Performance of constitutive models in predicting behavior of remolded clay

The performance of several soil constitutive models was evaluated by comparing experimental results and simulated behavior of a medium plasticity clay. Input parameters for the soil constitutive models were obtained from triaxial compression and extension tests on normally and overconsolidated medium plasticity clay. The soil models employed for this study were the Cam Clay, Modified Cam Clay, 3-SKH, and S-CLAY1 models. In order to investigate the influence of some of the input parameters on the performance of the models, sensitivity analyses were also performed. The comparisons demonstrate that the Cam Clay model was able to predict the normally consolidated compressive behavior of medium plasticity clay. Both 3-SKH and Cam Clay models were able to produce acceptable predictions for stress?Cstrain and stress path behavior for overconsolidated compression of the soil. The 3-SKH model did not perform satisfactorily for predicting pore pressure variations, while the Cam Clay model demonstrated fairly acceptable predictions. For the normally consolidated reduced extension test, the Modified Cam Clay and S-CLAY1 models performed better than the Cam Clay and 3-SKH models in predicting the stress?Cstrain curve, pore pressure variations, and stress path.     

Sensitivity Analysis of Selected Input Parameters for an Advanced Constitutive Model

Experimental results and simulated behavior of a medium plasticity clay were compared to investigate the performance of an advanced soil constitutive model. The soil model employed for this study was the 3-SKH model. The performance of this model was compared to the performance of two other critical state models; the Cam Clay (CC) and Modified Cam Clay (MCC) models. In addition, the influences of some of the input parameters on the performance of the 3-SKH model were investigated by performing sensitivity analyses. The comparisons demonstrated that the CC model was able to predict normally consolidated compressive behavior of the remolded medium plasticity clay. The compression behavior of overconsolidated clay was better captured by both the 3-SKH and CC models. For extension behavior of the normally consolidated samples, the MCC model performed better than the CC and 3-SKH models in predicting the stress path. The 3-SKH model was found to be very sensitive to varying the exponent in the hardening function. In particular, a difference between the evaluated values as small as 0.01 had a noticeable effect on the predicted stress–strain values for the overconsolidated compression and extension samples.     

Ground disturbance caused by shield tunnelling in a stiff, overconsolidated clay

Attewell, P.B. and Farmer, I.W., 1974. Ground disturbance caused by shield tunnelling in a stiff, overconsolidated clay. Eng. Geol., 8: 361–381.

Some of the factors affecting ground deformation around shield tunnelling excavations in stiff clays are considered. There is particular reference throughout the paper to an analysis and interpretation of measured ground deformation around a 4.146-m diameter, hand-excavated, shield-driven tunnel at a nominal axis depth of 29.3 m in the overconsolidated London Clay. The maximum surface settlement was found, by precise levelling, to be 6.1 mm but the shape of the transverse surface settlement profile conformed to a normal probability curve only up to the time of shield passage. Of the contributory ground losses at the tunnel, yield of the clay at the tunnel face appears to dominate to the extent of generating up to 50% of the eventual surface settlement. Measurement evidence suggests a rate of yield at the face that is 2 to 3 times the radial yield over the shield and implies that up to about one-fifth of the surface settlement could be attributed to radial yield into the grouted sections of the erected tunnel lining.     

Characteristics and distribution of clay minerals and their effects on reservoir quality: Huagang Formation in the Xihu Sag,East China Sea Basin

Abstract

The characteristics and distribution of clay minerals and their effects on reservoir quality in the Huagang sandstones in the Xihu Sag, East China Sea Basin were studied by using X-ray diffraction, casting thin-sections, scanning electron microscopy, electron microprobe analysis, fluid inclusion analysis, constant-rate mercury injection and nuclear magnetic resonance. Clay minerals consist of kaolinite, chlorite, illite and illite–smectite mixed layer (I/S); kaolinite forms from dissolved feldspars, chlorite occurs as clay coatings that are transformed from clay precursors owing to the flocculation of suspended detrital clays or the crystallisation of pore fluids, and illite forms from the illitisation of detrital smectite, authigenic kaolinite and K-feldspars. Clay distribution is controlled by sedimentary environments, burial history and lithologies. Typical reservoirs in the western sub-sag are thin and developed in braided river facies at relatively shallow burial depths with clays dominated by kaolinite. However, typical reservoirs in the central inversion tectonic zone are thicker and developed in a braided delta front facies at deeper burial depths with clays mainly consisting of chlorite, illite and I/S. High-quality reservoirs are characterised by coarse granularity, high quartz content and low clay content with widespread development of chlorite coatings that inhibit quartz cements at low temperatures. At higher temperatures, the high-quality reservoirs develop more pores providing growth space for quartz cements and result in the coexistence of chlorite coatings and quartz cements. The high-quality reservoirs are controlled by their lithological characteristics rather than chlorite coatings. Illite and I/S clays create severe damage to reservoirs by reducing the size and connectivity of pore-throats.     

Elastic shear stiffness anisotropy and fabric anisotropy of natural clays

Natural clays usually show anisotropic stiffness due to their deposition process and anisotropic in situ stress state. The stiffness anisotropy depends on both of the stress anisotropy and fabric anisotropy, while the latter can be quantified by the stiffness anisotropy at isotropic stress states. This paper measures the K₀ value (i.e., stress anisotropy) and elastic shear stiffness anisotropy of natural Shanghai clay in a triaxial apparatus with horizontal and vertical bender elements. The results show that the K₀ value of Shanghai clay lies in the range of 0.40–0.66, and an empirical equation is proposed to estimate the K₀ value based on the plasticity index and initial void ratio. The fabric anisotropy of natural Shanghai clay lies in the range of 1.2–1.4 with a stronger fabric in the horizontal plane. Moreover, the experimental data of the stiffness anisotropy and fabric anisotropy of different clays in the literature are reviewed and analyzed. It reveals that the stiffness anisotropy generally increases, while the fabric anisotropy remains nearly the same during K₀ consolidation. For normally consolidated clay, the fabric anisotropy generally lies in the range of 1.1–1.7. For overconsolidated clays, the fabric anisotropy generally increases as the overconsolidation ratio increases. Empirical equations are proposed to approximately estimate the fabric anisotropy of clays based on its stress normalized elastic shear stiffness.

     

A critical state model for overconsolidated structured clays

This paper presents a generalised critical state model with the bounding surface theory for simulating the stress–strain behaviour of overconsolidated structured clays. The model is formulated based on the framework of the Structured Cam Clay (SCC) model and is designated as the Modified Structured Cam Clay with Bounding Surface Theory (MSCC-B) model. The hardening and destructuring processes for structured clays in the overconsolidated state can be described by the proposed model. The image stress point defined by the radial mapping rule is used to determine the plastic hardening modulus, which varies along loading paths. A new proposed parameter h, which depends on the material characteristics, is introduced into the plastic hardening modulus equation to take the soil behaviour into account in the overconsolidated state. The MSCC-B model is finally evaluated in light of the model performance by comparisons with the measured data of both naturally and artificially structured clays under compression and shearing tests. From the comparisons, it is found that the MSCC-B model gives reasonable good simulations of mechanical response of structured clays in both drained and undrained conditions. With its simplicity and performance, the MSCC-B model is regarded as a practical geotechnical model for implementation in numerical analysis.     

THE LATE CRETACEOUS SHOSHONITIC ASSEMBLAGE OF CENTRAL KAMCHATKA

Soil materials used to construct the impervious core of the Camligöze dam consist of clay minerals, quartz, feldspar, and calcite. Smectite and chlorite + kaolinite appear to be the dominant clay minerals. Illite-smectite and illite also are present. The soils are predominantly of the CL (inorganic clays, silty clays, sandy clays of low plasticity) type, and to a lesser extent of the MH (inorganic silts of high plasticity) type and gravelly, sandy, and silty clay. The study-area soils are not active, provide low to medium swelling potential, and are impervious when compacted. According to the undrained shear strength when compacted, materials can be regarded as “soft.” Evaluation based on specific gravity, maximum dry unit weight, optimum water content, liquid limit, and the plasticity index results in an assessment rating of the relative desirability of the tested soils for core as 3 (very high level of desirability). For a small quantity (20%) of the soils that are of the MH type, the desirability is 9 (very low level of desirability). The materials are generally usable as impervious material for the clay core.     

黏土矿物古气候意义研究的现状与展望

系统地分析了利用海洋沉积物、古土壤、湖盆沉积物中黏土矿物进行古气候环境研究的现状、存在问题和发展趋势。海洋沉积物的物源范围广,影响因素复杂,其中的碎屑黏土矿物所指示的古气候参数只能用于解释母源区的气候变化,而只有自生黏土矿物才能指示沉积区的气候;古土壤形成于特定的地质背景条件下,尤其是发育于火山物质母岩之上的风化自生黏土矿物,可以准确地指示该区的古气候条件;湖盆沉积物的物源范围小,沉积物中的黏土矿物可以更有效地运用于古气候环境的分析。对于沉积物中黏土矿物来源的分析,可以借助晶体中cv空位和tv空位的精细结构特征进行判断;在风化改造的红土剖面研究中,因强烈的化学风化、淋滤和迁移,黏土矿物方法具有独特的优势。风化过程中形成的一些亚种或过渡性黏土矿物,以及同生沉积过程中形成的黏土矿物,对气候环境的变化更加敏感,应加强这方面的研究。此外,在造山带的气候环境演化研究中,自生黏土矿物稳定同位素可以更可靠地指示气候环境的变化。     

The geotechnical properties and behaviour of a pro-glacial lake clay, and its cementitious stabilization

The Tees Laminated Clay forms a continuous deposit around the Tees estuary in north east England and was deposited in a proglacial lake which occupied the Teesside area during late Devensian times, disappearing about 13 000 years ago. It overlies till, with which it has a sharp contact and reaches a maximum thickness in parts of central Middlesbrough of just over 9 m. The laminations consist of alternating thin layers of clay and silt, and represent seasonal increments of sediment. Kaolinite and illite, with lesser amounts of chlorite, dominate the mineral composition. Quartz tends to account for less than 35%. The notable carbonate content may be diagenetic in origin. The birefringence ratio of the clay minerals indicates a medium degree of particle orientation which suggests a partially flocculated fabric.This laminated clay has a high plasticity. However, the liquid and plastic limits of the silty layers differ significantly from those of the clay layers or the bulk samples. The clay fraction has a greater influence on these limits than the silty fraction. In addition, the linear shrinkage, liquidity indices and consistency indices of the silty layers differ appreciably from those of the clay layers and the bulk samples. The laminated clay is inactive. The orientation of the laminations in relation to the direction of maximum principal stress influences the shear strength, the lowest strengths being developed when these orientations are between 30° and 60°. The stress paths, anisotropic ratios and values of Af indicate that the Tees Laminated Clay is normally consolidated or lightly overconsolidated. The McLamore–Gray concept possibly can be applied to the anisotropic strength behaviour.Tees Laminated Clay can give rise to problems during construction operations since it can lose strength significantly on remoulding. Accordingly, these soils were treated with cementitious materials to determine the extent to which their engineering behaviour was enhanced by the addition of such materials. © Rapid Science Ltd. 1998     

A constitutive thermomechanical model for saturated clays

The structural deformation in clays results from microscopic phenomena involving the mechanical contact-stress change, the physico-chemical variation of repulsive forces in expansive clays, and thermal dilatancy of macropores. These textural strains are associated to three plastic mechanisms represented by respectively the yield surfacesf_Tm, f_R-A andf_T. Under a thermal cycle, the sizes of interlamellar spaces between clay platelets are not modified, hence the temperature cycle is expected to have no effect on repulsive forces and thus the second mechanism is not affected by temperature changes.

This paper suggests a formulation of a model of thermo-elasto-plastic behaviour of non-expansive saturated clays characterised by two plastic mechanisms. The mechanical yield surfacef_Tm of the contact-stress mechanism is based on a modified cam-clay model; the thermal softening yield surfacef_T is a plane separating two thermal domains. In normally consolidated conditions, the resulting response to an increase of temperature is compressive. However, in highly overconsolidated conditions, a small irreversible dilative volumetric strain is observed when the temperature is above a threshold value. In intermediate conditions, the material starts with an expansion and tends to a compression.

The constitutive model combines thermo-mechanical hardening, predominant in normally consolidated states (NCS) and absent in overconsolidated states (OCS) where the thermal softening occurs. The characterisation of the model requires information about rheological parameters obtained from oedometric and triaxial paths. Lastly, some numerical simulations of thermo-mechanical tests onremoulded Boom, ‘Bassin Parisien’ andPontida clays are presented, which show satisfactory agreement between experiments and model predictions.     

有机粘土化学研究进展与展望

有机粘土化学是研究粘土矿物与有机化合物之间相互作用的一门学科,其内容包括粘土矿物对有机化合物的吸附作用,粘土矿物对有机化合物转化的催化作用以及有机化合物对粘土矿物的合成与风化的影响。着重阐述了近20年来有机粘土化学的研究进展,并就这一学科领域的研究作了展望。     

粘土矿物钝化改性和钝化机理初步研究

在岩土工程中,进行灾害防治和软基处理的一个核心问题是把粘土矿物由亲水性改造成疏水性,抑制和降低其含水量,使得粘土遇水后不再吸水膨胀,从而保持土壤的稳定性。岩土力学工程中以保持土壤稳定性为目的对粘土的改性称为“钝化”。所谓钝化就是用 钝化剂与粘土矿物表面发生反应,改变粘土矿物的表面结构,提高憎水性,增加粘土整体的力学性质,从而达到稳定土体的目的。在粘土矿物钝化中表面活性剂是一个重要的组成部分 ,但是表面活性剂在固液界面的吸附,除决定于其本身的性质外,还决定于固体的性质、溶 液的pH值和温度等。因此,研究粘土矿物表面结构和性质以及与钝化剂的相互作用是提高粘 土矿物钝化效果的重要途经。利用近代分析方法（XRD、IR、Raman、MAS NMR、TEM、AFM和S EM）,对粘土矿物（高岭石、蒙脱石和伊利石）钝化前后的表面结构和性质进行了系统研究 ,并结合相关的土力学、工程力学参数的测定,阐明粘土矿物钝化的机理,剖析和研制粘土 矿物钝化剂,为解决各类工程和灾害问题中的土壤稳定提供理论依据。     

Climatic control on clay mineral formation: Evidence from weathering profiles developed on either side of the Western Ghats

Many physico-chemical variables like rock-type, climate, topography and exposure age affect weathering environments. In the present study, an attempt is made to understand how the nature of clay minerals formed due to weathering differs in tropical regions receiving high and low rainfall. Clay mineralogy of weathering profiles in west coast of India, which receives about 3 m rainfall through two monsoons and those from the inland rain-shadow zones (<200 cm rainfall) are studied using X-ray diffraction technique. In the west coast, 1:1 clays (kaolinite) and Fe—Al oxides (gibbsite/goethite) are dominant clay minerals in the weathering profiles while 2:1 clay minerals are absent or found only in trace amounts. Weathering profiles in the rain shadow region have more complex clay mineralogy and are dominated by 2:1 clays and kaolinite. Fe—Al oxides are either less or absent in clay fraction. The kaolinite—smectite interstratified mineral in Banasandra profiles are formed due to transformation of smectites to kaolinite, which is indicative of a humid paleoclimate. In tropical regions receiving high rainfall the clay mineral assemblage remains the same irrespective of the parent rock type. Rainfall and availability of water apart from temperature, are the most important factors that determine kinetics of chemical weathering. Mineral alteration reactions proceed through different pathways in water rich and water poor environments.     

Isotopic exchange of Zn with stable Zn adsorbed on reference clay minerals

For reference clays of low organic content, Zn adsorbed on the clay minerals is in kinetic equilibrium with ⁶⁵Zn in solution. Thus the specific activity approach applied to the transport of ⁶⁵Zn(II) at the water-reference clay interface is intrinsically valid.     

Geochemistry of continental margin sediments from northwest Africa

The geochemistry of the non-carbonate fraction of sediments from the continental shelf and slope off northwest Africa has been investigated using R-mode factor analysis. The geochemistry is governed by a small number of factors controlled quantiatively by glauconite, phosphorite, quartz, clay minerals and secondary iron oxides. The distributions of the factors and of their controls are governed by mineral provenance and by depositional processes. Clay mineral factors predominate in fine grained sediments. Phosphorite, glauconite and oxide factors and quartz predominate in sands. Provenance controls the relative abundances of the minerals of the sand fraction all of which, except a proportion of the glauconite which is clearly recent, are relict and detrital. The recent glauconite is concentrated in poorly oxidised sediments off the southern Sahara. Provenance differences are detected between Saharan and Moroccan clays, and also exist between the clays of north and south Morocco.     

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

Clay minerals formed through chemical weathering have long been implicated in the burial of organic matter (OM), but because diagenesis and metamorphism commonly obscure the signature of weathering-derived clays in Precambrian rocks, clay mineralogy and its role in OM burial through much of geologic time remains incompletely understood. Here we have analyzed the mineralogy, geochemistry and total organic carbon (TOC) of organic rich shales deposited in late Archean to early Cambrian sedimentary basins. Across all samples we have quantified the contribution of 1M and 1M_d illite polytypes, clay minerals formed by diagenetic transformation of smectite and/or kaolinite-rich weathering products. This mineralogical signal, together with corrected paleo-weathering indices, indicates that late Archean and Mesoproterozoic samples were moderately to intensely weathered. However, in late Neoproterozoic basins, 2M₁ illite/mica dominates clay mineralogy and paleo-weathering indices sharply decrease, consistent with an influx of chemically immature and relatively unweathered sediment. A late Neoproterozoic switch to micaceous clays is inconsistent with hypotheses for oxygen history that require an increased flux of weathering-derived clays (i.e., smectite or kaolinite) across the Precambrian-Cambrian boundary. Compared to previous studies, our XRD data display the same variation in Schultz Ratio across the late Neoproterozoic, but we show the cause to be micaceous clay and not pedogenic clay; paleo-weathering signals cannot be recovered from bulk mineralogy without this distinction. We find little evidence to support a link between these mineralogical variations and organic carbon in our samples and conclude that modal clay mineralogy cannot by itself explain an Ediacaran increase in atmospheric oxygen driven by enhanced OM burial.     

Evaluation and Prediction of the Swelling Characteristics of Nigerian Black Clays

The swelling characteristics of black clays from two major areas of occurrence in Borno State, Nigeria, were investigated in the laboratory. Clay samples derived from lagoonal clay deposits showed high swelling tendencies while those samples derived from Olivine basalts showed medium to high swelling tendencies. Although interparticle swelling was the dominant swelling mechanism, soil samples with higher clay contents exhibited higher swelling tendencies. The developed predictive models show that the free swell percentage as well as the swelling pressure can be predicted from measured values of electrical conductivity, specific gravity, clay content and plasticity index. On the other hand, swell percent can be predicted from measured values of electrical conductivity and specific gravity only.     

桩柱结构在地震作用下非线性响应研究

基于试验基础上建立的经典弹塑性模型--剑桥模型能够准确描述正常固结土的应力-应变关系。当土体的应力历史上经历过卸载或受到循环交变荷载作用即进入超固结状态,它作为土的应力历史的反映,相比正常固结土受力特性有着显著的差异。为研究超固结因素对土体加载特性的影响,在引入能考虑超固结状态影响的下负荷面剑桥模型后,通过三轴压缩和剪切试验对处于超固结状态下土体的受力特性进行了对比分析,并对循环剪切加载下的应力-应变关系以及超固结比的演化规律进行了研究。结果表明,下负荷面剑桥模型能准确反映超固结因素对土体力学特性的影响,相比原状土有着更高的屈服强度。而通过数值模拟自由场地基在地震作用下的动力响应可以看出,超固结因素对地基的动力响应起到了不可忽略的影响,尤其在强震下更需要考虑其影响。在自由场地基地震动力响应基础上,通过对桩柱结构桩-土耦合系统在地震作用下非线性动力响应的模拟对土体非线性以及超固结因素的影响进行了对比研究,研究表明：土体的非线性因素能显著降低结构振动响应中的高频成分,由于土体在交变加载下很快进入超固结状态,相对于剑桥模型,下负荷面剑桥模型在考虑超固结因素后土体的承载性能显著提高,尤其在强震作用下超固结因素带来的影响更加明显,因此,建议对桩基结构物地震响应研究考虑超固结因素影响,以提高桩基结构物地震响应模拟的精确度和可靠性。     

Associated and non-associated constitutive relations for undrained behaviour of isotropic soft clays

Incremental theory of plasticity has been used to derive associated and non-associated stress–strain relations for analysing the undrained stress-strain response of isotropic normally and lightly overconsolidated clays. A series of comparisons between the theoretical stress-strain-pore pressure response and laboratory test results for 3 in. × 6 in. samples of very high degree of repeatability and reproduceability have been described. From these comparisons it can be seen that both the associated and, non-associated Cam clay models appear to give consistently the best correlations between the theoretical results and the experimental observations.