Diagenetic alteration of a Mesozoic fluvial gold placer deposit,southern New Zealand

Gold paleoplacers become progressively more affected by diagenetic processes with age and burial. Mesozoic paleoplacer deposits in southern New Zealand display intermediate stages of diagenetic transformation compared to little-affected Late Cenozoic paleoplacers and strongly-affected Paleozoic and Precambrian paleoplacers. The Mesozoic (Cretaceous) diagenesis resulted in near-pervasive alteration, cementation and lithification of the paleoplacer. Lithic clasts and matrix have been extensively altered to illite, ferrous iron-bearing smectite-vermiculite, and kaolinite, and the cement consists mainly of clays and calcite. Diagenetic pyrite, marcasite, vivianite, and Mn oxide also contributed to cementation. Alteration occurred under near-surface (<500 m depth) conditions with groundwater that had circumneutral pH, high alkalinity, and elevated dissolved K, Mg and Ca. Detrital albite remained unaffected by alteration. Detrital gold has been variably dissolved and redeposited, with widespread formation of gold overgrowths on the 1–10 μm scales, with 1–3 wt% Ag. Gold mobility was driven by reduced sulphur complexes in the low redox, high pH diagenetic environment. The overgrowth gold locally contributed to cementation of fine clastic grains, and has intergrown with diagenetic clays and Mn oxide. Post-diagenetic oxidation of the paleoplacer deposit has transformed much of the pyrite to ferric oxyhydroxide and deposited some ferric oxyhydroxide coatings on gold. These oxidation processes have had only minor effects on gold mobility and textures. Hence, the low redox conditions of diagenetic gold mobility were distinctly different from those typically associated with oxidation-related supergene gold mobility. Diagenesis can affect economics of paleoplacer mining by hindering rock disaggregation during processing, coating gold particles with secondary minerals, and increasing the clay content of the deposit, all of which can lower the efficiency of gold recovery.     

Analysis of an illustrative interaction between structural features and earthworm populations in Brazilian ferralsols

Ferralsols are characterized by poorly-defined horizons, weak macrostructure and strong development of a fine granular structure comprising subangular micro-aggregates. In this study, the morphological and physical modifications caused by earthworm activity in a clayey ferralsol were analysed. After describing soil structures, undisturbed samples were taken for evaluating aggregates and determining clod bulk density and particle density. Soil water retention properties were measured and an inventory of soil invertebrate macrofauna was created. The structural and porous transformations were due to aggregates created by earthworm activity. Changes in bulk density can be associated with pore modifications caused by a change in the proportions of aggregate types, and a notable reduction of total porosity was measured, tending to increase soil volume with dense aggregates. Structural modifications affected the topsoil down to 0.5 m and water retention between −1 and −33 kPa, the principal water compartment of these soils.     

陕北晋西砂黄土的胶结物与胶结作用研究

在野外调查的基础上,采用SEM、XRD、土壤物理和土壤化学等方法,对陕北晋西12个县市的48个马兰黄土样品进 行了全分散法粒度组成测定、聚集体分析以及黄土胶结物组成的测试分析。结果表明,砂黄土中的粘土矿物和超细碳酸盐、 游离氧化物和有机质等胶结物质主要是以聚集体包膜的形式存在于碎屑颗粒表面,并构成砂黄土骨架间的结构联结,具有弱 胶结特性。砂黄土中粘土矿物和有机质为非水稳性胶结物质,仅在干燥状态具有较弱的胶结作用;黄土中的CaCO3呈粗颗粒 骨架和细分散超细碳酸钙两种形式存在,并分别起着骨架作用和胶结作用,超细碳酸钙为不可逆的胶结材料,在聚集体形成、 保存以及聚集体与碎屑颗粒之间的联结方面发挥着重要作用;非晶质游离氧化物(SiO2、Fe2O3、Al2O3)含量虽低,但由于他们 是以薄膜形式分布在颗粒表面,在黄土胶结作用中也是不可忽视的,并与上述各种胶结物共同起作用。通过统计分析认为, 凝聚系数可以较好地反映黄土的胶结程度,成为联系黄土胶结特性与黄土物理力学性质之间的桥梁。黄土胶结物组成及其 胶结机理的研究不仅有助于深化砂黄土工程特性的认识,而且对揭示黄土高原表生地球化学环境具有重要意义。     

Stress-strain behaviour of reconstituted illitic clay at different temperatures

Tests on specimens of reconstituted illitic clay have examined the influence of temperature on the mechanical behaviour of clay soils. The program involved consolidation to effective confining pressures up to 1.5 MPa, heating to 100°C, and tests on normally consolidated and overconsolidated specimens with OCR = 2. The tests included isotropic consolidation, undrained triaxial compression with pore water pressure measurement, drained tests along controlled stress paths to investigate yielding behaviour, and undrained tests which involved heating and measurement of the resulting induced pore water pressures. The large strain strength envelope is independent of temperature. However, peak undrained strengths increase with temperature because smaller pore water pressures are generated during shearing. An important contribution from the study is a series of results for the yielding of illitic clay at three different temperatures. For the first time, there is clear evidence of yield loci decreasing in size with increasing temperature. An associated flow rule can be assumed without serious error. The results contribute to the confirmation of a thermal elastic-plastic soil model developed by the authors from cam clay following the addition of a small number of extra assumptions. Depending on the initial stress state, heating under undrained conditions may produce shear failure.     

Improvement of a hypoplastic model to predict clay behaviour under undrained conditions

A shortcoming of the hypoplastic model for clays proposed by the first author is an incorrect prediction of the initial portion of the undrained stress path, particularly for tests on normally consolidated soils at isotropic stress states. A conceptually simple modification of this model, which overcomes this drawback, is proposed in the contribution. The modified model is applicable to both normally consolidated and overconsolidated soils and predicts the same swept-out-memory states (i.e., normal compression lines) as the original model. At anisotropic stress states and at higher overconsolidation ratios the modified model yields predictions similar to the original model.     

Particle orientation and its influence on the mechanical behaviour of isotropically consolidated reconstituted clay

The behaviour of naturally occurring geological materials such as clay and sand depends on many factors. For example, stresses, strains, previous stress history, mineralogy and the depositional environment all contribute in some degree to a characteristic that all natural soils share, namely “structure”. The structure of clay, or more generally, the microstructure of microscopically sized clay mineral particles, is just as important as the many other parameters that are used to quantify the performance of clays. This paper examines the microstructure that results from the particle arrangement brought about during reconstitution in the laboratory and considers its relevance to the resulting stress–strain behaviour.

Samples of reconstituted kaolin clay were produced using two different procedures. In the first series of tests, kaolin slurry was simply isotropically compressed in one increment. In the second series, the slurry was first isotropically compressed to a low pressure and then completely remoulded. This was followed by isotropic compression to the same pressure as the other series. Specimens were taken from the two series of samples, reconsolidated at various isotropic pressures, and sheared under undrained conditions.

Scanning Electron Microscope (SEM) images indicated that the monotonically compressed samples (Series 1) exhibited an anisotropic microstructure that was distinct from the remoulded (Series 2) samples. Significant differences were also found in the consolidation and stress–strain characteristics of the samples produced in the two series.     

Characterization of drying-induced deformation behaviour of Opalinus Clay and tuff in no-stress regime

The drying-induced deformation behaviour of Opalinus Clay and tuff, which are being investigated under international and local collaborative projects for nuclear waste disposal in Switzerland and Japan, was investigated under a no-stress condition in the laboratory to evaluate their generic susceptibility to the formation of excavation damaged zone. The cylindrical core samples of Opalinus Clay and tuff were prepared to a one-dimensional drying condition and submitted to an uncontrolled laboratory environment. The strain evolution, evaporative water loss and environmental entities, such as temperature and relative humidity, were recorded simultaneously and quasi-continuously. It was observed that the drying phase induced significant strain magnitude and damage in Opalinus Clay samples, which was evidenced by the formation of hairy cracks on the surface parallel to the bedding. On the contrary, the strain occurrences in tuff samples were relatively insignificant, and no tendency of cracking was observed. In addition, the quasi-continuous availability of volumetric strains was further used in poroelastic relation for the estimation of capillary suction evolution. The calculated results were validated with pore size distributions obtained from mercury intrusion porosimetry.     

Identifying parameters controlling soil delayed behaviour from laboratory and in situ pressuremeter testing

The aim of this paper is to present a methodology for identifying the soil parameters controlling the delayed behaviour from laboratory and in situ pressuremeter tests by using an elasto‐viscoplastic model (EVP‐MCC) based on Perzyna's overstress theory and on the elasto‐plastic Modified Cam Clay model. The influence of both the model parameters and the soil permeability was studied under the loading condition of pressuremeter tests by coupling the proposed model equations with Biot's consolidation theory. On the basis of the parametric study, a methodology for identifying model parameters and soil permeability by inverse analysis from three levels of constant strain rate pressuremeter tests was then proposed and applied on tests performed on natural Saint‐Herblain clay. The methodology was validated by comparing the optimized values of soil parameters and the values of the same parameters obtained from laboratory test results, and also by using the identified parameters to simulate other tests on the same samples. The analysis of the drainage condition and the strain rate effect during a pressuremeter test demonstrated the coupled influence of consolidation and viscous effects on the test results. The numerical results also showed that the inverse analysis procedure could successfully determine the parameters controlling the time‐dependent soil behaviour. Copyright © 2008 John Wiley & Sons, Ltd.     

An experimental study on static and dynamic bending behaviour of piles in soft clay

Static and dynamic lateral load tests were carried out on model aluminium single piles embedded in soft clay to study its bending behaviour. Model aluminium piles with length to diameter ratios of 10, 20, 30 and 40 were used. Static lateral load tests were conducted on piles by rope and pulley arrangement upto failure and load–deflection curves were obtained. Dynamic lateral load tests were carried out for different magnitudes of load ranging from 7 to 30 N at wide range of frequencies from 2 to 50 Hz. The load transferred to the pile, pile head displacement and the strain variation along the pile length were measured using a Data Acquisition System. Safe static lateral load capacity for all piles is interpreted from load–deflection curves. Dynamic characteristics of the soil–pile system were arrived from the acquired experimental data. The soil–pile system behaves predominantly in nonlinear fashion even at low frequency under dynamic load. The displacement amplitude under dynamic load is magnified by 4.5–6.5 times the static deflection for all piles embedded in soft clay. But, the peak magnification factor reduces with an increase in the magnitude of lateral load mainly because of increase of hysteretic damping at very soft consistency. The maximum BM occurs at the fundamental frequency of the soil–pile system. Even the lower part of the pile affects the pile head response to the inertial load applied at the pile head. The maximum dynamic BM is magnified by about 1.5 times the maximum static BM for model piles in tested consistency of clay. The maximum dynamic BM occurs at a depth of about 1.5 times the depth of maximum static BM for model piles, which indicates an increase of active pile length under dynamic load.     

A highlight of environmental and engineering geology in Fargo, North Dakota, USA

Several unfavorable environmental and engineering geologic conditions exist in Fargo, North Dakota. Dominantly, the behavior of smectitic clays within the proglacial Lake Agassiz sediments of the Sherack and Brenna Formations creates subsoil instability beneath engineered structures in the Fargo area and slope instability within cutbank meanders of the Red River of the North. Unfavorable engineering geologic conditions encountered include: the elastic deformation of clayey glaciolacustrine soils, shrink-swell properties, inadequate bearing capacities, and mass movements. These conditions are responsible for structural failures including the Fargo Grain Elevator in 1955 and the Northern Pacific railroad grade. Bank failures along the Red River are common due to the inherent instability of Brenna Formation smectitic clays which are subject to plastic deformation in the subsurface, with resultant block failure of overlying Sherack Formation. Recent alluvial sediments due to typical fluvial action and the continued seasonal saturation of cutbank meanders within the floodplain also add to soil instability.     

Elastic behaviour and structural evolution of topaz at high pressure

The elastic behaviour and the high-pressure structural evolution of a natural topaz, Al_2.00Si_1.05O_4.00(OH_0.26F_1.75), have been investigated by means of in situ single-crystal X-ray diffraction up to 10.55(5) GPa. No phase transition has been observed within the pressure range investigated. Unit-cell volume data were fitted with a third-order Birch-Murnaghan Equation of State (III-BM-EoS). The III-BM-EoS parameters, simultaneously refined using the data weighted by the uncertainties in P and V, are: V ₀=345.57(7) ų, K _T0=164(2) GPa and K′=2.9(4). The axial-EoS parameters are: a ₀=4.6634(3) Å, K _T0(a)=152(2) GPa, K′(a)=2.8(4) for the a-axis; b ₀=8.8349(5) Å, K _T0(b)=224(3) GPa, K′(b)=2.6(6) for the b-axis; c ₀=8.3875(7) Å, K _T0(c)=137(2) GPa, K′(c)=2.9(4) for the c-axis. The magnitude and the orientation of the principal Lagrangian unit-strain ellipsoid were determined. At P−P ₀=10.55 GPa, the ratios ε₁:ε₂:ε₃ are 1.00:1.42:1.56 (with ε₁||b, ε₂||a, ε₃||c and |ε₃| > |ε₂| > |ε₁|). Four structural refinements, performed at 0.0001, 3.14(5), 5.79(5) and 8.39(5) GPa describe the structural evolution in terms of polyhedral distortions.     

DEM analyses of one-dimensional compression and collapse behaviour of unsaturated structural loess

Natural loess is a kind of under-consolidated and unsaturated loose granulates (silts) with its microstructure characterized with large voids and inter-particle cementation. This paper presents a distinct element method (DEM) to investigate its macro- and micro-mechanical behaviour (compression and collapse behaviour) under one-dimensional (1D) compression condition. A relationship between bond strength in DEM model and initial water content is used to develop a bond contact model for loess. Then, DEM structural loess samples are prepared by the multi-layer under-compaction method, and cemented with the bond contact model. The effect of water content and void ratio on compression and collapse behaviour of loess is numerically investigated by simulating 1D compression and wetting tests on the DEM material. The DEM results agree qualitatively with available experimental observations in literatures. The wetting-induced deformation is independent of the sequence of wetting and loading under 1D compression condition. The macroscopic yielding and collapse behaviours are associated with bond breakage on microscopic scale. Moreover, bonds break in one of the two failure types in the simulations, i.e. tensile failure and shear failure (compression-shear failure and tension-shear failure), with bonds broken firstly mainly due to tension followed by shear when the samples are compressed, while mainly due to shear when the samples are wetted under a certain pressure. In addition, the contact orientations and deviator fabrics of contacts under 1D compression and wetting were also investigated.     

Optical processing of structural contour maps

Optical processing is a fast, easy, economical, and reliable method of quantifying map patterns. A reduced transparency of a zebra-like pattern, obtained by blackening in alternate contour bands of a map, is used as input to an optical system of a laser source and objective lens to produce Fourier transforms. By sampling the intensity distribution across the transform, information about the spatial frequency (periodicity) and orientation in the original map can be inferred. Parameters used for comparing the map images are the values of intensity distribution in the transform. These intensities are normalized and subjected to comparison using various similarity methods. Cross-multiplication of the intensities is suggested as a measure of vectorial comparison of the orientations. By plotting the directional intensities into a polar coordinate system, rose diagrams are prepared for visual comparison. Optically derived parameters are independent of scale and other restrictive and limiting requirements as demanded in techniques of map analyses based on numerically derived parameters. The technique can be used in analyzing other pictorial data including aerial photographs, rock-fabric diagrams, and thin sections.     

重庆地区煤田构造格局及控煤构造样式

重庆市域位于上扬子陆块之川中前陆盆地、米仓山-大巴山基底逆冲带和扬子陆块南部被动边缘褶冲带范围。区域构造格局和构造演化历史复杂,多期构造作用形成了渝东北NW-NWW-EW向和渝西-渝东-渝东南NE-NNE-NW向形态特征。根据重庆地区煤田构造格局,分析了该区控煤构造样式,阐述了构造对煤系赋存的控制程度。研究认为,重庆地区构造复杂,控煤构造样式以挤压压缩构造为主,煤层大部分赋存于褶曲两翼。     

塔北隆起古生代构造样式和构造反演

塔北隆起是塔里木盆地的主要含油气区,笔者利用地震剖面解释和平衡剖面技术对塔北隆起构造变形和构造演化进行了研究.塔北隆起构造变形存在明显的构造差异,西部英买力地区主要发育盖层滑脱式逆冲断层褶皱和y型压扭断层,而其东部主要发育基底卷入式逆断层及断层传播褶皱和反冲断层构造.造成塔北隆起东、西构造样式差异的机理主要有两点:一是东部地区对震旦纪一早奥陶世正断层在后期挤压作用下反转,二是英买力地区中寒武统盐层的发育.塔北隆起地区古生代构造演化也存在着一定的差异,塔北隆起东部震旦纪一早奥陶世发育大型正断层构造,而英买力地区为稳定碳酸盐岩台地;晚奥陶世至泥盆纪,塔北隆起东部牙哈断层发生反转,轮台凸起隆升,而英买力地区位于隆起的斜坡部位;石炭纪至二叠纪,塔北隆起东部大规模抬升,剥蚀严重,出露变质基底,英买力地区接受强烈的变形作用,主要发育盖层滑脱式逆断层和褶皱构造.     

Study on the swelling behaviour of blended clay–sand soils

Soils containing expansive clays undergo swelling that can be both detrimental and beneficial in various applications. In the Arabian Gulf coastal region, natural heterogeneous soils containing clay and sand (tills, shales, and clayey sands) support most of the civil infrastructure systems. Likewise, mixes of clay and sand are used for local earthwork construction such as roads and landfills. A clear understanding of the swelling behaviour of such soils is pivotal at the outset of all construction projects. The main objective of this paper was to understand the evolution of swelling with increasing clay content in local soils. A theoretical framework for clay–sand soils was developed using phase relationships. Laboratory investigations comprised of mineralogical composition and geotechnical index properties of the clay and sand and consistency limits, swelling potential, and morphology of clay–sand mixes. Results indicated that soil consistency of mixes of a local expansive clay and an engineered sand depends on the weighted average of the constituents. Mixes with 10% clay through 40% clay capture the transition from a sand-like behaviour to a clay-like behaviour. Influenced by the initial conditions and soil matrix, the swelling potential of the investigated mixes correlated well with soil plasticity (SP(%) = 0.16 (I _p)^1.188). The parameters sand void ratio and clay–water ratio were found to better explain the behaviour of blended clay–sand soils.     

结构性吹填软土蠕变模型研究

以天津滨海新区吹填场地经过真空预压处理的吹填软土为试验材料,对原状土及其重塑土基本性质与蠕变特性进行了试验研究。试验结果表明：经过真空预压处理后的吹填软土同时具有结构性和蠕变性,结构性的存在,对土体蠕变具有重要的影响。在修正的剑桥模型的基础上,对其进行改进,使其考虑结构性的影响,并以此作为蠕变屈服函数,建立了考虑结构性影响的吹填软土半经验半理论蠕变模型。计算结果表明,该模型符合结构性吹填软土实际,可在工程实践中应用。     

分形结构土体强度研究

分形结构是土体中普遍存在的一种现象,通过对分形结构连接部位所占据的影响域（Voronoi域）的分析,将颗粒连接承载能力换算为影响域上的应力强度,由此获得了一些有益的结果。土体颗粒系统结构分维值的大小对土体强度有着明显的影响;基本结构单元的颗粒数越多,形成的环的长度越长,占据的空间就越大,从力学上讲就越不稳定,因此,其强度越低;土体中分形结构的级数实际上决定了土体中最大孔隙的尺寸,分形结构的级数越多,其相应的最大孔隙越大,等效应力强度会越低;发现偶应力强度与基本结构单元的面积（二维）成反比,即基本结构体越大,偶应力强度就越小,由此可推论出：在其他条件相同的情况下,颗粒越大,偶应力强度就会越低。     

湛江强结构性黏土的物理力学性质指标及相关性分析

湛江黏土具有较强的结构性,结构性对土的力学特性影响不容忽视。对湛江黏土的物理力学指标的相关性和变异性进行统计分析,建立重要指标间的关系经验公式,这可为湛江地区地基基础可靠性设计的参数选取提供依据。结果表明,湛江黏土富含黏粒、黏土矿物含量高、含胶凝状的有机质成分、强胶结特性以及片状颗粒为主的絮凝结构是导致其具有不良物理性质和良好力学特性指标的根本原因。不同区域黏土的力学指标空间变异性较大,但物理指标变异性较小。e-、L-、 L-e、a1-2-e关系具有线性相关性,但相互关系明显不同于其他地区黏土特征,因此,工程设计中不能简单借鉴其他地区黏土经验公式。