Numerical model and flume experiments of single- and two-layered hillslope flow related to slope failure

A hillslope flow model is developed considering 3D saturated and unsaturated flow of water during rainfall events. A finite difference-based numerical model of hillslope flow processes is developed. Four different experiments are done to see the effects of a single- and double-layered soil in pore-water pressure dynamics and slope failure. Results from the numerical model are verified with experimental results. The numerical and experimental values of the pore-water pressure and moisture contents are in good agreement. The results show that the hillslope heterogeneity caused by multiple layers of soil has greater influence on hillslope pore-pressure dynamics and slope failure patterns. The depth of slope failure shows high dependency on layering characteristics of the soil slope and pattern of rainfall. The proposed model provides a perspective on failure mechanism of a single- or double-layered slope under rainfall infiltration.     

Geochemical soil sampling for deeply-buried mineralized breccia pipes,northwestern Arizona

Thousands of solution-collapse breccia pipes crop out in the canyons and on the plateaus of northwestern Arizona; some host high-grade uranium deposits. The mineralized pipes are enriched in Ag, As, Ba, Co, Cu, Mo, Ni, Pb, Sb, Se, V and Zn. These breccia pipes formed as sedimentary strata collapsed into solution caverns within the underlying Mississippian Redwall Limestone. A typical pipe is approximately 100 m (300 ft) in diameter and extends upward from the Redwall Limestone as much as 1000 m (3000 ft).Unmineralized gypsum and limestone collapses rooted in the Lower Permian Kaibab Limestone or Toroweap Formation also occur throughout this area. Hence, development of geochemical tools that can distinguish these unmineralized collapse structures, as well as unmineralized breccia pipes, from mineralized breccia pipes could significantly reduce drilling costs for these orebodies commonly buried 300–360 m (1000–1200 ft) below the plateau surface.Design and interpretation of soil sampling surveys over breccia pipes are plagued with several complications. (1) The plateau-capping Kaibab Limestone and Moenkopi Formation are made up of diverse lithologies. Thus, because different breccia pipes are capped by different lithologies, each pipe needs to be treated as a separate geochemical survey with its own background samples. (2) Ascertaining true background is difficult because of uncertainties in locations of poorly-exposed collapse cones and ring fracture zones that surround the pipes.Soil geochemical surveys were completed on 50 collapse structures, three of which are known mineralized breccia pipes. Each collapse structure was treated as an independent geochemical survey. Geochemical data from each collapse feature were plotted on single-element geochemical maps and processed by multivariate factor analysis. To contrast the results between geochemical surveys (collapse structures), a means of quantifying the anomalousness of elements at each site was developed. This degree of anomalousness, named the “correlation value”, was used to rank collapse features by their potential to overlie a deeply-buried mineralized breccia pipe.Soil geochemical results from the three mineralized breccia pipes (the only three of the 50 that had previously been drilled) show that: (1) Soils above the SBF pipe contain significant enrichment of Ag, Al, As, Ba, Ga, K, La, Mo, Nd, Ni, Pb, Sc, Th, U and Zn, and depletion in Ca, Mg and Sr, in contrast to soils outside the topographic and structural rim; (2) Soils over the inner treeless zone of the Canyon pipe show Mo and Pb enrichment anf As and Ga depletion, in contrast to soils from the surrounding forest; and (3) The soil survey of the Mohawk Canyon pipe was a failure because of the rocky terrane and lack of a B soil horizon, or because the pipe plunges. At least 11 of the 47 other collapse structures studied contain anomalous soil enrichments similar to the SBF uranium ore-bearing pipe, and thus have good potential as exploration targets for uranium. One of these 11, #1102, does contain surface mineralized rock. These surveys suggest that soil geochemical sampling is a useful tool for the recognition of many collapse structures with underlying ore-bearing breccia pipes.     

The influence of shallow landslides on sediment supply: A flume-based investigation using sandy soil

The impact of rainfall-induced shallow landslides on hillslope sediment discharge is not well understood. This paper reports experimental measurements of sediment discharge after water-induced shallow landslides are triggered on sandy soil in a flume under simulated rainfall. The principal aim of the research was to investigate how varying soil depth affects the location and occurrence of shallow slope failures, as well as how it affects sediment yields downslope. Four experiments were conducted using the same sandy soil and a 30° and 10° compound slope configuration under average rainfall intensity of 50 mm h^− 1 for up to 390 min. Soil depths were set to 200, 300, 400 and 500 mm. Engineering and geotechnical properties of the soil were examined. Sediment discharge and runoff were collected from the flume outlet at 15 minute intervals. Changes in the soil slope profiles after landslides and soil physical properties resulted from soil armouring, under continuous rainfall were also recorded. Results showed that sediment yields at the flume outlet, before landslides occurred, were very low and limited to the finer soil particles as would be expected for a sandy soil. However subsequent variations in sediment discharge were strongly related to failure events and their proximity to the outlet. Sediment yield was also affected by the original soil depth; the greater the depth, the higher the sediment yields. Post-failure reductions in sediment discharge were observed and attributed to post-failure slope stabilization under continuing rainfall and extensive soil armouring near the flume outlet. The results provide a clear linkage between landslides and sediment discharge due to hydrological processes occurring in the hillslope. This knowledge is being used to develop a model to predict sediment discharges from hillslopes following shallow landslide events.     

尼古拉兹圆管沿程水流阻力实验的新发现

关于圆管沿程阻力系数的变化规律至今一直沿用尼古拉兹(Nikuradse)的实验结果。在参考尼古拉兹实验的基础上,采用与其相似的制作工艺,设计并制作了相对粗糙度Δ/d为1/30.89的人工粗糙圆管,开展了不同流速条件下的水流阻力实验,验证了尼古拉兹关于圆管沿程水流阻力随水流流态变化的规律,同时发现尼古拉兹当年在人工粗糙圆管制作过程中采用的"二次刷胶"粘贴砂粒的方法,使得圆管内壁砂粒的实际粗糙度减小,其实验所得的圆管沿程阻力系数偏小。本次圆管沿程阻力系数确定的实验方法能够更准确地获得人工粗糙圆管沿程阻力系数,今后可以据此开展相应的系统实验,对尼古拉兹人工粗糙圆管沿程阻力系数及其变化规律加以修正。     

Monitoring of Over Cutting Area and Lubrication Distribution in a Large Slurry Pipe Jacking Operation

Slurry pipe jacking was firmly established as a special method for the non-disruptive construction of the underground pipelines of sewage systems. Pipe jacking, in its traditional form, has occasionally been used for short railways, roads, rivers, and other projects. Basically the system involves the pushing or thrusting of concrete pipes into the ground by a number of jacks. In slurry pipe jacking, during the pushing process, mud slurry and lubricant are injected into the face and the over cutting area that is between the concrete pipes and the surrounding soil. Next, the slurry fills voids and the soil stabilizes due to the created slurry cake around the pipes. Fillings also reduce the jacking force or thrust during operation. When the drivage and pushing processes are finished, a mortar injection into the over cutting area is carried out in order to maintain permanent stability of the surrounding soil and the over cutting area. Successful lubrication around the pipes is extremely important in a large diameter slurry pipe jacking operation. Control of lubrication and gaps between pipes and soil can prevent hazards such as surface settlement and increases in thrust. Also, to find voids around the pipes after the jacking process, in order to inject mortar for permanent stabilizing, an investigation around the pipes is necessary. To meet these aims, this paper is concerned with the utilization of known methods such as the GPR (Ground Penetrating Radar) system and borehole camera to maintain control of the over cutting area and lubricant distribution around the pipes during a site investigation. From this point of view, experiments were carried out during a tunnel construction using one of the largest cases of slurry pipe jacking in Fujisawa city, Japan. The advantages and disadvantages of each system were clarified during the tests.     

Effects of monsoonal rainfall on waste dump stability and respective geo-environmental issues: a case study

The stability of mine waste dumps is of critical importance and an issue, the mining industry of Goa, India is continually facing. The State of Goa receives high rainfall of around 3,000 mm annually. This heavy monsoonal rainfall is often believed to be the cause of dump slope instability. In light of several dump collapses encountered in the State and their damaging effects on both mine operations and local geo-environmental conditions, this paper examines their stability considering various geotechnical factors and the downstream environmental effects of a slope failure. The mechanical properties of dump waste material are reported at different compaction levels. The effect of these properties and changes in pore-water pressure are specifically examined using limit equilibrium analysis. An empirical formula is developed relating dump height, material mechanical parameters and pore-water pressure to the factor of safety of the slope. It was found that the level of compaction experienced by the material had a significant effect on the factor of safety.     

Use of continuous measurements of dissolved organic matter fluorescence in groundwater to characterize fast infiltration through an unstable fractured hillslope (Valabres rockfall, French Alps)

Continuous measurements of natural fluorescence of dissolved organic matter (DOM) in groundwater have been used to characterize infiltration through an unstable fractured hillslope at the event time scale. Within the gneissic Valabres rockfall area in the southern French Alps, two sites, in the unstable massif and in the collapse area, were continuously monitored for electrical conductivity and for DOM natural fluorescence using a field fluorometer, and analyzed weekly for hydrochemical data. Two main results were found. From a methodological point of view, DOM natural fluorescence was a relevant tracer of fast infiltration in fissured media, knowing that continuous measurements improved the study of infiltration processes at the event time scale. From a hydrogeological point of view, the unstable fractured massif showed delayed dilution phenomena and the collapse areas showed fast and slow infiltration by piston-type flows via more or less open fractures. Consequently, from this conceptual model one may suppose that, unlike the collapse zones, the fractured hillslope threatening the valley is not submitted to strong pore-water pressure variations.     

Incorporating hydro-mechanical coupling in an analysis of the effects of rainfall patterns on unsaturated soil slope stability

Rainfall-induced landslides can cause loss of life and damage to property, infrastructure, and the environment. Rainfall patterns affect the pore-water pressure of unsaturated soil slopes, and are related to the slopes’ stability. Four rainfall patterns were chosen to represent natural rainfall patterns for an examination of rainfall infiltration into soil slopes using numerical models incorporating coupled water infiltration and deformation in unsaturated soils. Our analysis showed that rainfall patterns play a significant role in the distribution of the pore-water pressure in soil slopes, and influence the slope stability. The pore-water pressure profile of soil slopes and the factor of safety are affected by the ratio of rainfall intensity and the coefficient of permeability. The depth and shape of the shallow sliding plane of the landslide is closely related to the rainfall pattern; moreover, the results showed a correlation between the factor of safety of the slope and the rainfall intensity. This relationship can be described by a dimensionless rainfall intensity. The nonlinear relationship can be used to estimate the slope stability resulting from rainfall infiltration when the hydro-mechanical coupling in unsaturated soil slopes is considered.     

黄土丘陵区土壤侵蚀链垂直带水沙流空间分布

采用多坡段组合模型、人工模拟降雨实验研究了土壤侵蚀链垂直带水沙流空间分布变化特征。结果表明:愈向下坡方向的坡段产流量愈大,单位面积、单位时间的产流量按坡段排列为:谷坡>梁峁坡下部>梁峁坡中部>梁峁坡上部。雨强为29.7mm/h时,坡面上没有沟蚀发育;雨强为60.5mm/h时,细沟主要分布在梁峁坡下部和谷坡处;雨强为90 2mm/h时,各种侵蚀形态在坡面均有较好发育,细沟的出现部位一直伸展到梁峁坡中部和上部之间。由于上坡来水来沙的作用,梁峁坡的产沙量增大了20.2%～63.5%,谷坡的产沙量增大了42.9%～74.5%。     

Erosion-creep-collapse mechanism of underground soil loss for the karst rocky desertification in Chenqi village,Puding county,Guizhou, China

Carbonate rocks distribute widely in China. The total area of the carbonate rocks is about 3,430,000 km², and the exposed area of the carbonate is approximately 13 % of China’s territory. In 2003, soil loss in Yunnan, Guizhou, and Guangxi provinces reached 179,600 km², which is almost 40.1 % of the total area, causing rocky desertification. In this study, the erosion-creep-collapse mechanism of underground soil loss for the karst rocky desertification in Chenqi village, Puding county, Guizhou province is proposed. The mechanism occurs under the following geological environment: slope surface undulation, underlying bedrock surface fluctuation and thin and inhomogeneous soil overlying, overlying soil generation by bedrock weathering, underground karst development, and large groundwater depth and lying water table under the bottom of soils. The erosion-creep-collapse mechanism of underground soil loss in the karst slopes is explained as follows: power loss due to human cultivation activities that destroy the soil structure, hydraulic force formed by rainfall infiltration, wet–dry cycle generated by rainfall, erosion effect caused by rainfall penetration, creeping and flowing of plastic-stream soil, and collapse. The erosion-creep-collapse mechanism of underground soil loss has seven steps: disturbance of soils filled in underground karst cave by human activities, internal soil erosion and partial collapse caused by hydraulic power, internal free surface formation within the soil in the filled karst cave, internal soil creeping, soil pipe formation, soil pipe collapse, and ground surface collapse and filling. Soil loss develops slowly, and sudden transportation occurs by collapse. Soil loss can be explained by the proposed mechanism, and soil loss can be prevented by controlling soil collapse.     

排水措施对黄河堤防淤背体排水效果试验研究

选择水平盲沟、土工席垫及水平和竖向PVC渗管3种排水方案,在黄河山东东明某堤段淤背体冲填施工中进行排水效果试验。通过埋设在淤区内的地下水位观测点,得到淤区内地下水位与时间的变化规律。试验结果表明,采取一定的排水措施能加速淤区土体的固结,但排水效果与排水方案有关;土工席垫排水与水平盲沟排水效果相差不大,且排水效果与淤土厚度有关,3 m左右淤土厚度设置一层水平排水盲沟或土工席垫,其排水效果较好;淤土厚度增加,其排水效果变差;但两者均不能有效防止横向围堤坍塌。采用水平和竖向PVC渗管方案,其排水效果好,特别是设置了竖向PVC渗管,能有效防止横向围堤坍塌。试验成果为黄河放淤固堤工程加速淤背体排水固结设计及施工提供参考。     

Changes in velocity profiles at roughness transitions in coarse grained channels

Heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness. Distinct spatial zones in terms of grain size have been reported, for example sand ribbons and bedload sheets. The transition from areas of finer to coarser grained surface sediment is often abrupt. However, the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels. This paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistancé on the structure of the turbulent boundary layer. Measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river. Immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface, boundary shear stress and roughness length increase considerably. Downstream from this transition, velocity profiles become concave upwards. Downstream and upstream sections show significant differences in terms of near bed velocities (deceleration downstream of the transition), velocity gradient and turbulence intensity of the streamwise velocity component. Comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity (or shear stress) associated with the pressure drag produced by large and isolated obstacles.     

Topography and volume effects on travel distance of surface failure

The authors investigated factors affecting the travel distance of a surface failure and developed a simple model for estimating the distance based on soil properties and topographic factor. The authors conducted field surveys and various soil tests for a number of surface failures that occurred during torrential rain at the end of August 1998 in southern Fukushima Prefecture, Japan. The authors studied the effects of various factors such as landslide volume, pore-water pressure, slope inclination, and the internal friction near the slip surfaces on the travel distances. The analyses showed no clear relationship between landslide volume and equivalent coefficient of friction, which was likely attributable to the very small range of volume compared to other studies on catastrophic landslides. The effect of excess pore-water pressure was likely negligible because undrained conditions were not to be maintained at the shallow flow depths. A positive correlation was shown between slope inclination and equivalent coefficient of friction. This correlation was attributable to two factors, one was positive correlation between the internal friction on the slip surface and slope inclination, and the other was the kinetic energy dissipation of moving mass that occurred at the inclination changing point between a slope and a sedimentary flat surface. The authors then developed a predictive model for critical flow inclination of landslide by installing the factors of soil properties and slope inclination factors. The model predicted equivalent coefficient of frictions, which were very similar to the observed values, thus verifying the effectiveness of the model.     

Performance of steel-reinforced high-density polyethylene pipes in soil during installation: a numerical study

Steel-reinforced high-density polyethylene (SRHDPE) pipe combines the advantages of the HDPE and steel pipes with good corrosion resistance and relative high load carrying capacity. However, no widely accepted method is available for design of SRHDPE pipes. The objective of this study is to evaluate effects of several key influence factors on performance of SRHDPE pipes in soil during the installation to facilitate future development of a design method for the SRHDPE pipes. To achieve this objective, a numerical study was conducted to investigate the effects of soil cover thickness, trench width, magnitude of compaction pressure, and friction angle of backfill on the performance of SRHDPE pipes during their installation. Numerical results indicate: (1) pipe peaking deflections increased with an increase in the soil cover thickness, the trench width, and the magnitude of the compaction pressure; (2) the friction angle of the backfill material had an insignificant effect on the pipe deflection; (3) the lateral earth pressure coefficient at the springline of the pipe was approximately 0.65 during the installation; (4) the vertical arching factors at the top of pipes in all cases were greater than 1.0, which means a negative soil arching effect occurring in the soil cover during the pipe installation; and (5) the location and magnitude of the maximum bending moment changed with the elevation of the backfill during the installation.

     

山坡地形曲率分布特征及其水文效应分析——真实流域的野外实验及相关分析研究

研究了山坡地形曲率的空间分布特征,发现流域略呈现出微凹、发散的形状特征,平均的剖面和水平曲率分别为-4.62×10-4和3.49×10-4。通过划分源头型和边坡型山坡,发现源头山坡多是收敛的,边坡型山坡则多为发散的;在山坡内部收敛、发散、凹和凸等地形地貌类型是同时存在的,即山坡一般由相互组合的各种坡型所组成。基于野外采样观测的成果,分析了和睦桥子流域90个采样点的土壤含水量及其对应曲率的相互关系。结果显示:一般来说,凹形山坡、收敛形山坡对应的土壤含水量更高,而凸形和发散形山坡对应的土壤含水量较低;不论是收敛的凹坡,还是发散的凸坡,其采样点土壤含水量都有随高程下降而升高的趋势,即愈靠近沟谷土壤含水量愈大。     

岩体应力应变曲线转型的孔压效应与降雨滑坡的机制分析

从宏观与微观机制方面探讨了孔隙水压力对岩体脆－延性转变的影响。定义了一个峰后破坏割线模量Epost来表征孔隙水压力对岩体应力应变曲线转型的效应,并结合突变理论得到的斜坡失稳的刚度判据,进一步分析了孔隙水压力对应力应变曲线峰后斜率的影响,随孔隙水压力的增加,峰后曲线斜率变陡,峰后刚度增大,即材料的均匀性、脆性增大与刚度比k减小。因此在系统内部条件不变时,刚度比k=1时就存在一个临界孔隙水压力pwcritical,即降雨等涨落因素引起滑面介质中的孔隙水压力大于这个临界值时斜坡就易于发生突变失稳。从而深化认识了降雨等外部涨落因素对斜坡系统失稳的重要触发作用。     

溯源侵蚀引发的拉裂-倾倒型黄土崩塌形成机制

溯源侵蚀是黄土丘陵区最强烈的动力地质作用。常导致沟头后退、坡脚掏蚀,进而引发崩塌灾害。文章通过对研究区崩塌灾害的统计分析,厘定出溯源侵蚀作用引发崩塌的典型类型为拉裂-倾倒型。该类型崩塌具有后壁陡直、粗糙、规模大、分布广、危害范围广,潜在危险性大等特点。分析认为溯源侵蚀引发拉裂-倾倒型崩塌是水力侵蚀和重力侵蚀耦合作用的结果。该类型崩塌的形成,主要是由于斜坡在地表水力侵蚀和重力作用下,坡体内拉应力分布区、剪应力分布区和强烈水力侵蚀区三个区发生了转化,经历了斜坡应力重新分布、拉应力的增大与范围扩大,剪应力状态改变与范围集中、坡顶拉张与孔隙水压力耦合作用下的崩塌体倾倒破坏三个阶段。控制和影响拉裂-倾倒型黄土崩塌稳定性主要因素包括:斜坡几何特征、土体物理力学参数及裂隙中的静水压力。随着土体含水率的上升,坡顶拉张裂缝深度加深,侵蚀凹槽深度加深,崩塌体稳定性逐渐降低。     

水位骤降时的非饱和坝坡稳定分析

从非饱和土抗剪强度理论出发,应用极限平衡法和以有限元应力计算为基础的极限平衡法,分别对库水位骤降时的坝坡稳定进行了计算。分析中采用非饱和-非稳定渗流理论,模拟了孔隙水压力的消散过程,并考虑基质吸力对土体抗剪强度的影响,将每个渗流分析结果调入稳定分析模块计算其稳定性。最后将两类极限平衡法稳定分析结果进行了比较,结果表明,库水位的骤降,易引起坝坡的滑动,随着超静孔隙水压力的消散,坝坡稳定性逐渐提高;随着孔压消散,基质吸力对抗剪强度的贡献增大。     

开口管桩高频振动贯入过程的ALE有限元分析

任意拉格朗日-欧拉(ALE)方法吸取了拉格朗日和欧拉法的优点,避免了常规有限元中拉格朗日方法的网格畸变问题,适用于开口管桩高频振动贯入过程的计算分析。采用ALE有限元方法,建立开口管桩高频振动贯入过程的数值模型,对沉桩过程中挤土效应、桩侧阻力和土塞效应的变化规律进行了详细研究。研究结果表明:挤土应力主要沿径向传播,且深层土体受到的挤土应力比浅层土体大;水平挤土位移随管桩贯入深度的增加而增大,而最大水平挤土位移与管桩贯入深度存在累积效应;挤土效应的影响范围约为10倍管径,因此在施工过程中要给以足够重视;桩外侧摩阻力随贯入深度增加呈近似线性增长,桩内侧摩阻力随贯入深度增加而呈非线性增长,增长速率随贯入深度增加而逐渐增大;管内土塞处于不完全闭塞状态,土塞程度由完全非闭塞向部分闭塞过渡。此外,研究了土体模量、桩土界面摩擦系数、振动频率和桩径对土体位移的影响。     

不同温度及排水条件下高温冻土孔隙水压力试验研究

在荷载的作用下孔隙水压力的消散是揭示冻土整体变形机理的关键,为了研究高温冻土中孔隙水压力变化规律,在不同温度、不同排水条件下,对高温冻土开展了压缩固结试验,并监测其在-1℃、-0.5℃和-0.3℃的条件下孔隙水压力及位移变化情况。结果表明：温度对孔压、变形有较大影响,温度越高,土体的变形速率越大,孔隙水压力峰值越大,消散速率也越快;而温度相同时,排水条件下的孔压峰值比不排水条件下的低,位移比不排水条件下的大;从试验结果中可以认识到,孔隙水压力在受骨架挤压增大的同时也在缓慢消散。