water content was more hydrophilic. So, it would be softening after immersed in water and the strength would be reduced. Contrastly, the soaking effect would be small when the soil was compacted under the wet side of optimum water content.(2) The multiple factors analysis showed that the compression strain was mostly influenced by the initial water content and that the shear strength was mostly influenced by vertical pressure. Influence of compaction energy was relatively small in all of the components, but the interplay between compaction energy and water content had the significant effect on compression strain and shear strength of compacted loess.(3) Hyperbolic hypothesis of εs-p relationships applied only to the pressure in the range of around400kPa. When the fit pressure range was large, the power function model was more consistent with the actual condition. The settlement calculation formula based on the compression stress-strain with the hypothesis of power function was given. Combined with the specific engineering, the results calculated by different methods were compared, and the SMM was used extensively.(4) When the compaction energy was the same, relationships between deviator and axial strain showed the variation trend of’strain softening’-’weak strain hardening’-’strong strain hardening’. The optimal water content should be used as the dividing value of distinction for the relationship between deviator and axial strain which was either’strain softening’ or’strain hardening’.(5) Crumb was the basic unit and the micro-structure of compacted loess was the type of crumb structure. There were two types of pores. Some were pores located between the crumbs and the others were pores located in the interior of crumb. When the compaction energy was same and the water content increased from small to large, changes of micro-structure of compacted loess included loose to dense density, unordered particle to ordered particle arrangement, and isotropic to anisotropic form.(6) Controlling the water content was particularly important to compacted loess during compaction because of its special water sensitivity and typical fine particles. Air porosity could be used as the quality control additional indicator for backfill compaction in the loess region on the condition of meeting the requirement of compaction degree. The control standards of air porosity should be set combined with the concrete soil compaction test. Generally, an air porosity value not more than10%was considered preferred. When the heavy machinery was used, controlling the construction water content within the optimal content (the water content value should not exceed wop+2%on the wet side was considered to be better. The deformation and strength should meet the design requirements under the premise that the compaction standard was achieved. It was more favorable to ensure the water stability of fill foundation or subgrade.