Interface creep behavior of grouted anchors in clayey soils: effect of soil moisture condition

This study aims at investigating the influence of moisture conditions on interface shear behavior of element-grouted anchor specimens embedded in clayey soils. The tests involved comparatively short embedment lengths and a device that was specially designed to facilitate moisture conditioning. Rapidly loaded pullout tests as well as pullout tests under sustained (creep) loading were conducted to characterize both the short-term and long-term ultimate shear strength of anchor–soil interfaces. Both values of the interface shear strength were found to decrease exponentially with increasing moisture content values, although their ratio was found to show a linearly decreasing trend with increasing moisture content. The interface shear creep response under pullout conditions was characterized by a rheological hybrid model that could be calibrated using experimental measurements obtained under increasing stress levels. The accuracy of the hybrid model was examined by evaluating the stress-dependent prediction model as well as its governing parameters. This investigation uncovers the coupled impact of soil moisture condition and external stress state on the time-dependent performance of grouted anchors embedded in clayey soils by correlating the interface shear strength with soil moisture content and associating the creep model with stress levels applied to the grout–soil interface.

     

Elastoplastic constitutive modelling of soil–structure interfaces under monotonic and cyclic loading

This paper is dedicated to the non-linear numerical modelling of the soil–structure interface. Thus, in a first part, after the presentation of the constitutive model, the soil–structure interface interaction is treated in terms of direct shear test simulations. A strategy for the interface model parameters’ identification is also presented. This strategy is linked to the similitude of soil–structure interface behavior and the soil behavior, regarding the interface surface roughness. In a second part, the performance of the numerical simulations are verified numerically against published results for soil–structure experimental shear tests. Finally, as an application, interface stress paths are studied in axially loaded pile–soil systems and load transfer mechanisms are identified.     

填埋场衬垫系统界面本构模型研究

典型垃圾填埋场衬垫系统由土/土工膜、土工膜/土工织物等多个界面组成,这些界面抗剪强度低,成为潜在的滑裂面,容易使填埋场发生沿界面的失稳破坏。本文根据大量土/土工膜界面直剪试验的研究,针对该界面的变形机理和应变软化特性,在前人研究的基础上确定了接触面的厚度,定义了接触面的切向应变和法向应变,将界面的变形分为3个阶段：弹性阶段,塑性阶段和残余阶段。同时提出了适合于土与土工膜界面的弹塑性本构模型。该本构模型参数较少,物理意义明确,运用简单方便。最后,结合Filz接触面直剪试验和钱学德单剪试验的结果,将通过该模型得到的拟合结果与之进行了对比验证分析。研究结果表明：本文提出的土与土工膜界面弹塑性本构模型是合理可行的,且建模方便,适用于工程实践。     

Cement-Stabilization of Sabkha Soils from Al-Auzayba,Sultanate of Oman

Sabkha soils are salt-bearing formations that are formed in arid regions. In their in situ states the sabkha soils have high compressibility and low shear strength. These soils are also heterogeneous and their properties depend on the type and amount of salt present. Thus, these soils are not suitable for support of infrastructures without the risk of high settlement and/or bearing capacity failure. This paper investigates the possibility of using cement to improve the shear strength of sabkha soils for possible use as a foundation-bearing soil. The sabkha soil used in this study is a sandy sabkha obtained from the coastal plains at Al-Auzayba, Sultanate of Oman. Cement was added in percentages of 2.5, 5, 7.5 and 10%, by dry weight of soil. The soil-stabilizer mixers were allowed to cure for 7, 14 and 28 days. Laboratory tests such as compaction, unconfined compression, consolidated undrained triaxial and durability tests were performed to measure the engineering characteristics of the stabilized material. The results showed substantial improvements in the shear strength of the sabkha–cement mixtures and the mixtures are also durable with small weight loss after 12 wetting/drying cycles. Thus, cement can be used to improve the shear strength of sabkha soils. Furthermore, the effective stress path and the tress-strain relation of the sabkha–cement mixtures follow trends similar to those of cemented calcareous soils.     

土工膜与土界面剪切特性细观研究

填埋场衬垫系统中,土与土工膜界面剪切强度较低,易造成失稳破坏。目前国内外学者主要采用室内试验对土与土工膜界面的宏观剪切特性进行研究,而对界面剪切特性的细观研究较少。为了从细观角度研究土与土工膜界面的剪切特性,本文采用EsyS-particle程序对土工膜与土界面直剪试验进行了离散元数值模拟分析。采用摩擦接触模型模拟砂土;采用黏结模型颗粒模拟土工膜,通过紧密排列土工膜颗粒以模拟土工膜的光滑表面。通过室内拟合试验,选取和校准材料的细观参数。分析结果表明,离散元模型能较好的模拟界面应力-应变关系;剪切带的厚度约为两倍平均土颗粒直径;剪切带中的土颗粒发生较大位移,孔隙比增大,而剪切带之外的土颗粒位移和孔隙比变化较小;随着剪切位移的增加,颗粒间接触力逐渐向左端集中,力链方向由垂直逐渐倾斜。     

Assessment of soil–pile–structure interaction influencing seismic response of mid-rise buildings sitting on floating pile foundations

The role of the seismic soil–pile–structure interaction (SSPSI) is usually considered beneficial to the structural system under seismic loading since it lengthens the lateral fundamental period and leads to higher damping of the system in comparison with the fixed-base assumption. Lessons learned from recent earthquakes show that fixed-base assumption could be misleading, and neglecting the influence of SSPSI could lead to unsafe design particularly for structures founded on soft soils. In this study, in order to better understand the SSPSI phenomena, a series of shaking table tests have been conducted for three different cases, namely: (i) fixed-base structure representing the situation excluding the soil–structure interaction; (ii) structure supported by shallow foundation on soft soil; and (iii) structure supported by floating (frictional) pile foundation in soft soil. A laminar soil container has been designed and constructed to simulate the free field soil response by minimising boundary effects during shaking table tests. In addition, a fully nonlinear three dimensional numerical model employing FLAC3D has been adopted to perform time-history analysis on the mentioned three cases. The numerical model adopts hysteretic damping algorithm representing the variation of the shear modulus and damping ratio of the soil with the cyclic shear strain capturing the energy absorbing characteristics of the soil. Results are presented in terms of the structural response parameters most significant for the damage such as foundation rocking, base shear, floor deformation, and inter-storey drifts. Comparison of the numerical predictions and the experimental data shows a good agreement confirming the reliability of the numerical model. Both experimental and numerical results indicate that soil–structure interaction amplifies the lateral deflections and inter-storey drifts of the structures supported by floating pile foundations in comparison to the fixed base structures. However, the floating pile foundations contribute to the reduction in the lateral displacements in comparison to the shallow foundation case, due to the reduced rocking components.     

多层软土地基中单桩沉降与内力位移分析

根据深厚多层软土地区地基土的物理力学性质,考虑桩侧土低荷载水平下的初始极限剪应力和高荷载水平下的应力软化特性以及桩端土承载力分段发挥特性,采用一种新的桩侧和桩端模型模拟单桩荷载传递机制。基于上述模型,利用递推迭代方法可计算单桩桩顶沉降、桩身轴力以及桩侧摩阻力。采取土工参数易于获取且适用于软土地区的经验p-y曲线描述桩-土界面力和位移的非线性关系。基于欧拉-伯努利梁和中心差分理论,考虑桩尖边界条件,采用数值计算方法对桩沿长度方向的转角、剪力、弯矩进行计算,以获取在特定荷载下这些变量的变化特征。最后,结合工程案例,对上述方法进行了验证,其计算简洁且与实际测试结果吻合良好。     

Constitutive modelling of a reinforced soil using hierarchical model

Drained triaxial tests are conducted on natural and reinforced sand under various stress paths. Direct shear tests and pull-out tests are conducted on soil–reinforcement interface and on reinforcement, respectively. The effects of two types of reinforcement, viz, woven and non-woven geotextile and number of layers of reinforcement are investigated. Hierarchical single surface model is used to depict the behaviour of natural and reinforced soil by treating the soil as a single composite material and by considering soil, reinforcement and interface as independent elements. It is shown that the material parameters are very much affected by the type and the number of layers of reinforcement. The hierarchical model provides satisfactory prediction for both natural and reinforced soil. Copyright © 1999 John Wiley & Sons, Ltd.     

注浆桩土接触面试验研究及后注浆抗拔桩承载特性数值分析

桩侧后注浆抗拔桩是一种新型的抗拔桩基础。借助于试验和数值手段对这种抗拔桩的承载变形特性开展了深入研究。首先进行了接触面的大型剪切试验,以揭示桩土接触面在注浆与未注浆前后其力学特性的差异性,并基于大型接触面剪切试验成果,建立了桩土接触面的修正库仑摩擦模型。将接触面模型应用到有限元数值分析,分别模拟了传统等截面抗拔桩与后注浆抗拔桩的承载变形特性。研究表明,相比传统的抗拔桩,桩侧后注浆抗拔桩的侧摩阻力得到了显著的改善,特别是深处的桩侧极限摩阻力得到了更充分的发挥,从而桩侧注浆抗拔桩的承载能力有了较大程度的提高。     

A frozen soil creep model with strength attenuation

A frozen soil creep model with shear strength attenuation was proposed based on a soft soil creep model. K ₀ compression and triaxial shear tests were conducted to obtain the model parameters. With triaxial creep tests, the performance of soft soil creep model was verified at first. It was shown that different creep stages can only be described separately, which was due to a constant failure line used in the model. After incorporating strength attenuation into the original model, relative position between stress point and failure line is time dependent, and the progressive development of creep strain from primary to tertiary stage can be captured reasonably.     

上海地区基坑开挖数值分析中土体 HS-Small模型参数的研究

数值分析方法已成为分析敏感环境下基坑开挖对环境影响的最重要手段,其关键是选择合适的土体本构模型和计算参数。HS-Small模型可以同时考虑土体剪切硬化和压缩硬化,而且还可以考虑土体剪切模量在微小应变范围内随应变衰减的行为,因此,HS-Small模型在基坑开挖数值分析中具有很好的适用性。但HS-Small模型的参数较多,不仅包含了HS模型的参数,还包含了小应变参数,因此,获取完整的HS-Small模型参数存在一定困难。通过上海地区5个基坑开挖数值分析的实例对基于室内试验得出的上海典型土层HS模型参数进行了检验。通过参数的敏感性分析,对HS-Small模型的部分参数的确定方法进行了修正。在此基础上,采用反分析法确定了敏感性最强的小应变参数,从而初步完整地获取了上海典型土层土体HS-Small模型参数。采用文中确定的上海典型土层HS-Small模型参数对该地区若干典型基坑工程实例进行计算,结果表明,计算值与实测值吻合得较好,从而验证了确定的上海典型土层HS-Small模型参数的适用性。     

土石混合体砾-土接触面参数的敏感性分析

土石混合体作为一种特殊的地质体在自然界中广泛分布,由强度较高的岩石块体、细土颗粒和充填其间的孔隙组成。随着各类大型工程建设的兴起和发展,与土石混合体相关的地质灾害严重影响和制约了工程的建设和人类的安全,因而对土石混合体的研究至关重要。本文从细观角度出发着重研究土石混合体中砾石和土体之间的接触性质,利用FLAC3D接触面单元模拟了土石混合体中的砾-土界面,首先构建了包含接触面的土石混合体三维数值模型,其次选取了接触面参数的合理变化范围,通过对数值模拟结果的分析,探讨了参数的敏感性,发现刚度,特别是法向刚度对土石混合体应力-应变曲线和砾-土的相对运动影响最大,其次是黏聚力,摩擦角的影响最小。通过与无接触面模型的对比,验证了包含接触面单元土石混合体三维模型的可行性和合理性。接触面单元的引入使精确描述土石混合体的宏观变形特征及砾-土间的相互作用机制成为可能。     

Application of unconstrained optimization and sensitivity analysis to calibration of a soil constitutive model

Large sets of soil experimental data (field and laboratory) are becoming increasingly available for calibration of soil constitutive models. A challenging task is to calibrate a potentially large number of model parameters to satisfactorily match many data sets simultaneously. This calibration effort can be facilitated by optimization techniques. The current study aims to explore systematic approaches for exercising optimization and sensitivity analysis in the area of soil constitutive modelling. Analytical, semi‐analytical and numerical optimization techniques are employed to calibrate a multi‐surface‐plasticity sand model. Calibration is based on results from a number of drained triaxial sample tests and a dynamic centrifuge liquefaction test. The analytical and semi‐analytical approaches and associated sensitivity analysis are applied to calibrate the model non‐linear shear stress–strain response. Thereafter, model parameters controlling shear–volume coupling effects (dilatancy) are calibrated using a solid–fluid fully coupled finite element program in conjunction with an advanced numerical optimization code. A related sensitivity study reveals the challenges often encountered in optimizing highly non‐linear functions. Overall, this study demonstrates applicability and limitations of optimization techniques for constitutive model calibration. Copyright © 2003 John Wiley & Sons, Ltd.     

基于冻融作用的氯盐渍土−钢块界面力学模型

在中国西北特殊气候环境下冻融作用是一种风化过程,反复改变着土体的微结构和物理性质,强烈影响着土体与结构的相互作用。针对冻融作用对盐渍土?结构接触面力学特性的影响,进行不固结不排水直剪试验,开展了冻融次数、含盐量、基质吸力等因素对非饱和氯盐渍土?钢块接触面力学性能影响的相关研究。试验结果表明,不含盐时接触面力学参数（黏聚力与内摩擦角）随冻融次数增加均先增大后减小,含盐时接触面黏聚力随冻融次数增加呈下降趋势,内摩擦角略有所增大;冻融前后接触面力学参数随着含盐量的增大先减小后增大,均存在含盐量阈值;未冻融时接触面力学参数含盐量阈值约为8%,随着冻融次数增加,该阈值有所变化;接触面基质吸力随冻融次数增加大体呈减小趋势并最终趋于稳定,随含盐量增加先减小后增大,基质吸力含盐量阈值约为10%;接触面剪应力?剪切位移分为线弹性变形阶段和强化阶段,竖向荷载较小时表现为弱硬化,未出现明显的应变软化现象。对接触面剪应力?剪切位移适用性模型进行评价发现龚帕兹模型能够与试验结果很好地吻合,据其建立了冻融作用的氯盐渍土?钢块界面力学模型,基于试验数据验证了其可靠性。     

地震荷载下长短桩复合地基的群桩效应系数分析

长短桩复合地基中,地震发生时,短桩在降低长桩所受剪力和弯矩影响中起到重要作用。本研究对弹性地基中的钢管桩基础实行了静态有限元分析,以及动态离心模型试验,通过桩径、桩长、桩间距等参数进行剪切波速,长桩剪力弯矩的对比分析,得到一种新的水平向群桩效应系数计算方法,并通过对模型建筑物的模拟,验证了该方法的有效性与实用性。结果表明,静态有限元数值模拟分析和离心模型试验结果有较好拟合,不同震级作用下通过该方法均能给出较为合理的系数。本文结果可为长短桩复合桩基的抗震性能提供重要参考,实际工程中,通过调整桩径,桩长,短桩数量等参数,可以提出更为合理的设计施工方案。     

注浆成型螺纹桩抗拔承载特性的数值分析

注浆成型螺纹桩为一种利用施工工艺创新,结合钻孔灌注和二次注浆技术的新型螺纹抗拔桩型,目前已在软土地区开展应用。为了对其受力承载特性深入研究,使该桩型得到广泛推广,通过数值分析方法对其抗拔性能和承载机制进行了三维有限元数值模拟。首先,通过数值模拟桩-土界面室内大型直剪试验得到了有限元分析需要的桩-土接触面参数,而后将得到的参数带入注浆成型螺纹桩抗拔三维有限元数值模型,通过计算得到了不同距径比S/D（即螺距与桩径的比值）螺纹桩的抗拔荷载-位移曲线和轴力分布,并观察了抗拔过程中桩周土体塑性变形的发展。数值分析表明,螺纹桩与桩周土体的机械咬合作用增大了桩侧摩阻力,从而使桩体极限抗拔承载力较等截面圆桩提高约2～5倍;同时,其承载能力与桩体的S/D有关,当S/D取最优时,荷载-位移曲线的初始切向刚度最大,极限承载力最高,桩周土体形成的连续拱形破坏区域最大。     

横-竖立体加筋地基中附加应力的分析计算

建立了横-竖立体加筋（H-V筋）地基的有限元模型,通过分析地基中的竖向应力分布、水平向位移分布以及筋-土界面相互作用,发现横-竖立体加筋地基中的竖向应力在筋材下方出现扩散和重分布,并逐渐向土体下部传递,使得土体中整体的应力分布更加均匀;同时,横-竖筋材中的竖筋类似于一个侧壁,其提供的垂直侧向力约束了介于竖筋间的土体,限制了土体的侧向水平位移,使得地基中筋材上部土体的侧向水平位移变小。基于有限元模拟对横-竖立体加筋地基加固机制的认识,将横-竖立体筋视为作用在地基上的一维弹性地基梁,通过弹性地基梁理论,根据弗拉曼解推导求解了横-竖立体加筋地基中任意一点竖向附加应力的计算表达式。将模型计算结果与有限元模拟所得结果进行对比发现两者吻合良好。     

基于三维数字图像相关技术钙质砂颗粒运动行为试验研究

桩基附近土颗粒的运动行为与宏观力学表现密切相关,对揭示界面剪切机制具有重要意义。利用自主研制的大型直剪仪,结合三维数字图像相关技术（3D-DIC）全场位移测量分析系统,开展了钢−钙质砂界面循环剪切试验,研究了界面附近砂颗粒的运动行为演化规律。结果表明：界面峰值剪切应力和发挥的界面摩擦角随循环次数的增大而增加,体变特性以剪缩为主;钙质砂颗粒左右移动的幅度与距界面的垂直距离成反比,钙质砂颗粒的位置随循环次数的增大逐渐向正剪切方向移动,试验结束时上剪切盒左侧区域的钙质砂颗粒向正剪切方向移动的距离最大;钙质砂颗粒在单个循环内出现有规律的上下移动,向下移动的幅值更大,位于上层的钙质砂颗粒向下移动的位移值大于下层;钙质砂颗粒的运动速度在沿单方向剪切时,呈现出慢−快−慢的变化规律,位于上剪切盒右侧区域的钙质砂的体变特性较左侧区域更显著;网格位移值和缺失数量随循环次数的增大而增加,在试验后期趋于稳定,试验结束时的破碎带厚度为 6.21 mm。     

基于冻融作用的非饱和黄土-混凝土界面力学模型

在季节冻土区,周期性的冻结与融化作用持续改变着浅层非饱和土体的微结构和物理力学性质,从而直接影响着土与结构物之间的相互作用。土体与结构物接触面的应力-应变关系及其强度特征是确定季节冻土区基础工程承载力、安全性和分析结构与土体相互作用的基础和关键。为了更好地服务于实际工程,通过对不同含水率、不同冻融循环次数的非饱和黄土-混凝土试样进行直剪试验,并同时采用滤纸法测试相应黄土的基质吸力,获取了不同试验条件下的应力-应变关系曲线以及接触面强度参数、基质吸力随冻融循环次数的变化规律。同时,基于试验数据,建立了非饱和黄土与混凝土界面剪应力-位移-冻融循环次数的本构模型,该模型对黄土-混凝土界面经过多次冻融循环后不同压力下的剪应力-位移的关系曲线起到了很好的描述作用,为解决实际工程中季节冻土区基础承载力的数值计算提供了参考。     

Performance of a compaction-grouted soil nail in laboratory tests

This study proposed a new soil nail known as the compaction-grouted soil nail, and a physical model was established to investigate its pull-out behaviour with different grouting pressures. The study on scale effect of the physical model was performed subsequently via numerical modelling. Additionally, interface shear tests were performed using the same boundary conditions as the physical model test. The strength parameters obtained were used to estimate the pull-out resistance of a conventional soil nail. The merits of these two soil nail types were compared based on their pull-out resistances. The physical model test results showed that the pull-out resistance of the compaction-grouted soil nail increases with increasing grouting pressure. In addition, the pull-out resistance exhibits hardening behaviour without a yield point, indicating that the compaction-grouted soil nail enables soils to remain stable against a relatively large deformation before ultimate failure. Furthermore, a higher grouting pressure results in a higher rate of increase for pull-out resistance versus pull-out displacement, which improves the performance of the compaction-grouted soil nail in the stabilization of large deformation problems. A comparison of the two types of soil nails suggests that the new compaction-grouted soil nail is more sensitive to grouting pressure than the conventional soil nail in terms of pull-out resistance improvement. In other words, the performance (pull-out resistance) of the compaction-grouted soil nail can be markedly improved by increasing the grouting pressure without inducing any accidental or undesired cracking or soil displacement.