Shear Resistance of Siltous Sands Improved with Bridelia Tannins

The ruin of several civil engineering works occurs due to shear rupture of the ground. When the stress is greater than the shear resistance, the internal friction angle and the cohesion of the soil loosen and rupture occurs. Cement and lime are often used to stabilize soils and improve soil strength. The costs and environmental problems of these technologies raise concerns and challenge researchers to innovate with clean, inexpensive materials, accessible to the most disadvantaged social classes. The question that this study seeks to answer is whether the binders derived from plant tannins, which also stabilize soils, improve the shear resistance of these soils. To do this, we determined for silty sand the shear parameters, notably the cohesion and the angle of internal friction in the non-stabilized state and when they are stabilized with the powder of the bark of the Bridelia under different water states. The results show that the addition of Bridelia powder to silty sand increases the cohesion of the soil by nearly 70.71% and the friction angle by 4.31%. But in unfavourable water conditions, the cohesion and internal friction angle of the silty sand material improved with Bridelia bark powder drops drastically by nearly 81.56%. but does not dissolve completely as for the same material. When it is not stabilized. This information is an invaluable contribution in the search for solutions to increase the durability of earthen constructions by improving the water-repellent properties of soils.

Failure Envelopes of Wide-Shallow Composite Bucket Foundation for Offshore Wind Turbines in Silty Sand

The wide-shallow composite bucket foundation(WSCBF) is a new type of offshore wind power foundation that can be built on land and rapidly installed offshore, there by effectively reducing the construction time and costs of offshore wind power foundation. In this study, the horizontal bearing capacity is calculated by finite element simulation and compared with test results to verify the validity of results. In this process, the vertical load and bending load are respectively calculated by the finite element simulation. Under the vertical load effect, the bucket foundation and the soil inside are regarded as a whole, and the corresponding buckling failure mode is obtained. The ultimate vertical bearing capacity is calculated using empirical and theoretical formulas; the theoretical formula is also revised by finite element results. Under bending load, the rotational center of the composite bucket foundation(in a region close to the bucket bottom) gradually moves from the left of the central axis(reverse to loading direction) to the nearby compartment boards along the loading direction. The H–M envelope line shows a linear relationship, and it is determined that the vertical and bending ultimate bearing capacities can be improved by an appropriate vertical load.

Influence of repeated freeze-thaw on dynamic modulus and damping ratio properties of silty sand

Under repeated freezing and thawing in deep seasonal frozen regions, the stability and strength of the soil are imposed in the form of large uneven settlement, instability and strength reduction, which affect the normal operation of railway lines. This study is to obtain the influencing rules of freeze-thaw on the dynamic properties （dynamic strain, confining pressure and compactness） of silty sand. Based on an amount of inner tests, the dynamic modulus and damping ratio properties of silty soil subjected to repeated freeze-thaw cycles were deeply researched and analyzed. The results are as follows： At the same dynamic strain, the relationship of dynamic stress and freeze-thaw cycles presents negative cor- relation, and the relationship of dynamic stress, confining pressure and compactness present positive correlation. The dynamic modulus double decreases while the damping ratio double increases with incremental increase in dynamic strain. The dynamic modulus sharply decreases while the damping ratio increases with incremental increase in freeze-thaw cycles, and then the changes level off after six freeze-thaw cycles. The dynamic modulus increases while the damping ratio decreases as the confining pressure and compactness increase at the same strain level.

Investigation on creep behavior of warm frozen silty sand under thermo-mechanical coupling loads

In cold regions,the creep characteristics of warm frozen silty sand have significant effect on the stability of slope and subgrade.To investigate the creep behavior of warm frozen silty sand under thermo-mechanical coupling loads,a series of triaxial creep tests were carried out under different temperatures and stresses.The test results reveal that the creep strains decrease as the consolidation stress increases,and finally tend to be equal under the same loading stress,regardless of whether the stress is isotropic or deviatoric.Additionally,warm frozen silty sand is highly sensitive to temperature,which greatly influences the creep strain both in the consolidation stage and loading stage.Furthermore,based on the creep test phenomena,a new creep model that considers the influence of the stress level,temperature,hardening,and damage effect was established and experimentally validated.Finally,the sensitivity of the model parameters was analyzed,and it was found that the creep curve transitions from the attenuation creep stage to the non-attenuation creep stage as the temperature coefficient and stress coefficient increases.The hardening effect gradually changes to the damage effect as the coupling coefficient of the hardening and damage increases.

Factors Influencing the Soil-Water Characteristics of Unsaturated Tropical Silty Sand

Fine grain soils have a complex engineering behaviour which depends but not limited to moisture content, changes in external pressure and characteristics of the pore medium. Sand often contains a considerable percent of silt which is expected to alter its natural behaviour. This composite matrix is referred to as silty-sand. To understand the behaviour of this matrix under varying moisture conditions, some of the factors influencing the soil-water characteristics of unsaturated silty sands were investigated. Representative samples were collected from a river bank after its index properties were predetermined in the laboratory. The samples were compacted at different moisture conditions and compactive efforts. With the pressure plate extractor device, the Soil-Water Characteristic (SWC) was obtained and SWC Curves plotted. Compaction at greater compactive effort (modified proctor) and optimum moisture content produced the largest air entry value and reduced air voids. The air entry values of the soils obtained ranged from 21 kPa to 57 kPa. Also changes in the shape of the SWCC were consistent with changes in pore size which occur by varying compaction conditions. Result shows that soil structure, compaction water content, compactive effort and percentage of fine particles are factors affecting the Soil-Water Characteristics.

Assessment of Cyclic Resistance Ratio (CRR) in Silty Sands Using Artificial Neural Networks

In this study, a backpropagation neural network algorithm was developed in order to predict the liquefaction cyclic resistance ratio (CRR) of sand-silt mixtures. A database, consisting of sufficient published data of laboratory cyclic triaxial, torsional shear and simple shear tests results, was collected and utilized in the ANN model. Several ANN models were developed with different sets of input parameters in order to determine the model with best performance and preciseness. It has been illustrated that the proposed ANN model can predict the measured CRR of the different data set which was not incorporated in the developing phase of the model with the good degree of accuracy. The subsequent sensitivity analysis was performed to compare the effect of each parameter in the model with the laboratory test results. At the end, the participation or relative importance of each parameter in the ANN model was obtained.

Migration process of ammonium ion in saturated silty sand and sandy loam

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Interaction of the Mining Environment on the Properties of Hydraulic Mortars in Silty Sands in Togo

The objective of this study is to determine the influence of the surrounding soils on the granular properties of the silty sands of Togo and on the resistance of the mortars. Sand compositions are made by substituting silty sands with clay soil, vegetal soil, lateritic soil or fine elements (<0.08 mm) which are the surrounding land polluting the sands in Togo. After identification tests, the mixtures were used to prepare test specimens of mortar which are subjected to bending and compression. It appears that additions of clay and plastic soils (ES = 0, VBM > 0.53 and IP > 19) from 10% to 35% cause drops in resistance of mortars from 7% to 96%;this loss is 8% to 70% for the rates of addition of less clayey soil (ES = 33, VBM = 0.40 and IP = 0) at rates of 10% to 100%. As for fine powdery soils (ES = 56.53 and VBM = 0.25), they have virtually no influence on resistance (loss of less than 3% for rates of 100%). Construction stakeholders thus have a decision-making tool for the choice of silty sand extraction zones according to the surrounding land and the quality of the desired concrete.

由石蜡基相变材料和煤渣改良的粉砂土的冻融性能

【目的】针对季节冻土区渠道的衬砌结构因冻融作用而受损的现状,同时为了解决石蜡基相变材料(phase change materials, PCM)的泄露,并满足煤渣的再利用需求,研究石蜡基PCM和煤渣对土体冻融性能的影响。【方法】以粉砂土为研究对象,选取石蜡基PCM和煤渣作为改良剂,制备石蜡基PCM改良土和石蜡基PCM-煤渣改良土;通过冻融循环作用下的体积变化率试验、无侧限抗压强度试验,微观结构及潜热研究土体的体积和无侧限抗压强度的变化。【结果】石蜡基PCM能抑制粉砂土的冻融变形,质量分数为8%时2种改良土的体积变化率最小;随着冻融循环次数的增加,土体的无侧限抗压强度均降低,降低速率先大后小;循环次数相同时,加入煤渣的改良土的无侧限抗压强度均大于未加入煤渣的;冻融循环9次后,石蜡基PCM质量分数为10%的石蜡基PCM改良土的无侧限抗压强度最低;2种改良土的结构比粉砂土更密实、稳定;潜热的吸收或释放延缓了土体冻融过程。【结论】将石蜡基PCM和煤渣共同掺入粉砂土,能控制土体的冻融变形且效果稳定,改善土体的冻融性能,减轻季节冻土区输水渠道衬砌结构的冻害,提高煤渣的利用率。

冻融对水泥改良路基土力学特性影响研究

以季冻区路基粉砂土为研究对象进行室内三轴压缩试验,分析水泥改良粉砂土与素土的应力-应变关系,探究冻融循环、水泥掺量和围压对土体抗剪强度的影响。试验结果表明:改良土的破坏以脆性破坏为主;围压增长不改变试样的破坏形式,土抗剪峰值随围压的增加而增大;冻融循环作用会削弱土体的黏聚力,内摩擦角的变化规律表现为在一定范围内波动;在水泥掺量相同的条件下,随冻融次数的增多,土的抗剪强度因水泥持续水化反应故呈先减小后增大再减小;当水泥掺量为2%时,土体已获得较大抗剪强度。

滩海吹填粉细砂地基长期承载力特性试验研究

对某运行10年以上出现沉陷冒水现象的滩海人工岛吹填粉细砂地基承载力进行分析,基于螺旋板载荷试验结果,研究了标贯击数、动探击数与地基承载力的关系,拟合了经验公式,在此基础上综合分析粉细砂地基承载力现状,探究地基长期承载力特性。研究结果表明,吹填粉细砂形成的滩海人工岛地基在长期反复的水力作用下可进一步密实,承载力有所提高,有利于场地的运行安全。但鉴于其松散特性,水力作用下易流失,防渗措施不到位可能导致沉陷脱空、冒水等工程问题,影响场地安全运行。因此,针对本工程沉陷冒水现象,需采取原位固化、帷幕封闭等防渗措施进行处理。

深井掘进技术在厚层高渗透性粉细砂层中适用性的数值模拟研究

科学合理高效开发深层地下空间是城市不断发展的必然趋势与重要难题。深井掘进技术(vertical shaft machine,VSM)是一种开挖超深竖井的新型工法,该技术施作的竖井结构形式简单、结构受力状态好、施工速度快、对周边环境扰动小、适用性广泛,可被广泛用于城市深层地下空间的点状开发。以中国首例使用VSM施工的南京市儿童医院沉井停车库项目为原型,建立在砂性土中开挖60 m竖井的数值模型,重点研究VSM竖井在厚层高渗透性的粉细砂层开挖过程的变形情况与开挖面的稳定性问题。数值模型结果表明:60 m竖井开挖过中地表变形量均控制在毫米数量级,且最大地表变形量不超过13 mm;在水位差1 m的水下施工过程中安全系数均较好控制在2.0以上。对于变形问题,分析不同开挖深度下的井底隆起变形和地表变形情况,表明在60 m开挖深度内,使用VSM开挖砂性土竖井在深度、安全性和对周边环境扰动上均具有明显优势。针对开挖面稳定性,计算分析与竖井内外水位差和竖井深度相关的开挖面安全系数,计算结果可用于指导实际施工,实现分段施工与精细化控制。

微胶囊相变材料改良粉砂土的导热系数及预测模型

【目的】针对季节冻土地区渠道冻融破坏,分析微胶囊相变材料(microencapsulated phase change materials,mPCM)改良粉砂土层渠基的温度场,对改良粉砂土的导热系数进行研究。【方法】以mPCM为改良剂,掺入渠基粉砂土形成mPCM改良粉砂土;对mPCM改良粉砂土进行导热系数实验和内部结构表征;采用多元线性回归和支持向量机(support vector machine,SVM)方法分别建立mPCM改良粉砂土的导热系数预测模型。【结果】mPCM改良粉砂土导热系数与含水率、干密度、mPCM掺量有关,且受冰水相对含量、冰水相变潜热、mPCM相变潜热和mPCM填充密实作用的影响,具有明显的温度效应;mPCM改良粉砂土导热系数的变化与实验温度和mPCM相变温度有关,可分为快速降低、缓慢降低和逐步上升3个阶段;多元线性回归和SVM模型均能较好地拟合预测mPCM改良粉砂土的导热系数,但SVM模型更适用于表征mPCM改良粉砂土导热系数各影响因素间的非线性关系。【结论】mPCM改良粉砂土的导热系数提高能够有效调控渠基土温度场,减轻渠道冻害,且SVM模型能更加准确地进行导热系数预测。

多源固废土壤固化剂固化粉土质砂力学性能试验研究

粉砂土易松散难压实,遇水易崩解,在填筑路基时易出现各种病害,严重影响公路的使用性能。采用水泥和石灰固化的粉砂土具有较高的强度和良好的路用性能,但水泥和石灰生产成本高且生产过程排放大量二氧化碳,环境污染严重。为改善粉砂土力学性能,降低固化成本及环境污染,将电石渣、粉煤灰等工业固废制备成固化剂,用于固化粉土质砂,并探究了固化剂掺量、压实度、养护龄期等因素对固化土力学性能的影响。试验结果表明:1)固化土的CBR、无侧限抗压强度和劈裂强度随固化剂掺量的增大先升高后降低,10%固化剂掺量的固化土力学性能最佳;固化土的CBR、无侧限抗压强度和劈裂强度都随压实度的增长而增加;2)固化剂掺量≥2%的固化粉土质砂,能满足《公路路基设计规范》(JTG D30—2015)规定的上路床的填筑要求;3)固化剂掺量≥6%的固化粉土质砂,能满足《公路路面基层施工技术细则》(JTG/T F20—2015)规定的底基层的强度要求。

粉细砂地层双线盾构施工实测地表沉降规律分析与预测

依托南通地铁二号线某区间盾构隧道施工,针对盾构穿越典型粉细砂地层条件,对双线盾构隧道施工引起的地表沉降开展现场测试和规律分析,对经典Peck公式在南通地区粉细砂地层中双线盾构施工的适用性进行探讨,并研究先行线和后行线土体损失率和地表沉降槽宽度系数的取值及影响因素。研究结果表明,在粉细砂地层中,后行线隧道施工对地层的二次扰动效应会引起先行线轴线上方地表沉降的显著增加,且二次扰动效应明显强于其他软土地层,但弱于砂土地层;相较于淤泥质土地层,在粉细砂地层中盾构停机对地表沉降的影响更为显著;先行线和后行线施工引起土体损失率比值η_(1)/η_(2)在1~5倍范围内,平均值为2.3,先行线和后行线地表沉降槽宽度系数比值K_(1)/K_(2)在1~2倍范围内,平均值为1.4;砂土、粉细砂和粉土地层中土体损失率比值η_(1)/η_(2)均大于1,η_(1)/η_(2)和K_(1)/K_(2)与隧道覆土深度比(H/D)相关性较弱。

超细水泥-水玻璃双液浆性能及注浆止水效果试验研究

北京地铁14号线高家园站的站厅层和站台层采用暗挖通道连接,地层为富水粉细砂层。为解决暗挖通道采用普通水泥-水玻璃双液浆注浆止水效果差且地表隆起较大的问题,提出采用超细水泥-水玻璃双液浆注浆止水的方案。首先对浆液流动性、凝结时间和固结体强度进行测试,根据测试结果确定了超细水泥-水玻璃双液浆的配合比:A液为超细水泥净浆,水灰比为1.0;B液为水玻璃溶液,波美度为20°Bé;缓凝剂为磷酸氢二钠,掺量为2.5%。A液约占总注浆量的80%,B液与缓凝剂各占约10%。采用颗粒流数值模拟方法,研究了超细水泥浆液中颗粒在地层中的运移距离与注浆压力的关系。结合浆液性能测试和数值模拟结果,提出了注浆压力0.5 MPa,注浆孔直径5 cm,间距50 cm,梅花形布置的注浆方案。经在暗挖通道现场注浆试验,采用超细水泥-水玻璃双液浆注浆止水效果良好。

富水粉细砂隧道失稳破坏特征分析及增稳固结技术研究

桃树坪隧道穿越富水粉细砂地层,施工过程中极易出现大变形、涌水涌砂和坍塌等一系列破坏特征。通过对隧道失稳破坏的分析,总结了隧道在富水粉细砂条件下失稳破坏的特征,分析了破坏特征发生的机制,认为隧道的破坏主要由围岩岩性和地下水造成;由隧道破坏机理提出了增稳固结措施,通过超前降水、超前小导管注高分子化学浆对掌子面前方砂层进行预先固结,提高钢架整体刚度,通过对扰动砂层进行灌浆和回填等措施进行固结,围岩的自承能力得到提高。从处理后围岩的情况、涌水量和监测情况分析发现,增稳固结技术能从根本上提高围岩的自承能力,抑制发生失稳破坏,为隧道在富水粉细砂地层中的施工提供借鉴。

粉砂与粉土固结蠕变特性对比的试验研究

为了解释基坑预降水引发的基坑周边地面沉降的滞后性,依托某地铁车站基坑预降水工程,开展场地中典型粉砂和粉土的一维固结蠕变试验。通过试验结果分析,得到土的压缩指数、回弹指数和次固结系数,给出了回弹指数与压缩指数和次固结系数与压缩指数的经验关系。蠕变试验结果表明,粉砂和粉土的次固结系数与固结荷载之间呈现良好的线性关系,提出了次固结系数与固结压力的经验公式。从固结压力下砂土天然结构破坏的角度,解释了次固结系数随着固结压力的增加而增大的原因,为解释预降水引起的基坑周边地面沉降滞后效应和地面沉降合理估算提供了实验依据。

土体置换率对预加固粉细砂地层掌子面失稳变形的影响

以佛山某道路盾构工程堤坝保护为背景,开展了粉细砂地层经旋喷桩预加固后的浅埋大直径盾构隧道工程的三维数值仿真分析,研究了土体置换率对掌子面失稳变形的影响规律。结果表明:(1)对粉细砂地层进行加固可提高掌子面支护力的调节范围,其中掌子面主动极限支护力随土体置换率提高而降低,土体置换率至30%后基本不再变化;(2)粉细砂地层加固对支护力不足产生的主动失稳变形有显著抑制作用,相比不加固工况,30%的置换率可使掌子面失稳时地层最大水平位移及地表最大沉降量分别降低90%及95%,依托工程所采用的4.52%土体置换率也可使最大水平位移及最大沉降量分别降低45%及30%。

不同制样方法条件下粉土质砂力学特性的试验研究

北疆供水一期工程渠基膨胀土受环境影响力学特性衰减严重并导致渠坡发生滑动破坏现象,采用粉土质砂对膨胀土进行换填处理,运行一定年限后渠坡仍发生滑动破坏。为深入分析渠坡破坏机理,采用静压、吸水、饱和-脱水三种制样方法模拟换填渠道完建期、通水期、停水期粉土质砂换填料力学状态,通过开展直剪、压缩、电镜扫描试验,从宏-微观角度分析其力学特性规律及物理机制。研究结果表明:(1)宏观力学试验:不同制样方法试样随着含水率增加,c、φ、E_(s)值整体呈现下降趋势,压缩指数呈上升趋势,回弹指数呈波动变化。随着含水率增加,吸水试样c、φ、E_(s)值始终小于另外两种制样方法试样,静压与饱和-脱水试样c、φ、E_(s)值之间的关系以最优含水率(ω=12.2%)为界限存在明显差异,当ω≤12.2%时,饱和-脱水试样c、φ、E_(s)值高于静压试样;当ω>12.2%时,静压与饱和-脱水试样c、φ、E_(s)值无明显差异。(2)微观物理机制:受结合水膜、易溶盐、黏土矿物等因素的影响,粉土质砂经历吸水/饱和-脱水后孔径会发生扩张/收缩,相较于静压试样,吸水/饱和-脱水试样中、大孔隙占比增加/减少,小孔隙占比减少/增加,宏观上表现为c、φ、E_(s)减小/增加,其中当ω>12.2%时饱和-脱水试样只是微孔隙发生变化,因此宏观上与静压试样强度指标无明显差异。由此表明经历吸水或饱和-脱水后粉土质砂换填料孔隙结构变化复杂,强度不稳定,是渠坡发生滑动破坏的一个重要诱因。(3)函数关系预测:通过建立静压与吸水、饱和-脱水试样强度指标之间的函数关系,预测换填渠道不同时期粉土质砂力学指标大小及规律,得出静压试样与吸水、饱和-脱水试样c、E_(s)值呈指数相关,φ值呈线性相关;采用粉砂土对静压与饱和-脱水试样c、φ值间的关系进行验证,发现其函数形式与粉土质砂一致,表明这可能是砂土共有的性质。研究结果可为北疆供水一期工程粉土质砂换填渠道的安全运行提供参考。