【作者】 聂影；

【导师】 栾茂田；

【作者基本信息】 大连理工大学 ， 岩土工程， 2008， 博士

【摘要】 随着海上能源的开发,海洋工程的数量和规模都在迅速发展,因此探索波浪荷载作用下海床的动力特性引起了研究者的极大兴趣。波浪荷载在海床中所产生的循环应力的主要特点之一是正应力偏差与剪应力所形成的循环偏应力的幅值不变而主应力轴方向发生了连续旋转。另外,海洋建筑物下地基内各个部位土单元的应力状态是各不相同的,沿着某一潜在滑动面,各点的初始主应力方向随其位置而改变。迄今,人们在研究海洋土体在主应力轴连续旋转条件下的变形与强度特性时,大多数都是采用的重塑砂土进行试验,成果也较多。然而在我国沿海地区海洋建筑物下覆地基表层土大都为数米厚的软黏土覆盖层,但饱和黏土在波浪作用下的动力特性及其强度的研究目前相对较少。作为海床与海洋建筑物地基稳定性评价中的一个基本而重要的问题,探讨复杂应力条件下饱和黏土的变形与强度特性,对于指导波浪荷载作用下饱和黏土海域海床稳定性分析,以及实际工程的设计和施工,具有重要的理论价值和实际工程意义。因此,为配合海洋工程设计与建设,必须进行黏土在复杂应力条件下的循环特性试验研究,发展海洋土动力学的实验技术与分析理论。国内外学者已对饱和黏土的动力特性进行了一定的研究,但由于试验条件的限制,一般采用常规动三轴试验仪和常规动扭剪仪进行试验,不能考虑主应力方向连续旋转的影响,无法很好地模拟波浪荷载作用下饱和黏土的复杂应力状态。基于对波浪荷载作用下饱和黏土动力特性研究的重要意义及其研究现状,本文利用土工静力—动力液压三轴—扭转多功能剪切仪,模拟海洋地基土单元的实际受力条件,对饱和重塑黏土的动模量与阻尼比,主应力轴连续旋转条件下的竖向—扭向循环耦合剪切特性进行了较为深入而系统的研究。全文主要包括以下几方面内容:(1)因现场取样费用高,地基土的空间变异度大,难以保证所获取土样的数量和均匀性,分析波浪、交通荷载等作用下土体参数的变化规律和系统探索土的基本规律性十分困难。因此在难以获得原状土样而又需要系统地探讨土样动力特性时,一定程度上是依靠室内制备重塑土来完成试验的。本文采用改进的真空抽吸法制备成批性质均一的黏土试样,作为土工试验所需的样品,这样既消除了土样的不均匀性,又保证了不同应力历史和不同类型试验之间的合理比较。改进的黏土制样方法具有操作简便、制样时间短,所制备试样成分均一、饱和度高,且易于切削制备圆柱或空心圆柱试样等优点。同时在较短时间内能以较低的成本获取大量试验土样,在室内对黏土进行试验特性的研究中可以采用。(2)采用单个试样在不同围压下多次固结(包括加卸压)的动扭剪方法来获取最大剪切模量和阻尼比等动力特性参数,并将这种方法获得的动剪切模量和阻尼比与采用多个试样的常规方法获得的试验结果进行了比较,探讨了两种试验方法获得的动剪切模量和阻尼比的主要差别,验证了这种简化方法的可靠性。单个试样的试验方法具有试验结果离散性较小,可以减小由于试验操作、试样均匀性等因素对试验结果分析的影响,节省取样数量并减少装样、拆样的工作量等优点。并且探讨了模拟波浪荷载等主应力轴连续旋转条件下饱和黏土的动模量和阻尼比的变化规律,建立了动模量和阻尼比的计算模式。(3)在实验室内模拟主应力轴连续旋转的重塑饱和黏土试验,对评价海床在波浪荷载作用下的稳定性,无疑具有重大的意义。因此针对饱和黏土开展了应力控制的双向耦合循环试验研究,详细研究了饱和黏土在主应力轴连续旋转条件下的动强度、孔压、变形、广义临界循环应力比、应力洛德参数等特性。基于试验结果探讨了不同剪应力幅值水平、振动波形、超固结比、振动频率等试验因素对土体性状的影响。评估黏土在波浪等主应力轴连续旋转条件下的动强度时,本文建议的综合应变式同时考虑了水平向剪切变形和轴向变形的共同效应,又反映了平均残余变形和循环变形的影响,可以作为主应力轴连续旋转的耦合循环剪切试验的破坏标准。由于黏性土在耦合循环荷载作用下孔隙水压力的量测存在着滞后现象,导致黏土土样的孔压在轴向和径向形成梯度,而且受到试验仪器本身的限制,孔压传感器安装在土样底部,测量到的孔压与土样中的实际孔压有一定的差距。为更好地反映轴向—扭转双向循环耦合作用下黏土的孔压,本文采用循环荷载停止后,将振动破坏的土样静置继续量测孔压,直到其达到一个稳定的最高值并记录下来,作为动载后黏土的孔隙水压力值。试验结果表明循环荷载停止后,试样的孔压在达到稳定前还会有较大的增加,采用循环荷载停止后静置继续采集孔压的方法可以更好的反映黏土的孔压发展规律。(4)针对饱和黏土在均等固结不排水条件下进行了应力控制的主应力方向交替变化和主应力方向连续旋转的不同应力模式下的循环剪切试验,通过对比探讨了模拟波浪荷载的不同应力模式对饱和黏土剪切特性的影响。研究表明:不同应力模式下的应力—应变关系曲线具有显著的不同,滞回曲线在循环初期均比较致密,随着循环次数的增加渐渐疏松,割线模量不断衰减:中主应力系数和应力洛德参数分别在主应力方向交替变化和主应力方向连续旋转变化的不同应力模式下变化规律也不同;主应力方向连续旋转的应力模式中,动剪切分量大的椭圆应力模式的动强度略小于动正向偏差分量大的椭圆应力模式的动强度。(5)针对饱和黏土进行了不同初始主应力方向角的单调扭剪试验,主要研究了不同初始主应力方向对单调剪切强度、轴向应变、剪切过程中主应力方向和孔隙水压力的影响。在三向非均等固结不排水条件下,进行了应力控制的不同初始主应力角的循环扭剪试验和主应力轴连续旋转的双向耦合循环剪切试验,通过对试验结果进行分析和探讨,考察了初始主应力角对重塑饱和黏土应变发展模式、动强度、孔隙水压力和广义临界循环应力等的影响。同时基于不同初始主应力角的循环扭剪试验和主应力轴连续旋转的耦合循环试验结果,建立了考虑初始主应力方向角的动强度经验关系式。更多还原

【Abstract】 With the exploitation and utilization of marine energy resource, the quantity and scale of ocean engineering are booming fast. Therefore, it takes great interests to the investigators about discovering the dynamic property under wave loading. The cyclic stresses induced by wave loading are characterized by the fact that the amplitude of the cyclic deviatoric stress originated from the deviation of normal stresses and the shear stress keeps unchanged but the orientation of principal stress axe rotate progressively. Moreover, the stress states of soil elements located at different parts in the foundation of ocean structures are all different. The orientation of initial stresses of all the points along a potential sliding surface was strongly depended on the location of the point. So far, with many successes, most experiments have been taken on remolded sand or silty soil on the study of deformation and strength characteristics about ocean soil in condition of continuous rotation of principal stress directions. Only fewer researches on the dynamic property and strength of saturated clay in wave loading have been taken. However, the topsoil under the foundations of marine structure in coastland is mostly soft clay cover layer of several meters. Consequently, as a basic and important problem in the foundation stability estimate of seabed and marine structure, to discuss the deformation and strength characteristics of saturated clay in condition of complex stress have great importance on theoretical value and practice engineering to further develop and perfect the research on ocean clay property. It is also to guide stability analysis of saturated clay sea area or seabed and practice technical engineering and construction under wave loading. Since above, to assort the design and construction of ocean engineering, the experimental investigation should be taken on the cycle performance of clay in condition of complex stress to develop the experimental technique and analyzing theory of ocean soil dynamics.The scholars abroad and at home have been already proceeded some definite studies on dynamic property of saturated clay. Due to the limitation of testing condition, the conventional dynamic triaxial test apparatus and conventional dynamic torsion shear apparatus were employed without considering influence of consecutive rotation in principal stress direction. So that the state of stress of saturated clay under wave load was hardly simulated. According to the significance and research status about the dynamic property on saturated clay under complex loading, the geotechnical testing equipments including the advanced apparatus for static and dynamic universal triaxial and torsional shear soil testing are employed to perform monotonic triaxial tests and cyclic shear tests under consolidated-undrained condition. Through a series of tests on saturated clay, the dynamic behavior including dynamic shear modulus and damping ratio and shear characteristic under continuous rotation of principal stress directions condition on clay are synthetically examined and studied. The main investigative contents and achievements consist of the following parts.(1) Due to the much specimens of clay needed in this research, considering the difficulty and huge expense to get the original state clay specimens of high quality, it is unpractical to get undisturbed specimens. It increases the discreteness of test results and many disadvantages in analyzing the geotechnical testing results for adopting the non-uniform undisturbed specimens. So it is an approach to investigate the strength and deformation behavior of saturated clays by utilizing repeatable and uniform saturated clay samples prepared in the laboratory. The author employed advanced vacuum suction method to prepare mass of clay samples of homogeneous properties, as the specimen required in soil tests, which removes the non uniformity and ensure the reasonable comparison between tests of different stress histories and types as well. The improved method possesses the advantages of convenient operation and shorter time of preparing samples. The prepared clay samples have the homogeneous property and highly saturated degree, and were easy to cut for preparing the cylinder or hollow-cylinder testing samples, which supplies a feasible way to uneasily obtain undisturbed samples and systematically to discuss engineering characteristic problems in clay.(2) The author conducted cyclic torsional shear test on single hollow cylindrical specimen to measure dynamic shear modulus and damping ratio under different confining pressures (including imposed and reduced pressure) to replace the conventional dynamic characterizes method. Also the differences of both test results should be discussed as well. The conclusion is that the results from single specimen method agree with those from conventional method. Furthermore, this method mentioned above could save amount of sampling, solve the problems of soil uniformity and supply a lot of advantages on base of reliable experimental result. In order to improve the precision of experiment, both the untouched and touched sensor are used together. The dynamic property on clay with rotation of principal stress under coupling cyclic shear with tiny amplitude strain behavior was examined. In addition, the computation of dynamic modulus and damping ratio was established.(3) Cyclic rotation of principal stress direction with constant amplitude is the cyclic stress characteristics in seabed deposit induced by waves. The apparatus for static and dynamic universal triaxial and torsional shear soil testing is employed to perform stress-controlled cyclic coupled vertical and torsional shear test under different amplitude of shear stress, waveforms of imitated wave, overconsolidated ratio and frequency. Through a series of tests on saturated clay, the dynamic strength, dynamic deformation behavior, pore water pressure, general critical cyclic stress ratio and Lode parameter of stress were systematically investigated. It was shown that a general strain fail criterion for clay was recommended for considering the integrative effects of torsional shear strain and normal deviatoric strain as well as cyclic strain and accumulative strain. Moreover, the general strain fail criterion was fit for the failure criterion of coupling cyclic test. The water pore pressure has hys-teretic phenomenon for clay as measured under cyclic loading, and it is believed that gradient of axial and radial of water pore pressure induced by it. However, the measurement of water pore pressure is proceeded at the bottom of the hollow specimen. Apparently, the measurement of water pore pressure can not reflect the truth increasing of water pore pressure completely under the cyclic loading. Thus, when coupling cyclic loading in cohesive soil is terminated at failure states, a continuous measurement of acquiring pore water pressure which was getting stable after ceasing coupling cyclic loading was adopt. The method of acquiring pore water pressure after ceasing coupling cyclic loading was able to better reflect the variation of water pore pressure.(4) The apparatus for static and dynamic universal triaxial and torsional shear soil testing is employed to perform stress-controlled cyclic test to simulate wave loading by alternative change of principal stress direction mode and continually rotation of principal stress direction mode under isotropic consolidation condition is examined. It was found that stress-strain relationship of shear stress and normal stress had obvious affected by cyclic stress mode and had distinguishing between consolidated-undrained and unconsolidated-undrained test by comparing the experiment data. Meanwhile, it was observed that stress-strain relationship curve gradually loosen and the secant modulus decayed as the cyclic number increased. Moreover, it was found that the variation of the coefficient of intermediate principle stress and Lode parameter of stress distinguishing between the alternate changed of principal direction mode and rotation of principal stress axes mode by comparing the experiment data. It was revealed that the dynamic strength and critical cyclic stress of alternative change of principal stress direction mode was more than the continually rotation of principal stress direction mode. The more far away between the deviatoric normal stress and the shear stress, the critical cyclic stress more is..(5) The typical test results under different orientation of initial principal stress with monotonic torsional shear test shows obviously features as undrained shear strength, various orientation of principal stress as well as pore water pressure evolution. The effect of initial orientation on dynamic strength, pore water pressure and critical cyclic stress are examined on the triaxial-torsional coupling shear tests and the cyclic torsional shear tests. In the other hand, an empirical formulation which considering the initial angle of principal stress effects on dy- namic strength of saturated clay was deduced from the experimental result of triaxial-torsional coupling shear tests and the cyclic torsional shear tests.更多还原