红层特殊岩土化学性质工程判别准则试验研究

为评价红层岩土化学特性对工程的影响,对红层岩土pH值、EC值、碳酸钙、芒硝、石膏等关键物质成分的变化规律进行了试验研究,提出了岩土化学稳定性的工程概念与分析思路,初步建立了pH值、EC值、碳酸钙、芒硝、石膏临界含量等指标组成的红层岩土化学稳定性综合判别标准,将红层岩土分为化学稳定性强、化学稳定性中等、化学稳定性差三类,为红层特殊岩土定量化评价与分类治理提供了参考依据。     

我国红层分布特征及主要工程地质问题

我国中新生代红层分布总面积约826 389km2,是工程建设中不可避免的岩土体。南方地区红层约占红层总面积的60%,以西南、中南地区红层分布较广,多为裸露型红层,受降雨等因素影响滑坡、风化等问题突出;北方地区红层约占40%,以甘肃以及蒙宁晋陕交界红层分布相对较多,多为埋藏型红层,工程地质问题具有隐蔽性。红层分布受控于分布区的区域断裂带,岩体结构破碎,水文网络发育,渗流、软化等问题突出。由于砂岩、泥岩、页岩、蒸发岩、杂色岩等多种软硬岩层互层结构和工程活动等因素的影响,沿着软弱结构面易发生滑坡、差异风化等工程地质问题。由于时代较新,岩土体的胶结性差,物理力学性能相对较差,岩石多属于软岩类别。稳定性差,易崩解软化。蒸发岩和红层岩土体中可溶成份在水作用下,容易发生岩溶、腐蚀、渗流等物理化学问题。建议在工程建设中将红层作为特殊岩体对待。     

Reconstruction of nearshore chemical conditions in the Mesoproterozoic: evidence from red and grey beds of the Yangzhuang formation,North China Craton

ABSTRACT

Mesoproterozoic red beds near ancient coasts have not aroused extensive interest. A new geochemical study of the alternating red and grey dolostones from the Yangzhuang Formation provides a better understanding of the redox conditions of nearshore sedimentary environments. In this contribution, whole-rock samples are characterized by positive correlations of rare earth elements (REE) vs. Th and Fe_T vs. Th and flat-type REE distribution patterns, indicating massive terrigenous input, which is considered to be inherited from felsic rocks. Relatively high (Fe_mag+Fe_ox)/Fe_T and Fe³⁺/Fe²⁺ ratios in red beds indicate more oxidized conditions in supratidal environments compared with the lower oxygen contents in intertidal environments. Under these two distinct chemical sedimentary conditions, acetic acid-leached red and grey samples both have HREE-depleted distributions, suggesting significant freshwater invasion. Moreover, limited terrigenous redox-sensitive elements (RSEs) can reach the coast where the red beds are deposited, whereas relatively high RSE enrichment factors originating from shallow oceans are recorded in grey beds. In the Mesoproterozoic, limited oxidative weathering, shallow seawater desalination, and low organic production occurred near the coast. Meanwhile, a prolonged period of low Mo and U availability preserved in carbonate minerals confirmed that marine oxygen levels failed to satisfy the deposition of offshore red beds. During the regression, potentially exposed sediments connected to atmospheric oxygen guaranteed the oxidation of iron and the formation of red beds, and these events were coupled with negative δ¹³C_carb shifts in the Yanliao rift zone.     

Size distribution produced due to attrition in fluidized beds,pneumatic pipelines and spouted beds

A comparative assessment of the attrition phenomena occurring in fluidized beds, pneumatic pipelines, and spouted beds has been made using the distributed fracture model. The controlling mechanisms responsible for attrition in these systems have been identified. The model is shown to correctly predict the interrelationship between the fraction of feed attriting and the distribution of sizes produced on attrition. A simple method is developed to estimate attrition in continuous fluidized and spouted beds. An analytical expression is also derived to calculate the size distribution of the product fines when the feed follows a specific distribution equation.     

Swelling behaviour of GPA-reinforced expansive clay beds subjected to swell-shrink cycles

Granular pile-anchor (GPA) technique has been found to be an innovative foundation technique for expansive clays posing the dual problem of swelling and shrinkage. Swelling occurs during absorption of water and shrinkage during evaporation of water. Generally, in field expansive clay beds, swelling takes place during rainy seasons and shrinkage during summers. GPA is a recent innovative foundation technique devised to ameliorate the dual swell-shrink problem of structures founded on expansive clay beds. The other innovative techniques are drilled piers, belled piers and under-reamed piles. Laboratory scale model studies and field scale experiments on GPAs yielded useful results and revealed that swelling of expansive clay beds was effectively controlled by GPA technique. Studies on swell-shrink behaviour of GPA-reinforced clay beds have not been performed so far. This paper presents results obtained from laboratory scale model studies on GPA-reinforced expansive clay beds subjected to alternate cycles of swelling and shrinkage. The data presented in this paper pertain to the swelling of test clay beds under the influence of three swell-shrink cycles (N) spanning a time period of 300 days. The test clay beds were reinforced with varying number of GPAs (n = 0, 1, 2 and 3). Heave (mm) in a given swell-shrink cycle decreased with increasing number of GPAs. Further, for a given number of GPAs (n), heave (mm) also decreased with increase in depth from the top of the clay bed. It was found that the resultant thickness of the clay bed (H_r) for swelling increased with increasing number of cycles. However, the percentage heave (ΔH/H_r) decreased as the number of swell-shrink cycles (N) increased.     

Scour holes and ripples occur below the hydraulic smooth to rough transition of movable beds

Scour holes often form in shallow flows over sand on the beach and in morphodynamic scale experiments of river reaches, deltas and estuarine landscapes. The scour holes are on average 2 cm deep and 5 cm long, regardless of the flow depth and appear to occur under similar conditions as current ripples: at low boundary Reynolds numbers, in fine sand and under relatively low sediment mobility. In landscape experiments, where the flow is only about 1 cm deep, such scours may be unrealistically large and have unnatural effects on channel formation, bar pattern and stratigraphy. This study tests the hypotheses that both scours and ripples occur in the same conditions and that the roughness added by sediment saltation explains the difference between the ripple–dune transition and the clear‐water hydraulic smooth to rough transition. About 500 experiments are presented with a range of sediment types, sediment mobility and obstructions to provoke scour holes, or removal thereof to assess scour hole persistence. Most experiments confirm that ripples and scour holes both form in the ripple stability field in two different bedform stability diagrams. The experiments also show that scours can be provoked by perturbations even below generalized sediment motion. Moreover, the hydraulic smooth to rough transition modified with saltation roughness depending on sediment mobility was similar in magnitude and in slope to ripple–dune transitions. Given uncertainties in saltation relations, the smooth to rough transitions modified for movable beds are empirically equivalent to the ripple–dune transitions. These results are in agreement with the hypothesis that scours form by turbulence caused by localized flow separation under low boundary Reynolds numbers, and do not form under generalized flow separation over coarser particles and intense sediment saltation. Furthermore, this suggests that ripples are a superposition of two independent forms: periodic bedforms occurring in smooth and rough conditions plus aperiodic scours occurring only in hydraulic smooth conditions.     

Isotopic remobilization in the Mount Isa tuff beds

The water-laid tuff beds in the mine sequence at Mount Isa, Queensland, yield anomalous whole-rock isochron results. The RbSr “age” is significantly younger than expected, the apparent initial ratio is high, and the scatter about the regression line is significantly greater than experimental. In the UThPb system, the scatter is greater, the “age” is even lower, and the initial ratios approximate to the ore values. The results are interpreted in terms of continued mobility of minor elements in the water-rich sediments while deep-burial conditions persist.     

Grain-size distributions of bed load: Inferences from flume experiments using heterogeneous sediment beds

The grain-size fractions in the bedload transported over the five heterogeneous sediment beds of different values of bed roughness were computed from the flume experiments. The existence of an entrapment factor associated with the sorting observed from the bed to active layer was modeled based on the modified critical shear stress to estimate the grain-size fractions in the transport layer under given hydraulic conditions. The efficiency of these models was tested with the observed data. Subsequently, the patterns of observed grain-size distributions in the transport layer were tested to identify the distributions developed in the active layer due to sorting using three probability density functions (pdf), such as, log-normal, log-hyperbolic and log-skew-Laplace. Tests indicated that a log-skew-Laplace distribution fitted best for 49%, a log-hyperbolic for 31%, and a log-normal for 20% out of forty-five bedload samples collected under unidirectional flow with changes in flow discharge and bed roughness. The results of this study would be useful to specify the grain-size distributions in the bedload formed under different hydrodynamic conditions in various sedimentary environments.     

Effects of base geogrid on geocell-reinforced foundation beds

The effect of a base geogrid layer below the geocell mattress in sand, sand overlying soft clay, and soft clay foundation beds has been studied using a series of laboratory scale load tests. A rigid circular model footing was used to apply static monotonic loading on the foundation beds with and without geocell reinforcement. The model test results show that the provision of an additional layer of planar geogrid at the base of the geocell mattress further increases the performance of the footing in terms of bearing capacity and reduction in surface heave of the foundation bed. It is also observed that the beneficial effect of the base geogrid layer becomes negligible with increasing height of the geocell mattress beyond the influence of the depth of the footing. Strain measurements in the base geogrid layer indicate that it is more effective with the limited heights of the geocell mattress. The earth pressure cells embedded in the subgrade soil provide evidence that when the base geogrid is provided below the geocell mattress, the footing pressure is distributed more uniformly over an extended area.     

红层软岩结构特征与工程评价初探

通过研究红层软岩风化程度、完整性、饱和单轴抗压强度、结构灵敏性、分散性、胶结系数、崩解性、不同尺度结构类型等工程参数与岩土体结构的关系,提出了岩土"结构稳定性"的工程概念与分析思路,初步建立了由结构类型、风化系数、完整性系数、坚硬程度、崩解模式、结构灵敏度、分散度、胶结系数等工程参数组成的红层岩土结构稳定性综合判别标准,将红层岩土分为结构稳定性强、结构稳定性中等、结构稳定性差三类,为理解红层岩土工程性能、工程安全设计等提供了参考。     

万州区红层软弱夹层蠕变试验研究

在万州侏罗纪红层中,连续分布多层产状稳定、厚度较大的软弱夹层,容易成为岩质滑坡的滑动面。万州区近水平滑坡的成因一直存在争议,为了从力学角度阐明万州区滑坡的成因机制,对万州区红层软弱夹层进行了排水蠕变试验,以研究软弱夹层的流变性能。采用Singh-Mitchell模型描述了软弱夹层的蠕变特性,并对模型中的参数进行了探讨,为从力学机制研究万州区近水平滑坡的成因机制提供模型和参数。     

有关粉尘释放模型的应力分配模式存在问题的讨论

准确模拟粉尘释放通量对评价粉尘气溶胶的气候效应、沙漠化和沙尘暴防治具有重要意义。敏感性试验、室内和野外风洞实验表明被广泛使用的DPM（Dust Production Model）粉尘释放模型的2个应力分配模式——Marticorena模式和 Alfaro模式存在明显问题：由于目前空气动力学粗糙度模式还不能考虑粗糙元间隙率和粗糙元分布状况空气动力学粗糙度的影响,以空气动力学粗糙度为主要参数的Marticorena模式和 Alfaro模式还不能考虑粗糙元间隙率和粗糙元分布状况对应力分配的影响,而相关实验表明这些因素对应力分配存在显著影响;Marticorena模式和 Alfaro模式预测的应力分配系数和起动摩阻风速存在显著差异,目前实验结果还不能判断二者正确与否;Marticorena模式和 Alfaro模式预测的能够有效控制风蚀的应空气动力学粗糙度与实际观测相矛盾。针对上述问题提出了通过风洞实验改进当前应力分配模式的主要途径。     

万州侏罗纪红层软弱夹层特征

在万州侏罗纪红层中,连续分布多层产状稳定、厚度较大的软弱夹层。经X射线衍射分析、红外光谱分析及扫描电镜观察：软弱夹层主要矿物成分为蒙脱石、长石、α-石英、伊利石、绿泥石等;其微观结构以片状结构为主,微裂隙和微孔隙发育,有少量网格结构,局部可见粘土矿物定向排列。野外调研表明：软弱夹层曾经受较强的构造挤压作用,部分已发生层间剪切破坏,从而导致剪切强度降低。对软弱夹层的物理力学性质进行试验研究可知：软弱夹层为中等膨胀性土;且一旦遇水饱和,其剪切强度将大幅度下降。这些特征说明,在特定的条件下,软弱夹层容易成为基岩滑坡的滑动面。     

Characterizing coal beds in western Kentucky with the AlLaSc coherent triad

Cyclic sedimentation and lateral facies changes make coal bed correlations inconclusive and difficult. This uncertainty can be further complicated if a coal basin has been structurally deformed.Coal macerals can be studied to indicate the nature and degree of coalification. Their use in coal bed correlation, however, is limited. Most of the trace elements and their ratios that have been studied show significant within-bed lateral and stratigraphic variations, and thus are not effective in correlating coal beds regionally.Geochemically coherent groups of elements, such as rare earth elements (REE) and platinum group elements (PGE), appear to be highly differentiated in coal-forming environments. Geochemical coherent elemental triads appear to be useful for coal bed identification or fingerprinting. The best triad which was demonstrated to be effective in coal bed characterization in western Kentucky, is that of Al, La and Sc. These three elements are highly correlated with one another and they can be determined accurately and simultaneously with instrumental neutron activation analysis (INAA).The elemental triad AlLaSc is used to identify and fingerprint three key coal beds in western Kentucky: the Springfield (western Kentucky No. 9), the Davis (western Kentucky No. 6), and the Mining City and Dawson Springs are both considered to be the No. 4 coal bed in western Kentucky). Four distinct groupings can be recognized by use of the AlLaSc triad. The Dawson Springs coals have the highest Al/(La + Sc) ratios, followed by the Springfield, the Davis and the Mining City. The Mining City coal bed generally has the highest La/Sc ratio. However, the Dawson Springs is not correlated with the Mining City using the triad analysis, even though they have reportedly similar stratigraphic positions in the western Kentucky coal basin.The AlLaSe triad appears to be effective in discriminating between the Springfield and the Davis coal beds throughout the entire Illinois Basin. Furthermore, the range of concentration variation of the ALLaSc triad suggests individual groupings of the No. 4 coal in western Kentucky. In addition to characterizing these coal beds, the AlLaSc triad may be used to confirm stratigraphic correlations.     

Initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds: experiments and modelling

Experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady/oscillatory flows. Derived critical shear stresses are compared with the mixed bed entrainment model of Wiberg & Smith (1987). As predicted by the model, coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains. Entrainment occurs generally at lower shear stresses than predicted by the model, especially under unidirectional flows. This may be the result of grains resting in unusually unstable positions during the experiments because the beds are ‘unworked’ at the start of the experiments. The model of Wiberg and Smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined. The pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data. The measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture. The results are poorly described by the pivoting angle model presented by Wiberg & Smith (1987) and are better represented by a model of the form Φ = αD^γ(D_i/D₅₀)^β (after 21 ), where α, γ and β are empirical constants. The threshold model is found to be more effective using the improved pivoting relationship. The entrainment of grains is found to be easier beneath unidirectional flows than combined flows, in accordance with previous authors’ findings. A suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made. Wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow. The results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near-threshold flows.     

The invaded zone in producing beds

The results of the invaded-zone investigation described in this article revealed that in the producing beds the zone ccnsists of 2 parts-- a part adjoining the wall of the well (which contains considerable amount of filtrate), and a more remote part in which capillary replacement of rock fluids by the filtrate was observed. In the producing beds, the latter part of the invaded zone had a high oil-gas saturation; this phenomenon was related to the spread of the filtrate over the layer as the result of capillary displacement and to the difference in the densities of filtrate, oil and gas. At a high gas-oil saturation of the invaded zone, the producing layers can be separated on the neutron-gamma logs or on electrical logs registered with small size sondes. It was not always possible to evaluate maximum yield of a producing bed according to the residual oil-gas saturation. In order to improve the technique of porosity evaluation or of the evaluation of the residual gas saturation coefficient, according to the geophysical parameters of the invaded zone, it was necessary to account for the nature of filtrate penetration into the producing layers. -- Petroleum Abstracts.     

萨北油田北二西区点坝内部侧积层定量表征

萨北油田北二西区萨Ⅱ1＋2b小层层内非均质性强,剩余油分散。本文依据研究区的地质特征和沉积环境,认为该区点坝内部侧积层的坝面分布为水平斜列式,分布形态呈＂缓-陡＂模式。基于研究区的密井网资料,在点坝内部侧积层分布模式的指导下,利用经验公式、岩心以及密井网剖面的方法对点坝内部侧积层进行了定量表征。得出研究区位于点坝顶部的侧积层倾角多在5°以内,而位于点坝中部的侧积层角度在4～10°之间,侧积层间距为40m,单一侧积体水平宽度约70m。以这些参数为基础,结合研究区侧积层的＂缓-陡＂模式,建立了能够用于油藏数值模拟的三维构型模型,分析了侧积层控制的剩余油分布模式,为厚油层剩余油挖潜提供了更准确的地质依据。     

Arsenic and lead concentrations in the Pond Creek and Fire Clay coal beds,eastern Kentucky coal field

The Middle Pennsylvanian Breathitt Formation (Westphalian B) Pond Creek and Fire Clay coal beds are the 2 largest producing coal beds in eastern Kentucky. Single channel samples from 22 localities in the Pond Creek coal bed were obtained from active coal mines in Pike and Martin Countries, Kentucky, and a total of 18 Fire Clay coal bed channel samples were collected from localities in the central portion of the coal field. The overall objective of this study was to investigate the concentration and distribution of potentially hazardous elements in the Fire Clay and Pond Creek coal beds, with particular emphasis on As and Pb, 2 elements that are included in the 1990 Clean Air Act Amendments as potential air toxics. The 2 coals are discussed individually as the depositional histories are distinct, the Fire Clay coal bed having more sites where relatively high-S lithologies are encountered. In an effort to characterize these coals, 40 whole channel samples, excluding 1-cm partings, were analyzed for major, minor and trace elements by X-ray fluorescence and proton-induced X-ray emission spectroscopy. Previously analyzed samples were added to provide additional geographic coverage and lithotype samples from one site were analyzed in order to provide detail of vertical elemental trends.The As and Pb levels in the Fire Clay coal bed tend to be higher than in the Pond Creek coal bed. One whole channel sample of the Fire Clay coal bed contains 1156 ppm As (ash basis), with a single lithotype containing 4000 ppm As (ash basis). Most of the As and Pb appears to be associated with pyrite, which potentially can be removed in beneficiation (particularly coarser pyrite). Disseminated finer pyrite may not be completely removable by cleaning. In the examination of pyrite conducted in this study, it does not appear that significant concentration of As or Pb occurs in the finer pyrite forms. The biggest potential problem of As- or Pb-enriched pyrite is, therefore, one of refuse disposal.     

Influence of wetting–drying cycles on swell–shrink behaviour of GPA-reinforced expansive clay beds

ABSTRACT

Granular pile-anchor (GPA) technique is an innovative tension-resistant foundation technique which can effectively ward off the dual swell–shrink problem posed by expansive clays. The other tension-resistant foundation techniques are drilled piers, belled piers and under-reamed piles. Laboratory scale model studies and field scale experiments on GPAs revealed that swelling of expansive clay beds could be effectively controlled by GPA technique. This paper presents results obtained from laboratory scale model studies on GPA-reinforced expansive clay beds subjected to alternate cycles of wetting and drying. Swelling and shrinkage of the clay beds were monitored for three wetting–drying cycles (N) spanning a time period of 300 days. The clay beds were reinforced with varying number of GPAs (n = 0, 1, 2 and 3). Swelling (mm) and shrinkage (mm) of the clay beds in a given wetting–drying cycle decreased with increasing number of GPAs. Further, swelling (mm) and shrinkage (mm) significantly decreased with increasing number of wetting–drying cycles (N) also. For a given number of GPAs (n), swelling and shrinkage decreased with increase in depth from the top of the clay bed too.     

Changes in velocity profiles at roughness transitions in coarse grained channels

Heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness. Distinct spatial zones in terms of grain size have been reported, for example sand ribbons and bedload sheets. The transition from areas of finer to coarser grained surface sediment is often abrupt. However, the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels. This paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistancé on the structure of the turbulent boundary layer. Measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river. Immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface, boundary shear stress and roughness length increase considerably. Downstream from this transition, velocity profiles become concave upwards. Downstream and upstream sections show significant differences in terms of near bed velocities (deceleration downstream of the transition), velocity gradient and turbulence intensity of the streamwise velocity component. Comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity (or shear stress) associated with the pressure drag produced by large and isolated obstacles.