波浪作用下海床液化-重固结移动边界分析模型及离心模型试验验证

改进了已有的移动边界分析模型,考虑了流体黏度和有限海床深度对波浪作用下海床液化-重固结行为的影响。采用层流Navier-Stokes方程替换原模型中的势流方程,从而合理描述液化/流态化海床和外部流体形成的黏性双层流体系统;同时,在控制方程中采用了有限深度海床的波致剪应力计算模型。通过与砂土和黏土中波浪–海床相互作用离心模型试验验证了该模型的有效性。结果表明液化海床尤其是砂土海床具有较大黏度,忽略流体黏度会高估液化/流态化海床表层的运动幅值;海床深度边界显著影响海床内部波致孔压分布规律,与以往无限深度海床计算结果存在较大区别。同时,该模型能够较好反映波浪作用下海床地基重固结后硬壳层的形成机制,以及海床地基内部孔压幅值放大效应。     

波浪作用下海床土体强度非均匀化数值分析

波浪在传播的过程中,产生的附加应力会使海床土体强度沿深度产生非均匀性变化。利用Biot固结理论,分析了波浪作用下海床土体的体积应变随深度分布情况,在此过程中考虑了土体的渗透系数、剪切模量、海床厚度、波浪周期和水深对海床土体的体积应变的影响。分析结果表明:对于无限厚度的粉质海床,波浪作用下在0.12个波长深度处有强度硬层存在;当沉积物的厚度小于0.5个波长时,波浪引起的强度硬层位于海床表面,当沉积物的厚度大于0.5个波长时,波浪引起的强度硬层所处的深度随着海床厚度的增加而增加;波浪引起的土体强度随着土体的剪切模量、水深、波浪周期及其渗透系数的影响在深度上呈现明显的非均匀变化。     

驻波作用下粉土海床累积液化机制分析

波浪被防波堤或岸壁反射时形成驻波,使其前面的水域发生强烈振荡,海床发生液化,可能导致结构物失稳破坏。以黄河三角洲粉土海床为研究对象,进行一系列驻波水槽试验,研究驻波作用下波腹剖面土体累积液化机制,在此基础上,采用数值模型进一步研究驻波作用下波浪参数(水深、波陡)和土体参数(饱和度)对海床累积液化的影响规律。结果表明:驻波作用下海床累积液化的发生与循环应力水平有关,当循环应力比χ达到累积液化所需的临界值χ_(cr)时,发生累积液化,深层土体累积液化所需的χ_(cr)大于浅层土体。波腹剖面土体液化所需的χcr远大于波节剖面,发生初始液化所需的时间大于波节剖面,液化深度小于波节剖面。波腹剖面土体累积液化是由波节及相邻位置向波腹剖面传递的超孔隙水压力P_(res1),和波腹剖面土体受循环正应力影响发生静压屈服引起的超孔隙水压力P_(res2)两部分共同作用所致。本试验中,在深度z=-0.05 m,波浪作用时间t≈600 s时,前者贡献比α1≈54.3%,后者α2≈45.7%。波节和波腹剖面土体累积孔隙水压力沿深度分布模式有差异,随水深减小,波陡增加,饱和度减小,海床液化深度在波节和波腹剖面均增大。     

双向耦合剪切条件下饱和松砂的液化特性试验研究

为了模拟海床及海洋建筑物遭受波浪荷载时所引起的循环应力,进行了一系列均等固结条件下的应力控制式轴向–扭转双向耦合循环剪切试验。加载路径在σd/2-τ应力空间内为椭圆。试验在保证椭圆面积不变的情况下,分别变化竖向和扭转向的荷载分量幅值,以此来探讨双向耦合剪切试验中各个分量的变化对饱和松砂的循环强度特性的影响。试验结果表明砂土在双向耦合荷载作用下,其液化强度与加载椭圆路径的面积和两个荷载分量比值密切相关。当轴向应力与剪应力幅值的比值保持不变时,砂土液化强度随着椭圆面积的增大而降低。而在椭圆面积保持不变时,当竖向与扭转向荷载分量的比值小于某一临界值0.6～0.75时,砂土液化强度随着比值的增加而增大,当竖向与扭转向荷载分量的比值大于某一临界值0.6～0.75时,砂土的液化强度随着比值的增加而减小,在临界值0.6～0.75之间表现出最高的强度。另外,在一个周期内孔隙水压力的循环变化与轴向应力相位一致,与循环剪应力相位相差90。     

海床波浪响应的积分变换解及其分析应用

考虑土骨架和孔隙水的压缩性,基于平面应变条件下的Biot方程,研究线性波浪荷载作用下海床的动力响应。采用积分变换的方法,推导突加波浪荷载引起的海床中的孔隙水压力、应力和位移解。以北海的波浪和海床特征为参数的算例表明:在进行近海结构物设计时应注意到突加波浪荷载作用下海床的瞬态响应与稳态响应有较大的差别,其最大瞬时液化深度和区域,较稳态响应计算得到的结果要大;对行波和驻波作用下海床响应的主应力旋转进行描述,指出对于有限厚度海床两种波浪响应的应力路径有显著差异;同时研究土性参数对孔压响应的影响。     

波流耦合影响下分层海床隧道周围砂土渗流孔压及液化分析

目前针对波浪作用下海底盾构隧道周围渗流场的既有理论研究一般将衬砌考虑为不透水介质,较少考虑隧道衬砌的渗透性,尤其是较少考虑波浪与海流共同作用对隧道的影响。此外,既有理论一般将海床视为均质且各向同性工况,忽略了实际情况下分层海床的影响。首先,基于波流共同作用下的海床表面的动力边界条件,采用传递-反射矩阵法得到波流共同作用下自由分层海床的孔压响应;其次,采用镜像法建立了由于隧道存在引起的砂土摄动压力控制方程,并利用砂土与衬砌间渗流连续条件获得了该方程的Fourier级数展开解析解;接着,采用叠加原理得到了波流共同作用下分层海床中隧道周围砂土的渗流压力响应及液化判定解答。最后,将理论解析解与数值结果及已有的试验结果进行对比,获得了较好的一致性。此外,针对海床渗透性和隧道衬砌渗透性进行了影响因素分析。结果表明：海流顺流会增大海床中的孔压和液化程度,逆流会减小海床中的孔压并抑制海床的液化,且流速相同时海床对逆流响应的相对差异总体上也大于顺流;当分层海床上层渗透系数较大时(k+s>1×10^-2 m/s),海床整体孔压较大,且第一次分层处孔压变化明显;当隧道衬砌渗透系数较...     

非标准椭圆形应力路径下饱和松砂动强度的试验研究

基于线性规则波浪作用下有限厚度海床动力响应的解析解,证明了非饱和、各向异性海床中一点土体单元上由主应力轴旋转所引起的应力路径应为非标准椭圆形而不是传统认知的圆形,并推导了量化该非标准椭圆形应力路径大小和形状的3个特征参数。通过27组不排水条件下的循环主应力轴旋转试验,讨论了轴向偏差应力和剪应力的幅值及初始相位差对饱和砂土动强度的影响。试验结果表明轴向偏差应力对动强度的影响高于剪应力的影响,同时初始相位差对动强度的影响显著。随着初始相位差的增大,破坏周次的对数基本呈线性减小,进而得出了非标准椭圆形和标准椭圆形两种应力路径下砂土动强度的关系。研究成果克服了传统圆形应力路径下土体动强度特性难以直接用于分析有限厚度海床稳定性的不足,可为近海海洋工程设计提供技术支持。     

轴向应力控制偏差对砂土液化特性影响研究

初始应力状态对饱和砂土动力液化特性有较大影响,应力控制偏差会导致液化试验结果出现较大离散。基于最新研制的高精度DSZ–Ⅱ型静–动统一三轴试验系统,辅之以精细试验技术,试验并分析初始轴向应力控制偏差对砂土液化特性的影响规律。初始轴向应力控制偏差会导致均等固结的液化振次发生明显变化,给砂土抗液化强度分析带来较大不确定性。初始轴向应力小于理想均等固结的初始轴向应力时,砂土在动力作用下更容易因拉伸而液化;而前者大于后者时,砂土在小控制偏差应力情况下的液化难度增加,但随着偏差应力增大,液化又变得较易发生。该结论仅限于均等固结轴向应力控制存在偏差的情况下,不涉及大固结应力比的情况。理想均等固结条件下,动应变在拉压两侧均能快速较大发展,初始轴向应力控制偏差会使得砂土动力变形幅度受到约束。轴向应力控制偏差对平均有效应力–偏应力(p'-q)关系曲线的中间发展阶段影响较大,但全部试验的p'-q关系曲线均被约束在相同的应力空间范围内。本研究对准确测定砂土抗液化强度指标具有参考意义。     

基于地基液化效应的深海吸力式基础失稳破坏机理研究

与陆地建筑物相比,海洋工程中构筑物的受力情况非常复杂,因此,对深水海洋结构物的稳定性与安全性提出了更高的要求。本文通过考虑地基液化效应,采用数值计算分析方法,开展海洋吸力式基础失稳机理研究,研究结果表明:①地震荷载导致地基液化地层土体强度在空间上分布不均匀,从而降低了地基承载力性能;②随着地震荷载作用时间的增加,海床砂土液化地层孔压逐渐消散,土层承载性能降低,致使吸力式基础结构原有的应力状态发生改变,易造成吸力式基础结构失稳破坏。     

砂土液化动稳态强度分析

根据动三轴实验数据，砂土的应力路径最终达到一个稳定的阶段，此时，在不同的振动周期，砂土的孔隙水压幅值不再增加，应力路径相互重叠，但是应变幅值却以恒定的速率不断增加，这种相对稳定状态中的极限强度是评估砂土抗液化能力的重要指标，并结合饱和砂土的孔隙水压力发展和应力路径，使砂土液化强度更加明了。     

Liquefaction analysis of sandy soil during strong earthquake in Northern Thailand

Northern Thailand has experienced several earthquakes which led to soil liquefaction in the past few decades. Traditional methods of evaluating liquefaction potential involve standard penetration test (SPT) or cone penetration tests. This research augmented experimental results with numerical methods to evaluate the liquefaction potential of Mae Lao Sand in Chiang Rai province of northern Thailand. SPT and downhole seismic test data collected during a field investigation at the Mae Lao site were compared to a 1D site response model analysis of the site. A series of undrained monotonic and cyclic triaxial tests was conducted on Mae Lao Sand specimens with different initial void ratios and confining pressures. Cyclic triaxial test results with varying deviator stress amplitudes were used to draw liquefaction resistance curves. Results from numerical simulation of sand liquefaction were used to characterise the stress–strain-pore water pressure response of Mae Lao Sand. 1D site response analysis determined seismic responses with different geological and groundwater conditions. All put together, the results showed that pore water pressure ratio decreases with increasing sand stiffness, the thickness of a soil layer significantly increases its liquefaction potential, and in-situ conditions and groundwater depths have major influences on the liquefaction potential of sand layers.     

Liquefaction and post-liquefaction resistance of sand reinforced with recycled geofibre

The present study provides an insight into the effect of recycled carpet fibre on the mechanical response of clean sand as backfill material subjected to monotonic loading and cyclic loading as well as post-liquefaction resistance of both unreinforced and carpet fibre reinforced soils. To achieve these goals, a series of multi-stage soil element tests under cyclic loading event resulting in liquefaction followed by undrained monotonic shearing without excess pore water pressure dissipation as well as a series of monotonic undrained shear test is conducted. All the specimens are isotropically consolidated under a constant effective confining stress of 100 kPa by considering the effect of cyclic stress ratio and carpet fibre content ranging from 0.25% to 0.75%. The obtained results revealed the efficiency of carpet fibre inclusion in increasing the secant shear modulus and ductility of clean sand under monotonic shearing without previous loading history. The impact of carpet fibre inclusion on the trend of cyclic excess pore water pressure generation and cyclic stiffness degradation was minimal. However, adding carpet fibre significantly improved both liquefaction and post-liquefaction resistances of clean sand. The liquefaction resistance of clean sand, at a constant 15 loading cycles, improved by 26.3% when the soil was reinforced with 0.75% recycled carpet fibre. In addition, the initial shear modulus of the liquefied specimen significantly increased by adding recycled carpet fibre.     

Liquefaction of Unsaturated Sand Considering the Pore Air Pressure and Volume Compressibility of the Soil Particle Skeleton

A series of cyclic triaxial tests of unsaturated soils was conducted to get a better understanding of the general liquefaction state of unsaturated soils. In the tests, cyclic shear strain was applied to fine clean sand with the same dry density but different initial suction states under the undrained condition. During cyclic shear, the volume change of the soil particle skeleton, the pore air pressure and the pore water pressure were measured continuously. Having used the effective stress defined by Bishop (Bishop et al., 1963), where the net stress and suction contribute to the effective stress, our test results showed that unsaturated sand specimens with quite a low degree of saturation lose their effective stress due to cyclic shear. At a zero effective stress state, unsaturated specimens behaved similarly to liquids in much the same way as saturated specimens. From experimental and theoretical considerations, the zero effective stress state (i.e., liquefaction) for unsaturated sand was found to have been established when both the pore air and water pressures build up to the point where it is equal to the initial total pressure. A volume change of pore air under the undrained condition, if a volume change of pore water is negligible, is equal to that of the soil particle skeleton. Therefore, it can be concluded that the liquefaction of unsaturated soil generally depends on the volume compressibility of the soil particle skeleton and the degree of saturation. On the other hand, according to the ideal gas equation of Boyle-Charles law, the volume change required to bring about a zero effective stress state can be calculated from the initial pore air pressure (usually the atmospheric pressure) and the final pore air pressure (the initial confining pressure). Therefore, the liquefaction of unsaturated soils also depends on the initial confining pressure. Based on this concept, the liquefaction potential of unsaturated soil can be evaluated by comparing the volume compressibility of the soil particle skeleton and the volume change of the pore air required to bring about a zero effective stress state.     

黄河口粉质土海床液化过程的现场试验研究

在黄河口海床若干深度处埋设孔压探头,海床表面利用活塞施加水压循环荷载以模拟波浪荷载,观测土体内孔隙水压力变化过程,研究黄河口原状和重塑粉质土在波浪循环荷载作用下的液化特征。通过研究发现,原状土和重塑土的孔压激发曲线模式不同,原状土孔压经历四个阶段,即先上升、后下降、再上升和剧烈波动;而重塑土孔压只有上升和剧烈波动两个阶段,这两种模式与循环荷载作用下粉土的微结构变化有关。分析波浪荷载的特点,将孔压出现剧烈波动的时刻作为完全液化的标志。土体液化与固结程度有关,固结程度越大,越不易液化。实际波浪荷载作用下,土体液化时间比文中试验测得的要短。     

动态土工真三轴仪在砂土液化研究中的应用

为了模拟实际场地中土体变形和强度特性,研究开发了揭示侧限条件下饱和砂土震动液化机理的试验方法。文中采用了动态真三轴试验系统,该设备采用刚性板加柔性面的混合边界加载装置,并应用先进的CATS试验控制系统。试验结果表明应用动态土工真三轴仪进行砂土液化实验是可行的,并初步分析了侧限条件下的液化机理,即土体将在轴向压力、侧压和孔隙水压力相等的条件下发生液化,表明振动荷载过程中的应力重分布对砂土液化强度和孔隙水压力发展等具有显著的影响。     

Nonlinear standing wave-induced liquefaction in loosely deposited seabed

Wave-induced residual liquefaction in loose seabed floor brings great risk to the stability of offshore structures in extreme climates. Understanding the characteristics of wave-induced residual liquefaction due to pore pressure buildup in loose seabed is meaningful for engineers involved in the design of offshore structures. In this study, standing wave-induced residual liquefaction is investigated deeply and comprehensively adopting a validated integrated numerical model. The time history of standing wave-induced pore pressure, effective stress, shear stress, lateral pressure coefficient \(K_0,\) stress angle, and displacement of seabed surface are all quantitatively demonstrated. The variation process of progressive liquefaction, stress path, as well as the stress-strain relation also are illustrated in detail. It is shown that the integrated numerical model FSSI–CAS 2D (FSSI: fluid–structures–seabed interaction, CAS: Chinese Academy of Sciences) incorporating the PZIII soil model can effectively and precisely capture a series of nonlinear dynamic response characteristics of loose seabed floors under standing wave loading. The computational results further confirm that the wave-induced liquefaction in loose seabed soil is progressive downward, initiating at the seabed surface. In addition, it is found that two physical processes, including vertical distribution of oscillatory pore pressure and time history of stress angle possibly could be used to judge the occurrence of wave-induced residual liquefaction in loose seabeds. Furthermore, it is also found that the progressive liquefaction process is significantly affected by wave height, permeability and saturation of seabed soil.     

Liquefaction response of loose gassy marine sand sediments under cyclic loading

Gassy sediments have often been encountered in the marine seabed, and they have different features from common saturated and unsaturated soil. By developing and improving an effective methodology, triaxial gassy sand specimens with different initial gas content (saturation ≥85%) were prepared in the laboratory. Their state parameters can be controlled in real time. A series of undrained dynamic triaxial tests by a Global Digital System (GDS) dynamic testing apparatus were conducted to investigate the liquefaction characteristics of gassy sand sediments. The results show that the gassy sand can liquefy the same as the fully saturated sand, but gas existence monotonically increases the sand liquefaction resistance. The occluded gas bubbles have significant influences on sand liquefaction properties. The dynamic pore pressure of gassy sand shows obvious features of slower accumulation, greater amplitude fluctuation, and deeper groove shape in time history curves of pore water, resulting from the effects of gas compression/expansion, migration, and dissolution/exsolution. By introducing a parameter of saturation, a modified model was proposed to describe the evolution of dynamic pore pressure of gassy sands. It was found that the model parameter θ is linearly dependent on the initial gas content (or initial saturation degree S_r).     

Liquefaction Resistances of Unsaturated Non-Plastic Silt

It has been pointed out that there are two possible mechanisms that enhance liquefaction resistances of unsaturated sand. The first mechanism is where air in a partially saturated sand mass plays a role of absorbing generated excess pore pressures by reducing its volume. Okamura and Soga (2005) derived the influential factors of the liquefaction resistance for partially saturated sand from theoretical consideration and effects of the factors were examined through a series of triaxial tests on a clean sand. They found a unique relationship between liquefaction resistance ratios and the potential volumetric strain, which allows the estimation of the liquefaction resistance for partially saturated sand. The second is the matric suction of unsaturated sand which increases the effective stress and thus the strength of the soil mass. In this study two series of cyclic triaxial tests on non-plastic silt were carried out to observe the liquefaction resistance in both mechanisms. In the first series, a top cap with an accumulator tank was used to study the effect of compressibility of pore fluid on the liquefaction resistance. The empirical relationship derived by Okamura and Soga is found to be valid even for the silt provided that the matric suction is negligible. In the second test series an ordinary cap was used. The liquefaction resistance increased linearly with the matric suction, with the increasing ratio being higher than that for the net stress. A unique linear relationship is found between the normalized liquefaction resistance and the matric suction. Results are summarized in the form which can be easily applied to evaluate the liquefaction resistance of a partially saturated soil.