Study On The Regularities Of Water Migration In Unsaturated Loess

Loess landslide is one of the main geological disasters in loess area, and rainfall orirrigation infiltration is a main factor for loess landslide. Generally, it is believed that largescale loess landslides occurred due to the rise of groundwater table caused by rainfall orirrigation, but the groundwater levels are often very deep in loess area, most of which varybetween20and80m, take Zaosheng tableland in Longdong loess plateau as an example,which groundwater depth is about120m. The permeability of loess is also very low, whichsaturated permeability coefficient varies from10-5to10-4cm/s. The depth of surface waterinfiltration is limited in loess indicated by field observations, which has been not more than4m so far. It is difficult to explain the relations between rainfall and large loess landslides, aswell as the phenomenon that the water level in wells fluctuated and the spring flow at the sideof loess platforms changed with rainfalls and irrigations. Therefore, someone think that largeamounts of surface water pours into the deep through vertical fissures, sinkholes or someother water paths. However, according to field investigation, these water paths only occur inunloading area which is mostly in the edge of loess tableland, and water easily drains fromexports due to the large hydraulic gradient. There is also study showing that even if surfacewater pores into through fissures, which can only lead to shallow soil moisture contentincreased. Considering that the existing monitoring depths were shallow and the periods wereshort, and the infiltration regularities and the affected depth of rainfall in loess were unknownyet, a monitoring site was set up in Zhengning county, Gansu province. Controlled manualdrip experiments were planed to simulate the natural rainfall condition, the changes of soilmoisture content within10m under the condition of manual dripping and natural rainfall wereobserved, the rules of water migration and the affected depth in loess were analyzed combinedwith the local stratums and soil characteristics, geographic and meteorological conditions. Theresults show that soil moisture is recharged by rainfall and drains through evaporation in thearea. Shallow soil moisture is affected greatly by rainfall and evaporation, and changing inannual cycle. Shallow soil moisture content will occur saltatory change if the precipitation ismore than a certain threshold, while on the contrary, almost no obvious fluctuations on thebackground trend, the circulation of soil moisture occurs mainly in surface layer, the trend isconsistent with the change of evaporation. The growth of moisture content lessens and theaffected range deepens when precipitation increased, conversely, the growth decreases and thechange of moisture content lags with the depth. In unsaturated loess in the deep, water movesmainly in the form of unsaturated seepage or vapor form, the amount of vapor is very small, resulting that soil moisture content grow not only weakly but non-annually, but cannot beignored. The water will gather on the surface as encountering layers with low permeability,leading to soil moisture content increased.Numerical model was built based on stratigraphic conditions of the test site, boundaryconditions were defined according to the intensity and duration of manual dripping, theprocess of drip infiltration was simulated by finite element software. Infiltration boundary wasdefined according to the precipitation data of a whole year, the process of rainfall infiltrationwas simulated. The results of both simulations fit well with the results of in-situ observation,preferably reflecting the rules that under the condition of rainfall infiltration, shallow soilmoisture content varies greatly with the change of precipitation, while moisture content ofmiddle loess layer almost remain stable, soil moisture content in the surface of deep aquicludegrows continuously.The discussion on the regularities of water migration in unsaturated loess in this paperhas an effect on promoting and offering basic data to the use of unsaturated soil theory inloess study and engineering, and is crucial to apply unsaturated seepage theory to studies onthe forming mechanism of rainfall-induced landslides.