| 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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