Uplift Capacity of Single Bent Pile and Pile Group Considering Arching Effects in Sand

In this paper an analytical method has been proposed to predict the net ultimate uplift capacity of the single bent pile and pile group with a bent embedded in sand considering arching effects. Arching develops due to relative compressibility of sand relative to pile which activates the soil-pile friction. The method takes into consideration the embedded length (L), diameter of the pile (d), bent angle, surface characteristics of pile, group configuration, spacing of the pile group and the soil properties. Log spiral failure surface with parabolic arch shape was assumed in the analysis. Theoretical investigation for uplift capacity was been carried out for the single bent pile and group of pile (2 × 1, 2 × 2) embedded in sand. The variable used in the analysis were embedded length to pile diameter (L/d = 15, 20 and 25), spacing in the group (3d, 4d and 6d) and angle of bent (6°, 14° and 20°). Typical charts for evaluation of net ultimate uplift capacity for pile groups are presented through the figures. Comparison of theoretical results shows good agreement with established experimental results.     

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.     

Experimental investigation of the behavior of pile groups in sand under different loading rates

The behavior of pile groups in sand under different loading rates is investigated. A total of 60 tests were conducted in the laboratory using model steel piles embedded in a medium dense sand. The model piles have an outside diameter of 25 mm and embedment length of 500 mm. Five different configurations of pile groups (2 × 1, 3 × 1, 2 × 2, 2 × 3, 3 × 3) with center to center spacing between the piles of 3d, 6d and 9d (d is the pile diameter) were tested. The piles were subjected to axial compressive loads under four different loading rates: 1.0, 0.5, 0.1 and 0.05 mm/min. Test results indicated that the axial compressive capacity of pile group increases with the loading rate such that the pile capacity versus logarithm of loading rate data plot approximately along a straight line. The slope of this line increases as the number of piles in a group increases and it decreases by increasing the spacing between piles in a group.     

Model Pile Groups Under Oblique Pullout Loads – an Investigation

Experimental investigations on model pile groups of configuration, 1 × 1(single pile), 3 × 1, 2 × 2 for embedment length to diameter ratio, L/d = 38, were conducted in uniform dry medium dense Ennore sand. The spacing of piles in the groups varied from 3 to 6 pile diameter. Soil–pile friction angles were δ = 16° and 28°. The pile groups were subjected to oblique pulling loads at angles θ = 0°, 30°, 60° and 90° with the vertical central axis of the groups. The load–displacement response, oblique ultimate pulling resistances have been qualitatively and quantitatively studied. The inclinations of the load, at which maximum oblique resistance for the groups were observed, have been reported. Predictions of ultimate resistance of pile groups under uplift, lateral and oblique pulling loads have been carried out respectively by the methods of Patra and Pise (2002) (Electronic Journal of Geotechnical Engineering, 8, Bundle B), Patra and Pise (2001) (Journal of Geotechnical and Geoenvironmental Engineering ASCE, 127(6), 481–487) and Chattopadhyay and Pise (1986c) (Proceedings of IST East Asian Conference on Struct Engineering and Const., Vol. 1, pp. 1632–1641). A comparison of the measured values of the Writers and others with the predicted values showed reasonable agreement.     

渤海西部海域新生代盖层特征及对油气的控制作用

渤海西部海域新生代盖层发育情况与油气富集密切相关。利用渤西地区丰富的钻井资料、测井资料、取心分析化验资料等对研究区新生代盖层发育特征、封闭性及其对油气成藏的控制作用开展了研究。研究结果表明,该区主要发育东二段下部、明下段下部及明下段上部3套区域泥质岩盖层,东二段下部区域泥岩盖层相比于明下段两套区域泥岩盖层具有单层泥岩厚度大、色暗、质纯、高泥地比、横向分布稳定及更高成岩阶段等特点,东二段下部泥岩盖层封闭性好于明下段泥岩盖层;东二段下部区域泥岩盖层控制了油气在凹陷区域深部沙河街组、东营组的聚集成藏并决定了油气朝沙垒田古隆起上倾方向运移,明化镇组下段区域"终极盖层"与断裂共同作用控制了整个渤西地区新近系的油气富集程度;提出了"周边凹陷东二下区域泥岩盖层底面构造脊输导体系"、"凸起斜坡及内部明下段泥岩盖层、断裂‘耦合’油气输导体系"两种区域盖层控制下的油气输导模式。      

Effect of Arching on Uplift Capacity of Single Piles

An analytical method has been proposed to predict the ultimate uplift capacity of single vertical piles embedded in sand considering arching effect. The present analysis takes into consideration of various pile and soil parameters such as length (L), diameter (d) of the pile, angle of internal friction of soil (ϕ), soil pile friction angle (δ) and unit weight of soil (γ). A modified value of coefficient of lateral earth pressure in uplift has been developed considering the arching effect of soil. A comparative assessment of the uplift capacity of piles predicted by using proposed theory and the existing available theories is made with the existing field and model test results. It has been observed that the present model considering the arching effect predicts the results closer.     

Uplift capacity of single piles: predictions and performance

The paper pertains to the development of a simple semi-empirical model for predicting the uplift capacity of piles embedded in sand. Various pile and soil parameters such as length (L), diameter (d) of the pile and angle of friction (ϕ), soil–pile friction angle (δ) and unit weight (γ) of the soil which have direct influence on the uplift capacity of the pile are incorporated in the analysis. A comparative assessment of the ultimate uplift capacity of piles predicted by using the proposed theory and some of the available theories are made with respect to each other and with reference to the measured values obtained from model tests in the laboratory. For this purpose experimental data have been collected from the literature and also from model tests conducted as a part of the present investigation. The study shows the proposed model has an excellent potential in predicting the uplift capacity of piles embedded in sand that are consistent with model pile test results.     

部分粘结预应力抗拔(浮)桩的优化设计

介绍了一种新型的抗拔（浮）桩——部分粘结预应力抗拔（浮）桩。该新型桩有效改善了抗拔桩的桩身结构,能充分发挥抗拔桩的承载力,改变了混凝土受拉时的受力性状,克服了普通抗拔桩容易产生裂缝的弊病。结合有关地下结构浮力方面的研究成果及现场实际的部分粘结预应力抗拔桩荷载试验结果,对抗拔（浮）桩进行了优化设计。     

基于极限承载力试验的扩底抗拔桩承载特性数值模拟分析

基于天津于家堡南地下车库工程扩底桩抗拔极限承载力试验,结合数值模拟手段对扩底桩抗拔承载特性、破坏模式和受力机制开展了分析研究。计算分析表明,扩底桩（有效桩长19 m）相比等截面桩抗拔极限承载力提高约50%,材料增加仅8.5%,扩底桩扩大头周边土体提供的抗力显著提高了抗拔承载力;荷载较小时抗拔力主要由等截面段侧摩阻力提供,扩头段抗拔力占桩顶加载的比值随加载近似呈线性增加;扩底桩等截面段沿桩土界面先发生剪切破坏,扩头段周边土体后发生受压破坏,抗拔承载力达到极限;扩头段位于同一土层时,不同桩长扩头段提供的极限抗拔力相差不大,桩长越长,扩头段抗拔力贡献率越低;扩头段抗拔力主要由自重、扩头段法向力竖向分力和侧摩阻力组成,其中法向力竖向分量提供了扩头段的主要抗拔力,占扩头段总抗拔力约70%。     

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。     

高临界循环荷载水平下钻孔桩的抗拔性状分析

为了研究抗拔桩在循环加载过程中桩身的性状,选取杭州萧山某工地两根抗拔桩作为试桩进行循环加载试验。在抗拔桩桩身埋设钢筋应力计,测量循环加载过程中桩身应力的变化情况;在桩身预埋钢管,测量桩顶及桩端上拔量。通过对桩身应力及桩顶、桩端上拔量的分析,研究循环加载对抗拔桩承载力及上拔量的影响。综合分析抗拔桩承受循环荷载时的桩顶、桩端上拔量、桩身轴力以及桩侧摩阻力分布可知,当抗拔桩承受循环荷载时,且临界循环荷载水平较大（接近或者达到1）,抗拔桩经过2～3次循环加载后就发生破坏,其研究结果可供其他类似工程参考。     

扩底抗拔桩桩端阻力的群桩效应研究

通过颗粒流数值模拟,从桩端阻力随上拔位移的发展与桩端周围土体颗粒位移表现等角度,研究了扩底抗拔桩端阻力的群桩效应问题。研究比较了单桩（墩）与群桩（墩）的抗拔性状以及不同墩距下中心墩与边墩阻力随上拔位移发展的情况。研究表明,在归一化上拔量 0.1时,单桩（墩）与群桩（墩）的上拔特性无明显差别;此后,随着上拔位移的发展,单桩（墩）的上拔端阻力要大于群桩中的桩（墩）的端阻力,桩（墩）周围土体颗粒的相互影响开始显现。在归一化上拔量 0.5的情况下,群桩（墩）中中心桩（墩）的端阻力要略大于边桩（墩）。在归一化上拔量 0.5的情况下,群桩中边桩的桩端阻力较中心桩的要大,而群墩中边墩的墩端阻力较中心墩要小,体现了桩身侧限对抗拔桩群中端阻力发挥的影响。随着墩距的增大,在较大的位移量以后群墩才与单墩的受力有显著差别。     

Vertical uplift resistance of circular plate anchors in clays under undrained condition

The vertical uplift resistance of circular plate anchors, embedded horizontally in a clayey stratum whose cohesion increases linearly with depth, has been obtained under undrained ( = 0) condition. The axi-symmetric static limit analysis formulation in combination with finite elements proposed recently by the authors has been employed. The variation of the uplift factor (F_c) with changes in the embedment ratio (H/B) has been computed for several rates of increases of soil cohesion with depth. It is noted that in all the cases, the magnitude of F_c increases continuously with depth up to a certain value of H_cr/B beyond which the uplift factor becomes essentially constant. The proposed static limit analysis formulation is seen to provide acceptable results even for the two other simple chosen axi-symmetric problems.     

等截面桩与扩底桩抗拔承载特性数值分析研究

建立了等截面桩与扩底桩的抗拔分析模型。采用直接约束处理的接触算法与库仑摩擦模型来模拟桩-土界面,土体选用抛物线型Mohr-Coulomb本构模型。通过程序的二次开发,实现初始应力场的合理计算,并采用位移边界对扩底抗拔桩的上拔过程进行动态的非线性有限元模拟。以现场足尺试验中的试桩为数值分析对象,通过调整计算参数使计算得到荷载-位移关系曲线与试验结果接近,来验证数值模型的正确性并确定计算参数。在此基础上,研究了上拔过程中轴力、侧摩阻力的大小与分布、桩身与扩大头附近土体变形与塑性区的发展规律。数值分析加深了对扩底桩抗拔承载特性的进一步认识。     

抗拔桩极限平衡方程及其应用

将抗拔桩侧阻力分解为与桩侧正压力不相关的桩-土黏结强度 、与桩侧有效正压力成正比的摩擦力 两部分,采用摩擦定律计算摩擦力 。基于轴对称条件,假定土体为半无限弹性体,以Mindlin公式积分计算分析极限平衡状态下桩-土共同作用,依据平面应变条件下柱状孔扩张的弹性力学解建立桩-土界面位移协调方程,推导出抗拔桩极限平衡方程,给出了求解方法及计算参数确定方法。该方程能反映桩与土的材料特性、桩体尺寸、桩顶埋深、群桩效应、卸荷效应等多因素对抗拔桩极限承载力的影响。结合海上风电大直径超长抗拔钢管桩足尺试验进行验证。对比分析结果表明,该方法计算的抗拔极限承载力与实测值接近,计算精度远高于现行规范推荐方法,其结果可为工程应用及抗拔桩承载力机制研究提供参考。     

抗拔（浮）桩的发展历程和研究方向

综述了由最初的普通钢筋混凝土抗拔（浮）桩发展到预应力抗拔（浮）桩的历程；总结了对于抗拔桩性能研究的3种方法：现场试验、模型试验和理论分析法。在此基础上为了降低成本，方便施工，环保高效，提出了一种新型的预应力抗拔（浮）桩，即部分粘接预应力抗拔（浮）桩，这将是预应力抗拔（浮）桩一个有意义的发展方向。     

A modulus‐multiplier approach for non‐linear analysis of laterally loaded pile groups

A modulus‐multiplier approach, which applies a reduction factor to the modulus of single pile p–y curves to account for the group effect, is presented for analysing the response of each individual pile in a laterally loaded pile group with any geometric arrangement based on non‐linear pile–soil–pile interaction. The pile–soil–pile interaction is conducted using a 3D non‐linear finite element approach. The interaction effect between piles under various loading directions is investigated in this paper. Group effects can be neglected at a pile spacing of 9 times the pile diameter for piles along the direction of the lateral load and at a pile spacing of 6 times the pile diameter for piles normal to the direction of loading. The modulus multipliers for a pair of piles are developed as a function of pile spacing for departure angle of 0, 90, and 180sup>/sup> with respect to the loading direction. The procedure proposed for computing the response of any individual pile within a pile group is verified using two well‐documented full‐scale pile load tests. Copyright © 2006 John Wiley & Sons, Ltd.     

Morphological characterization of cluster microforms

A field study was conducted on two mountain streams in the Cascade Mountains of Washington State on the morphological characterization of cluster microforms. Morphological characterization of clusters is presented in terms of: (i) cluster shape; (ii) cluster geometric properties; and (iii) the spatial arrangement of clusters in the horizontal plane. Clusters were differentiated from other microtopography features such as reticulate structures and transverse ribs, and identified clusters were categorized by shape as being of pebble, line, comet, heap or ring type. The complex spatial arrangement of clusters at the sites was characterized by using a two‐dimensional correlation function, which allowed for measurement of the average cluster‐spacing properties. For the rivers examined, pebble‐shaped clusters were the most frequently observed cluster shape. Cluster geometric properties were found to be controlled by particles of the largest size fraction in the bed and the projected frontal width of the cluster – with cluster length being linearly related to cluster width for cluster width‐to‐height ratios <3·5. Results of the cluster‐spacing analysis suggest that cluster spacing increases with cluster size and decreases with local slope. Application of this principle to the available spacing data shows that cluster spacing λ scales with the ratio of S/d₀ such that λS/d₀ = constant, where S is the local slope and d₀ the diameter of the largest particle in the cluster.     

Vertical uplift capacity of two interfering horizontal anchors in sand using an upper bound limit analysis

The vertical uplift resistance of two interfering rigid rough strip anchors embedded horizontally in sand at shallow depths has been examined. The analysis is performed by using an upper bound theorem of limit analysis in combination with finite elements and linear programming. It is specified that both the anchors are loaded to failure simultaneously at the same magnitude of the failure load. For different clear spacing (S) between the anchors, the magnitude of the efficiency factor (ξ_γ) is determined. On account of interference, the magnitude of ξ_γ is found to reduce continuously with a decrease in the spacing between the anchors. The results from the numerical analysis were found to compare reasonably well with the available theoretical data from the literature.     

Ultimate bearing capacity of equally spaced multiple strip footings on cohesionless soils without surcharge

The ultimate bearing capacity of a group of equally spaced multiple rough strip footings was determined due to the contribution of soil unit weight. The analysis was performed by using an upper bound theorem of limit analysis in combination with finite elements and linear programming. Along the interfaces of all the triangular elements, velocity discontinuities were considered. The value of ξ_γ was found to increase continuously with a decrease in S/B, where (i) ξ_γ is the ratio of the failure load of an interfering strip footing of a given width (B) to that of a single isolated strip footing having the same width and (ii) S is the clear spacing between any two adjacent footings. The effect of the variation of spacing on ξ_γ was found to be very extensive for small values of S/B; ξ_γ approaches infinity at S/B=0. In all the cases, the velocity discontinuities were found to exist generally in a zone only around the footing edge. Copyright © 2008 John Wiley & Sons, Ltd.