插入式基础真型抗拔试验研究

对一种新型杆塔基础--插入式基础进行了大量抗拔室内真型试验,分析了影响插入式基础抗拔极限承载力的各种因素。试验结果表明：角钢在发生屈服前,角钢与混凝土间的拉拔力与角钢的种类无关,荷载-相对位移曲线几乎重合;而一旦角钢发生屈服,由于抗拉屈服强度的不同,角钢的种类对抗拔极限承载力的影响将很大;角钢的抗拔力随着角钢埋入长度的加大而增加;混凝土强度越高,埋入角钢间的摩擦力越大,则抗拔极限承载力越大;配筋率对埋入角钢的抗拔极限承载力影响不大。     

自由段先注浆对预应力锚索抗拔试验结果影响研究

为了解决无法采用已注浆锚索进行抗拔试验问题,基于界面脱黏剪切模型,利用荷载传递和弹性变形理论,考虑锚索锚固段与自由段长度比Lb /La的影响,建立了采用自由段已注浆锚索抗拔试验结果评价锚索抗拔性能的方法。结果表明：自由段注浆对锚索抗拔力有较大幅度的提高,尤其是当界面剪切阶段由弹性向脱黏过渡时,提高比例快速增加,并在脱黏后逐渐趋于稳定。其次,锚索自由段与锚固段交界面处剪切位移和轴力最大,并向两端逐渐降低。随着拉拔力的提高,界面剪切位移和轴力都不断增大,同时向锚索两端的衰减速度也加快。此外,弹性阶段时剪应力曲线为山峰型,而脱黏阶段为马鞍型,并随抗拔力的提高,马鞍开口逐渐扩大。最后,锚固段与自由段长度比Lb /La对锚索极限抗拔力的影响较小,其极限抗拔力折减系数?Q取值介于0.81～0.83之间,且Lb /La =2.5时,?Q计算值仅比实测结果小6.02%,验证了本文方法的可行性。     

Load and resistance factor design (LRFD) calibration for steel grid reinforced soil walls

This paper reports the results of load and resistance factor design (LRFD) calibration for pullout and yield limit states for steel grid reinforced soil walls owing to soil self-weight loading plus permanent uniform surcharge. The calibration method uses bias statistics to account for prediction accuracy of the underlying deterministic models for reinforcement load, pullout capacity and yield strength of the steel grids, and random variability in input parameters. A new revised pullout design model is proposed to improve pullout resistance prediction accuracy and to remove hidden dependency with calculated pullout resistance values. Load and resistance factors are proposed that give a uniform probability of failure of 1% for both pullout and yield limit states. The approach adopted in this paper has application to a wide variety of other reinforced soil wall technologies.     

The compaction effect on the performance of a compaction-grouted soil nail in sand

In this study, two series of physical modeling experiments, with and without a grouting process, were conducted under different grouting pressures to study the effect of compaction grouting on the performance of compaction-grouted soil nails. In addition, a hyperbola-based model was proposed to describe the variation of the pullout forces with and without grouting. Some of the main conclusions drawn are as follows. First, the compaction effect initially influences the mobilized pullout force, but not the final stage of pullout; the large difference between the two series of tests in regard to the pullout force at the initial stage led to the first part of this conclusion. However, the final pullout force results of the tests, both those with and those without grouting, were similar. Second, once the soil condition changes, the compaction effect on the performance of a soil nail depends on the grouting pressure rather than the diameter of the grout bulb. Third, the difference in the soil response (i.e., vertical dilatancy and the vertical and horizontal squeezing effects) derived from the compaction grouting effect will result in the initial difference in the increased rate of the pullout force between the tests with a grouting process and those without. Finally, a hyperbola-based model was proposed to describe the variation of the pullout force of the model tests with and without grouting, through which the pullout force is available of prediction for the given diameter of grout bulb and pullout displacement.

     

Inextensible reinforcement on non‐linear elasto‐plastic subgrade under oblique pull

In this paper, a rational analysis of pullout resistance of inextensible sheet reinforcement subjected to oblique end force has been presented considering a non‐linear (hyperbolic), elasto‐plastic, normal stress–displacement relationship of the subgrade. Under an oblique pull, high normal stresses develop on stronger subgrades, thus mobilizing high shearing resistance at the reinforcement–soil interface. The higher the bearing resistance of the subgrade, the higher the horizontal component of pullout force and the lower the end displacement of the reinforcement. On the other hand, the end displacement at pullout can become very high for weaker subgrades especially at high values of the angle of obliquity. Also, the pullout capacity under oblique loading for weaker subgrades may approach or even fall below the axial pullout capacity at high values of the angle of obliquity. These adverse pullout responses owing to a low value of bearing resistance of subgrade are magnified when the subgrade stiffness is also small. On weaker subgrades, improvement in angle of interface shear is not advisable as this leads to further reduction in the pullout force and increase in the end displacement. Results are compared with back analysis of published test data on model reinforced soil walls. The comparison suggests that the present model leads to a more rational and better prediction of the pullout failure. Copyright © 2008 John Wiley & Sons, Ltd.     

带加强肋土工格栅的极限拉拔阻力分析

在立体加筋体系研究基础上,提出了带加强肋土工格栅加筋,即通过对普通土工格栅的肋进行加强处理,使其肋具有一定的竖向厚度,构成具有立体加筋效果的带加强肋土工格栅。采用染色砂法,通过带加强肋土工格栅的拉拔试验进行了拉拔力机理分析。在拉拔过程中,加强肋前方土体会首先被挤密加强,成为一个刚性的楔体,然后刚性楔与筋材一起运动成为一个土筋共同体。推导了带加强肋土工格栅加筋土的拉拔阻力模型,分析中将加强肋的侧阻力作用简化为侧向土压力的作用,采用极限平衡理论,通过两侧的主动土压力与被动土压力进行计算。根据所建立的拉拔阻力模型计算出带加强肋土工格栅加筋的极限拉拔阻力,并与试验结果进行了对比,两者基本吻合     

传感光纤-砂土界面力学性质的试验研究

近年来分布式光纤感测技术在岩土与地质工程领域的应用研究已成为一个研究热点,但工程实践表明,埋入式传感光纤与被测岩土体之间的变形协调性会对监测结果产生显著的影响,该问题一直制约着光纤感测技术在工程监测中的推广。本文通过一系列室内拉拔试验,对比了3种预埋于砂土中的单模光纤在不同上覆压力的作用下的拉拔力-拉拔位移曲线,并建立了一个用于描述光纤-砂土界面力学性质的三段式的拉拔模型,分析峰值拉拔力、残余拉拔力、有效拉拔位移和残余拉拔位移4个参数在不同上覆压力作用下的变化情况。分析结果揭示传感光纤-砂土界面的力学特性和应力传递规律,为分布式光纤传感技术在岩土体变形监测中的应用,尤其在变形监测光纤的选用标准方面,提供了参考依据。     

低围压下锚固点应变传感光缆与土体变形耦合性试验研究

应变传感光缆与被测土体之间的耦合性是决定分布式光纤监测准确性的关键。为解决土工模型试验中传感光缆与被测松填砂土间的变形耦合性问题,本文研制了一种锚固点应变传感光缆,并利用自制的光缆-土体耦合性试验装置,开展了相关的光缆拉拔试验,探究了低围压下锚固点对应变传感光缆与砂土之间耦合性的影响及作用机理。研究结果表明:光缆的轴向应变随拉拔位移的增大而增大,但应变传递被限制在0.35 m以内;相同拉拔位移下,锚固点应变传感光缆比普通传感光缆需要更大的拉拔力;锚固点大大降低了应变梯度,使应变在一定范围内呈现出平均化的特征,影响范围为锚固点前后0.05~0.075 m以内。提出了耦合性的提升率指标,以此定量评价锚固点对增强光缆与土体耦合性的作用效果。本次试验中锚固点的提升率为97.41%,表明锚固点可有效增强光缆与土体的耦合性;同时,提出了一个力学模型来模拟锚固点应变传感光缆在松填砂土中的拉拔过程,准确地描述了光缆拉拔力与锚固点受力和光缆表面摩擦力之间的关系。研究成果对于土工模型试验中分布式光纤测试技术的应用提供了科学依据。     

土工格栅与土界面作用特性试验研究

土工格栅与土的界面摩擦特性指标是加筋土工程设计的关键。通过分析土工格栅与土的界面摩擦作用和进行了直剪摩擦试验和拉拔摩擦试验,测试了两种试验条件的界面摩擦特性。在两种试验条件下,土工格栅加筋土复合体的抗剪强度均有界面摩擦角φsq和界面凝聚力csq,且土工格栅与土相对位移量的不同,其复合体的强度机理有区别。在拉拔摩擦试验中,剪应力峰值强度对应的剪切变形值高于直剪摩擦试验中剪应力峰值强度的剪切变形值5～10倍以上。两种试验均有其适用性,而土与土工格栅的相对位移较小时直剪摩擦试验较能反映实际;土与土工格栅相对位移较大时土与格栅双面均发生相对位移,拉拔摩擦试验更为合适。随法向应力的增大,直剪摩擦和拉拔摩擦试验的剪应力峰值以及剪应力峰值对应的位移均提高。直剪摩擦的剪切速度小,剪应力峰值强度高,且达到峰值强度的剪切位移大;增加剪切速度,剪应力峰值强度降低,且对应的位移也减少,其原因是界面上的孔隙水压力消散和筋材的应力松弛。应根据具体工程的需要选择直剪摩擦试验和拉拔摩擦试验确定设计参数。     

基于Hoek-Brown破坏准则的浅埋条形锚板抗拔力上限分析

现有的锚板极限承载力研究大多是采用线性或非线性Mohr-Coulomb破坏准则在砂质地基中进行的,然而Mohr-Coulomb破坏准则并不适合分析岩质地基中的抗拔结构。采用Hoek-Brown破坏准则构建了一个曲线型的破坏机制,根据极限分析上限定理求出了条形锚板抗拔力的表达式。通过变分计算,得到了极限状态下条形锚板的抗拔力和岩体破裂面的上限解。为了证明所采用方法的有效性,当材料参数B =1时,采用与Mohr-Coulomb破坏准则等效的土体参数,计算了曲线型破坏机制下条形锚板的极限抗拔力,并与已有计算结果进行了比较。结果表明,采用曲线型破坏机制得到的锚板极限抗拔力与直线型多块体破坏机制的结果基本一致,证明了所采用的曲线形破坏机制是正确的。参数研究表明：在其他参数不变的情况下,锚板极限抗拔力和破坏面都随岩体参数B的增大而减小。     

细砂中钢带-填土废塑料瓶抗拔性能试验研究

研究利用填土废塑料瓶作为横向构件附在加筋土挡墙（MSEWs）中钢带上可行性。配筋中的横向构件对增加抗拔力有重要影响。该加筋系统是由作为纵向构件的钢带和作为横向构件的填土废塑料瓶组成。为更好地研究以填土废塑料瓶作为横向构件对抗拔性能的影响,对一条独立的钢带和分别带有1～7个瓶子的钢带进行了拉拔试验。在不同的法向应力下进行了超过18次的实验室大型拉拔试验（例如拉拔箱尺寸为长1.2 m、宽0.6 m、高1.0 m）来评估新提出的加固元件的性能。试验结果表明,在扁钢带上建立三维空间对提高抗拔阻力影响很大。提高抗拔阻力最有效的方法是在距离S与直径D之比为3的钢带上加入4个横向构件,其平均抗拔阻力约为单独钢带的5倍。有横向构件和无横向构件钢带的极限抗拔阻力随竖向应力的增大而增大。     

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。     

Pullout test model for extensible reinforcement

A formulation for the analysis of pullout test on highly extensible planar reinforcement is presented. The non-linear differential equation for pullout mechanism was expressed in non-dimensional form and solved numerically using the Gauss–Siedel technique. Parametric study was carried out for various ranges of relative stiffnesses, and relative bond resistances. Normalized load–displacement relations and the variations of pullout force and reinforcement displacements along the length of reinforcement are presented graphically. A method for the estimation of the interface interaction parameters from a pre-failure test is also given. The numerical predictions compare well with the available experimental pullout test results for various geotextiles, polymers and nylon geosynthetics. Copyright © 1999 John Wiley & Sons, Ltd.     

Adequacy of field pullout testing of soil nails

In practice, field pullout testing of soil nails is a widely accepted method for the construction quality control and the performance assessment of soil nail walls. At the design stage, field pullout tests provide useful information about the design bond strength of the nail–soil interface, which is critical in the determination of minimum soil nail length. To obtain a reliable and representative estimate of the design bond strength, it is necessary to conduct a sufficient number of field pullout tests. This study demonstrates the use of the concepts of simple random sampling and stratified random sampling to assess the adequacy of the ultimate bond strength adopted for design based on the minimum recommended number of field pullout tests.     

Reliability Based Earthquake Resistant Design for Internal Stability of Reinforced Soil Structures

This paper presents a method to evaluate reliability for internal stability of reinforced soil structures using reliability based design optimization. Using limit equilibrium method and assuming the failure surface to be logarithmic spiral, analysis is conducted to maintain internal stability against both tensile and pullout failure of the reinforcements. Properties of backfill soil and strength of the geosynthetic reinforcement are considered as random variables. For the seismic conditions, reliability indices of all the geosynthetic layers in relation to tension and pullout failure modes are determined for different magnitudes of seismic accelerations both in the horizontal and vertical directions, surcharge load and design strength of the reinforcement. The efforts have been made to obtain the number of layers, pullout length and total length of the reinforcement at each level for the desired target reliability index values against tension and pullout modes of failure. The influence of horizontal and vertical earthquake acceleration, surcharge load, design strength of the reinforcement, coefficient of variation of soil friction angle and design strength of the reinforcement on number of layers, pullout length and total length of the reinforcement needed for the stability at each level is discussed.     

A numerical study of the pullout behavior of grout anchors underreamed by pulse discharge technology

Pulse discharge technology (PDT) is an innovative construction method used to enhance the bearing capacity of piles and the resisting capacity of anchors by underreaming using a high-pressure shockwave induced by an underwater electric discharge. This study numerically analyzes the pullout behavior of a grout anchor underreamed by PDT. A series of finite element analyses were performed to examine the pullout behavior of the anchor based on successive simulations from underreaming to subsequent pullout tests. The electric blasting and shockwave generation by PDT was equivalently modeled using the underwater explosion (UNDEX) model, and the appropriate UNDEX parameters were determined by benchmarking the laboratory PDT tests. Full-scale PDT underreaming and the subsequent pullout tests in dry sand deposits reported in the literature were then simulated on the basis of fluid–structure interaction (FSI) analyses and static uplift analyses. The predicted expansion of the borehole and the pullout behaviors were compared with field test results to validate the numerical model. Moreover, the results from a parametric study conducted to investigate the influence of soil and anchor characteristics on the uplift behavior of the PDT underreamed anchor are discussed.     

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。     

土工织物与吹填土界面作用特性试验研究

筋-土界面强度参数是加筋结构设计和稳定分析的关键技术指标。拉拔试验能较好模拟现场加筋行为而得到广泛的应用。基于拉拔和直剪试验研究了拉拔（剪切）速率对筋-土界面特性和吹填砂强度特性的影响规律及机制,同时探讨了不同填料界面、筋材类型的加筋效果。结果表明：随着拉拔（剪切）速率的增大,吹填砂-筋材-吹填砂界面的抗剪强度下降明显,而软土-筋材-吹填砂界面以及砂本身强度则变化不大。从强度指标来看,拉拔速率增大,筋-砂界面强度的降低主要表现为似黏聚力的降低,筋-软土界面抗剪强度的增加表现为内摩擦角增大,剪切速率对吹填砂则基本无影响。筋-土界面特性受拉拔速率和正应力的共同影响,与筋材类型和填料特性有关。筋-土界面内摩擦角小于填料摩擦角,但在一定正应力下低速剪切时（如<0.53 mm/min）可获得高于填料的抗剪强度。宜根据似黏聚力大小合理选择摩擦参数进行加筋结构的设计与评价。     

吸力式沉箱基础极限拉拔承载力的数值分析

作为新型的深水海洋基础型式,吸力式沉箱基础被广泛地用于系泊深水海洋设施中,从而承受巨大的倾斜上拔荷载。在上拔荷载水平分量与竖向分量的共同作用下,吸力式沉箱的承载特性及其工作性能评价是海洋工程设计与建设中的关键技术问题之一。然而现有的理论分析与试验研究并不能满足工程实践的需要,因此,对吸力式沉箱基础的极限承载力分析建立了有限元数值计算方法。当沉箱基础在快速拔出过程中,正常固结黏土处于完全不排水状态,沉箱基础发生整体破坏时表现出反向地基承载力失稳模式,此时沉箱基础所发挥的极限承载能力往往最大。为此,在数值计算中直接假定沉箱基础及其周围土体处于完全不排水状态,针对不同的沉箱长径比,分别确定了在竖向上拔荷载和水平拉拔的单独作用下沉箱基础极限承载力。对比发现：竖向上拔极限承载力有限元解能够较好地与理论计算结果相符合,而水平极限承载力解与理论计算结果存在一定的差异。     

粘土与筋带直剪试验与拉拔试验对比分析

在土工合成材料加筋土工程中,土工合成材料与填土的界面作用特性直接决定加筋土工程的内部稳定性,所以土工合成材料与填料的界面作用特性指标是最关键的技术指标。对两种加筋材料与高液限粘土之间分别进行了直剪试验和拉拔试验,通过分析可得筋土之间的剪应力随着垂直压力的增大而增加,但随着垂直压力的增大筋土之间剪应力增长率减小,即随着垂直压力的增加,筋土之间的剪应力收敛于某一上限值;由于加筋材料在试样制作过程中不可避免地在土中发生凹凸变形使土体对筋带产生一定的锚固作用,拉拔试验所测得的似摩擦系数及似粘聚力均大于直剪试验所测得的似摩擦系数及似粘聚力。因此,实际加筋土工程中,拉拔试验更能较好地反映其真实的工作状态。