【作者】 王勇；

【导师】 孔令伟；

【作者基本信息】 中国科学院研究生院（武汉岩土力学研究所） ， 工程力学， 2009， 博士

【摘要】 正在兴建的杭州地铁1号线在勘查过程中遭遇到含有压力较高的可燃性气体土层,这种分布广、埋深浅的含气地质条件势必会给正在修建的地铁工程带来不利影响。然而,目前国内外在此类地层中修建地铁的工程经验和相关研究成果甚少;如何认清它的工程性状和灾变机理,采取何种针对性措施才能有效确保地铁工程安全施工和顺利运营,已成为当前地铁建设中亟待解决的难题。本文以杭州地铁典型区段浅层气藏中的储气砂土作为研究对象,基于非饱和土力学理论,采用室内试验、模型试验和数值模拟的方法,对储气砂土的土层特性、工程性状演化规律和成灾机理展开较为系统的研究,并就浅层储气地层在地铁工程中的灾变模式和防治进行了初步探讨,主要研究内容和结论如下:第一,分析总结了地铁工程所遇含浅层气地层的成因机理、气藏类型、气体组分和气压力分布特征及基本赋存规律,对气体的成藏过程和赋存分带性做出推断。杭州地区广泛存在的超浅层气藏是由其特殊的地理位置和气候条件所决定的,气藏属常压气藏,埋深较浅、气体成分以甲烷为主,富含有机质的淤泥层系主要的气源层和封盖层,未胶结砂层和贝壳层系气体的主要储集层;气藏中砂体沿深度由干变湿,由非饱和至饱和,气体赋存具有明显的分带性。第二,针对不同饱和度的砂土,系统开展了持水特性、渗水、渗气、剪切、固结等基本物理力学特性的研究,分析了上述特性随基质吸力或饱和度的变化规律。试验结果表明:储气砂土系粉细砂,含少量粘粒,孔隙率较高,气藏中砂土的基质吸力变化范围约在0～100kPa之间;VG模型能较好的描述储气砂土的持水特性,借助其土水特征曲线可以在工程缺乏试验条件下,预测得到储气砂土的气藏压力、抗剪强度、渗水系数等工程指标。利用自行设计的试验装置测定了储气砂土的气渗透特性,发现饱和度相对密度变化对储气砂的渗气性影响更为显著,VGM模型和Parker模型可分别用来描述储气砂土中水相和气相的渗透特性;储气砂土的表观凝聚力符合幂指函数形式,压缩和回弹性随吸力的增大而减小。第三,针对地铁建设中的常见应力路径,通过室内应力路径试验,研究了不同工程路径、不同气压水平的储气砂土的强度变化规律,并给出了气体释放后砂土强度的预测方法。结果表明:不同应力路径下,储气砂土的应力-应变特性具有显著差异,其强度受应力路径影响显著;不同应力路径下的储气砂土抗剪强度可用统一公式来描述;储气砂土中气体释放后强度将会提高,给出了气体释放后预测砂土强度的方法;实际工程中若采用饱和砂土强度指标时应考虑折减。第四,气体释放是储气砂土在地铁工程中最常经历的一条卸荷路径,通过室内试验研究了放气路径下储气砂土的变形特性。试验结果表明:原始地层压力越高,因气体释放导致的土层变形越大,有控分级放气可以有效减小含气土层的变形;干湿历史对砂土的变形特性影响显著,脱湿后再吸湿过程中土体变形小于脱湿过程中的变形,随净平均应力的增大,吸力对砂土变形的影响逐渐减弱,砂土逐步趋于更加稳定的结构;放气路径中,储气砂土变形受基质吸力和净平均应力的耦合影响,但在原始气藏条件下,气体释放中因吸力或含水量变化引起的砂土变形量很小,变形主要源于气压减小引起净平均应力增大而导致的土体变形;气体重新回聚引起的砂土回弹量很小,对实际工程影响不大。第五,基于室内试验,建立了工程放气路径下描述储气砂土固相宏观变形行为的应力应变本构模型,模型参数均可由室内试验确定;在VGM和Parker模型基础上,给出了不同含水饱和度下砂土中的水、气相对渗透曲线,并由此建立了描述水、气运移规律的两相流分析模型;与放气路径储气砂本构模型耦合,实现了对工程当中、气体释放条件下储气砂土变形的数值描述,揭示了变形过程中固、液、气三相耦合效应机理。第六,为了解气体释放对地铁隧道的影响,自行设计了一套模型试验系统,进行了隧道穿越储气层和隧道穿越封盖层两种工况下,储气砂层中气体释放对地铁隧道受力和稳定性影响的室内模型试验。研究结果表明:气体释放将引起隧道产生附加变形和管片附加应力,浅层气体释放对地铁隧道结构应力和结构相对变形影响较小,而对隧道整体位移影响较大,易引起不均匀沉降,导致隧道裂缝或断裂,进而引发危险;气体重新回聚时,隧道产生的附加应力和回弹位移量较小,工程中可不予考虑;地铁隧道施工前,宜对浅层气体进行预先排放。最后,分析了浅层储气地层在不同工程阶段的灾变模式,针对性的提出了地铁工程在施工期和运营期的防治对策。储气地层在地铁工程不同阶段的危害表现形式和灾变特征有所差异,防治需针对勘探阶段、设计阶段、施工前准备阶段、施工阶段和地铁运营期等不同时期,制定有效措施,来确保地铁建设安全顺利进行。更多还原

【Abstract】 Currently, a type of soil layer stored with high pressure and flammable gas was encountered during the exploration of Hangzhou metro line 1 project under construction. Such the wide distribution and shallow buried gassy ground condition is bound to bring adverse effects on the construction of this subway engineering. However, experiences and related research achievements about building subway under such condition are lacking both at home and abroad. What engineering properties and hazard mechanisms of the layer and which control measures should be adopted to ensure the construction safety and the operation of subway smoothly have become the problems solved urgently in the current metro design and construction.Taking the gassy sand in shallow gas reservoirs of the typical section in Hangzhou metro as an object, based on the theory of unsaturated soil mechanics, and with the laboratory test, model experiment and numerical simulation analysis, the evolution law of its engineering properties and hazard mechanism are studied systematically. Intrinsic mechanisms, models of gassy sand causing disasters and measures taken for controlling disasters in the subway construction are discussed in this thesis. The main contents and the conclusions are as follows:Firstly, geologic origin, type of gas reservoir, gas component, feature of gas pressure distribution and law of basic occurrence of the shallow gas stratum are summarized; the formation mechanism of such shallow gas, the history of gas migration and accumulation and the zone theory of its occurrence are deduced. The shallow gas reservoir existing widely in Hangzhou area is determined by its special geographical position and climate condition. It is buried shallowly and belongs to that of normal pressure reservoirs. The gas composition is main methane. Silt layer with rich organic matter is the main source and sealing layer; and uncemented sand shell bed is the main gas reservoir. The state of its occurrence in gas reservoirs is from dry to wet, and saturation degree of it is from low to high until the sand is saturated along the depth; the gas occurrence has an obvious feature of zonation.Secondly, on account of the different saturation degree of sand, basic mechanical properties such as moisture holding capacity, water permeability, gas permeability, shearing and consolidation features are studied, and their variations with the suction or saturation degree are analyzed. Results show that the gassy sand with high porosity is silty fine sand with a small quantity of clay fraction, and the matric suction of the sand in gas reservoirs varies from 0 to100kPa. Van Genuchten equation can favorably describe the soil water characteristic curves of gassy sand (SWCC), and other engineering indexes of the gassy sand such as gas pressure in reservoirs, shearing strength and coefficient of permeability can be predicted by the curve of SWCC under lacking test conditions. Gas permeability of the gassy sand is studied by using a self-designed experimental device, and results show that influences on gas permeability from the variation of saturation degree is more significant than that of density. VGM and Parker models can be used to describe the water and gas permeability of the gassy sand satisfiedly. The apparent cohesion of gassy sand can be described with the power exponent function and its compressibility and rebound resilience decrease with the increasing of suction.Thirdly, on account of the common stress path in metro construction, strength properties of gassy sand under different stress paths and with different gas pressure levels are studied through lab stress path experiments, and a method to forecast the strength of gassy sand after being released of gas is proposed. Results show that the stress-strain relationships of gassy sand are distinct under the different stress paths, which dramatically affects its features of strength. The shear strength of gassy sand under different stress paths can be described by a unified formula. The strength of gassy sand will be enhanced, and a method is proposed to forecast it after the release of gas in it. It tends to be unsafe if the strength indexes of saturated sand are adopted and it should be considered to be reduced in practical engineering.Fourthly, gas release is one of the most common unloading paths of gassy sand in subway engineering construction. Deformation behaviors of gassy sand under the path of release of gas are studied through the lab tests. Results show that the higher the original reservoir pressure is, the larger the soil deformation caused by gas release is. The way to release of gas under controlling can effectively reduce the deformation of gassy layers. The wetting and drying history of gassy sand has a significant influence on its deformation behaviors. Deformations of gassy sand in the re-wetting process are less than those in drying process. With the increasing of net stress, influences on deformation of gassy sand due to the variation of suction weaken gradually, and the structure of sand gradually becomes more stable. In the path of gas release, both the matric suction and the net stress affect deformation behaviors of gassy sand. But in the original gas reservoirs, the deformation of gassy sand caused by the variation of suction or saturation degree is very small, and the dominating deformation arises from the increasing of net stress because of the decreasing of gas pressure. Re-accumulating of the gas only leads to a small swelling deformation, and has little influence on the deformation of gassy sand in practical engineering.Fifthly, a stress and strain constitutive model describing the solid-phase macro deformation behavior of gassy sand in the path of gas release under controlling is established based on the lab experiments and parameters of the model can be determined through the experiments. A relative permeability curve between the water and gas under different saturation degrees are given, by which a two-phase flow model describing the migration law of water and gas in gassy sand is established. Coupling with the constitutive model, behaviors of deformation influenced by the effects coupling with the three phases such as solid, liquid and gas can be calculated with the numerical analysis for gassy sand in the path of gas release.Sixthly, in order to investigate influences on subway tunnels in the path of gas release, two working conditions which are that the tunnel passes through the gas reservoir bed and that the tunnel passes through the cap bed are carried out respectively with the self-designed experimental device system for model tests. Results show that gas release in gassy sand will cause the additive deformation of tunnel and additive stress of the shield segment, and make important impacts on the global displacement of tunnel but little influences on the stress of shield segments and the relative displacement of tunnel. It is inclined to arouse the differential settlement and cracks on the tunnel so as to cause hazardousness. Re-accumulating of the gas can also produce additional stress and the additive resilience, but it can be ignored in practical engineering because the influence degree is small. The shallow gas in reservoirs is appropriate to be discharged before construction of the subway tunnel.Finally, the disaster mode of the gassy ground at different stages of the subway engineering is analyzed, and some counter measures during construction and operation period of the metro project are proposed. According to the analysis, the gassy ground has different disaster mechanisms and manifestations at the different stage of construction, and the corresponding measures for prevention should be taken with distinguishing different periods such as exploration, design, construction preparation, construction and metro operation. Only thus can it ensure the safety of metro engineering and operation smoothly.更多还原