The Research On Moisture Diffusion Of Unsaturated Subgrade Soil

Southern China, especially Southwest region, has disadvantages in highway building particularly in unsaturated soil subgrade construction, which have been limited by the terrain and the natural environment. The difficulties are as follows:(1)There always forms high fill subgrade and high side protective slope when building road with complicated topography, as well as long construction period, low slope safety and easily spoiled by the scour of rainfall.(2)Rainy season is a significant phenomenon in South China. However, the distribution of rainfall is extremely uneven, the alternative of sunny and rainy is uncertain, which create the huge temperature difference between roadbed surface and its deep inside, especially in Southwest region.(3)Affected by the design ideas and current codes, the long residual action of waterproof and drainage measures are lacking of consideration so that the accumulated water makes moisture diffusion.Powers that makes moisture diffusion in soil constituted by many factors, such as matric potential, gravity potential, pressure potential, temperature potential, solute potential. The diversity of the powers and the randomness of subgrade immigration make varieties of moisture dynamitic change in soil. According to the current theories, the moisture of subgrade soil diffuses inside with liquid state--gaseous state-- liquid state, under the control of temperature field. Besides, the movements are also effected by soil,compactness,temperature,rainfall and time. Based on it and combine with actual situations of subgrade construction in South China, we carried out the research with three testing programs. Plan A: We built a relative constant temperature and humidity surrounding by adopting thermostatic water bath and water resisting insulation materials, observing the changeable situation of moisture diffusion in the condition of thermostatic water beneath soil's bottom, exploring the rules of the moisture diffusion in vertical direction with different moisture content, compactness and temperature. Plan B: We applied radiation heating device and rainfall simulator to imitate long-time raining and dry-wet cycle respectively, then discussed about the rules of the moisture diffusion in horizontal direction of different subgrade slope compactness under the control of long-time raining and dry-wet cycle. Plan C:We used radiation heating device and rainfall simulator, viewing the moisture diffusion inhorizontal direction in different dry-wet cycle and 96% compactness of subgrade slope, with the help of three axis test instrument, exploring laws of moisture diffusion and strength variety of shallow soil layer in different dry-wet cycle of high compactness subgrade slope.Eventually,the evidence shows the main reason that makes the rate of water content vary may not relative with the moisture diffusion in vertical direction,but related to the moisture diffusion which caused by the dry-wet cycle and unsaturated subgrade soil matric potential of subgrade's sides.