【作者】 朱建群；

【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， 2007， 博士

【摘要】 纯净砂在自然界中并不多见,而含细粒砂土却广泛分布,尤其是水力冲积填土和海底沉积土。迄今为止,细粒含量对砂土的强度特性和应力应变关系的影响在岩土界尚未形成共识。故而,本论文以此为研究背景,以具有片状颗粒特征的南京砂为主要研究介质,围绕细粒含量对砂土工程性状的影响和规律深入探讨,主要研究内容与成果如下:1.对具有片状颗粒特征的南京粉细砂进行了较为系统的固结不排水剪切试验,发现在低应力水平下随着相对密实度增大,南京砂残余偏应力逐渐增大;相同密实度的砂土在低围压(<100kPa)下表现出不稳定,但随着围压的增大而进入稳定阶段,在高围压下又进入不稳定阶段。极松散南京砂在剪切过程中出现了完全静态液化现象,初始孔隙比e 0为1.075,其“亚稳定”结构是其在低围压下产生静态液化现象的主要原因。通过应力路径修正的方法,建立了南京砂峰值强度和残余强度间的关系:2.通过测定10种不同细粒含量的砂土的基本物理性质指标,把握不同细粒含量砂土的颗粒级配特征。细粒对含细粒砂的干密度、不均匀系数和曲率系数、孔隙比等物理状态指标均存在影响,且随细粒含量的增加,含细粒砂土级配由不良变为良好。3.系统开展了低围压下不同细粒含量砂土的固结不排水剪切试验,认为在峰值强度之前,细粒含量对其应力应变关系影响不大,但在峰值之后受细粒含量的影响程度明显加强;细粒含量对砂土的稳态线位置和斜率均有显著影响,且在细粒含量小于25%范围内,砂土稳态线随细粒含量增加而下移,当细粒含量超过25%时,砂土稳态线随细粒含量增加而上移,该砂土的临界粉粒含量为25%。4.利用粒间状态参量e s、e f和D rs论证含细粒砂土的力学性状机制,分析出其随细粒含量变化的演化规律。当砂粒占控制优势时,围压对砂土力学性状的影响不大,且粉粒含量对残余强度的影响也不大;当粉粒占控制优势时,围压对砂土的峰值强度和残余强度的影响均十分明显;在试验范围内围压的增大使砂土出现了不同程度的剪胀,土体的稳定性有所增强;当砂粒和粉粒共同承担土体颗粒间力链的传递时,砂土强度特性受控于细粒含量和围压大小,其剪切强度均随细粒含量的增加和围压增大而增大。5.利用标贯击数与相对密实度、平均粒径的经验公式,探讨了细粒含量对砂土流滑失稳的影响,初步确定了各含细粒砂土处于稳态线上的临界标贯击数,并验证了其他研究者关于《建筑抗震设计规范》过高估计了南京砂的抗液化能力的结论。6.利用优势砂模型(superior sand model)对三轴压缩条件下南京砂的力学性状进行模拟。与试验结果对比可知,该模型较好地反映了中等围压下南京砂的力学性状,但对极松散南京砂在低围压下的模拟结果却不理想,这反映了模型对砂土结构性参数的具有较高敏感性。更多还原

【Abstract】 It was known that clean sandy isn’t in extension all the while, and a significant amount of fines exist in alluvial and hydraulic deposit silty sand. And the role of nonplastic fines content on the strength properties and steady-state behavior of sandy soils has been a topic of debate for some time. In this paper the research was done with CU tests on the Nanjing sand with sheet particle to analyze and evaluate the effects under low confining pressures. The main research of this dissertation is as follows:1. The undrained compression tests were performed on Nanjing sand under lower confining pressures.The residual strength is increasing with the increasing of relative density at the same confining pressure and the samples at the same relative density are from instability to stability and to instability again under the higher confining pressure. The complete static liquefaction has occurred for the looser Nanjing sand at 1.0750e =. The metastable particle structure of Nanjing sand makes for the static liquefaction. By normalizing the stress path, the relationship of peak strength and residual strength is:2. The basic physical properties indices of 10 kinds of silty sand are measured to know their particle distribution. The change of specific gravity is little with fine content; neither are the changes of the changes of dry density, uniformity coefficient, coefficient of curvature and void ratio. All show the particle size distribution of silty sand is from worse to better when the fine contents are increased.3. The CU tests on 10 kinds of silty sand are performed under lower confining pressures. The fine contents have small effect on the deviatoric stress which arrives at the peak stress, but after the peak stress, the effect on the stress becomes obvious very much. And it shows there is different not only the position of the steady state lines but also the slope of theirs when the fines are changed. The steady state line of clean sand is on top of that of silty sands and the steady state lines move down as the fines increasing up to 25%, then move upwards up to 60%. So the limit fine content of silty sands is 25%.4. Contribution from the coarser and the finer grains in silty sand to stress-strain ralationship is affected by the intergranular matrix structure. New intergranular state parameters, such as intergranular relative density ( D rs), intergranular and interfine void ratio( e sand e f), are induced to characterize the behavior of silty sands. As e s < emax,HSthe soil behavior of silty sand at an intergranular void ratio e s is similar to that of the host sand at a void ratio e equal to e sand the fine content and confining pressure have little effects on the stress-strain-strength relations. When the fine content exceeds 25%, the peak strength and residual strength are affected by the confining pressure. And the residual strength is more sensitive to the confining pressure than the peak strength. Even more, at e s in the vicinity of emax, HS, the fines and confining pressure have obvious effect on the stress-strain behavior of silty sand. The stability of specimen is better with the increasing fines and confining pressure.5.The effect of fines on the stability was discussed by the empirical correlation between SPT N-value and relative density, average grain diameter for sandy soils. The critical SPT N-values in steady state line of silty sands with different fines were calculated. It is validated that the flow potential of Nanjing sand is overestimated in Code for Seismic Design of Building.6. The results are analyzed for triaxial compression tests by superior sand model. Compared with the test results, the superior sand model can reflect the stress-strain behavior of Nanjing sand with medium confining pressures. But the result of the looser Nanjing sand under lower confining pressure can’t be simulated when the test was terminate at a shortfall axial strain of 20%.更多还原