| As a kind one of the most common geological disasters, the landscapes not only cause a great economic damage, but also make a threat to the environment. The rainfall type of landscape is more than90%of total landscape, have the characteristics of wide distribution, small size, high frequency, sudden strong, etc. The slopes made of tens of meters of igneous rock weathering crust that is widely distributed in Fujian province, which will turn to soil-like material landslides under the effect of typhoon. Most of those landslides are in the sliding state, which seriously threaten the safety of geological environment fragile areas. Under the control of the internal microstructure and engineering characteristics of unsaturated, the effect of rainfall infiltration and sliding characteristics of soil-like slope are different from the homogeneous soil landslide. The micro hydraulic characteristics of three kind of soil-like materials (tuff residual soil, kaligranite-porphyry residual soil and granodiorite residual soil) are tested and observed in this article. On the basis of the design rainfall conditions according to the actual weather conditions, the characteristics of underground water seepage and the macroscopic failure of soil-like material landslides are analyzed, which play the key role to reveal the start and resurrection mechanisms of landslides. According to the existing research achievement of various rainfall type landslide disasters in this area, the macro and micro engineering characteristics of three kinds of slip soil are analyzed through the indoor unsaturated tests, in order to get the relationship between the water content of slope body and the shear strength. And the Yongtai landslide is chosen as an example to research the internal seepage field change cause by dry-wet circulation caused by different soil influence on landslide macro morphology to reveal the soil landslide mechanism and internal damage evolution process. (1) The regional geological tectonic environment in the area including topography, lithology, structure environment and meteorological hydrological conditions are expounded in the first place, and the associated effect of soil landslide and rainfall are analyzed. The total rainfall, short-term rainfall, rainfall intensity are analyzed the time and spatial distribution of the rainfall and the soil-like landslide. And the results showed that the number of small to moderate rain days plays the most important role in initiation mechanisms of landslides, the secondary influence factors are the number of days of the continuous rain and daily rainfall^25.0mm. In addition the total rainfall x1, rain days of≥25.Omm x6, and the longest continuous rain days x9are chosen as the factors to curve fitting analysis with landslides probability y, and estimated the y on the basis of the rainfall factors.(2) The rock mineral and chemical composition of tuff, potassium long granite porphyry, granodiorite and monzonitic granite porphyry are analyzed based on flake observation, the results show that mica subtle tuff are with tuff structure, the other three extrusive rock are with medium-fine grains structure, all compose chiefly of plagioclase and quartz, and the main mineral composition SiO2content more than68%. The results of X-ray diffraction tests show that the main component minerals of sliding zone soils are quartz, feldspar, calcite and clay minerals such as kaolinite and chlorite, illite and chlorite, etc. the total amounts of clay minerals respectively are15%,30%,52%. And the trace element types and content are different due to the types of parent rock.(3) On the basis of the distribution rule of the sliding zone and the fracture in the researched region, the sliding zone is mainly located in soil rock interface or the soil interface between different structures. The existing of gully, tension crack and dry wet bilge shrink crack have played an important role in strengthening rainfall infiltration and seepage field redistribution. The landslides form of soil-like material, material composition and the rule of crack distribution are all considered to partition the landslide according to the development degree, the result can be learned that the rainfall landslides mainly distributed in mountainous hilly terrain(4) Internal soil-like material slopes are in the unsaturated and saturated dry-wet circulation state with the rainfall infiltration in the researched region. In this article, the soil water characteristic curve of the soil-like material samples with different mineral compositions and different initial state are tested by one-dimensional stress controllable volumetric pressure plate extractor. And the results show that the order of curve fitting parameters of three samples SWCC as following:an1>ay2>ay1in drying process. The difference of the air-entry value cause by the diversity of the clay mineral composition, that is the more that contain hydrophilic clay minerals, the inlet, the greater the value, but the trend of a is the opposite of n,which is controlled by the pore distribution. In the moisture adsorption, the air-entry value is mainly controlled by fine particles content:the higher the clay content, the greater the value of the are-entry. While the value of n and mineral composition has a high degree of correlation.(5) The influence rule of different initial states of SWCC, the dry density and initial water content of soil play the most important role and have the interaction relations. In the same initial the higher the coefficient, the stronger the water-holding capacity of the internal microscopic pore, which shows the higher value of air-entry and well-distributed pore size. Compare with the three samples, those all show similar variation, only with the different clay particle content and composition, the increase degree is different.(6) The unsaturated seepage characteristics of three soil-like material samples are researched with constant flow test, and the results shows the order of saturated permeability coefficient as following:ksy1>ksy2>ksn2, which is consistent with the content of clay mineral composition order, while is in contrast to the fine grain content results. With decreasing soil water content, mineral composition, especially clay mineral component’s contribution to the permeability coefficient is higher than the influence of grain size distribution in the water absorption process, this conclusion further illustrates the particle microstructure is the greatest contribution to the value of unsaturated infiltration coefficient. The permeability coefficient is the ultimate cause of soil moisture state change rate. The different drying, moisture process, mineral composition and grain composition lead to difference in SWCC and permeability coefficient of unsaturated soil, which will directly cause the difference of seepage characteristics under rainfall action. In addition, due to the existence of hysteresis effect, the water in soil-like material with high content of coarse particle discharge rate is not equal. As the result, the sliding zone will be formed in the fine particle soil and extension.(7) The shear strength of different station of matric suction and dry-wet circulation path was obtained respectively of three kinds of soil-like material samples in the research zone by unsaturated direct shear test. The results shows that the change rules of three are similar to the loess or clay, namely with the increase of matrix suction, internal friction angle is hardly changed, mainly manifested in the change of shear strength. The only difference is that change ranges of various kinds of shear strength is quit differ with other clay. With increase of matrix suction in high level, the effect of suction on shear strength is beginning to weaken, soil sample shear strength started to decline after the peak at the same time. What we can obtain in SWCC is that the matrix suction values of samples corresponding to peak shear strength are closed to each other, in addition the cohesion of three kinds of material increase with water content in terms of Gauss Function, while the internal friction angle increase with water content on the basis of Hill1Function. The peak shear strength of Y1and N1is close to75KPa, while the one of Y2is about100KPa, ands the corresponding water content are respectively21.49%,26.7%,9.61%. The water content is proportion to the clay mineral and hydrophilic mineral content of soil-like material samples.(8) The law of shear strength along with the change of matrix suction exist big differences with different conditions of vertical load. Such as the effect of matrix suction on the shear strength of Y1and N2is the largest when the vertical load is200KPa, however the one of Y2is100KPa. For rainfall landslides, the sensitivity of shear strength to the matrix suction or water content changes in samples corresponding to separate depth samples. When Y1is about12m in depth, the shear strength attenuation rate is the largest along with increasing water content, while when Y2is in depth of5m, the rate is the largest. The influences of different initial conditions on the unsaturated shear strength were test and the results as follows. One hand with the increase of initial moisture content, cohesive force increases firstly and then decrease, while the internal friction decrease which can be disregard. On the other hand the cohesive force has an obvious decrease after dry-wet circulation path, however with the increase of cycling times, the amplitude decreases slowly.(9) The modifying classification of the shear strength of soil-like material under unsaturated condition by unified Mohr-Coulomb shear strength criterion is established on the base of suction stress theory and F&X fitting equation. Comparing the direct shear tests results of remolded sliding zone soils with different saturation with the predict results on the base of suction stress theory, we can find that the predict results have acceptable accuracy, but still needs a large number of trials to improve the correction factors. To the rainfall landslides, slopes are in dry-wet circulation or unsaturated state cause of suffering both rainfall and evaporation in rain season. Analyzing this kind of problem with traditional strength theory for saturated soils can’t obtain results conform to real situations. Predicting the shear strength of soils under unsaturated condition by unified Mohr-Coulomb shear strength criterion on the base of suction stress theory can adequately consider the shear strength of sliding materials under unsaturated conditions conveniently and accurately which is benefit for reasonably evaluation of the stability of soil slope and hazard prediction.(10) An observation by Field emission scanning electron microscopy indicates that:the microscopic particles tuff residual soil is made of columnar and massive single grains and granulating of schistose minerals polymer. Furthermore, most of the single grains (quartz particles) surrounding grains, the mineral particles exist in form of edge-to-surface contact and plane to plane contact. Granite residual soil particles distribution has better layers, thin and flat, lined up tightly and has the advantages of orientation, directional property, in addition, the mineral particles exist in form of edge-to-surface contact and plane to plane contact, which mainly form of symbiosis between chlorite and quartz. However the micro structure of granodiorite residual soil is a bit different from two soils mentioned above, which is made of flake and columnar mineral particles mainly consist of granule-flocculent structures, with a few quartz phenocrysts in clumps filling in pore, and the image of thin platy aggregates appear obvious orientability, rather than messy configuration of feldspathoid.(11) One hand, in the condition of same water content, the microporosity and pore areas decrease along with increasing of vertical loads, at the same time sample granules rearrangement of material particles is in a denser way and stronger orientability, circularity of pore is on rise with vertical loads. When the vertical loads reach300KPa, there are a sign of failure of three samples micro structure, the particles become disorder. In the other hand, n the condition of same vertical loads, the higher the initial moisture content, structures and tissues change more of micro structure. The pore orientability of the initial water content for24.9%of the sample is more intensive than the samples of32.22%and9.8%, which is consistent with the results of direct shear. After one time of wet-drying process, the soil micro structure state is hardly changed, only lead to increase porosity slightly and degree of orientability. After three times of the dry-wet circulation, the porosity increase massively, orientability become worse, the particles is out of order, flake particle breaking into smaller particle size, folding sheet structure of samples turn to flocculation structure.(12) The structure unit and pore of three kinds of soil-like materials have obvious fractal structure characteristics, and the fractal dimensions are all1.88-1.97. The fractal dimensions of the soil-like material of tuff eluvial soil shear plane in unsaturated direct shear test is1.9-2.0, moreover the fractal dimensions rise with the increases of the times of the dry-wet cycling. This conclusion helps illustrate that when soil-like material experiences drying-wetting cycles, the particles become loose, compactness is reducing, thus reduce the shear strength of soil, which is consistent with the disintegration of the soil-like material. The relationship between pore fractal dimension and moisture content shows that with the increase of initial moisture content, the shear surface pore distribution dimension is also in rise. When the dry samples turn to unsaturated, the pore fractal dimension increased more obviously than the one nearly saturated state. In the process of test, vertical load increases, the dimension is in a linear decrease trend. From another angle, The increase of the vertical load, sample density increase gradually, but when the load increases to a certain extent, the readjustment of the pore is becoming more and more difficult, soil structure gradually close to a new equilibrium, so the fractal dimension is nearly stabilized.(13) There is a quadratic function relationship between fractal dimension of pore distribution Dp and the unsaturated shear strength, which is that when Dp increase, cohesive force and internal friction angle is quadratic increase and decrease respectively. The correlation formula of pore fractal dimension and macro engineering parameters of soil-like material based on experimental results. On this account we can estimate the unsaturated shear strength of soil-like material in this research area from the microscopic view.(14) The groundwater seepage field of the landslide shows that under different rainfall conditions, the absolute value of pore water pressure (matrix suction) of soil-like material all in decreasing, the reduction rate and range are relative to the rainfall intensity and duration. Under the action of Low rainfall intensity and long durations, the matrix suction decreases gradually with the increase of rainfall duration, when the soil close to the saturated state, rate is reduced, until the matrix suction reduced to0. While with the high intensities and short durations, matrix suction is reduced in a short time rapidly, the slope surface soil tends to saturation state. Furthermore there will appear water in face of the slope.(15) The comparison of the displacement field in the landslide under different rainfall conditions show that the displacement of landslide correlates well with the rainfall, the greater total rain, the larger displacement will be, and most of the landslide displacement is formed in the early stage of the rainfall process, late rains the slope is tending towards stability. After rain, the displacement is the maximum in the middle of slope, the second place is in trailing edge the minimum in front. In the aspect of landslide stability, the results show that the Qishan landslide is in critical state with stability coefficient is1.08. One hand when the rainfall duration is same, the stability decreases with rainfall intensity. The other hand when the rainfall intensity is same the stability decreases with duration increasing, and the coefficient decrease rapidly at the beginning of the rain, as the rain continued, the coefficient tends to constant. Under the condition of same total rainfall, the influence of the type of the low intensity, long time rain on slope stability is larger than shorter time, high strength type of rainfall. This calculation results are consistent with the second chapter statistical results.(16) The atmosphere-unsaturated soil interaction model is used to establish the longtime calculation model of landslide stability considering soil surface evaporation and transpiration. The calculation shows that the influence of climate change on landslide stability is mainly through the change of matrix suction soil under the condition of dry-wet circulation. When rain start, the soil-like material is in the moisture adsorbing, result in matrix suction decrease with the shear strength, furthermore result in the stability decreasing, while the evapotranspirationt makes the soil into the dehumidifying process again, the matrix suction of soil gradually restored, but because of the capillary hysteresis of SWCC, The equivalent water content under the condition of the corresponding matrix suction decreases. So in long-term dry-wet circulation, makes the soil shear strength gradually decay, and eventually lead to slope instability. |
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