【作者】 姚涛；

【导师】 邵龙潭；

【作者基本信息】 大连理工大学 ， 岩土工程， 2008， 博士

【摘要】 黄土在我国有比较广泛的分布。它作为一种特殊的土,除了具有一般土的工程性质外,还具有其独特的力学特征,主要如湿陷性、结构性和非饱和等。为满足黄土地区岩土工程建设的需要,黄土的工程性质研究一直是我国岩土工程和岩土力学研究的重要领域。但是受到三轴试验手段的限制,使用常规三轴试验方法研究非饱和黄土湿陷变形和强度特性时难以准确测定土体变形,也因为黄土的力学试验周期长,比较耗时,所以有关黄土试验资料的积累还不够丰富。将基于亚像素角点检测的数字图像测量方法应用于三轴试验试样变形测量中,为三轴试验试样变形测量提供了一种新方法,该方法通过捕捉橡皮膜上方块标记的角点位移对试样变形进行测量,测量结果能够达到较高的亚像素精度。应用该方法能够更加准确地测量饱和土和非饱和土三轴试验试样的局部变形,进而深入研究土在三轴应力状态下的变形与强度特性。本文使用GDS三轴试验仪,并结合数字图像测量系统对陕西杨凌Q₃黄土进行了三轴条件下的分级固结试验、三轴湿陷试验、不同应力比条件下的三轴增湿试验以及固结不排水剪切试验,分析和探讨了黄土在固结、湿陷和剪切阶段的变形特性和强度特性。通过研究,本文主要取得了如下成果:首先,通过分析黄土分级固结试验数据,得到了不同围压下土体的应变和时间之间的关系,探讨了三轴固结过程中,试样帽和底座对试样产生端部约束的影响范围,尝试使用围压和体应变的关系曲线确定土体的临界压力,为进一步预测土体在剪切试验中的应力—应变形式提供一定的依据。研究发现:在某一级围压下,无论是轴向应变、径向应变还是体积应变都和时间呈良好的双曲线关系。坐标变换后可以得到每级围压下的最终应变值,拟合发现最终应变值和围压呈良好的幂函数关系。应用通过拟合得到的应力—应变—时间的经验公式不但可以得到给定应力水平下的土体最终固结变形量,而且可以得到任意时刻的变形量。将土样划分为11个土段并对其径向应变进行分析发现,土体在中间五段的变形比较均匀,其他部分变形差异较大。因此,为减少端部约束对试样变形的影响,建议采用土体中间五段数据进行分析整理。基于重塑黄土三轴湿陷试验结果得到了试样各截面达到给定湿陷径向应变值所需要的时间,分析了土体累积湿陷体变率与湿陷时间的关系,并对重塑黄土湿陷变形影响因素进行了探讨。研究发现:土体湿陷应变与围压、初始含水率和初始干密度均呈反比例关系。使用试样中间五段数据进行处理得到的湿陷应变明显高于使用试样整体数据得到的湿陷应变值,但由于端部约束对不同初始条件的土体影响程度不同,因此两种方法得到的曲线形式并不完全一致。基于重塑黄土不同应力比条件下的三轴增湿试验,定义了单位球应力下的湿陷体应变ε_vp^*,以此表明球应力对湿陷体应变的贡献率。研究发现:在相同含水率的情况下,围压越大,湿陷体应变的值也越大,但同时,围压越大,单位球应力下的湿陷体应变却越小。提出了由试验结果得到的s_vp^*与含水率w之间关系的数学表达式,该表达式中包括峰值单位球应力下的湿陷体应变ε_vpmax^*和表征曲线关系发生转折时的特征含水率w^*这2个待定参数,可以通过试验确定。通过归一化处理发现在4种k值和4种围压组合的16种情况下的试验结果都分布在S型条带内,通过平均化处理,给出了以特征含水率w^*的平均值,最大值和最小值确定的平均特征曲线、下包络线和上包络线。研究发现对于参数ε_vpmax^*,围压水平越低,在不同k值条件下的ε_vpmax^*之间差异越大,反映出在低围压,高偏压固结条件下单位球应力下重塑黄土会产生更大的湿陷体应变,对于实际工程而言不利于安全,因此在工程中应尽量避免低围压,高偏压固结情况的产生。最后通过对不同初始条件下的重塑黄土进行固结不排水剪切试验分析了不同初始条件下土体的应力—应变表现形式,并对围压、初始含水率和初始干密度等影响因素进行了探讨,研究了土体抗剪强度与含水率、干密度的关系。研究发现在不同的初始含水率和初始干密度条件下,重塑黄土应力—应变关系曲线既有软化型又有硬化型。对比局部变形和整体变形的结果,认为在较高围压条件下,围压对于端部约束对测量结果的影响起到了限制作用。研究还表明采用局部变形数据进行数据处理,其优势在于对软化型曲线的测量上更加准确,体变的突变点更加清晰。对于体变的测量更加准确,试验数据整理方法的差异对于体变—轴向应变关系曲线而言比应力—应变关系更加敏感。研究重塑黄土粘聚力和内摩擦角与含水率之间的关系后发现:由整体变形和局部变形确定的粘聚力和内摩擦角都随着含水率的增大而减小,随着干密度的增大而增大,且由局部变形确定的内摩擦角比由整体变形确定的值要小。含水率对粘聚力和内摩擦角都有很大影响,粘聚力和内摩擦角随着含水率的增加而下降,相关分析表明粘聚力与含水率二者呈指数关系,内摩擦角与含水率之间呈二次多项式关系。水分状态变量对重塑黄土抗剪强度指标粘聚力的影响在含水率水平较低时起主要作用,在含水率水平较高时,对土体抗剪强度的弱化作用并不显著。水分状态变量对抗剪强度指标内摩擦角的影响在含水率水平较高时起主要作用,在含水率水平较低时,对土体抗剪强度的弱化作用并不显著。更多还原

【Abstract】 Loess is widely distributed in China.Besides the engineering properties of general soil, loess has particular mechanics characteristics,such as collapsibility,structural property,and unsaturated.In order to meet requirements in the geotechnical engineering construction in loess area,study on engineering characteristics of loess is an important field of research on the geotechnical engineering and geotechnical mechanics.But accumulation of loess’s test data is not enough abundant,because the limitation of triaxial test means,and the long test period,and much time consuming of the mechanics tests of loess.Digital image measurement method,which is based on sub-pixel comer detection,is a new technique of triaxial specimen deformation measurement.This technique measures the specimen deformation through the tracking of comer detection displacement,and the result precision can be up to sub-pixel level.This new technique can measure the triaxial specimen deformation of both saturated and unsaturated soil more exactly,and can deeply analyze the soil stress-strain property under triaxial stress state.As a typical unsaturated soil,it is difficult to accurately confirm the deformation of loess sample in order to study the characteristics of collapsible deformation and strength using conventional triaxial test method.A new deformation measurement method is provided in triaxial test of loess by using digital image measurement system.In this paper,deformation and strength characteristics of loess in the process of consolidation,collapse and shearing are analysed and inquired by means of triaxial staged consolidation test,triaxial collapsibility test, triaxial moistening test under different stress ratio and consolidation-undrained triaxial shear test of Yangling Q₃ loess in Shanxi by using of GDS triaxial testing apparatus along with digital image measurement system.Firstly,the relationship between strain and time of loess under different confining conditions is obtained by analyzing data of triaxial staged consolidation,the influence scope of end restraint is discussed,and the critical strength is determined through analysis of the relation between confining pressure and volumetric strain.The research shows that,to axial strain,radial strain and volumetric strain,each of which and time can be described with hyperbolic curve under the certain confining pressure.The final strain under each grade pressure can be obtained through coordinate transformation,and the final strain has an exponential relationship with the confining pressure by fitting.The formula found by fitting that can predict the consolidation deformation of soil under next grade load.The Soil specimen is divided into 11 segments and the radial strain is analyzed,what can find that the deformation difference of the middle five segments is uniform,and the deformation of other parts is great different.Therefore,in order to reduce the effect of end constraint on the deformation of specimen,using the data of middle five segments is proposed to analysis.The results based on the remolded loess triaxial collapsibility test can educe the time that every cross-section of specimen achieved required collapsible radial strain.The relationship between the accumulated collapsible volumetric strain rate of soil and the time is analyzed, and the factors of collapsible deformation for remolded loess are discussed.Study shows that collapsible strain with confining pressure,initial water content and initial dry density have reciprocal proportion relations.The collapsible strain can be gained by using the data of middle five segments of the specimen to process that significantly higher than the collapsible strain of overall specimen.However,due to the effect of end constraints is different on the soil under different initial conditions,so the curves of two methods are not exactly same.Based on the triaxial moistening test of remolded loess under different deviatoric stress,ε_vp^*,the collapsible volumetric strain in the unit spherical stress is defined in order to show the contribution rate of stress on the collapsible volumetric strain.Under the same water content condition,the greater confining pressure,the greater collapsible volumetric strain,but at the same time,the collapsibile volume strain under the unit spherical stress was smaller when the confining pressure was greater.The mathematical expression ofε_vp^* and water content w is made by the test results,which included two parameters those are the collapsible volumetric strain under the peak unit spherical stress and the characteristic water content w^*. Through normalization processing found that 4 kinds of k value and 4 kinds of confining pressure combined 16 kinds of circumstances,the test results are distributed in S-band.By an average of treatment,the average characteristic curve,the lower envelope and the upper envelope are determined by the average,maximum and minimum value of the characteristic water content.To parameterε_vpmax^*,the lower confining pressure,the greater the difference of eachε_vpmax^* under different k values conditions.The results reflect that the remolded loess can generate more collapsible volumetric strain in the unit spherical stress under the low confining pressure and the high deviatoric pressure consolidation,which is not conducive to security at actual projects,so it should try to avoid generating low confining pressure and high deviatoric pressure consolidation in the project.Finally,stress-strain curve types of remolded loess under different initial conditions are analyzed,and the influence factors such as confining pressure,initial water content,initial dry density are discussed,the relationships between shear strength and water content,dry density are studied.It is found that strain softening and hardening both appeared in the stress-strain curves of remolded loess under different initial water content and initial dry density. Compared the result of local deformation with overall deformation,confining pressure restricts the influence which end restraint to measurement result under higher confining pressure.The study also indicates that the advantages of data processing with the local deformation are more accurate measurement of strain softening curve,much clearer break point of volumetric strain.The relationships between the cohesive strength,the friction angle and water content of remolded loess are studied.And the research finds that the cohesive strengths and the friction angles confirmed with the local deformation and overall deformation both minish along with the accretion of water content,increase along with the increase of dry density,and the friction angle obtained from the local deformation is smaller than the overall deformation.Analysis indicates that the cohesive strength has an exponential relationship with the water content,the relationship between the friction angle and water content conforms to quadratic polynominal function.The influence on cohesive strength from water state variable plays main effect only when the level of water content is lower,the debilitation is not significant on the shear strength when the level of water content is higher. The influence on the friction angle from water state variable plays main effect only when the level of water content is higher,the debilitation is not sigcificant on the shear strength when the level of water content is lower.更多还原