Entrapment of clay particles enhances durability of bacterial biofilm-associated bioclogging in sand

Acta Geotechnica - Utilization of bacterial biofilms and extracellular polymeric substances (EPS) for engineered bioclogging has recently garnered increasing attention in various geotechnical...     

Conditions for upscalability of bioclogging in pore network models

In this work, we model the biofilm growth at the microscale using a rectangular pore network model in 2D and a cubic network in 3D. For the 2D network, we study the effects of bioclogging on porosity and permeability when we change parameters like the number of nodes in the network, the network size, and the concentration of nutrients at the inlet. We use a 3D cubic network to study the influence of the number of nodes in the z direction on the biofilm growth and on upscalability. We show that the biofilm can grow uniformly or heterogeneously through the network. Using these results, we determine the conditions for upscalability of bioclogging for rectangular and cubic networks. If there is uniform biofilm growth, there is a unique relation between permeability and porosity, K ～ ?², this relation does not depend on the volume of the network, therefore the system is upscalable. However, if there is preferential biofilm growth, the porosity-permeability relation is not uniquely defined, hence upscalability is not possible. The Damköhler number is used to determine when upscalability is possible. If the Damköhler number is less than 10¹, the biofilm grows uniformly and therefore the system is upscalable. However, if the Damköhler number is greater than 10³, the biofilm growth exhibits a deviation from uniform biofilm growth and heterogeneous growth is observed, therefore upscalability is not possible. There is a transition from uniform growth to preferential growth if the Damköhler number is between 10¹ and 10³.     

Retarding effect of concentration of cementation solution on biocementation of soil

Concentration of cementation solution (CCS) is one of the key factors influencing the cementation effect on soil improvement through the microbially induced carbonate precipitation (MICP) process. To precipitate more calcium carbonate per treatment, a higher CCS is needed. However, the MICP process may be retarded or even terminated with an increase in CCS. This retarding effect can be a major limitation for the MICP-based soil treatment and thus needs to be understood properly. This paper presents a systematic study on the conditions causing retarding and its effect on biocementation. The test results of this study have identified that there is retarding effect of CCS on the MICP process, showing that the calcium conversion efficiency, which represents the amount of calcium that has been converted into calcium carbonate in each treatment, reduces with the increase in CCS, and the concentration of calcium is the control factor. The retarding effect will dominate increasingly when CCS is higher than 1.0 M and the amount of calcium carbonate precipitation will reduce for the given amount and type of bacteria used in this study and become zero with CCS of 2.5 M. For the same calcium carbonate content, the unconfined compressive strength is greater for sand treated using a lower CCS as the contribution to the bonding strength by the calcium carbonate generated under a lower CCS is greater than that under a higher CCS.

     

Impact of recharge water temperature on bioclogging during managed aquifer recharge: a laboratory study

To investigate the effect of recharge water temperature on bioclogging processes and mechanisms during seasonal managed aquifer recharge (MAR), two groups of laboratory percolation experiments were conducted: a winter test and a summer test. The temperatures were controlled at ~5±2 and ~15±3 °C, and the tests involved bacterial inoculums acquired from well water during March 2014 and August 2015, for the winter and summer tests, respectively. The results indicated that the sand columns clogged ~10 times faster in the summer test due to a 10-fold larger bacterial growth rate. The maximum concentrations of total extracellular polymeric substances (EPS) in the winter test were approximately twice those in the summer test, primarily caused by a ~200 μg/g sand increase of both loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS). In the first half of the experimental period, the accumulation of bacteria cells and EPS production induced rapid bioclogging in both the winter and summer tests. Afterward, increasing bacterial growth dominated the bioclogging in the summer test, while the accumulation of LB-EPS led to further bioclogging in the winter test. The biological analysis determined that the dominant bacteria in experiments for both seasons were different and the bacterial community diversity was ~50% higher in the winter test than that for summer. The seasonal inoculums could lead to differences in the bacterial community structure and diversity, while recharge water temperature was considered to be a major factor influencing the bacterial growth rate and metabolism behavior during the seasonal bioclogging process.     

透明砂土与天然砂土动力特性对比

人工合成透明土材料在支撑岩土工程可视化模型试验技术领域得到了良好的应用。然而,关于该新型材料的动力特性研究仍相对缺少,一定程度上影响了其在动力相关模型试验中的推广应用。基于共振柱、动扭剪试验仪器,开展人工合成透明砂土的动变形与动强度特性试验研究,并与天然砂土及福建标准砂的相关动变形与动强度特性进行对比分析;透明砂土由折射率一致的熔融石英砂和混合油配制而成,混合油由15号白矿物油和正12烷按比例混合而成。试验测定并分析了透明砂土的动剪切模量-应变、阻尼比-应变、动剪切模量-阻尼比等关系曲线,以及孔压、动强度特征。试验结果表明,由熔融石英砂与混合油配制而成的透明砂土的动变形和动强度特性与天然砂土相似,可以模拟天然砂土材料开展动力相关模型试验研究。     

钙基液态水硬性加固剂用于潮湿土遗址的加固保护

通过对钙基液态水硬性加固剂的研制,对潮湿环境土遗址的加固保护进行了探索性研究。X射线衍射（XRD）和扫描电镜（SEM）分析结果表明,已成功制出了潮湿土样水硬性加固所需的纳米材料。同时,研究发现,氢氧化钙在650 ℃下用马弗炉煅烧1.5 h后,可得到直径约50 nm、长度约200 nm的针状二次生石灰;这种二次生石灰经水消化后,会成为粒径在200～300 nm左右、大小十分均匀的扁平椭圆状纳米氢氧化钙颗粒;氧化钙在密封陈化一段时间后,也可使氢氧化钙的粒径达到纳米级。土样加固试验的结果显示,纳米石灰的乙醇分散液的渗透性优于分析纯氢氧化钙,钙基水硬性加固剂对潮湿土样的加固效果较为明显,有望成为一种适合于潮湿环境土遗址加固的新型材料。     

光透法定量粗糙裂隙生物堵塞下的隙宽

基于光透法定量测量裂隙隙宽的原理及在两相流中的应用,设计了一组二维粗糙裂隙模拟装置研究微生物在裂隙地下水系统中的运移行为,首次定量研究了生物堵塞过程中隙宽及分布的变化。研究结果表明:微生物大多以离散菌簇状形态存在,未能形成明显的连续生物膜,不同空间位置微生物分布具有一定的随机性,本试验中微生物接种和营养注入方式决定了微生物集中分布在裂隙底部区域;微生物生长造成裂隙平均隙宽的下降(从1.181 μm降低为1.086 μm,下降8.04%),此外,基于光透法测量的不同空间位置隙宽(除2个点外)在微生物生长过程中都有所下降,两次微生物接种后点2处隙宽下降最明显(从1.275 μm减少到1.081 μm),下降了194 μm,约为15.22%。光透法作为一种高效无损的室内监测手段,将其应用于裂隙介质的生物堵塞尤其是定量计算生物膜厚度的研究中,对裂隙中生物堵塞研究具有重要的参考意义。     

砂体及其研究价值

本文论述了砂体的意义，砂体的研究价值，砂体的分类，并从多方面阐明了砂体在可地浸砂岩型铀矿找矿工作中的作用。抛砖引玉，力图推进砂体的研究工作，为可地浸砂岩型铀矿的找矿工作服务。     

初始孔压非均布下一维地基砂垫层优化研究

针对吹填土地基水平排水砂垫层铺设深度优化问题,建立了初始孔压非均布下含砂垫层地基的一维固结模型,利用有限Fourier正弦变换法得出了超静孔压和地基整体平均固结度的解答并通过退化验证了解答的正确性。采用二分法分析了砂垫层的最优铺设深度随时间的演化规律,给出了砂垫层最优铺设深度与时间的关系图。研究表明：在固结初期,砂垫层宜放置在土中初始超静孔压较大的位置;在固结后期,砂垫层的最优设置深度为土体厚度的2/3（单面排水）或者1/2（双面排水）的位置。以预压地基平均固结度达到90%所需时间最短为例,当初始孔压倒三角形、正三角形与梯形分布时,单面排水条件下,应分别在0.52、0.72、0.62倍地基土深度处设置砂垫层;双面排水条件下,应分别在0.42、0.58、0.46倍地基土深度处设置砂垫层。最后通过算例分析了当地基土中砂垫层采用最优铺设深度时,与在中间位置铺设砂垫层、不设砂垫层的情况分别进行对比,当时间因子取0.09时,地基土平均固结度分别提高6%和54%。     

Characteristics of wind-blown sand on Gobi/mobile sand surface

The characteristics of wind-blown sand on Gobi/mobile sand surface have been investigated through field observation and wind tunnel experiments. On moving sand surface, the pattern of wind speed profile in sand flow follows the power function and its power exponent is about 0.20. While on Gobi surface, due to collision of sand grains with gravels, the structure of sand flow differs from that on moving sand surface, which decreases exponentially with height. The height of blown sand activities on Gobi surface is mainly concentrated below 20 cm. In addition, the structure of sand flow can reach a peak at a certain height and increases with inlet wind velocity.     

粉细砂层中锚索施工涌水涌砂的处理技术

广州市良业大厦基坑支护工程存在较厚的淤泥质粉细砂层,预应力锚索锚点位置处于粉细砂层中时,锚索施工易发生涌水涌砂现象而引发基坑事故。施工过程中通过采取跟管钻进干钻成孔、墙后孔口设置止浆袋及墙后注浆的处理措施,减少了锚索施工过程中涌水涌砂现象的发生,避免了因涌水涌砂水土流失造成基坑外地面下沉而引发的基坑事故。     

Recent sand volcanoes in the sand of a dike under construction

Sand volcanoes are small volcano-like features. They range in diameter from a few centimetres to several metres. Five volcanoes with a maximum diameter of 2 m were observed during the construction of a dike on the coast of Northwestern Germany. The cones reached heights of about 15 cm with craters in the middle. The rise of the volcanoes is explained by de-watering of sediment.     

Variations in wind velocity and sand transport on the windward flanks of desert sand dunes

The magnitudes of increases in wind velocity, or speed-up factors, have been measured on the windward flanks of transverse and linear dunes of varying height. On transverse dunes, velocity speed-up varied with dune shape and height. For linear dunes, speed-up factors varied principally with wind direction relative to the dune, with dune shape and dune height. The main effect of velocity speed-up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas. This is greatest for large dunes, with winds of moderate velocity blowing at a large angle to the dune. Changing ratios of base to crest sand-transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase. On linear dunes, the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi-directional wind regime.     

Wind tunnel experiments on vertical distribution of wind-blown sand flux and change of the quantity of sand erosion and deposition above gravel beds under different sand supplies

Wind tunnel experiments were carried out with respect to the vertical distributions of wind-blown sand flux and the processes of aeolian erosion and deposition under different wind velocities and sand supplies above beds with different gravel coverage. Preliminary results revealed that the vertical distribution of wind-blown sand flux was a way to determine whether the gobi sand stream was the saturated one or not. It had different significances to indicate characteristics of transport and deposition above gobi beds. Whether bed processes are of aeolian erosion or deposition was determined by the sand stream near the surface, especially within 0–6 cm height, while the sand transport was mainly influenced by the sand stream in the saltating layer above the height of 6 cm. The degree of the abundance of sand supply was one of the important factors to determine the saturation level of sand stream, which influenced the characteristic of aeolian erosion and deposition on gravel beds. Given the similar wind condition, the sand transport rates controlled by the saturated flow were between 2 and 8 times of the unsaturated one. Those bed processes controlled by the saturated flow were mainly of deposition, and the amount of sand accumulation increased largely as the wind speed increased. In contrast, the bed processes controlled by the unsaturated flow were mainly of aeolian erosion. Meanwhile, there was an obvious blocking sand ability within the height of 0–2 cm, and the maximal value of sand transport occurred within the surface of 2–5 cm height.     

Physical and chemical properties of sound-producing and soundless sand particles from booming sand dunes,northern China

This study here is the result of a comparative study of the geomorphic features, grain size distribution, major mineral components, and micro-textures of sound-producing and soundless sands from three geoparks in north China, and discusses the possible causes of this naturally occurring, physical phenomenon. The sound-producing sand dunes we have investigated are situated along a curvilinear belt of deserts and experience variable precipitation–evaporation rates on a yearly basis. “Singing sands” occur mainly on barchan-type dunes and adjacent to lakes or springs, whereas soundless sands are mainly located in desert areas where there is no nearby surface or groundwater sources. We have analyzed samples from nine sound-producing and two soundless sand dunes using grain size, X-ray diffraction and fluorescence, and SEM analyses. All sand types are composed largely of quartz and feldspar, but sound-producing sands also contain secondary minerals such as kaolinite (3–5%), albite (2–6%), microcline (2–5%), and calcite (5%) that are lacking in the soundless sand samples. Sound-producing sands are generally fine-grained whereas soundless sands are coarse-grained, and all sand types are generally sub-rounded to rounded indicating long transport distance from their provenance. Sphericity values of both sand types are nearly identical with predominantly oblate shapes. Surface pitting is not a unique feature of either sand types, and hence can be ruled out as a major cause of the acoustic properties of sound-producing sands. Densely distributed dissolution features such as scale-like upturned plates and silica scales on grain surfaces contribute significantly to the sound emission of singing sands. Thus, the physical–climatic conditions in deserts such as the presence of surface water and groundwater and precipitation–evaporation rates, which collectively control the formation and distribution of dissolution features on sand surfaces, have a first-order control on the production of sound-producing sands.     

循环单剪下珊瑚钙质砂和普通硅质砂剪切特性对比研究

对普通硅质砂和西沙群岛珊瑚钙质砂开展多级正应力下循环单剪试验,对比多循环周期下两种砂土剪切性质和颗粒破碎的差异。研究发现,两种砂样在循环剪切下体积变化都比较微弱,剪切过程中存在阶段性剪胀,使得试样的轴向位移呈波动变化;循环单剪下剪应力峰值包络线可以用联合型指数函数表达;剪应力随循环周期变化分为同步阶段和差异阶段,两个阶段转变节点对应的循环周期数随着正应力的增大而迅速减小;同步增长阶段珊瑚钙质砂和普通硅质砂的剪应力变化一致,在差异阶段普通硅质砂的剪应力要高出珊瑚钙质砂。两种砂剪应力差异程度随试验正应力不同而变化,普通硅质砂剪应力最大可高出珊瑚钙质砂14.7%;珊瑚钙质砂和普通硅质砂的颗粒破碎存在明显差异,珊瑚钙质砂全粒径范围内颗粒破碎分布更均衡,普通硅质砂在特定粒径区间内出现了剧烈的颗粒破碎,颗粒级配曲线存在明显拐点。普通硅质砂和珊瑚钙质砂滞回曲线的形状及随循环次数的变化规律有显著差异,是两种砂样剪切性质不同的重要体现。     

Volumetric sand production model and experiment

A sand production model was developed for volumetric sand production predictions that take into account the effects of the external stresses and fluid flow rate. The model couples the poro‐mechanical behaviour of the solid–fluid system with the erosion behaviour of the solids due to fluid flow. It predicts reasonably experimental volumetric sand production data from a hollow cylinder test on a weak sandstone. The test results show that in weak and compactive sandstones, sand production is associated with decohesioning and plasticification of a zone around the inner hole which can then be mobilized by the hydrodynamic forces of the fluid flow. The sand production rate increases both with external applied stress and fluid flow rate but it is constant with time under constant external stress and fluid flow rate. In both cases a critical lower limit has to be exceeded for sand production initiation. Copyright © 2001 John Wiley & Sons, Ltd.     

Sun, sand and sea urchins

Jamaica, land of holiday dreams, is also fascinating to the geologist. In particular, the fossil record is diverse, the ancient fauna containing many rarities. The fossil ochinoid fauna is especially large and varied for such a small island, with about 130 nominal species ranging from the late Cretaceous to the Pleistocene.