Stochastic modelling of erosion and deposition in cohesive soils

A new stochastic method of detachment rate estimation was used in erosion modelling. This method was based on calculating the probability of driving forces exceeding resistance forces in the interaction of oscillating flow and structured soil. Knowledge of the probability density functions for flow velocity, soil cohesion, aggregate size and soil integrity makes it possible to calculate theoretically the erosion rate of cohesive soil for any combination of these stochastic variables. The proposed theory explains the variability in relationships between rate of detachment and flow velocity. With flow velocity, detachment rate increases more rapidly for more integrated soil with higher cohesion and larger aggregates. This theory also shows the great difference between soil erosion type for relatively high and relatively low flow velocities, and explains rather high errors, even with detailed models, in the calculation of low soil erosion rate. Copyright © 2004 John Wiley & Sons, Ltd.     

Stochastic modelling of dispersion in shallow water

A random walk model to describe the dispersion of pollutants in shallow water is developed. By deriving the Fokker-Planck equation, the model is shown to be consistent with the two-dimensional advection-diffusion equation with space-varying dispersion coefficient and water depth. To improve the behaviour of the model shortly after the deployment of the pollutant, a random flight model is developed too. It is shown that over long simulation periods, this model is again consistent with the advection-diffusion equation. The various numerical aspects of the implementation of the stochastic models are discussed and finally a realistic application to predict the dispersion of a pollutant in the Eastern Scheldt estuary is described.     

Effects of slope position and land use on the stability of aggregateassociated organic carbon in calcareous soils

Topography and land use affect soil organic carbon(SOC) storage, stabilization, and turnover, through several biogeochemical processes. This study investigated the aggregate composition and SOC content of bulk soils and aggregates at different slope positions under different land uses in a typical karst catchment of southwestern China. Our results show that the proportion of macro-aggregates and the SOC content of bulk soils and aggregates at different slope positions decreased from the upper to the lower slope. The SOC content generally increased with an increase in the mean weight diameter and proportion of macro-aggregates under different land uses. Our results indicate that macro-aggregates in forest and grassland soils make a greater contribution to both aggregate composition and SOC content than that in arable land soils. Therefore,converting farmland to forest or grassland can facilitate the accumulation of macro-aggregates as well as the storage of SOC.     

Undrained behaviour of two silica sands and practical implications for modelling SSI in liquefiable soils

The aim of the present study is twofold. Firstly, the paper investigates the undrained cyclic and post-cyclic behaviour of two silica sands by means of multi-stage cyclic triaxial tests. Secondly, based on the post-cyclic response observed in the element test, the authors formulate a simplified stress–strain relationship that can be conveniently used for the construction of p–y curves for liquefiable soils. The multi-stage loading condition consists of an initial cyclic loading applied to cause liquefaction, followed by undrained monotonic loading that aimed to investigate the post-cyclic response of the liquefied sample. It was found that due to the tendency of the liquefied soil to dilate upon undrained shearing, the post-liquefaction strain–stress response was characterised by a distinct strain–hardening behaviour. The latter is idealized by means of a bi-linear stress–strain model, which can be conveniently formulated in terms of three parameters, i.e.: (i) take-off shear strain, γ_to, i.e. shear strain required to mobilize 1 kPa of shear strength; (b) initial secant shear modulus, G₁, defined as 1/γ_to; (c) post-liquefied shear modulus at large strain, G₂ (γ⪢γ_to). Based on the experimental results, it is concluded that these parameters are strongly influenced by the initial relative density of the sample, whereby γ_to decreases with increasing relative density. Differently both shear moduli (G₁ and G₂) increases with increasing relative density. Lastly, the construction of new p–y curves for liquefiable soils based on the idealized bi-linear model is described.     

Stochastic analysis of unsaturated transport in soils with fractal log-conductivity distribution

Within the framework of stochastic theory and the spectral perturbation techniques, three-dimensional dispersion in partially saturated soils with fractal log hydraulic conductivity distribution is analyzed. Our analysis is focused on the impact of fractal dimension of log hydraulic conductivity distribution, local dispersivity, and unsaturated flow parameters, such as the soil poresize distribution parameter and the moisture distribution parameter, on the spreading behavior of solute plume and the concentration variance. Approximate analytical solutions to the stochastic partial differential equations are derived for the variance of asymptotic solute concentration and asymptotic macrodispersivities.     

Stochastic modelling of seasonal and yearly rainfalls with low-frequency variability

Stochastic rainfall models are important for many hydrological applications due to their appealing ability to simulate synthetic series that resemble the statistical characteristics of the observed series for a location of interest. However, an important limitation of stochastic rainfall models is their inability to preserve the low-frequency variability of rainfall. Accordingly, this study presents a simple yet efficient stochastic rainfall model for a tropical area that attempts to incorporate seasonal and inter-annual variabilities in simulations. The performance of the proposed stochastic rainfall model, the tropical climate rainfall generator (TCRG), was compared with a stochastic multivariable weather generator (MV-WG) in various aspects. Both models were applied on 17 rainfall stations at the Kelantan River Basin, Malaysia, with tropical climate. The validations were carried out on seasonal (monsoon and inter-monsoon) and annual basis. The third-order Markov chain of the TCRG was found to perform better in simulating the rainfall occurrence and preserving the low-frequency variability of the wet spells. The log-normal distribution of the TCRG was consistently better in modelling the rainfall amounts. Both models tend to underestimate the skewness and kurtosis coefficient of the rainfall. The spectral correction approach adopted in the TCRG successfully preserved the seasonal and inter-annual variabilities of rainfall amounts, whereas the MV-WG tends to underestimate the variability bias of rainfall amounts. Overall, the TCRG performed reasonably well in the Kelantan River Basin, as it can represent the key statistics of rainfall occurrence and amounts successfully, as well as the low-frequency variability.     

Stochastic modelling of rainfall occurrences in continuous time

Abstract

The paper is concerned with the modelling of rainfall occurrence in continuous time. The Alternating Renewal Process is employed for the evaluation of probability distribution functions for total wet and dry periods over a homogeneous time interval (0, t). The derived general solution is simplified by assuming that the individual wet and dry intervals are random variables following an Erlang distribution, in particular an exponential distribution. Data on a continuous time scale from the Mikra Station in Greece are used to illustrate the proposed methodology.     

Characterizing incipient motion of low fines content soils with varying compositions,water contents,and relative densities

Laboratory flume experiments were conducted to quantify the effects of the soil characteristics on the critical shear stress of low fines content soil samples collected from the Montauk shores in New York. The collected soils were reconstituted at five different fines contents, ranging between 0 and 20%. These soil mixtures were composed of two initial water contents, dry of optimum and optimum moistures, and two relative densities, one moderate dense and the other dense. The strength indices of...     

Stochastic modelling of the pulse-like near-fault ground motions with time-frequency representation

Journal of Seismology - Seismic near-fault ground motions differ from those in the far-field. They could contain large amplitude and long period velocity pulses and cause damage to structures. In...     

Monitoring and modelling of pore water pressure changes and riverbank stability during flow events

Pore water pressures (positive and negative) were monitored for four years (1996–1999) using a series of tensiometer‐piezometers at increasing depths in a riverbank of the Sieve River, Tuscany (central Italy), with the overall objective of investigating pore pressure changes in response to ?ow events and their effects on bank stability. The saturated/unsaturated ?ow was modelled using a ?nite element seepage analysis, for the main ?ow events occurring during the four‐year monitoring period. Modelling results were validated by comparing measured with computed pore water pressure values for a series of representative events. Riverbank stability analysis was conducted by applying the limit equilibrium method (Morgenstern‐Price), using pore water pressure distributions obtained by the seepage analysis. The simulation of the 14 December 1996 event, during which a bank failure occurred, is reported in detail to illustrate the relations between the water table and river stage during the various phases of the hydrograph and their effects on bank stability. The simulation, according to monitored data, shows that the failure occurred three hours after the peak stage, during the inversion of ?ow (from the bank towards the river). A relatively limited development of positive pore pressures, reducing the effective stress and annulling the shear strength term due to the matric suction, and the sudden loss of the con?ning pressure of the river during the initial drawdown were responsible for triggering the mass failure. Results deriving from the seepage and stability analysis of nine selected ?ow events were then used to investigate the role of the ?ow event characteristics (in terms of peak stages and hydrograph characteristics) and of changes in bank geometry. Besides the peak river stage, which mainly controls the occurrence of conditions of instability, an important role is played by the hydrograph characteristics, in particular by the presence of one or more minor peaks in the river stage preceding the main one. Copyright © 2004 John Wiley & Sons, Ltd.     

边坡地震稳定性分析探讨

传统的拟静力法和安全系数时程分析法在评价边坡地震稳定性时存在一定的局限性。在提出准确的评价边坡地震稳定性必需因素的基础上,建议对边坡地震稳定性分析方法重新进行分类。根据动力分析得到的边坡在地震作用下的破坏机制和破裂面的性质和位置,提出基于拉-剪破坏的动力时程分析法和强度折减动力分析法。第一种方法将FLAC计算得到破坏时刻的动应力施加到静力情况下边坡上,采用动力分析得到的拉-剪破裂面,结合极限平衡法求解边坡地震安全系数,是一种改进的动力有限元时程分析法;第二种方法考虑了拉-剪破坏的FLAC强度折减动力分析法,是完全动力的方法。最后通过算例分析验证了新方法的可行性,为边坡地震安全系数计算提供了一种新的思路。     

Stochastic modelling of groundwater flow and solute transport in aquifers

This paper presents an introductory overview of recently developed stochastic theories for tackling spatial variability problems in predicting groundwater flow and solute transport. Advantages and limitations of the theories are discussed. Lastly, strategies based on the stochastic approaches to predict solute transport in aquifers are recommended.     

地震边坡稳定性分析的拟静力方法适用性探讨

应用极限平衡拟静力方法进行地震边坡稳定性分析时,对粘聚力较小、内摩擦角较大的浅层滑坡而言,随着地震水平设计加速度值的增大,其临界滑动面基本保持不变;然而,对粘聚力较大而摩擦角较小的深层滑坡而言,随着地震水平设计加速度值的增大,临界滑动面越深、越宽,会呈现出发散现象,选取两个典型的均质土坡,设计了由浅及深的多条滑动面,分别采用极限平衡瑞典方法进行了相应安全系数的计算,同时比较了自重、地震力、粘聚力在抗滑力矩以及滑动力矩中所占比重的变动情况。研究发现:对于浅层滑坡,随着滑动面由浅及深的变化,粘聚力产生的抗滑力矩所占百分比下降幅度较大,自重产生的滑动力矩与总体抗滑力矩的比值下降幅度较大;而对于深层滑坡,随着滑动面由浅及深变化,自重产生的抗滑力矩所占百分比增长的幅度较大,地震力产生的滑动力矩与总体抗滑力矩比值增长幅度也较大。     

南海北部陆坡稳定性定量分析

随着海洋工程的发展,海底滑坡作为一种潜在的地质灾害逐渐成为人们关注的热点.本文采用二维极限平衡法计算并分析了海底斜坡稳定性问题.通过对斜坡模型在各种条件下安全系数的计算,定量分析了斜坡内在因素(如斜坡角度、主要土力学参数)和主要触发机制(地震、快速堆积等)对安全系数的影响.理论计算表明,静态条件下,均质斜坡角度小于20°时,均处于稳定状态;对于含软弱层的斜坡,快速堆积等引起的不排水状态下斜坡安全系数明显降低,斜坡角度大于14°时就会发生失稳.拟静态条件下,当地震动峰值加速度(PGA)小于0.15g时,对于角度小于20°的均质斜坡处于稳定状态,但PGA大于0.25g时,角度大于13°的斜坡即处于失稳状态;对于含软弱层斜坡,PGA为0.1g时,角度大于10°的斜坡即处于不稳定状态;当PGA大于0.3g时,3°以上的海底斜坡即处于失稳状态,发生海底滑坡.结合南海北部陆坡海底地形、地貌特征,在静态条件下,均处于稳定状态;但在地震加载的拟静态下,根据南海北部地震动峰值加速度分布,台湾浅滩段则处于不稳定状态.这解释了该区域大陆坡折带处海底滑坡广泛发育的原因,也表明了地震是引发南海北部滑坡最主要的触发机制之一.     

Salt accumulation in semi-arid floodplain soils with implications for forest health

Dieback of native Eucalyptus largiflorens forests is an increasing problem on the floodplains of the lower River Murray, southern Australia. Salinisation of floodplain soils, as a result of the changed hydrological management of the River Murray, appears to be a primary cause of the dieback. Regulation of the River Murray has reduced the frequency of large flood events by a factor of approximately three and caused groundwater levels beneath floodplains to rise. The higher water tables have resulted in increased discharge of the naturally saline groundwater in the floodplains by evapotranspiration, and the decreased incidence of large floods has reduced floodwater recharge and hence leaching of salt from floodplain soils. Use of soil physical properties for a range of floodplain soils, combined with measurements of groundwater discharge from bare and vegetated sites, suggests that the time-scale for complete soil salinisation can, at worst, be less than 20 years. Moreover, salt accumulation at most sites will continue to occur as the present flooding regime (of which there is limited scope for improvement) appears incapable of providing the leaching required to counteract accumulation. The analyses carried out here suggest that the ‘critical’ water table depth (below which groundwater discharge is balanced or exceeded by floodwater recharge) needs to be increased by 14–55% (the more clayey the soil, the larger the increase) to prevent salt accumulation. Failure to implement schemes which lower the water tables beneath the floodplain may, in the long term, cause serious damage to these important riparian forests.     

Effects of vertical seismic acceleration on 3D slope stability

The conventional pseudo-static approach often neglects the effect of the vertical seismic acceleration on the stability of a slope, but some analyses under plane-strain (2D) conditions show a significant effect on the slope stability. The purpose of this study is to investigate the effect of the vertical acceleration on the safety of three-dimensional (3D) slopes. In the strict framework of limit analysis, a 3D kinematically admissible rotational failure mechanism is adopted here for 3D homogeneous slopes in frictional/cohesive soils. A set of stability charts is presented in a wide range of parameters for 3D slopes under combined horizontal and vertical seismic loading conditions. Accounting for the effects of the vertical seismic acceleration, the difference in safety factors for 3D slopes can exceed 10%, which will significantly overestimate the safety of the 3D slopes.     

Comparison of relative permeability–fluid saturation–capillary pressure relations in the modelling of non-aqueous phase liquid infiltration in variably saturated,layered media

The characteristic functions relating relative permeabilities and capillary pressures to fluid saturations (k_r–S–P_c models) are of great importance for the modelling of sub-surface multi-phase flow and transport. In order to test their performance and to identify their important parameters, four well-known three-phase k_r–S–P_c models have been tested against published experimental data on non-aqueous phase liquid (NAPL) migration in the unsaturated zone. Both homogenous systems and systems with embedded heterogeneities have been analysed.     

堆石坝坝坡的抗震稳定分析

目前分析堆石坝下游坡的抗震稳定性仍采用瑞典圆弧法和简化毕肖普法，土体的抗剪强度多用莫尔－库仑准则。试验表明堆石材料的抗剪强度更符合非线性表达式。已有文献采用非线性抗剪强度分析堆石坝的抗震稳定性，但该方法的安全标准少见论证。本文对采用线性与非线性抗剪强度的有限元法和极限平衡法分析堆石坝坝坡稳定性进行了比较。结果显示，毕肖普法安全系数比瑞典法高2％－10％，采用非线性抗剪强度计算的安全系数比采用线性抗剪强度高15％－25％。与毕肖普法容许安全系数比瑞典法高同理，用非线性抗剪强度准则进行堆石坝坝坡稳定计算，其容许安全系数也应在线性准则的基础上提高。提高的幅度有待进一步积累经验。     

层状结构岩质边坡动力稳定性试验研究

介绍了所进行的层状结构岩质边坡动力稳定性试验。结果表明，在水平地震动加速度达到0．4g时，层状结构岩质边坡就会出现局部的层间错动现象，当水平地震加速度达到0．8g时，层间结合力较弱的边坡将发生大面积的表面滑动和崩塌。而在铅直地震力作用下，当地震加速度达到1.0g时，才会出现破坏现象。因而对层状结构岩质边坡来说，其水平地震力造成的危害是主要的。