微生物沉积碳酸钙固化珊瑚砂的试验研究

向珊瑚砂中注入巴斯德芽孢杆菌菌液、氯化钙和尿素的混合液,利用微生物沉积碳酸钙固化珊瑚砂;并对珊瑚砂固化体进行了渗透、强度及微观结构等试验。试验结果表明,巴斯德芽孢杆菌的活性随时间呈衰减趋势,但衰减速度缓慢,能较好地满足珊瑚砂固化的需要。随着菌液、氯化钙和尿素的混合液注入次数的增加,珊瑚砂柱渗透性逐步降低,最终渗透性降低了1～2个数量级。微生物固化后的珊瑚砂柱应力-应变曲线大致可分为3段,即应力随应变缓慢增加段、快速增加段以及突降段。试样发生压裂脆性破坏,无侧限抗压强度最高达到14 MPa左右。抗压强度随干密度增加而增大,随渗透性降低而增大。微生物固化后珊瑚砂颗粒被生成的碳酸钙完整的包裹,孔隙间极少见生成的碳酸钙,与普通硅砂微生物固化后的微观结构不同,较好地解释了渗透性降低不多的原因。     

The spatial and temporal variability of sand erosion across a stabilizing coastal dune field

This study aimed at quantifying the temporal and spatial variability in sand erosion and deposition over a coastal dune field in Israel. These were measured monthly over 2 years using 315 erosion pins over four transects that were placed perpendicular to the coastline. Vegetation cover was estimated based on aerial photographs and Landsat satellite images, whereas the relative height was based on a digital elevation model. These variables were calculated for the area upwind (south west) of the erosion pins, at various lengths, ranging from 15 to 400 m. Nine geomorphologic units were defined, five related to active units, and four to stabilized units. In active units at least 65% of the temporal variance in the annual absolute changes in sand level was explained by the index of Resultant Drift Potential, with most of the sand movement occurring during winter storms. Local rainfall had no apparent impact on sand mobility, due to the low coincidence of sand carrying winds and rainfall in Israel during the passage of frontal cyclones. As for the spatial variables, only a weak correlation was found between sand mobility with the distance from the coastline (R² = 18%). Rather, sand erosion and deposition were influenced by vegetation cover and the relative height of an area of 100–200 m upwind. The values of Soil Adjusted Vegetation Index were significantly negatively correlated with annual absolute changes (R² = 40%), whereas the relative height was significantly positively correlated (R² = 36%). Applying a multiple regression model, 68% of the spatial variability in sand mobility was explained. The resulting map of sand activity clearly shows that at this stage of the stabilization process, most of the dunes are now disconnected, and movement of sand grains from the beach or between the dunes, is very limited. These methods can be applied into spatial and temporal models of sand mobility, thus assessing the impact of different management practices on coastal dunes.     

微生物沉积碳酸钙固化砂土试验研究

砂土固化对增加砂土强度、减小渗透性极为有利,可提高和改善砂土的力学特性。通过培养基L进行菌种繁殖,研究了不同条件下的细菌生长特性;分析流出液的pH值和Ca~(2+)浓度的内在关系,揭示了固化砂柱的渗透性、无侧限抗压强度变化规律;通过微观结构阐释微生物固化效果,并研究了Ca~(2+)浓度与试样强度的关系。结果表明:2mL细菌母液加入100mL培养液并在30℃、pH为6,150rpm振动速度的条件下最适宜菌种生长;固化过程中,pH值逐渐降低,而Ca~(2+)浓度则增大;采用0.5mol/L胶凝液固化效果较好且固化周期短;固化后砂土颗粒间孔隙被生成的碳酸钙填充;试样渗透性最终降低3~4个数量级;流速越小,固化时间越长,固化效果越好;且固化试样破坏模式都是脆性破坏;采用尿素和醋酸钙混合溶液作为胶凝液,更能提高Ca~(2+)利用率。     

黄豆脲酶诱导碳酸钙沉淀多变量试验研究

植物源脲酶诱导碳酸钙沉淀（enzyme induced carbonate precipitation,简称EICP）可以显著改善砂土的工程力学特性,但在具体操作时,参数取值无对应规范,固化效果有待提升。基于黄豆脲酶,研究了温度、脲酶浓度、尿素浓度、钙浓度、pH值、钙源种类等变量对脲酶活性与碳酸钙沉淀的影响,并进行了沉淀物（碳酸钙晶体）的扫描式电子显微镜（scanning electron microscope,简称SEM）与X射线衍射（X-ray diffraction,简称XRD）测试,在此基础上开展了黄豆脲酶固化砂的无侧限抗压强度与固化效果试验研究。结果表明：脲酶活性随脲酶浓度的增加而线性增长,但存在温度阈值,温度超过阈值后,脲酶将完全失活,且阈值随脲酶浓度的增大而降低;尿素浓度与pH值共同影响脲酶活性,二者存在一个最优组合,当尿素浓度在0.1～1.0 mol/L时最优pH值为7,当尿素浓度在1.0～1.5 mol/L时最优pH值为8。脲酶是沉淀反应的催化剂,脲酶浓度越高,反应越完全,碳酸钙沉淀率越高;尿素与钙溶液则主要通过掺入量影响碳酸钙沉淀量,掺量比例宜为1:1,且二者浓度与pH值可通过影响脲酶活性来影响碳酸钙的沉淀情况;不同钙源对碳酸钙沉淀量的影响幅度不大。不同钙源沉淀碳酸钙晶体的成分与密度基本相同,但晶体结构差异较大,氯化钙沉淀碳酸钙晶体以块状为主,表面分布球状、类球状晶体,胶结面大,可作为EICP技术中较为理想的钙源。基于黄豆脲酶和氯化钙钙源固化砂的无侧限抗压强度约为掺粉煤灰砂样的6倍,通过SEM图像可发现,沉淀碳酸钙晶体包裹并黏结砂粒成为整体,固化效果非常理想。     

Effect of activated carbon on microbial-induced calcium carbonate precipitation of sand

Microbial-induced calcium carbonate precipitation (MICP) is a sustainable technique to transform or improve physical and mechanical properties of soils. This paper aims to study the effect of activated carbon (AC) on the property of bio-treated China Standard sand. Six sample groups were prepared considering various dosages of cementation solutions and bacterial suspensions. In each group, samples were prepared at six different AC ratios (0%, 0.2%, 0.5%, 1%, 2%, and 3% by weight of sand). Bacterial retention ability, calcium carbonate mass, unconfined compression strength (UCS), and microstructures of bio-treated samples were examined and evaluated in the presence of AC. It was found that the improved yield of calcium carbonate crystals and increased UCS were correlated to the enhanced bacterial retention ability attributed to AC. In addition, the test results showed that the amount of cementation solution played an important role in the MICP process, but volume variation of the bacterial suspension had a little effect on the bio-treated samples.     

Assessment of temporal changes in coastal sand dune environments using the log-hyperbolic grain-size method

Abstract Three cores (<9 m long) recovered from late Holocene coastal sand dunes in Northumberland, north‐eastern England, were examined stratigraphically and dated by the infrared stimulated luminescence (IRSL) method to the time period between the Medieval Warm Period (MWP; ≈ 900–1200 AD) and Little Ice Age (LIA; ≈ 1550–1800 AD). Grain‐size variability and CaCO₃ content were determined at 10‐mm intervals throughout parts of the cores. The sahara computer program was used to plot the log‐hyperbolic distribution (LHD) of grain‐size spectra from different stratigraphic levels and estimate the LHD's statistical components. Core samples plot variously in the depositional and erosional domains of the hyperbolic chi (χ) vs. xi (ξ) shape triangle, which describes components of sediment sorting. Depositional and erosional phases, discriminated within the LHD's shape triangle, cannot readily be resolved from core stratigraphies or correlated between cores. IRSL dates from single cores show a clustering about the period 1430–1540. Between the dated horizons, dunes accreted at ≈ 0·02–0·05 m year^?1, enabling the timing of excursions in χ and ξ to be estimated. These excursions, which can be resolved to the subannual level between the dated horizons, may be related to deposition by episodic storms or storm surges in the North Sea region, which is supported by historical documentary evidence. This innovative use of the LHD method and its derived statistics can be used successfully to describe temporal trends in coastal depositional environments and identify likely storm events in dune sediment records.     

Geochemistry of carbonate precipitation from the groundwater in a coastal region

Groundwater samples were collected from a coastal region of Andhra Pradesh to assess the possible conditions of the formation of carbonates. The area experiences a semi-arid climate and is underlain by khondalites, over which the Quaternary sediments occur. The study of the geochemistry of groundwater indicates that groundwater is mostly of fresh, with alkaline nature. The study further suggest that the breakdown of feldspars as kaolinite during rock-water interaction, releases Ca²⁺. Soils/weathered products contribute high CO₂ under the open system. The Ca²⁺ and CO₂ are added to the groundwater through the infiltrating recharge water. They subsequently precipitate as fine-grained carbonates in the weathering profile due to evapotranspiration under a freshwater environment.     

Performance of microbial-induced carbonate precipitation on wind erosion control of sandy soil

Wind erosion is a serious problem throughout the world which results in soil and environment degradation and air pollution. The main objective of this study was to evaluate feasibility of microbial-induced carbonate precipitation, as a novel soil-strengthening technique, to reduce wind erosion risk of a sandy soil. For this purpose, the erosion of biocemented soil samples was investigated experimentally in a wind tunnel under the condition of wind velocity of 45 km h^?1. The weight loss of treated samples relative to the weight loss of control treatment was 1.29 and 0.16 % for low and high bacterial mix concentrations, respectively, indicating a significant improvement in erosion control in biologically treated samples. The effect of biological treatment on wind erosion control was even superior at the higher velocities. Thereafter, the penetration resistance of the surface layers as a simple index of resistance against wind erosion was measured. Significant improvements in the penetration resistance of the treated soil samples were observed. Although low bacterial mix concentrations did not significantly improve the penetration resistance of the samples, significant improvements in the penetration resistance of the treated soil samples were observed reaching to the highest measured strength (56 kPa) in high bacterial mix concentrations samples. Finally, the morphology of precipitated CaCO₃ crystals using scanning electron microscopy and X-ray powder diffraction analysis showed that the CaCO₃ was mainly precipitated as vaterite crystals forming point-to-point contacts between the sand granules.     

Improvement in silty sand with enzyme-induced carbonate precipitation: laboratory model experiment

Acta Geotechnica - Enzyme-induced carbonate precipitation (EICP) is a promising method for the improvement in soil properties. In this study, a model test using a meter-scale soil model was carried...     

Bio-mediated calcium carbonate precipitation and its effect on the shear behaviour of calcareous sand

Calcareous sands have abundant intraparticle pores and are prone to particle breakage. This often leads to poor engineering properties, which poses a challenge to coastal infrastructure construction. A study using bio-cementation to improve the engineering properties of calcareous sand is presented in this paper. The macro- and microscopic properties of bio-cemented calcareous sand were characterized by drained triaxial tests and scanning electron microscopy observations. Experimental results show that the precipitated calcium carbonate can effectively fill the intra- and interparticle pores and bond adjacent particles, thus enhancing the shear strength of calcareous sand. The special structures (e.g. abundant intraparticle pores and rough surface) and mineral components (i.e. calcium carbonate) of calcareous sand are beneficial for improving bacterial retention in soil, which leads to a relatively uniform and dense calcium carbonate distribution on the sand particle surface, exhibiting a layer-by-layer growth pattern. This growth pattern and the abundant interparticle pores would result in less effective calcium carbonate. The strength enhancement of bio-cemented calcareous sand is significantly lower than that of bio-cemented silica sand at the same calcium carbonate content, which may be caused by the differences in the following: (a) soil skeleton strength; (b) the amount of effective calcium carbonate; and (c) interparticle pore-filling of calcium carbonate.

     

Vulnerability and fate of a coastal sand dune complex,Rosetta-Idku,northwestern Nile Delta,Egypt

Types, distribution, and origin of recent sand dunes between Rosetta and Idku, in the western sector of the Nile Delta, Egypt were investigated. Sand samples from the dunes, beach, and seafloor were studied for grain size distribution and mineralogical composition. It has been found that most of the dunes in the study area have been subjected to deterioration and removal due to the construction of buildings and the International Coastal Highway. The remnant constitutes a damaged belt of foredunes that extends from El Bouseily village to the west of Idku town. The dune’s origin is interpreted to be the result of coastal drifting and the subsequent transport of sediments of the former Canopic Nile branch eastward by the predominant longshore current and by aeolian processes. The blown sand grains accumulated to form a belt of coastal sand dunes of original longitudinal and crescentic forms. Urbanization of the coast has severely altered the landscape. The study area is considered vulnerable to the impacts of climate change and the expected rise in sea level. The outcome of potential sea level rise is serious; erosion problems are expected to be exacerbated and vast areas from land and property would be lost. Thus, protection and preservation the remaining dunes in the study area are vital requirements for shore protection.     

A field quantification of coastal dune perennial plants as indicators of surface stability, erosion or deposition

Dune plants both modify the wind field around them and are impacted by various stress factors, among them sand erosion and sand deposition. As coastal dunes are being either stabilized or remobilized, in response to the changes in the rates of sand and dune movement, the vegetation cover and composition are expected to vary reflecting the differences in the sedimentary conditions. In this field study 315 quadrats of 100 m², in which the perennial plant species were sampled, were analysed with respect to annual rates of sand erosion and deposition that were measured using erosion pins. A visual exploratory data analysis was introduced, based on selective filtering of samples according to their vegetation cover. This method, combined with established statistical tools, enabled the authors to uncover the inclination and indicative power of nine perennial dune plants to either a stabilized or a mobile environment, and to establish whether they are more prevalent in places undergoing sand erosion or sand burial. Two species were found to be clear indicators of a stabilized environment, Stipagrostis lanata , and Retama raetam . Of the species indicating a mobile environment, only one may be stated as a clear indicator of sand erosion: Silene succulenta , with Cyperus macrorrhizus coming close to being an indicator of a less mobile erosive environment. The best indicator species for sand burial was found to be, as expected, Ammophila arenaria , with Artemisia monosperma also indicating high rates of sand mobility especially when its relative cover is higher than 80%. Such information can be used to monitor natural processes of dune stabilization or reactivation, or to assess the success of a management plan that aims at stabilizing a dune, or at remobilizing it by removing vegetation.     

Insight into the mechanism of microbially induced carbonate precipitation treatment of bio-improved calcareous sand particles

Acta Geotechnica - The use of the calcareous soil as a backfill material in ocean constructions faces pervasive challenges due to the significant rate of particle fracture. To meet the requirements...     

瓜尔豆胶固化黄土的工程特性及抗冲蚀试验研究

为了减少暴雨冲刷条件下黄土边坡侵蚀的发生,提出采用瓜尔豆胶固化黄土对边坡坡面进行防护。基于直剪试验、渗透试验以及模拟暴雨边坡冲刷试验,研究了瓜尔豆胶固化黄土的工程特性及抗冲蚀能力,并对比素黄土与固化黄土的微观结构,探讨了瓜尔豆胶对黄土的加固机制。试验结果表明：瓜尔豆胶可有效增强黄土的抗剪强度和抗渗透性,固化黄土的黏聚力...     

含碳酸盐混合砂的三轴剪切试验研究

含碳酸盐混合砂作为一种特殊的岩土材料主要分布于热带沿海地区,独特的成因和组构使得其具有不同于陆源砂的岩土工程特性。针对取自我国南海的珊瑚砂与取自长江流域石英砂按不同质量百分比配制而成的混合砂,开展不同围压下的三轴固结排水剪切试验,研究不同碳酸钙含量混合砂的剪切特性。结果表明:(1)混合砂峰值强度随有效围压和碳酸钙含量的增加呈近似线性增长;(2)围压及碳酸含量是影响混合砂剪胀特性的主要参数,随着碳酸盐含量的增加,胀缩转换点及其对应的轴向应变量增大;(3)随着碳酸盐含量的增加,混合砂的黏聚力呈线性增加趋势,峰值内摩擦角?f缓慢增长;(4)混合砂的相对破碎率Br随碳酸钙含量及围压的增加而增加。     

微生物诱导碳酸盐沉积改善裂隙岩石防渗性能和强度的试验研究

微生物诱导碳酸盐沉积(MICP)技术主要利用微生物生命活动与环境反应形成的碳酸盐来修复岩土体。为了研究该技术改善含裂隙岩石防渗性能和强度的效果,利用巴氏芽孢杆菌开展了裂隙黄砂岩的修复试验,并对修复后的裂隙黄砂岩进行了无侧限抗压、核磁共振和电镜扫描(SEM)等测试,分析了巴氏芽孢杆菌对裂隙黄砂岩的修复效果和修复机制。研究表明:巴氏芽孢杆菌对裂隙黄砂岩具有较好的修复效果;修复时间越长,巴氏芽孢杆菌的修复效果越好。修复42d后,裂隙黄砂岩的孔隙率下降36.41%,防渗性能提升94.62%,抗压强度增加30.52%。巴氏芽孢杆菌具有较好修复效果原因在于,其诱导产生的碳酸钙能够胶结填充物与试样,大幅降低试样的孔隙率,改善其内部孔隙结构的均质性。     

扁铲探头贯入干砂的位移特征试验研究

扁铲侧胀试验（DMT）已在国内外岩土工程勘察、地基加固效果评价等领域中得到广泛应用。DMT的测试过程在扁铲探头贯入到测试位置后进行,因此探头的贯入过程对土体造成的扰动在一定程度上将直接影响测试结果。目前关于扁铲探头的贯入机理,以及贯入过程可能引起的土体扰动及其对测试结果的影响尚未研究清楚。通过室内模型试验手段,进行扁铲探头贯入不同初始密实状态下均质干砂的试验研究,分析了探头贯入过程中产生的土体位移场分布特征。试验结果表明：扁铲探头的楔形部和膜片所在的侧胀部贯入产生的挤压作用是引起土体变形扰动的主要原因;探头楔形部的贯入过程表现为向下和向斜侧面挤压土体,竖向位移量很小,产生向两侧扩展为主的扁状位移场,而探头侧胀部的贯入过程主要表现为向两侧面水平向挤压土体,产生半椭圆状水平位移场且分布范围明显更大,同时探头侧面表现为一定的剪切作用而产生较窄范围的竖向位移场。另外,扁铲探头贯入干砂产生的位移场受砂土初始密实状态的影响较小,主要表现为探头楔形部周围的位移场分布范围随密实度增大而扩大。     

Development of coastal erosion and management

Coastal erosion and management attract much conern all around the world as coastal erosion is a problem at many coastal sites exacerbated by human activities and its adaptability through reasonable mitigation measures. This paper summarizes the main factors causing coastal erosion including reduction of sediment discharged by rivers trapped coastal structures, sand mining and reclaimation, relative sea-level rise, destruction of coastal ecosystem and weakening of coastal defenses. Mechanisms causing erosion of coastal dune, soft rock coast and muddy coast are identified and discussed. Sandy coastal erosion, soft rock coastal erosion, muddy coastal erosion, biological coastal erosion and coastal structural erosion are identified according to the characteristics of erosion in China. This study supplements the theory and methodology for coastal erosion management and provides information for managers and stakeholders.     

强夯砂土桩复合地基承载力的试验研究

借鉴砂井法加固粘性土地基的机理, 提出了用强夯砂土桩法加固饱和粉质粘土地基, 对某工程试夯区进行的现场试验表明,不仅砂土桩本身密实性好、强度高, 而且桩间土的物理力学性能也得到了很好的改善, 复合地基承载力得到了大幅提高。工程应用结果也证实加固效果显著。     

微生物沉积碳酸钙固化砂质黏性紫色土试验研究

重庆紫色土是一种砂质黏性土,地区降雨集中,水力冲蚀作用剧烈,极易产生水土流失,微生物诱导方解石沉积(MICP)技术因能耗低、污染小而广泛应用于土体加固中。通过正交试验优化了巨大芽孢杆菌(BNCC 336739)的培养基和培养条件,活菌数增长126%,活性良好。采用巨大芽孢杆菌,进行低水压(9.8kPa)灌注固化砂质黏性紫色土试验,探究了固化效果的变化规律。结果表明:随固化次数增加,碳酸钙生成量和干密度逐级增加,无侧限抗压强度与碳酸钙生成量正相关;碳酸钙有效沉积越来越少,强度提高趋于稳定,固化9次后强度提高77%;随孔隙被碳酸钙填充和上下碳酸钙硬壳的形成,渗透性不断降低,最终下降两个数量级;通过试样上、中、下三部分碳酸钙生成量C的样本标准差s来反映碳酸钙分布离散程度,发现割线弹性模量在s的影响下随C增加而波动上升,波动表现为在C相近或s相差很大时,s越小割线弹性模量越大。研究成果可以为MICP技术在紫色土地区的地基、边坡加固和水土流失防护等工程应用提供科学依据和参考。