Soil moisture causes dynamic adjustments to root reinforcement that reduce slope stability

In steep soil‐mantled landscapes, the initiation of shallow landslides is strongly controlled by the distribution of vegetation, whose roots reinforce the soil. The magnitude of root reinforcement depends on the number, diameter distribution, orientation and the mechanical properties of roots that cross potential failure planes. Understanding how these properties vary in space and time in forests remains a significant challenge. Here we test the hypothesis that spatio‐temporal variations in root reinforcement along a hillslope occur as a function of topographic soil moisture gradients. To test this hypothesis we compared root reinforcement measurements from relatively dry, divergent noses to relatively wet, convergent hollows in the southern Appalachian Mountains, North Carolina, USA. Our initial results showed that root reinforcement decreased in areas of higher soil moisture because the tensile strength of roots decreased. A post hoc laboratory experiment further demonstrated that root tensile strength decreased as root moisture content increased. This effect is consistent with other experiments on stem woods showing that increased water content in the cell wall decreases tensile strength. Our experimental data demonstrated that roots can adjust to changes in the external root moisture conditions within hours, suggesting that root moisture content will change over the timescale of large storm events (hours–days). We assessed the effects of the dynamic changes in root tensile strength to the magnitude of apparent cohesion within the infinite slope stability model. Slopes can be considerably less stable when precipitation‐driven increases in saturated soil depth both increase pore pressures and decrease root reinforcement. Copyright © 2016 John Wiley & Sons, Ltd.     

Seedling growth dynamic of Haloxylon ammodendron and its adaptation strategy to habitat condition in hinterland of desert

Through measuring the above/below-ground growth data of Haloxylon ammodendron seedlings at different stages in hinterland of the desert the results show that the H.ammodendron seedling growth has demonstrated different adaptation characteristics in the continued arid environment with time and space. In May, July, September and October, the growth speed of vertical root is 0.607 cm/d,0.809 cm/d,0.155 cm/d and 0.394 cm/d, respectively; the growth speed of height is 0.093 cm/d,0.076 cm/d, 0.408 cm/d and 136 cm/d, respectively. It is explained that seedlings root system has the growth superiority in space. The maximum growth speed of below-ground (vertical root and horizontal root) of seedling is earlier than that of above-ground (height and horizontal of shoot). In the different periods, the vertical growth speed and the horizontal growth speed of below-ground is 2-10 times and 3-5 times than the height increase speed and the shoot growth speed, respectively. In the whole season, the growth speed of above/below-ground of seedlings shows the alternation growth tendency. At the different periods, the root/shoot ratio of H. ammodendron seedlings is 0.41,0.3,0.39 and 0.88. All these characteristics are the comprehensive performance of seedlings' strategy selection to adapt to the continued arid environment.     

Modelling increased soil cohesion due to roots with EUROSEM

As organic root exudates cause soil particles to adhere firmly to root surfaces, roots significantly increase soil strength and therefore also increase the resistance of the topsoil to erosion by concentrated flow. This paper aims at contributing to a better prediction of the root effects on soil erosion rates in the EUROSEM model, as the input values accounting for roots, presented in the user manual, do not account for differences in root density or root architecture. Recent research indicates that small changes in root density or differences in root architecture considerably influence soil erosion rates during concentrated flow. The approach for incorporating the root effects into this model is based on a comparison of measured soil detachment rates for bare and for root‐permeated topsoil samples with predicted erosion rates under the same flow conditions using the erosion equation of EUROSEM. Through backwards calculation, transport capacity efficiencies and corresponding soil cohesion values can be assessed for bare and root‐permeated topsoils respectively. The results are promising and present soil cohesion values that are in accordance with reported values in the literature for the same soil type (silt loam). The results show that grass roots provide a larger increase in soil cohesion as compared with tap‐rooted species and that the increase in soil cohesion is not significantly different under wet and dry soil conditions, either for fibrous root systems or for tap root systems. Power and exponential relationships are established between measured root density values and the corresponding calculated soil cohesion values, reflecting the effects of roots on the resistance of the topsoil to concentrated flow incision. These relationships enable one to incorporate the root effect into the soil erosion model EUROSEM, through adapting the soil cohesion input value. A scenario analysis shows that the contribution of roots to soil cohesion is very important for preventing soil loss and reducing runoff volume. The increase in soil shear strength due to the binding effect of roots on soil particles is two orders of magnitude lower as compared with soil reinforcement achieved when roots mobilize their tensile strength during soil shearing and root breakage. Copyright © 2008 John Wiley & Sons, Ltd.     

港珠澳跨海工程沉管隧道三维地震反应分析

以港珠澳大桥沉管隧道为工程研究背景,考虑管节接头GINA止水带的橡胶材料特性、场地的初始地应力平衡以及上覆动水压力作用等,分析了水平及竖向地震作用下沉管隧道三维动力反应。结果表明:动水压力对隧道结构的竖向及水平方向的动力响应均有一定影响,尤其是对隧道结构的竖向反应影响较水平方向更加明显,最大可达70%;隧道接头GINA止水带竖向剪切变形较水平纵向的拉伸变形及水平横向剪切变形明显偏大,尤其两侧止水带竖向剪切变形较大;混凝土隧道管节上顶板及边墙较管节底部更易受到明显的拉应力。     

Modeling the functional influence of vegetation type on streambank cohesion

The important role of vegetation in adding cohesion and stabilizing streambanks has been widely recognized in several aspects of fluvial geomorphology, including stream restoration and studies of long‐term channel change. Changes in planform between braided, meandering, and anabranching forms have been attributed to the impacts of vegetation on hydraulic roughness and bank stability. However, these studies focus either on flume studies where analog vegetation is used, or case studies featuring one species, which is commonly invasive. We present functional differences of bank‐stabilizing root characteristics and added cohesion, with vegetation categorized as woody and non‐woody and by the vegetation groups of trees, shrubs, graminoids, and forbs. We analyzed root morphology and tensile strength of 14 species common to riparian areas in the southern Rocky Mountains, in field sites along streambanks in the montane and subalpine zones of the Colorado Front Range. Using the vegetation root component (RipRoot) of a physically‐based bank stability model (BSTEM), we estimated the added cohesion for various sediment textures with the addition of each of the 14 species. Significant differences exist between woody and non‐woody vegetation and between the four vegetation categories with respect to the coefficient of the root tensile strength curve, lateral root extent, and maximum root diameter. Woody vegetation (trees and shrubs) have higher values of all three parameters than non‐woody species. Tree roots add significantly more cohesion to streambanks than forb roots. Additionally, rhizomes may play an important role in determining the reach‐scale effects of roots on bank stabilization. Differences in root characteristics and added cohesion among vegetation categories have several important implications, including determining the likelihood of planform change, developing guidelines for the use of bank‐stabilizing vegetation, and linking the effect of vegetation to geomorphic structure that can benefit ecosystem functioning. Copyright © 2014 John Wiley & Sons, Ltd.     

Seedling growth dynamic of <Emphasis Type="Italic">Haloxylon ammodendron</Emphasis> and its adaptation strategy to habitat condition in hinterland of desert

Through measuring the above/below-ground growth data of Haloxylon ammodendron seedlings at different stages in hinterland of the desert the results show that the H. ammodendron seedling growth has demonstrated different adaptation characteristics in the continued arid environment with time and space. In May, July, September and October, the growth speed of vertical root is 0.607 cm/d, 0.809 cm/d, 0.155 cm/d and 0.394 cm/d, respectively; the growth speed of height is 0.093 cm/d, 0.076 cm/d, 0.408 cm/d and 136 cm/d, respectively. It is explained that seedlings root system has the growth superiority in space. The maximum growth speed of below-ground (vertical root and horizontal root) of seedling is earlier than that of above-ground (height and horizontal of shoot). In the different periods, the vertical growth speed and the horizontal growth speed of below-ground is 2–10 times and 3–5 times than the height increase speed and the shoot growth speed, respectively. In the whole season, the growth speed of above/below-ground of seedlings shows the alternation growth tendency. At the different periods, the root/shoot ratio of H. ammodendron seedlings is 0.41, 0.3, 0.39 and 0.88. All these characteristics are the comprehensive performance of seedlings’ strategy selection to adapt to the continued arid environment.     

Seedling growth dynamic of Haloxylon ammodendron and its adaptation strategy to habitat condition in hinterland of desert

Through measuring the above/below-ground growth data of Haloxylon ammodendron seedlings at different stages in hinterland of the desert the results show that the H. ammodendron seedling growth has demonstrated different adaptation characteristics in the continued arid environment with time and space. In May, July, September and October, the growth speed of vertical root is 0.607 cm/d, 0.809 cm/d, 0.155 cm/d and 0.394 cm/d, respectively; the growth speed of height is 0.093 cm/d, 0.076 cm/d, 0.408 cm/d and 136 cm/d, respectively. It is explained that seedlings root system has the growth superiority in space. The maximum growth speed of below-ground (vertical root and horizontal root) of seedling is earlier than that of above-ground (height and horizontal of shoot). In the different periods, the vertical growth speed and the horizontal growth speed of below-ground is 2–10 times and 3–5 times than the height increase speed and the shoot growth speed, respectively. In the whole season, the growth speed of above/below-ground of seedlings shows the alternation growth tendency. At the different periods, the root/shoot ratio of H. ammodendron seedlings is 0.41, 0.3, 0.39 and 0.88. All these characteristics are the comprehensive performance of seedlings’ strategy selection to adapt to the continued arid environment.

     

基于拟动力法的顺层岩质边坡稳定性极限分析

基于极限分析上限法的原理,结合随时间变化的拟动力法,考虑地震、水力和坡顶超载作用下建立含张拉裂缝的顺层岩质边坡极限分析模型,并推导出边坡安全系数的表达式。结构面采用合理的水压力分布形式,且强度参数采用非等比例折减,分析边坡前缘是否堵塞,水平和竖向地震系数、张裂缝积水深度和坡顶超载对边坡安全系数的影响。分析表明:边坡前缘堵塞、水平地震系数、张裂缝积水深度和坡顶超载的增加,降低边坡的稳定性;竖向地震系数的增加提高了边坡的稳定性。随着非等比例折减相关系数的增加,边坡稳定性显著提高。因此结构面参数的变化规律需要得到关注。     

Large roots dominate the contribution of trees to slope stability

Tree roots provide surface erosion protection and improve slope stability through highly complex interactions with the soil due to the nature of root systems. Root reinforcement estimation is usually performed by in situ pullout tests, in which roots are pulled out of the soil to reliably estimate the root strength of compact soils. However, this test is not suitable for the scenario where a soil progressively fails in a series of slump blocks – for example, in unsupported soils near streambanks and road cuts where the soil has no compressive resistance at the base of the hillslope. The scenario where a soil is unsupported on its downslope extent and progressively deforms at a slow strain rate has received little attention, and we are unaware of any study on root reinforcement that estimates the additional strength provided by roots in this situation. We therefore designed two complementary laboratory experiments to compare the force required to pull the root out. The results indicate that the force required to pull out roots is reduced by up to 50% when the soil fails as slump blocks compared to pullout tests. We also found that, for slump block failure, roots had a higher tendency to slip than to break, showing the importance of active earth pressure on root reinforcement behaviour, which contributes to reduced friction between soil and roots. These results were then scaled up to a full tree and tree stand using the root bundle and field-measured spatial distributions of root density. Although effects on the force mobilized in small roots can be relevant, small roots have virtually no effect on root reinforcement at the tree or stand scale on hillslopes. When root distribution has a wide range of diameters, the root reinforcement results are controlled by large roots, which hold much more force than small roots. © 2019 John Wiley & Sons, Ltd.     

Modelling biome‐scale root reinforcement and slope stability

Rapid changes in the composition of hillslope vegetation due to a combination of changing climate and land use make estimating slope stability a significant challenge. The dynamics of root growth on any individual hillslope result in a wide range of root distributions and strengths that are reflected as up to an order of magnitude variability in root cohesion. Hence the challenge of predicting the magnitude of root reinforcement for hillslopes requires both an understanding of the magnitude and variability of root distributions and material properties (e.g. tensile strength, elasticity). Here I develop a model for estimating the reinforcement provided by plant roots based on the distribution of biomass measured at the biome level and a compilation of root tensile strength measurements measured across a range of vegetation types. The model modifies the Wu/Waldron method of calculating root cohesion to calculate the average lateral root cohesion and its variability with depth using the Monte Carlo method. The model was validated in two ways, the first against the predicted depth‐reinforcement characteristics of Appalachian soils and the second using a global dataset of landslides. Model results suggest that the order of magnitude difference in root cohesions measured on individual hillslopes can be captured by the Monte Carlo approach and provide a simple tool to estimate root reinforcement for data‐poor areas. The model also suggests that future hotspots of slope instability will occur in areas where land use and climate convert forest to grassland, rather than changes between different forest structures or forest and shrubland. Copyright © 2018 John Wiley & Sons, Ltd.     

Influence of root characteristics and soil variables on the uprooting mechanics of Avena sativa and Medicago sativa seedlings

The success of seedlings and rejuvenated woody debris growing on river bedforms depends on the resistance to uprooting by flow provided by their simple root architecture. Avena sativa and Medicago sativa seedlings were used in flume experiments as prototypes for juvenile riparian plants. Very little is known about the magnitude of root anchoring forces and the role of secondary roots of such simple root systems. We performed 1550 vertical uprooting experiments on Avena sativa and Medicago sativa seedlings grown in quartz sand. Seedlings were pulled up by direct traction using a wheel driven by a computer‐controlled motor and the force was recorded. Roots were scanned and architectural parameters (root length and number of roots) determined. Uprooting force and work (the integral of the applied force times the distance over which it is applied) were then related to root architecture and soil variables. Resistance to uprooting increased with decreasing sediment size and sediment moisture content. The initial response of the root–soil system to uprooting showed linear elastic behaviour with modulus increasing with plant age. While the maximum uprooting force was found to increase linearly with total root length and be mainly dependent on the length of the main root, uprooting work followed a power law and has to be related to the whole root system. Thus, for the young plants we considered, secondary roots are responsible for the ability to withstand environmental disturbances in terms of duration rather than magnitude. This distinction between primary and secondary roots can be of crucial importance for seedlings of riparian species germinating on river bars and islands where inundation is a main cause of mortality. Beyond clarifying the biomechanical role of soil and root variables, the uprooting statistics obtained are useful in interpreting and designing ecomorphodynamic flume experiments. Copyright © 2014 John Wiley & Sons, Ltd.     

Three-dimensional EM computer simulation on sprite initiation above a horizontal lightning discharge

The purpose of this paper is to study the three-dimensional (3D) effects of the source current and the electromagnetic (EM) pulse on the distribution of upper atmospheric electric field and ionization created. A new lightning model has been employed; i.e., we have included a horizontal channel in addition to the conventional vertical channel. The effects of the horizontal lightning channel are summarized as follows: (1) the effect of a vertical channel plays a fundamental role in the whole view of sprite initiation; (2) the position of a sprite is shifted from the position of its parent vertical channel in response to the length of a horizontal channel; and (3) we observe very fine structures with local maxima and minima in the “reduced” electric field (the electric field divided by the neutral gas density). These theoretical inferences are discussed and compared with the observational facts so far reported (such as lateral shift of sprites, morphological difference of sprites, etc.).     

铁路桥梁根式挖井基础桥墩的水平承载特性研究

根式基础作为一种新型变截面结构形式,已被证明对于提高沉井基础与桩基础的竖向及横向承载力有明显影响,但其对挖井基础抗震性能的改善作用尚不明确。为明确根键对挖井基础桥墩承载力特征的影响,对一铁路根式挖井基础桥墩进行拟静力试验,研究其滞回曲线与骨架曲线特性,通过建立符合试验的有限元模型,讨论不同根键参数对挖井基础桥墩耗能能力及承载力的影响。结果表明根键的存在充分带动了周围土体的参与度,可大大提高挖井基础桥墩的承载力,且随着墩顶位移荷载的增加承载力呈非线性增加。试验及数值模拟均表明：根式挖井基础的破坏主要是由基础周围的土体失效引起的,基础自身并未发生损坏;增加根键的长度可以明显提高根式挖井基础桥墩的承载能力与耗能能力;当根键之间的间距大于根键自身宽度时,增宽根键可明显提高根式挖井基础桥墩的承载能力与耗能能力,而当根键的间距小于自身宽度时,加宽根键对承载力的影响不明显;增加根键的数量能明显提高根式挖井基础桥墩的承载力;在基础底部布置根键的效果并不弱于在侧壁布置,且根键布置在侧壁时不宜靠近土体表面。研究成果可为根式挖井基础在我国铁路桥梁中的应用提供科学依据。     

Relationship between tree height and landslide characteristics obtained by GIS assessment

Landslides in forested landscapes have far-reaching implications, beyond that of just destroying the forest itself, sometimes initiating large-scale sediment disasters. Although vegetation increases slope stability through its root network, it is hard to evaluate its contribution to slope stability over a wide area. In this study, the relationship between tree height and landslide characteristics in the Ikawa catchment, central Japan, was investigated to develop a method for evaluating the effects of forest cover on slope stability over a regional extent. Catchment-wide tree height was obtained using airborne LiDAR point cloud data and used in conjunction with the root depth profile, measured for trees of various height by digging trenches. Root tensile strength per unit area of soil was calculated from individual root diameters and empirical power law equations on the relationship between root diameter and root tensile force in order to better understand the effect that tree height has on slope stability. Landslide density in the Ikawa catchment shows that landslides occur more frequently in forests with shorter trees, with occurrence decreasing as tree height increases. This is likely due to the stabilizing features of larger trees having a greater network of roots, which is supported by the general increase in total root area and the deeper penetration of root biomass into the soil as the height of trees surveyed increases. Landslide density was not solely affected by tree height, but also by slope gradient and plane curvature. Decreasing landslide occurrence and landslide area as tree height increases suggests that slope stability increases with tree height, while the random distribution of results when comparing landslide depth to tree height suggests that while tree height has an impact on relative slope stability, the landslide failure depth is independent of tree height, and thus controlled by other factors. © 2020 John Wiley & Sons, Ltd.     

南北地震带川滇甘陕地区竖向与水平向加速度反应谱比的统计分析

本文收集了从南北地震带川滇甘陕地区263个有详细场地资料的强震台站获得的802组强震动记录,按照场地、震级和震中距等因素统计分析了竖向与水平向加速度反应谱比的形状以及不同场地、震级和震中距下不同周期段内的谱比平均值,并与建筑抗震规范GB50011—2010规定的0.65进行了比较。结果表明:竖向与水平向加速度反应谱比受场地、震级和震中距等因素的影响; Ⅰ类和Ⅱ类场地在全周期段的谱比均值基本上大于0.65;无论何种分组情况谱比均值在0.1—1.0 s周期范围内基本低于定值0.65,周期大于1.0 s的谱比均值基本上远高于0.65。因此对于大部分抗震规范直接把竖向地震作用取为水平向地震作用的0.65倍,有待商榷。本文建议根据水平向反应谱的标定方式来确定现有竖向强震动记录的竖向地震作用。      

Reliability assessment of internal stability of reinforced soil structures: A pseudo-dynamic approach

A methodology for reliability based optimum design of reinforced soil structures subjected to horizontal and vertical sinusoidal excitation based on pseudo-dynamic approach is presented. The tensile strength of reinforcement required to maintain the stability is computed using logarithmic spiral failure mechanism. The backfill soil properties, geometric and strength properties of reinforcement are treated as random variables. Effects of parameters like soil friction angle, horizontal and vertical seismic accelerations, shear and primary wave velocities, amplification factors for seismic acceleration on the component and system probability of failures in relation to tension and pullout capacities of reinforcement have been discussed. In order to evaluate the validity of the present formulation, static and seismic reinforcement force coefficients computed by the present method are compared with those given by other authors. The importance of the shear wave velocity in the estimation of the reliability of the structure is highlighted. The Ditlevsen's bounds of system probability of failure are also computed by taking into account the correlations between three failure modes, which is evaluated using the direction cosines of the tangent planes at the most probable points of failure.     

地震动参数对斜坡加速度动力响应规律的影响

2008年‘5.12’汶川大地震诱发斜坡地质灾害在空间分布上表现出了明显的高程效应和岩性效应。本文采用上硬下软和上软下硬两种典型岩性组合斜坡模型,完成了1:100比尺的振动台试验。文中重点分析了地震波类型(频谱)、激振方向和地震动三参数对斜坡模型水平向加速度动力响应规律的影响。分析结果表明:(1)水平单向激振时,15Hz正弦波和汶川地震波作用下的高程放大效应主要体现在斜坡模型中上段,两者在上软下硬组合斜坡模型中产生了近乎相同的水平向加速度动力响应规律,原因主要在于两者的卓越频率接近。(2)模型对合成向汶川地震波的放大作用依次超过单向水平向和竖直向汶川波的作用,且合成向与水平单向汶川地震波的作用规律基本相同。(3)随着振动强度增加,模型对低频波的放大作用增强。(4)在合成向汶川地震动作用下,随着振动强度增加,模型各高程处的水平向加速度峰值(PGA)逐渐增加,其相应的放大系数在模型中上段逐渐降低至2.0以下,最终趋于平缓,表明模型沿高程向的放大效应逐渐减弱。此外,各参数对模型的水平向加速度响应因模型自身的岩性组合结构而异,随着振动强度增加,上硬下软斜坡模型中上部的水平向速度响应值基本保持在1.0～2.7倍于上软下硬斜坡模型中上部的水平向加速度响应值这一水平。     

带有楔形轨道的滑移支座力学性能数值模拟研究

针对结构隔震装置在较大的倾覆力作用下抗拉能力不足的问题,提出了一种带有楔形分离式轨道的新型滑移支座。新支座能够承受上部结构传递的荷载以及适应较大水平位移,同时利用楔形轨道与楔形滑块之间的相互作用提供抗拉能力。采用通用有限元程序ABAQUS建立了支座的三维实体有限元模型,对其抗压、抗拉、压剪和拉剪力学性能进行数值模拟研究,同时考虑了楔形角度的改变对支座力学性能的影响。通过分析表明：在竖向新支座具有较好的抗压和抗拉能力,抗拉性能受楔形角度的影响较大,角度大于60°时,支座的抗拉能力随着角度的减小不断提升;而当角度减小到60°时,楔形滑块与上部连接部分出现薄弱点,导致支座的抗拉能力开始降低。在压剪与拉剪的作用下,支座水平向的滞回曲线比较饱满,具有较好的水平向摩擦耗能能力且能够适应较大的水平位移。     

Optimal root profiles in water-limited ecosystems

The vertical distribution of roots in the soil is of central importance to the mass and energy exchange between the land and the atmosphere. It has been demonstrated that the vertical root profiles which maximize transpiration in numerical experiments reflect well the characteristics of root profiles observed in nature for water-limited ecosystems. Previous research has demonstrated how the optimal vertical root profile depends on both the mean annual precipitation (MAP) and the soil texture. Recently, in the climate literature, it has been suggested Chou et al. (2012) [5] that increased greenhouse forcing in the tropics can lead to a simultaneous decrease in the frequency and increase in the intensity of precipitation. In this paper we demonstrate how such a change in the statistical structure of rainfall, even with no change to MAP, requires deeper root distributions to maintain optimal water use. These results raise interesting questions for future studies of nutrient dynamics, the cost of additional below ground carbon allocation, and inter plant functional type competition.