南阳膨胀土的工程地质特征和填筑适宜性

施工阶段的工程地质调查发现,宁（南京）西（西安）铁路内乡-镇平段南阳膨胀土成因类型上不仅有第四系冲积洪积粘土,而且发育上第三系湖相沉积粘土。不同成因的膨胀土具有不同的空间分布特征和工程性质,X射线衍射测试表明,南阳膨胀土的主要膨胀性粘土矿物是伊利石/蒙脱石混层矿物。物质组成和物理化学性质差异导致击实土在含水量变化条件下膨胀力和膨胀变形有明显的差别;对同一种膨胀土材料,其击实含水量越小,膨胀性越强,甚至含水量降低1 %～2 %,膨胀力就可以增加到200 %以上;击实膨胀土的直剪强度受到钙质结核含量的影响,对于膨胀势为高或强的膨胀土,结核含量小于15 % 时,经过干湿循环后直剪强度衰减量为40 %～65 %,结核含量为25 %～30 % 的强膨胀土直剪强度衰减量则低于30 %;对膨胀土的地质特征、工程性质、击实土的膨胀性和直剪试验结果进行分析,探讨其用于填筑铁路路基的适宜性,进行了填筑适宜性分级。依据研究成果制定的路基施工工艺取得了比较理想的工程效果     

Osmotic Swelling and Osmotic Consolidation Behaviour of Compacted Expansive Clay

Compacted clay soils are used as barriers in geoenvironmental engineering applications and are likely to be exposed to salinization and desalinization cycles during life of the facility. Changes in pore fluid composition from salinization and desalinization cycles induce osmotic suction gradients between soil–water and reservoir (example, landfill/brine pond) solution. Dissipation of osmotic suction gradients may induce osmotic swelling and consolidation strains. This paper examines the osmotic swelling and consolidation behaviour of compacted clays exposed to salinization and desalinization cycles at consolidation pressure of 200 kPa in oedometer assemblies. During salinization cycle, sodium ions of reservoir fluid replaced the divalent exchangeable cations. The osmotic swelling strain developed during first desalinization cycle was 29-fold higher than matric suction induced swelling strain of the compacted specimen. Further, the diffusion controlled osmotic swelling strain was 100-fold slower than matric suction-driven swelling process. After establishment of ion-exchange equilibrium, saturated saline specimens develop reversible osmotic swelling strains on exposure to desalinization cycles. Likewise the saturated desalinated specimen develops reversible osmotic consolidation strains on exposure to cycles of salinization. Variations in compaction dry density has a bearing on the osmotic swelling and consolidation strains, while, compaction water content had no bearing on the osmotic volumetric strains.     

Influence of Cyclic Wetting Drying on Collapse Behaviour of Compacted Residual Soil

The climatic zones where residual soils occur are often characterized by alternate wet and dry seasons. Laboratory studies of earlier workers have established that the alternate wetting and drying process affects the swell-shrink potentials, water content, void ratio and particle cementation of expansive soils. The influence of cyclic wetting and drying on the collapse behaviour of residual soils has not been examined. This paper examines the influence of alternate wetting and drying on the collapse behaviour of compacted residual soil specimens from Bangalore District. Results of such a study are useful in anticipating changes in collapse behaviour of compacted residual soil fills. Experimental results indicated that the cyclic wetting and drying process increased the degree of expansiveness of the residual soils and reduced their collapse tendency. Changes in the swell/collapse behaviour of compacted residual soil specimens from wetting drying effects are attributed to reduction in water content, void ratio and possible growth of cementation bonds.     

Impact of Carpet Waste Fibre Addition on Swelling Properties of Compacted Clays

Municipalities and recycling and environmental authorities are concerned about the growing amount of carpet waste produced by household, commercial and industrial sectors. It is reported that 500,000 tonnes of carpet waste fibre are plunged into landfills annually in the UK. In the United States of America, around 10 million tonnes of textile waste was generated in 2003. In geotechnical engineering, expansive clay soils are categorised as problematic soils due to their swelling behaviour upon increase in the moisture content. The problematic nature of such soils is intensified with the increase in the plasticity index. This paper presents results of a comprehensive investigation into utilisation of carpet waste fibres in order to improve the swelling characteristics of compacted cohesive soils. Therefore, two different clay soils with markedly different plasticity indices (i.e. 17.0 and 31.5 %) were treated with two different types of carpet waste fibre. Waste fibres were added to prepare specimens with fibre content of 1, 3 and 5 % by dry weight of soil. Soil specimens with different dry unit weights and moisture contents were prepared so as to the swelling behaviour of fibre reinforced compacted clays is completely attained under various scenarios. The results indicated that the behaviour of the fibre reinforced soils seems highly dependent on the initial compaction state and secondary on the moisture content. It was found that the swelling pressure drops rapidly as the percentage of fibre increases in samples prepared at the maximum dry unit weight and optimum moisture content. Reducing the dry unit weight, while maintaining constant moisture content or increasing the moisture content at constant dry unit weight was found to reduce the swelling pressure.     

Shear strength of a compacted scaly clay in variable saturation conditions

Scaly clays are stiff and highly fissured clays often used as construction materials. This paper presents the results of triaxial compression tests carried out on saturated and unsaturated samples of a compacted scaly clay. Complementary investigation on the microstructural features and their evolution with the amount of water stored into the material are also presented in order to shed light on the evolution of the micro- and macroporosity with suction. The water retention behaviour of the compacted scaly clay is also addressed. The results from the controlled suction triaxial tests are used to discuss the applicability of a single-shear strength criterion to compacted double-structured clays when the effective stress concept for unsaturated soils is used. The choice of the degree of saturation to be included in the effective stress definition for obtaining a satisfactory representation of the shear strength is addressed. It is shown that the best results are obtained when the macropore degree of saturation is considered along with its evolution during the applied stress path.     

An advanced hydro-mechanical constitutive model for unsaturated soils with different initial densities

This paper presents an advanced constitutive model for unsaturated soils, using Bishop’s effective stress (σ′) and the effective degree of saturation (S_e) as two fundamental constitutive variables in the proposed constitutive model. A sub-loading surface and a unified hardening parameter (H) are introduced into the σ′–S_e modelling framework to interpret the effects of initial density on coupled hydro-mechanical behaviour of compacted soils. Compared with existing models in the literature, the main advantage of the proposed model that it is capable of modelling hydro-mechanical behaviour of unsaturated soils compacted to different initial densities, such as the dependence of loading–collapse volume on initial void ratio and density effect on the shearing-induced saturation change. The proposed model requires 13 material parameters, all of which can be calibrated through conventional laboratory tests. Numerical studies are conducted to assess the performance of the model for a hypothetical soil under two typical hydro-mechanical loading scenarios. The proposed advanced unsaturated soil model is then validated against a number of experimental results for both isotropic and triaxial conditions reported in the literature.     

Determination of Swelling and Shrinkage Properties of Undisturbed Expansive Soils

Expansive soils exhibit large volume changes when their water content changes. Alternate heave and settlement due to seasonal climatic variations result in distress and damage in civil infrastructure systems. This research focuses on the understanding of swelling and shrinkage phenomenon in the surface layer of expansive soils. Undisturbed field samples were used to capture the effect of in situ conditions (geologically induced fissuring and environmentally caused saturation) on volume change properties of Regina clay. Based on laboratory investigations, the swelling potential and swelling pressure of the native clay at S = 82% were found to be 1.5% and 3.5 kPa, respectively. The swell-shrink path during progressive soil drying followed an S-shaped curve comprising of an initial low structural shrinkage followed by a sharp decline during normal shrinkage and then by a low decrease during residual shrinkage. The soil microstructure correlated well with the observed volume change behaviour as well as with the consistency limits. The presence of fissures in field samples at various degrees of saturation confirmed that the investigated deposit is at an equilibrium condition with respect to the swell-shrink phenomenon. The swelling properties at any initial saturation state were estimated using the free swelling test and the swell-shrink test data in conjunction. The swelling potential increased 12 times (from 2 to 24%) and the swelling pressure increased by two orders of magnitude (from 27 to 2500 kPa) with a change in the degree of saturation from 80% (at the plastic limit) to 60% (at the shrinkage limit).     

路基粒状填土的旋转压实试验

尽管击实试验是目前最流行的室内土体压实试验方法,但实际上与现场任何一种压实方法都无相似之处,且不适用于无粘性土（粒状土）的压实试验。无粘性土的振动击实试验方法也存在很多问题。现代重型压实设备的使用,路基粒状填土的压实密度达到了目前室内规范试验方法不可能达到的水平。介绍了美国采用供高性能沥青路面混合料设计与质量控制之用的旋转压实机对粒状土进行的压实试验。其结果表明,旋转压实试验能有效模拟路基粒状填土的现场压实特征;可采用压实压力200 kPa、旋转角为1.25°、旋转次数为90、旋转速度为20 rpm的试验结果来控制粒状填土路基的现场压实质量。     

Collapse behaviour of a lateritic soil

Collapsible soils are problematic by nature. They undergo collapse or sudden settlement or subsidence under a given stress when their water content is increased. Collapse is characterised by collapse potential expressed as ΔH/H x 100, where ΔH is collapse compression. The amount of stress applied and the water content at the time of collapse govern the amount of collapse. In other words, collapse potential depends upon the amount of stress and the water content. Loess and other wind-blown silts are examples of collapsible soils. This paper presents a parametric study on the collapse behaviour of a lateritic soil. Remoulded specimens of a lateritic soil passing through a 425 µm sieve were compacted in 1-D consolidometer or oedometer at a density of 13 kN/m³ for studying collapse under varied initial moisture contents and initial surcharge pressures. Collapse compression of the samples was induced by saturating the specimens with water contents corresponding to 100% degree of saturation. After collapse occurred, the 1-D consolidation tests were continued up to an applied stress of 160 kPa. Collapse behaviour was studied for the applied initial pressures (σ_i) of 10, 20 and 40 kPa and for the initial water contents (w_i) of 5%, 10%, 15% and 20%. Collapse compression and collapse potential decreased with increasing w_i for all σ_i.     

宽级配砾质土作为GCls防渗垫保护层的抗剪强度试验研究

水化后的土工织物黏土垫层(GCls)是良好的复合防渗材料,但同时表现出较低的抗剪强度特性。为改善水化后的GCls抗剪强度低的缺陷,拟采用取自天然并经人工调配的宽级配砾质土料代替黏土作为GCls防渗垫的保护层,共同构成复合防渗系统。文中利用TGH直剪摩擦拉伸仪对筛选出的宽级配砾质土及其与不同水化条件下的GCL接触面进行了试验研究,得到宽级配砾质土样及其与质量含水率分别为50%和完全水化的GCL接触面的抗剪强度试验数据并进行了整理分析。结果显示,宽级配砾质土样的抗剪强度大于其与不同水化条件下的GCL接触面的抗剪强度,且宽级配砾质土与GCL接触面的抗剪强度随GCL含水率的增加而减小。因此,可以根据外荷载作用下土中应力会发生扩散的原理,利用抗剪强度高和压缩性低的砾质土来承担部分甚至全部荷载,以弥补水化后的GCls抗剪强度低的缺陷。     

路基煤矸石填料土-水特征曲线试验研究

煤矸石作为煤层开采和洗选过程中产生的废弃料,常用于路基填筑。压实的路基煤矸石填料存在双孔隙群结构,其土-水特征曲线（SWCC）具有双台阶现象。而SWCC与土体非饱和渗透系数密切相关,对路基内部水分运移以及路基长期累积变形有重要影响。为了探究压实度、粉粒含量、最大颗粒粒径对双台阶SWCC的影响,对路基煤矸石填料进行了一系列压力板试验,并采用描述双台阶SWCC的Bi-V-G公式拟合实测SWCC,分析了模型参数变化规律。试验结果表明,粉粒含量为5%时,不同压实度试样SWCC均存在双台阶现象;粉粒含量为50%时,随着压实度增大,试样SWCC由双台阶形过渡为单台阶形,在0.90和0.95压实度之间存在一个界限值,双台阶现象消失;增大压实度、提高粉粒含量或减小最大颗粒粒径,均会使SWCC台阶高差减小。随着压实度增大或粉粒含量提高,模型参数 、 （与进气值有关的参数）和 、 （与排水程度有关的参数）值均逐渐减小,试样大、小孔隙部分均进气值增大,持水能力提高。     

Effects of Post-compaction Residual Lateral Stress and Electrolyte Concentration on Swelling Pressures of a Compacted Bentonite

The effects of post-compaction residual lateral stress and salt concentration in the hydrating fluid on swelling pressures of compacted MX80 bentonite is brought out in this paper. In order to release the residual lateral stresses, following the static compaction process during preparation of specimens, compacted bentonite specimens were extruded from the specimen rings and then inserted back prior to testing them for swelling pressures in isochoric condition. The swelling pressure tests were carried out at several dry densities of the bentonite with distilled water and solutions of NaCl (0.1 and 1.0 M) as the hydrating fluids. With water, the test results showed that specimens that underwent extrusion and insertion processes exhibited about 10–15 % greater swelling pressures as compared to the specimens those that were compacted and tested. The influence of saline solutions was found to reduce the swelling pressure of the bentonite, but their impact was less significant at high compaction dry densities.     

Experimental Set-up for Determining Soil Water Retention Curves for Granular Soils During Drying

Abstract: Soil water retention curves (SWRCs) provide an important means of describing the response of unsaturated soils during drying / wetting processes in terms of variations of degree of saturation, water content or void ratio with suction. A key consideration in generating these curves is how to measure the suction. Frequently the filter paper technique is adopted, especially when high suctions are developed, e.g. with plastic clays. As each measurement takes at least a week with this technique, it can take months or years to generate a full SWRC in drying and wetting. Developments in laboratory tensiometers now allow matrix suctions up to about 1.5 MPa to be measured. With such a device it is possible to develop SWRCs for granular soils such as silts and clays in hours or days by continuous measurement. This paper describes an experimental set-up that was developed to measure changes in volume, water content and matrix suction during drying of three granular soils. Limitations of the apparatus and usefulness of the curves are discussed.     

压实粉土非线性应力-应变关系的试验研究

对京九线路基粉土用GDS三轴仪进行固结不排水剪试验,研究不同围压、含水量、压实度下压实粉土样的应力-应变关系。结果表明:压实粉土在低围压下为应变软化,高围压下为应变硬化,存在一个屈服应力;含水量和压实度对粉土强度和应力-应变关系影响显著,随着含水量的减小、压实度的增大,压实粉土样强度增大,应力-应变曲线上升。对围压小于屈服应力的应变软化曲线,用常规的土的软化关系式进行拟和;对围压大于屈服应力的应变硬化曲线,用邓肯-张模型进行拟和;这二者的结合能描述压实粉土的应力-应变关系曲线,效果良好。     

广西红粘土击实样强度特性与胀缩性能

通过对广西贵港红粘土重型击实样的室内试验研究,探讨了其力学特性、胀缩性能、孔径分布特征与含水量之间的关系。结果表明：干密度指标总体上能反映红粘土击实样的强度规律,但非饱和击实样强度峰值对应的含水量因基质吸力作用而偏小,饱和后土体由于吸水膨胀与基质吸力的消失,使得强度峰值对应含水量较饱和前明显增大,红粘土在最优含水量下压实,虽可获得很高的压实度,但饱和后的强度并非最大;红粘土击实样的胀缩性能主要由含水量决定,同时,受到干密度的影响;孔隙主要以孔径在0.01～0.05μm范围内的小孔隙为主,为进一步掌握红粘土的工程力学特性提供了帮助。     

荆门非饱和膨胀土的变形与强度特性试验研究

通过压力板试验,对比分析了荆门原状膨胀土与石灰改良土持水特征的差异性;应用非饱和土三轴仪,开展了原状膨胀土、石灰改良膨胀土与重塑膨胀土的变形及强度特性试验。结果表明：①石灰改良膨胀土与原状膨胀土相比,其进气值有显著降低,残余含水率显著升高,土-水特征曲线两个特征点的斜率较为平缓,说明其水稳定性较好,土体性状更为稳定;②原状膨胀土和石灰改良膨胀土在非饱和与饱和状态的应力-应变关系曲线均呈应变软化型,随净围压的增大,应变软化的程度趋缓;随含水率的减小,峰值应力增大,应力峰值点有所提前,应变软化现象更加显著。饱和重塑膨胀土的应力-应变关系呈应变强化型,非饱和重塑膨胀土则呈现为应变软化型,但其应变软化的程度较前两类土大为趋缓;③经石灰改性后,膨胀土强度参数值有大幅度提高,即使在湿化饱和后,石灰改良膨胀土仍保持了相对稳定的力学性质和较高的抗剪强度参数值。相比之下,无论是重塑膨胀土,还是原状膨胀土,对湿化作用均十分敏感,其强度参数值或波动较大,或整体水平较低。     

非饱和填土侧限压缩变形特性试验研究及应用初探

为研究吸力及压实度对非饱和压实填土压缩变形特性的影响,并建立脱湿（吸力增长）状态下的填方土体工后沉降变形修正计算应用模型,开展了控制吸力和压实度的一维非饱和土侧限压缩试验。结果表明：压缩曲线平缓段随压实度和吸力的提高而增长,表明土样的结构屈服强度同步得到提高;在比容v变化差值序列上,饱和土压缩土样最大、常规压缩土样次之、脱湿土样最小,并且吸力越大,比容v变化差值越小。经历脱湿（吸力增长）后的压实土压缩性降低,定义和建立了吸力压缩系数 及其经验模型,用以表征和度量吸力和压实度对压缩特性的影响规律及程度,发现 随竖向应力增加呈现指数型衰减。对模型参数与压实土相关参量间的关联性进行了分析探讨,同一压实度土样不同吸力所对应的参数 均值随压实水平提高而线性减小,参数λ总体上随吸力的增加而增大,但随压实度的提高吸力对于土体抗压性增强的贡献水平降低且参数 试验数值点靠拢线性趋近线,土样的压缩性随压实度和吸力大小变化而动态调整。基于分层总和法的基本原理初步构造建立了高填方非饱和填土压缩变形修正计算模型（MS）,其应用途径应建立在进一步对非饱和压实填土受荷状态下脱湿土-水特征关系的研究基础之上。     

Evaluation of Engineering and Cement–Stabilization Parameters of Clayey–Sand Mixtures under Soaked Conditions

Clay soils, especially clay soils of high or very high swelling potential often present difficulties in construction operations. However, the engineering properties of these clay soils can be enhanced by the addition of cement, thereby producing an improved construction material. Higher strength loss of cement stabilized clay soils after soaking in water is attributed to water absorbing capacity of the clay fraction (e.g. montmorillonite). Kaolinite and illitic soils are largely inert and resist to water penetration. These clays generally develop satisfactory strengths resulting to low strength reduction [Croft, 1967]. The swelling clays such as bentonite soaked in water, due to environmental conditions, result to volume increase causing macro and micro-fracturing in engineering structures. These fractures accelerate water penetration and consequently cause greater strength loss [Sällfors and Öberg-Högsta, 2002]. The water intrusion during soaking creates swelling and disrupts the cement bonds. The development of internal and external force systems in soil mass, due to soaking conditions, establish the initiation of slaking. Internal force system of a stabilized clayey soil consists of the resultant stresses established by the bonding potential of a cementing agent and the swelling potential of a clay fraction. In an effort to study this influence of soaking conditions and final absorbed water content on the stabilization parameters (cement, compaction, curing time), both unconfined compressive strength and slaking (durability) tests were carried out on two different cement stabilized clayey mixtures consisted of active bentonite, kaolin and sand.     

Global representation of the drying–wetting curves of four engineering soils: experiments and correlations

The main objective of this paper is to examine how different engineering soils react to environmental variations and to provide correlations to characterize their behaviour under null external mechanical stress. Two French and two Algerian soils with liquid limits ranging from 36 to 112 were prepared under both slurry and Proctor compaction conditions, and then subjected to drying–wetting paths with suction controlled from several kPa to several hundreds of MPa. Experimental results are presented in five diagrams to show globally and simultaneously the shrinkage–swelling, saturation–desaturation and water retention characteristics. A reasonable consistency was observed between the oedometric and drying curves of slurry, confirming the equivalence between hydraulic loading (suction) and mechanical loading (consolidation stress) on the volume change behaviour of different soils. As an intrinsic parameter of soil nature, liquid limit was found to have a significant influence on the shrinkage limit, air-entry suction and compressibility of both slurry and compacted samples. For that reason, correlations between these characteristics and liquid limit were set up, providing a good basis for a first estimation of the drying–wetting curves. At the micro-scale, new experimental results were obtained: either on drying or wetting path, the micro-pores were almost unaffected, whereas, when matrix suction increased from 0.1 to 8 MPa, the volume of macro-pores decreased to quasi-closure. At last, the analogy between the compaction and drying–wetting curves, and the comparison of different methods to determine the water retention curve were addressed. Such analogies and comparisons contribute to a better understanding of the mechanisms of mechanical stress and suction.     

Modeling soil collapse by artificial neural networks

The feasibility of using neural networks to model the complex relationship between soil parameters, loading conditions, and the collapse potential is investigated in this paper. A back propagation neural network process was used in this study. The neural network was trained using experimental data. The experimental program involved the assessment of the collapse potential using the one-dimensional oedometer apparatus. To cover the broadest possible scope of data, a total of eight types of soils were selected covering a wide range of gradation. Various conditions of water content, unit weights and applied pressures were imposed on the soils. For each placement condition, three samples were prepared and tested with the measured collapse potential values averaged to obtain a representative data point. This resulted in 414 collapse tests with 138 average test values, which were divided into two groups. Group I, consisting of 82 data points, was used to train the neural networks for a specific paradigm. Training was carried out until the mean sum squared error (MSSE) was minimized. The model consisting of eight hidden nodes and six variables was the most successful. These variables were: soil coefficient of uniformity, initial water content, compaction unit weight, applied pressure at wetting, percent sand and percent clay. Once the neural networks have been deemed fully trained its accuracy in predicting collapse potential was tested using group II of the experimental data. The model was further validated using information available in the literature. The data used in both the testing and validation phases were not included in the training phase. The results proved that neural networks are very efficient in assessing the complex behavior of collapsible soils using minimal processing of data. This revised version was published online in July 2006 with corrections to the Cover Date.