不同加载路径下玻璃纤维复材筋黏结性能试验研究

玻璃纤维复材(GFRP)筋与砂浆的黏结性能是其应用于土钉支护结构中重要的参数。对6组不同直径和不同埋深的GFRP筋砂浆拉拔试件进行单调和循环拉拔试验,研究GFRP筋与砂浆黏结性能。结果表明:GFRP筋在砂浆中的拉拔破坏模式与长度锚固相关;砂浆强度对20筋体极限黏结强度影响不大,而对22、25筋体的黏结强度发挥有影响;分级循环加载拉拔中,黏结滑移曲线均会发生斜率的变化,且在拉拔力卸载后出现一段残余变形,存在非线性变形,在化学胶着力消失后,滑动摩擦力与机械咬合力呈现线性特征;GFRP筋与砂浆黏结滑移本构模型中,τ-s曲线上升段与改进的BPE模型的上升段趋势吻合,但还需进一步研究模型参数。     

深基坑土钉墙支护施工仿真分析

针对深基坑土钉墙支护，采用三维非线性有限元分析方法，模拟研究了深基坑的开挖与支护过程。计算中考虑了初应力和土体材料的非线性及钉土之间的相互作用。通过实例分析，给出了深基坑开挖与支护过程中土钉的轴力、钉土界面剪应力及基坑位移场等的情况，同时证明了选用经反分析得到的土体力学参数其结果更加准确。     

土钉墙变形的实用计算方法

本文在大量的土钉墙现场实测、模型试验、拉拔试验等资料的基础上,假定土钉轴力沿土钉轴线呈双抛物线形分布、土钉剪应力-位移关系满足理想弹塑性模型以及土钉面层不受力,推导出钉土相对位移、土钉墙坡面侧向变形和被动区土体内沿土钉轴线任意点土体位移的实用计算公式,给出了多根土钉及分步开挖后土钉坡面侧向位移增量的计算方法,讨论了本文提出的土钉变形计算方法中主要参数的计算模式,包括土压力、钉土间剪切变形系数、土钉整体稳定性计算等。通过三个工程的实测资料对本文计算理论进行验证,结果表明了本文土钉墙变形计算理论方法的有效性和适用性。本文所提出的土钉墙变形计算方法可以利用工程实践中大量的土钉抗拔实测资料,比较容易地推算土钉墙开挖后的变形情况,从而为土钉墙变形的预测和控制提供了设计和施工方面的依据和参考。     

光纤光栅传感器在FRP土钉现场拉拔试验中的应用

FRP不同于钢筋材料特性,FRP土钉与灌浆材料的黏结力分布与发展规律等作用机理一直是研究的难题和急需解决的问题。通过在FRP土钉中埋设光纤光栅传感器,并将FRP土钉用于实际边坡支护,在施工完成后进行现场拉拔力测试试验,测定短期拉拔力作用下FRP土钉的受力特性,分析表明:FRP土钉承担了支护作用;受力初期钉体近端先承力;随着拉拔力增大,钉体各点受力增长率不一致,近端初期增长较快,到一定拉拔力后,钉体各点拉拔力增长率趋于常数;钉体各位置承载力的变化趋势最后均呈现一致性,但大小不均匀。     

锚杆锚固体与土体界面特性室内测试新方法

为了更真实地反映锚杆受拉时锚固体与土体界面的特性,较准确地获得包含界面剪切残余段的剪应力–位移(?–s)全过程曲线,自行研发了一种锚–土界面摩阻性能测试仪及相应的锚杆拉拔试样制作装置和方法。该测试仪器和测试方法简便易行,可成批模拟多种环境条件进行室内锚杆拉拔试验。利用该仪器完成了4批次27个不同条件的锚固体拉拔试验,深入研究了锚固体养护龄期、拉拔速率等因素对锚–土界面剪切强度特性的影响,提出了一种锚–土界面?–s全过程本构模型。研究结果表明:界面剪切强度在锚固体养护14 d后增长缓慢;锚杆以0.1~2.5 mm/min拉拔时,速率对剪切强度的影响不大;提出的锚–土界面模型计算曲线与试验曲线吻合良好。     

压密效应对新型压密注浆土钉的强化研究

注浆压密效应是影响新型压密注浆土钉抗拔性能的一个重要因素。基于大型模型箱,开展了有压密效应和无压密效应的两组对比拉拔试验,研究了注浆压密效应对新型土钉性能的影响。另外,针对抗拔力随拉拔位移的演化规律,提出了抗拔力计算的双曲模型。研究表明:注浆压密效应对拉拔前期的抗拔力影响较大,而对最终抗拔力影响甚微。当土体条件发生变化时,注浆压密效应对抗拔力的影响取决于注浆压力,而非节泡直径。注浆压密效应引起的土体响应差异,其中包括垂直剪胀效应以及垂直和水平的挤压效应,是引起两种试验条件下抗拔力增加速率差异的根源。引入压缩模量和极限拉拔应力两个参数,建立的抗拔力计算双曲模型,可对新型压密注浆土钉的抗拔力进行有效计算。     

基于幂函数模型的钢丝网筋土界面关系研究

在双绞合六边形钢丝网加筋红砂岩粗粒土大型拉拔试验的基础上,分析双绞合六边形钢丝网筋材抗拔力的发展过程,研究采用理想弹塑性模型、双曲线模型和指数模型描述拉拔剪应力-位移曲线的不足,建立适合描述网状筋材筋土界面关系的幂函数模型,通过Fish函数对FLAC3D中Geogrid单元的筋土界面关系进行修正,并进行拉拔试验数值模拟...     

土钉支护季节冻土区边坡冻胀效应耦合模型建立及求解

为了研究季节冻土区边坡冻胀效应下土钉支护结构沿土钉长度方向的剪应力和轴力分布变化规律，首先，基于传热学理论及固液两相共存的导热微分方程得到冻结锋面移动变化规律，结合冻胀率公式进一步推导得到寒区边坡自由冻胀变形计算公式；其次，将土钉对边坡的支护作用视为无穷多个沿着土钉长度方向上的剪切分布力，考虑到边坡冻结时冻结层与未冻土层的刚度差异性，通过坐标变换后将刚度当层法与Mindlin解相结合推导双刚度层边坡内部有剪切集中力作用时钉土相互作用接触面上任一点的位移解析解；最后，基于钉土二者相互作用位移协调条件，建立关于土钉长度方向剪切分布力的黎曼积分方程，通过闭Newton-Cotes法的复合梯形积分公式及MATLAB软件编程对算例求解分析，获得土钉轴力分布规律，并利用数值解和实测值进行对比验证，此外还分析冻胀率、初始含水量等参数对土钉长度方向剪应力及轴力分布影响。结果表明：本文耦合计算方法得到土钉轴力分布形式与现场实测、数值模拟结果在总体趋势上基本一致，表明耦合计算法是合理可行的；土钉长度方向的剪应力及轴力分布规律符合中性点理论，轴力曲线呈现出两头小中间大的抛物线状，其最大值对应出现在剪应力为零...     

考虑锚固体不均匀及杆体脱黏效应的GFRP抗浮锚杆杆体荷载分布函数

基于荷载传递理论及Kelvin位移解,推导得出理想条件下GFRP抗浮锚杆杆体与锚固体的剪应力、轴力沿锚固深度的分布函数,并通过对2根同型号GFRP抗浮锚杆的拉拔试验进行验证。试验结果表明:试验锚杆杆体轴力及剪应力分布曲线与理论值形式相似,证明提出理论方法应用于求解GFRP锚杆杆体荷载分布函数的可行性。但由于锚固体成型后的不均匀性及杆体的脱黏效应,导致试验剪应力值低于理论值且实际曲线主要分布范围较深,同时造成轴力下降较慢,消失深度较深的现象。通过固定脱黏长度下的平均剪应力衰减法及下移弹性段起点的方法对理想条件下的杆体剪应力分布函数进行修正,并利用杆体轴力分布与剪应力分布之间的函数关系,对理想条件下的轴力分布函数进行修正,修正后剪应力及轴力分布曲线精度大大提高。     

垃圾填埋场土工合成材料的界面特性试验方法研究

拉拔、直剪、单剪试验常用于材料间界面特性研究,通过对垃圾填埋场中组成复合衬垫的土工膜、土工布和砂土、黏土界面分别进行了土工膜与砂土、土工膜与黏土、土工布与砂土、土工布与黏土的拉拔试验,土工布与砂土、土工膜与黏土的直剪试验及土工网–土工膜–黏土组合界面的单剪试验。研究结果表明:土工合成材料与土界面特性主要取决于土工合成材料的表面性质,直剪和单剪试验得到应力–位移曲线的初始斜率高于拉拔试验;法向应力由低向高变化时控制滑动面也发生了变化,界面特性受到防渗结构层中相邻材料的影响;在3种试验中拉拔试验得到的界面强度包线最高,单剪次之,直剪最低;单剪试验能较合理地模拟垃圾填埋场复合防渗结构的工作条件,建议利用单剪试验技术确定界面的强度参数,直剪试验也是可选择的试验方法。     

System reliability analysis of soil nail wall using random finite element method

Soil nail wall is a compound system which for safety margin determination, consideration of safety factors of its components and their correlations is required. In this paper, considering a real site using the random finite element method (RFEM), the reliability indices of global stability, lateral displacement stability, tensile strength, and pullout resistance stability as components of the soil nail wall system are obtained. In another section of the paper, using the sequential compounding method (SCM), the importance of the mentioned stability modes and their effects on system reliability and system probability of failure are represented. Results show that the most considerable interdependence is between the global and lateral displacement stabilities. Among the reliability indices of the components, the minimum one is attributed to the pullout resistance. Furthermore, the uppermost row of the nails has the most critical reliability index compared with the others. The locations of the slip surfaces and nail intersections varied from 0.05–0.90 of the nail length, which means that the uncertainty of the soil parameters has the most significant effect on the pullout resistance safety factor of the nails. The performance level of the soil nail wall decreases from below average to poor when the soil nail wall is considered to be a system with series components.

     

变形协调情况下土钉墙内力和位移的计算方法

 土钉墙的内力主要包括土钉轴力、钉土剪力和面层受力,而土钉墙变形的过程是土钉、面层和土体之间相互协调共同作用的过程,传统的计算模型都是从力矩平衡的力学角度出发,不能合理解释土钉内力和变形的相互关系。通过对土钉墙受力和变形机制的分析,利用Mindlin解计算在虚拟开挖应力和钉土剪力共同作用下土体侧向位移,结合钉土剪力和钉土相对位移的线性关系以及钉土剪力在最危险滑动面位置等于0的特点,计算土钉轴力、钉土剪力和土钉墙在开挖面位置的变形。土钉被动区段钉土剪力超出其极限值的部分转移至面层,假定面层受力增量呈三角形分布,根据静力平衡得到面层受力分布。通过与工程实测资料的对比分析,初步验证土钉墙内力和变形计算方法的合理性和可行性。     

Pullout performance of GFRP anti-floating anchor in weathered soil

Anchors are often used as anti-floating reinforcements in civil engineering structures. However, conventional steel bars present disadvantages concerning corrosion and poor adaptability to aggressive environments. Glass fiber-reinforced polymer (GFRP) components could provide a solution to these problems. In this paper the feasibility of GFRP anti-floating anchors is evaluated. Four full scale pullout tests were performed in moderately decomposed granite (MDG). Bare Fiber Bragg grating (FBG) sensors were embedded into the specimens during the pultrusion process to monitor the stress–strain distribution along their lengths. Based on the results the behavior of the anchors was assessed, including the relationships between the pullout force and the head displacement, the axial strain along anchors and the shear stress at the GFRP-grout interface. The stress distribution of anchors showing interlaminar shear failure was then analyzed based on a maximum shear stress criterion. It was proved that the load transfer mechanism of GFRP and steel anti-floating anchors differs significantly. GFRP anti-floating anchors reach failure due to interlaminar shear failure, while conventional steel anchors generally fail as a result of shear at the grout–soil interface. The test results also showed that the embedded FBG technique is reliable for monitoring the stress–strain state of an anisotropic material.     

Modeling the pullout behavior of short fiber in reinforced soil

Fiber reinforcement is an effective method for improving engineering properties of soil. However, the interaction mechanism of the fiber and the surrounding soil is not well understood. Based on mechanical analysis of fiber-soil interface under pullout condition, a tri-linear model is proposed to describe the shear stress-displacement relationship. The progressive pullout process of a short fiber in soil is divided into five consecutive phases: (1) the initial pure elastic phase (Phase I); (2) the elastic-softening phase (Phase II); (3) the pure softening phase (Phase III); (4) the softening-residual phase (Phase IV); and (5) the final pure residual phase (Phase V). For each phase, the analytical solutions of the distributions of tensile force, interfacial shear stress and displacement are derived. Through a comparison between the pullout test results of polypropylene fiber (PP-fiber) and the predicted results, the effectiveness of the proposed model in capturing the progressive load-deformation behavior of a short fiber in soil is verified. Moreover, the effects of water content and dry density of soil on the model parameters are analyzed in detail. It is found that the interfacial peak/residual shear resistance and shear stiffness of fiber reinforced soil significantly depend on soil compaction conditions. In general, two transition phases (Phase II and Phase IV) are not evident during the whole pullout process of PP-fiber.     

Simplified Procedure to Evaluate Earthquake-Induced Residual Displacement of Geosynthetic Reinforced Soil Retaining Walls

Based on a series of shaking table model tests, it was found that the effects of 1) subsoil and backfill deformation, 2) failure plane formation in backfill, and 3) pullout resistance mobilized by the reinforcements on the seismic behaviors of the geosynthetic reinforced soil retaining walls (GRS walls) were significant. These effects cannot be taken into account in the conventional pseudo-static based limit equilibrium analyses or Newmark's rigid sliding block analogy, which are usually adopted as the seismic design procedure. Therefore, this study attempts to develop a simplified procedure to evaluate earthquake-induced residual displacement of GRS walls by reflecting the knowledge on the seismic behaviors of GRS walls obtained from the shaking table model tests. In the proposed method, 1) the deformation characteristics of subsoil and backfill are modeled based on the model test results and 2) the effect of failure plane formation is considered by using residual soil strength after the failure plane formation while the peak soil strength is used before the failure plane formation, and 3) the effect of the pullout resistance mobilized by the reinforcement is also introduced by evaluating the pullout resistance based on the results from the pullout tests of the reinforcements. By using the proposed method, simulations were performed on the shaking table model test results conducted under a wide variety of testing conditions and good agreements between the calculated and measured displacements were observed.     

Investigation of soil deformation characteristics during pullout of a ribbed reinforcement using X-ray micro CT

Steel-strip reinforced earth walls stabilize through the pullout resistance of the reinforcements. Soil dilation during the pullout of ribbed reinforcements may contribute to the evolution of pullout resistance; however, few studies have clarified this mechanism by investigating how soils behave with increasing pullout displacement. The ribs of the reinforcements enhance the pullout resistance, although the influence of the rib dimensions on the evolution of pullout resistance with increasing pullout displacement has not been sufficiently revealed. In the present study, a triaxial pullout apparatus is developed and pullout tests are conducted using ribbed reinforcements with different rib-inclination angles under isotropic stress. The displacement and strain fields in the soils during the pullout of the reinforcements are investigated by X-ray micro CT and a digital image correlation technique. It is found that larger rib-inclination angles provide higher pullout resistance at an early stage of the pullout because of the higher bearing resistance related to the more significant soil densification above the ribs. With increasing pullout displacement, the reinforcements with different rib-inclination angles come to behave as almost one in the same since a rigid soil wedge related to the passive soil failure is generated above the ribs. This tendency results in similar soil deformation characteristics and pullout resistance levels for every reinforcement beyond the soil failure state, although the rib-inclination angles are different.     

Pullout of soil nail with circular discs:A three-dimensional finite element analysis

An internal failure mode for a soil-nailed system consists of failure at nail heads,slope facing,nail strength,along groutesoil interface and pullout failure.A better understanding of pullout of soil nail thus becomes important to assess the stability of a soil-nailed system.In the present study,an investigation into the pullout behaviour of soil nail with circular discs along the shaft has been carried out by a threedimensional finite element analysis using Abaqus/Explicit routine.A total of 67 simulations have been performed to accurately predict the pullout behaviour of soil nail.The soil nail under study has circular discs along its shaft varying in numbers from 1 to 4.The pullout of this soil nail in a pullout test box has been simulated with a constant overburden pressure of 20 kPa acting on the nail.The pullout load edisplacement characteristics,stresses around soil nail and failure mechanism during pullout are studied.Variations of dimensionless factors such as normalised pullout load factor and bearing capacity factor have been obtained with different combinations of parameters in terms of relative disc spacing ratio,anchorage length ratio,embedment ratio,diameter ratio and displacement ratio.From the results of analyses,it is found that nail with more circular discs requires higher pullout load.There are critical relative disc spacing ratio and diameter ratio which significantly affect the pullout behaviour of nail.     

风化岩地基GFRP抗浮锚杆力学与变形特性现场试验

利用光纤光栅传感技术,对10根GFRP抗浮锚杆进行现场拉拔破坏性试验,研究了风化岩地基中GFRP抗浮锚杆的承载性能与变形特性。试验结果表明:发生滑移破坏的锚杆杆体、锚固体荷载-位移差曲线高于同型号发生断裂破坏的锚杆;锚固长度接近临界锚固长度的试验锚杆荷载-位移差曲线上升较平稳;增加杆体直径有助于提高锚杆承载能力、限制杆体位移并且降低杆体、锚固体的位移差。此外,杆体横截面轴应力沿锚固深度呈"反S型"分布,由孔口沿锚固深度方向递减;锚杆轴向界面剪应力沿锚固深度呈先增大后减小的趋势,且剪应力在锚固体内按斜向上方向由第一界面传递至第二界面。最后,利用剪应力分布简化模型求得杆体、锚固体位移差与发生滑移破坏的锚杆试验结果较为一致,可为GFRP锚杆的推广应用提供理论基础。     

Displacement and load transfer mechanisms of geogrids under pullout condition

Reinforcing elements embedded within soil mass improve stabilization through a load transfer mechanism between the soil and the reinforcement. Geogrids are a type of geosynthetic frequently used for soil reinforcement, consisting of equally spaced longitudinal and transverse ribs. Under pullout conditions, the longitudinal ribs are responsible for tensile resistance, while transverse ribs contribute to a passive resistance. This paper describes a new analytical model capable of reproducing both load transfer and displacement mechanisms on the geogrid length, under pullout conditions. The model subdivides the geogrid into rheological units, composed by friction/adhesion and spring elements, mounted in line. Friction/adhesion elements respond to the shear component mobilized at the soil–geogrid interface. Spring elements respond to the geogrid's tensile elongation. Model parameters are obtained through tensile strength tests on geogrids and conventional direct shear tests on soil specimens. The need for instrumented pullout tests becomes therefore eliminated. Results predicted from this new model were compared to instrumented pullout test data from two types of geogrids, under various confining stress levels. The results revealed that the new model is capable of reasonably predicting load and displacement distributions along the geogrid.