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小南海坝基软弱夹层发育规律及三维可视化

The Development Regularities and Three-dimensional Visualization of the Weak Intercalations in the Foundation of the Xiaonanhai Reservoir

【作者】 聂琼

【导师】 项伟;

【作者基本信息】 中国地质大学 , 岩土工程, 2014, 博士

【摘要】 重庆长江小南海水电站坝址位于重庆市江津区珞璜镇下游约1.4km,是国务院批准的《长江流域综合利用规划简要报告》推荐梯级开发方案的重要枢纽。坝址控制流域面积70.5万km2,多年平均流量8650m3/s,相应多年平均径流量2685亿m3。小南海水电站大中坝坝址水库正常蓄水位为196m,死水位194m,校核水位以下总库容为13.0亿m3;中坝址枢纽布置代表方案的最大坝高为67m,装机容量1680MW,水电站对带动重庆地区经济可持续发展及提高重庆境内长江干流航道标准具有重要意义。小南海水电站中坝址建坝地层为侏罗系上统遂宁组(J3s)红层,是一种软硬相间的不等厚的地层组合岩体。红层坝基中软弱夹层众多,在后期的构造变动中不同程度地遭受剪切破坏而泥化,引起大坝抗滑稳定、沉陷变形、渗透变形及长期演变等问题,是工程勘察过程中亟待研究和解决的工程问题。软弱夹层的分布规律是布置勘探、指导勘探的前提和依据,是对一个地区地基地质条件作出正确评价和结论的基础,科学地解释软弱夹层的形成规律,可避免工程勘察的盲目性,对今后类似平缓红层地区的工程地质研究有着重要的意义。坝区地质构造复杂、地质信息众多,给坝址选择、枢纽布置、地下工程设计与施工等各方面带来了很大的困难,建立反映工程地质信息的三维地质模型,能提供对工程地质问题的正确判断和分析的大量信息。论文结合小南海水电站实际情况及项目任务书,研究坝址区红层地层的岩性组合、物质成分、结构特点、粒度参数、沉积构造、古生物等沉积环境特征,分析地层旋回特征,揭示坝址区软弱夹层的沉积规律及发育特征。通过对软弱夹层样品粒度组成、物质成分、物理化学特性、微结构特征及力学性质的室内试验研究,结合水利水电工程地质勘察规范中夹层工程地质分类方法,建立坝址区软弱夹层的工程分类标准,综合提出不同类型软弱夹层的强度参数建议取值。采用PFC2D软件模拟软硬互层岩体中,软弱夹层在层间剪切作用下剪切带的形成与破坏过程。最终选用GOCAD软件建立小南海坝基地层及软弱夹层空间分布三维地质结构模型,建立岩性参数模型,并分析参数在地质体中的分布规律,为坝基抗滑稳定评价提供基础。论文的主要研究内容及结论为:(1)结合钻井资料和室内试验研究坝基沉积环境判别标志,进一步分析得到坝基岩层水平向和垂向演变及软弱夹层的分布规律。小南海水电站建坝地层为侏罗系上统遂宁组(J3s)红层,基岩主要为砂岩、粉砂岩、黏土质粉砂岩、粉砂质黏土岩和黏土岩。砂岩矿物成分以石英为主,占矿物总含量90%以上,其次为白云母,含微量长石、铁质、燧石、绿泥石等,砂岩成分成熟度高。坝基软岩(包括黏土质粉砂岩、粉砂质黏土岩、黏土岩)的矿物成分主要由绿泥石、伊利石、石英、长石、方解石及赤铁矿组成,化学成分以SiO2为主,含量为50~60%,其次为Al2O3。以采用薄片粒度统计法和筛分-虹吸比重瓶联合测定法分析坝基岩石的粒度特征,认为J3s6段细粒沉积物在剖面上有上细下粗的韵律变化,与之相比J3s5段地层沉积物粒度更粗,主要为长石岩屑砂岩—黏土质粉砂岩、粉砂质黏土岩二元结构。坝基岩层主要发育的层理类型有大型交错层理、中小型交错层理、水平层理、波状层理、卷曲层理、波痕等。生物化学特征表现为粉细砂岩及粉砂岩中见虫孔构造和植物根系及植屑化石。粉砂岩或黏土质粉砂岩中含有大量灰绿色斑点或团块,可能为小生物遗骸生成的有机质在氧化条件下造成局部还原的效果。综合分析得到,小南海坝基岩层属于典型的河流沉积,主要建坝地层J3s6~J3s4段地层中共识别出以下4种次一级的沉积环境,即河床滞留、边滩、天然堤及洪泛平原(盆地)沉积,其中洪泛平原(盆地)发育最为普遍。J35s~J3s4段地层剖面可划分为5个明显的沉积旋回,每个旋回厚度约10m,具有典型的二元结构。分别统计了坝址区研究范围内J3s6~J3s4段黏土岩类(粉砂质黏土岩及黏土岩)的厚度及占该段地层总厚度的百分比,并利用Surfer软件绘制了厚度等值线图及厚度百分比等值线图。结果可知坝址区J3s6段地层中黏土岩类厚度最大为22m,大多钻孔揭露的黏土岩类厚度在5m以内,局部区域黏土岩类厚度占J3s6段地层总厚度的70~80%,而左岸溢流坝局部区域不含黏土岩类;坝址区J3s5段地层中黏土岩类总体较少,仅在左岸厂房钻孔MZK232附近的地层中出现一段厚度大于15m的黏土岩,其他大部分区域黏土岩类厚度均在5m以内:坝址区内仅少量钻孔完全揭露J3s4段地层。对野外J3s4-1剖面地层的沉积环境进行了马尔可夫链分析,得出J3s4-1段地层的韵律结构模式主要分2种:①粉砂岩-厚层黏土质粉砂岩互层半韵律沉积,韵律层厚度大,代表了典型的洪泛平原细粒沉积;②黏土质粉砂岩-黏土岩薄互层半韵律沉积,韵律层厚度小,为泥化夹层的形成提供了良好的条件。软弱夹层的产出受到沉积环境的制约,洪泛盆地沉积的软弱夹层厚度大、连续性好、分布稳定,对工程的影响最大,常构成控制结构面;天然堤沉积如未经改造,分布也较稳定,夹层薄、性状较差;洪泛平原沉积连续性好,分布较稳定,但粒度较天然堤中形成的夹层粗,性状稍好;边滩沉积中的夹层不稳定、连续性差,但性状较好。漫长的地质时期中,同一地区的沉积环境是不断演变的,演变过程中新老地层的不同组合对软弱夹层的分布有着重要的影响,前期形成的软弱夹层受后续水动力较强的河道、边滩冲刷改造作用,连续性将被破坏。(2)论文通过试验研究了软弱夹层工程地质特征,建立了坝址区软弱夹层的工程分类标准,并提出各类软弱夹层强度参数建议取值。粒度分析结果看出大部分软弱夹层黏粒质量分数(<0.002mm)介于10%~30%之间,母岩多为黏土质粉砂岩或粉砂质黏土岩。软弱夹层矿物成分以伊利石(水云母)、绿泥石等黏土矿物为主,次为石英、方解石、长石和赤铁矿。定向片数据分析结果显示坝址区软弱夹层内伊利石含量为65~85%,蒙脱石含量除编号为3S101的夹层高达55%之外多在5%~20%之间,绿泥石一般含量5~10%,高岭石含量5~10%。综合考虑软弱夹层矿物成分、粒度成分、泥化程度及构造特征等因素的基础上,结合《SL55-2005中小型水利水电工程地质勘察规范》附录C软弱夹层工程地质分类方法将坝址区软弱夹层划分为三大类:Ⅰ类岩块岩屑型、Ⅱ类岩屑夹泥型和Ⅲ类泥化夹层(包括Ⅲ1泥夹岩屑型和Ⅲ2全泥型)。采用Quanta200环境扫描电子显微镜观察软弱夹层的微区形貌和微观结构,结果看出平行于层理方向,软弱夹层内泥化面多见于黏土岩、粉砂质黏土岩夹层内,泥化表面起伏度较小,见明显条纹状挤压擦痕;片状黏土矿物呈高度定向排列,黏土矿物单片及片簇较为密集,主要为面-面接触;厚层泥化夹层通常包含两个或两个以上主错动面,说明夹层曾遭受多期构造挤压作用影响;夹层内摩擦光面多附方解石薄膜。垂直于层理方向,泥化夹层具有明显分带性,可分为泥化带和劈理带,泥化带较薄,见两个错动面,超微结构为分散结构,错动面附近有较强平行层面排列趋势。室内慢剪试验获得性状最差的泥化夹层在饱水状态下的摩擦因数为0.223,按规范用粒度成分定量指标选取的夹层强度参数明显偏低,相当于接近液限状态时室内剪切峰值强度的0.72~0.86倍,最终类比其他相似地质条件下的工程经验,提出各类软弱夹层强度参数建议值。(3)软弱夹层剪切破坏过程的PFC2D模拟。层间剪切带的形成需具备物质条件、动力条件和环境条件。坝基沉积物多属洪泛平原沉积,厚度大,分布广泛,沉积物较细,黏粒质量分数及黏土矿物含量较高,为层间剪切带的形成提供了物质基础;地层以粉砂岩-黏土岩不等厚互层的岩性组合为主,在构造运动或岩层屈曲时,由于软硬岩层强度差异,夹层发生层间剪切位移而引起结构破坏;在一定地下水条件下,水岩物理化学作用导致夹层的矿物成分和结构连接特性发生变化,发生软化和泥化。论文建立软硬相间岩层软弱夹层简化PFC2D模型,用水平剪切速度产生的水平应力作用来加载,比较不同剪切时步夹层内孔隙比的变化、剪切裂隙发育位置、宽度、角度及间距,直观地反映其剪切破坏过程。建立不同试样尺寸的PFC2D模型,对比不同厚度及延伸长度的夹层破坏特征:夹层厚度及上下所夹硬岩尺寸一定时,试样长度越长,裂隙与层面夹角越小、裂隙间距越大,微裂隙越容易集中在夹层左右两端;试样长度和上下所夹硬岩厚度相同时,裂隙与层面的夹角相近,但夹层厚度越大,裂隙间距越大,模拟结果为解释红层坝基不同发育规模软弱夹层的剪切破坏特征提供依据。(4)应用GOCAD三维地质建模软件建立坝基三维地层结构模型,分析软弱夹层空间延展特征,建立控制性软弱夹层的三维地质模型,评价各主要建坝地层的强度参数分布特征。论文应用GOCAD三维地质建模软件建立了研究区钻孔揭露的9个主要地层界面,包括地表、Q4、J3s6、J3s4-2、J3s4-1、J3s3、J3s2、J3s1,利用软件强大的三维空间数据处理功能解决了建模过程中地层的缺失、尖灭等问题,取得了形象的、直观三维地质模型。通过GOCAD软件中的DSI插值法将各个钻孔的参数属性(岩体粒度、黏粒质量分数)值插入到整个三维地质模型中,建立坝区工程地质参数模型,可通过鼠标点击模型获得相应位置的属性值,得到各种地质信息在整个模型中的分布特征及规律。采用GOCAD软件中的属性截面法和Slicer法,从平行于3个坐标轴的方向对参数模型任意位置进行切割,得到不同剖面上地层岩性及参数属性的空间分布及变化。采用GOCAD软件中的X-Section法,在研究区任意钻孔之间建立剖面,简单快捷地形成工程地质剖面图。建立三维地质体模型及岩石力学参数模型,分别评价各主要建坝地层的强度参数分布特征。通过钻孔岩芯编录资料的核对、沉积相分析及钻孔录像分析三种方法判定软弱夹层发育的位置、类型、性状及连通性等特征。提出延展长度、延展宽度、纵向连通率、横向连通率、面积、面积比几个与延展性有关的概念,用于分析坝区内软弱夹层的空间延展特性。论文考虑建基面20m以下连续性较好的软弱夹层为控制性软弱夹层,会影响坝基稳定性。计算得出不同水工建筑物(右岸溢流坝、右岸电站厂房、左岸电站厂房、左岸溢流坝及船闸)下的控制性软弱夹层的发育深度及延展性特征,并应用GOCAD三维地质建模软件,按照不同建筑物坝段分别建立地层面模型及软弱夹层面模型,实现软弱夹层的三维可视化。

【Abstract】 The Xiaonanhai Reservoir is a proposed dam on the Yangtze River in Chongqing Provience, which is40km upstream of the city. The basin area controlled by the dam site is around705,000square kilometers. The amount of water flowing on average every year is8,650cubic meters, which is the equal of268.5billion of the runoff per year. The normal storage level is196meters, and the level of dead water is194meters. The total storage capacity below the check water level is1.3billion cubic meters. This reservoir is a big issue for the sustainable development of Chongqing Provience. The Xiaonanhai Reservoir is located in Chongqing Province. The hard and weak interbedded strata with low strength and great variation belong to the Upper Jurassic Suining Formation. Weak intercalations, which are suffered shear failure and argillization owing to the tectonic events, can lead to a series of geotechnical problems, such as the stability, deformation, seepage and long-term evolution of the foundation. In this case, analyzing the distribution of the weak intercalations is the premise for arranging and guiding the exploration, and the basis of making correct evaluation and conclusion for geotechnical conditions. It has great significance for the researches in similar red bed area in future. A lot of complicated geological structure information has brought great difficulties to choose the dam location, layout of the project and design of the underground constructions. It is necessary to build three-dimensional geological model to show the information of the geological body. The thesis began with the analysis of the environment properties of the red beds in research area, including the lithological association, material composition, grain size parameter, sedimentary structure and palaeobios of the deposits. Markov chain analysis was applied to the stratigraphic succession in order to point out any depositional regularity of the weak intercalations. Considering the grain size, mudding degree and structure characteristics, weak intercalations were divided into several types and the depositional regularities of each type was discussed. Strength parameters were obtained through the physic-mechanical properties. Based on the particle flow theory, PFC2D models of the hard and weak interbedded layer were constituted to study the failure process of weak intercalations. Finally, the thesis studies on the geosciences modeling and3D visualization of the dam formation. Parameter models were further constructed to show their distribution regularities. The research contents and results of this thesis are shown as follows:(1) Based on the drilling data and laboratory experiments results, indicators for distinguishing depositional environments were revealed, and the distribution regularities of weak intercalations in horizontal and vertical direction were further discussed.The bedrocks were characterized by alternations of sandstone, siltstone, argillaceous siltstone and mudstone. Thin section tests show that red sandstones contain more than90%of quartz, followed by muscovite, and the rest minerals are feldspar, irony, flint, and chlorite, et al. X-ray tests reveal that the soft rock, including clayey siltstones, silty claystones, and mudstones, are composed of chlorite, illite, quartz, feldspar, calcite and hematite. The chemical composition are dominated by SiO2with a percentage of50-60%, followed by Al2O3. The grain size properties of the sediments are obtained by thin section test and sieving-siphon pycnometer method. The results show the grain size of J3s6changes from fine to coarse with depth. However, there are more coarse-grained sediments in J3s5, which showing the dual structure of feldspathic litharenite-clayey siltstones.The bedding structures include cross bedding, horizontal bedding, wavy bedding, curled bedding and ripple marks. Biochemical characteristics are obvious in the siltstones, such as burrow structure, plant roots and fossils. Some celadon spots or agglomerates should be the result of local reduction in organic matters under oxidized conditions. According to the comprehensive analysis, there are four sub-sedimentary environment recognized in J3s6-J3s4: channel lag deposits, point bar, natural levee and flood plain or flood basin. The succession is interpreted to be fluvial facies, dominated by the point bar and flood plain (basin) sub-facies. However, channels and levee deposits are rarely to be found in this area. Five vertically repeated cycle rhythms are recognized in the strata of J3s6-J3s4. A cyclical variation is normally10meters thick, which shows a typical dual structure.This paper studied the thickness and proportion of clay rocks in J3s6-J3s4of the research area. Contour maps are presented by Surfer software. The results show that most of the thickness of clay rocks is less than5meters. The thickest layer reaches22meters in J3s6. Clay rocks are rare in the overflow dam on left bank. However, only a few of drillings expose clay rocks no further than5meters in J3s5, but there are15meters of that around the MZK232drill. Most of the drills cannot show the layers in J3s4because the limit of drilling depth.Cyclic processes are clear in J3s4-1strata. Taking these strata as an example, the Markovian simulation results show that the semi-rhythmic structures of the given section are mainly of two types:one is siltstone and argillaceous siltstone alternation with great rhythmic thickness, the other is thin bedded argillaceous siltstone with interbedded mudstone.The distribution of weak intercalations is controlled by sedimentary environment. Continuous intercalations deposited in flood basin can easily turn to controlled structural surface, which lead to the unstability of the dam foundation. Thin weak intercalations in natural levee always have poor mechanical properties. By contrast, the intercalations in flood plain have high strength and stable distribution. The worst and most volatile ones exist in the point bar. As the evolution of the sedimentary environment, the distribution of weak intercalations should be transformed as the stratum combination alters. For example, their continuities would be destroyed as the result of river revolution and beach erosion.(2) Study on the engineering geological properties of the weak intercalations, and establish their standard classification, finally suggest the values for strength parameters.Grain size analysis results show that the clay content of the weak intercalations is between10%and30%. The minerals of them are dominated by clay minerals of illite, and follow by quartz, calcite, feldspar and hematite. The oriented strand thin section test reveals that the main clay mineral of the weak intercalations is illite with a proportion of65-85%. Montmorillonite accounts for5-20%, but except for the sample of number3S101, in which the content reaches up to55%. Both of chlorite and kaolinite have a percentage of5-10%.Considering the grain size, mudding degree and structure characteristics, the in-competent beds are divided into three types:(i) cracked rock;(ii) cracked rock with mud interbedded;(iii) mudstone with cracked rock interbedded, or mudstone.Micro-structures of the weak intercalations are described by means of scanning electron microscopy (SEM). The results show that the mudding surfaces with small relief are frequently seen in mudstones and silty claystones. The schistose clay minerals which contact by surface to surface, are highly oriented arranged. Thick weak intercalations normally have two steep planes caused by multi-period tectonic deformations. Calcite films attached to the smooth friction surface. In addition, the intercalations with dispersed structures have obviously zonations, including muddy zones and cleavage zones.Slow shear test reveals that the lowest strength parameter of weak intercalations is0.223, and the value of strength parameter according to the quantified index of granularity composition is0.72to0.86times of that obtained from the slow shear test. In addition, recommended values of typical weak intercalations were given by considering other similar engineering experience.(3) Numerical simulation of the failure process of intercalated shear zone by PFC2D software.Materials, tectonics and environments are three essential elements for the formation of the intercalated shear zone. The flood plain deposits with high clay content provide abundant material basis. Shear zones are formed when shear rupture occurs in interbedded hard and weak rocks. The minerals and structures will change under certain groundwater condition. As a result, weak intercalations become softening and mudding.Based on the particle flow theory, PFC2D models of shear zone were constituted to study the failure process of shear band. The simulation test focused on the void ratio change, as well as the position, width, angle and spacing of the shear cracks. The results show that the shear failure process of the model is consistent with the development of slip surface in clay which was proposed by A.W. Skempton. The failure experiences a process of fissures formed, grew lager, and developed until large scale sliding occurs. Soil structure is seriously damaged and bearing capacity decline after slide occurs. Compare eight PFC2D models with different thickness and length, the simulation results reveal that when the thickness of hard and weak rock is fixed, the longer sample leads to the gentler angle of the cracks and larger spacing between them. Meanwhile, micro cracks are prone to be concentrated initially in both ends of shear zone. Maintaining the length and the thickness of hard rock, the spacing of the cracks becomes larger with the increasing thickness of shear zone, but the angle remains unchanged.(4) Build three-dimensional geological modeling of the dam foundation by GOCAD software. Analyze the spatial extension features of weak intercalations by means of drilling videos, build the three-dimensional geological models and evaluate the strength parameters distribution regularities.Three-dimensional geological models of9formation interfaces were intuitively established by GOCAD software, including ground, Q4, J3s6, J3s5, J3s4-2, J3s4-1, J3s3, J3s2and J3s1. Visual geological models were obtained and stratigraphic lost and pitch-out problems were solved using spatial data processing functions. DSI method was applied to build engineering geological parameter models, such as rock granularity and clay content. As a result, parameter values at any point can be obtained by clicking the place in the model, distribution regularities of the parameters can be further obtained. Spatial distribution properties of any geological information were shown in three directions which parallel to the axis by using the Section or Slicer method in GOCAD. In addition, engineering geological profiles between any drilling can be speedily established by using X-Section method.The position, type, property and connectivity and other features of weak intercalations are obtained through the logging data, sedimentary facies analysis and drilling videos. Several new concepts related to extensibility, such as extended length, extended width, vertical connectivity rate, lateral connectivity rate, are proposed to analyze spatial extension characteristics of weak intercalations in the research area. Continuous weak intercalations, which are20m below the foundation surface, have impact on the stability of the dam. Calculate the depth and extending parameters of these controlled weak intercalations in different marine constructions, including overflow dam, power-house and ship lock. Build the three-dimensional geological models and parameter models and evaluate the distribution regularities of the parameters in each layer.

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