| In the loess plateau area of northwest China, irrigation water is usually pumped from the Yellow River for agriculture, which causes frequent loess landslide activities, especially in Heifangtai area. Irrigation water infiltrates to the deep, resulting in the rise of ground water level. Deep soil is saturated for a long time, and soluble salt which plays a role in cementation is dissolved. So the micro-structure of loess is broken, and chemical composition in the pore solution is changed at the same time. Tests of water chemistry prove that the content of soluble salt is higher in this region.Based on field investigation, a large number of experiments are made to study how underground water affects the strength of loess in a long time. From the perspective of interaction between water and soil, the time effect of strength weakening of loess is analyzed, as well as the influence made by the content of soluble salt. Finally, by using numerical software, the influence on the stability of loess slope made by underground water is analyzed.(1) Time effect is obvious when underground water affects the strength of loess. The curve of strength(internal friction angle)-soaking time is like a “spoon”, which could be divided into 3 stages as following. Initially(0-3d), internal friction angle decreases rapidly; then it increases slightly at 3-5d; finally(5-20d) it decreases slowly.(2) Particle distribution and salinity of loess are both changed by desalting experiments on the soil, which makes influence on the microscopic geometry fabric and internal connection of soil particles, affecting the strength of loess.(3) The compression deformation at later stage of loess collapsibility can be seen as the quantitative measurement of water softening. Softening coefficient is defined as the ratio of the deformation and initial height. Both collapsibility and softening coefficient increase at first and then decrease with soil depth(consolidation pressure).(4) Groundwater makes a big difference on soil by the way of physical-chemical effects in long-term. Images of SEM(Scanning Electron Microscopy) show that water dissolves soluble salts rapidly as soon as the so il meets water; “Point to point” contact between particles transforms into “point to face” or “face to face” gradually, and particles are arranged in haphazard, becoming fine-grained during soaking in water for a long time; Results of laser particle size analysis show that clay co ntent decreases first and then increases as soaking time goes by; Zeta probe tests indicate that the thickness of adsorbed water layer follows the same discipline as clay content; It can be seen from the experiment of XRD(X-ray Diffraction) that the mineral composition of loess presents different variation with time: illite becomes reduced but quartz becomes increased; Chemical tests show that variation process of content of major cations can be divided into five phases, corresponding to the three stages o f the changing of internal friction angle with the soaking time.(5) A series of complex physical-chemical effects in microscopic view make big influence on macroscopic shear strength of loess(internal friction angle). The curve of strength(internal friction angle)-soaking time is ike a “spoon”.The mechanism analysis is as follows. First, the cements(soluble salt) between particles of loess dissolve in water rapidly, breaking the microstructure and making the internal friction angle decrease; at the same time, ion concentration in the pore water increases, and it exchanges ion with outer layer of clay particles. So the thickness of adsorbed water of clay particle decreases, making the internal friction angle increase slightly; With the increase of soaking time, gypsum dissolves in sodium chloride solution and coarse particles are dispersed into clay particles further, then the total thickness of electrical double layer increases, so the internal friction angle decreases slightly.(6) When considering the influence of underground water, the result of stability calculation by numerical analysis shows that Dangchuan 2# landslide was in unstable state for its stability coefficient was 1.027 before the first sliding. Small shallow loess landslides may occur under the influence of various external factors.(7) Dangchuan 2# Landslide is the epitome of loess landslides in Heifangtai area. Vast agricultural irrigation makes ground water level rise, and deep soil of the slope is saturated for a long time, so that the strength reaches the weakening limit. In the special topographic conditions, small shallow landslides occur when disturbed by external affecting factors. In the process of rising of ground water, when stress path crosses the state interface, the incomplete drained shear happens suddenly. Then loess debris flow or fluid loess landslide occurs. |
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