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.     

Effects of inter-helix spacing and short-term soil setup on the behaviour of axially loaded helical piles in cohesive soil

Axial compressive load tests performed on piles instrumented with strain gauges were completed in order to investigate the effects of inter-helix spacing on the behaviour of helical piles. The test piles had two helices with varied values of helix spacing. The helix-bearing soil layer consisted of a homogeneous clay with an average undrained shear strength of 65 kPa. The test pile failure mechanisms were determined by comparing the measured load distributions to the distributions predicted by the individual bearing and the cylindrical shear models. The results suggest that the individual bearing model dominates the pile behaviour for piles with an inter-helix spacing to helix diameter ratio greater than or equal to 1.5. The helix bearing capacity factor and the shaft adhesion factor were evaluated by comparing the measured pile component resistances to the theoretical estimations. The back-calculated bearing capacity and the adhesion factors were below the values traditionally used in helical pile design. The effects of the soil setup on the pile behaviour were evaluated by comparing the load-settlement response of a pile tested immediately after the pile installation to equivalent piles tested many days after the installation. A piezometer installed near the upper helix edge was used to measure the excess pore pressure generation and dissipation induced by the installation. The results suggest that the pore pressure generation induced by the pile installation was minimal and had little influence on the short-term ultimate capacity.     

浅层土体加固对倾斜桩竖向承载力影响研究

桩身倾斜可能导致在小于对应于相同条件下竖直桩的极限承载力的荷载下提前发生弯曲破坏,并可在弯曲破坏前产生较大的桩顶水平位移和桩身挠曲。根据某工程现场载荷试验结果,对倾斜桩桩周土体进行浅层加固数值模拟,以研究其改善倾斜桩竖向承载性状的机理。分析结果表明：加固体深度、加固体面积及加固体尺寸均可对加固效果产生影响。加固体对竖向荷载作用下倾斜桩的承载性状的改善体现在对被动区土体强度和压缩性的改善以及约束桩身上部桩体转动和挠曲的作用。进一步研究了加固体压缩性的影响,结果表明：加固体弹性模量越大,加固效果越明显;但随着弹性模量的增大至一定值后,加固效果基本不再变化。     

小型桩基竖向循环加载模型试验系统研制与应用

介绍了一种小型桩基竖向循环加载模型试验系统,包括加载系统、压力室、起吊装置、模型桩、数据采集系统。该系统通过伺服电机驱动联轴器带动滚柱丝杠转动,进而带动加载板对模型桩施加竖向位移荷载;通过水压加载法对土样进行围压加载,模拟不同深度土层的应力状态及固结情况。可针对不同固结状态的地基土和多种桩基型式,开展不同荷载组合下桩基竖向循环加载模型试验研究,适用于饱和软土、一般黏性土、粉土、砂土等均质或非均质地基。应用该设备进行了单桩竖向循环加载模型试验,并与数值模拟进行对比。研究结果表明,动荷载幅值、地基土固结压力对桩基承载力特性影响很大;动荷载幅值较小时,桩基承载力几乎没有弱化;地基土固结压力升高时,桩基承载力提高显著,桩基承载力弱化速度减慢;随着振次及振幅的增加,桩顶轴力逐渐弱化至残余值;位移循环荷载作用下,桩周土的弱化导致桩基产生了负摩阻力,进一步降低了桩基承载力。模型试验结果与数值模拟结果吻合较好。通过初步应用,证明了该试验系统弥补了常规1g小比例尺模型试验中低围压的不足,可以较好地反应出桩顶荷载和位移的非线性关系和承载力循环弱化现象。同时该系统输出荷载波形精确,量测系统灵敏度高、稳定性好...     

Effects of specimen density and initial cyclic loading history on correlation between shear wave velocity and liquefaction resistance of Toyoura sand

Some previous studies have shown a good correlation between the shear wave velocity, V_s, and the cyclic resistance ratio, CRR. Recently, however, a V_s-based liquefaction assessment method has become an alternative and supplementary method to the conventional N_SPT-based method. It is known that the CRR is influenced not only by the specimen density, but also by the soil fabric. Unfortunately, there are concerns that different combinations of the effects of the specimen density and the soil fabric may generate different relations between V_s and the CRR even if the tested specimens are of the same soil material. In the current study, a series of V_s measurements and undrained cyclic triaxial tests is performed on Toyoura sand specimens with different soil fabrics for three different specimen densities. The fabric of the specimens is varied by applying initial cyclic loading. The results of the V_s measurements indicate that the V_s of the specimen is affected by the initial cyclic loading histories, and the results of the undrained cyclic triaxial tests show that there is a good correlation between V_s and the CRR. However, the correlation varies depending on the specimen density even when the tested material is Toyoura sand only. In other words, the soil-type specific correlation between V_s and the CRR depends on the specimen density. Therefore, the results indicate that both V_s and the specimen density are necessary parameters for an accurate assessment of the CRR.     

静载荷试验确定水泥土桩复合地基承载力

以淮盐高速公路淮安段工程为例，就其14组静力载荷试验进行了分析，对利用静力载荷试验确定地基承载力的方法进行了一定的探讨，并总结了几点体会。     

竖向下压循环荷载作用下单桩承载力及累积沉降特性模型试验研究

竖向循环荷载作用下,单桩的承载力及变形特性均不同于静荷载情况。利用浙江大学自行研制的大型地基与边坡工程模型试验系统开展了单桩基础竖向下压循环荷载作用下的大比例模型试验,获得了不同侧/端阻比下桩基的循环承载力及桩顶循环累积沉降。试验发现:存在一最小循环荷载比,当循环荷载比小于该值时,桩顶不会发生循环累积沉降;一定循环次数内桩顶循环累积沉降基本与循环次数成正比,且循环累积沉降与循环次数的比值近似与桩顶循环荷载比呈线性关系。通过给定单桩循环破坏标准,绘出了模型桩的循环承载力曲线,发现侧/端阻比较大的桩基更易发生桩顶循环累积沉降;通过拟合桩顶无量纲沉降,可预测桩基循环累积沉降的发展。最后提出了长期循环荷载作用下单桩基础的工程设计方法。     

竖向荷载作用下螺杆灌注桩受压承载机理的试验研究

螺杆灌注桩是一种上部为圆柱形、下部为螺丝形的组合式灌注桩,由于具有承载力高、节约材料、文明施工程度高等特点,在工程中得到广泛应用.螺杆灌注桩由于其特有的施工工艺及截面形式,使得其承载机理不同于等截面灌注桩.通过室内模型试验,采用图像处理技术直观地得到螺杆桩和直杆桩的破坏形态,进而对其承载机理和承载力计算进行探讨,并采用...     

PHC管桩水平承载特性现场试验研究

针对PHC管桩水平承载特性研究的不足,以泗许高速亳州段工程为依托,对管桩水平载荷进行了试验,研究了PHC管桩的水平承载特性,得到了PHC管桩的水平承载力大小以及管桩在水平荷载作用下的变形规律。     

自平衡法在大直径灌注桩承载力检测中的应用

以某工业厂房为例,介绍了自平衡法在大直径灌注桩静载荷试验中的应用,并与传统的锚桩法、堆载法相比较,分析了自平衡法的优点和使用前景,为自平衡法的推广应用提供了技术支持。     

兰州地区旋挖钻孔灌注桩竖向承载力特性试验研究

通过兰州某交通枢纽项目旋挖钻孔灌注桩静载荷试验与内力测试,分析了该旋挖钻孔灌注桩的荷载传递机理和承载特性。研究表明,桩顶荷载的大部分由桩侧阻力承担,桩端阻力只承担竖向荷载的一小部分;桩侧阻力优先于桩端阻力发挥;荷载-沉降(Q-S)曲线呈缓变型;该试验桩的竖向承载力明显高于一般钻孔灌注桩。     

Model testing study on engineering performances of circular helicoid piles during the whole process of installation and bearing in sandy soil

The circular helicoid pile (CH pile) is a new type of special-shaped pile that has been developed in Japan and South Korea in the past decade and has been widely used in the fields of construction, transportation, natural energy and agriculture due to its excellent compressive and pullout bearing performances. Consequently, this new type of pile has good engineering application prospects. However, as an innovative engineering structure, the CH pile is not widely known by geotechnical engineers worldwide. The geometric structure of the CH pile is similar to a circular helicoid in differential geometry. Therefore, the pile-soil interaction problem cannot be reduced to a plane strain problem or an axisymmetric problem in theoretical research. In view of this, dry silica sand was used as the model foundation in this study, and a model test device and method that can effectively reflect the installation process and loading-bearing service state of CH piles were developed. Under different installation methods, pile structures and foundation soil conditions, 90 model tests were carried out to evaluate the engineering performances of CH piles during the whole process of installation and bearing, including the installation performances during the installation process, the compressive bearing performances under axial compressive loading and the pullout bearing performances under axial pullout loading. Compared with steel sheet piles and steel pipe piles, CH piles have better engineering performance and more economic benefits from the aspects of installation, construction, recycling, timeliness of engineering application, and the relative relationship between bearing capacity and pile mass.     

Performance of geosynthetic-encased stone column-improved soft clay under vertical cyclic loading

This paper presents the results of a laboratory investigation into the performance of geosynthetic-encased stone column-improved (GESC-improved) soft clay under vertical cyclic loading. A reduced-scale model is adopted to perform a series of tests considering the principal parameters, such as the cyclic loading characteristics, including the loading frequency and amplitude, and the encasement length. The results indicate that, among other things, the overall benefit of the geosynthetic encasement of stone columns installed in soft clay is greater under cyclic loading than under static loading, and that the cyclic effect tends to lead to a stress concentration ratio that is smaller than that under static loading. The effectiveness of this encasement in improving the performance of GESCs becomes greater when subjected to cyclic loading with a lower loading frequency and/or a smaller amplitude. The settlement and pore pressure variations with the encasement length, together with the exhumed GESCs taken after the tests, suggest that full encasement is necessary to maximize the performance of GESCs under cyclic loading.     

Responses of single and group piles within MSE walls under static and cyclic lateral loads

To investigate the behavior of piles and the performance of the mechanically stabilized earth (MSE) walls under static and cyclic lateral loading, six reduced-scale model tests of single and group piles within the MSE walls were conducted inside a test box. In the single pile tests, a hollow aluminum tube as a pile was placed at a distance of 2D or 4D (D is pile diameter) behind the wall facing, while in the group pile tests, the piles were only placed at the distance of 2D with a spacing of 3.3D between the piles. The piles were subjected to static lateral loading only and cyclic lateral loading followed by static loading until failure. The test results showed that the lateral load capacity of each pile in the group pile test was approximately 60% that of the single pile, while the wall facing displacements and the geogrid strains in the group pile test were larger than those in the single pile test. The lateral pile capacity, the wall facing displacement, the strain in the geogrid, and the lateral earth pressure behind the wall facing in the static and cyclic loading tests were evaluated at the pile head displacement equal to 20%D.     

基于沉降控制的竖向下压循环荷载作用下的桩基设计方法

低幅值、大周次竖向循环荷载作用下桩基发生累积沉降及承载力弱化,影响到风机塔架、高速铁路路基等沉降敏感结构物的服役性能。首先总结了竖向循环荷载作用下桩基设计要求及若干设计关键问题。基于模型试验结果,分析了竖向循环荷载作用下的桩基承载力和累积沉降演化规律;定义了3种桩顶累积沉降发展模式及对应的循环荷载比和静荷载比组合,建立了桩基循环累积沉降分析图,提出了组合循环荷载作用下的桩基循环加载性状简化判定公式。提出了针对竖向循环荷载作用下桩基的设计方法及设计流程,并给出了设计实例。     

膨胀灌注桩竖向承载力试验研究

本文结合膨胀灌注桩静载试验,探讨膨胀灌注桩增载机理,给出便于工程设计的增载计算公式,计算结果与实测吻合较好。     

海洋高桩基础水平单调及循环加载现场试验

开展了海洋软黏土中 2 根大直径高桩基础的现场水平单调和循环加载试验,实测获得了桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力和孔隙水压力,揭示了水平单调和循环荷载作用下桩土相互作用规律及桩基水平位移和桩身弯矩发展规律。利用实测桩身水平位移推算了桩周土反力,在此基础上提出了相应的双曲线型 p – y 曲线,通过引进 Poulos 循环弱化模型建立了水平循环荷载作用下的桩基双曲线型 p – y 曲线分析模型,水平单调及循环荷载作用下桩顶荷载 – 位移关系、桩身变形和桩身弯矩及桩侧土压力等计算结果与实测值均吻合良好。通过现场试验发现规范 p – y 曲线法计算结果偏保守的主要原因是所采用的 p – y 曲线的刚度偏小;不同时段的循环荷载对桩基循环累积变形有叠加效应。 建议设计中应考虑桩基全寿命服役期内所承受的所有循环荷载的影响, 对于重要工程应开展相应的现场水平加载试验,实测桩身水平位移或桩身弯矩,进而利用所推算的桩周土反力来分析桩基受力变形及承载力。     

软土地基倾斜桩竖向承载力研究

在软土地区,由于土方开挖引起桩侧土压力不平衡,或由于基坑支护结构变形过大,常导致已施工的工程桩产生侧移,成为斜桩。结合珠海市某工程项目,利用有限元程序模拟土方开挖过程桩侧不平衡土压力对桩受力和变形的影响,分析桩侧移、倾斜的原因。结果表明,施工阶段桩的侧移与桩侧土层性状及分布厚度密切相关,也与桩的直径、配筋相关。在此基础上,对斜桩在使用阶段的受力及变形情况进行分析,指出桩顶水平约束对桩的受力性状以及竖向承载力会产生很大的影响。选取典型斜桩进行现场静荷载试验,与有限元模拟分析的结果进行对比验证,试验结果与有限元模拟分析的结果基本吻合。     

Research on vertical bearing capacity of inclined pile in soft soil foundation

在软土地区，由于土方开挖引起桩侧土压力不平衡，或由于基坑支护结构变形过大，常导致已施工的工程桩产生侧移，成为斜桩。结合珠海市某工程项目，利用有限元程序模拟土方开挖过程桩侧不平衡土压力对桩受力和变形的影响，分析桩侧移、倾斜的原因。结果表明，施工阶段桩的侧移与桩侧土层性状及分布厚度密切相关，也与桩的直径、配筋相关。在此基础上，对斜桩在使用阶段的受力及变形情况进行分析，指出桩顶水平约束对桩的受力性状以及竖向承载力会产生很大的影响。选取典型斜桩进行现场静荷载试验，与有限元模拟分析的结果进行对比验证，试验结果与有限元模拟分析的结果基本吻合。     

大灌注桩的原型试验与承载力及嵌岩深度的判定

通过对嵌岩灌注桩的原型测试,将理论计算成果与实测数据进行对比,分析了该桩的受力机理,并推断出该桩在竖向荷载时的合理承载力与嵌岩深度。