Domestic Well Capture Zone and Influence of the Gravel Pack Length

Domestic wells in North America and elsewhere are typically constructed at relatively shallow depths and with the sand or gravel pack extending far above the intake screen of the well (shallow well seal). The source areas of these domestic wells and the effect of an extended gravel pack on the source area are typically unknown, and few resources exist for estimating these. In this article, we use detailed, high-resolution ground water modeling to estimate the capture zone (source area) of a typical domestic well located in an alluvial aquifer. Results for a wide range of aquifer and gravel pack hydraulic conductivities are compared to a simple analytical model. Correction factors for the analytical model are computed based on statistical regression of the numerical results against the analytical model. This tool can be applied to estimate the source area of a domestic well for a wide range of conditions. We show that an extended gravel pack above the well screen may contribute significantly to the overall inflow to a domestic well, especially in less permeable aquifers, where that contribution may range from 20% to 50% and that an extended gravel pack may lead to a significantly elongated capture zone, in some instances, nearly doubling the length of the capture zone. Extending the gravel pack much above the intake screen therefore significantly increases the vulnerability of the water source.     

浆固碎石桩成桩注浆渗透影响分析

浆固碎石桩作为一种新型软土地基处理技术,其主要通过注浆改善桩体的加固效果,同时通过浆体渗透来改善桩周土体的物理力学性质,从而减小浆固碎石桩复合地基沉降。针对浆体对桩周土体的渗透作用,按照平面轴对称问题,推导出注浆渗透影响范围的计算方法和浆固区压缩模量计算公式,并通过室内模型试验研究,验证了浆固区压缩模量计算公式的正确性。随后,利用数值计算分析,对浆固区影响范围进行量化分析,并通过数值拟合得到了考虑注浆渗透影响的桩体等效半径计算公式。所得结果对工程设计具有重要的指导意义。     

Special Issue: Gravel Bed Rivers 8

Theoretical, modeling, experimental and monitoring of sediment transport in gravel bed rivers are introduced. The key findings of 27 papers on this theme were published in Earth Surface Processes and Landforms during the past year. Among these, 18 papers were published by attendants of the 8th Gravel Bed River Workshop held in Japan in 2015. This Commentary introduces the main themes associated with these papers including innovations in sediment transport conceptualization and modeling and monitoring, and also three wider themes: the importance of sediment supply; relationships between gravel bed river science and restoration; and the growing recognition of the critical role played by wood in gravel bed rivers. Copyright © 2018 John Wiley & Sons, Ltd.     

Permeability of Triaxially Compressed Sandstone: Influence of Deformation and Strain-rate on Permeability

— The influence of differential stress on the permeability of a Lower Permian sandstone was investigated. Rock cylinders of 50 mm in diameter and 100 mm length of a fine-grained (mean grain size 0.2 mm), low-porosity (6–9%) sandstone were used to study the relation between differential stress, rock deformation, rock failure and hydraulic properties, with a focus on the changes of hydraulic properties in the pre-failure and failure region of triaxial rock deformation. The experiments were conducted at confining pressures up to 20 MPa, and axial force was controlled by lateral strain with a rate ranging from 10^?6 to 10^?7 sec^?1. While deforming the samples, permeability was determined by steady-state technique with a pressure gradient of 1 MPa over the specimen length and a fluid pressure level between 40 and 90% of the confining pressure. The results show that permeability of low-porosity sandstones under increasing triaxial stress firstly decreases due to compaction and starts to increase after the onset of dilatancy. This kind of permeability evolution is similar to that of crystalline rocks. A significant dependence of permeability evolution on strain rate was found. Comparison of permeability to volumetric strain demonstrates that the permeability increase after the onset of dilatancy is not sufficient to regain the initial permeability up to failure of the specimen. The initial permeability, which was determined in advance of the experiments, usually was regained in the post-failure region. After the onset of dilatancy, the permeability increase displays a linear dependence on volumetric strain.     

地震大小的度量

本文首先对地震大小度量的研究历程作了简要回顾。在此基础上着重指出以下几点: ① 近几十年广泛使用的M_L、m_b(m_B)、M_S震级标度,作为对地震大小的度量,不仅都存在着“以偏概全”和“震级饱和”的问题,而且由于未充分顾及地震波衰减的区域差异,尤其台站场地对地震动放大作用的差异,因此都只是对地震大小的不精确的度量。 ② 地震矩M₀不仅物理含义明确,而且克服了M_L、m_b(m_B)、M_S震级标度所存在的各种问题,是最适合科学度量地震大小的物理量。为了继续延用“震级”这一术语,Kanamori定义了矩震级标度M_W,虽然其假定的前提条件仍有待研究,但也是对地震相对大小的较合理的度量。 ③ 为更加科学地度量地震的大小,必须充分利用现代区域数字地震台网产出的大量波形数据,加强地震波衰减特征、场地效应和震源参数测定及有关定标关系的研究,在完善M_L、m_b(m_B)、M_S震级测定方法的基础上,重点改进、完善M_W标度,逐渐推进以M_W作为统一度量地震相对大小的物理量,为地震科学研究和地震预测研究奠定更加坚实的基础。     

国内外液化砂砾土土性对比分析

砂土液化问题的研究特别是砂土液化的判别目前已经取得了较大的进展,并在国内外现行的规范中充分反映,而砂砾土由于颗粒大、透水性好,普遍认为地震时孔压不至于上升至液化的程度,往往将其划分为非液化土类。通过调查汶川地震砂砾土液化情况以及勘察试验获取其土性资料,对比分析了国内外液化砂砾土的地质背景及土性特征。主要认识为：①以往国内外砂砾土液化实例虽然有限,但已经表明松散-稍密的砂砾土在一定的地震强度下仍有可能发生液化,而2008年汶川地震中的大量砂砾土液化的事实,说明笼统地将砂砾土划归为非液化土类的做法有误;②汶川地震液化砂砾土的颗粒级配范围涵盖了国内外其他地震的颗粒级配,研究汶川地震的砂砾土液化问题具有代表性和普遍性,以此建立的砂砾土液化评价方法在国际上应具有通用性。     

浆固碎石桩单桩轴向荷载传递分析

浆固碎石柱是一种新型的桩基技术,已成功应用于铁路、高速公路等工程中。但由于浆固区的存在,浆固碎石桩荷载传递机理的复杂性制约了其发展和工程应用。荷载传递分析法是桩荷载传递机理分析的重要方法之一。本文采用线性弹塑性和双折线传递函数,分别模拟桩侧土体的非线性和桩端土的硬化特性,同时考虑浆固区存在对桩荷载传递的影响,推导出一种浆固碎石桩单桩荷载传递分析方法。最后,通过对工程实例进行计算,验证了本文计算方法的实用性。结果表明,浆固碎石桩除具有一般刚性桩的桩体置换作用外,浆固区的存在对提高浆固碎石柱承载力、减小沉降量也有着重要作用。     

A Poroelastic Description of Permeability Evolution

Pore pressure changes in a geothermal reservoir, as a result of injection and/or production of water, result in changes of stress acting on the reservoir rock and, consequently, changes in the mechanical and transport properties of the rock. Bulk modulus and permeability were measured at different pressures and temperatures. An outcropping equivalent of Rotliegend reservoir rock in the North German Basin (Flechtinger sandstone) was used to perform hydrostatic tests and steady state fluid flow tests. Permeability measurements were conducted while cycling confining pressure; the dependence of permeability on stress was determined at a constant downstream pressure of 1 MPa. Also, temperature was increased stepwise from 30 to 140 °C and crack porosity was calculated at different temperatures. Although changes in the volumes of cracks are not significant, the cracks control fluid flow pathways and, consequently, the permeability of the rock. A new model was derived which relates microstructure of porosity, the stress–strain curve, and permeability. Porosity change was described by the first derivative of the stress–strain curve. Permeability evolution was ascribed to crack closure and was related to the second derivative of the stress–strain curve. The porosity and permeability of Flechtinger sandstone were reduced by increasing the effective pressure and decreased after each pressure cycle.     

含砾量对饱和砂砾土液化特性的影响

利用GDS循环三轴仪进行一系列饱和砂砾土不排水动三轴液化试验,研究其在循环荷载作用下的液化特性,分析含砾量对饱和砂砾土动强度和动孔压的影响规律。研究表明:含砾量对砂砾土液化性能影响较大,随着含砾量的增加砂砾土抗液化强度呈单调增加趋势;随循环周次的增加孔隙水压力不断升高,增长速率与所施加的循环应力幅值有关,同一固结压力下,振次比相同时循环动应力幅值越大动孔压比越大;破坏振次对动孔压增长模式存在影响,破坏振次较小时砂砾土动孔压增长模式呈双曲线型发展,破坏振次较大时砂砾土的动孔压增长模式可用反正弦函数来表示,且含砾量越大循环荷载引起的孔隙水压力越高;含砾量对砂砾土液化特性的影响可从砂砾土的微细观结构特征得到阐释,并借助其粒间状态参量进行分析。     

Modeling the Permeability Loss of Metallic Iron Water Filtration Systems

Over the past 30 years the literature has burgeoned with in situ approaches for groundwater remediation. Of the methods currently available, the use of metallic iron (Fe⁰) in permeable reactive barrier (PRB) systems is one of the most commonly applied. Despite such interest, an increasing amount of experimental and field observations have reported inconsistent Fe⁰ barrier operation compared to contemporary theory. In the current work, a critical review of the physical chemistry of aqueous Fe⁰ corrosion in porous media is presented. Subsequent implications for the design of Fe⁰ filtration systems are modeled. The results suggest that: (i) for the pH range of natural waters (>4.5), the high volumetric expansion of Fe⁰ during oxidation and precipitation dictates that Fe⁰ should be mixed with a non‐expansive material; (ii) naturally occurring solute precipitates have a negligible impact on permeability loss compared to Fe⁰ expansive corrosion; and (iii) the proliferation of H₂ metabolizing bacteria may contribute to alleviate permeability loss. As a consequence, it is suggested that more emphasis must be placed on future work with regard to considering the Fe⁰ PRB system as a physical (size‐exclusion) water filter device.     

Axial and Radial Permeability Evolutions of Compressed Sandstones: End Effects and Shear-band Induced Permeability Anisotropy

The influence of hydrostatic and uniaxial stress states on the porosity and permeability of sandstones has been investigated. The experimental procedure uses a special triaxial cell which allows permeability measurements in the axial and radial directions. The core sleeve is equipped with two pressure samplers placed distant from the ends. They provide mid-length axial permeability measure as opposed to the overall permeability measure, which is based on the flow imposed through the pistons of the triaxial cell. The core sleeve is also equipped to perform flows in two directions transverse to the axis of the sample. Two independent measures of axial and complementary radial permeability are thus obtained. Both Fontainebleau sandstone specimens with a porosity of about 5.8% to 8% and low permeability ranging from 2.5 mD to 30 mD and Bentheimer sandstone with a porosity of 24% and a high permeability of 3D have been tested. The initial axial permeability values obtained by each method are in good agreement for the Fontainebleau sandstone. The Bentheimer sandstone samples present an axial mid-length permeability 1.6 times higher than the overall permeability. A similar discrepancy is also observed in the radial direction, also it relates essentially to the shape of flow lines induced by the radial flow. All the tested samples have shown a higher stress dependency of overall and radial permeability than mid-length permeability. The effect of compaction damage at the pistons/sample and radial ports/sample interfaces is discussed. The relevance of directional permeability measurements during continuous uniaxial compression loadings has been shown on the Bentheimer sandstone until the failure of the sample. We can efficiently measure the influence of brittle failure associated to dilatant regime on the permeability: It tends to increase in the failure propagation direction and to decrease strongly in the transverse direction.     

砾石掺量及侧限压力对黏土类土石混合体动力特性的影响分析

黏土类土石混合体常作为路基填料在工程中广泛应用,然而其在交通荷载作用下的动力特性方面研究较少。为此,本文以0.05 Lc(Lc为圆柱试样直径)为土体、块石分类阈值,根据土石混合体中砾石掺量、侧限压力的不同,采用自振柱仪对其在小应变(10^-6~10^-4)下的动剪切模量和阻尼比进行研究。结果表明:土石混合体最大动剪切模量随砾石掺量的增加而不断增大,且在20%~40%掺量区间内增幅最大,随侧限压力的增加而增大,且增幅逐渐变小;在应变幅值相同的条件下,动剪切模量衰变程度随砾石掺量的增加而不断减少,且在20%~40%掺量区间内减幅最大,随侧限压力的增加而减小,且减幅逐渐变小;最小、最大阻尼比随砾石掺量和侧限压力的增加而减少。结合试验结果分别从颗粒“骨架”结构性、动态结构稳定性、材料密实度、能量耗散等方面阐述块石掺量和侧限压力对土石混合体动剪切模量和阻尼比的影响机理。最后采用优化Hardin-Drnevich模型建立砾石掺量、侧限压力与土石混合体动力特性参数(最大动剪切模量、参考剪应变、最大阻尼比、最小阻尼比)之间的估算公式,以期为土石混合路基的动力设计和施工提供指导。     

Permeability hysterisis of limestone during isotropic compression

The evolution of permeability hysterisis in Indiana Limestone during application of isotropic confining pressures up to 60 MPa was measured by conducting one-dimensional constant flow rate tests. These tests were carried out either during monotonic application of the confining pressure or during loading-partial unloading cycles. Irreversible permeability changes occurred during both monotonic and repeated incremental compression of the limestone. Mathematical relationships are developed for describing the evolution of path-dependent permeability during isotropic compression.     

碎石桩加固液化粉土地基的数值模拟分析

碎石桩是目前工程中用于处理饱和粉土液化地基最有效、最经济、最普遍的方式之一,但是饱和粉土地基上碎石桩的抗震液化特性还有待于进一步研究。本文利用三维有限差分程序FLAC3D结合安徽某高速公路路基工程对碎石桩加固后的粉土地基的抗震液化特性进行数值分析。通过研究不同埋深距离碎石桩中心不同距离处的孔压增长、消散规律,得到高烈度地震荷载作用下碎石桩加固饱和粉土地基的合理桩间距及加固深度,为类似工程提供了借鉴。     

碎石桩加固液化粉土地基的数值模拟分析

碎石桩是目前工程中用于处理饱和粉土液化地基最有效、最经济、最普遍的方式之一,但是饱和粉土地基上碎石桩的抗震液化特性还有待于进一步研究。本文利用三维有限差分程序FLAC3D结合安徽某高速公路路基工程对碎石桩加固后的粉土地基的抗震液化特性进行数值分析。通过研究不同埋深距离碎石桩中心不同距离处的孔压增长、消散规律,得到高烈度地震荷载作用下碎石桩加固饱和粉土地基的合理桩间距及加固深度,为类似工程提供了借鉴。