离心模型试验中深水防波堤上波浪循环荷载的模拟研究

 研制离心模型试验中新型的非接触式循环荷载加载系统,用于模拟深水防波堤上的波浪荷载,解决了波浪荷载模拟研究中的消能困难、产生附加约束和附加质量以及难以模拟波谷反向荷载等问题,通过计算机控制加载,能够达到模拟风暴荷载的要求。开发动力离心试验的新测控系统,包含高速数据采集、低通数字滤波去除噪声和干扰、改进比例积分微分(PID)加载控制、控制信号生成等功能模块,优化动力离心试验中加载控制和数据采集的功能。通过在离心模型试验中对箱筒型基础防波堤施加非接触式波浪荷载的作用,表明该加载设备可精确、稳定地施加不同的波浪荷载,并可任意调节波浪荷载幅值。得出波浪荷载作用下箱筒形基础防波堤的动位移特征：动倾角的累积值和幅值均有一定的增长,水平位移量以幅值增大为主,而竖向位移量以累积值增大为主;箱筒形基础防波堤主要表现为振动–摇摆耦合运动模式。     

作用于防波堤上波浪荷载的离心机模拟

随着港口码头向深水大吨位化的发展,新扩建防波堤逐渐向深水区延伸,防波堤受到的波浪荷载强度越来越高,当堤基位于海底软黏土层上时,由于地基承载力低,往往考虑轻型防波堤结构,在某一防波堤延伸段工程中就推出了薄壁箱筒型基础防波堤新结构。然而,这类轻型防波堤能否经得住设计波浪荷载的作用以及它的性状为工程师所关注,故希望通过土工离心模型试验,在高速旋转的离心力场中模拟作用于防波堤结构上波浪荷载作用,观察其动态反应特性。根据电磁激励器原理研制开发了一套非接触式循环波浪荷载模拟器系统,能提供不同频率、幅值高达1200 N的不对称周期性往复荷载,满足了波浪荷载模拟需要。文中随后介绍了该装置在箱筒型基础防波堤结构与地基的波浪荷载作用试验中的应用,试验测量了防波堤结构的水平变位、沉降和倾斜以及地基中孔隙水压力等性状反应,基于模型试验结果,探讨了波浪荷载作用下箱筒型基础防波堤的破坏模式,结果发现,主要破坏模式为防波堤倾斜过度而失稳或水平变位超出容许值而发生侧向滑动破坏。     

Stability of composite-type breakwaters reinforced by rubble embankment

Details of the failure characteristics of composite-type breakwaters reinforced by rubble have not been clarified, including the prehension of some failure modes and the conditions that cause each mode. In this study, details of the failure characteristics and a performance assessment method were studied using two approaches, namely, centrifuge model tests and a circular slip analysis. The model tests showed that sliding failure could occur when the friction between the mound and the caisson was low, the caisson was wide, and the reinforcing embankment was high and/or wide. In contrary cases, bearing capacity failure was liable to occur. Basically, the larger the reinforcing embankment size, the greater the strength and stiffness. The performance assessment using a circular slip analysis and considering failure modes was verified through discussions. The analysis resulted in the underestimation of the stability of the reinforced breakwaters when the rubble was angular. However, the caisson moved to a certain degree even with a relatively lower load and the displacement reached that which is allowable by the structure. Consequently, a circular slip analysis was deemed suitable for evaluating the performance of reinforced breakwaters.     

Settlements in Fine-Grained Soils Under Cyclic Loading

Based on the methods previously presented by the authors (Yasuhara et al., 1992, 1994, 1996) for predicting the degradation in strength and stiffness of soft clays in the course of cyclic loading, a methodology has been developed to estimate the cyclic loading-induced settlements. The method also includes not only immediate settlements but also post-cyclic long-term settlements due to dissipation of cyclically induced excess pore pressures in soft soils. The simplified formulae included in the proposed methodology are given as functions of the amplitude of cyclic-induced excess pore pressure normalized by the confining pressure, u/p'_c, plasticity index I_p and factor of safety against bearing capacity failure, F_s. The calculations of cyclic-induced settlements were conducted for soft soil deposits with different index and geotechnical properties. The results calculated using the proposed methodology are presented in the form of a design chart to give the settlement versus normalized excess pore pressure ratio relations including the effects of the plasticity index and safety factor for bearing capacity. An example of the calculated results using the proposed procedure for the earthquake-induced settlements of embankments founded on soft clay, is presented to demonstrate the practicality of the method for design at fields.     

水平-竖向-扭转荷载下吸力沉箱基础承载特性数值研究

海上风机吸力沉箱基础承受结构自重及环境因素引起的竖向、水平、扭转荷载,处于复合加载状态。以位于饱和黏土地基上长径比为0.25～1.0的单桶沉箱基础为对象,ABAQUS为计算工具,研究沉箱基础的承载特性,并考虑沉箱与地基土的摩擦接触作用。首先分析沉箱基础在单一的竖向荷载、水平荷载及扭转荷载作用下的极限承载力,并给出沉箱基础在这些荷载组合下的破坏包络面,其中倾覆力矩通过施加偏心水平荷载实现。结果表明,沉箱基础的竖向承载力可用改进的经典承载力公式计算,而水平承载力系数在给定的接触条件下随沉箱长径比增加而减少,扭转承载力与理论预测结果非常吻合;扭转荷载对沉箱的竖向承载力影响较大,但对水平承载力影响较小;不同扭转荷载下的V-H包络面可用椭圆曲线拟合。     

饱和细砂中裙式吸力基础水平单调加载模型试验——承载力及变形分析

海上风电资源的开发和利用是当今世界关注的热点问题,作为其塔架的基础,主控荷载是水平荷载。裙式吸力基础具有更高的水平承载能力和控制水平位移的能力,故非常适合作为海上风电塔架的基础。通过饱和细海砂中裙式吸力基础的水平单调加载模型试验,探究基础水平承载力的影响因素及转动点位置的变化规律,并分析了地基土的变形影响范围及规律。研究发现：与传统吸力基础相比,裙式吸力基础的水平承载力提高显著,且能有效控制水平位移;水平承载力随基础的裙高、裙宽的增加而增大,随加载高度的增加而减小;在水平荷载作用下基础主要是绕某一点（即转动点）发生转动,转动点位于主桶长度的0.45～0.7倍之间;达到极限荷载时,地表隆起范围远远大于沉降范围,沿加载轴线方向,隆起范围约为2.5倍主桶直径。     

Model testing of tripod caisson foundations in silty clay subjected to eccentric lateral loads

In this study, model tests were conducted to investigate the bearing capacities of tripod caisson foundations subjected to eccentric lateral loads in silty clay. Lateral load–rotation curves of five eccentric-shaped tripod suction foundations were plotted to analyze the bearing capacities at different loading angles. It was observed that the loading angle significantly influenced the bearing capacity of the foundations, particularly for eccentric tripod caisson foundations. Compared with eccentric tripod caisson foundations, the traditional tripod foundation has a relatively high ultimate lateral capacity at the omnidirectional loading angle. By analyzing the displacement of the caissons, a formula for the rotational center of the tripod caisson foundation subjected to an eccentric lateral load was derived. The depth of the rotation center was 0.68–0.92 times the height of the caisson when the bearing capacity reached the limit. Under the undrained condition, suction was generated under the lid of the “up-lift” caisson, which helps resist lateral forces from the wind and waves.     

Optimal cost design with sensitivity analysis using decomposition techniques. Application to composite breakwaters

Minimizing the expected total cost of a structure, including maintenance and construction is a difficult problem because of the presence in the objective function of the yearly failure rates, which have to be calculated by an optimization problem per each failure mode. In this paper, a new method for the design of structures that minimizes the total expected costs of the structure during its lifetime based on Benders’ decomposition is presented. In addition, some tools for sensitivity analysis are introduced, which make it possible to determine how the cost and yearly failure rates of the optimal solution are affected by small changes in the input data values. The proposed method is illustrated by its application to the design of a composite breakwater under breaking and non-breaking wave conditions.     

沉井基础设置根键及压浆后处理原位试验分析

研究设置根键及注浆对沉井基础承载特性的影响,采用自平衡原位试验的方法对某一沉井基础设置根键前后及压浆处理进行现场测试与分析。整个测试过程分普通沉井承载力测试、顶入根键后承载力测试、压浆后测试3个阶段进行。根据测试结果,分析了沉井基础在3种不同情况下的荷载与位移关系曲线(Q-s)、轴力的分布情况及侧摩阻力分布规律。研究结果表明:沉井基础在设置根键的情况下,通过根键的挤密、叠加及扩散作用,大大改善了沉井基础与土体的相互作用,使得沉井基础承载能力得到极大提高,提高幅度达到50%,且通过井壁后压浆处理可进一步提高其承载能力;从承载力的分担比例来看,根键的设置使得沉井基础的侧摩阻力所占比重明显增大。与普通沉井相比,设置根键后,沉井基础的侧摩阻力占总的极限承载力的比例提高了28.7%,通过后压浆的处理可进一步增加4%;从轴力分布的规律来看,设置根键及后压浆处理的沉井,两者轴力分布曲线都较为平缓,这对发挥沉井基础材料本身力学性能较为有利,达到了充分利用材料的目的。     

性能参数对混凝土柱破坏指数的影响

不同破坏准则对混凝土柱的破坏程度评价结果存在一定差异。针对已有抗震研究多考虑最终破坏指数与构件破坏程度的对应关系,未考虑不同破坏准则与构件性能参数选取对评估结果合理性的影响。以混凝土柱为研究对象,计算了极限状态下位移角与位移角限值比值、承载力与峰值承载力比值、变形与变形能力比值与双参数准则、延性准则破坏指数之间的相关系数,分析了破坏指数与破坏现象、破坏规律的吻合程度,探讨了几种性能参数对不同破坏准则的适用性。结果表明:极限状态下混凝土柱的变形能力值离散性最小,与位移角比值、承载能力退化值的相关性最低,更适合应用于破坏准则研究; 双参数破坏准则计算结果受组合系数β的取值影响显著,反映的混凝土柱破坏趋势与承载力变化规律、试验现象存在较大差异; 由延性破坏准则计算得出的破坏指数能反映位移角比值、承载力退化值、变形能力值对破坏的影响规律,考虑变形能力极限的延性准则能更好地量化混凝土柱的破坏。     

Prediction Method on Deformation Behavior of Caisson-Type Sea Walls Covered with Armored Embankment on Man-Made Islands During Earthquakes

In this paper, centrifuge shaking table tests were conducted in order to understand the performance of seawalls during a seismic event. The model tests showed that the displacement of the caisson was much affected by the seaward shear deformation of the sand seabed beneath it during shaking. It was also confirmed that an armored embankment played an important role in the displacement of the caisson during shaking. Based on these test results, a two-dimensional DEM-FEM coupled analysis method was newly developed to numerically predict the deformation of seawalls covered with armored embankments during earthquakes. The movements of the armor units were calculated by DE analysis and deformations of the caisson, rubble mound, sand seabed and backfill were calculated by FE analysis considering the non-linearity of the soil materials based on the effective stress. Dynamic interaction was taken into account by delivering the nodal displacements of the finite elements or the nodal forces converted from the contact forces through the imaginary distinct elements defined at the boundaries between the DE and FE domain. The applicability of this method to the prediction of the deformation of seawalls was verified through numerical simulations of the centrifuge model test.     

Geosynthetic-sheet pile reinforced foundation for mitigation of earthquake and tsunami induced damage of breakwater

Earthquake and tsunami impose great threats on the stability of a breakwater. Foundation of the breakwater is weakened by these forces, and it may result in collapse of the breakwater. Lateral flow of seabed soils take place beneath the breakwater, and excess pore water pressure is generated in the foundation by an earthquake that precedes tsunami. These factors may lead to excessive settlement and horizontal displacement of the breakwater. Tsunami introduces additional instability to the deformed breakwater. Due to water level difference between seaside and harborside of the breakwater during a tsunami, seepage occurs through its foundation, and it may cause pipping of seabed soils. Tsunami induced scouring of mound is also a big problem for the stability of the breakwater foundation. Finally, these result in failure of the breakwater foundation. Due to failure of its foundation, the breakwater may collapse and cannot block the tsunami. It results in entering of the tsunami in coastal areas. In order to make a breakwater resilient against earthquake and tsunami induced damage, reinforcing countermeasures were developed for foundation of a breakwater. Geogrid, gabions and sheet piles were used for reinforcing a foundation model. The effectiveness of the model is evaluated through physical modeling for mitigating the earthquake and tsunami induced damage. Shaking table tests were performed to determine the effectiveness of the reinforced model under different earthquake loadings. Tsunami overflow test was conducted on the same deformed model in order to see the effects of tsunami on the model. Comparisons were made between the unreinforced and reinforced foundations, and it was observed during the tests that the reinforced foundation performed well in reducing the damage of the breakwater brought by the earthquake and tsunami. Overall, this study is useful for practice engineering, and the reinforced foundation model can be adopted for designing a breakwater foundation to reduce damage triggered by an earthquake and tsunami in the future.     

桥梁吸力式沉箱基础承载特性试验研究

吸力式沉箱基础是跨海桥梁基础的一个新选择。基于桥梁基础的受荷特点,考虑不同的荷载作用方式,通过一系列模型试验研究了砂土中吸力式沉箱基础的承载特性。试验结果表明:沉箱的长度越长,吸力式沉箱基础的竖向承载力越大,侧壁摩阻力的贡献越大,而沉箱端部阻力可忽略不计;沉箱的长度越长,吸力式沉箱基础的水平承载力也越高,但相对于竖向承载力而言,基础的水平承载力很小,一般不超过其竖向承载力的5%。预加一定的竖向荷载,可显著提高吸力式沉箱基础的水平承载力,且预加的竖向荷载越大,基础的水平承载力越高,由于桥梁工程中基础承受的竖向荷载很大,所以有必要考虑这一因素对基础水平承载力的提高。     

加筋风砂土扩展基础的抗拔试验与位移控制计算

承受上拔荷载的扩展基础,可以用上拔位移或上拔荷载作为设计控制条件。在典型沙漠地区进行调研和现场采取土样,进行土工格栅与风砂土的摩擦特性试验,通过室内模型试验研究了上拔荷载作用下土工格栅加筋风砂土地基扩展基础的力学性能,包括荷载、位移、变形、破坏机理和承载能力的研究,提出了有效的土工格栅加筋形式:平铺一层和二层土工格栅。在上述研究基础上,对上拔位移机理进行了分析研究,提出了上拔位移计算模型和上拔位移控制的分析计算方法。     

扩底桩的抗拔承载力试验及计算

通过对干旱地区黄土中扩底桩的抗拔试验 ,测试了扩底桩在上拔荷载、水平荷载作用下的上拔位移和水平位移以及位移与荷载的关系。研究了极限上拔承载力和抗拔桩的破坏机理。在相同条件下 ,增加扩大端的高度对提高桩的极限上拔承载力是有效的 ,破坏机理为土的减压软化和损伤软化的渐进性破坏。提出了极限上拔承载力的理论计算模式 ,并与实测资料进行了对比 ,理论计算结果与实测值是吻合的     

碎石垫层沉箱基础水平循环加载模型试验与分析

碎石垫层沉箱基础是一种适于强震区深厚软弱土地基的深水基础形式,具有抗震性能良好、承载力高、便于施工、造价合理等优点。采用自主设计的大周数水平循环加载装置对碎石垫层沉箱基础开展了室内1g模型试验研究,探讨了基础在水平静力加载、循环加载以及循环后静力加载下的承载变形特性。试验研究表明,基础在长期水平循环荷载作用下具有安全可靠的工作性能。静载下基础的荷载–水平位移关系可视为刚塑性、滑动破坏前为双曲线关系。循环荷载下基础的累积位移发展呈现收敛的趋势,并随着荷载幅值的增大而增大;基础在单向循环荷载下的累积位移大于同等大小荷载静载下的位移。循环加载后基础沿各方向的水平承载力和刚度均有所提高。然后,对基于边界面理论的修正G-M弹塑性接触面本构模型进行改进,利用ABAQUS接触面子程序UINTER将其嵌入ABAQUS中,对碎石垫层沉箱基础的模型试验进行了有限元数值分析,模拟得到的规律与试验结果相近。     

预制圆孔板剪力墙轴心抗压试验与分析

通过两个预制圆孔板剪力墙试件的轴心抗压试验,结果表明：其破坏形态为压坏,受压承载力很大,平面外位移小,不会发生失稳破坏。采用混凝土结构设计规范的公式计算试件的正截面受压承载力,计算结果与试验结果符合较好。最后进行了非线性有限元分析,进一步为预制圆孔板剪力墙装配整体式住宅建筑体系的推广应用提供理论依据。     

Failure envelope considering the ultimate tensile capacity of suction caissons in sand

Little analytical work has been done to elucidate the ultimate capacity of suction caissons under vertical tensile (V), lateral (H), and moment (M) loads in soils. In this paper, in order to reveal the effect of vertical tensile, lateral, and moment loads on the ultimate capacity of suction caissons in sand, an analytical investigation was made using a traditional bearing capacity theory. Taking account of the vertical equilibrium of an annular element of a skirt, through the vertical tractions inside and outside the skirt of a suction caisson when a vertical tensile load is applied, the vertical displacement of the soils adjacent to the skirt of the suction caisson was presented. The most appropriate bearing capacity equation for predicting the experimental results was shown for suction caissons having an embedment larger than a diameter in sand. For the deformation-load responses of suction caissons with various embedment ratios in sand, subjected to inclined tensile loads, there was a good agreement between the results obtained from laboratory tests and those predicted by the present method. The failure surfaces, considering the ultimate tensile capacity in the H-M, H-V, and M−V planes, and in the H-M−V space, for suction caissons in sand, were presented.     

大直径扩底桩承载力及沉降变形的计算

大直径扩底桩(墩)在我国已得到广泛的应用,其设计计算方法目前仍沿用传统的单一极限应力设计方法,有许多不合理之处。本文根据近几年的研究成果,参考国外一些设计理论,提出一套适合我国具体情况的大直径扩底桩(墩)在变形和极限应力双控制条件下的设计方法。     

可变式扩底支盘注浆灌注桩的控制与检测

介绍了可变式扩底支盘注浆灌注桩的技术优势，并通过具体事例，对可变式扩底支盘注浆灌注桩的承载力和桩身完整性作了检验测试，得出可变式扩底支盘注浆灌注桩可满足工程上高层建筑承载力高、沉降低的要求的结论。