Numerical Simulation of Heat Transfer Characteristics of Horizontal Ground Heat Exchanger in Frozen Soil Layer

A simplified numerical model of heat transfer characteristics of horizontal ground heat exchanger (GHE) in the frozen soil layer is presented and the steady-state distribution of temperature field is simulated. Numerical results show that the frozen depth mainly depends on the soil′s moisture content and ambient temperature. The heat transfer loss of horizontal GHE tends to grow with the increase of the soil′s moisture content and the decrease of ambient temperature. Backfilled materials with optimal thermal conductivity can reduce the thermal loss effectively in the frozen soil. The applicability of the Chinese national standard “Technical Code for Ground Source Heat Pump (GB 50366-2005)” is verified. For a ground source heat pump project, the feasible layout of horizontal GHE should be determined based on the integration of the soil′s structure, backfilled materials, weather data, and economic analysis.

Analytical expressions for determining the stability of cohesionless soil slope under generalized seismic conditions

In recent major earthquakes, the researchers have found the need for consideration of vertical seismic acceleration for the stability analysis of the man-made and natural slopes. However, in most past studies, the performance of slopes has been assessed by accounting only the horizontal seismic component of the ground motion, without giving due weightage to the effect of vertical component. In the present study, analytical expressions are derived to determine the factor of safety, yield seismic coefficient and consequently the seismic displacement of cohesionless soil slope under combined horizontal and vertical components of the ground motion. The derivation uses the Newmark's sliding block approach, in which the soil slope with a planar failure surface within the framework of conventional pseudo-static analysis is assumed to follow the Mohr-Coulomb failure criterion. The effects of vertical seismic coefficient on the stability of cohesionless slope have been studied through a set of graphical presentations for a specific range of soil parameters. It is observed that overlooking the effect of the vertical component of the ground motion on factor of safety and the displacement while designing the slope may be detrimental, resulting in the slope failure. The general expressions presented in this paper may be highly useful in the field of earthquake geotechnical engineering practice for designing the cohesionless soil slopes under combined horizontal and vertical seismic loads.

Henry’s Equilibrium Partitioning between Ground Water and Soil Air: Predictions versus Observations

Humans spend 64% - 94% of their time indoors;therefore, indoor air quality is very important for potential exposure to volatile organic compounds (VOC). The source of VOC in the subsurface may come from accidental or intentional releases, leaking landfills or leaking underground and above-ground storage tanks. Once these contaminants are present near or beneath buildings, they may move as a vapour through soil gas and enter the building. A large number of vapour intrusion (VI) algorithms have been published in peer-reviewed publications that link indoor VOC concentrations to the contamination of soils. These models typically include phase partitioning calculations of VOC based on Henry’s law to estimate the concentration of a particular contaminant in soil gas. This paper presents the results from a series of laboratory experiments concerning the use of the Henry’s Law constant for the calculation of toluene concentrations in equilibrium between ground water and soil air. A series of column experiments were conducted with various toluene concentrations in artificial (ground) water to contrast the predicted and observed (soil) air concentrations. The experiments which exclude soil material show a toluene fugacity behaviour roughly in line with Henry’s law whereas the experiments which include soil material result in equilibrium soil concentrations which were around one order-of-magnitude lower than was expected from a Henry Law-based estimation. It is concluded that for toluene inclusion of Henry’s Law in VI algorithms does not provide an adequate description of volatilisation in soils and may lead to an overestimation of health risk. Instead, a model based on a simple description of the relevant intermolecular interactions could be explored.

An analytical p-y curve method based on compressive soil pressure model in sand soil

With the high-quality development of urban buildings,higher requirements are come up with for lateral bearing capacity of laterally loaded piles.Consequently,a more accurate analysis to predict the lateral response of the pile within an allowable displacement is an important issue.However,the current p-y curve methods cannot fully take into account the pile-soil interaction,which will lead to a large calculation difference.In this paper,a new analytical p-y curve is established and a finite difference method for determining the lateral response of pile is proposed,which can consider the separation effect of pile-soil interface and the coefficient of circumferential friction resistance.In particular,an analytical expression is developed to determine the compressive soil pressure by dividing the compressive soil pressure into two parts:initial compressive soil pressure and increment of compressive soil pressure.In addition,the relationship between compressive soil pressure and horizontal displacement of the pile is established based on the reasonable assumption.The correctness of the proposed method is verified through four examples.Based on the verified method,a parametric analysis is also conducted to investigate the influences of factors on lateral response of the pile,including internal friction angle,pile length and elastic modulus of pile.

Study of the Bearing Capacity at the Variable Cross-Section of A Riser- Surface Casing Composite Pile

Reducing the cost of offshore platform construction is an urgent issue for marginal oilfield development.The offshore oil well structure includes a riser and a surface casing.The riser,surface casing and oil well cement can be considered special variable cross-section piles.Replacing or partially replacing the steel pipe pile foundation with a variable cross-section pile to provide the required bearing capacity for an offshore oil platform can reduce the cost of foundation construction and improve the economic efficiency of production.In this paper,the finite element analysis method is used to investigate the variable cross-section bearing mode of composite piles composed of a riser and a surface casing in saturated clay under a vertical load.The calculation formula of the bearing capacity at the variable section is derived based on the theory of spherical cavity expansion,the influencing factors of the bearing capacity coefficient N_(c) are revealed,and the calculation method of N_(c) is proposed.By comparing the calculation results with the results of the centrifuge test,the accuracy and applicability of the calculation method are verified.The results show that the riser composite pile has a rigid core in the soil under the variable cross-section,which increases the bearing capacity at the variable cross-section.

Soil resistivity and ground resistance for dry and wet soil

In this paper, soil resistivity and ground resistance at two different sites near an electrical substation are measured using a grounding system grid with and without rods. With the Wenner four-pole equal-method, the soil resistivity is measured at both selected sites, one of which contains wet soil while the other contains dry soil. Cymgrd simulation software is then used to determine the acceptability of these measured resistivity values by finding out the root mean square error between the measured and calculated values for both wet and dry soil sites. These values for wet and dry soil sties were found to be only 0 %and 4.92 %, respectively, and deemed acceptable. The measured soil resistivity values were then used to evaluate the ground resistance values of a grounding grid ‘with rod’for the wet soil site and ‘without rods’ for the dry soil site,and then compared with the simulated ground resistance values. These comparisons were also found to be in good agreement. In addition, ground potential rise, maximum permissible step and touch potentials have also been estimated using the simulation software.

An Efficient Method for Earth Resistance Reduction Using the Dead Sea Water

To protect humans, devices and maintain high reliability of electrical equipments, efficient earthing system has to be provided for residential and commercial buildings. The use of chemical elements around the electrode of earthing systems reduces the earth resistance which improves the efficiency of these systems. However, the use of these elements can’t achieve very small earth resistance especially in the case of dry clay. In addition, they are expensive and not available all the time. This paper presents a method that treats the soil with adding the Dead Sea water instead of the chemical elements to further reduce the earth resistance and to avoid the expenses of these elements. In our approach, the Dead Sea water is added to a hole around the electrode with a distance not exceeding 10 cm to avoid the corrosion layer on the electrode. Practical measurements are demonstrated to show the efficiency of the proposed approach.

基坑中软土的水平抗力系数几种确定方法探讨

该文围绕基坑中软土的的水平抗力系数的确定方法开展了探讨,分析了基坑规程中土的水平反力系数的比例系数m值的来源,分析了水平抗力系数确定的理论,罗列了现行规范中软土的水平抗力系数的经验值,最后通过案例分析,将3种土的水平抗力系数的计算方法(m值、k值和c值)的计算结果与实测进行了比较,发现按目前基坑规程公式计算的软土的水平反力系数的比例系数m值偏小的现象。

土壤冻结时考虑导体传热的 接地装置散流失效模型

西藏地区新能源送出走廊不可避免的会途经极寒冻土地区,接地装置面临着冻结失效的风险。以往的研究中,接地装置散流性能计算前提是将冻土考虑成静态水平分层结构,忽略接地装置与杆塔相连时整体的散热效应对土壤冻结结构的影响,导致实际运行中接地装置散流性能分析不准确。因此,该文首先建立考虑接地导体传热的土壤水热场耦合模型,计算得到杆塔散热条件下土壤的冻结结构;然后,在此冻结结构下构造接地装置冲击散流失效模型;最后,利用该模型与传统模型对比,探究接地导体的半径、长度和土壤初始温度等因素对接地装置散流性能的影响。研究表明,由于接地导体具有良好的导热性能,其所接触的土壤易形成局部冻结,与传统模型相比,-10℃低气温下相同接地装置失效时刻提前约18 d;若低温持续时间相同,半径6 mm的单根垂直接地导体达到3.3 m就不会失效,而传统模型下则是1.6 m,相差约2倍。此外,减小接地导体半径或降低接地导体导热系数均可减缓局部冻结。

水合氧化物对抗生素抗性基因水平转移的影响

研究了水合氧化铝与水合氧化锰对氨苄青霉素抗性基因(负载于pUC19质粒)向大肠杆菌水平转移(转化)过程的影响.结果表明,2.5~200mg/L浓度水平的水合氧化铝与水合氧化锰均显著抑制了抗生素抗性基因(ARGs)转化,两者对转化频率的抑制率最高分别达79.44%和57.64%.水合氧化铝析出的Al (III)是其导致ARGs转化频率发生变化的原因之一,而水合氧化锰几乎无离子析出.此外,水合氧化铝可与p UC19质粒的磷酸骨架以及碱基腺嘌呤、胸腺嘧啶与鸟嘌呤发生结合,与之形成大尺寸加合物,进而降低ARGs进入胞内的效率.对于水合氧化锰,其暴露后宿主细胞膜通透性显著减小,并且该颗粒也与pUC19质粒交缠形成了大尺寸团聚物,这可能是抑制ARGs转化的主要原因.

土壤环境中抗生素抗性基因污染研究进展和热点分析

近年来,土壤环境中抗生素抗性基因(ARGs)污染引起广泛关注。以Web of Science核心数据库和万方数据库文献资料为数据源进行文献计量学分析,从年发文量变化、不同国家贡献及研究主题演变(基于关键词)等方面对土壤环境中ARGs污染相关研究进行剖析,以此探讨国内外该领域的研究现状、热点和发展。结果表明,土壤中ARGs污染相关研究的发文数量快速增长。我国文献在发文数量、论文被引频次方面具有重要影响,表明我国在该领域有较强的国际学术影响力。关键词聚类分析表明该领域的研究方向主要集中在以下5个方面:(1)土壤中携带ARGs的微生物及其环境行为;(2)土壤环境中ARGs的来源和持久性;(3)土壤环境中ARGs的传播和消减方法;(4)土壤中共存物质对ARGs丰度和迁移行为的影响;(5)农业生产活动对土壤环境中ARGs污染的影响。同时,在介绍ARGs常用检测方法以及土壤环境中ARGs污染来源和分布的基础上,剖析影响ARGs在土壤中传播的多种因素,探讨土壤环境中ARGs的消减方法,并指出当前研究尚存的不足之处以及今后的研究方向。该文可为未来土壤环境中ARGs污染领域的研究和风险管控提供参考。

土壤特性对圆环型直流接地极暂态温升的影响

温升是影响直流接地极安全运行的重要因素,为研究土壤特性对圆环型直流接地极暂态温升的影响,建立了水平圆环型直流接地极的有限元模型,对比分析了不同土壤特性下水平双圆环直流接地极的温升变化规律。仿真结果表明:土壤导热系数、比热容和土壤电阻率是重要影响因素;在水平双层土壤中,接地极的最高温升与土壤电阻率成正比,上层土壤电阻率的影响较下层土壤电阻率更为明显,而下层土壤电阻率对内环与外环的最高温升差值影响较大;上层土壤超过一定厚度后,上层土壤厚度的变化对极环最高温升的影响较小。研究成果可为特高压直流接地极选址及设计提供参考。

蒙西地区典型季节变化对接地网接地阻抗影响研究

季节变化改变土壤电阻率值,土壤电阻率的改变对接地装置阻抗值造成影响。在某场区土壤电阻率监测的基础上,研究该场区埋设的不同类型接地装置阻抗变化情况。结果表明:经相关性分析,该地区全年土壤电阻率与气温变化呈强负相关,表现出明显的季节变化特性;在设计受季节变化影响较大的区域接地网时,可以通过增大接地网面积、接地网埋深,以及采用磁导率小、柔韧性好的石墨接地装置,来减小季节变化对接地阻抗值的影响,为受季节变化影响较大地区接地网设计提供参考。

不同桩-土界面摩擦系数对水平荷载下大直径单桩变形特性影响研究

为研究不同桩-土界面摩擦系数对水平荷载下大直径单桩变形特性的影响,根据现场试验及室内循环剪切试验结果,采用Abaqus软件及可考虑软土软化效应的本构模型进行数值模拟。结果表明:提高桩-土界面摩擦系数可在一定程度上减少桩顶位移,其减少程度与桩土相对位移呈正相关。桩-土界面摩擦系数变化区间为0.1~0.5,在水平静载作用下,桩-土界面摩擦系数对桩顶位移的影响随荷载的增大而增大,随桩径的增大而减小。水平循环荷载作用下,桩-土界面摩擦系数对桩顶位移及桩周土体软化程度的影响随荷载峰值、荷载频率、循环次数的增大而增大,桩顶位移减小值最大可达25.85%,桩周土体软化程度最大可达17%。

长沙市某区浅层地埋管岩土体热响应试验性能研究

长沙市某区为高新技术发展示范基地,能源冷热需求量大,该片区地质条件复杂,邻近少有地源热泵热响应测试数据,通过性能研究,揭露研究区地层以第四系覆盖层及元古界泥岩、砂岩、板岩为主,10~40 m存在上层少量上层滞水;通过线热源模型热响应试验,测试岩土体综合平均导热系数为1.78~1.95 W/(m·K),钻孔热阻值为0.18~0.23(m·K)/W,并与钻孔取芯和室内热物性测试进行对比,吻合度较好。考虑长沙地区气候特征,在不考虑大量钻孔换热过程中存在热干扰和长时间持续运行情况下,通过换热计算,夏季每延米换热量最大可达51~53 W/m,冬季每延米换热量最大取值为47~49 W/m。

复杂土壤条件高压直流接地极与埋地油气管道防护距离研究

为保障埋地管道附近人员的安全,对高压直流接地极对埋地管道的电流干扰及人身安全距离进行了研究。首先,利用CDEGS仿真软件建立了埋地管道受高压直流接地极的干扰模型,筛选了影响安全距离的主要因素;其次,研究了埋地管道在不同均匀土壤和多层复杂土壤的安全距离;最后,建立了均匀土壤与复杂土壤的安全距离折算关系,提出了采用最大管地电位并利用系数修正得到复杂土壤安全距离的方法。研究结果表明,土壤电阻率对安全距离的影响最大;若均匀土壤的安全距离小于8km,修正系数采用0.85,可以得到复杂土壤的安全距离;若均匀土壤安全距离为8~12 km,修正系数与安全距离符合幂函数的关系;若均匀土壤安全距离大于12 km,建议修正系数取0.53。所提方法可供高压直流接地极及管线设计提供参考,并且可为未来拟建高压直流接地极选址提供借鉴。

基于离心机实验的海上风力机大直径单桩桩土作用研究

地基基础是海上风电结构所受载荷的最终承载者,针对中国沿海软弱超固结软土地基开展力学特性研究。通过离心模型试验方法对单桩基础形式的静力特性进行分析和对比。研究结果揭示通过离心模型试验模拟桩基础的有效性,该方法能准确捕捉现场地基的应力分布及桩与土体之间的相互作用特性。在软黏土环境中,针对不同直径的单桩进行的分析表明,其变形行为及桩土相互作用机制呈现出显著的差异性。规范中的p-y曲线不适用于中国近海土质,据试验结果提出地基无量纲水平抗力系数随埋深变化和与循环应力幅值相关的循环弱化因子计算模型。

基于CDEGS软件的高土壤电阻率地区主变电所接地网设计

[目的]为解决高土壤电阻率地区主变电所接地电阻难以降低的问题,有必要对高土壤电阻率地区主变电所的接地网设计进行研究。[方法]结合该主变电所的地理位置条件及土壤电阻率特点,通过单一变量法确定了水平接地体的合理数量;基于CDEGS仿真软件分析了水平接地体按指数非均匀布置时,接触电势、跨步电压及接地电阻的改善效果;提出了降低接地电阻的优化措施。[结果及结论]当水平接地网导体数量增加到一定程度后,导体的边际效应会大幅下降;垂直接地极对降低该主变电所的接地电阻作用不大,设计中不建议采用;采用深井接地极后,主变电所的接触电势、跨步电压均大幅降低,接地电阻也降至0.5Ω以下,均满足规范要求。

考虑地质影响的深基坑复合土钉墙最优支护参数选择技术

通过勘察一区域综合办公写字楼主楼获取土层详细指标,利用土钉抗力参数计算选取最佳土钉墙材料,并设计深基坑复合土钉墙支护方案与施工流程。将设计好的施工方案输入到FLAC3D软件中模拟深基坑支护结构,模拟不同地质影响下的侧压力系数。经试验验证:当土钉参数为5排、钢管嵌固深度参数为2 m时,支护结构可以最大程度降低深基坑的水平位移与沉降;在侧压力系数较大情况下,可以保持较低的竖直位移与深基坑坑底隆起值,具有较好的支护效果。