Numerical modelling of free field traffic-induced vibrations

This paper deals with the numerical modelling of free field traffic-induced vibrations during the passage of a vehicle on an uneven road. The road unevenness subjects the vehicle to vertical oscillations that cause dynamic axle loads. The latter are calculated from the vehicle transfer functions and the frequency content of the road profile as experienced by the vehicle axles. A transfer function between the source and the receiver that accounts for the dynamic interaction between the road and the soil is used to calculate the free field response. Its calculation is based on a dynamic substructure method, using a boundary element method for the soil and an analytical beam model for the road. The methodology is validated with analytical results and is finally illustrated by a numerical example where the free field vibrations during the passage of a vehicle on a traffic plateau are considered.     

Theory and applicability of a recrystallized grain size paleopiezometer

An approximate theoretical relation is derived which relates stress during steady state creep to both subgrain size and dynamically recrystallized grain size. The relation results from equating the dislocation strain energy in the grain boundary to that in the enclosed volume. Available data on metals and silicates are in excellent agreement with the theory. For paleopiezometry, the recrystallized grain size must be preserved by quenching, by cooling under stress, or by inhibition of grain growth by intimate mixture of two or more phases. In general, stress may be underestimated using rocks in which grain size has been reduced by dynamic recrystallization, especially if the grain size is very small. Stress may be overestimated using coarse grained rocks in which the grain size has increased toward the steady state value. Quantitative limits remain to be established. The theoretical relation can in principle be applied to any metal or mineral if only the effective isotropic elastic moduli and the Burgers vector are known. When used as a paleopiezometer, the technique indicates that high stresses on the order of 100 MPa are not infrequently associated with mantle diapirism and with large scale thrust faulting. Consideration of the Mt. Albert ultramafic body suggests that texturally inferred stresses from peridotite massifs and from ultramafic xenoliths in alkali olivine basalts might reflect either horizontal variations in stress across a rising diapir or else a vertical variation in stress as defined by the pyroxene geobarometer (Mercier et al. 1977). In either case the stresses are probably characteristic of local diapirism. Stresses characteristic of global upper mantle flow might be inferred from xenoliths originating from above kimberlite-producing diapirs.     

Numerical modelling of strain in lava tubes

Received: 5 November 1998 / Accepted: 17 November 1999     

BFO-PSO算法下的弹性波数值模拟

地震波正演模拟是地震反演与成像的基础和关键,有限差分算法广泛应用于地震波数值模拟,差分算子的精度直接影响数值模拟的质量和效率.本文提出一种BFO-PSO算法下的有限差分算子优化方法,并应用其进行弹性波数值模拟.首先,将BFO算法中的趋化、复制、驱散三个步骤引入PSO算法,形成具有更好全局搜索能力和更快收敛速度的BFO-...     

The threshold effect of image resolution on image‐based automated grain size mapping in fluvial environments

It has recently been demonstrated that surficial grain sizes in fluvial environments could be derived with automated methods applied to airborne digital imagery having a ground resolution of 3 cm. This letter seeks to further examine the potential of digital imagery for automated grain size mapping. In order to broaden the application of automated grain size mapping from airborne imagery, the effect of image resolution needs further study. Automated grain size mapping was attempted on an airborne digital image with a ground resolution of 10 cm. The results show that meaningful grain size information can be derived from 10 cm imagery. However, the ground resolution of the image acts as a size threshold below which no grain size information is detectable. Therefore, these results strongly suggest that future applications of automated grain size mapping will always be dependent on the ground resolution made available by the technology in use at the time of image acquisition. Copyright © 2005 John Wiley & Sons, Ltd.     

Development of a vector-based 3D grain entrainment model with application to X-ray computed tomography scanned riverbed sediment

Sediment transport equations typically produce transport rates that are biased by orders of magnitude. A causal component of this inaccuracy is the inability to represent complex grain-scale interactions controlling entrainment. Grain-scale incipient motion has long been modelled using geometric relationships based on simplified particle geometry and two-dimensional (2D) force or moment balances. However, this approach neglects many complexities of real grains, including grain shape, cohesion and the angle of entrainment relative to flow direction. To better represent this complexity, we develop the first vector-based, fully three-dimensional (3D) grain rotation entrainment model that can be used to resolve any entrainment formulation in 3D, and which also includes the effect of matrix cohesion. To apply this model we use X-ray computed tomography to quantify the 3D structure of water-worked river grains. We compare our 3D model results with those derived from application of a 2D entrainment model. We find that the 2D approach produces estimates of dimensionless critical shear stress ( ) that are an order of magnitude lower than our 3D model. We demonstrate that it is more appropriate to use the c-axis when calculating 2D projections, which increases values of to more closely match our 3D estimates. The 3D model reveals that the main controls on critical shear stress in our samples are projection of grains, cohesive effects from a fine-grained matrix, and bearing angle for the plane of rotation (the lateral angle of departure from downstream flow that, in part, defines the grain's direction of pivot about an axis formed by two contact points in 3D). The structural precision of our 3D model demonstrates sources of geometric error inherent in 2D models. By improving flow properties to better replicate local hydraulics in our 3D model, entrainment modelling of scanned riverbed grains has the potential for benchmarking 2D model enhancements. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Numerical modelling of channel migration with application to laboratory rivers

The paper presents the development of a morphological model and its application to experimental model rivers.The model takes into account the key processes of channel migration,including bed deformation,bank failure and wetting and drying.Secondary flows in bends play an important role in lateral sediment transport,which further affects channel migration.A new formula has been derived to predict the near-bed secondary flow speed,in which the magnitude of the speed is linked to the lateral water level gradient.Since only non-cohesive sediment is considered in the current study,the bank failure is modelled based on the concept of submerged angle of repose.The wetting and drying process is modelled using an existing method.Comparisons between the numerical model predictions and experimental observations for various discharges have been made.It is found that the model predicted channel planform and cross-sectional shapes agree generally well with the laboratory observations.A scenario analysis is also carried out to investigate the impact of secondary flow on the channel migration process.It shows that if the effect of secondary flow is ignored,the channel size in the lateral direction will be seriously underestimated.     

Paleoclimatic significance of grain size of loess-palaeosol deposit in Chinese Loess Plateau

Particle size analysis for samples from three last glacial cycle loess-palaeosol profiles along a northwestsoutheast transection on the Chinese Loess Plateau has been carried out. The paleoclimatic significance of grain size indices has been obtained by analyzing the fraction content variations and their comparisons with global ice volume and solar radiation variations. The results show that (i) paleoclimatic significance of the grain size indices of loess-palaeosol deposit is different with grain size fraction content and sampling points in Chinese Loess Plateau; (ii) the sub-coarse grain fraction is a good proxy index of East Asia winter monsoon strength and therefore can be used to detect the global climate changes; (iii) the content of sub-fine and fine grain fractions is influenced by both the input of sub-coarse grain fraction and pedogenesis; (iv) the sub-coarse fraction exhibits a negative relationship with the sub-fine and fine fractlon. Project supported by the 9.5 major project of Chinese Academy of Sciences and the open grant of the State Key Laboratory of Loess and Quaternary Geology, Chinese Academy of Sciences.     

兴凯湖沉积物粒度特征揭示的27.7kaBP以来区域古气候演化

结合孢粉分析,对兴凯湖一根长269cm的沉积岩芯研究表明:沉积物粒度分布特征可以较好地反映区域气候变化,即粗粉砂和砂增多对应于降水减少的低湖面时期,细粉砂增多对应于降水增加的高湖面时期,而粘土增加则对应于气候干燥的静水沉积环境。约27740-25540cal aBP时期,沉积物颗粒较粗,湖区处于低湖面的冷干气候;而随后25540-23650 cal aBP时期,中粉砂增加,粘土较少,分选性较好,湖区处于冷湿气候环境23650-19940cal aBP阶段,湖面封冻期长,沉积物粘土较多,湖区气候冷干,对应于末次盛冰期。19940-14510cal aBP阶段,细粉砂增加,比前期降水增多,温度也有所升高。14510-10800cal aBP时期,本地区进入晚冰期,沉积物粒径波动频繁,依次出现较高含量的细粉砂-粗粉砂-粘土阶段,反映了暖湿到干燥的气候变化,对应于北欧的B(o|¨)lling/Older Dryas/Aller(o|¨)d/Younger Dryas气候波动期。10800-1050cal aBP,总体上沉积物粒径变化不大,中粗粉砂含量基本保持低值,细粉砂逐渐增加,粘土逐渐减少,显示深水稳定沉积环境,降水逐渐增多,处于全新世暖湿期;其中8330-7000cal aBP阶段,粗颗粒迅速增加较多,对应于8.2 cal kaBP冷事件。约1050cal aBP以来,沉积物中值粒径大幅度增加,湖面水位下降幅度较大,气候变凉干,也反映了人类活动增强造成水土流失加剧。     

Evolution of grain size distributions and bed mobility during hydrographs in gravel-bed braided rivers

Evolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding gravel-bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally-stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel-bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high-resolution (~1 mm precision) digital elevation models generated from close-range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50% of the channel-forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near-equal mobility at peak channel-forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D₉₀ and active widths were narrow (< 3% of wetted width). Above this discharge, both increased so that at channel-forming discharge, the active depth had a local maximum of 9D₉₀ while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes in the morphological active layer with discharge are directly connected to the conditions of bed mobility, and strongly correlated with bedload transport rate. © 2018 John Wiley & Sons, Ltd.     

Environmental records of lacustrine sediments in different time scales: Sediment grain size as an example

Understanding the past is the key to know thepresent and to predict the future. As a core project ofthe International Geosphere Biosphere Program, PastGlobal Changes (PAGES) aims to reconstruct theevolution history of global climate, global environ-ment and global ecosystem with resort to all kinds ofnatural records, so as to reveal the change rules(change processes, change amplitude and change rate)of the global environment and thus provide scientificbasis for predicting climate changes p…     

Aeolian deposition of dust over hills: the effect of dust grain size on the deposition pattern

Wind tunnel experiments were conducted to investigate the effects of topography on the grain size characteristics of aeolian dust deposits. Experiments were performed on three isolated hills having various size and aspect ratios. The longitudinal profile of the median grain diameter was investigated for each hill. The longitudinal dust deposition profile was also studied for nine grain size classes of between 10 and 104 µm, as were wind and dust concentration profiles in the atmosphere upwind of, over and downwind of a hill. The wind tunnel experiments show that the grain size characteristics of aeolian dust deposits are affected by topography. Most apparent is the occurrence of a zone of reduced grain size on the leeside of hills, which extends from just upwind of the summit to a distance of several times the height of the hill. Slightly coarser than normal dust is deposited on the concave windward hill slope and in a zone downwind of the area of reduced grain size, but the increase in grain size in these zones remains very small. Although the normalized dust deposition profile for a hill does not vary substantially as a function of grain size, systematic trends are observed. The most important tendencies are: (1) a progressive extension, in the downwind direction, of a zone of decreased dust deposition on the leeside of a hill (the coarser the grains, the further downwind the zone of reduced deposition extends); (2) a progressive increase in dust deposition immediately upwind of a hill (the finer the grains, the higher the deposition value upwind of a hill becomes). Both tendencies are explained by the difference in inertia of the grains, which is controlled by grain size. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical modelling of the transverse dynamic behaviour of circular tunnels in clayey soils

In this paper, different approaches aimed at investigating the dynamic behaviour of circular tunnels in the transverse direction are presented. The analysed cases refer to a shallow tunnel built in two different clayey deposits. The adopted approaches include 1D numerical analyses performed modelling the soil as a single-phase visco-elastic non-linear medium, the results of which are then used to evaluate the input data for selected analytical solutions proposed in the literature (uncoupled approach), and 2D fully coupled FE simulations adopting visco-elastic and visco-elasto-plastic effective stress models for the soil (coupled approach). The results are proposed in terms of seismic-induced loads in the transverse direction of the tunnel lining. The different constitutive hypotheses adopted in the coupled numerical approach prove to play a significant role on the results. In particular, the plasticity-based analyses indicate that a seismic event can produce a substantial modification of loads acting in the lining, leading to permanent increments of both hoop force and bending moment.     

The role of mineralogy,geochemistry and grain size in badland development in Pisticci (Basilicata,southern Italy)

Mineralogical, geochemical and grain‐size composition of soil and pore‐water chemistry parameters were characterized on both eroded (south‐facing) and non‐eroded (north‐facing) clayey‐silt slopes in the Basilicata region (southern Italy). Only a few grain‐size parameters and clay mineralogy discriminate eroded from non‐eroded substrates. Compared with the latter, the former have fractions of over 63 µm and 1–4 µm lower and fractions 4–63 µm higher. Grain‐size characters of crusts did not discriminate with respect to substrate. Bulk rock mineralogy was not distinctive, but the clay mineral assemblage shows that the eroded slope is enriched in kaolinite, mixed layers (illite–smectite) and chlorite, whereas illite decreases, although overlaps are common. Chemical data enable discrimination between eroded and non‐eroded slopes. pH, SAR (sodium adsorption ratio), TDS (total dissolved salts) and PS (percentage of sodium) are distinctive parameters for both eroded and non‐eroded slopes. TDS increases in depth in the non‐eroded slope, whereas the maximum TDS is just below the crust in the eroded one. On average, eroded substrates are higher in pH, SAR and PS than non‐eroded ones. The ESP (exchangeable sodium percentage) of the eroded slope has a higher value than the non‐eroded one. Crusts are less dispersive than eroded substrates, and non‐eroded substrates behave as crusts. This suggests that the portion of the slope most severely exposed to weathering tends to stabilize, due to strong decreases in SAR, PS and ESP. Several diagrams reported in the literature show similarly anomalous crust samples on eroded slopes, compared with other samples coming from greater depths on eroded slopes. In the present case study, the exchangeable form of Na characterizes crusts more than the soluble form. This study describes the erosional mechanism, which involves morphological and geographic exposure and climatic elements, as well as grain size, mineralogy, chemistry and exchangeable processes of soils. Copyright © 2006 John Wiley & Sons, Ltd.     

偶应力理论框架下的弹性波数值模拟与分析

在经典地震学理论框架下,先人发展了数不胜数的地震技术,为社会发展做出了巨大贡献;可是,当前的技术仍有难以逾越的障碍,亟需破局.高铁地震学联合研究组,在河北定兴采集到大量数据;其中可见含有大量的旋转运动分量.由于基于经典连续介质力学推导弹性波动方程时,从理论出发点上就去除了旋转项,且在其理论框架内,介质被视为一个连续的质...     

Aerial photosieving of exposed gravel bars for the rapid calibration of airborne grain size maps

In recent years, fluvial remote sensing has seen considerable progress in terms of methods capable of system scale characterisation of river catchments. One key development is automated grain size mapping. It has been shown that high resolution aerial photography can be used to automatically produce grain size maps over entire rivers. However, current aerial grain size mapping procedures all require field calibration data. The collection of such data can be costly and problematic in the case of remote areas. This paper presents a method developed to remove the need for field based calibration data. Called ‘aerial photosieving’, this method consists of using the same very high resolution aerial imagery intended for grain size map production to visually measure particle sizes on‐screen in order to provide calibration data. The paper presents a rigorous comparison of field‐based photosieving calibration data and aerial photosieving calibration data. Statistical tests are used to demonstrate that aerial photosieving gives similar results when compared with field‐based data with only a slight systematic overprediction. The new aerial photosieving method therefore simplifies the overall procedure required for the production of grain size maps and thus improves the cost‐effectiveness and potential availability of this new fluvial remote sensing technology. Copyright © 2010 John Wiley &Sons, Ltd.     

沉积物金属元素变化的粒度效应--以太湖沉积岩芯为例

水体沉积物中金属元素含量变化除了与人为污染有关之外,在很大程度上受沉积物粒度、矿物组成等沉积物性质的影响．在进行沉积物金属元素研究中,要充分考虑沉积物金属元素含量变化的粒度效应．本文通过对太湖MS岩芯中17种金属元素、沉积物粒度、矿物组成等指标的分析,研究了金属元素变化特征以及与沉积物粒度组成的关系．MS岩芯金属元素变化可分为两类,第一类主要为Al、Fe、K等,该类元素在沉积岩芯中下部含量较高,岩芯上部含量较低;第二类为Na元素,在沉积岩芯中下部含量较低,岩芯上部含量升高．MS岩芯中金属元素与粘土含量之间具有显著相关关系,经沉积物粒度(粘土含量)校正后,沉积岩芯中金属元素含量趋于稳定．因此,沉积物粒度组成(粘土含量)是影响金属元素含量的主要因素,在研究太湖沉积物金属元素变化规律及进行金属污染评价时,应对金属元素含量进行粒度校正．     

近百年来洞庭湖粒度指示的沉积环境演变特征

湖泊沉积物储存着长时期的流域环境信息,是百年尺度湖泊环境演变研究的高分辨率记录器。本文通过²¹⁰Pb_ex、¹³⁷Cs组合定年法,分析放射性核素剖面特征,研究沉积速率时空变化规律,并解析沉积物粒度指示的环境信息,揭示百年尺度湖泊沉积环境演变。结果表明:洞庭湖²¹⁰ Pb_ex剖面分布呈现多段衰变型、混合—衰变—平衡型、衰变—本底型和无沉积型4种类型。3大湖体200 cm的沉积深度对应的年代跨度为127～162年,构建了洞庭湖沉积物深度—年代关系。洞庭湖平均质量沉积速率为1.47～3.11 g/(cm²·a),整体上呈现南洞庭湖>西洞庭湖>东洞庭湖的空间特征;自1900年后,质量沉积速率增加了3～7倍,呈时序增加趋势。沉积年代上,沉积物粒度特征表明洞庭湖沉积环境呈3个阶段2个过渡期的演变特征,与同时期区域内毁林开荒、水利建设、植树造林等人为活动有较好的对应关系,说明近百年来人为活动是洞庭湖泥沙沉积的重要因素。本研究结果可为强化湖泊及其流域生态环境保护...