海底粉质土波致液化的判别程序与计算方法

海底土体在波浪作用下能否产生液化是海岸工程所关心的问题。借鉴地震液化判别使用的砂土液化判别方法,将海底粉质土波致液化的判别分为初判和复判2个阶段。初判以所致海床土体发生破坏的临界循环应力比界限指标来判别,以土质基本特征和波浪条件为参数,对某海域海底液化形成判断;复判以波致海床土体中剪应力与实际土体的动剪切强度比较来判别。结合已有研究成果给出了波致土体液化判别的具体方法。     

海域工程场地可液化土的地震液化效应分析

能否对砂土液化进行准确的判断评价,与工程的安全性、经济效益和社会效应关系密切。目前,勘察规范和抗震规范中均没有专门针对海底岩土层饱和砂土液化判别方法的规定和要求。文章对目前饱和砂土可液化性判别方法进行了分析总结,给出了海域工程场地可液化土的液化判别方法及存在的问题,提出海域工程建设场地饱和砂土层的判别宜采用标准贯入试验法和室内液化振动三轴试验抗液化剪应力判别法相结合的方式进行综合评价。     

波浪作用下埕岛海域海底土液化分区

根据埕岛海域表层沉积物特征,结合该区的波浪实测资料推算的波浪要素,利用动三轴实验得到研究区土体在循环荷载作用下孔隙水压力的增长与振动次数的关系,计算研究区内海底土层的液化可能性和液化所需的时间,并根据土体在不同水深情况下达到液化所需的时间对研究海域进行了液化分区。结果显示,7-8 m等深线之间的海底土体由于受到波浪破碎作用的影响,最易发生液化,液化影响深度也最深,自该海域向近岸和远海,液化可能性降低;土层埋深为2.5 m以浅时,研究区大部分区域液化可能性为高,而到埋深为4 m时土层液化可能性明显降低。     

黄河水下三角洲浅表土体的风暴液化问题

黄河水下三角洲的地质勘察揭示了海底浅表地层发生的各种灾害地质现象.以风暴浪导致海底土体液化观点,结合土体动力三轴试验、波浪水槽试验,对黄河水下三角洲浅表地层土体的液化发生条件、形成模式、液化土体运动以及地层发生的重新层化问题进行了分析,指出黄河水下三角洲的灾害地质由于风暴浪导致海底粉质土液化运动而形成,液化后土体运动形...     

海底管线区浅表层土孔压静力触探参数特征及应用

利用东海陆架海底输油管道路由调查获得的孔压静力触探数据和钻探资料等,采用统计分析的方法,对管道路由区5 m以浅土体的孔压静力触探曲线特征、土的分类方法鉴别作了探讨,结果表明:研究区5 m以浅黏性土的锥尖阻力(qc),侧摩阻力(fs)值较小,且随深度呈线性增加,粉质土的qc,fs值增大,但随深度线性增加规律不明显,砂类土的qc值急剧增大,但fs值变化不大;Robertson法和Eslami-Fellenius法两种土类划分方法均适用于研究区浅表层软土的土类划分,但Robertson法在判别粉质土时受到一定限制,Eslami-Fellenius法判别较为准确,应用简单。本研究可为我国在海底管线工程路由勘察中直接利用孔压静力触探(CPTU)参数划分土层和判别土类作铺垫。     

黄河水下三角洲浅表土体的风暴液化问题探讨

黄河水下三角洲的地质勘察揭示了海底浅表地层发生的各种灾害地质现象。本文以风暴浪导致海底土体液化观点,结合土体动力三轴试验、波浪水槽试验,对黄河水下三角洲浅表地层土体的液化发生条件、形成模式、液化土体运动以及地层发生的重新层化问题进行了分析,指出黄河水下三角洲的灾害地质由于风暴浪导致海底粉质土液化运动而形成,液化后土体运动形式与波浪运动一致,液化土体运动造成的土颗粒分异而使地层重新层化,并初步指出了风暴浪导致海底土体液化在地学、环境、工程等方面的研究问题。     

地震作用对琼州海峡海缆路由区海底稳定性影向的分析

以琼州海峡的北海—临高段海缆路由为例,利用计算对比的方法,分析了地震作用下,路由区3m以浅海底土液化或滑移的可能性,评价了路由区海底的稳定性。结果发现,在地震烈度为Ⅶ度的地震作用下,砂性土海底的地震剪应力平均值(0.86kPa,1.72kPa和2.57kPa)小于砂性土的液化剪应力(1.60kPa,3.20kPa和4.81kPa);粘性土海底的地震剪应力平均值(1.25kPa,2.49kPa和3.74kPa)小于粘性土的抗滑剪应力平均值(4.07kPa,4.61kPa和4.49kPa),路由区海底稳定性良好;在地震烈度为Ⅷ度的地震作用下,砂性土海底的地震剪应力平均值(1.73kPa,3.45kPa和5.12kPa)大于砂性土的液化剪应力;粘性土海底的地震剪应力平均值(2.49kPa,4.98kPa和7.47kPa)除表层外,大于粘性土的抗滑剪应力,路由区海底会发生失稳破坏。     

波浪作用下海底粉土孔隙水压力响应过程监测研究

波浪引起的海底土体内部孔压累积是导致液化发生的主要原因,研究波浪作用下土体内部孔压响应过程对于明确液化机理、预测液化发生具有重要作用。在黄河口使用自行研发的孔压监测设备对海底粉土孔隙水压力进行了有效监测。监测结果显示,海底粉土的孔压变化主要受波浪影响且存在一定的影响范围,超出该范围则波浪对海底粉土的孔压无影响。同时,基于监测过程内的孔压变化对海底粉土进行了液化评判,并对波浪作用效果和液化影响因素进行了探讨。波浪对海底粉土内部孔压影响效果主要有3种:(1)有孔压振荡但不发生累积;(2)有孔压振荡且发生累积;(3)无孔压振荡且不发生累积。     

波浪作用下黄河三角洲海底粉土液化特征对比研究

作为一种常见的近海海底灾害地质现象,波致海床液化严重威胁着黄河三角洲地区海底工程设施的安全。粉质海床液化后,海底粉土的结构、物理和力学性质均发生了改变,研究该变化规律尤其是评估液化后海底粉土再次发生液化的可能性具有重要的理论意义和应用价值。本文利用室内动三轴仪对取自黄河三角洲已液化和未液化海底粉土开展了液化试验对比研究,讨论了已液化和未液化海底粉土在孔压增长模式和轴向动应变发展趋势方面的异同,对比分析了二者的液化势。研究结果表明:应变标准比孔压标准更适用于评估黄河三角洲地区海底粉土的液化势;孔压和动应变发展模式均表明与未液化粉土相比,已液化海底粉土再次发生液化的抗力有所提高;已液化和未液化海底粉土归一化孔压比u_d/σ₃与循环加载次数比N/N_f间相关关系可采用双曲线或指数函数模型进行定量化描述;未液化海底粉土的波致液化临界循环应力比约为0.20,已液化海底粉土的临界循环应力比约为0.35。研究成果有助于加深对海底粉土波致液化特性的认识,亦可为循环应力历史影响下的土体力学性质研究提供参考。     

辽东湾北部浅海区海洋工程地质特征

通过对浅地层剖面仪、侧扫声纳和海底取样等实测资料的详细分析,揭示了辽东湾北部浅海区的自然地质状况和复杂的海洋工程地质特征。该海区海底地形地貌较为复杂,存在海底冲刷深槽、埋藏古河道、浅层气、潮流沙脊与潮沟、埋藏古陡坡、软弱土层及可能的砂土液化层等潜在灾害地质因素,对海上构筑物存在直接或潜在的危险性,应引起高度重视。     

波浪作用下粉质土海床液化界面波压力的研究

波浪作用下粉质土海床的液化是影响海上平台、海底管线等海洋构筑物安全的灾害之一。在进行构筑物设计中应考虑海床液化的深度问题,而液化土体对下部海床的界面波压力是计算海床孔隙水压力增长以及液化深度的重要参量。本文基于波致粉土海床自上而下的渐进液化模式,利用双层流体波动理论,推导了考虑海床土体黏性的海床界面波压力表达式,并与不考虑黏性时的界面波压力进行了比较分析。结果表明,计算液化后土体界面波压力时,是否考虑液化土体的黏性对结果影响较大,进而可能影响粉质土海床液化深度的确定。     

Wave-induced seabed response in shallow water

To simulate the wave-induced response of coupled pore fluids and a solid skeleton in shallow water, a set of solutions with different formulations (fully dynamic, partly dynamic, and quasi-static) corresponding to each soil behavior assumption is presented. To deal with Jacobian elliptic functions involved in the cnoidal theory, a Fourier series approximation is adopted for expanding the boundary conditions on the seabed surface. The parametric study indicates the significant effect of nonlinearity for shallow water wave, which also enhances the effect of soil characteristics. The investigation of the applicability of reduced formulations reveals the necessity of a partly or even fully dynamic formulation for the wave-induced seabed response problem in shallow water, especially for thickened seabed. The analysis of liquefaction in the seabed indicates that the maximum depth of liquefaction is shallower, and the width of liquefaction is broader under cnoidal wave loading. The present analytical model can provide more reasonable result for the wave-induced seabed response in the range of shallow water wave.     

Parametric study of the wave-induced residual liquefaction around an embedded pipeline

Wave-induced liquefaction in a porous seabed around submarine pipeline may cause catastrophic consequences such as large horizontal displacements of pipelines on the seabed, sinking or floatation of buried pipelines. Most previous studies in relation to the wave and seabed interactions with embedded pipeline dealt with the wave-induced instaneous seabed response and possible resulting momentary liquefaction (where the soil is liquefied instantaneously during the passage of a wave trough), using theory of poro-elasticity. Studies for the interactions between a buried pipeline and a soil undergoing build-up of pore pressure and residual liquefaction have been comparatively rare. In this paper, this complicated process was investigated by using a new developed integrated numerical model with RANS (Reynolds averaged Navier–Stokes) equations used for governing the incompressible flow in the wave field and Biot consolidation equations used for linking the solid–pore fluid interactions in a porous seabed with embedded pipeline. Regarding the wave-induced residual soil response, a two-dimensional poro-elastoplastic solution with the new definition of the source term was developed, where the pre-consolidation analysis of seabed foundation under gravitational forces including the body forces of a pipeline was incorporated. The proposed numerical model was verified with laboratory experiment to demonstrate its accuracy and effectiveness. The numerical results indicate that residual liquefaction is more likely to occur in the vicinity of the pipeline compared to that in the far-field. The inclusion of body forces of a pipeline in the pre-consolidation analysis of seabed foundation significantly affects the potential for residual liquefaction in the vicinity of the pipeline, especially for a shallow-embedded case. Parametric studies reveal that the gradients of maximum liquefaction depth with various wave and soil characteristics become steeper as pipeline burial depth decreases.     

波浪作用下单桩基础周围海床液化机制研究

建立波浪作用下单桩周围三维海床动力响应模型,考虑自重影响下的海床长时间固结过程。采用已有物理模型试验数据对模型进行验证,证实其具有较好的适用性。模拟波浪作用下单桩周围三维海床液化区域,通过定量分析超孔隙水压力和土体初始有效应力的变化,讨论单桩插入深度对海床液化的影响机制。研究表明,单桩插入深度发生变化时,土体初始有效应力对海床液化的影响要大于超孔隙水压力,且影响程度随着插入深度的增加而逐渐增大。     

Effects of cross-correlated multiple spatially random soil properties on wave-induced oscillatory seabed response

The evaluation of seabed response under wave loading is important for prediction of stability of foundations of offshore structures. In this study, a stochastic finite element model which integrates the Karhunen-Loève expansion random field simulation and finite element modeling of wave-induced seabed response is established. The wave-induced oscillatory response in a spatially random heterogeneous porous seabed considering cross-correlated multiple soil properties is investigated. The effects of multiple spatial random soil properties, correlation length and the trend function (the relation of the mean value versus depth) on oscillatory pore water pressure and momentary liquefaction are discussed. The stochastic analyses show that the uncertainty bounds of oscillatory pore water pressure are wider for the case with multiple spatially random soil properties compared with those with the single random soil property. The mean pore water pressure of the stochastic analysis is greater than the one obtained by the deterministic analysis. Therefore, the average momentary liquefaction zone in the stochastic analysis is shallower than the deterministic one. The median of momentary liquefaction depth generally decreases with the increase of vertical correlation length. When the slope of the trend function increases, the uncertainty of pore water pressure is greatly reduced at deeper depth of the seabed. Without considering the trend of soil properties, the wave-induced momentary liquefaction potential may be underestimated.     

波浪诱发松散海床渐进式液化的数值分析

近海区域广泛分布着第四纪新沉积的松散海洋土,波浪荷载作用下松散海床会发生液化进而对近海结构物的稳定性存在巨大威胁。本文采用中国科学院流体-结构-海床相互作用数值计算模型FSSI-CAS 2D,选用Pastor-Zienkiewicz-Mark Ⅲ（PZⅢ）弹塑性本构研究了波浪诱发的松散海床液化问题。分析了波浪荷载引起的松散海床内超孔隙水压力、有效应力以及应力角的时程变化特性,并预测了松散海床的渐进液化过程。计算结果表明,波浪荷载作用下松散海床内残余孔压会累积增长,海床表面最先发生液化,然后逐渐向下发展至液化最大深度。同时指出海床内超孔隙水压力的竖向分布特征和应力角的变化时程均可以作为判断海床液化的间接参数。最后,通过应力状态分析,讨论了海床渐进式液化的发展过程和趋势。     

The effect of structures on the wave-induced liquefaction potential of seabed sand deposits

One of the important design considerations for marine structures situated on sand deposits is the potential for instability caused by the development of excess pore pressure as a result of wave loading. A build-up of excess pore pressure may lead to initial liquefaction. The current practice of liquefaction analysis in marine deposits neglects the effects of structures over seabed deposits. However, analyses both in terrestrial and marine deposits have shown that the presence of a structure, depending on the nature of the structure and initial soil conditions, may decrease or increase the liquefaction potential of underlying deposits. In the present study, a wave-induced liquefaction analysis is carried out using mechanisms similar to earthquake-induced liquefaction. The liquefaction potential is first evaluated using wave-induced liquefaction analysis methods for a free field. Then by applying a structure force on the underlying sand deposits, the effect of the structure on the liquefaction potential is evaluated. Results showed that depending on the initial density of the sand deposits and different structures, water depths and wave characteristics, the presence of a structure may increase or decrease the liquefaction potential of the underlying sand deposits.     

波浪荷载及土体特性对风电单桩基础水平变形影响规律

大直径单桩基础是海上风电应用广泛的一种基础形式,严格控制桩基泥面处的位移是保证基础稳定和风机安全运营的关键因素.通过数值方法建立了单桩—海床的三维模型,将可以描述海洋砂土超固结性和结构性的弹塑性本构模型通过UMAT子程序嵌入有限元软件ABAQUS中,桩基承受的波浪荷载通过Morison方程进行计算模拟.针对无波浪荷载、仅作用于海床的波浪荷载、同时作用于桩基和海床的波浪荷载三种情况,分析了海床土的动力响应以及桩基的水平位移之间的差异,探讨了海床土体参数对桩基水平变形的影响.研究结果表明海床土体液化会导致桩基水平变形增加,海床土渗透性、超固结性、结构性对桩基水平位移影响显著,研究成果可为海上风电单桩基础的设计与运维提供参考.     

Analysis of seabed instability using element free Galerkin method

Wave-induced seabed instability, either momentary liquefaction or shear failure, is an important topic in ocean and coastal engineering. Many factors, such as seabed properties and wave parameters, affect the seabed instability. A non-dimensional parameter is proposed in this paper to evaluate the occurrence of momentary liquefaction. This parameter includes the properties of the soil and the wave. The determination of the wave-induced liquefaction depth is also suggested based on this non-dimensional parameter. As an example, a two-dimensional seabed with finite thickness is numerically treated with the EFGM meshless method developed early for wave-induced seabed responses. Parametric study is carried out to investigate the effect of wavelength, compressibility of pore fluid, permeability and stiffness of porous media, and variable stiffness with depth on the seabed response with three criteria for liquefaction. It is found that this non-dimensional parameter is a good index for identifying the momentary liquefaction qualitatively, and the criterion of liquefaction with seepage force can be used to predict the deepest liquefaction depth.