单层立体加筋砂土性状的三轴试验研究

针对加筋土中传统的筋材布置特点,提出了立体加筋土的概念,并设计了采用轴对称布置的单层立体加筋砂土的试验方案,进行了48组镀锌铁皮和橡胶板两种筋材的单层立体加筋砂土的室内三轴试验,探讨了不同立体加筋方式、不同围压作用下应力–应变及强度变化规律。通过试验结果对比,分析了立体加筋砂土同传统水平加筋砂土之间应力–应变关系和强度指标的差异规律,竖向筋高度对立体加筋砂土的强度影响,单侧和双侧布置竖向筋材等两种布筋方式对立体加筋砂土强度的影响,以及不同变形模量的加筋材料对立体加筋土强度的影响。试验结果表明:立体加筋砂土的强度随竖向筋的高度增加而增大;立体加筋不仅能提高砂土的粘聚力,同时也能增加砂土的内摩擦角,尤其是双侧立体加筋砂土;在竖向筋总高度相同时,双侧立体加筋形式比单侧立体加筋能更有效提高砂土的强度。     

条形荷载下 H-V加筋砂土地基模型试验研究

提出了一种新的H-V立体加筋(horizontal-vertical)模式,这是一种在传统水平条带式筋条上布置竖向或空间筋材形成的组合立体形式,其显著特点是所加竖向筋材能有效地限制筋材间土体的侧向位移,在竖筋侧面产生侧阻力,而且在竖筋间将形成"土体加强区",从而有效改变加筋土的受力状况,提高土体的强度和稳定性.本文分别对水平加筋和H-V加筋两种加筋方式进行了多组的加筋地基模型试验,并将这两种加筋方式对承载力和沉降的影响进行了对比分析.试验结果表明,H-V加筋方式能明显提高地基承载力和减小地基沉降,而且其加筋效果随着竖筋高度和水平方向筋材长度的增加不断提高.在模型试验的基础上,初步分析了H-V加筋方式的作用机理.     

低超载下条带式带齿加筋界面特性

在提出立体加筋的基础上,对条带式带齿加筋砂土进行了低超载下大量的拉拔试验来研究筋土的界面特性,分析了带齿筋的条带式加筋对拉拔性能的影响,探讨了不同上层覆压下筋条的拉拔力与水平位移的关系以及似摩擦系数f*的变化规律。在试验成果分析的基础上,分析了条带式带齿加筋与砂土的相互作用机理,建立了条带式带齿加筋砂土的拉拔力模型。并将试验结果与理论值比较,二者基本吻合。     

轴对称条件下水平-竖向加筋黏性土的强度分析

水平-竖向(H-V)加筋是在传统水平加筋基础上布置不同形状的竖筋所形成的空间加筋体系之一。针对包含(非满布)环状竖筋的典型H-V加筋方式,结合已进行的H-V加筋黏性土三轴试验,分析H-V加筋黏性土的工作机理,即H-V加筋的作用不仅体现在水平加筋的摩擦效应,而且还表现在竖向加筋的侧向约束作用,在H-V加筋中被竖筋包围的土体会形成挤密区,当荷载达到极限值时,竖筋将发生倾覆。分析中将竖筋的侧阻作用简化为侧向土压力,按极限平衡状态计算竖筋两侧的主动土压力与被动土压力,并将其转换为筋条上的水平径向拉力。在水平加筋强度理论基础上,推导H-V加筋黏性土摩擦型破坏条件下的强度公式,并与试验值进行对比验证,结果表明二者较为接近。     

H-V加筋土性状的颗粒流细观模拟

正交水平-竖向(H-V)加筋是一种新型的土工加筋技术,在H-V加筋土中除了有传统的水平筋条外,还在水平筋条上设置了竖向齿筋。结合已完成的加筋土三轴试验,采用基于离散元理论的颗粒流软件(PFC2D)对试验进行了仿真模拟,较好地拟合了H-V加筋砂土三轴试验的应力–应变曲线,并通过观测颗粒的受力情况分析了H-V加筋砂土的受力机理。通过引入颗粒流理论,对H-V加筋砂土试样的剪切带形成进行了细观数值模拟,揭示了H-V加筋土中剪切带产生、扩展的渐进破坏规律。     

刚性条带式带齿加筋土的极限拉拔力模型

"立体加筋"体系的核心就是在传统水平筋条的基础上布置不同形状的竖向筋条或三维形式的空间加筋。作为一种典型的立体加筋方式,刚性筋材的条带式带齿加筋土经三轴压缩试验、拉拔试验以及挡墙和地基模型试验证明,可以显著地提高加筋土体的强度和地基极限承载力,增强土筋的综合摩擦特性,限制土体变形。采用染色砂法,对带齿筋材的平面应变拉拔试验过程进行了观察。通过分析齿筋前方染色砂土标记线的运动情况和水平标记线的变化情况,发现在拉拔过程中,齿筋前方土体会首先被挤密加强,成一个刚性的楔体,然后周围其他土体会产生绕刚性楔体与齿筋的流动。根据齿筋的宽高比,假定了刚性楔体的形状,从而确定了绕流阻力的分布情况。借鉴沈珠江院士对于抗滑桩阻力的分析方法,推导了刚性条带式带齿加筋土的拉拔阻力模型。在应变控制式拉拔试验机上,进行了法向应力分别为25,50和75kPa的条带式带齿加筋土的拉拔试验,将试验结果与模型计算值进行比较,二者比较接近。     

布袋注浆桩加筋材料作用试验分析

通过布袋注浆桩现场工程低应变反射波法试验、钻探取芯无侧限抗压强度试验、单桩竖向静载试验、单桩水平静载试验结果,分析了布袋注浆桩加筋材料对桩体成桩质量、桩体强度、承载力的影响.结果表明,加筋材料对提高成桩质量有积极影响,且引入加筋注浆管简化了施工工艺,加筋材料对桩体的竖向抗压承载力影响较小,但对提高桩体的水平强度作用明显.     

单向网格状带齿加筋砂垫层加固道路软基的变形特性研究

基于立体加筋(Horizontal-Vertical,简称H-V)概念,设计单向网格状带齿加筋,即由平面上的网格状加筋和竖向齿筋组成的三维立体加筋形式。针对这种新型加筋形式,进行四组单向网格状带齿加筋砂垫层加固道路软基的室内模型试验,主要研究软土地基的承载力、沉降及路堤边坡的侧向位移等情况,并在模型试验的基础上,初步分析单向网格状带齿加筋砂垫层的作用机理。试验结果表明:较水平加筋方式,单向网格状带齿加筋砂垫层能显著提高软基承载力、减少软基沉降及不均匀沉降,并能减少路堤的侧向位移。     

土工格栅加筋砂土复合体极限承载能力分析

土工合成材料加筋土桥台可以有效减小桥梁与路基之间的差异沉降,避免“桥头跳车”现象的发生。为了计算土工合成材料加筋土复合体在设计中承受荷载的安全冗余度,对其极限承载能力进行了分析。首先讨论了评价加筋土复合体极限承载能力的计算公式,并提出了该公式是否适用于评价加筋细颗粒土复合体承载性能的问题。然后在平面应变的条件下,进行了5组土工格栅加筋砂土模型试验和1组无加筋模型试验,考虑了加筋间距和筋材强度对加筋砂土复合体极限承载能力的影响,并将试验结果与公式的计算结果进行对比,发现该公式低估了加筋砂土的承载能力。基于莫尔库仑破坏准则,并假定加筋土的破坏面符合朗肯破坏面,提出了预测加筋砂土极限承载能力的分析模型,并将模型的计算值与试验值进行对比,发现两者基本吻合。     

基于Goodman界面单元的加筋砂土变形破坏有限元数值分析

利用非线性弹塑性有限元,对无加筋砂土和采用光滑铜板的加筋砂土平面应变压缩试验结果进行从小变形到破坏的全过程数值解析.考虑到光滑铜板与砂土之间存在滑移,在有限元分析中采用Goodman连接单元作为界面单元.在有限元分析中还需考虑影响砂土变形强度的诸多特性,包括:(1) 围压大小的相关性;(2) 强度的各向异性;(3) 峰值前应变硬化及峰值后应变软化的非线性特性;(4) 剪胀性;(5) 应变局部化及剪切带形成特性等.结果表明,利用建议的有限元方法得到的加筋砂土平面应变压缩的应变-应力关系与室内试验结果吻合较好,最大应力比和峰值前刚度非常接近试验结果;提出的有限元分析方法可以较准确地捕捉到砂土的剪切带发生、发展状况,合理地模拟加筋砂土的渐进性破坏特性以及加筋材和砂土之间的相互作用机制.     

Analytical modeling of geogrid reinforced soil foundation

This paper aims at developing analytical solutions for estimating the ultimate bearing capacity of geogrid reinforced soil foundations (RSF) for both sand and silty clay soils. Failure mechanisms for reinforced soil foundations are proposed based on the literature review and the results of experimental study on model footing tests conducted by the authors. New bearing capacity formulas that incorporate the contribution of reinforcements to the increase in bearing capacity are then developed for both reinforced sand and silty clay soil foundations based on the proposed failure mechanisms. The predicted bearing capacity values are compared with the results of laboratory model tests on reinforced sand and silty clay soil. The proposed analytical solutions were also verified by the results of large-scale model tests conducted by the authors for reinforced silty clay and the data reported in the literature. The predicted bearing capacity values from analytical solutions are in good agreement with the test results.     

加筋软岩粗粒土路堤填料大型三轴试验研究

 为研究加筋粗粒土填料的强度变形特性及加筋效果,进行加筋强风化软岩粗粒土固结不排水和固结排水大三轴试验。试验表明：加筋填料的应力–应变关系表现为应变硬化型;轴向应变较小(ea＜1%)时,加筋填料效果不明显,随着轴向应变的逐渐增大加筋效果逐渐发挥。加筋填料的孔隙水压力均高于素填料,随着加筋层数的增加均有不同程度的提高。加筋效果系数均＞1.0,一层加筋填料加筋效果系数为1.09～1.21,二层加筋填料加筋效果系数为1.30～1.71,三层加筋填料加筋效果系数为1.31～1.72。加筋前后填料的内摩擦角j基本不变,填料的黏聚力增大。加筋填料的本构关系可以用Duncan-Chang模型来描述,依据试验结果求得模型参数。     

聚丙烯纤维加筋砂土的剪胀特性

随机分布的聚丙烯纤维在土体中彼此交错形成网状,可显著提高砂土的抗剪强度,减少其峰值强度折减。描述土体剪胀规律的剪胀方程是建立纤维加筋土的本构模型的核心。以聚丙烯纤维加筋南京细砂为主要研究对象,通过不同围压下的常规三轴压缩试验,研究了不同纤维质量百分比含量对南京细砂强度、变形以及剪胀特性的影响。对比聚丙烯纤维加筋砂土与纯砂土的应力应变关系和强度特性,分解出聚丙烯纤维对砂土的内在约束应力。将外力克服剪胀阻力需要做的额外功分解为由土颗粒运动和纤维变形两部分所消耗的能量,利用纤维对土的内在约束应力表示与加筋纤维有关的耗散能,基于最小比能原理建立了聚丙烯纤维加筋砂土的剪胀方程,揭示了聚丙烯纤维加筋砂土的特殊剪胀特性。经过对比发现常规三轴压缩试验结果与模型结果具有良好的一致性。     

A Laboratory Investigation on the Behaviour of Reinforced Sand Samples Under Plane Strain and Triaxial Conditions

An experimental study on the behaviour of reconstituted samples of sand reinforced with geotextile layers is presented using standard triaxial and plane strain tests. After describing the main features of the plane strain device and of the data acquisition system, details are provided on the procedure developed for preparing plane strain samples containing inclined reinforcements with respect to the principal stress directions. Comments are also made on the influence of the preparation procedure on the overall behaviour observed during loading. Finally, the results of the experimental investigation are described with reference to standard triaxial tests with horizontal reinforcements, and to plane strain tests with horizontal and inclined reinforcements. The study complements data already available in the literature, for the development of constitutive laws applicable to the numerical analysis of reinforced earth structures.     

Laboratory tests on geosynthetic-encapsulated sand columns

Granular columns have been introduced into engineering practice to improve the bearing capacity and reduce settlement of sand column in a weak or soft soil. The improvement can be enhanced by encapsulating the column with tensile resistant material. The improvement depends on the confinement offered by the surrounding soil, the reinforcing material and the granular column material. In this study, the extent of improvement for a sand column subjected to constant confining pressures is studied through laboratory experiments. A series of triaxial compression tests were carried out in laboratory to investigate the response of sand columns encapsulated by geotextiles. The tests consisted of triaxial compression tests on sand columns with two different densities and encapsulated by sleeves fabricated from three different geotextiles. The increase in deviatoric stress, the reductions in volumetric and radial strains, and the increase in confining pressure generated by the encapsulating reinforcement were measured and analyzed. The mobilized pseudo-cohesion and friction angle corresponding to various axial strains are analyzed to interpret the reinforcing effect. The experimental results are compared with data obtained from analytical method reported in the literature.     

Bearing capacity of horizontally layered geosynthetic reinforced stone columns

In very soft soils, the bearing capacity of stone columns may not improve significantly due to very low confinement of the surrounding soil. Therefore, they may be reinforced with geosynthetics by using vertical encasement or horizontal layers. Very limited studies exist on horizontally reinforced stone columns (HRSCs). In this research, some large body laboratory tests have been performed on horizontally reinforced stone columns with diameters of 60, 80, and 100?mm and groups of stone columns with 60?mm diameter. Results show that the bearing capacity of stone columns increases by using horizontally reinforcing layers. Also, they reduce lateral bulging of stone columns by their frictional and interlocking effects with stone column aggregates. Finally, numerical analyses were carried out to study main affecting parameters on the bearing capacity of HRSCs. Numerical analysis results show that the bearing capacity increases considerably with increasing the number of horizontal layers and decreasing space between layers.