Prediction of Swelling Kinetics of Expansive Soils of Rufisque (Senegal, West Africa)

The disorders caused by the swelling of the soil on the structures have been observed for several years in the city of Rufisque. This article presents the results of the study of swelling kinetics of expansive soils in Rufisque and their prediction based on the hyperbolic rule. The odometer is used as an instrument for measuring swelling and the tests are carried out on some intact samples at their sampling water content. The present study shows that in Rufisque the most swelling layer is marl. The results show two phases of development. The first phase is very fast and represents 77% of the final deformation and the second one is slower. The prediction of the issue by the hyperbolic rule shows that it underestimates the first phase but gives a good prediction of the second phase of the swelling rate. There is a good correlation between the final swelling rates. However, the model gives a bad approximation of the half-swelling time.

Factors Predisposing and Triggering the Phenomenon of Shrinkage-Swelling of Clay Soils in the Urban Center of Diamniadio

The phenomena of shrinkage and swelling of clay induce damage to housing structures every year. Precipitation, climatic changes and drought are the cause of wall cracks due to subsidence or swelling of the supporting soil. This movement alters the balance between the soil and the structures. To explain this defection, the soil is made up of three elements: the solid, the liquid and the gas. Sometimes in a natural way or following a human intervention, one of these elements undergoes an abnormal variation that causes the loss of the balance between land and works. It is in this sense that this article deals on the one hand with the factors of predisposition and triggering of the phenomena of shrinkage-swelling of the clay soils of Diamniadio and on the other hand, the factors of aggravation linked to the lithological heterogeneity and the variation in the thickness of the layers susceptible to shrinkage-swelling. The studies carried out have enabled a deeper understanding of the behavior of expansive soils following their interactions with climate, vegetation, hydrology, hydrogeology, constructions among others, but also the influence of lateral and vertical variations of fine soil facies.

SAND: An Additive for Stabilzation of Swelling Clay Soils

An experimental program carried out in this study seeks to understand the physical mechanisms of stabilization of an expansive soil by adding an inert material (sand) at various forms;mixing and intercalation layers of sand. The first aim of the study is to analyze the effect of stabilization on the variation of soil consistency. The results show a marked improvement in soil consistency. Then a study of the effect of the additives on the variation of the swelling in order to solve the lifting structures caused by this phenomenon is carried out. It can be seen that the addition of sand is capable of reducing the swelling action. We complete the study using an ultrasonic device, measuring the propagation velocities of ultrasonic pulse through the tested clay samples. This shows a change occurring in the porosity of the clay when sand is added to it. As a result some interesting conclusions concerning the expansive soil-material amendment are drawn.

Hydromechanical Behavior of Low-Swelling Soils Compacted at Low Water Content: Laboratory Study

Fine unsaturated soils are used in many applications, particularly in road infrastructure and in construction. These materials undergo deformations according to the stresses to which they are subjected. The purpose of this paper is to study the influence of hydromechanical stresses on the behavior of low swelling soils compacted at low water content in accordance with the French standard GTR 92 (Guide des Terrassements Routiers). Then, various experimental tests on an oedometer were carried out in the laboratory. Two types of low swelling soil sampled in Nasso on the outskirts of the town of Bobo Dioulasso (Burkina Faso) were used. After shuffling, each sample was moistened to its optimum water content and then compacted to 90% and 95% of its optimum density. Behavior tests show that these soils deform very little when subjected to hydromechanical stresses. However, these deformations are swelling in nature for low mechanical stresses and when the stresses are high, they tend to collapse. When these soils are subjected to a vertical stress of 420 kPa, the primary consolidation time is of the order of one minute for NH2 (a silty soil) and about ten minutes for NH3 (a silty-clayed soil).

Improved prediction of clay soil expansion using machine learning algorithms and meta-heuristic dichotomous ensemble classifiers

Soil swelling-related disaster is considered as one of the most devastating geo-hazards in modern history.Hence,proper determination of a soil’s ability to expand is very vital for achieving a secure and safe ground for infrastructures.Accordingly,this study has provided a novel and intelligent approach that enables an improved estimation of swelling by using kernelised machines(Bayesian linear regression(BLR)&bayes point machine(BPM)support vector machine(SVM)and deep-support vector machine(D-SVM));(multiple linear regressor(REG),logistic regressor(LR)and artificial neural network(ANN)),tree-based algorithms such as decision forest(RDF)&boosted trees(BDT).Also,and for the first time,meta-heuristic classifiers incorporating the techniques of voting(VE)and stacking(SE)were utilised.Different independent scenarios of explanatory features’combination that influence soil behaviour in swelling were investigated.Preliminary results indicated BLR as possessing the highest amount of deviation from the predictor variable(the actual swell-strain).REG and BLR performed slightly better than ANN while the meta-heuristic learners(VE and SE)produced the best overall performance(greatest R2 value of 0.94 and RMSE of 0.06%exhibited by VE).CEC,plasticity index and moisture content were the features considered to have the highest level of importance.Kernelized binary classifiers(SVM,D-SVM and BPM)gave better accuracy(average accuracy and recall rate of 0.93 and 0.60)compared to ANN,LR and RDF.Sensitivity-driven diagnostic test indicated that the meta-heuristic models’best performance occurred when ML training was conducted using k-fold validation technique.Finally,it is recommended that the concepts developed herein be deployed during the preliminary phases of a geotechnical or geological site characterisation by using the best performing meta-heuristic models via their background coding resource.

Study and Comparison of Swelling and Compressibility Characteristics of Crumb Marl, Flaky Marl with Attapulgite and Sandy Clay from the Diamniadio Urban Pole at the Oedometer

In Senegal, the Diamniadio, Sebikhotane and Bargny sector contains clay soils that are problematic for construction. In order to have more information on the behavior of the clay soils of Diamniadio, free swelling tests followed by load-discharge cycles were carried out according to standard NF P 94-090-1. These tests were carried out using an Oedometric device on the three samples from the study site (sandy clays with calcareous concretion, marls with crumbs and laminated marls with attapulgite) to apprehend their swelling aspects in saturated conditions. For the free swelling test, a determination of the different swelling phases will be carried out followed by a comparison of the rate of evolution of the phases for the three samples from the site. In the same vein, the compressibility characteristics of the samples will also be provided from load-unload Oedometric tests. Thereafter, we proceed to a comparison of the void index at the initial state of the samples after two charge-discharge cycles and the influence of the cycles on the reorganization of the internal structure of the samples. These studies will provide more information on the swelling behavior of Diamniadio soils in the presence of water.

Microstructural, Chemical and Mineralogical Analyses for Understanding the Geotechnical Properties of Clayey Soils

This paper presents a study research in the laboratory of three clayey soils from the depression of the Lama, a tropical climate area where disorders appear on construction and roads. Samples were tested to establish the relationship between their mineralogy and their geotechnical properties. On the three clayey soils, analyses were performed to characterize the macroscopic behavior (physical tests, free swelling test and compressibility tests) and the microstructural, chemical and mineralogical characterizations by thermal analysis, chemical analysis, X-Ray diffraction, Scanning Electron Microscopy observations complementary X-ray EDS analyses. At first glance, the results of physical and compressibility tests appear not to be consistent with the free swelling test results. Indeed, these results highlighted differences in the behaviors of the samples because the swelling potential revealed by the results of physical and compressibility test does not follow the same trend as the one given by the free swelling test result. The main clay minerals in the studied clayey soils varied from fibrous clays (palygorskite) to best-known clays such as smectite and kaolinite. Microstructural, chemical and mineralogical analyses allowed to understand and explain the different trends obtained by the different types of geotechnical characterization tests of studied clayey soils.

膨胀土微结构对膨胀行为的影响

评估膨胀土的膨胀行为对于膨胀土地区的结构设计十分重要。因降雨入渗,膨胀土在垂直方向和水平方向均产生膨胀变形,当膨胀变形受抑制时将产生膨胀压力,影响其周围结构的稳定性。为揭示膨胀土在增湿膨胀过程中出现各向异性的原因,从微观角度出发,通过电镜扫描(SEM)对百色中膨胀土和枝江弱膨胀土的微观结构进行观察,并通过图像处理技术统计分析膨胀土内黏土矿物颗粒的层状排列。通过研发的二维膨胀仪和改进的试件制备方法,从宏观角度测得了侧限条件下的两向膨胀规律。研究结果表明,膨胀土的微观结构呈片状且面-面相叠;当膨胀土处于天然松散状态时,其内部的黏土矿物颗粒随机定向排列且集聚;压实后,因受各向不均等应力作用,黏土矿物颗粒开始趋向于水平层状排列;干密度越大,土样越密实,其水平层状排列越显著。侧限条件下,两向的膨胀规律表现出显著的差异,干密度越大,膨胀性越强,这种差异就越明显。然而,对黏土矿物颗粒来说,其膨胀的方向垂直于其长轴,高度的水平定向是造成膨胀土在宏观上表现出膨胀各向异性的原因,这种两向的膨胀差异受到干密度和膨胀性的影响,难以做出预测,建议在工程实践中实测两向的膨胀规律。

含泥量对砂类硫酸盐渍土工程特性的影响分析

为了明确含泥量对砂类硫酸盐渍土的盐胀和力学特性的影响,人工配制了不同细粒土含量的砂类硫酸盐渍土,在1%和3%(质量分数,下同)含盐量单向冻结盐胀试验的基础上,选取了细粒土含量为5%、15%、30%和40%的砂样进行常温、低温三轴剪切试验。研究结果表明:此试验条件下,不同级配砂类硫酸盐渍土的冻结温度为-0.7~-0.1℃,当砂样孔隙溶液浓度在冻结温度之上达到饱和时,降温过程中会首先生成盐结晶;1%含盐量条件下,高细粒土含量(≥30%)砂样的起胀温度在4~9℃之内,而低细粒土含量砂样的起胀温度在0℃附近,3%含盐量砂样的起胀温度为20~23℃;试验含水率和细粒土含量通过影响土体中自由水的含量对盐冻胀产生显著影响。在力学特性方面,随着细粒土掺量的增加,砂类硫酸盐渍土的抗剪强度表现出先增大后减小的趋势,细粒土由增强摩擦转变为颗粒间的“润滑”作用;此外,冻结后砂土转变为承载能力更强的“土-盐-冰骨架结构”,抗剪强度大幅提高,并呈现出明显的脆性破坏特征,由于冻结砂土受相对温度的影响,随着含盐量的增加,破坏应力呈先减小后增大的趋势。

Utilization of Local Available Materials to Stabilize Native Soil (Earth Roads) in Tanzania—Case Study Ngara

The native soil behavior of unpaved low volume roads and their mode of failure were studied to establish proper method of stabilizing native soils using local available materials. The use of lime and its positive effect to modify and stabilize Ngara soil in Tanzania is presented in this paper. There are many methods of stabilizing soil to gain required engineering specifications. These methods range from mechanical to chemical stabilization. Most of these methods are relatively expensive to be implemeted by slowly developing nations and the best way is to use locally available materials with relatively cheap costs affordable by their internal funds. Tanzania is a country having abundantly amount of Lime. Ngara native soil roads (unpaved rural roads) was studied under preliminary investigation and found to have higher plasticity, tendency of swelling and shrinking, low bearing capacity when wet, compressive strength of soil to have higher sensitivity to moisture and lower shear strength when wet. These soil behaviors deny road access to about 32 million people in Tanzania (80% of population) during rainy season. The detailed investigations is still going on at China university of Geosciences but other results from similar researches show that Ngara soils can be stabilized by lime and gain the required engineering properties.

覆盖型岩溶区高速铁路综合选线研究

南玉高铁沿线覆盖型岩溶和膨胀岩(土)广泛发育,岩溶发育程度强烈,膨胀岩(土)多呈弱—中等膨胀性,给高速铁路勘察设计、施工运营全生命周期均带来较大风险和挑战。通过分析沿线地形地貌、特殊的岩溶地质特征和工程环境特点,提出针对性的勘察方法和选线原则,前期选线贯彻“短距通过,傍山绕避”原则,针对确实无法绕避的岩溶强烈发育区,提出“双向跨越”思路,安全、经济地跨越深溶槽,以期为覆盖型岩溶区高速铁路勘察设计及工程建设提供借鉴。

A Slip-Force Device for Maintaining Constant Lateral Pressure on Retaining Structures in Expansive Soils

Expansive soils can pose tough issues to civil engineering applications. In a typical year, expansive soils can cause a greater financial loss than earthquakes, floods, hurricanes and tornadoes combined. Various means have been studied to tackle problems associated with expansive soils. The majority of the methods are based on treatment of the soils. While the methods may be effective in some cases, their limitations are also obvious: The treatment normally involves complex processes and may not be eco-friendly in the long run. In many cases, the effectiveness of the treatment is uncertain. A retaining system that maintains a constant lateral pressure is proposed, which consists of three components: the retaining sheet, the slip-force device and the bracing column. The retaining sheet bears the pressure exerted by expansive backfills and is not embedded into the soils. Placed between the retaining sheet and bracing column, the slip-force device permits displacement of the retaining sheet but keeps the force on the sheet and the bracing column constant. The governing equation of the motion of the piston in the slip-force device is derived and a numerical simulation of a practical case is conducted based on the derived governing equation. Numerical results show that as the expansive soil swell, the spring force will increase and the piston will move accordingly. When the pressure of the oil in chamber reaches the open threshold of the unidirectional relief valve, the valve will open and the spring force and the oil pressure in the chamber will keep constant. The results also show that some parameters, such as damping ratio, have very slight influences on the device behavior, say 2 × 10-6 or even 4.8 × 10-9. Theoretical and numerical studies prove the effectiveness of the proposed retaining system.

巴东组泥岩重塑土湿胀特性宏细观跨尺度研究

针对泥岩地区高速铁路路基上拱问题,以郑万高速铁路沿线巴东组泥岩风化土为研究对象,开展不同初始含水率、干密度、上覆荷载及干湿循环次数条件下泥岩重塑土湿胀特性宏细观试验。基于多因素从宏细观角度对泥岩膨胀性进行了试验验证,对维护膨胀土区域工程稳定性有一定借鉴作用。结果表明:泥岩重塑土无荷膨胀率随着初始含水率的增加线性减小,随着初始干密度的增加呈指数增长;上覆荷载作用抑制了泥岩重塑土的膨胀变形;干湿循环作用下,泥岩重塑土膨胀率及吸水量均随干湿循环次数的增多而总体上先增加后减小,强度劣化,泥岩内部结构由致密变为疏松,矿物颗粒吸水膨胀,试样内部大尺寸孔隙逐渐形成。

土工袋包裹膨胀土复合体抗渗与变形特性研究

膨胀土具有吸水膨胀性强、压缩性大、易产生裂缝等特点,危害较大,需要采取相应措施进行治理和防范。研究提出了一种采用土工袋包裹膨胀土的治理措施并进行室内模型试验,探究土工袋包裹膨胀土对膨胀土的抗渗与控制膨胀变形的效果。结果表明:在模型箱内进行淋雨试验后,有土工袋包裹的膨胀土内部含水率低于无土工袋包裹的膨胀土,在土工布内缝制一层无纺布后防渗效果得到进一步提升;土工袋对淋雨后膨胀土的膨胀变形有显著的控制效果,且随着上覆荷载的增加变形量逐渐减小。因此土工袋装膨胀土用于边坡防护可有效减少雨水入渗,同时能够限制土体膨胀,说明以此对膨胀土问题进行治理是可行的。研究对膨胀土边坡的浅层滑动防治提供了参考。

膨胀土流态固化改性试验与配合比研究

流态固化改性是弃方资源化新方式,有助于解决道路工程中异形或狭隘空间难以回填压实问题.选取南宁地区中等膨胀土弃方,与水、水泥及标准砂混合配制流态固化土试样并开展室内土工试验,分析掺配比对试样膨胀性、强度、流动度及泌水率的影响规律.结果表明:试样膨胀性由掺灰比和掺砂比决定,受水固比影响小,当掺灰比大于18%且掺砂比超过6%时,试样呈现非膨胀特征.无侧限抗压强度主要来自水泥水化作用,随掺砂比的增加呈先增大后降低的变化,掺砂比上限宜控制在10%;水固比增加导致强度大幅下降,水固比大于80%后干湿循环条件下的强度衰减趋势显著.初始流动度与水固比、掺砂比及掺灰比正相关,受掺灰比影响最小;试样拌合完成后的90 min是流动度损失的主要阶段,流动度损失率为23%~32%,随后变化放缓.泌水率取决于水固比,受掺灰比影响较小.基于试验规律,经归一化分析得到膨胀土流态固化改性配合比设计流程.

不同膨胀潜势等级的膨胀土残余强度环剪试验研究

为探讨不同膨胀潜势等级的膨胀土残余强度与其成分特征的相关性,采用经自主改装的环剪仪开展Denver强膨胀土、荆门中、弱膨胀土环剪试验研究,并与排水反复直接剪切试验结果进行系统对比分析,在此基础上,用实测结果探讨残余强度与土成分特征相关关系的适用性。结果表明:(1)环剪试验测得Denver强膨胀土、荆门中、弱膨胀土有效残余强度内摩擦角分别为11.3°,12.3°,23.7°,比排水反复直接剪切试验测得的低1.7°~3.3°,并无应力相关性且具很好精度,更适于在工程中应用。(2)相关经验关系和公式存在高估或者低估膨胀土有效残余强度内摩擦角的情况。(3)残余强度与土成分特征间定性关系非常明确,上述经验关系对一般土性状况下残余强度预测较为准确;但在土成分复杂状况下,定量准确预测较为困难,这也反映出实测的重要性与必要性。

考虑非饱和土基质吸力-回弹指数相关性的弹塑性本构模型及有限元实现

非饱和土压缩指数受基质吸力影响明显,回弹指数是土的临界状态本构模型中的重要变形参数。但在本构模型理论研究中,回弹指数随基质吸力的变化影响往往被忽略。为解决这一矛盾,以杭州地区重塑粉土为试样,使用GDS非饱和直剪系统的竖向加载功能,对试样进行50、100、200 kPa 3种定吸力条件下一维固结回弹试验,获取不同基质吸力非饱和回弹指数κ(s),并拟合得到κ(s)函数关系,对比已有试验研究,验证了拟合结果的合理性。进而将κ(s)表达式引入非饱和土扩展剑桥弹塑性本构模型,形成一种考虑回弹指数随基质吸力变化的非饱和土弹塑性本构模型。应用子增量步显示积分算法将该模型编辑成ABAQUS有限元UMAT子程序,最后基于ABAQUS有限元平台结合自编UMAT子程序进行固结回弹数值试验,模拟实际试验过程,对比两者试验结果,验证了该UMAT子程序的正确性。实现了一种考虑回弹指数随基质吸力变化的非饱和土弹塑性本构模型的数值化,为其进一步应用建立基础。

原状膨胀土二维胀缩各向异性试验研究

膨胀土的胀缩变形具有方向性,存在各向异性的问题,横向胀缩变形对膨胀土边坡工程影响不可忽视。为研究原状膨胀土的二维胀缩各向异性,采用简易方法制备竖向、横向原状土样,进行干湿循环、不同初始状态、荷载作用下的膨胀土二维膨胀与收缩各向异性试验。结果表明,竖向胀缩变形大于横向胀缩变形,竖向、横向最大膨胀率与最大收缩率不相等,原状膨胀土二维胀缩变形是不可逆的;干湿循环过程中,原状膨胀土的绝对膨胀率、相对膨胀率、绝对收缩率与相对收缩率,竖向数值均大于横向数值;原状膨胀土的胀缩各向异性与含水率、干密度等初始状态无关。

膨胀土地基膨胀特性对无砟轨道路基变形性能的影响

为研究膨胀土地基浸水膨胀行为对无砟轨道路基力学与变形特征的影响,首先,基于温度场等效湿度场理论,引入材料热属性,通过热-力耦合分析方法模拟膨胀土的膨胀特性;其次,建立双线铁路无砟轨道-基床-膨胀土地基的三维数值模型,并验证其可靠性;最后,研究膨胀土地基的膨胀潜势(以热膨胀系数表征)和膨胀区域(以膨胀区域纵向长度表征)对无砟轨道路基力学与变形性能的影响。研究结果表明:在局部浸水情况下,膨胀土地基的膨胀行为引发了无砟轨道结构的隆起、纵向不平顺和侧向倾斜现象;这些现象随着膨胀土地基的膨胀潜势增大和膨胀区域扩大而更加明显;当膨胀土地基中的膨胀区域沿路基纵向长度超过10 m后,无砟轨道结构的侧倾角度增幅很小;基床层填料的压缩引起路基隆起变形衰减,其衰减率随地基膨胀潜势增大而增大,随地基膨胀区域扩大而减小。本文提出将无砟轨道结构的隆起量、隆起纵向不平顺性以及侧倾角度同时作为膨胀土地区无砟轨道路基的性能控制指标;建议采取降低路基隆起变形横向分布不均匀性和膨胀土地基湿度分布梯度的措施,以减小无砟轨道结构的侧倾程度。

膨胀性裂土病害产生机理及综合防治措施研究

膨胀土是指吸水后显著膨胀、失水后显著收缩的高塑性黏土,随着膨胀土问题的日渐加剧,膨胀土对施工环境的影响已成为当今岩土工程中亟待解决的全球性技术难题之一。在文献调研、资料收集及成果汇总的基础上,对膨胀土的基本性质、胀缩机理进行了总结,从病害产生部位、形态特征、成因及破坏速度等角度系统划分了膨胀土“四类”破坏类型;通过堑坡深度、垂直方向、坡顶及坡底水平方向变形量提出了膨胀土堑坡“四个”活动层面;结合膨胀土地区路基、边坡、建筑物及其他各类病害特点,总结了相对应的技术措施,研究成果对于膨胀土地区在建和拟建工程有参考意义。