Estimation of residual shear strength ratios of liquefied soil deposits from shear wave velocity

For sites susceptible to liquefaction induced lateral spreading during a probable earthquake, geotechnical engineers often need to know the undrained residual shear strength of the liquefied soil deposit to estimate lateral spreading displacements, and the forces acting on the piles from the liquefied soils in order to perform post liquefaction stability analyses. The most commonly used methods to estimate the undrained residual shear strength （Su~） of liquefied sand deposits are based on the correlations determined from liquefaction induced flow failures with SPT and CPT data. In this study, 44 lateral spread case histories are analyzed and a new relationship based on only lateral spread case histories is recommended, which estimates the residual shear strength ratio of the liquefiable soil layer from normalized shear wave velocity. The new proposed method is also utilized to estimate the residual lateral displacement of an example bridge problem in an area susceptible to lateral spreading in order to provide insight into how the proposed relationship can be used in geotechnical engineering practice.

Seismic stability safety evaluation of gravity dam with shear strength reduction method

A new method of numerical seismic stability safety evaluation for a rock slope is proposed based on the analysis of a gravity dam foundation subjected to earthquake loading. The shear strengths of the weak discontinuities are divided by different shear strength reduction ratios （K） and numerical seismic analysis is carried out after the static analysis is completed. With different K values, the curves of the permanent horizontal displacement of key points of the dam foundation （K-displacement curves） are studied. According to the curve change, the distribution of plastic zones in the foundation, and the slow convergence of the finite element method （FEM）, the seismic stability safety factor is defined as Kwhen the gravity dam is in the limit equilibrium state subjected to earthquake loading. These concepts were applied to the evaluation of seismic stability safety of a gravity dam for a hydropower project. The analysis of the example shows that the proposed method is feasible and is an effective method of seismic stability safety evaluation.

Effects of Polypropylene Fibers on the Shear Strength of Sandy Soil

This paper presents the effect of two types of polypropylene fibers on shear strength parameters of sandy soil. To achieve the goals of this research, a sandy soil was obtained from a depth of 40 cm from the natural ground surface around American University of Sharjah. Two types of polypropylene fibers;one highly flexible with flat profile and the other with relatively high stiffness and crimpled profile were used in this study with four different aspect ratios (L/D) for each type. The initial physical properties of the sand such as specific gravity, angle of internal friction and shear strength were obtained in accordance with American Standard for Testing and Materials (ASTM). The sandy soils were mixed with the two types of fibers at different percentages by dry weight of the sand and four different aspect ratios. The test results of the study showed that the shear strength of the sand increased with increasing the flexible flat profile fibers content. Also it was noticed that by increasing the aspect ratio of the flexible flat profile the angle of internal friction and the shear strength increased. The crimpled profile fiber increased the shear strength of the sand under high normal load and has small or no effect on shear strength of the sand at low aspect ratio under low normal load.

Using Piezocone Dissipation Test to Estimate the Undrained Shear Strength (<i>s_u</i>), <i>N_kt</i>and <i>N_Δu</i>Factors in Cohesive Soils

The current practice of geotechnical engineering commonly uses a combination of theoretical and empirical correlations to estimate the soil undrained shear strength in clays from the piezocone test. In order to complement the use of such correlations, the application of a method to estimate the soil undrained shear strength, using measures of the excess pore pressure in dissipation tests of piezocone is presented. In cohesive soils, excess pore pressure and undrained shear strength are dependent on the same variables (stress state, stress history, soil stiffness), which allows them to be related by the theoretical cavity expansion-critical state framework. This paper mentions the mathematical formulation that supports the theoretical framework used, its relationship with the Nkt and NΔu factors and their estimation in a case studied. The results obtained are consistent within the dispersion found in the international literature and encourage the use of the method in engineering practice.

Shear behavior of sand-expanded polystyrene beads lightweight fills

Through direct shear and triaxial compression tests, effects of expanded polystyrene (EPS) mass ratios in sand-EPS mixtures and stress status on materials' shear behavior were investigated. Hyperbolic curves were used to fit relationship between shear stress and shear displacement. The shear behavior is marginally associated with the EPS ratios and normal/confining stresses. Increases of EPS ratios and decreases of normal/confining stresses result in shear strength decreases. The shapes of Mohr-Coulomb's envelope include linear and piecewise linear types, which are basically determined by the EPS ratio. Such difference is thought related to the embedding or apparent cohesion effect under relatively high EPS ratio conditions. Shear strength parameters can be used for further modeling and design purposes.

Experimental studies on behavior of fully grouted reinforced-concrete masonry shear walls

An experimental study is conducted on fully grouted reinforced masonry shear walls (RMSWs) made from concrete blocks with a new configuration. Ten RMSWs are tested under reversed cyclic lateral load to investigate the influence of different reinforcements and applied axial stress values on their seismic behavior. The results show that flexural strength increases with the applied axial stress, and shear strength dominated by diagonal cracking increases with both the amount of horizontal reinforcement and applied axial stress. Yield displacement, ductility, and energy dissipation capability can be improved substantially by increasing the amount of horizontal reinforcement. The critical parameters for the walls are derived from the experiment: displacement ductility values corresponding to 15% strength degradation of the walls reach up to 2.6 and 4.5 in the shear and flexure failure modes, respectively; stiffness values of flexure- and shear-dominated walls rapidly degrade to 17%–19% and 48%–57% of initial stiffness at 0.50 Dmax (displacement at peak load). The experiment suggests that RMSWs could be assigned a higher damping ratio (～14%) for collapse prevention design and a lower damping value (～7%) for a fully operational limit state or serviceability limit state.

不同冲击气压下煤样动态剪切强度的长径比效应

采用Φ50mm分离式霍普金森压杆(SHPB)试验系统,开展了不同冲击气压下直径75mm,长径比分别为0.20,0.27,0.33,0.40和0.47的5组煤样的动态剪切试验,划分了煤动态剪应力时程曲线的阶段,探讨了冲击气压对煤样动态剪切强度的影响,分析了煤样动态剪切强度和加载率的长径比效应,并建立了长径比效应理论模型。研究结果表明:①煤样动态剪应力时程曲线可分为应力初始上升、应力线性增长、应力缓慢上升和应力下降4个阶段;②煤样动态剪切强度与冲击气压呈正线性相关,但不同长径比下增加幅度存在差异,具体表现为:相同冲击气压增量下,煤样长径比越小,动态剪切强度的增加幅度越大;③煤样动态剪切强度和加载率均与长径比有关,在0.25,0.35 MPa较低冲击气压与0.45,0.55 MPa较高冲击气压下分别呈现出正、负长径比效应,并通过方差分析确定了长径比对其影响最小的冲击气压为0.376MPa;④建立了不同冲击气压下煤样动态剪切强度长径比效应理论模型,通过加载率效应推导出加载率长径比效应理论模型,并验证了模型的合理性和准确性。

大荷载矿石堆场中深厚吹填软土的地基处理及后评价

吹填淤泥形成陆域是资源利用和环境保护方面的优质解决方案,但当前吹填软土形成陆域的工程中,多数为荷载较小的堆场、工业园区道路、绿化区等。以青岛港董家口港区矿石码头工程20万吨级散货泊位后方预留堆场区的地基处理工程为例,对该工程的设计、施工及使用后6年的情况进行详细阐述,从多个角度展示深厚吹填软土的演变情况,详细介绍土体含水量、孔隙比、抗剪强度等指标在不同时期的变化,对比施工期沉降计算和监测数据,分析不同时期抗剪强度指标下矿石堆存的高度。结果表明,在大荷载矿石堆场中采用吹填软土形成陆域是可行的,通过合理的地基处理方式和有序增加堆存荷载,最终可使矿石堆场达到满载堆存。

Seismic Performance of Steel Reinforced Ultra High-strength Concrete Columns

The seismic performance of steel reinforced ultra-high-strength concrete columns(SRSHC) with various shear-span ratios(λ) were studied through a series of experiments.The concrete compressive cube strength value of experimental specimens ranged from 92.9 MPa to 108.1 MPa.The main experimental variables affecting seismic performance of specimens were axial load ratio and stirrup reinforcement ratio.The columns(λ=2.75) subjected to low cyclic reversed lateral loads failed mainly in the flexural-shear mode failure and columns(λ≤2.0) subjected to low cyclic reversed lateral loads failed mainly in the shear mode failure.Shear force-displacement hysteretic curves and skeleton curves were drawn.Coefficient of the specimen displacement ductility was calculated.Experimental results indicate that ductility decreases with axial pressure ratio increasing,and increases with stirrup reinforcement ratio increasing.Limit values of axial pressure ratio and minimum stirrup reinforcement ratio of columns are proposed to satisfy definite ductility requirement.The suggested values provide a reference for engineering application and for the amendment of the current Chinese design code of steel reinforced concrete composite structures.

斜截面抗剪强度上限值和预应力混凝土梁最小配筋率计算理论研究

文章遵循基本力学原理,从结构理论出发,推导修正后的混凝土梁斜截面抗剪强度上限值理论与公式,首次给出考虑斜箍筋特殊效应后的斜截面抗剪强度上限值的理论与方法;抗剪实验梁结果证明所提出的抗剪强度上限值理论与方法的正确性;基于预应力束应力全过程分析,首次推导并提出了预应力混凝土梁双值最小配筋率理论与计算方法,明确了预应力混凝土梁最小配筋率存在两个临界点,合理的预应力混凝土梁配筋率在两个临界点形成的范围之内,计算实例证明预应力混凝土梁双值最小配筋率理论的正确性。

循环荷载下渭北地区黄土动力特性分析

为研究渭北地区的黄土动力学特性分析,通过TYS-20型土动三轴试验机,研究不同围压下黄土的动本构关系以及动骨干曲线,分析黄土的动剪模量比和阻尼比的关系,提出动剪切模量衰减模型和阻尼增长模型;研究不同围压对黄土动强度和残余应变的影响,从理论上解释黄土动力特性变化的原因,从而更好地保证黄土震陷对地下空间影响最小化;最后对比黄土震前震后微结构,并进行机理分析对该地区地下空间发展提供理论意义.研究结果显示:黄土的动本构关系符合双曲线模型;动剪模量比随动剪应变呈负指数衰减关系,而阻尼比随动剪应变呈对数关系增长;动强度和动强度指标受试验条件的影响较大,其中动强度随围压的增大而增大,表明动强度分析能较好地说明黄土的动力特性变化规律;根据电子显微镜对比土体震前震后微结构,并进行机理分析,为该地区地下空间发展提供理论支持.通过研究表明,渭北地区黄土在循环荷载作用下不易产生振动变形,属于轻微震陷区.

超固结黏土不排水剪切的屈服与临界状态

以研究超固结黏土的屈服与临界状态为目的,通过GDS三轴剪切试验仪(GDS triaxial shear testing apparatus)开展了一系列不同超固结状态的黏土不排水剪切试验。为对黏土的应力路径进行分析,选取了能描述超固结状态不同时土的屈服特性的ALPHA(α)模型。分析了黏土的偏应力/孔压-应变曲线变化趋势,并确定了孔压达到峰值时土体为破坏状态。根据应力破坏点拟合出临界状态线,提出了针对不同超固结比的不排水抗剪强度的计算公式。试验结果表明:不排水剪切试验中,前期固结压力相同时,超固结比越小,抗剪强度越高,但是屈服强度并非随超固结比的降低而单调递增变化;超固结比相同时,前期固结压力越大,抗剪强度越高,屈服强度也越高;正常固结和轻超固结土的破坏应变皆在5%左右,而重超固结土则呈现明显的剪胀现象和脆性破坏,破坏应变小于2.5%;正常固结和轻超固结土的应力路径呈“S”形,重超固结土的应力路径呈不断增长的趋势。根据破坏标准,重超固结土和轻超固结土、正常固结土共用一条临界状态线,不排水抗剪强度计算公式参数较少,易获取,易嵌入本构模型中,应用方便。

上硬下软层状土中锚板抗拔承载特性研究

锚板基础是一类在海工系泊系统中广泛应用的锚固基础,其特点是具有较高的承载力和稳定性。锚板的承载能力是基础设计的重要参考因素,而承载能力与土体的性质密切相关。现有关于锚板承载力的研究多集中在单一均质土层中,而对层状土尤其是上硬下软这类层状土体中的承载特性认识不足。基于数值分析,综合考虑层状土的分层性质,对锚板基础在硬黏-软黏层状土体及砂土-软黏土复合土体中的承载特性展开研究,分析锚板埋深、土层抗剪强度比、层厚比及土体性质等关键因素对锚板周围土体破坏模式和承载力的影响。结果表明:在硬黏-软黏土体中,随着抗剪强度比、层厚比的增加,极限承载力均显著增加;在砂土-软黏土体中,极限承载力同样随着层厚比的增加而增加,当下层软黏土中的锚板破坏进入局部破坏后,层厚比对承载力的影响逐渐减弱。研究可以为上硬下软层状土中锚板基础的承载力确定提供计算方法和理论参考。

带压环境下泥浆-泡沫组合改良级配不良砂剪切特性研究

盾构穿越富水砂性地层时易出现排渣不畅或喷涌等风险,仅采用泡沫改良往往无法有效改善级配不良渣土的流动性与抗渗性,归因于泡沫易消散和流失,故须引入膨润土泥浆进行组合改良。正确认识土舱压力环境泥浆-泡沫组合改良级配不良砂土的剪切大变形行为有助于优化满足盾构安全掘进的渣土组合改良方案。因此,开展了泥浆-泡沫组合改良级配不良砂土的加压十字板剪切试验,研究结果表明:在土舱压力环境下,组合改良砂土的初始孔隙比和饱和度随泥浆注入比(bentonite injection ratio,简称BIR)增加而增大,有效应力降低,进而导致峰值强度和残余强度降低;然而剪切强度的剪切率相关性和有效内摩擦角对BIR的敏感性较弱;BIR增加还使大变形后改良砂土的屈服应力有所降低。此外,对比常压下流动性相似的3类不同改良砂土带压剪切大变形行为,表明带压环境下提高BIR或泡沫注入比(foam injection ratio,简称FIR)均可提高孔隙比且减小有效应力,但相较于纯泡沫改良砂土,组合改良砂土具有更低的有效应力和剪切强度。该类级配不良砂土的合适组合改良参数为FIR=20%、BIR=10%~15%。最后,细观成像结果揭示了膨润土对改良砂土剪切强度的双重削弱作用:(1)吸附砂土颗粒表面,降低颗粒间摩擦力;(2)延缓泡沫衰变,使得小粒径气泡较均匀分布于孔隙中,进而有效削弱土骨架强度。

循环剪切作用下土工袋组合体动力特性试验研究

土工袋具有承载力高、阻尼大、造价低等诸多优点,是一种适用于村镇地区中、低层房屋的减/隔震材料。通过开展室内循环剪切试验对土工袋组合体的剪切应力⁃剪切应变关系、动力特性参数以及抗剪强度进行分析讨论,探究不同竖向应力与剪切应变幅值条件下土工袋组合体动剪切模量与等效阻尼比的变化规律,同时研究了循环次数对土工袋组合体动力特性参数以及抗剪强度参数的影响。试验结果表明:在小变形情况下,土工袋组合体的峰值剪切应力和动剪切模量随着循环次数的增加而逐渐增大,等效阻尼比则随着循环次数的增加而逐渐减小;在大变形情况下,土工袋组合体发生层间滑移并出现界面软化现象,此时的峰值剪切应力(抗剪强度)随着循环次数的增加而逐渐减小,动力特性参数随循环次数的变化与小变形情况下完全相反。随着循环次数的增加,土工袋组合体的抗剪强度参数似摩擦角逐渐减小,似黏聚力逐渐增大并趋于稳定。在剪切应变幅值较大的情况下,土工袋组合体通过袋内土体的剪切变形以及土工袋层间滑移产生摩擦耗能,具有较好的减振消能效果。

减载伸长路径下棕榈纤维加筋砂力学响应

为探究棕榈纤维对石英砂力学性能的增强作用,采用减载三轴伸长试验方法测试了饱和状态下棕榈纤维加筋石英砂的剪切力学性能,计算了棕榈纤维加筋石英砂在不同纤维长度和掺量下的应力-应变曲线、初始弹性模量和割线模量,基于摩尔库伦准则分析了棕榈纤维加筋石英砂剪切强度随纤维配比的变化规律,通过对比素砂和加筋砂在剪切过程中的体积应变及孔隙得出了棕榈纤维对石英砂力学性能的增强作用。结果表明:纤维加筋会增强石英砂在伸长破坏中的应力应变强度,对石英砂峰值应力和残余应力提升效果最佳的纤维掺量和长度为0.9%和12 mm;棕榈纤维加筋石英砂的初始弹性模量和峰值割线模量均随有效围压的增大而增大,整体刚度与有效围压呈正相关关系;纤维对石英砂剪切强度的增强效果只能在低围压状态时显现;在石英砂中添加棕榈纤维减小了石英砂的临界孔隙比及剪胀性。研究结果有助于促进纤维加筋砂的工程应用并提供相应理论参考。

纤维增强橡胶砂复合土动力学特性室内试验研究

针对橡胶颗粒与土混合而成的散粒体作为地基回填材料时承载力可能下降和变形较大的缺点,本文提出在该混合料中加入聚丙烯纤维进行改良,通过共振柱试验研究了橡胶含量、纤维含量和有效围压对混合复合土动剪切模量和阻尼比的影响。结果表明:随着橡胶含量R_(C)、纤维含量F_(C)和固结围压σ_(3)的增大,复合土动剪切模量的衰减及阻尼比的增长均有所减缓。复合土的最大动剪切模量G_(max)受R C和σ3的影响较大,受F C的影响较小,提出了G_(max)与R_(C)、F_(C)、σ_(3)的经验拟合公式。同时,本文给出了复合土G/G_(max)~γ曲线在Davidenkov模型中参数A和B的建议值,及参数γ_(0)与F_(C)的线性增长公式。并对复合土的阻尼比增长曲线进行拟合,给出了不同工况下λ_(min)、λ_(0)、β与F_(C)线性拟合公式。

不同充填度结构面抗剪强度尺寸效应试验研究

针对现有岩体结构面抗剪强度尺寸效应研究较少考虑充填程度差异影响的不足,首先基于渐进扩大法选取平均起伏差和平均起伏角相近的4种连续尺寸天然结构面形貌,并通过3D打印技术制作结构面面板,然后结合类岩石材料配制方法和毫米级充填装置制备5种充填度的充填结构面,最后通过课题组自主研发的大型结构面直剪试验系统开展3种法向应力条件下的室内直剪试验,获得充填结构面的剪切力学与破坏特征,并分析临界充填度和抗剪强度尺寸效应特性。结果表明:随着充填度的增加,结构面剪切破坏模式主要由“岩-岩”剪断破坏逐渐转变为“岩-土界面”滑动破坏,且结构面归一化峰值抗剪强度逐渐减小,直至达到临界充填度时趋于稳定;随着结构面尺寸的增加,临界充填度逐渐增加,变化范围为1.6~2.0;无充填结构面抗剪强度存在较明显的正尺寸效应,而充填结构面抗剪强度尺寸效应较不明显。通过尺寸效应机理分析发现,在不考虑结构面平均起伏角的影响下,平均起伏高度是影响临界充填度和抗剪强度尺寸效应的主要因素。上述研究可为充填结构面抗剪强度尺寸效应模型建立奠定试验基础。

考虑土石混合体“颗粒破碎-剪切参数-含水率”相关性的力学特性研究

抽水蓄能电站建设产生大量土石混合料堆积形成弃渣坝,渣体结构松散、级配差、重度大、透水性强,若遇强降雨,将严重影响其安全稳定性。现以弃渣坝土石混合体为研究对象,探究其在不同应力状态下应力-应变发展规律、变形强度特性,以及剪切前后颗粒级配定量研究。试验结果表明:随着含水率增加,混合体试样的抗剪强度降低,应力-应变曲线整体呈现硬化趋势,含水率的增加降低了非线性内摩擦角,黏聚力呈现先减小后增大的趋势;随着试样含水率增大,表征其变形特性的相变应力比减小,体变量减小,剪胀特性影响减弱,随着围压增大,结果与上述一致。通过对混合体剪切前后颗粒级配测定,引入颗粒破碎指标探究土石混合体颗粒破碎-剪切参数-含水率相关性分析,结果表明,当含水率增加且颗粒破碎增大时,抗剪强度减小幅度变大,对坝体的安全影响更为明显;当含水率和破碎程度减小时,结果反之。最后,通过引入破碎参数进行了邓肯-张模型修正,并基于试验结果论证了模型的适用性,以期为后续弃渣坝结构安全稳定提供借鉴。

异性结构面剪切特性及抗剪强度计算方法

天然结构面常存在两侧岩体岩性不一致的情况,对该类异性结构面抗剪特性进行研究有利于更全面地分析复杂地层中岩石的稳定性。首先,通过光学扫描技术获取天然结构面形貌点云数据并构建结构面网格模型;其次,将模型进行3维实体重构以浇筑多组含不同异性结构面的人工试样;然后,开展不同法向应力及剪切方向条件下的直剪试验;最后,通过有限差分仿真模拟软件FLAC3D进行数值模拟。基于JRC–JCS(joint roughness coefficient–joint wall compressive strength)结构面剪切模型,采用2维剖面法量化处理结构面粗糙度系数JRC,引入结构面组合壁面强度CJCS(combined joint wall compressive strength)及壁面强度系数比λ的概念以体现不同强度组合情况下的结构面抗剪特性差异。结果表明:数值模拟与直剪试验结果的规律性表征情况较为一致;在结构面单轴抗压强度较高或者较低一侧被确定时,强度较低侧对结构面峰值抗剪强度起主要的控制作用,强度较高侧则起到一定的增强作用,且壁面强度系数比λ对峰值抗剪强度的影响与法向应力的数值成正比。基于上述现象,利用相关函数对试验结果进行非线性曲面拟合,推导出一种表征不同岩石强度组合情况下的结构面峰值抗剪强度计算公式,并得出有效影响岩石结构面强度的强度比区间。该公式是对JRC–JCS模型的扩展且可在工程实践中广泛应用。