Experimental studies of the geosynthetic anchorage – Effect of geometric parameters and efficiency of anchorages

The soil reinforcement by geosynthetic is widely used in civil engineering structures: embankments on compressible soil, slope on stable foundations, embankments on cavities and retaining structures. The stability of these structures specially depends on the efficiency of the anchors holding the geosynthetic sheets. Simple run-out and wrap around anchorages are two most commonly used approaches. In order to improve the available knowledge of the anchorage system behaviour, experimental studies were carried out. This paper focuses on a three-dimensional physical modelling of the geosynthetics behaviour for two types of anchors (simple run-out and wrap around). The pull-out tests were performed with an anchorage bench under laboratory controlled conditions with three types of geosynthetic (two geotextiles and one geogrid) and in the presence of two types of soil (gravel and sand).The results show that there is an optimum length for the upper part of the geosynthetic for the wrap around anchorage.     

Experimental studies on inclined pullout behaviour of geosynthetic sheet Vis-À-Vis geogrid - Effect of type of anchor and sand

This paper presents the details of an experimental investigation using large-scale inclined pullout apparatus on sheet geosynthetic and geogrid embedded in run-out, I-type, and L-type anchors. The influence of the type of sand on the behaviour of the sheet and the geogrid is also investigated. The results show that in both the sheet and the geogrid, I-type anchor provides approximately 50% and L-type anchor provides 90% higher pullout force than the run-out anchor. The maximum pullout force increases by more than 20% as the inclination of pullout force increases from 0° to 30° for both the sheet and the geogrid.     

含水量对红黏土中土工格栅拉拔性能影响的试验研究

 通过室内大型拉拔试验设备,对土工格栅在8组不同含水量的红黏土中的拉拔特性进行系统测试。结果发现,土工格栅在黏性填料中主要表现为拔出破坏,含水量对于拉拔力的影响显著,拉拔极限荷载随含水量的增加逐渐降低,在塑限附近趋于一常数,且此时格栅与填料之间的似摩擦因数接近0。格栅的应变分布特征证明含水量的增加导致筋土摩擦力的显著减小。除了影响极限拉拔力,含水量还影响格栅的拉拔过程,它的增加使得格栅应变的线性增长结束后很快达到其极限承载力。     

Interaction between geogrid reinforcement and tire chip–sand lightweight backfill

This paper deals with the interaction between the geogrid and the tire chip–sand mixture including the determination of the index properties of the backfill materials, the shear strength parameters, the interaction coefficients, and the efficiency of geogrid reinforcements in tire chip–sand backfills. Numerous experiments including index tests, compaction tests, pullout tests, and large-scale direct shear tests were conducted. Saint–Gobain (geogrid A) and Polyfelt (geogrid B) were selected as reinforcing materials. Tire chip–sand mixtures with mixing ratios of 0:100, 30:70, 40:60, and 50:50 by weight were used as fill materials. The test results revealed that the dry unit weight of tire chip–sand mixtures depended more on the sand content, and less on the water content. The mixture at the mixing ratio of 30:70 by weight or 50:50 by volume was found to be the most suitable fill material compared to other mixing ratios. The pullout resistance and the pullout interaction coefficients of geogrid A were slightly higher than those of geogrid B. In contrast, in the direct shear resistance, the direct shear interaction coefficients, and the efficiency values of geogrid B were slightly higher than those of geogrid A. Since geogrid B has the needed uniaxial reinforcement properties and its sufficient interaction characteristics with tire chip–sand mixture, the geogrid B was utilized in this study. The interaction coefficients between the tire chip–sand backfill with 30:70 mixing ratio by weight were found to be 0.71 in pullout mode and 0.92 in direct shear mode for geogrid B.     

Experimental and numerical analysis of large scale pull out tests conducted on clays reinforced with geogrids encapsulated with coarse material

The pullout test is one of the methods commonly used to study pullout behavior of reinforcements. In the current research, large pullout tests (i.e. 100 × 60 × 60 cm) have been conducted to investigate the possibility of pullout resistance enhancement of clays reinforced with HDPE geogrid embedded in thin layers of sand. Pullout tests on clay–geogrid, sand–geogrid and clay–sand–geogrid samples have been conducted at normal pressures of 25, 50 and 100 kPa. Numerical modeling using finite element method has also been used to assess the adequacy of the box and geogrid sizes to minimize boundary and scale effects. Experimental results show that provision of thin sand layers around the reinforcement substantially enhances pullout resistance of clay soil under monotonic loading conditions and the effectiveness increases with increase in normal pressures. The improvement is more pronounced at higher normal pressures and an optimum sand layer thickness of 8 cm has been determined for maximum enhancement. Results of numerical analysis showed the adequacy of the box and geogrid length adopted as well as a relatively good agreement with experimental results.     

A new approach to evaluate soil-geosynthetic interaction using a novel pullout test apparatus and transparent granular soil

Geosynthetic reinforced soil walls and slopes are now a mature technology in geotechnical engineering. Nevertheless, the mechanisms of soil-geosynthetic interaction are not fully understood for pullout of a geogrid material in the anchorage zone of a reinforced structure. It is also difficult to quantify the interactions between the geogrid and the soil. A new strategy to overcome these difficulties is to use a pullout box with a transparent glass bottom, a transparent soil, and non-contact measurement technology. This paper describes such a pullout box apparatus which is used in combination with a recently developed transparent granular soil. Embedded geogrid specimens are visible through the transparent bottom of the box and the surrounding soil. The displacements of the geogrid and seed (target) particles placed in the transparent soil are tracked using digital images captured by a row of synchronized cameras located below the apparatus. Digital processing is carried out using the Digital Image Correlation (DIC) technique to quantify the in-situ displacement of the geogrid specimen and surrounding soil. The displacements are used to compute continuous longitudinal strain profiles in the geogrid specimen over the duration of each pullout test and relative shear displacements between the geogrid and the soil. Also reported are lessons learned to improve the method of clamping geogrid specimens at the front of the pullout box which are also applicable to conventional pullout box equipment.     

火灾作用下锚具对预应力钢棒锚固性能退化规律研究

为研究火灾作用下锚具对预应力（PC）钢棒锚固性能退化规律,完成65组由JXM型锚具、混凝土块、抗拉强度标准值为970 MPa的φ7高强PC钢棒及反力架组成的预应力锚固系统高温性能试验,其中29组为裸露锚具高温瞬态试验,18组为裸露锚具高温稳态试验,18组为带防火保护的锚具高温瞬态试验。研究PC钢棒应力水平（0.2~0.7）、温度（20~800℃）、防火保护措施（灌浆料封锚、涂抹防火涂料、灌浆料封锚后再涂抹防火涂料）和升温速率（10.0、26.7℃/min）对锚固性能的影响。裸露锚具高温瞬态试验结果表明：高温下锚固系统破坏模式为PC钢棒在锚具中发生滑移;PC钢棒的应力水平越高、温度越高、升温速率越大,锚固系统破坏时间越短。高温稳态试验结果表明：温度低于400℃时,锚固系统破坏模式为PC钢棒被拉断,温度高于400℃时,破坏模式为PC钢棒从锚具中滑移;高温后锚具不能再使用。带防火保护的高温瞬态试验结果表明：锚固系统破坏模式为PC钢棒从锚具中滑移,防火保护不改变锚固系统的破坏模式;与无防火保护措施相比,三种保护措施下锚固系统破坏时间均提高100%以上;若将GB 14907-2002《钢结构防火涂料》关于钢结构防火涂料厚度的要求直接应用于预应力锚具防火,将偏于不安全。     

Effects of transverse members on geogrid pullout behavior considering rigid and flexible top boundaries

Geogrid pullout tests have been regarded as the most direct and effective way to describe the interfacial behavior between geogrid and soil. To investigate the coupled effects of geogrid transverse members and top-loading boundaries on the geogrid-soil interaction, numerical simulations of geogrid pullout tests using the Discrete Element Method (DEM) were carried out in this study. The rigid top boundary was simulated by a rigid wall, while the flexible top boundary was modeled with a string of bonded particles that could rotate and move up and down freely. The coupled effects of geogrid transverse members and top boundary conditions on the geogrid-soil interaction under pullout loads were visualized not only by the force distributions along the geogrids and in the specimens but also by the displacements of soil particles and geogrids. Additionally, the quantitative geogrid force and strain distributions along the geogrids, the lateral force distributions on the front walls, and the vertical displacements of top boundaries also showed the influence of transverse members on the geogrid pullout behavior considering the rigid and flexible top boundaries. The DEM investigation results of this study may provide helpful guidelines for regulating the geogrid pullout test apparatus and methods.     

Modelling of debonding along the fixed anchor length

Debonding of the fixed length of ground anchorages is presented, concentrating on the failure behaviour of the bond at the proximal end of the fixed length. The dynamic effects of debonding along the fixed length are discussed in particular, with specific reference to dynamic testing. The static effects in terms of load distribution are also reviewed by comparing pull out tests undertaken on laboratory anchorages bonded using resin grout to results from a bond model included in a dynamic model. Bond models, which allow the dynamic effect of the debonding to be observed, are proposed for inclusion in the dynamic model. These models are an elastic–plastic yielding model with some residual load capacity and a model with reduced stiffness once the plastic-yielding regime has been entered. Simulations of an anchorage system including these bond models show both a change in load distribution and a drop in the first natural frequency due to debonding. Simulation of a case study is undertaken using the model and the results show good agreement with the measured field data.     

Influence of longitudinal and transverse members on geogrid pullout behavior during deformation

A series of pullout tests were carried out by using a highly extensible geogrid with the different longitudinal member and transverse member ratios to investigate the influence of the longitudinal and transverse members of a highly extensible geogrid on the pullout behavior. From the results, the following were made clear: the mobilization of bond stress depends on the strain of the geogrid; the influence zones of longitudinal members become isolated with the increase of the longitudinal member spacing; the mobilization of transverse member resistance depends on the displacement of the geogrid at the location of that transverse member; and the contribution of longitudinal members to the pullout force is more significant than that of transverse members during the deformation stage in the case of the highly extensible geogrid, since large elongation occurs in the geogrid, which restricts the mobilization of the full effect of transverse members. Furthermore, the mobilization mechanisms of the longitudinal member and transverse member effects are discussed.     

基于夹持效应的普立特大桥隧道锚现场模型试验研究

 为揭示隧道锚围岩夹持效应的力学机制,认识隧道锚围岩破坏模式并确定极限抗拔能力,进行圆台形锚体和圆柱形锚体的夹持效应对比试验。试验选择在普立特大桥隧道锚碇区勘探斜洞的2条平行支洞内进行,2种模型试体的侧面积和高度相同,以定量比较因夹持作用引起的隧道锚围岩极限抗拔能力的差别。试验结果表明,与圆柱形锚体模型相比,圆台形锚体模型因存在夹持效应,破坏前的变形量、围岩变形影响范围、破坏时的极限荷载明显增大。圆柱形锚体发生锚体混凝土与围岩接触面破坏,脆性破坏明显。圆台形锚体发生围岩沿不利结构面破坏,且破坏前经历了很长的屈服变形阶段,两者破坏模式完全不同。提出了夹持效应系数的概念及其计算模式,得出弹性阶段夹持效应系数、极限强度夹持效应系数分别为4.48,4.54。夹持效应产生的抗拔能力远远大于混凝土与岩体接触面的抗剪强度,是隧道锚抗拔能力的主要支撑。     

影响格栅加筋膨胀土拉拔试验的新因素分析

不少学者开展过土中加筋拉拔试验,受测试仪器所限,大多通过改变填土含水率、厚度(上覆法向压力)、类型(粗、细粒土)及拉拔速度等因素来研究对试验结果的影响,除考虑筋材类型(土工格栅、带、网或布)外,对筋土中另一重要影响因素—筋材的初始张拉状态少有研究。本文采用长沙理工大学自行研发的大型数控拉拔试验系统,发挥其尺寸大、双向气囊加载、消除侧壁摩擦等优势,开展膨胀土中格栅加筋拉拔试验,探究筋材尺寸、初始张拉状态、温度、界面残余强度及拉拔方式等新因素对测试值的影响。结果表明:格栅尺寸有一定影响,尤其宽度影响较大;对最大拉拔力而言,格栅应力释放的影响近8.8%;拉拔方式的影响约12.1%;温度的影响为15.9%;残余强度的影响占23.6%。研究结果可供加筋膨胀土工程设计参考。     

基于透明土的4种锚杆拔出对比模型试验

采用透明土材料和粒子图像测速法(PIV)技术,通过物理模型试验研究锚杆拔出机理。在试验中采用普通圆柱型和手榴弹型、糖葫芦型和圣诞树型3种异型共4种不同形状锚固段锚杆,测试获得各锚杆随锚杆锚固段上移对周围土体的扰动规律、锚杆位移与锚固力变化曲线,分析各锚杆锚固段的极限承载力与锚固段破坏机理。试验结果分析表明:在本文试验条件下,在锚杆拔出破坏前,除了手榴弹型锚杆外,各种形状锚杆对土体水平位移影响相对于竖向位移均较小,糖葫芦型和圣诞树型锚杆对土体的竖向位移影响范围最大,达到6.0倍锚杆半径,比普通圆柱型锚杆对土体竖向位移影响范围大1.5倍;异形锚杆可以很有效的提高锚杆极限承载力,比普通圆柱型锚杆可提高66%~91%,其中圣诞树型锚杆的极限承载力最大;锚杆锚固段的有效长度、有效横截面积、有效直径对其极限承载力有直接影响;锚杆极限承载力值是发生在锚杆锚固段与土体接触的界面开始破坏到完全破坏之间,锚杆锚固段与土体接触界面发生破坏,是从锚杆自由段部位开始逐渐往锚杆底部发展,以A型锚杆为例,当破坏发展到距锚杆底部1/3的位置时,锚杆极限承载力达到峰值。     

悬索桥隧道锚工程应用与研究现状分析

基于国内外重要文献数据库,采用文献计量技术,整理完善了隧道锚内外工程应用现状,分析了30座隧道锚建设地点、应用数量、主跨度、锚塞体轴向长度、围岩性质等特点,推测了未来隧道锚建设趋势。通过文献统计手段,概括了隧道锚材料与结构选型,并对20多年来隧道锚国内外相关中英文论文发表情况进行了调研,统计了隧道锚文献来源期刊与学位论文、分析了相关论文的发表机构,并采用可视化技术构建关键词共现网络,对目前隧道锚研究热点与重点进行整理。针对隧道锚应用地质特点以及相关研究进行分析评价,基于隧道锚的上述研究现状,对隧道锚未来的研究方向提出简要的思考,认为隧道锚的应用会更加广泛,同时还需要对复杂地质作更深入全面的研究。     

Prediction of pullout interaction coefficient of geogrids by extreme gradient boosting model

Geogrids embedded in fill materials are checked against pullout failure through standard pullout testing methodology. The test determines the pullout interaction coefficient which is critical in fixing the embedment length of geogrids in mechanically stabilized earth walls. This paper proposes prediction of pullout interaction coefficient using data driven machine learning regression algorithms. The study primarily focusses on using extreme gradient boosting (XGBoost) method for prediction. A data set containing 220 test results from the literature has been used for training and testing. Predicted results of XGBoost have been compared with the results of random forest (RF) ensemble learning based algorithm. The predictions of XGBoost model indicates 85% accuracy and that of RF model shows 77% accuracy, indicating significantly superior and robust prediction through XGBoost above RF model. The importance analysis indicates that normal stress is the most significant factor that influences the pullout interaction coefficients. Subsequently pullout tests have been performed on geogrid embedded in four different fill materials at three normal stresses. The proposed XGBoost model gives 90% accuracy in prediction of pullout interaction coefficient compared to laboratory test results. Finally, an open-source graphical user interface based on the XGBoost model has been created for preliminary estimation of the pullout interaction coefficient of geogrid at different test conditions.     

端部机械锚固FRP-混凝土黏结界面承载力试验研究

采用胶-机械混合锚固体系可以有效防止纤维增强复合材料 (FRP)与混凝土之间的早期剥离破坏,提高FRP材料的强度利用率。为明确端部锚固对黏结界面性能及破坏形式的影响,进行了不同锚固形式的FRP-混凝土黏结结点的单面剪切试验,对比分析了纯外贴锚固（EB）、普通混合锚固（HB）和自锁混合锚固(SLHB)三种不同端部锚固形式的锚固性能。对于HB-FRP加固方法,探究了FRP黏贴长度和机械锚固扣件施加扭矩大小对加固效果的影响。结果表明：采有EB、HB、SLHB锚固形式的FRP加固构件破坏形式分别为界面剥离、FRP滑动拉出和FRP拉断（强度利用率为1.0）;采用SLHB-FRP和HB-FRP加固方法的试件相比采用EB-FRP加固方法的加固构件在15N·m的扭矩下可以提高承载力约122.9%和56.4%,采用EB-FRP和HB-FRP加固方法试件的FRP强度利用率分别为0.448和0.702;随着扭矩的增大,采用HB-FRP加固方法的试件极限荷载也有不同程度的提升,且失效荷载与扭矩线性相关;对于自锁混合锚固形式,在15kN·m的扭矩作用下,FRP强度均能得到100%利用,失效荷载均为FRP拉断荷载,与黏结长度无关。     

Pullout testing and Particle Image Velocimetry (PIV) analysis of geogrid reinforcement embedded in granular drainage layers

The paper investigates the feasibility of using fine-grained soil as backfill material of geosynthetic-reinforced walls and slopes, through a laboratory study on pullout behavior of geogrids in granular layers. A series of pullout tests was carried out on an HDPE uniaxial geogrid in thin sand and gravel layers that were embedded in clay specimens.Aside from different soil arrangements, the influences of moisture content and overburden pressure on the geogrid pullout behavior is assessed and discussed. The tests were carried out at four different gravimetric water contents (GWC) on the dry and wet sides of the clay optimum moisture content (OMC), and overburden pressure values within the range σ_v = 25–100 kPa. Particle Image Velocimetry (PIV) was used to capture digital images during the tests, which were processed to help with the interpretation and improved understanding of the soil-geogrid interactions at different GWC values. Results show that embedding geogrid reinforcement in layers of sand or gravel can significantly increase the pullout resistance in an otherwise moist clay backfill, and this improved pullout efficiency is greater at higher overburden pressures. The improvement in pullout capacity was observed in clay specimens compacted at both the dry and wet sides of the OMC.     

Influence of cyclic tensile loading on pullout resistance of geogrids embedded in a compacted granular soil

This paper deals with some results of a wide experimental research carried out in order to study factors affecting cyclic and post-cyclic pullout behaviour of different geogrids embedded in a granular soil. The new test procedure developed (multistage pullout test) and the relative results are described. In particular, test results obtained using the constant rate of displacement (CRD) and the multistage pullout tests highlighted the influence of the different factors involved in the research (cyclic load amplitude and frequency, vertical confining stress, geogrid tensile stiffness and structure) both on the peak pullout resistance and on the peak apparent coefficient of friction mobilized at the interface.     

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.     

The influence of a cyclic loading history on soil-geogrid interaction under pullout condition

The knowledge of soil-geosynthetic interface behaviour is a key point in the design of geosynthetic-reinforced soil structures. The pullout ultimate limit state can be reproduced conveniently by means of pullout tests performed with large-size laboratory apparatuses, which allow studying the interaction mechanisms that develop in the anchorage zone. During the service life of geosynthetic-reinforced soil structures, reinforcements may be subjected to long-term cyclic vehicular loads or short-term seismic loads in addition to dead loadings, such as the structure's self-weight and other sustained loads. In order to study the influence of a cyclic loading history (a sinusoidal function with fixed amplitude A, number of cycles N and frequency f) on the post-cyclic peak pullout resistance, the writers carried out a series of multi-stage pullout tests on a high density polyethylene extruded uniaxial geogrid embedded in a compacted granular soil for different vertical effective stress σ′_v values. Moreover, the stability of the soil-geosynthetic interface from a point of view linked to the cyclic loading application has also been investigated. Test results showed that the design pullout resistance parameters are affected by the applied cyclic loading history for specific combined conditions (A, N and σ′_v) and it should be taken into account for designing geosynthetic reinforced soil structures.