非完全粗糙平板锚旋转安装过程中的丢失埋深研究

吸力贯入平板锚被用于系泊深海浮式结构,实际应用中必须预估平板锚旋转安装过程中的丢失埋深。采用大变形有限元方法探索非完全粗糙平板锚在正常固结黏土中的旋转过程。大变形有限元法通过网格重剖避免平板锚大幅值平动和转动引起的土体单元扭曲。根据平板锚旋转到达的埋深,实时更新“锚-土”界面上的摩擦剪切强度。将数值模拟结果与离心机模型试验进行对比验证,表明高岭土中平板锚表面的粗糙系数约为0.3。平板锚丢失埋深随粗糙系数的减小而增大,但粗糙系数对丢失埋深的影响受平板锚厚度比和拉力偏心比的耦合作用。厚度比和偏心比越小,粗糙系数对丢失埋深的影响越显著。     

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。     

Numerical Investigation of the Inclined Pullout Behavior of Anchors Embedded in Clay

Two-dimensional plane strain finite element analysis has been used to simulate the inclined pullout behavior of strip anchors embedded in cohesive soil. Previous studies by other researchers were mainly concerned with plate anchors subjected to loads perpendicular to their longest axis and applied through the centre of mass. This paper investigates the behavior of vertical anchors subjected to pullout forces applied at various inclinations with respect to the longest anchor axis, and applied at the anchor top and through the centre of mass. The effects on the pullout behavior of embedment depth, overburden pressure, soil–anchor interface strength, anchor thickness, rate of clay strength increase, anchor inclination, load inclination and soil disturbance due to anchor installation were all studied. Anchor capacity is shown to increase with load inclination angle for anchors loaded through the centre of mass; greater effects are found for higher embedments. The results also show that anchor capacity improves at a decreasing rate with higher rates of increase of soil shear strength with depth. In addition, the capacity of vertically loaded anchors is shown to approximately double when the soil–anchor interface condition changes from fully separated to fully bonded. Similarly, disturbed clay strengths adjacent to the anchor following installation cause a significant reduction in anchor capacity. The results showed a significant effect of the point of load application for anchors inclined and normally loaded. The effects of other parameters, such as anchor thickness, were found to be less significant.     

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.     

软土中浅埋法向承力锚位移破坏标准模型试验

为确定浅埋软黏土中法向承力锚的位移破坏标准,利用实验室自行开发的电动伺服加载控制装置,对埋置在饱和软黏土中深度为3倍锚板宽度、系缆力角度为30°的法向承力锚模型进行了荷载和位移控制的静力加载试验。依据试验结果,以极限承载力与锚板宽度为参考,对锚板法向荷载与位移进行归一化处理,通过归一化曲线确定了法向承力锚的位移破坏标准,即破坏位移约为0.38倍锚板宽度。另外,依据锚板的尺寸、埋深及试验土体的强度条件,利用经验公式对锚的极限承载力进行了计算。结果表明,以该位移破坏标准确定的极限承载力与经验公式计算结果基本一致,两者相差均不超过10%,初步验证了该位移破坏标准的合理性。     

Vertical Uplift Capacity of Equally Spaced Multiple Strip Anchors in Sand

The ultimate uplift resistance of a group of multiple strip anchors placed in sand and subjected to equal magnitudes of vertical upward pullout loads has been determined by means of model experiments. Instead of using a number of anchor plates in the experiments, a single anchor plate was used by simulating the boundary conditions along the planes of symmetry on both the sides of the anchor plate. The effect of clear spacing (s) between the anchors, for different combinations of embedment ratio (λ) of anchors and friction angle (ϕ) of soil mass, was examined in detail. The results were presented in terms of a non-dimensional efficiency factor (ξ_γ), which was defined as the ratio of the failure load for an intervening strip anchor of a given width (B) to that of a single strip anchor plate having the same width. It was clearly noted that the magnitude of ξ_γ reduces quite extensively with a decrease in the spacing between the anchors. The magnitude of ξ_γ for a given s/B was found to vary only marginally with respect to changes in λ and ϕ. The experimental results presented in this study compare reasonably well with the theoretical and experimental data available in literature.     

Seismic vertical uplift capacity of horizontal strip anchors using pseudo-dynamic approach

This paper presents the pseudo-dynamic analysis to determine the seismic vertical uplift capacity of a horizontal strip anchor using upper bound limit analysis. However, in the literature, the pseudo-static approach was used by few researchers to compute the seismic vertical pullout resistance, where the real dynamic nature of earthquake accelerations cannot be considered. Under the seismic conditions, the values of the unit weight component of uplift factor f_γE are determined for different magnitudes of soil friction angle, soil amplification, embedment ratio and seismic acceleration coefficients both in the horizontal and vertical directions. It is observed that the uplift factor f_γE decreases significantly with the increase in seismic accelerations and amplification but increases with the increase in embedment ratio. The results are compared with the existing values in the literature and the significance of the present methodology for designing the horizontal strip anchor is discussed. In presence of vertical earthquake acceleration and amplification of vibration, the present values of f_γE compare reasonably well with the existing pseudo-static values obtained by modifying the horizontal acceleration coefficient.     

Estimation of Pullout Capacity of Vertical Plate Anchors in Cohesionless Soil Using MARS

Vertical plate anchors provide an economical solution to safely resist the large horizontal forces experienced by the foundation of different structures such as bulkheads, sheet piles, retaining walls and so forth. This paper develops a multivariate adaptive regression spline (MARS) model-based approach for the determination of horizontal pullout capacity (P _u ) of vertical plate anchors buried in cohesionless soil by utilizing experimental results reported by different researchers. Based on the collection of forty different pullout experimental test results reported in the literature for anchors buried in loose to dense cohesionless soil with an embedment ratio ranges from 1 to 5, a predictive approach for P _u of vertical plate anchors has been developed in terms of non-dimensional pullout coefficient (M _γq ). The capability of the proposed MARS model for estimating the values of M _γq is examined by comparing the results obtained in the present study with those methods available in the literature. Using different statistical error measure criteria, this study indicates that the present approach is efficient in estimating the horizontal pullout capacity of vertical plate anchors as compared to other methods. The sensitivity analysis indicates that the embedment ratio (H/h, where H = embedment depth of anchor, and h = height of anchor) and internal friction angle (?) of soil mass are the two most important parameters for the evaluation of non-dimensional pullout coefficient (M _γq ) using the proposed MARS model.     

Seismic interference of two nearby horizontal strip anchors in layered soil

In the present analysis, an attempt is made to explore the seismic response of two nearby horizontal strip anchors embedded in non-homogenous c-? soil deposit at different depths. The analysis is performed by using two-dimensional finite-element software PLAXIS 2D. Each anchor carries equal static safe-working load without violating the ultimate uplift capacity under static condition. The soil is assumed to obey the Mohr?CCoulomb failure criterion. The behavior of single isolated anchor subjected to an earthquake loading is determined first to study the interference effect between two anchors. The horizontal acceleration response obtained from the Loma Prieta Gilroy Earthquake (1989) is considered as the input excitation in the analysis. A parametric study is performed by varying the clear spacing (S) between the anchors at different embedment ratios (??). The magnitude of vertical displacement, shear stress, and shear strain developed at different locations of the failure domain is determined for different clear spacings between the anchors.     

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.     

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。     

Seismic uplift capacity of inclined strip anchors in sand using upper bound limit analysis

Pseudo-static approach is adopted in this paper to determine the seismic uplift capacity of an inclined strip anchor using upper bound limit analysis. Two different failure mechanisms are considered to obtain the magnitudes of unit weight component of uplift factor f_γE for different values of soil friction angle, interface friction of anchor plate, anchor inclination, embedment ratio and horizontal seismic acceleration coefficient. The failure mechanism 1 consists of a triangular and quadrilateral rigid blocks; whereas the failure mechanism 2 comprises a logarithmic spiral failure zone with varied focus, sandwiched between a triangular and quadrilateral rigid blocks. It is observed that the magnitude of uplift factor f_γE decreases significantly with the increase in seismic acceleration but increases with the increase in embedment ratio and roughness of the anchor surface. However, a mixed trend in the values of f_γE can be observed for different inclination of the anchor, which is clearly discussed in this paper. The results are compared with the existing values in the literature and the significance of the present methodology for designing the inclined strip anchor is discussed.     

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.     

方形平板锚抗拉承载力的大变形有限元分析

基于网格重分和改进的REP应力恢复技术,建立了三维大变形有限元方法研究拉力作用下方形平板锚与黏性土地基的相互作用。与常规的小变形有限元不同,大变形分析能够完整模拟平板锚的上拔过程,如果平板锚底面与土体始终保持接触,三维大变形计算得到的方板与圆板抗拉力相差很小;在无重土中的平板在加载初始即与土体脱离时,方板的承载力略低于圆板。大变形分析给出的立即脱离承载力系数与模型试验数据基本吻合,而小变形有限元与下限分析忽略了方形平板锚的长距离上拔过程对其抗拉力的影响,可能高估深锚的承载力。改进估计方形平板锚抗拉承载力的简化方法,方便于工程应用。     

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。     

Uplift Capacity of Horizontal Strip Anchors in Cohesionless Soil

This paper presents the details of the theoretical analysis of net uplift capacity of horizontal strip anchor in cohesionless soil using Kötter’s equation. A plane failure surface inclined at a characteristic angle with the ground surface is assumed. Results obtained using the proposed method are compared with the available experimental results of 30 cases for dense to loose cohesionless soil, with the maximum embedment ratio of 8. It is observed that the proposed method leads to the predictions of net uplift capacity of horizontal strip anchor that are very close to the experimental results in 93% cases. The comparison of results with available theoretical solutions shows that, proposed method makes better predictions for anchor embedment ratio less than 8 in dense cohesionless soils.     

The ultimate uplift capacity of multi-plate strip anchors in undrained clay

Soil anchors are commonly used as foundation systems for structures requiring uplift resistance such as transmission towers, or for structures requiring lateral resistance, such as sheet pile walls. Anchors commonly have more than one plate or bearing element and therefore there is a complex interaction between adjacent plates due to overlapping stress zones. This interaction will affect the failure mode and ultimate capacity. However, no thorough numerical analyses have been performed to determine the ultimate pullout loads of multi-plate anchors. By far the majority of the research has been directed toward the tensile uplift behaviour of single anchors (only one plate). The primary aim of this research paper is to use numerical modelling techniques to better understand plane strain multi-plate anchor foundation behaviour in clay soils. A practical design framework for multi-plate anchor foundations will be established to replace existing semi-empirical design methods that are inadequate and have been found to be excessively under or over conservative. This framework can then be used by design engineers to more confidently estimate the pullout capacity of multi-plate anchors under tension loading.     

UPPER BOUND SOLUTION FOR PULLOUT CAPACITY OF ANCHORS ON SANDY SLOPES

By making use of limit analysis, an upper bound solution in a closed form for determining the ultimate pullout capacity of plate anchors buried in sandy slopes has been established. The anchor plate orientation has been considered either horizontal or parallel to the slope, with the pullout force applied perpendicular to the plate. It has been found that the pullout capacity for horizontal anchors, even on slopes, remains the same as that on horizontal ground surface as long as the average embedment ratio is kept constant. Whereas for anchors which are aligned parallel to the slope the collapse load decreases continuously with the increase in the inclination of slope. © 1997 John Wiley & Sons, Ltd.     

充气锚杆在砂土中的模型试验研究

为探讨充气锚杆的承载特性,通过建立室内试验模型,改变锚杆充气压力大小、埋设深度、土体密度、橡胶膜长度、厚度等条件,进行了一系列的抗拔试验,得到相应的荷载-位移曲线。研究结果表明：在影响充气锚杆抗拔承载力的诸多因素中,充气压力及土体密实度与承载力成一定倍数增长关系,是最显著的影响因素;其次是橡胶膜长度,成显著线性增长关系;充气锚杆的极限位移主要来自橡胶膜的弹塑性变形,反映了充气锚杆在抗拔过程中橡胶膜的力学行为;通过与常用锚杆的对比分析,得出充气锚杆的极限抗拔承载能力约是单锚片螺旋锚的4.3倍,是双锚片螺旋锚的1.9倍,其极限侧阻力约为一般注浆扩大头锚杆的2～4倍     

Three-dimensional analysis of vertical square anchor plate in cohesionless soil

ABSTRACT

Vertical anchor plates are often provided to increase the performance of various geotechnical engineering structures such as sheet pile walls, bulkheads, bridge abutments and offshore structures. Hence, the safe design of such structures needs a better understanding of the 3D behaviour of the anchor plate. This paper presents and discusses the results obtained from a series of 3D finite-difference analyses of vertical square anchor plate embedded in cohesionless soil. The 3D model is found to closely predict experimental pullout load–displacement relationship. The failure mechanism observed in the numerical model is found to be very similar to the failure reported in experimental studies. For a given embedment depth, the stiffness of the breakout factor–displacement response substantially reduces with increase in anchor plate size. However, the ultimate reduction in anchor capacity is found to approximately 8% with an increase in anchor size from 0.1 to 1 m. Numerical analysis reveals that at deeper embedment depth, the friction angle of sand is the critical parameter in enhancing the performance of anchor plate. The obtained 3D model results are then compared with the published results and are found to be reasonably in good agreement with each other.