Modelling the poroelastoplastic behaviour of soils subjected to internal erosion by suffusion

Granular soils subjected to seepage flow may suffer suffusion, ie, a selective internal erosion. Extending the classical approach of poromechanics, we deduce a new form of the Clausius-Duhem inequality accounting for dissipation due to suffusion, and we deduce restrictions on the constitutive laws of the soil. We suggest (a) a possible coupling between the seepage forces and the suffusion kinetics and (b) an extension of an existing elastoplastic model for the skeleton mechanical behaviour. Numerical integrations of the elastoplastic model are carried out under drained axisymmetric triaxial and oedometric conditions. As a result, we prove that the extended model is able to qualitatively reproduce the suffusion induced strains and the strength reduction experimentally observed. Predictions on the oedometric behaviour of suffusive soils are also provided.     

A review of landsliding and coastal erosion damage to historic fortifications in South East England

Coastal fortifications in South East England have been continuously built and rebuilt over the centuries according to the ebb and flow of politics in Europe. This paper covers an extensive historical period from the influx of the Romans to the present day, detailing how landslides and coastal erosion have affected the numerous constructions along this stretch of coastline. Principal landslide-prone strata form extensive sections of this part of the coast with the tendency to be weak and highly susceptible to marine attack and cliff retreat. Fortifications have been built out of necessity in strategic positions without full consideration of the ground upon which they have been constructed. The continual construction of fortifications along this section of coastline constitutes an unparalleled archaeological record, but it is one that is under continuous threat from coastal erosion and landsliding.     

巨厚松散层下煤层开采的抗渗透性破坏和煤柱安全性研究——以口孜东矿为例

采掘诱发裂隙导通顶板上覆含水砂层中产生的水力坡度是渗透型溃砂的关键判断依据之一,但是,到目前为止主要是采用定流量下的稳定流水力坡度表达式计算,而实际上,上覆含水层中水动力条件和涌水量在溃水溃砂过程中都是非稳定的。本文依据非稳定流理论,给出了二维潜水、承压水单井定降深模型下的水力坡度J的解析公式,并分析了J随时间和距离的演变特征。结果表明,对于潜水,J随与井的距离r的增加而下降,下降速度随r的增加而减小;存在拐点r′,当r<r′时,J随时间延长而下降,在r>r′时,J随时间延长而增加;随着地下水头H₀的增加,J是增加的;本文获得的潜水的非稳定流条件下的J大于潜水稳定流的J。对于承压水,J随r的增加而下降,下降速度随r的增加而减小;与潜水不同的是,在不同r处,J随时间延长而下降,不存在拐点;随着H₀的增加,J是增加的;本文获得的承压水的非稳定流条件下的J不总大于承压水稳定流的J。最后,在伊犁河谷某矿对本文方法进行了初步应用。     

A physical statistical approach to erosion

Erosion is a complex process consisting of many components such as surface runoff, impact of raindrops, wind forces, soil and rock mechanics, etc. Trying to integrate all these processes into a physical model seems to be hopeless. In order to understand the variety of natural shapes and patterns produced by erosion we present an integrated statistical approach. Our model is based on simple physical constraints for the separation of amalgamated particles (abrasion) and for the movement of loose particles (denudation) and on the laws of statistics. After some simplifications, we obtain a nonlinear system of partial differential equations which is solved using finite volume techniques. The model is suitable for the formation of different types of rill systems and the episodic behaviour of erosion processes, a kind of self-organized criticality. Besides effects of inhomogeneities, e.g. the formation of terraces can be investigated.     

隧道渗流侵蚀的颗粒流模拟

建造在砂土环境中的盾构隧道渗漏水会将砂土颗粒带入隧道内部,导致隧道周围砂土流失,引起隧道的不均匀变形与沉降。建立了隧道–土体离散元计算模型,模拟隧道不同部位发生局部渗流侵蚀时隧道中心位移、隧道表面土压力分布及地表沉降的变化。结果表明,隧道中心位移、地表沉降量均与颗粒流失比例呈线性增加关系;受颗粒流失及流失缝周围形成的颗粒力拱影响,隧道表面土压力重分布,会使流失缝宽度进一步扩大而加剧渗流侵蚀进程,且渗流侵蚀发生于隧道底部产生的影响大于渗流侵蚀发生于隧道腰部与顶部。     

On the effect of compaction on the progression of concentrated leaks in cohesive soils

Acta Geotechnica - The purpose of this study is to conduct hole erosion tests (HETs) to better understand the progression of concentrated leaks in compacted soils. While samples with high levels of...     

沙漠公路风沙土路基风蚀破坏试验研究

以沙漠公路风沙土路基为研究对象,通过室内风蚀风洞试验研究路基的风蚀破坏规律,以及路基不同断面对风沙流运动的影响。以路基高度、路基边坡坡率和路基宽度作为路基断面主要设计参数,研究不同路基断面下风沙流扰动、增速、减速、恢复的过程,以及路基周围风速流场的变化特征,分析路基病害较未病害时路基周围流场的变化。试验结果表明：路基高度和边坡坡率对风沙流运动的影响较大。随路基高度增加,路基对风沙流流场扰动增强,迎风坡坡顶处吹蚀破坏和背风坡坡底处堆蚀破坏越显著,在确定的路基边坡坡率下,路基模型高度为250 mm较模型高度为60 mm时,迎风坡坡顶风速增加1.13倍,背风坡坡底风速减小2.53倍,建议沙漠公路路基高度宜小于2.5 m。进一步,在确定的路基高度下,比较不同的边坡坡率对路基沿程风速的影响,发现当路基边坡坡率为1:1.75时,路基沿程风速变化不明显,沙漠公路风沙土路基不宜被风蚀破坏。     

黏性泥石流沟床冲刷深度研究

沟床冲刷深度是泥石流灾害防治工程设计最重要的参数之一,但到目前为止,关于黏性泥石流沟床冲刷的研究较少,沟床冲刷深度还没有权威可信的计算方法,是泥石流防治工程设计急需解决的技术问题。详细分析了黏性泥石流及可能冲刷沟床运动过程中受力情况,推导出黏性泥石流沟床最大冲刷深度计算公式。公式表明黏性泥石流沟床冲刷深度随泥石流泥深、泥石流重度和沟床纵比降及沟床堆积土体黏性的增大而增大,随沟床堆积土体内摩擦角的增大而减小。与现有计算方法相比,公式基于严格理论推导,计算结果更为精确,可用于计算已发生泥石流地区的不同频率的泥石流的冲刷深度,并举例说明了计算公式的实用价值,其结果为泥石流防治工程设计提供技术支撑。     

水泥改良土强度的海水侵蚀效应与数值模拟

滨海地区改良软土会受到海水的长期浸泡作用,为获得其力学性能随时间的演化规律,依据海水的主要物质成分和含量制备了盐水溶液。选取天津滨海新区的软土,掺入15%的水泥进行养护,为便于对比分析,选用有机质含量低的粉土作比较。为了说明盐溶液浸泡的作用,进行了盐水和淡水两种浸泡方式的对比试验。无侧限抗压强度试验结果表明,改良软土历经淡水溶液和盐溶液浸泡3 d后其强度差值达到最大,7 d和28 d养护龄期试样分别相差0.40、1.28 MPa,建议利用28 d的标准养护龄期强度作为设计参考值。利用Geo-Studio中的Seep/W和CTRAN模块模拟了盐溶液离子浸入试样的过程,发现离子在试样内部迁移2～3 d趋于平稳,主要迁移至离试样表层5 mm的区域范围内,数值计算结果与宏观力学试验结果相一致。     

Evaluation of probabilistic flow in two unsaturated soils

A variably saturated flow model is coupled to a first-order reliability algorithm to simulate unsaturated flow in two soils. The unsaturated soil properties are considered as uncertain variables with means, standard deviations, and marginal probability distributions. Thus, each simulation constitutes an unsaturated probability flow event. Sensitivities of the uncertain variables are estimated for each event. The unsaturated hydraulic properties of a fine-textured soil and a coarse-textured soil are used. The properties are based on the van Genuchten model. The flow domain has a recharge surface, a seepage boundary along the bottom, and a no-flow boundary along the sides. The uncertain variables are saturated water content, residual water content, van Genuchten model parameters alpha (α) and n, and saturated hydraulic conductivity. The objective is to evaluate the significance of each uncertain variable to the probabilistic flow. Under wet conditions, saturated water content and residual water content are the most significant uncertain variables in the sand. For dry conditions in the sand, however, the van Genuchten model parameters α and n are the most significant. Model parameter n and saturated hydraulic conductivity are the most significant for the wet clay loam. Saturated water content is most significant for the dry clay loam. Electronic Publication     

Organic carbon stocks and erosion in the soils of Guangdong,South China

Soil organic carbon (SOC) storage and erosion in South China at the regional scale in the past decades remains far from being understood. This paper calculated the SOC density, storage and erosion in 14 soil classes in Guangdong Province, South China, based on statistical data from the soil survey and soil erosion survey of Guangdong, which was performed in the 1990s. The purpose of this study is to understand the relationships between soil classes and SOC erosion at the regional scale. The results indicated that the SOC density in the soils of Guangdong varied from 12.7 to 144.9 Mg ha^?1 over the entire profile and from 12.6 to 68.4 Mg ha^?1 in the top 20-cm soil layer. The average area-weighted SOC density in the topsoil (0–20 cm) and the entire profile was 32 ± 3 and 86 ± 4 Mg ha^?1, respectively. The total SOC storage was 1.27 ± 0.06 Pg, with 35.6 % (0.46 ± 0.04 Pg) located in the topsoil. The average area-weighted strength of the SOC erosion in the 1990s was 20.6 ± 0.8 Mg km^?2 year^?1. The results indicated that SOC erosion was strongly related to soil class.     

Subaqueous soils: A pedological approach to the study of shallow-water habitats

Science-based management of shallow-water habitats is limited by information on the spatial distribution of properties of sediments. This limitation in part stems from the lack of an adequate model or system to classify and delineate subaqueous soil types (sediments). Present classification systems are inadequate because the existing paradigm does not actually consider them as “soils” but merely as “sediments”. Field observations suggest that these sediments could be better understood as “soils”, and the present paradigm could be modified to incorporate a new one—a pedological paradigm. We propose the application of a pedological paradigm for subqueous soils of subtidal habitats to develop ecological interpretations of subaqueous soil types and apply an inventory of subaqueous soil resources for management of estuarine shallow-water habitats. *** DIRECT SUPPORT *** A01BY074 00009     

Variations in soils derived from an erodible sandstone formation and factors controlling their susceptibility to erosion and landslide

Environmental degradations caused by erosion and landslide within an area in the South-eastern part of Nigeria were assessed, and also an attempt to characterize the underlying soils based on their degree of susceptibility to erosion and landslide. Factors affecting soil erodibility such as physical and chemical properties of soil, as well as vegetation density and slope gradient was determined in 20 different areas. Simple linear regression and principal component analysis were employed to relate the factors controlling erosion and landslide to the erosion and slide densities, and to determine the pattern that exists in the data and as well express the data in such a way as to highlight their similarities and differences. Four vertical horizons and two horizontal zones of soil were distinguished based on their degree of weathering as well as their chemical and mineralogical composition; hence the soils were classified into six different types based on their degree of susceptibility to erosion and landslide. Variations in their degree of susceptibility to erosion and landslide were majorly controlled by its chemical and mineralogical composition rather than its particle size distribution.     

Fractal models for the fragmentation of rocks and soils: a review

Fragmentation, the process of breaking apart into fragments, is caused by the propagation of multiple fractures at different length scales. Such fractures can be induced by dynamic crack growth during compressive/tensile loading or by stress waves during impact loading. Fragmentation of rocks occurs in resoonse to tectonic activity, percussive drilling, grinding and blasting. Soil fragmentation is the result of tillage and planting operations. Fractal theory, which deals with the scaling of hierarchical and irregur systems, offers new opportunities for modeling the fragmentation process. This paper reviews the literature on fractal models for the fragmentation of heterogeneous brittle earth materials. Fractal models are available for the fragmentation of: (1) classical aggregate; (2) aggregates with fractal pore space; and (3) aggregates with fractal surfaces. In each case, the aggregates are composed of building blocks of finite size. Structural failure is hierarchical in nature and takes place by multiple fracturing of the aggregated building blocks. The resulting number-size distribution of fragments depends on the probability of failure, P(1/bⁱ) at each level in the hierarchy. Models for both scale-invariant and scale-dependent are reviewed. In the case of scale-invariant P(1/bⁱ)< 1, theory predict: D_f = 3 + log [P(1/bⁱ)]/log[b] for classical aggregates; D_f=D_m+log[P(1/bⁱ)]/log[b] for aggregates with fractal pore space; and D_f=D_s for aggregates with fractal surfaces. where b is a scaling factor and D_f, D_m and D_s are the fragmentation, mass and surface fractal dimensions, respectively. The physical significance of these parameters is discussed, methods of estimating them are reviewed, and topics needing further research are identified.     

地下水流系统研究中的方法论探讨:以CUG-武汉地下水流系统研究为例

地球系统科学和地下水流系统理论的出现,标志着水文地质学进入新的发展时期.地下水流系统理论已成为水文地质学的新范式,其产生和发展,从方法论上为地下水问题的研究提供了新的启示.自20世纪80年代初以来,中国地质大学(武汉)地下水流系统组持续开展了地下水流系统理论与应用研究.试图从方法论的角度回顾与总结这些研究成果.地下水问题受多因素影响,研究应以"目标与问题导向相结合",靶向准确,才能在信息浩渺中不失方向,向着目标前行;研究时应采用"假设演绎法",先依据已有认识和资料演绎出应有的现象,再有目的 地观察和寻求证据,或修改假设继续求证,直到假设被证实或证伪;演绎寻证过程,可以采用"控制性实验""信息提取与组织""多学科方法与手段融合"等技术方法.实例分析证实,控制性模拟实验使我们得出了地下水流模式的新认识;采用多通道的信息提取、加工和组织,构建地下水流系统模式,能够有效解决各类工程中的应用问题;多学科与手段融合、各种信息相互验证,提高了结果的可信度.以实例研究为基础,从研究方法上的总结能为正确认识和理解地下水流系统理论、推进新理论和新思路在水文地质研究中的应用提供参考.     

Head-based isogeometric analysis of transient flow in unsaturated soils

An isogeometric analysis (IGA) is introduced to obtain a head-based solution to Richards equation for unsaturated flow in porous media. IGA uses Non-Uniform Rational B-Spline (NURBS) as shape functions, which provide a higher level of inter-element continuity in comparison with Lagrange shape functions. The semi-discrete nonlinear algebraic equations are solved using a combination of implicit backward-Euler time-integration and Newton-Raphson scheme. The time-step size is adaptively controlled based on the rate of changes in the pore pressure. The results from the proposed formulation are compared and verified against an analytical solution for one-dimensional transient unsaturated flow in a homogenous soil column. The proposed method is then applied to four more complex problems including two-dimensional unsaturated flow in a two-layered soil and a semi-circular furrow. The test cases in two-layered soil system involve sharp variations in the pressure gradient at the intersection of the two media, where the pore water pressure abruptly changes. It is shown that the proposed head-based IGA is able to properly simulate changes in pore pressure at the soils interface using fewer degrees of freedom and higher orders of approximation in comparison with the conventional finite element method.     

一维稳态渗流时非饱和土中的基质吸力分布

首先对非饱和土中的土水势和干土吸水过程中土水势的变化进行了定性分析。然后利用水相连续性条件、广义Darcy定律及Gardner关于非饱和土导水系数和基质吸力的经验公式,建立了非饱和土一维稳态渗流情况下基质吸力沿深度变化的关系表达式,并利用差分方法对稳态入渗和稳态蒸发两种情况进行了求解。根据提出的方法,不仅可以方便地得到基质吸力沿深度的分布,而且可以求出水头和导水系数沿深度的变化。     

冻土区管土相互作用研究综述

管道是长距离输送天然气或石油的最经济有效的工具之一,当管道穿越冻土区时,将面临土体冻胀融沉作用引起的管道弯曲变形和破坏,管土之间的相互作用为冻土区管道设计和运营的重要考虑因素。简述目前世界上主要的穿越冻土区管道工程（罗曼井管道、俄罗斯远东地区管道、美国阿拉斯加管道及中国寒区管道网）的设计理论发展,归纳总结了管土相互作用室内外相关试验、数值模拟分析理论和方法等方面的研究成果和发展现状,并针对冻土区管土相互作用的研究提出进一步的研究展望。