Mechanisms involved in triggering debris flows,within a cohesive gravel soil mass on a slope:a case in SW China

The triggering mechanisms of debris flows were explored in the field using artificial rainfall experiments in two gullies, Dawazi Gully and Aizi Gully, in Yunnan and Sichuan Provinces, China,respectively. The soils at both sites are bare, loose and cohesive gravel-dominated. The results of a direct shear test, rheological test and back-analysis using soil mass stability calculations indicate that the mechanisms responsible for triggering debris flows involved the decreases in static and dynamic resistance of the soil. The triggering processes can be divided into 7 stages: rainfall infiltration, generation of excess runoff, high pore water pressure, surface erosion, soil creep, soil slipping, debris flow triggering and debris flow increment. In addition, two critical steps are evident:(i) During the process of the soil mass changing from a static to a mobile state, its cohesion decreased sharply(e.g., the cohesion of the soil mass in Dawazi Gully decreased from 0.520 to0.090 k Pa, a decrease of 83%). This would have reduced the soil strength and the kinetic energy during slipping, eventually triggered the debris flow.(ii) When the soil mass began to slip, the velocity and the volume increment of the debris flow fluctuated as a result of the interaction of soil resistance and the sliding force. The displaced soil mass from the source area of the slope resulted in the deposition of a volume of soil more than 7-8 times greater than that in the source area.

Effect of clay content to the strength of gravel soil in the source region of debris flow

The production of runoff in the source area of a debris flow is the consequence of a reduction in soil strength. Gravel soil is widely distributed in the source region, and the influence of its clay content on soil strength is one of the important questions regarding the formation mechanism of debris flows. In this paper, the clay content in gravel soil is divided into groups of low clay content(1%, 2, 5%), moderate clay content(3.75%, 5.00%, 6.25%, 7.5%) and high clay content(10.0%, 12.5%, 15%). Tests of the unconsolidated undrained shear strength and consolidated drained shear strength were performed. The unconsolidated undrained shearing(UU) experiment simulates the rapid shear failure of loose gravel soil under the conditions of brief heavy rainfall. The consolidated drained shearing(CD) experiment simulates creep failure of consolidated sediment during extended rainfall. The pore water pressure first increased and then decreased as the clay content increased, and the increase in pore pressure was relatively high in the gravel soil sample when the clay content is in the range of 3.25-7.50%, and stress in the gravel soil is relatively low for a moderate clay content. Gravelly soils with a moderate clay content are moreprone to debris-flow initiation. This paper presents a mathematical formula for the maximum shear stress and clay content of gravel soil under two conditions. The key processes whereby the soil fails and triggers a debris flow—volume contraction of soil, expansion of clay soil, and rise of pore pressure―cause reductions in the soil friction force and enhancement of the water content in the clay particles, and subsurface erosion of soil reduces the soil viscosity, which eventually reduces the soil strength so that the soil loses its stability, liquefies and generates a debris flow.

Analysis of influence factors of unconfined compressive strength for composite soil stabilizer-stabilized gravel soil

In order to investigate the effect o f some factors on the unconfined compressive strength(UCS)for composite soil stabilizer-stabilized gravel soil(CSSSGS),the orthogonal test is adopted to set up the experimental scheme.Three levels o f each factor armconsidered to obtain the change laws o f UCS,in which the binder dosages are8%,10%,and12%;the curing times ae7,14and21d;the gradation nae0.3,0.35and0.4;and the degrees of compaction are95%,97%,and99%.The range analysis clearly indicates that the influence degree o f the four factors on UCS is in such an order:dosage,age,gradation,and degree o f compaction.The variance analysis shows that only the composite soil stabilizer dosage can significantly affect UCS.In road construction,the examination o f composite soil stabilizer dosage and base-course maintenance should be given much more attention to obtain satisfactory base-course strength,compared w ith gradation floating and the change of degree o f compaction.

Influence law of modified glutinous rice-based materials on gravel soil reinforcement and water erosion process

A large number of loose piles formed by mountain hazards are highly susceptible to hydraulic erosion under rainfall conditions.The use of ecological substrate materials for erosion control and ecological restoration of gravel soil slopes has become a current research hotspot and the study difficulty.The post-earthquake slump accumulation gravel soil in Jiuzhaigou was selected as the research object,and the self-developed modified glutinous rice-based material was used to reinforce the gravel soil.The variable slope flume erosion test and rainfall simulation test were carried out to study the water erosion resistance of the material reconstructed soil under the influence of runoff erosion and raindrop splash erosion.The results show that:As the material content reached 12.5%,the reconstructed soil did not disintegrate after 24 hours of immersion,the internal friction angle was increased by 42.26%,and the cohesion was increased by 235.5%,which played a significant reinforcement effect.In the process of slope erosion,the soil rill erodibility parameter Kr was only 3‰ of the gravel soil control group,the critical shear force τ increased by 272%,and the soil erosion resistance was significantly improved.In the process of rainfall and rainfall on the slope,the runoff intensity of the reconstructed soil was stable,and the ability to resist runoff erosion and raindrop splash erosion was enhanced.The maximum value of soil loss rate on different slope slopes is 0.02-0.10 g·m^(-2)s^(-1),which is significantly lower than that of the control group and has better erosion reduction effect.

Evaluation of the Impact of Truck Overloading on Flexible Compacted Gravel Lateritic Soil’s Pavements by FEM with Cast3M^©

Within the framework of a FEM of the nonlinear behavior of lateritic pavement of Senegal, the effect of truck overloading is studied to estimate its impact on the deformability of road pavement on compacted gravel lateritic soils. For that purpose, various loading conditions were tested to measure the impact on the critical response parameters of road pavement design. The implementation of the models was realized with Cast3M©. This study allowed us to point out that the observed variations are linear and would help to plan in advance the impact of axle overloads for a better evaluation within the framework of the mechanistic (M. E.) design of pavements.

Effects of Different Gravel Mulched Years on Soil Microbial Flora and Physical and Chemical Properties in Gravelsand Mulched Fields

Soil microbial flora and influencing factors of soil microbes in soil and gravel-sand mixed layer（ SGSML）,roots denseness layer（ RDL）,eluviate layer（ EL） and calcium accumulation layer（ CAL） in gravel-sand mulched fields（ GSMFs） with different gravel mulched years（ 1,6,12,19 and 25 years） were studied. The results showed that in the composition of soil microbes in the GSMFs,the quantity of bacteria was the largest,followed by actinomycetes,while the number of fungi was the smallest. The total quantity of soil microorganisms in the GSMFs dropped rapidly with the increase of soil depth,which was related to the sudden decrease in the quantity of bacteria. The number of microbes in the RDL was larger than that in the SGSML with few roots due to the effects of root distribution. The number of bacteria and actinomycete in the growing season was larger than that in the non-growing season,while the quantity of fungi in the growing season was smaller than that in the non-growing season. The quantity of bacteria and fungi was the largest in the GSMFs which had been mulched with gravel for 6-12 years. With the increase of mulching time,the GSMFs aged gradually,so their quantity reduced gradually. The quantity of actinomycetes was the smallest in the GSMFs which had been mulched with gravel for 6-12 years and increased with the increase of mulching time. The number of soil microbes in the GSMFs had a good correlation with soil moisture content,p H and mulching time. Soil total carbon content was an important factor restricting the quantity of soil microbes in the GSMFs.

Grain Size Influence on the Compaction Aptitude and the Bearing Strength of the Gravel Lateritic Soils

This study is carried out to appreciate the coarse elements influence of the gravel lateritic soils on the pavement behavior. The material involves not only the 0/20 mm granular particles considered in the specifications and on the laboratory tests, but also all of the material that is actually used in the construction sites. Geotechnical characterization was leaned on five differentiated granular classes of 0/20;0/25;0/31.5;0/40 and 0/63 mm diameters. The sample of 0/40 mm gets the best compaction aptitude, while that of 0/31.5 mm reaches the greatest bearing strength and a good Optimum Moisture Content. According to the requirements and its bearing strengths, the material is usable up to subbase layer and its behavior depends on the grain size.

Frost heave control of fine round gravel fillings in deep seasonal frozen regions

Fine round gravel soil is widely employed in the subgrade of high and thawing. The lower the fines content in fine round gravel soil, but compaction difficulty increases. This study is to obtain the speed railways in cold regions to prevent frost heaving the smaller the quantities of frost heaving and thawing, optimum fines content and limited frost heaving and thawing. The fine round gravel soil filling （FRGSF） used in the Harbin-Qiqihaer Passenger Dedicated Line is taken as the study object. Influence of fines content on optimum water content, maximum dry density and frost heaving properties of FRGSF were studied by means of compaction and frost heaving tests. Results show that the maximum dry density of the FRGSF increases first and then decreases with an increase of fines content, namely there is an optimum fines content for easy compaction. The method of surface-vibratory instrument is fit for coarse-grained soils, and wet state of coarse-grained soil is in favor of compaction. Considering the relationship of fines content with maximum dry density and the frost heaving ratio of FRGSF, the fines content should be limited to within the range of 9%-10%, so that the frost heaving ratio is less than 1%, and the FRGSF is easily compacted. Water supply is proved to be an important factor influencing the amount of frost heaving of FRGSF. We also conclude that in the field, it is imperative to control waterproofing and drainage measures.

A fundamental procedure and calculation formula for evaluating gravel liquefaction

Field investigations following the 2008 Ms8.0 Wenchuan earthquake identified 118 liquefaction sites, most of which are underlain by gravelly sediment in the Chengdu Plain and adjacent Mianyang area, in the Sichuan Province. Gravel sediment in the Sichuan province is widely distributed; hence it is necessary to develop a method for prediction and evaluation of gravel liquefaction behavior. Based on liquefaction investigation data and in-situ testing, and with reference to existing procedures for sandy soil liquefaction evaluation, a fundamental procedure for gravel liquefaction evaluation using dynamic penetration tests （DPT） is proposed along with a corresponding model and calculation formula. The procedure contains two stages, i.e., pre-determination and re-determination. Pre-determination excludes impossible liquefiable or non-liquefiable soils, and re-determination explores a DPT-based critical N120 blows calculation model. Pre-determination includes three criteria, i.e., geological age, gravel contents, gravel sediment depths and water tables. The re-determination model consists of five parameters, i.e., DPT reference values, gravel contents, gravel sediment depths, water tables and seismic intensities. A normalization method is used for DPT reference values and an optimization method is used for the gravel sediment depth coefficient and water table coefficient. The gravel liquefaction evaluation method proposed herein is simple and takes most influencing factors on gravel sediment liquefaction into account.

Resilient Modulus of Compacted Lateritic Soils from Senegal at OPM Conditions

Repeated load triaxial tests were performed on five compacted gravel lateritic soils collected from different locations in Senegal: Sébikotane, Dougar, Pa Lo, Mont-Rolland and Ngoundiane. The study revealed that resilient modulus decreases with the increase of the bulk and deviatoric stress in constant confining pressure. In addition, resilient modulus increases with the percentage of cement for appreciably equal contents of moisture. This effect tends to stop for higher stress. Besides, correlations were made with some models of resilient modulus such as the Uzan-Witczack model (Witczack and Uzan, 1988 [1]) and the National Highway Research Program (NCHRP) model (2004 [2]). The study confirms that both models give very good results with the best correlations being obtained with the Uzan-Witczack model.

Triaxial shear behavior of a cement-treated sand——gravel mixture

A number of parameters,e.g.cement content,cement type,relative density,and grain size distribution,can influence the mechanical behaviors of cemented soils.In the present study,a series of conventional triaxial compression tests were conducted on a cemented poorly graded sandegravel mixture containing 30% gravel and 70% sand in both consolidated drained and undrained conditions.Portland cement used as the cementing agent was added to the soil at 0%,1%,2%,and 3%(dry weight) of sandegravel mixture.Samples were prepared at 70% relative density and tested at confining pressures of 50 kPa,100 kPa,and150 kPa.Comparison of the results with other studies on well graded gravely sands indicated more dilation or negative pore pressure in poorly graded samples.Undrained failure envelopes determined using zero Skempton’s pore pressure coefficient (= 0) criterion were consistent with the drained ones.Energy absorption potential was higher in drained condition than undrained condition,suggesting that more energy was required to induce deformation in cemented soil under drained state.Energy absorption increased with increase in cement content under both drained and undrained conditions.

基于拉拔试验的加筋土界面摩擦特性

加筋土具有良好的工程性能,被广泛应用于加固路基和边坡支挡等领域,筋土界面摩擦强度对支挡结构的整体性和稳定性具有重要影响.为研究不同加筋土的界面摩擦特性,自制拉拔装置,对黏土-土工格栅、黏土-碳纤维布、黏土-土工格室和碎石土-土工格室等4种加筋土进行拉拔试验.研究结果表明,4种加筋土中,土工格室与碎石土的界面摩擦强度最高,构成的加筋体承受变形的能力最强;黏土-碳纤维布构成的加筋体界面摩擦强度最低,但具有筋土间相互作用发挥较快、允许变形小的特点;不同加筋体的允许变形差别较大,在实际工程中应充分考虑.提出了一种考虑土工格室加强区的无黏性土-土工格室界面最大摩阻力计算理论模型,研究结果可为加筋土工程筋材选取提供参考.

Stress Ratio-Strain Relation of Pile and Soil in Composite Foundation

A series of triaxial compression tests were arried out by means of composite-reinforced soil samples to simulate the interaction between soil and pile. The samples are made of gravel or lime-soil with different length at the center. The experiment indicates that the strength of the composite samples can not be obtained by superimposure of reinforcing pile and soil simply according to their replacement proportion. It also indicates the law for stress ratio of reinforcing column to soil. The stress ratio of reinforcing column to soil increases and reaches peak rapidly while load and strain is small. Then the ratio decreases. This law is in accordance with the measuring resuits in construction site.

基于电阻的冻结砂砾土孔隙冰压融效应研究

为验证冻结砂砾土压缩过程中是否存在压融效应,对不同含水(冰)状态与不同冻结温度的砂砾土进行无侧限压缩试验和电阻测试,通过核磁共振测定饱和砂砾土的未冻水含量。结果表明:(1)砂砾土压缩过程电阻均先快速降低后趋缓,仅有干燥样品在应力峰值点后出现电阻增大的现象;(2)电阻快速降低阶段干燥样品的电阻降低率小于饱和冻结样品,−4℃饱和样品该值为26.8%,其值为相同温度下干燥样品的4倍;(3)随温度降低,电阻快速降低阶段的降低率先增加后减小;(4)随温度降低,自由水与毛细水的相对含量快速减小,吸附水的相对含量先小幅增加后缓慢降低。分析认为:冻结砂砾土压缩过程中存在压融效应,这导致样品在受荷过程中高应力区未冻水含量增大,而融水会沿未冻水膜向低应力区的孔隙迁移、复冰进而改变孔隙结构;冻结温度在−2~−4℃区间附近,压融效应更容易发生。

玄武岩纤维遏制作用下的砾类土管涌试验研究

管涌如果无法及时得到遏制,会导致大量土体流失以致堤防溃决。为研究管涌发生环节时使用玄武岩纤维的遏制作用,拟通过室内管涌试验,从孔压、水力梯度、渗透系数、流速、颗粒流失等方面,研究不同纤维含量和纤维长度的玄武岩纤维对管涌发生发展的遏制效应。结果表明,玄武岩纤维的加入使得缺级配砾类土体的迁移通道变得更加曲折和狭窄,从而遏制整个土体的管涌发展。同等玄武岩纤维含量下玄武岩纤维缺级配砾类土存在最佳的纤维长度遏制管涌发展。纤维长度过长,增大了土体的迁移路径同时也使迁移通道相对变宽;纤维长度越短使得细颗粒的迁移通道变得更加狭窄,更加容易造成局部堵塞,进一步使得管涌继续从其他方向发展,造成颗粒的进一步流失。

砂砾土渗流侵蚀中细粒迁移-沉积-滤通的物理水力临界条件

细粒迁移机制是理解砂砾土渗流侵蚀过程的基础与关键,对研究砂砾土斜坡雨水侵蚀过程的细观致灾机制具有重要意义。目前其运移模式及运移状态发生转变的临界条件并不清晰,不同物理水力条件下的细粒运动类型有所不同。为掌握砂砾土侵蚀过程中细粒的整体运动类型及其发生改变的临界条件,采用可视圆柱入渗试验和离散元数值模拟,分析了细粒迁移的影响因素和内部机理。结果表明:(1)细粒迁移受级配和水力梯度影响显著,而受初始孔隙率影响不显著,且级配的影响大于水力梯度;(2)水力作用下细粒整体运动状态可分为沉积和滤通2种模式,内部结构不稳定的砂砾土细粒运动处于滤通状态,内部结构稳定和稳定性过渡型砂砾土随水力梯度升高细粒的运动状态从整体沉积转变为整体滤通;i=3.4-(3)细粒运动状态在粒径比和水力梯度共同作用下存在明显界限,最终得到细粒沉积-滤通转变的临界条件为0.12e(D15/d85)/1.5。研究可为砂砾土斜坡渗蚀失稳防护提供理论指导。

砾石覆盖下的农田土壤环境效应:研究进展与展望

砾石覆盖是旱区特有的一种保护性耕作模式。长期的农业生产实践已经证明,地表覆砂能够很好地适应旱区严酷、恶劣的自然环境,并为当地农业发展和效益提升带来极大助力。然而,砾石覆盖农田退化所带来的一系列负面效应愈加凸显,成为当地农业发展的障碍因子。文章以砾石覆盖的土壤环境效应为脉络,归纳总结了砾石覆盖对土壤水热环境、土壤盐分、土壤有机质、土壤微生物和酶活性,以及水土保持等方面的影响效应和相关机制。同时,进一步指明旱区砂田当前所面临的问题与挑战,并提出该领域未来可持续深化和拓展的方向,主要包括:(1)砾石覆盖农田退化过程中土壤环境的综合演变机制;(2)“砾石-土壤-作物”系统间的协同作用机制;(3)应对砂田退化的农业修复模式及可持续发展路径;(4)砾石覆盖对局部区域气候的影响机制。

干旱砾漠区不同微地貌单元土壤性状及真菌群落变化特征

探明干旱砾漠区不同微地貌单元土壤性状、真菌群落组成特征及其变化驱动因素,对于该区土壤真菌群落构建机制的理论研究和针对性生态损伤修复策略制定的实践指导具有重要意义。本文首先对比了干旱砾漠区4种微地貌单元(风蚀残丘、砾漠戈壁、河谷和风沙地)土壤理化性质、真菌α多样性及群落组成的变化特征,再结合不同微地貌单元植物群落特性和微气象因子测定,探究了各微地貌单元影响土壤真菌群落的主要生态因子。结果表明:(1)干旱砾漠区土壤均为砂质土壤,其中风沙地砂粒含量最大且黏粒含量最小,而河谷土壤粒径组成则相反;河谷和砾漠戈壁之间土壤容重和有机质含量无显著差异但显著高于其他两种微地貌单元;河谷土壤可溶性盐含量显著低于其他微地貌单元21.4%,但土壤含水量显著高出39.3%;速效养分中除砾漠戈壁的速效氮与速效钾、风蚀残丘的速效磷含量显著较低外,其他微地貌单元速效养分并未表现出显著性差异。(2)土壤真菌α多样性中,Shannon-Wiener指数、Pieloue指数和Simpson指数均表现为在风沙地显著降低而其他微地貌单元差异不明显,但Chao1多样性指数没有显著差异;真菌群落组成在门水平上,不同微地貌单元都以子囊菌门和担子菌门为优势菌门,其中子囊菌门在砾漠戈壁和风蚀残丘优势度最大,担子菌门在河谷占比最大;在属水平上,风蚀残丘的新凸轮孢菌属、暗茎草属,砾漠戈壁的新凸轮孢菌属、光黑壳,河谷的曲霉属、链格孢属,风沙地的金银花属、新凸轮孢菌属分别为各地貌单元土壤真菌群落优势属。(3)土壤含水量、有机质、速效氮、可溶性盐是影响干旱砾漠区不同微地貌单元土壤真菌群落结构变化的共同关键因子,风沙地的土壤机械组成、风蚀残丘的地表温度、光辐射强度、砾漠戈壁的地表风速和河谷的地表植被生物量分别为各自地貌单元影响土壤真菌群落的差异化生态因子。

不同覆砂年限果园土壤水盐及矿质元素变化与砂田衰退关系研究

通过研究不同覆砂年限砂田土壤水分特征和矿质元素变化规律,探索砂田衰退的机理。对可用砂田和废弃砂田的砂土混合层变化、土壤含水量、矿质元素及盐分含量进行了长期定位的分析测定。结果表明,种植1、3、7、15a砂田砂土比由9.16降低至1.60,而荒地为3.59。砂覆盖3a后土壤平均水分含量较原生地高13.1mg/g;砂覆盖15a后土壤0~20、20~40、40~60、60~80、80~100mm土层含水量比原生地分别高2.4、3.5、4.0、4.4、4.7mg/g。不同覆砂年限果园0~60cm土壤有机质、土壤全氮与碱解氮、全磷与速效磷、全钾与速效钾及全盐含量变化随覆砂年限增加均呈先增加后降低的趋势,这与砂田功效及衰退密切相关,也是中老砂田衰退的主要特征。

充填型岩溶隧道仰坡滑塌机制及稳定性控制技术——以宜宾巡场至玉和公路下坝隧道出口段为例

为解决充填型岩溶隧道进洞过程中引起的仰坡滑塌问题,利用现场调查和MultiFracS多物理场断裂分析软件,对隧道进洞过程中仰坡体内裂缝发展过程和围岩变形情况进行分析,提出洞口仰坡加固措施并进行效果评价,得出如下结论:1)连续降雨是引起仰坡滑塌的直接原因,随着含水率的增加,碎石土充填物抗剪强度降低,溶腔区位移变化显著且出现微裂隙;当含水率增加至20%时,溶腔区裂缝进一步发展,局部出现开裂,地表滑动位移不断增加。2)隧道进洞施工扰动加速了地表覆盖层与下伏基岩间贯通裂缝的形成,仰坡体呈现整体滑动趋势。基于现场调查和数值分析,采用“缓坡+钢锚管/锚索框架梁”的仰坡防护形式,用注浆钢锚管替代锚杆和部分锚索,能够避免溶腔内碎石土在雨水下渗中产生变形破坏。经现场实测数据验证,加固后的仰坡在隧道二次进洞过程中处于稳定状态,隧道洞内变形和地表沉降均得到了有效控制。