Swelling characteristics of East-Africa black cotton soil based on computer molecular simulation

Black cotton soil in East Africa is not a stable engineering material for highway and railroad projects. Its strong swelling potential when it absorbs water causes distresses in subgrade of highway and railroad, and thus leads to failures of the projects. This paper presents study on the swelling characteristics of black cotton soil in East Africa. Lab tests were conducted to obtain its basic engineering properties, and the results show that black cotton soil contains high amount of montmorillonite and exchangeable cations and is strong expansive soil. Molecular modelling was exploited to further investigate water absorption ability of montmorillonite. Three different molecular models of montmorillonite were constructed and used for simulations, among which Types I and II montmorillonite represent the expansive soil montmorillonite in China, and Types II and III montmorillonite represent black cotton soil montmorillonite in East Africa. The results showed that the interlayer cations of Type III montmorillonite possessed the strongest water absorption ability based on analysis of radial distribution function(RDF) of cations. Interlayer compensatory cations of Na+ enhance the hydration ability of the other major cations, thus resulting in the strong swelling potential of East-Africa black cotton soil.

Plasticity,strength,permeability and compressibility characteristics of black cotton soil stabilized with precipitated silica

The suitability of using precipitated silica(PS) from the burning of rice husk was investigated to improve the geotechnical engineering properties of a black cotton soil. A laboratory experimental program consisting of series of specific gravity, Atterberg limits, compaction, California bearing ratio(CBR), unconfined compression and consolidation tests was conducted on the untreated and PS treated soil samples. The application of PS to the soil significantly changed its properties by reducing its plasticity and making it more workable, improving its soaked strength, and increasing its permeability and the rate at which the soil gets consolidated. An optimal PS content of 50%, which provided the highest soaked strength, is recommended for the improvement of the subgrade characteristics of the BC soil for use as a pavement layer material.

Clay mineral expansion of black cotton soil based on interlayer cation hydration

To analyze the water swelling characteristics of black cotton soil(BCS),X-ray fluorescence and X-ray diffraction characterizations were performed to investigate the chemical compositions and types of clay minerals in BCS.A montmorillonite crystal lattice was established to simulate the hydration of interlayer cations by applying the SPC/E potential energy model,universal force field,algorithm of charge balance,and periodic boundary.Results indicated that the main clay mineral found in the BCS was montmorillonite(32.6%)with small amounts of interstratified illite-montmorillonite(10.9%),illite(2.3%),and kaolinite(1.5%).The high expansive potential of BCS comes from the strong adsorption property of montmorillonite with a high content of magnesium and sodium ions to water molecules.The exchangeable cations of Na^(+)in BCS were only 3.73%,but they enhanced the adsorption capacity of clay to water molecules and accelerated the hydration of Mg^(2+)(47.1%)and Ca^(2+)(4.78%).The free swell index can be used as a classification index of the swelling potential of BCS.

Research on replacement depth of black cotton soil based on cracking behavior of embankment

In order to analyze the influence of replacement depth of black cotton soil(BCS)foundation on the initial cracking depth of a highway embankment,the laboratory tests were performed to construct the constitutive relationship between state variables and stress variables of BCS,and the coupled consolidation theory for unsaturated soils was employed to simulate the change in the major principal stress of the subgrade soils caused by water loss shrinkage of BCS with the help of Abaqus 6.11 codes.The simulation results indicate that the water losing shrinkage of BCS causes tensile stress within the subgrade,which leads to embankment cracking.The crack depth decreases with the increase in the BCS replacement depth and the embankment height,and increases with the increase in the burial depth of BCS.In the distribution area of deep BCS,the key values of foundation replacement depth for controlling the crack depth of the embankment with the height of 1 to 4 m are 1.2 and 1.5 m.In the low filling section,when the buried depth of BCS is 2,3 and 4 m,the key values of the foundation replacement depth to control the crack depth of the embankment are 0.8 and 1.2 m.In order to control the embankment cracking induced by the water losing shrinkage of BCS,a reasonable replacement depth of the foundation should be selected while slope protection is carried out well.

Adaptability evaluation of pavement structure to replacement treatment subgrade of black cotton soil

Aiming at the typical engineering problem of black cotton soil(BCS)subgrade under the alternation of dry and wet climate in the region of Nairobi,Kenya,this paper takes the pavement structure as the research object,and the numerical calculation model of BCS subgrade is established based on the consolidation coupling theory of unsaturated soil.Taking the modulus and thickness of the subbase as variables,the deformation characteristics and additional stresses of different pavement structures are analysed.Then the adaptability of different pavement structures to replacement treatment subgrade of BCS is evaluated by gray incidence decision analysis method.The results show that whatever the pavement structure is,neither subgrade modulus nor thickness is sensitive to the pavement surface deformation,and the deformation differences between each pavement structure are more obvious in wet season;the additional stress at control layer bottom and pavement surface decreases with the increase of subbase modulus,whereas the stress may increase at subbase bottom;the additional stress at subbase bottom,control layer bottom and pavement surface all decreases with the increase of subbase thickness for pavement Structure I and II.For pavement Structure III,the change of subbase thickness is not sensitive to the additional stress at the control layer bottom and pavement surface,whereas the stress at subbase bottom increases with the increase of subbase thickness.It is concluded that the most adaptable structure is pavement Structure I,which can minimize the comprehensive level of pavement settlement and additional stress.

非洲黑棉土高速路基病害数值仿真及处治对策分析

文章以内罗毕东环路黑棉土地段路基为研究对象,通过建立高速路基病害分析有限元模型,充分分析病害产生的影响因素,并提出了黑棉土路基病害处治对策。研究发现,随着蒸发的持续,路堤土体结构变形逐渐增加,进而引起土体结构内最大主应力随之逐渐增大;随着换填材料渗透系数的减小,最大主应力峰值逐渐增加;当使用弱膨胀土直接填筑时,可使用弱膨胀土填筑下路堤;考虑对中、强膨胀潜势黑棉土进行物理处治,主要进行包边处治和土工膜处治。

黑棉土地基中的桩基隔胀处理技术研究及效果评价

对黑棉土地区进行桩基隔胀处理具有重要的现实意义。文章提出了一种桩基隔胀处理技术,并用解析方式和数值模拟进行了研究。研究发现,在钻孔灌注桩和土之间设置双层分离式圆筒结构隔胀效果明显,在东非黑棉土地区工程中采用降低桩-黑棉土界面刚度的方法,土体胀缩引起的桩头位移将明显减小;在减小黑棉土胀缩对桩轴力的影响方面,界面折减的效果也比较明显。研究结果对今后类似地区工程建设有重要的参考和借鉴价值。

Load Settlement Characteristics of Model Footings Resting on Surface Treated Coir Fiber Reinforced BC Soil Overlying Loose Stratum

In nature there are situations where in foundation had to be placed over loose soil deposits. In such cases, deep foundations are recommended which increases cost of the structure. Black cotton soils, because of its high swelling and shrinkage characteristics, have been a challenge to geotechnical engineers, increasing the strength of Clayey soil which is formed introducing randomly distributed fibers results in comparatively more homogeneous soil mixture and is one of the popular methods of soil stabilization. In the present study, model footing resting on BC soil reinforced with optimum percentage of treated and untreated coir fiber underlain by loose soil deposit was conducted to study its influence on bearing capacity and settlement. Loose soil deposit has been simulated by compacting sawdust in prefabricated steel tank. BC soil reinforced with optimum percentage of treated and untreated coir fibers has been compacted over loose sawdust and load settlement characteristics were determined at various D/B ratio of 1,2, 3 and for different sized model footings. The results indicated that settlement of model footings is significantly small at lower D/B = 1 and 2 ratios. Bearing capacity of BC soil is also found to be significantly affected by fiber reinforcement.

Strength and compressibility behaviors of expansive soil treated with coffee husk ash

Various factors can affect the durability of roads, such as the strength of sub-grade, the quality of the subbase and base course, the environments and properties of the soil used. Particularly, roads built on expansive soil are susceptible to early damage due to the swelling and shrinkage characteristics of this kind of soil under changing moisture conditions. The most common technique used to improve the properties of problematic soil is stabilization with additives. Using waste materials to improve the properties of expansive soil is a recent trend in soil stabilization. This study deals with the treatment of expansive soil with coffee husk ash(CHA). Coffee husk is a by-product of coffee production, and CHA is the resulting ash after burning it. In this study, the bearing capacity and compressibility characteristics of expansive soil(specifically black cotton(BC) soil) stabilized with varying percentages of CHA(5%, 10%,15%, and 20%) are investigated. Then, scanning electron microscopy(SEM) and energy dispersive X-ray(EDX) were used to analyze the influence of CHA on surface morphology and chemical composition of the studied soil. The results showed that the soil treated with CHA is generally improved in terms of strength.Addition of 20% CHA increases the bearing capacity of the soil by three-fold. In addition, the morphological studies of the soil samples treated with 10% and 15% CHA indicated the formation of hydrated particles and cementitious compounds as a result of the reaction between the soil and CHA. This indicates the potential usage of CHA as a stabilization agent and subsequently, it can address the disposal and environmental concerns related to coffee husk.

Quantification of the effects of management factors on maize (Zea mays L.) and cotton(Gossypium hirsutum L.) residues decomposition rate

Efforts to quantify management effects on decomposition rate of added substrates to the soil is important especially where such information is to be used for prediction in mathematical or simulation models. Using data from a short term (60 days) greenhouse simulation study, a procedure for quantifying effects of management on SOM and substrate decomposition is presented. Using microbial growth rate u (q), microbial efficiency in substrate utilization e (q), specific decomposition rates for added plant residues to two contrasting soils, red earth (Ferrasol) and black earth (Acrisol) were estimated. The treatments included straw addition + buried, (T1); straw addition + mineral N (T2); and straw addition + tillage, (T3). Sampling was done every 15 days. Straw decomposition rate was affected by external mineral N sources (Urea 46% N). Addition of an external N source significantly increased decomposition rates. The study could not, however, fully account for the effect of tillage on residues because of the limited effect of the tillage method due to the artificial barrier to mechanical interference supplied by the mesh bags. It is concluded that using few decomposer parameters, decomposition rates and consequently SOM trends in a soil system can be monitored and quantification of the influence of perturbations on decomposition rate of added substrates possible.

6种植物在室外黑棉土绿墙中的应用

以“桃色火焰”矾根(Heuchera micrantha‘Peach Flambe’)、细叶萼距花(Cuphea hyssopifolia)等6种植物为试验材料,在室外黑棉土绿墙上进行栽培试验,以存活率、生长状态、覆盖度及观赏效果为考察指标,对生长适应性、观赏效果进行综合分析。结果表明:银香科科(Teucrium fruticans)、大吴风草(Farfugium japonicum)、“花叶”薜荔(Ficus pumila‘Variegata’)3种植物适应性强,观赏效果好,适宜应用于成都地区室外黑棉土绿墙;“桃色火焰”矾根(H.micrantha‘Peach Flambe’)不耐夏季强光照直射,可考虑应用于耐阴环境的黑棉土绿墙;细叶萼距花(C.hyssopifolia)枝叶易受低温影响,冬季观赏效果较差,可考虑用于小气候较好的黑棉土绿墙;“火焰”南天竹(Nandina domestica‘Fire Power’)生长适应性差,不适宜应用于成都地区室外黑棉土绿墙。

埃塞俄比亚黑棉土胀缩特性及大气影响深度研究

研究目的:埃塞俄比亚在建亚的斯亚贝巴至吉布提铁路Sebeta-Dire Dawa段,地处东非大裂谷内,为火山喷发物覆盖,地表广泛分布黑棉土,多具强膨胀性,对铁路工程影响大。黑棉土的胀缩特性及大气影响深度直接影响路基设计措施和路堑换填深度,进而影响工程设计质量和投资。因此,有必要对黑棉土的胀缩特性及大气影响深度进行研究,为黑棉土地段路基设计提供依据。研究结论:(1)埃塞俄比亚黑棉土天然密度、干密度小,天然孔隙比大,自由膨胀率、蒙脱石含量、阳离子交换量高,含有机质(灼烧失量),多具强膨胀性等特性;(2)埃塞俄比亚雨季黑棉土在荷载作用下主要表现为压缩,旱季地表土在浸水条件下具有较强的膨胀性;(3)埃塞俄比亚黑棉土(膨胀土)地区大气影响深度为3.5 m,大气影响急剧层深度为1.6 m;(4)本文对埃塞俄比亚类似工程项目的地质勘察和路基设计具有一定的参考意义。

埃塞俄比亚铁路黑棉土勘察方法及工程特性研究

研究目的：黑棉土是埃塞俄比亚及非洲部分国家大面积分布的一种特殊的土层，呈灰黑色、灰褐色，当地俗称黑棉土，一般具有膨胀性。目前中资公司大量进入埃塞俄比亚勘察设计及施工市场，对黑棉土的研究较少，因此，有必要在无既有经验可循的条件下，研究一套适合该国国情的勘察方法，查清黑棉土的工程特性，为铁路工程选择合适的工程措施提供基础资料。研究结论：（1）黑棉土具有中～强膨胀性、中等一高压缩性等工程特性；（2）提出加强路基排水系统设置；堑坡高度≤3m，边坡采用灌草防护；堑坡高度〉3m，边坡采用骨架护坡等防护；边坡较高时，设挡土墙；弱中膨胀土路堑基床底层换填不小于0．5m，中-强膨胀土路堑基床底层换填不小于1m等处理措施；（3）本文研究成果可为埃塞俄比亚的铁路、公路、轻轨交通建设等的勘察设计工作提供指导及借鉴。

高液限膨胀性黏土基桩工作性状的离心机试验研究

通过离心机试验研究高液限强膨胀性黑棉土地基中4根模型基桩在浸水条件下桩头位移和桩身轴力的变化规律,考虑的影响因素包括桩长、桩头荷载以及是否采用隔胀措施。通过自主设计制作的离心机内降雨装置和模型土层中的渗流通道实现黑棉土地基浸水条件。试验发现,黑棉土浸水后对桩身轴力的影响分为两阶段:前期土体刚度降低阶段,黑棉土遇水刚度降低后能提供的摩阻力明显降低,使得桩身轴力增大;后期胀拔阶段:黑棉土遇水膨胀隆起,对桩基产生向上的胀拔力,桩身轴力减小。此外,浸水引起的桩头位移变化与桩顶受荷情况密切相关:未受荷桩发生上拔位移,而荷载为625 k N的桩发生沉降位移。在桩周采取隔胀措施可以削弱甚至消除黑棉土变形对桩基的不利影响。

砂姜黑土区棉花不同叶枝留量对产量的影响

１９９７ －１９９８ 年研究了砂姜黑土区不同叶枝留量对棉花产量的影响，结果表明，留叶枝与常规整枝相比单株成铃增多，铃重大，成铃时空分布合理，烂铃减少，优质铃增加－ 但不同叶枝留量效果不同，Ｔ４ 效果最好，皮棉增产３１－５％ ，霜前花率提高１６－６％ －

黑棉土路基病害特征及成因机制分析

为分析黑棉土路基的主要病害及成因,测试了路基开裂位置基底黑棉土的含水率和路基弯沉,计算了地基胀缩变形量,研究了路堤高度、黑棉土埋深和换填深度对地基变形量的影响.结果表明:深宽裂缝是黑棉土路基的主要病害形式;路堤自重对黑棉土胀缩变形的抑制作用不明显,地基变形主要发生在黑棉土大气影响深度范围内;地基换填材料渗透性大,不能阻止雨水入渗到基底黑棉土中,黑棉土地基发生了显著的胀缩变形;黑棉土换填深度接近大气影响深度时,地基变形量方可满足规范要求;排水顺畅路段雨季后路基弯沉小于设计值,反之,路基强度不能满足设计要求;地基换填深度不足、换填材料渗透性大与路基防排水设计缺陷是诱发黑棉土路基开裂的主要原因.

埃塞俄比亚黑棉土工程特性及路基处理技术研究

黑棉土作为一种特殊岩土有着其自身固有的工程特性,由于国内外勘察方法、技术标准的不同,要求对其重新进行认识、分析。针对非洲特殊的气候特征,分析了该黑棉土的工程特性,并根据国外膨胀土的判定标准对非洲黑棉土的膨胀性进行了判定,得出该黑棉土为膨胀土,具有高膨胀性,低CBR值等特征。为消除黑棉土的膨胀性对公路路基的影响,研究了该区域黑棉土路基处理技术,并根据膨胀土不同的膨胀潜势对路基基底提出了不同的处理措施。

强膨胀土——桩基相互作用及隔胀效果数值模拟

黑棉土是一种强膨胀土,其受湿度变化影响呈现很强的胀缩特性,对桩基的承载力和变形不利,易导致断桩等多种问题。本文建立了强膨胀土桩基有限元模型,分析了胀缩条件下桩基内力和位移的变化规律,与桩土共同作用解析解对比,验证了数值模型的有效性并解释离心机试验现象。利用该模型进行参数分析,指出黑棉土胀缩对桩的不利影响及工程中采用隔胀措施的必要性,进而模拟局部换填和界面隔胀措施。结果表明:浅表层黑棉土适合采用局部换填隔胀,换填宽度宜不小于膨胀土厚度;深厚层黑棉土适合采用双层分离式隔胀,隔胀套管应保证界面折减程度达到土体参数的30%且布置至黑棉土和普通土分界面附近;桩基承台底部宜增加柔性垫层,预留胀缩空间。

肯尼亚黑棉土胀缩等级判定方法研究

以肯尼亚内罗毕南环线黑棉土为依托,在介绍不同膨胀土胀缩等级判定方法的基础上,结合X衍射试验、阳离子交换量试验和室内土工试验结果,对黑棉土的涨缩等级进行了判定。在此基础上针对当地现状,提出了合理的黑棉土涨缩等级分类方法。

谈非洲地区黑棉土地基处理

分析了非洲地区公路建设项目中黑棉土地基的形成及病害特点,并结合埃塞俄比亚高速公路工程实例具体阐述了黑棉土地基处理方案及施工控制措施,为今后海外施工企业处理同类地基提供了借鉴和参考。