均质黏土中圆形平板锚的抗拉承载力分析

基于网格重新生成和场变量映射的大变形有限元模型,探索了立即脱离和无脱离两种典型条件下均质黏土中圆形平板锚的抗拉承载力。与小变形有限元比较,大变形分析克服了锚周围土体初始网格畸变的不利影响,能够追踪平板锚整个拔出过程中抗拉力的变化。通过具体算例,考察平板锚表面摩擦性质和上覆土重等因素对立即脱离工况承载力的影响程度,指出有重土中深锚的承载力小于无重土中对应的承载力与上覆土重之和,其上限是无脱离条件下的承载力。计算结果表明：土重对无脱离条件下的承载力影响很小,进而给出了无脱离承载力系数与初始埋深的关系曲线。     

锚板在正常固结黏土中的承载力

在岩土工程中,锚板通常被用来提供竖直或水平抗拔力,比如发射塔的基础、板桩墙结构和悬浮式海洋平台的基础。采用弹-塑性有限元方法对正常固结不排水黏土中的条形锚板进行数值分析,以图表形式给出了不同埋深率、不同上拔倾角、不同锚-土黏结形式下条形锚板的承载力系数和周围土体的流动机构,分析了土体自重对锚板承载力的影响,并给出了不同情况下锚板的极限承载力系数。采用基于重新划分网格并插值状态变量的大变形分析方法（RITSS）,分析了正常固结黏土中锚板在连续拔出过程中的承载力变化以及土重对锚-土分离模式的影响。     

基于耦合欧拉-拉格朗日法的锚板在黏土中的极限承载力数值分析

当结构在土体中运动时,往往导致土体发生较大的变形,此类问题采用大变形数值分析方法更为恰当。耦合欧拉-拉格朗日（Coupled Eulerian-Lagrangian, 简称CEL）法是大变形数值分析方法中的一种,在分析大变形问题时具有很强的适用性,但在国内尚未开展CEL法分析锚板承载力的研究。以方形锚板在均质土及线性土中的拔出试验为原型,基于CEL法建立数值模型,对锚板的极限承载力及破坏机制进行研究,并通过用户自定义子程序,实现了线性土的强度分布随锚板拔出而变化。计算结果表明,土体杨氏模量越大,锚板的极限承载力越大;随着位移增大,锚板的抗拔力先增大,后降低;当埋深小于临界埋深时,土体发生整体破坏;当埋深大于等于临界埋深时,土体发生局部破坏。数值计算反映的规律与试验结果基本吻合,体现了CEL法模拟锚板在海床中大位移响应的出色能力。     

正常固结黏土中平板锚基础的吸力和抗拉力

平板锚是新近出现的一种系泊深海浮式结构的基础型式。当黏土地基中的平板锚承受上拔力时,平板上、下表面超静孔压差形成的吸力使其抗拉承载力显著增加,对于风浪等快速加载条件尤其如此。利用有限元软件ADINA建立有效应力形式的轴对称动力有限元模型,研究圆形平板锚在缓慢加载与快速加载时的超静孔压分布与地基破坏型式。快速加载时重黏土和高岭土两组典型正常固结土样所得极限承载力系数与塑性极限分析解一致。进而通过变动参数分析,讨论加载速率和埋深对吸力和总抗拉力极限值的影响,并给出排水和不排水加载条件对应的临界加载速率。结果表明,不排水加载条件的总抗拉力可能达到排水总抗拉力的3倍。     

土工格栅加筋砂土中水平锚板抗拔特性试验研究

土工格栅加筋能够有效改善锚板的抗拔承载力,然而锚板在上拔过程中的破坏机制及其影响因素尚需进一步研究。针对砂土中水平锚板的抗拔特性,开展了多组锚板上拔试验,分析了砂土密实度、锚板埋深、土工格栅布设层数和位置等因素的影响,结合粒子图像测速（particle image velocimetry,简称PIV）技术探究了锚板周边土体的变形破坏机制。研究结果表明：单层接触式格栅加筋对锚板的抗拔承载力有明显的提升,且其对土体性能的改善优于非接触式格栅加筋情况,其原因与土工格栅变形量和上覆土体重力有关;当采用双层土工格栅加筋时,下层格栅可充分发挥限制土体侧向变形和均化应力分布的作用,上层格栅相对而言贡献不大;采用土工格栅加筋后,锚-土界面附近土体的变形模式发生了明显的变化,其破坏面相比未加筋前向内侧收敛,且剪应变分布更为均匀。     

非均质地基浅埋水平条形锚板承载力上限分析

考虑地基土体的非均质特性,采用非线性Mohr-Coulomb强度准则及其关联流动法则构造了浅埋水平条形锚板的曲线型破裂机制与机动许可速度场,根据极限分析上限定理推导了条形锚板抗拔承载力的表达式。利用变分极值原理求得了锚板抗拔承载力及其上方土体破裂面的上限解,分析了锚板埋深、土体非均质和非线性强度特性对锚板抗拔承载特性的影响,并将该上限解与已有计算方法进行了对比。结果表明:锚板埋深、土体非均质和非线性强度特性对其抗拔承载力与破裂面特征具有明显的影响。锚板埋深和土体非均质系数越大以及土体非线性强度系数越小,锚板抗拔承载力和土体破裂面深度、宽度均是越大。该上限解与极限平衡和极限分析有限元方法的计算结果基本一致,验证了所采用的曲线型破裂机制和地基非均质变化规律有效性,为条形锚板设计提供了一定的参考。     

锚板抗拉破坏机制试验研究

锚板上拔过程是一个复杂的锚土相互作用过程,锚板周围土体在上拔过程中的变形破坏机制对于锚板抗拔力的可靠预测具有重要意义。为了对锚板破坏机制进行量化分析,基于LabVIEW软件开发环境,开发了力、位移和图像同步采集系统,该系统由力传感器、位移传感器、相机和一台计算机组成,可对锚板上拔过程中的力、位移和图像进行自动同步采集,从而保证了力、位移和图像的一一对应关系。基于PIV（particle image velocimetry）无干扰测量技术对砂土中锚板在上拔过程中的图像进行了测量分析,得到了锚板周围土体的位移场、剪切应变场和体积应变场。变形场试验结果表明：锚板上拔过程中,锚板上部土体中间部分位移大、两边小,最终形成一个倒置的梯形;剪切应变场显示锚板上拔过程经历了局部剪切带形成,扩展并最终在锚板两侧形成一个倾斜向上并贯通到地面的对称剪切带,剪切过程中剪切带内伴随着剪涨。在峰后阶段,剪切带形状由峰值点内倾转为外倾,锚板两侧边缘处出现局部土体流动软化。该试验结果可为锚板上拔预测模型建立以及设计提供参考依据。     

浅埋平板圆锚竖向拉拔极限承载力计算方法

采用自制可视化试验装置开展了平板圆锚的拉拔模型试验,基于数字照相测量技术对极限拉拔下锚周土体的位移变形场进行了量化分析。在本次试验的埋深比范围内,极限承载力随埋深比增加而非线性增大,但增长速率逐渐减缓;观测到的锚周土体滑动面与地面、锚板所围区域整体呈现出“底大、顶小、径长”的倒喇叭形状;滑动面可用两条直线段来近似描述;极限拉拔力学模型由一个截面直径上小下大的倒圆台和一个等截面圆柱体组成。根据极限平衡条件推导建立了砂土中浅埋平板圆锚竖向拉拔极限承载力的计算方法,该方法对4组试验数据的计算较其他4种方法与试验实测值更为接近,且离散性更小,效果较好。     

黄土中圆形抗拔锚板基础承载力分析

在三维状态下运用极限平衡理论,对黄土中圆形抗拔锚板基础在受到垂直于板面荷载作用下的承载力进行理论分析,该理论研究考虑了锚板在上拔过程中板周土体的破裂方程、破裂面上的正应力和剪应力、埋深率等影响因素。理论公式计算结果表明：抗拔锚板承载力系数随着锚板埋深的增加而增大,并且当深度达到一定值时,承载力系数将达到其极限值;当埋深率h/D=1~2之间时,上覆土重对承载力系数影响较小;当h/D=4时,土体和锚板之间的吸力对承载力系数的影响大于锚板上覆土重;当h/D>4时,上覆土重对锚板抗拔承载力起着决定作用。理论计算公式与已有学者的试验结果进行对比表明提出的模型理论计算结果和其他学者的试验结果有良好的一致性,验证了该理论的正确性,为工程中抗拔锚板的设计提供了有价值的参考。     

斜坡浅埋水平条形锚板抗拔承载力的极限分析

针对山区和丘陵等复杂地形下浅埋锚板抗拔承载力计算问题,基于极限分析上限定理、非线性Mohr-Coulomb强度准则及其关联流动法则,构造了斜坡浅埋水平条形锚板的曲线型破裂机制和机动许可速度场,采用变分极值原理获得了其上方土体破裂面方程和抗拔承载力的上限解,分析了斜坡倾角和锚板埋深对锚板抗拔承载力的影响。结果表明:随着斜坡倾角的增大,锚板抗拔承载力逐渐减小,此时其上方两侧土体破裂面不再对称且整体向下坡侧偏移;锚板抗拔承载力及其上方两侧土体破裂面宽度均随着埋深增大而增加;锚板埋深越小,斜坡倾角对其抗拔承载力的影响越大,应在计算中予以考虑,以更合理地反映斜坡浅埋水平条形锚板的抗拔承载特性。     

Three-dimensional numerical analysis of the keying of vertically installed plate anchors in clay

Plate anchors, such as suction embedded plate anchors and vertically driven plate anchors, offer economically attractive anchoring solutions for deep/ultra-deep water offshore developments. The rotation/keying processes of plate anchors will cause embedment losses, which lead to decreases of the uplift resistances of the anchors in normally consolidated soil. In the present paper, the keying processes of vertically installed strip and square plate anchors are simulated using the 3-D large deformation finite element method. The effects of loading eccentricity and pullout angle on the embedment loss during keying are investigated. Both the development of the uplift resistance and the soil flow mechanisms are presented. The numerical results show that the loading eccentricity e/B has a much larger effect on the embedment loss than the pullout angle does. The anchor shape has a minimal effect on the loss in anchor embedment. The shape factors (square/strip) are 1.05–1.09 for loss of embedment and 1.10–1.19 for capacity.     

Effect of shape on holding capacity of plate anchors buried in soft soil

Plate anchor is one of the most common varieties of anchors used in the construction and maintenance work of various on-land and offshore structures. An accurate estimation of the uplift capacity of anchor foundations is necessary for an economical design as well as for the safety and stability of structures. This paper outlines the effect of shape of anchor plates on their breakout capacity, through a series of model tests. Both shallow and deep anchor behaviours were investigated under conditions developing suction force and without suction force. The results of these tests are presented in terms of load-displacement behaviour, variation of breakout factors (with and without suction force) with depth of embedment, the critical embedment depth of anchors and variation of suction force with embedment ratio. Further, the variations of breakout factor ratio with aspect ratio and embedment ratio are reported. Based on the experimental results and the model test results of other investigators an empirical relationship has been suggested to determine the shape factor and holding capacity of plate anchors buried in saturated cohesive soils.     

Uplift response of symmetrical anchor plates in reinforced cohesionless soil

The uplift response of symmetrical anchor plates with and without geogrid reinforcement layers has been evaluated in model tests and numerical simulations using PLAXIS. Many parameters of the reinforcement layers were used to reinforce the sandy soil over circular, square, and rectangular symmetrical anchor plates of various sizes. In the current research, different parameters, such as relative density of sand and embedment ratios, in conjunction with geogrid reinforcement layer parameters including size, number of layers, and the proximity of the layer to the circular anchor plate, were investigated in a scale model. The failure mechanism and the associated rupture surface were observed and evaluated. Test results showed that using geogrid reinforcement layers significantly improves the uplift capacity of symmetrical anchor plates. It was found that inclusion of one geogrid layer resting directly on top of the symmetrical anchor plate was more effective in enhancing the symmetrical anchor capacity than the layer itself. It was also found that the inclusion of one geogrid layer on the symmetrical anchor plate improved the uplift capacity more than the same symmetrical anchor plate embedded without a reinforcement layer. The single geogrid layer was also more effective in enhancing the uplift capacity compared to the multiple geogrid layer reinforcement approach. In general, the results show that the uplift capacity of symmetrical anchor plates in loose and dense sand can be significantly increased by the inclusion of geogrid layers. It was also observed that the inclusion of geogrid layers reduces the requirement for a higher L/D ratio to achieve the required uplift capacity. The results of the laboratory and numerical analysis are found to be in agreement in terms of the breakout factor and failure mechanism pattern.     

Influence of overburden pressure and soil rigidity on uplift behavior of square plate anchor in uniform clay

A numerical study incorporating three-dimensional Eulerian large deformation finite element analyses is performed to investigate the pullout process of horizontal square plate anchors in both hypothetical weightless soil and soil with self-weight. The validity of the numerical model is established through verification against published experimental and numerical results. The failure mechanisms during the pullout process under different conditions are then investigated. Three types of failure mechanism are observed; of which only two have been reported in the literature. The third mechanism identified in this study, which is a partially localized flow mechanism, is operative when the soil overburden ratio is not high enough to mobilize the full flow mechanism. The influence of soil self-weight is directly investigated by incorporating the density of the soil in the finite element model and maintaining the gravitational acceleration field throughout the analysis. The critical overburden ratio corresponding to the full transition to a localized plastic flow mechanism is identified in this study. The effect of the soil rigidity index (E/s_u) on the anchor uplift capability has not been systematically investigated in earlier studies. Contrary to the general failure mechanism and the full flow mechanism described in the literature, the capacity factor corresponding to this new mechanism increases with increasing E/s_u. The capacity factors for square plate anchors corresponding to different anchor embedment ratios, overburden ratios and E/s_u are provided in the form of design charts.     

Computations of Seismic Passive Resistance and Uplift Capacity of Horizontal Strip Anchors in Sand

In this paper, the limit equilibrium method is used to compute seismic passive earth pressure coefficients and the vertical uplift capacity of horizontal strip anchors in presence of both horizontal and vertical pseudo-static earthquake forces. By considering a simple planar failure surface, distribution of soil reaction is obtained through the use of Kötter’s equation. Presence of pseudo-static seismic forces induces a considerable reduction in the seismic passive earth pressure coefficients. The reduction in seismic passive earth pressure coefficients increases with increase in magnitude of the earthquake accelerations in both horizontal and vertical directions and with increase in wall friction angle. The vertical uplift capacity of horizontal strip anchor is obtained for various values of soil friction angle, embedment ratio and seismic acceleration coefficients in both horizontal and vertical directions by using rigorous computational optimization. Proper justification for selected value of wall friction angle is established. Results are presented in the form of non-dimensional breakout factor for anchor. A significant reduction in breakout factor is observed in presence of both the seismic acceleration coefficients whereas breakout factor increases with increase in soil friction angle and embedment ratio even under the seismic condition. Angles of failure planes keep changing with change in seismic acceleration coefficients and failure zone shifts towards the critical direction of seismic acceleration coefficients. Present results are compared and found in good agreement with some specific available results in literature.     

砂土中群锚锚周土体变形特性模型试验研究

群锚是常见的基础形式应用较为广泛,由于群锚之间的相互作用,群锚上拔过程中锚周土体的变形破坏机制比较复杂。采用非接触式数字图像相关方法（DIC）对群锚上拔过程开展模型试验研究,分析了群锚上拔过程中上拔力-位移关系曲线特征和锚周土体变形破坏机制。试验结果表明,密实度和埋深对群锚上拔力-位移关系曲线特征具有显著影响,在相同密实度、相同埋深率下浅埋与深埋群锚与同条件下的单锚具有相似的上拔力-位移关系曲线特征;群锚抗拔承载力具有明显的叠加效应,且砂土密实度、埋深和锚间距等参数因素对群锚效应具有显著影响。通过变形场的研究,得出了砂土密实度、埋深以及锚间距对群锚效应的影响规律。     

Serviceability limit state design for uplift of helical anchors in clay

Presently, no displacement-based design methodology exists for helical anchors subjected to tensile or uplift loading. This study investigates the statistical and probabilistic aspects of the load-displacement uncertainty associated with a database of thirty-seven uplift loading tests of helical anchors founded within cohesive soils. Initially, an ultimate resistance model is identified, and the semi-empirical uplift breakout factor statistically characterized. A relationship between ultimate resistance and slope tangent capacity is established, and used to form the basis for normalizing the load-displacement response. Hyperbolic and power law models are statistically evaluated for use in serving as a reference load-displacement model; the hyperbolic curve was selected based on goodness-of-fit statistics. Monte Carlo reliability simulations are used to establish an equivalent-deterministic load factor that associates the selected load factor with a probability of exceeding a pre-determined allowable uplift displacement, given uncertainty in the undrained shear strength, ultimate resistance model, transformation uncertainty, uncertainty in the allowable displacement, and variability in uplift loading. A practical example is provided to show the intended use of this probabilistic helical anchor displacement model.