| As a common geological hazard recently, debris flow causes great economic damage and casualties with features of business and seriousness. However, considering its complicated formation and high cost of treatment, now there is no effective measure to manage and control it. Therefore, for many years, scholars at home and abroad initiate a lot studies on debris flow, in which one of the most important preventing and mitigating way is the study on the trigger mechanism and critical condition, and it possesses a great directive meaning to the prevention and mitigation of debris flow. As to gully congeries triggered debris flow, so few cases could be found that few studies are on this type of debris flow in present studies. At the same time, studies on this type are mainly from the respects of topography, rain condition and other factors, lacking of sufficient laboratory tests and physical model studies. Hence study on gully congeries trigger mechanism by artificial rainfall experiment on gully congeries is of great significance.Luanchuan County locates in the mountain area of Funiu Mountains. Here the mountain area is larger than the plain area, and human activity is frequent, so that the topography is altered mostly. As well as the microclimate in mountain area, in the flood season rain comes intensively and frequently, so all the factors facilitate the debris flow in this area. Moreover, because of the smooth development of agriculture, mining industry and tourism, the tourism now is well-managed with perfect facilities benefiting by its early development, now it is the pillar industry of Luanchuan County. Most of scenic spots are in the gully, so they are influenced by the debris flow seriously, and the personal safety of residents and tourists are at risk of the possibility of debris flow. Luanchuan County pertains to the area prone of debris flow. In history many debris flow occurred in Luanchuan, and some debris flow gullies characterize in intermittency and large-scale. In 24th July 2010, debris flow occurs in 29 locations in Luanchuan by the result of continuous rainstorm, causing 89 casualties and direct economic loss 1.98 billion. By a close investigation in 2012, some congeries left by previous debris flow in some gullies is found. This congeries could be triggered as new debris flow once in the presence of heavy rainfall.Basing on field investigation, a large amount of congeries scatter in Shishu gully, the most spreads over the middle and downstream area where the terrain is comparatively flat. In 24th July 2010, debris flow occured in Shishu gully, and the material source was mainly the congeries left by previous debris flow. A serious damage was caused by this bursting debris flow. Therefore, study on the trigger mechanism and critical condition in Shishu gully benefits the prevention and mitigation of congeries triggered debris flow, reducing the loss of lives and property of residents, and providing some direct to the forecast and prevention of debris flow. Meanwhile, this study could also be a reference to the starting, forecast and prevention of same type of debris flow.Therefore, for the startup of channel deposit of debris flow in Shishu gully Luanchuan county, using ground surveys, laboratory tests for parameters, artificial rainfall models tests and theories analysis were combined to give the physical and mechanics parameters of the debris flow deposits in channel, and the variation of simultaneously of the shear strength, stress-strain, pore water pressure, moisture content in different conditions were obtained. Then combined with the results of debris flow starting test and previous research, the channel debris flow deposits starting mechanism were analyzed, the startup mode of debris flow fan under different conditions were discussed. Finally, with the use of SPH2D which is granular flow simulation software, the startup mode of Shishu gully debris flow in different rainfall intensities was simulated, then comparison with the tests result, a good coincidence with the actual situation was got. Based on the above, the study of debris flow fan in channel’s starting model is established, and the critical condition of startup in Shishu gully is explained. The main research work and results are as follows:1、Based on the investigation of debris flow which was known in Luanchuan county, its topography, geological environment conditions and outbreaks in history were found out, the causes and characteristics were analyzed. The landform of Luanchuan county, mainly is middle mountain and middle-low mountain, complex formation, complex geologic structure, abundant rainfall, human activities for transforming the environment are relatively frequent. For these conditions are conducive to the formation of debris flow, it has a multiple bursting in county history. By the investigation of Shishu gully, rainfall surface runoff scoured the deposit as the role of heavy rainfall, the it instability and form the debris flow, the source of this debris flow mainly came from the channel deposit which was retained by the onset of the debris flow in history.2、The development characteristics of debris flow in Shishu gully were discussed from its formation condition and characteristics of this debris flow bursting in July 24th,2010. The conditions of Shishu gully are beneficial to the startup of debris flow, as the abundant rainfall, drainage area and the gully slop ration are in the range of easy-happening of debris flow. The source of "7.24" debris flow were came from the channel deposit, which were mainly distributed in the zone-relief channel formed by the past debris flow whose scale was less than "7.24" debris flow. During the formation of debris flow, at first soil strength was weaken caused by the antecedent rainfall, then as the result of continuous high intensity rainfall, the strong surface runoff which carrying sand scoured, drive the fan to starting. The "7.24" debris flow has the characteristics of bottom tearing scouring to channel deposit by surface runoff, it’s a type of gully bed startup debris flow.3、By grain analyses test, X-ray diffraction test, and permeability test, the basic physical and mechanical parameters of provenance soil were got. Through laboratory tests on soil samples of basic physical, chemical and mechanical properties, which taken from the debris flow in Shishu gully, it came to the conclusion that the provenance soil can be considered to the coarse grained soil. And the uniformity coefficient of the soil samples is 26.92, the curvature coefficient is 4.95, the two coefficients show that the particle distribution of soil samples is not uniform, while the particle size distribution is well. The results of permeability test show that, the hydraulic conductivity of soil samples locates within the coarse sand, and it is conducive to the rapid infiltration of rainwater with the good permeability.All these features help the provenance soil quickly reach saturation state in rainfall, the formation of surface runoff, and the debris flow initiation may be quick and easy.4、By using large shear test and triaxial shear test, the variation between the value of c, (?) and the stress and strain of the provenance soil in different conditions is studied. Test results show that there have been an inverse relationship between the cohesion and saturation of provenance soil, cohesion decreases with the increasing saturation, and the shear strength of soil is inversely proportional to the saturation, the shear strength decreases with the increasing saturation. With the provenance soil gradually reaching to the saturation state in rainfall conditions, the water holding capacity of the soil reduces, the cohesion decreases sharply, and the shear strength decreases, and with the rainfall going on, surface runoff occurs which is provided with strong erosion and sediment carrying capacity, it could easily lead to the occurrence of debris flow. In CU shear test, the soil samples with small dry density mostly shear contraction destroyed, and the soil samples with large dry density mostly shear dilatancy destroyed. In addition, the value of c, (?) increases while the dry density of the soil samples increase. The friction angle of soil samples in CU shear test is much greater than the friction angle in UU shear test under the same conditions, and this gap became smaller with the dry density increasing.5、For debris flow deposit under the condition of different gully bed gradient, saturability of the soil and rainfall intensity, using artificial rainfall,analyze and research the change of the provenance strength and the launched result in the process. Through physical model experiment, it is concluded that when debris flow started, the soil of provenance saturated basically,the water content of the soil rising quickly in the downstream, the final moisture content is higher, more likely to be damaged.The increase of the pore water pressure has a great influence on soil strength, the pore water pressure with the trend of increasing first then decreasing and remained stable at last in the process of the experiment, and was effected sensitive mostly for the rainfall intensity at present. Soil has an intense energy exchange in the process of moisture content increased gradually and eventually achieve shear failure but the energy exchange is smaller after the shear failure. Through analyzing the test results of orthogonal design, the primary and secondary relations of factors in gully’s forming time is rainfall intensity> saturability> gully bed gradient; the primary and secondary relations of rushing out of the influence of the volume in provenance is saturability> rainfall intensity> gully bed gradient. For ShiShu gully,the debris flow is not going to happen when the rainfall intensity get the value of 30 mm/h at present. debris flow deposit launched partly when the rainfall intensity get the value of 60 mm/h at present. Saturability of the soil is 50% when gully bed gradient get the value of 17°,and debris flow occurred when current cumulative rainfall get the value of 70 mm; when saturability of the soil is 100%, and debris flow occurred when current cumulative rainfall get the value of 20 mm even the gully bed gradient get the value of 12°; When the current cumulative rainfall reaches 90 mm, debris flow deposit launched with a wide range of parts,even cracked the ground,when the initial saturability of the soil is 75%,gully bed gradient get the value of 12°nd debris flow occurred when current cumulative rainfall get the value of 40.5 mm. when the initial saturability of the soil is 100%,gully bed gradient get the value of 17°nd debris flow occurred when current cumulative rainfall get the value of 30.5 mm. when the initial saturability of the soil is 50%,gully bed gradient get the value of 15°nd debris flow occurred when current cumulative rainfall get the value of 85.5 mm.This launching critical condition can play a role as a reference and guidance for the launch forecasting work of debris flow deposit in ShiShu gully for the future.6、According to the test results, we can analyze the debris flow deposit in channel resurrection start-up mechanism, and use the SPH simulation software to verify the findings. Through experimental observation, what the debris flow deposit in channel resurrection start-up process is:antecedent precipitation-current rainfall, shaping the runoff, soil surface erosion, forming gullies-soil strength continuing to decline, approaching critical stable state-soil body reaching a critical steady state and then being unstable, mudslides fan start-up-forming debris flow. However, it is different in start-up and failure modes in the form of moderate rain and heavy rain. In moderate intensities,when the deposit reaches a critical steady state, the soil in the toe of the slope produces liquefaction, then stacked body starts to break down from the foot of the slope and partially starts forming debris flow; but in heavy rain,and the deposit reaches a critical stable state, provenance soil begins to liquefy,when saturated soil is in the further rain strike and vibration and surface runoff erosion, it will be unstable.and the deposit begins to start up from the foot of the slope rapidly, then it appears start-up in large areas, and even brings about the planning bottom phenomenon. It can be seen from numerical simulation results of Pei Shu Channel debris flow, when the loose deposit in trench under the moderate rain condition, slope shape produces only partial adjustment, and the deposit starts up partly, also surface runoff erosion only affects the deposit surface, but it does not affect the overall shape; when under the heavy rain conditions, slope shape does change, and the deposit begins to shift, the rapid development of slope deformation, then the deposit starts up entirely. The simulation results are basically consistent with the site survey and physical model test results.The main innovation points in this paper are as follows:1.. Based on taking the test of physical and mechanics parameters of the debris flow whose source is debris flow fan in channel, the feature parameters are obtained. And according to the results of large direct shear test and the triaxial shear test, a detailed analysis and research on strength and characteristics of the source soil were got. Furthermore, a physical model test with artificial rainfall is implemented on this type of debris flow.This test condition uses the SPSS software to take an orthogonal design which has greatly reduced the number of experimental groups. During the formation of debris flow, the variation of the accumulation body in moisture content, pore water pressure and temperature etc is obtained. Combined with the start condition of debris flow which has been observed, this paper takes an orthogonal evaluation on the formation time of gully and the runout quantity of source soil, then obtains the importance ranking of the three factors and the start condition of debris flow.2, Combining with parameter test and physical model test results and analyzing the starting process and genetic mechanism of debris flow that derives from the provenance of accumulation body, a conclusion about the trigger mechanism of this debris flow is reached. The different starting-damage modes of debris flow derived from Shishu channel accumulation body both in the medium rainfall intensity and maximum rainfall intensity are analyzed and stated. At the end of this dissertation, the author conducts a numerical simulation of the starting process of debris flow derives from Shishu channel accumulation body by SPH software. Combining with the practical survey and test results, the author analyzes and verifies the trigger mechanism that comes from the preceding test.All in all, in this dissertation the author studys the re-starting process and formation mechanism of Shishu channel debris flow accumulation body basing on the properties of soil mass of provenance by results of model test, and concludes the critical condition of Shishu channel accumulation -body debris flow and the re-trigger mechanism of channel debris-flow accumulation body. This study benefits the forecast, and prevention of debris flow of this kind a lot, and it’s conducive to the further development of disaster prevention and mitigation. |
PDF Full Text Request