黄土覆盖层水–气耦合运移土柱试验及数值模拟

基于水分储存与释放机理的土质覆盖层在干旱和半干旱地区垃圾填埋场具有较好的应用前景。前人研究土质覆盖层大多侧重于其雨水存储及防渗性能,忽略了填埋气在覆盖层中的运移及其影响。通过在黄土土柱底部通入甲烷和二氧化碳等比例混合气体模拟填埋气在覆盖层运移,在土柱顶部施加3 cm常水头模拟雨水入渗对黄土覆盖层中气压分布以及各气体组分分布的影响。基于试验结果,利用商业软件Geo-Studio中的Air/W模块对黄土覆盖层水–气耦合运移进行了数值模拟与分析。研究结果表明:模拟雨水入渗降低了上层黄土的导气系数,导致土柱中气压显著增加,呈现出先增加至"突破值"然后回落至"稳定值";气压"突破值"约等于上部土体进气值;数值模拟能较好地模拟水分入渗过程并捕捉到气压变化趋势,但由于其无法模拟多组分填埋气在土柱中的复杂物理过程,导致气压的模拟结果与实测值存在差距。本文为更准确模拟分析土质覆盖层中水与填埋气耦合运移过程提供了一些建议。     

Laboratory assessment of factors affecting soil clogging of soil aquifer treatment systems

In this study the effect of soil type, level of pre-treatment, ponding depth, temperature and sunlight on clogging of soil aquifer treatment (SAT) systems was evaluated over an eight week duration in constant temperature and glasshouse environments. Of the two soil types tested, the more permeable sand media clogged more than the loam, but still retained an order of magnitude higher absolute permeability. A 6- to 8-fold difference in hydraulic loading rates was observed between the four source water types tested (one potable water and three recycled waters), with improved water quality resulting in significantly higher infiltration. Infiltration rates for ponding depths of 30 cm and 50 cm were higher than 10 cm, although for 50 cm clogging rates were higher due to greater compaction of the clogging layer. Overall, physical clogging was more significant than other forms of clogging. Microbial clogging becomes increasingly important when the particulate concentrations in the source waters are reduced through pre-treatment and for finer textured soils due to the higher specific surface area of the media. Clogging by gas binding took place in the glasshouse but not in the lab, and mechanical clogging associated with particle rearrangement was evident in the sand media but not in the loam. These results offer insight into the soil, water quality and operating conditions needed to achieve viable SAT systems.     

Effects of subbase materials on phosphorous attenuation in permeable pavement system and correlation between pollution attenuation and subbase materials

In this study, laboratory models of six permeable pavement system (PPS) columns with various subbase configurations were examined for phosphorus attenuation. Experiments were carried out under varying intensities and durations of rainfall to investigate possible PPS modifications to enhance the performance of phosphorus attenuation and the likelihood of phosphorus leaching back to infiltration. The results show the inefficiencies of phosphate attenuation in the conventional PPS structure. During long-duration rainfall events, phosphates can be leached back to the infiltrate through the conventional PPS structure. The intensity of rainfall affects the attenuation of total phosphorus through the six permeable pavement columns. To achieve an enhanced phosphate attenuation, the findings of this study suggest modifying the PPS structure by incorporating thin sand layers in the subbase. Also, the correlation between pollutant attenuation and subbase materials is found. Finally, design modification of PPS is recommended to enhance the pollutant attenuation capacity of the PPS.     

降雨强度对松散堆积土斜坡破坏的模型试验研究

通过在室内构建泥石流源区松散堆积土体斜坡模型,采用三种不同降雨强度对其稳定性进行研究。得出了不同降雨强度下斜坡土体流失规律与斜坡的失稳方式。结果表明：在低雨强（26mm/h）作用下,松散斜坡的土体流失量最少,并以坡面侵蚀为主;在中、高雨强（51mm/h,68mm/h）作用下,首先表现为坡面侵蚀和沟道侵蚀,进而是大规模土体流滑破坏,在土体累积流失量与时间曲线上有突变点;在高雨强作用下,形成了阵性泥石流。     

A laboratory study of landfill-leachate transport in soils

Continuous flow experiments were conducted using sand-packed columns to investigate the relative significance of bacterial growth, metal precipitation, and anaerobic gas formation on biologically induced clogging of soils. Natural leachate from a local municipal landfill, amended with acetic acid, was fed to two sand-packed columns operated in upflow mode. Degradation of the influent acetic acid resulted in the production of methane and carbon dioxide, and simultaneous reduction of manganese, iron, and sulphate. Subsequent increase in the influent acetic acid concentration from 1750 to 2900 mg/l, and then to 5100 mg/l, led to rapid increase in the dissolved inorganic carbon, solution pH, and soil-attached biomass concentration at the column inlet, which promoted the precipitation of Mn(2+) and Ca(2+) as carbonate, and Fe(2+) as sulphide. An influent acetic acid concentration of 1750 mg/l decreased the soil's hydraulic conductivity from an initial value of 8.8 x 10(-3)cm/s to approximately 7 x 10(-5)cm/s in the 2-6 cm section of the column. Increasing the influent acetic acid to 5100 mg/l only further decreased the hydraulic conductivity to 3.6 x 10(-5)cm/s; rather, the primary effect was to increase the length of the zone experiencing reduced hydraulic conductivity from 0-6 cm to the entire column. As bioaccumulation was limited to the 0-5 cm section of the column, and the effect of metal precipitation was negligible, the reduction on the deeper sections of the column is attributed to gas flow, which was up to 1440 ml/day. Mathematical modelling shows that biomass accumulation and gas formation were equally significant in reducing the hydraulic conductivity, while metal precipitation contributed only up to 4% of the observed reduction.     

地电场作用下煤中甲烷气体渗流性质的实验研究

针对地球物理场中地电场对煤层瓦斯渗流性质的影响，在实验室采用三轴渗流实验装置和电场实施装置，研究了电场作用下煤中甲烷气体的渗流性质。实验结果表明：(1)加电场后，甲烷气体渗流速度比无电场时要大，并且渗流速度随甲烷气体压力梯度增大成线性增加；(2)加电场后，煤中甲烷气体渗流量随电压(电场强度)升高近似成线性增大：(3)煤化程度越高的煤导电性越好，且甲烷气体在煤中的电动效应越明显；(4)同时，加电场后，煤的等效渗透系数可表达成ke=k(1 βE)。此项实验研究探索为电场作用下的煤层中瓦斯渗流及运移规律的进一步研究奠定基础。     

离子型稀土原地浸矿注液孔周含水率分布的计算模型

考虑注液孔内积水产生的压力水头在土体中的传递,建立了积水条件下的一维垂直入渗的基本方程;将实测的饱和导水率等效为孔底一维入渗的饱和导水率,采用积水条件一维垂直入渗的基本方程计算注液孔孔底中心垂向的含水率分布;假设入渗过程中形成的湿润锋簇上的含水率增量相等,根据累积注液量等于湿润体内的水分增加量,确定湿润锋簇中各湿润锋的位置,结合注液孔孔底中心垂向的含水率分布,确定湿润体内含水率分布。在龙南足洞某稀土矿山选择一场地较平整的矿块进行单孔注液试验,在注液孔周布置4个测点,采用FDS-100水分传感器记录测点位置在入渗过程中不同时刻的含水率,与模型的计算值进行比较,结果发现,4个测点的实测值与计算值相关系数基本在0.900以上,模型的计算误差均在10%以下,满足工程误差要求,说明此方法能较为准确的计算出注液孔内水分入渗过程及湿润体内含水率分布。     

路面结构层非饱和渗流参数敏感性分析

根据非饱和土的渗流理论,采用有限元软件对不同降雨强度、不同降雨历时下路面结构进行渗流模拟,结果表明在同样的降雨量前提下,降雨历时长的雨型入渗量大,降雨历时比降雨强度对表面水的渗入量影响更为重要。     

Experimental Study on the Behavior of Unsaturated Compacted Silt Under Triaxial Compression

Most of the experimental investigations conducted on unsaturated soil have been performed under a constant air pressure. Changes in air pressure during deformation are in some cases important in practice. For example, in order to explain the stability problems of embankments during earthquakes and seepage flow, and grounds containing gas associated with the dissociation of methane hydrates, it is necessary to consider the interaction between the soil and the pore fluids. In the present study, we carried out fully undrained tests as well as drained tests, namely, constant water and constant air shearing tests. We performed the fully undrained tests using an air-controlled valve to measure the pore air pressure. For the stress variables of the unsaturated soil, skeleton stress values were used to describe the experimental results. From triaxial compression tests on silty soil, we found that the initial suction, the confining pressure, and the strain rate of unsaturated soil strongly influence the stress-strain behavior of unsaturated silt.     

黄土–碎石毛细阻滞覆盖层储水能力实测与分析

中国西北地区气候较干旱,黄土分布广泛。就地取材采用当地的黄土作垃圾填埋场封场土质覆盖层具有技术可行性和良好的经济性。在西安江村沟垃圾填埋场建造了国内首个20m×30m大尺寸黄土–碎石毛细阻滞覆盖层现场试验基地,在基地开展了极端降雨试验。水量分配测试结果表明:总降雨量214.8 mm;坡面径流1.7 mm,占总降雨量的0.8%;土层存储(含蒸发)199.57 mm,占总降雨量的92.9%;渗漏13.53 mm,占降雨量的6.3%。基质吸力与水份运移规律分析结果表明:持续降雨条件下毛细阻滞覆盖层(900 mm)细粒土中表层土(15 cm深度以上)和底层土(85cm深度以下)的孔压(或体积含水率)均较高;底层土孔压(或体积含水率)较高是由于碎石–黄土界面间毛细阻滞效应对水份下渗的阻滞作用,这是有别于单一土层降雨入渗水份运移的显著特征。储水能力评估结果表明:极端降雨试验实测黄土–碎石毛细阻滞覆盖层有效储水量为251.95 mm。采用室内吸湿土水特征曲线评估覆盖层有效储水能力,有效储水量理论值S_(fac)为218.75 mm,实测值较理论值大15.18%,结果偏于安全。采用现场吸湿土水特征曲线评估覆盖层有效储水能力,有效储水量理论值Sfac为278.32 mm,实测值比理论值小9.47%,偏于危险。防渗设计中建议采用室内吸湿土水特征曲线。     

降雨条件下一维土柱垂直入渗模型试验研究及其渗透系数求解

 为研究降雨条件下一维土柱的入渗规律及求解非饱和土体的渗透系数,开发一套模拟降雨条件下一维土柱垂直入渗模型试验装置,对非饱和重塑黄土土柱做了4组不同降雨强度下的一维垂直入渗试验,得到不同降雨强度下垂直土柱的入渗率时程曲线、浸润峰深度时程曲线及监测点体积含水率的变化规律;提出计算非饱和土体渗透系数的新方法,并结合试验结果,得到试验土样非饱和渗透系数与基质吸力的关系曲线。结果表明：(1) 降雨强度对垂直土柱的入渗影响较大,当降雨强度小于土柱最小入渗能力时,入渗率等于降雨强度;当降雨强度大于土柱最小入渗能力时,入渗率时程曲线呈无压入渗、有压入渗和饱和入渗3阶段变化。(2) 不同降雨强度下,土柱出现积水点和饱和点的历时不同,降雨强度越大出现积水点和饱和点的时间越短,有压入渗阶段越长。(3) 在同一降雨强度下,监测点距土柱上表面越远,其体积含水率时程曲线越密集;而同一监测点,降雨强度越大,其体积含水率时程曲线越稀疏。(4) 非饱和重塑黄土渗透系数随基质吸力的增大而减小,且渗透系数的对数与基质吸力呈线性变化。     

非饱和砂土坡面降雨非正交入渗试验与数值模拟研究

传统降雨入渗分析仅以降雨强度在坡面上的正交分量作为边界条件,不符合实际降雨非正交入渗规律。为了研究非饱和砂土的非正交入渗规律性,采用自行研制的室内降雨试验装置对非饱和砂土坡面进行了一系列不同降雨强度、坡角和孔隙比的降雨入渗试验,并对应地进行了正交入渗条件下的数值模拟。测量了入渗率、出渗速率及砂土储水增量随时间变化的关系曲线,分析了雨强、坡角和孔隙比对试验结果的影响。试验结果显示各试验中均无坡面径流现象,与正交入渗边界理论差异显著。通过分析非饱和砂土在传统坡面降雨正交入渗边界条件下的入渗率、出渗速率及砂土储水增量等数值模拟结果与对应的降雨入渗试验结果的差异,证明按正交入渗边界理论计算得到的砂土坡面土体含水率、入渗能力及坡面边界条件转化的判别机制均与实际情况不符。     

Experimental investigations on effects of gas pressure on mechanical behaviors and failure characteristic of coals

The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction. In this study, triaxial compression and seepage tests were carried out on coal at different gas pressures. The mechanical properties and failure process of coal were studied, as well as the acoustic emission (AE) and strain energy. The influence of gas pressure on the mechanical parameters of this coal was analyzed. Based on the conventional energy calculation formula, the pore pressure was introduced through the effective stress formula, and each energy component of coal containing gas was refined innovatively. The contribution of gas pressure to the total energy input and dissipation during loading was quantitatively described. Finally, the influence of gas pressure on coal strength was theoretically analyzed from the perspectives of Mohr–Coulomb criterion and fracture mechanics. The results show that the total absorbed energy comprises the absorbed energy in the axial pressure direction (positive) and in the confining pressure direction (negative), as well as that induced by the pore pressure (initially negative and then positive). The absorbed energy in the axial pressure direction accounts for the main proportion of the total energy absorbed by coal. The quiet period of AE in the initial stage shortens, and AE activity increases during the pre-peak stage under high gas pressure. The fractal characteristics of AE in three stages are studied using the correlation dimension. The AE process has different forms of self-similarity in various deformation stages.     

Evaluation of methane emissions from Taiwanese paddies

The main greenhouse gases are carbon dioxide, methane and nitrous oxide. Methane is the most important because the warming effect of methane is 21 times greater than that of carbon dioxide. Methane emitted from rice paddy fields is a major source of atmospheric methane. In this work, a methane emission model (MEM), which integrates climate change, plant growth and degradation of soil organic matter, was applied to estimate the emission of methane from rice paddy fields in Taiwan. The estimated results indicate that much methane is emitted during the effective tillering and booting stages in the first crop season and during the transplanting stage in the second crop season in a year. Sensitivity analysis reveals that the temperature is the most important parameter that governs the methane emission rate. The order of the strengths of the effects of the other parameters is soil pH, soil water depth (SWD) and soil organic matter content. The masses of methane emitted from rice paddy fields of Taiwan in the first and second crop seasons are 28,507 and 350,231 tons, respectively. The amount of methane emitted during the second crop season is 12.5 times higher than that emitted in the first crop season. With a 12% reduction in planted area during the second crop season, methane emission could be reduced by 21%. In addition, removal of rice straw left from the first crop season and increasing the depth of flooding to 25 cm are also strategies that could help reduce annual emission by up to 18%.     

三轴应力条件下煤层气储层渗流滞后效应试验研究

 利用自主研发的含瓦斯煤岩热–流–固耦合三轴伺服渗流装置,以型煤试件为研究对象,进行不同温度和不同围压条件下煤层气储层渗透率演化规律的试验研究。研究煤层气在煤层气储层中的运移规律并采取相应的煤层气抽放措施,可以预防煤与瓦斯突出或者对煤层气储层中赋存的煤层气进行合理利用,对于矿井建设和实现煤与煤层气共采均具有重要的实际意义。试验结果表明：(1) 在不同试验条件下,煤岩三轴压缩试验过程中普遍存在着煤层气渗流速度变化滞后于应变和应力的现象,煤岩的体积最小点滞后量较大;(2) 煤岩的体积最小点滞后量和应力峰值点滞后量在温度越高的试验条件下呈减小趋势;(3) 煤岩所受围压通过压缩煤样侧壁,导致其内部结构变化而对煤岩的煤层气渗流速度起到阻碍作用,围压越大,体积最小点滞后量越大。     

蜗形滞流器应用条件试验与模拟

为明确蜗形滞流器在排水管网中的应用条件,采用物理模型进行试验,探究蜗形滞流器发挥有效滞流作用的最小压力水头,并建立SWMM雨洪模型,模拟研究加载蜗形滞流器后在不同重现期(3、5、10、20、50年)以及不同降雨历时(1、2和3 h)下研究区域的滞水改善情况。结果表明:蜗形滞流器的滞流效果与相对水头有关,型号为D200R300的蜗形滞流器产生滞流作用的最小相对水头在15 cm左右;总滞水量和滞水时间削减率随着重现期的增大有所减少,且蜗形滞流器对重现期≤10年的降雨具有更好的雨水调控作用,但受降雨历时的影响较小;蜗形滞流器更适宜安装在汇流后下游过水能力不足的管段,在不同降雨条件下削减率能保持在60%以上,最大可达到100%。     

Theoretical analysis and model test for rainfall-induced shallow landslides in the red-bed area of Sichuan

Heavy rainfall is a key cause of shallow landslides in red-bed terrains with steep topography and residual soils of degradable strength. In this study, laboratory model tests were carried out to examine the characteristics of rainfall infiltration, deformation, and failures of slopes in the red-bed area of Sichuan. The hydrological response and deformation of the slope soil during rainfall are addressed. Based on a modified Green–Ampt infiltration model, ponding along the bedrock surface is incorporated. A physically based model for shallow landslides caused by rainfall is developed. The theoretical analysis and the model test results indicate that the slope failures are related to erosion in the shallow soil layer and rainwater infiltration, particularly along preferential seepage channels. The process of rainfall-induced shallow landslides can be separated into three stages: erosion at the slope toe, tension crack formation at the slope crest, and shallow sliding. When initial underground water level is located at the bedrock surface or the preferential seepage flow quickly reaches the bedrock surface, it is easier for the soil slopes to slide along the bedrock surface than along the wetting front.     

CO2驱替煤层CH4试验研究

 通过对大煤样试件(100 mm×100 mm×200 mm)进行注CO2驱替煤层CH4试验,较真实地模拟在煤层中储存CO2以及驱替开采煤层气的过程。研究发现：CH4在煤体中渗透率与体积应力呈负指数相关规律;煤体对CO2的渗透率高于对CH4气体的渗透率2个数量级以上;随体积应力及驱替压力的不同,单位体积煤体可储存17.47～28.00体积CO2,CO2/CH4置换体积比可达7.03～13.91;在恒定体积应力及驱替压力条件下,CO2注入、CH4置换、产出均能够平稳进行;2种不同煤层CH4含量条件与驱替置换方式下,产出气体中初期CH4含量高达20%～50%,随时间延续产出气体中CH4含量有所下降,但仍能持续保持在10%～16%;驱替压力、驱替速度、注入倍数、煤层CH4含量、储层结构及其渗透性等因素共同决定着CO2/CH4驱替置换效果;在CO2注入煤体进行置换吸附期间,受气体吸附解吸、煤基质自身变形等因素影响,煤体会发生膨胀现象。该研究成果对CO2煤层处置及驱替置换开采煤层气实践具有重要理论意义与指导价值。     

Research on the protection of expansive soil slopes under heavy rainfall by anchor-reinforced vegetation systems

An anchor-reinforced vegetation system (ARVS) is a new type of flexible slope protection system. ARVSs are composed of vegetation, anchors and high-performance turf reinforcement mats (HPTRMs) and can maintain the stability of expansive soil slopes while producing superior ecological effects. Through in situ comprehensive monitoring and comparative analyses, we systematically discuss the impact of heavy rainfall on ARVS-protected expansive soil slopes during the local wet season in Nanning. The results show that during the local wet season, the soil temperature and water content in vegetation-covered areas are lower than those in bare areas. The soil temperatures in different areas on the expansive soil slopes decrease with increasing depth. The ARVSs significantly affected the regulation of water transport and soil temperature, which limited soil deformation in different directions. The subsequent heavy rainfall would cause greater deformation of the expansive soil slope when less antecedent rainfall fell. Low-intensity and long-duration heavy rainfall events have greater negative impacts on expansive soil slopes than high-intensity and short-duration rainfall events. Under the action of severe rainfall-evaporation, expansive soil slopes protected by ARVSs can maintain stability.     

Virus removal within a soil infiltration zone as affected by effluent composition, application rate, and soil type

The column studies presented in this paper simulated the infiltrative surface of onsite wastewater systems where effluent is applied and where a biomat may form. Two bacteriophages, MS-2 and PRD-1, were used as surrogates for human pathogenic enteric viruses during two tracer tests. A vacuum manifold was used to simulate the drainage effects of an underlying unsaturated soil profile, allowing for the collection of percolate samples at 4 cm immediately below the infiltrative surface. The impact of effluent applied (septic tank effluent (STE) or a simulated ground water), soil type (medium sand or sandy loam), hydraulic loading rate (5 or 25 cm/day) and method of application (four equivalent daily doses or 24 equivalent micro-doses per day) on the removal of viruses were investigated. These unsaturated mini column experiments demonstrated that the removal of viruses within an infiltrative surface zone (of approximately 4 cm) generally improved over time under the conditions studied. An exception occurred in sand-filled columns dosed with STE where the removal of PRD-1 decreased after a period of effluent application. Statistical analysis conducted on the calculated percent removal demonstrated that the quality of the effluent applied to the infiltrative surface is important for removal of MS-2 and PRD-1. Hydraulic loading rate also proved important in the removal of viruses. At the time of tracer test 2, columns dosed at the higher HLR (25 cm/day) had higher percent removals for both MS-2 and PRD-1. Soil type altered the removal of PRD-1 at the time of the second tracer test, at which time sandy loam had higher removal rates for PRD-1. No significant differences were observed between columns dosed four times daily and those dosed 24 times daily for either bacteriophage at either of the tracer test time points. These data suggest that over a relatively short period of operation the infiltrative surface of soil based wastewater treatment systems can achieve much higher removal then initially measured shortly after startup.