随机波浪作用下海床动力响应及液化的理论分析

运用随机波浪理论,基于广义Biot动力固结理论,建立了能够考虑波浪荷载随机特性影响的计算海床动力响应及液化深度的解析数值模型,并从时域上进行求解分析.通过与传统线性规则波浪荷载作用下海床动力响应及液化深度的数值结果的对比分析,详细讨论了考虑波浪荷载的随机特性对于求解海床动力响应及液化深度的影响程度.结果表明,根据随机波浪理论计算得到的海床中的超孔隙水压力、水平有效应力、竖向有效应力、剪应力表现出较强的不规则性,其幅值沿着海床深度方向的变化趋势与根据线性规则波浪理论计算得到结果的变化趋势基本相同,但在同数量级上更大,同时计算得到的海床最大液化深度明显大于根据线性规则波浪理论计算得到的结果.因此,在海洋地基设计和自由场地安全评估时应该合理地考虑波浪荷载的随机特性.     

非饱和砂质海床在复合防波堤下固结的数值研究

本文以Biot动态方程(u-p公式)为控制方程,采用有限元方法和Generalized Newmark-时间积分方法,在SWANDYNE的基础上发展了用于分析海床基础固结和动态响应分析的计算程序PORO-WSSI 2D。利用太沙基一维固结理论验证了POR-WSSI 2D中固结模块的有效性。利用该计算程序,深入研究了大型海床基础在大型复合式防波堤和静水压力作用下的固结过程以及最终的固结状态。计算结果表明计算程序PORO-WSSI 2D能够有效地分析评价海床基础在海洋结构物作用下的固结过程以及预测海床基础的剪切破坏行为。所确定的最终固结状态还可以为后续研究海床基础在波浪、地震等动力荷载作用下的动态液化和剪切破坏提供可靠的初始条件。     

波浪作用下黄河三角洲粉质土海床动力响应分析

应用饱和土动力固结理论和Pastor-Zienkiewicz III动力本构关系,对波浪作用下黄河三角洲粉质土海床的动力响应特征进行了有限元分析,应用总超孔压准则对海床进行了液化判别,并将计算结果与现场观测资料进行对比。结果表明：波浪导致的海床超孔压由瞬态孔压和累积孔压两部分组成;相比均质海床,拥有表面硬层的海床瞬态孔压沿深度衰减更快,累积孔压在表层增长速度更大;不同波浪条件下,瞬态孔压值及其变化趋势较为一致,累积孔压则具有较大的不同。年平均波浪条件下海床不会发生液化;5 a和50 a一遇极端波浪条件下,考虑三维效应和具有表面硬层的海床更容易液化,最大液化深度在海床表面以下2～3 m范围内。计算所得的海床最大液化深度与实测的黄河三角洲海底灾害地貌深度有较好的一致性,表明了文中方法的有效性和合理性。     

波浪作用下饱和砂质海床土体与管线相互作用规律研究

针对波浪引起的饱和砂质海床土体和管线相互作用问题,将Biot动力固结理论与笔者课题组提出的砂土液化变形弹塑性本构模型相耦合,较为合理地再现了简谐波浪作用下较浅饱和砂质海床中管线周围可液化海床土体的超静孔隙水压力瞬态累积变化规律与液化过程。数值计算结果与Sumer等的试验规律一致。结果表明：由于管线的存在,改变了饱和砂质海床液化区域的空间分布。液化首先由管线下部土体开始产生,随着波浪荷载的持续作用,液化区域沿着管线外壁向上演化;同时海床表层土体产生液化并向深层发展,最终管线周围土体都发生液化,这是导致空管上浮的主要原因。当饱和砂质海床中存在管线时,管线附近海床土体液化深度明显变深。超静孔压累积和渗透力变化的耦合作用是导致饱和砂质海床土体产生液化的原因。与将海床土体视为饱和弹性多孔介质相比,可考虑液化全过程的弹塑性动力分析能更为合理地揭示实际波浪作用下饱和砂质海床土体的渗流场和应力场的瞬态时空演变规律。     

非线性波浪荷载作用下海上风机管桩基础周围海床液化研究

为研究近浅海领域内海上风机大直径单桩基础周围的砂质海床在非线性荷载作用下的瞬时液化的稳定性,在OpenFOAM平台上建立波浪-单桩-海床三维数值模型(WSSI模型),采用olaFoam求解器求解RANS方程模拟波浪的非线性运动,将Biot方程作为海床模型的控制方程研究单桩周围海床的液化规律。对以往的数值解析结果和实验结果进行对比分析,验证本文建立三维波浪模型和海床模型的准确性和有效性。将建立的波浪-管桩-海床相互作用三维数值模型应用到实际工况中,对数值模型在实际工程中的海床响应进行分析评估。研究结果表明,单桩周围海床的孔隙水压力场和有效应力场在水深方向发生较大变化,在水平方向变化较小。在该工况下海床的最大液化深度可达到10 m,且在垂直于波浪运动方向的海床更容易发生液化,为实际工程预测海床液化提供一定的技术支持。     

粉质海床波浪响应的数值模拟及试验研究

针对波浪荷载作用下粉质海床的动力响应问题,结合波浪槽模型试验研究的结果,提出了一种粉质海床在波浪作用下超静孔隙水压力增长的数值分析方法。计算结果表明：该方法能较好地拟合试验数据,揭示粉质海床孔隙水压力发展的机理,能运用于海床液化势的评价分析。     

Effects of principal stress rotation on wave-induced soil response in a poro-elastoplastic sandy seabed

In this study, a constitutive model is developed in order to investigate wave–seabed interactions. This model takes into account the impact of principal stress rotation (PSR) and is based on the generalized plasticity theory, in which plastic strain generated by PSR is considered an additional item in the constitutive relationship of soil. The normalized loading direction and plastic flow direction were determined based on the stress tensor invariant. Comparisons between the present model and previous Hollow Cylinder Apparatus tests and geotechnical centrifugal wave tests show good agreement. Numerical results show the effects of PSR on predictions of liquefaction potential due to: (a) the cumulative impact of plastic strain in the seafloor and (b) the buildup of pore pressure. Parametric study shows that the model parameters, including the wave and seabed parameters, have significant effects on the wave-induced soil liquefaction.

     

Effect of wave non‐linearity on the standing‐wave‐induced seabed response

A standing wave in front of a seawall may reach a height more than twice of its incident component. When excess pore pressure occurs, it may even induce seabed instability, hence endangering the structure. This issue was studied previously using only linear wave theory. In this paper, standing‐wave theory to a second‐order approximation is applied, in order to demonstrate the differences between these two solutions. The spatial and temporal variations in the instantaneous pore pressure are first calculated, in addition to their vertical distributions. The effects of wave height, water depth and the degree of soil saturation on pore pressure distributions are then discussed, followed by the net pore pressure averaged over one wave cycle. The results suggest the existence of a residual pore pressure in the seabed and its net pore pressure can be used to estimate the wave‐induced liquefaction potential in a soil column. It also indicates that, in deep water, the second‐order solution predicts that a negative pore pressure at an antinode which may be greater than a positive pressure. Overall, the second‐order solution is found to agree better with the experimental results of the pore pressures available, compared to the linear solution. Copyright © 2000 John Wiley & Sons, Ltd.     

Three‐dimensional nonlinear finite element analysis of pile groups in saturated porous media using a new transmitting boundary

The dynamic behaviour of pile groups subjected to an earthquake base shaking is analysed. An analysis is formulated in the time domain and the effects of material nonlinearity of soil, pile–soil–pile kinematic interaction and the superstructure–foundation inertial interaction on seismic response are investigated. Prediction of response of pile group–soil system during a large earthquake requires consideration of various aspects such as the nonlinear and elasto‐plastic behaviour of soil, pore water pressure generation in soil, radiation of energy away from the pile, etc. A fully explicit dynamic finite element scheme is developed for saturated porous media, based on the extension of the original formulation by Biot having solid displacement (u) and relative fluid displacement (w) as primary variables (u–w formulation). All linear relative fluid acceleration terms are included in this formulation. A new three‐dimensional transmitting boundary that was developed in cartesian co‐ordinate system for dynamic response analysis of fluid‐saturated porous media is implemented to avoid wave reflections towards the structure. In contrast to traditional methods, this boundary is able to absorb surface waves as well as body waves. The pile–soil interaction problem is analysed and it is shown that the results from the fully coupled procedure, using the advanced transmitting boundary, compare reasonably well with centrifuge data. Copyright © 2007 John Wiley & Sons, Ltd.     

近海欠密实砂质海床内波致渐进液化特征研究

利用一个经过广泛验证的数值模型FSSI-CAS 2D为计算工具,采用砂土的高级本构模型Pastor-Zienkiewicz-Mark III (PZIII) 描述海床砂土的动态力学行为,定量研究松散海床地基土在波浪作用下,其内部的液化过程和特征,以加深对波致海床液化特征、性质的认识。计算结果分析表明,开发的耦合数值模型FSSI-CAS 2D能够很好地捕捉到波浪作用下欠密实海床的动力响应特征,以及海床内的累积液化过程等一些列的非线性物理现象。研究表明,波浪导致的松砂海床液化是一个渐进过程,海床表面首先液化,并逐渐向下扩展。     

波浪导致黄河口海床沉积物超孔压响应现场试验研究

黄河口海床特殊的工程地质性质与复杂的工程动力稳定性问题,均与海床沉积物在波浪荷载作用下的孔压动力响应密切相关。在现代黄河水下三角洲潮间带岸滩选择4个典型研究点,现场模拟波浪作用对原状海床沉积物实施循环加载,利用孔隙水压力观测、沉积物强度测试、样品采集与实验室土工测试等方法手段,测定黄河口原状海床沉积物在循环荷载作用不同阶段的孔压响应与强度变化。研究发现,黄河口原状海床沉积物在经历循环加载过程中,典型的超孔压响应可分为逐渐累积、部分消散、快速累积、累积液化和完全消散5个阶段,分别对应沉积物强度的衰减、增大、衰减、丧失和恢复过程,沉积物的粒度组成与结构性强弱决定了超孔压的具体响应模式。波浪导致原状海床液化深度受沉积物的干密度、孔隙比、饱和度等初始物理性质影响显著,细颗粒组分的相对含量高低也在很大程度上控制着沉积物的液化特性。     

考虑波致孔压累积与应力耦合效应的埋置管线周围海床响应特征与机制分析

波浪导致的海床液化是埋置管线失稳的一个重要因素。超静孔隙水压力累积引起的液化深度较深,对海底管线稳定性的影响较大,因此波浪作用下管线-海床系统的累积响应特征一直受到研究者的重点关注。本文基于考虑孔压累积与海床应力耦合发展的数值计算模型,对非线性行进波作用下含埋置管线的海床累积响应特征进行了模拟计算,并与非耦合模型的计算结果进行了对比分析,结果表明,当考虑孔压累积与海床应力的耦合效应时,管线附近累积孔压在水平方向上的不均匀分布会导致海床循环剪应力的增大,从而会极大地促进管线周围海床累积孔压的发展,增大管线的影响范围; 忽略孔压累积与海床应力的耦合效应,会在一定程度上低估管线周围海床的液化深度,不利于管线的安全。     

Oscillatory soil response and liquefaction in an unsaturated layered seabed

The subject of wave-induced soil response in a real seabed has attracted the attention of geotechnical and coastal engineers over the last three decades, for which several basic theories have been developed. However, the evaluation of soil liquefaction has not been attempted theoretically in a seabed with multiple sub-layers, in which homogeneity in soil properties can be assumed within each layer. In this study, a semi-analytical approach is presented for obtaining solutions for the pore pressure and effective stresses in a non-cohesive layered seabed of finite thickness subject to a system of three-dimensional waves. Based on the numerical results for a layered seabed, influences of soil characteristics (relative layer thickness, permeability ratio and shear modulus) on seabed responses are described. Special attention is given to the effect of placing a coarser material as a top layer for protecting an underlayer of finer sediment. Although only a three-layered seabed is explicitly solved in this study, the procedure outlined can readily be extended to a multi-layered soil system. The three-dimensional solutions can also be applied to the two-dimensional progressive or standing wave systems.     

砂性地基中防波堤地震残余变形机制分析与液化度预测法

海港工程中防波堤地震残余变形的预测以及震损机制的分析是较复杂的问题。采用定义在应变空间中考虑土体动主应力轴方向偏转影响的多重剪切机构塑性模型,分别从砂性土的黏粒含量和标贯击数2个主要影响因素对防波堤地震变形的变化机制进行了有效应力分析,得出防波堤较大的残余变形是由于地震作用下土体中孔隙水压力的升高致使土体软化而产生的。然后,采用液化度单一指标从物理本质上来间接表征防波堤的残余变形,得到防波堤残余变形与液化度之间的函数预测关系,预测值与震害调查值以及数值分析值基本相符,表明所得液化度预测公式具有一定的可靠性。残余变形的液化度预测法可为类似防波堤地震灾害设计与评价提供参考依据。     

表层液化地基中结构与桩惯性相互作用简化分析方法

传统桩基础结构抗震设计中一般认为忽略结构与桩基础相互作用影响使得结构所受惯性作用计算结果偏于保守,但对几次强震中地震动反应谱和桩基础结构震害特征的总结与分析表明,液化作用和结构与桩基础相互作用可能导致桩基础结构惯性作用增强。通过可液化地基中桩基础结构的动力离心模型试验手段,分析了地基液化对结构与桩惯性相互作用的影响,发展了适用于表层液化地基中的结构与桩惯性相互作用的简化计算方法,并利用该方法对文中及Wilson等的离心模型试验进行分析,得到的结构加速度与试验结果吻合较好,从而验证了方法的有效性。     

Elementary empirical model to assess seismic soil liquefaction

Liquefaction potential (LP) assessment plays a significant role in damages due to earthquake. The spirit underlying the present work is the evaluation of LP by correlating most significant parameters reflecting the dynamic response of soil with actual field behavior wherein an attempt of integrating the effect of dynamic soil properties and ground motion parameters simulating the actual site conditions is being made. Accordingly, a dynamic response–based Elementary Empirical Liquefaction Model (EELM) is proposed by processing a total of 314 reported case records covering a wide range of parameters demarcating “yes” and “no” zones of liquefaction. The method to develop the EELM essentially consists of evaluation of liquefaction potential, defining functional form of EELM representing dynamic response of soil to earthquake shaking, collection of data, computation of model parameters and formulation followed by validation of the model. The proposed empirical model though in fundamental form is found to perform fairly well resulting into an overall success rate of 86 % for both liquefaction and non-liquefaction points with significantly high success rate of 98 % for liquefied cases. Comparison of predictive performance of the proposed EELM with other approaches shows higher efficiency and thus signifies the theme of employing integrated approach.     

黄河口潮滩粉土体固结非均匀性研究

通过分析直立堤前海床地貌和深度剖面方向贯入强度特性及相邻自由海床剖面方向贯入强度特性,研究了黄河口潮滩粉土体动力响应的非均匀性。研究发现：沿海床剖面不仅存在水平硬层,同时也存在竖直硬层,且竖直硬层与水动力条件有关。结合现场循环冲击荷载试验和直立堤前多孔介质海床数值模型,解释粉土海床非均匀性形成的机制：由于粉土体中细小无黏性颗粒在波浪作用下逐渐脱离土骨架,在土体中缓慢流动汇聚,在后期波浪改造累积作用下固结排水强度增大,形成强度不同的地带,导致土体动力响应的非均匀性。     

Dynamic responses of unsaturated half‐space soil to a moving harmonic rectangular load

The problem of the dynamic responses of a semi‐infinite unsaturated poroelastic medium subjected to a moving rectangular load is investigated analytical/numerically. The dynamic governing equations are obtained with consideration of the compressibility of solid grain and pore fluid, inertial coupling, and viscous drag as well as capillary pressure in the unsaturated soil, and they can be easily degraded to the complete Biot's theory. Using the Fourier transform, the general solution for the equations is derived in the transformed domain, and then a corresponding boundary value problem is formulated. By introducing fast Fourier transform algorithm, the unsaturated soil vertical displacements, effective stresses, and pore pressures induced by moving load are computed, and some of the calculated results are compared with those for the degenerated solution of saturated soils and confirmed. The influences of the saturation, the load speed, and excitation frequency on the response of the unsaturated half‐space soil are investigated. The numerical results reveal that the effects of these parameters on the dynamic response of the unsaturated soil are significant.     

行波激励下防波堤地震动力响应分析

考虑到行波效应对防波堤地震动力响应的影响,建立了基于弹塑性本构的防波堤-地基系统三维有限元模型。分析中考虑了结构间的动态接触作用,并采用基于接触负载均衡的显式并行计算方法,解决了大规模非线性有限元问题的求解困难。计算给出了不同视波速下行波效应对防波堤挡浪墙沉降、防波堤残余变形以及挡浪墙截面弯矩的影响程度及规律。结果表明：挡浪墙震后沉降随视波速增大而增大,并逐步逼近一致激励响应;行波效应对防波堤残余变形模式几乎没有影响,行波激励与一直激励下所得的残余变形模式基本一致,横波地震动输入下引起的震后残余变形要大于纵波输入下的残余变形;多数情况下,行波激励下的挡浪墙弯矩会比相应的一致激励下的响应大。分析方法及结果为防波堤三维整体抗震设计提供了参考。