Direct shear tests of coarse-grained fillings from high-speed railway subgrade in cold regions

In order to study the shear behavior of coarse-grained fillings taken from the subgrade bottom layer of a cold region high-speed railway,large scale direct shear tests were conducted with different normal pressures,water contents and temperatures.The results indicate that the relationship between shear displacement and shear stress changed from strain-softening at lower normal pressures to strain-hardening at higher normal pressures,in both unfrozen and frozen states.This phenomenon was mainly due to the shear dilatation deformation effect.The shear displacement-shear stress curves show similar stages.Besides,the shear stress rapidly increased and there was not an increment in the shear displacement during the initial stage of the shear process in the frozen state.In both the unfrozen or frozen states at the same water contents,the shear strength increased with increasing normal pressure.

Study on Shear Strength Characteristics of Columnar Jointed Basalt Based on in-situ Direct Shear Test at Baihetan Hydropower Station

Columnar jointed basalt(CJB) widely distributes in the dam site of the Baihetan Hydropower Station.The columnar joint structure and fracture development of CJB have significant influence on the mechanical properties of rock mass,and the mechanical properties of CJB are of great significance to the Baihetan Hydropower Project.Therefore,in-situ direct shear tests were carried out on ten test adit at different locations in the dam site area to study the shear behavior of CJB.In this study,21 sets of in-situ direct shear tests were conducted for rock types of type Ⅱ_(2),type Ⅲ_(1)and type Ⅲ_(2),with horizontal and vertical shear planes and two different specimen sizes of CJB.Shear strength parameters of CJB were obtained by linear fitting of in-situ direct shear test results based on the Mohr-Coulomb strength criterion.The results indicate that the shear strength parameters of CJB with horizontal shear plane increase as the increase of rock type grade.The shear strength parameters of CJB show obvious anisotropy and the friction coefficient of the horizontal shear plane is greater than the vertical shear plane.The friction coefficient in the horizontal direction of the shear plane is 1.27 times that in the vertical direction of the shear plane.With the increase of rock type grade,the difference of friction coefficient becomes larger.However,the cohesion changes little whether the shear plane is horizontal or vertical.In addition,the size effect of CJB in this area is significant.The shear strength parameters of large size(100 cm × 100 cm) specimens are lower than those of regular size(50 cm × 50 cm) specimens.The reduction of cohesion is greater than that of the friction coefficient.For rock type Ⅲ_(2),the cohesion of large-size specimens is 0.637 of the regular-size specimens.The reduction percentage of the friction coefficient for type Ⅲ_(2)is 1.66 times that of type Ⅲ_(1).The reduction percentage of the cohesion for type Ⅲ_(2)is 1.27 times that of type Ⅲ_(1).The size effect decreases with the increase of rock type grade.The research results of this study can provide an important basis for the selection of rock mechanics parameters in the dam site area of Baihetan Hydropower Station and the stability analysis of the dam foundation and rocky slopes.

Scale dependence of direct shear tests

Direct shear test has been widely used to measure the shear strength of soils and other particulate materials in industry because of its simplicity. However, the results can be dependent on the specimen size. The ASTM (American Society for Testing and Materials) publications suggest that for testing soils the shear box should be at least ten times the diameter of the largest particle and the height of the box should be no more than half of its diameter. These guidelines are empirically based. A series of two-dimensional numerical direct shear tests are performed to investigate this scaling effect. By analyzing the bulk friction, particle translation and rotation, percentage of sliding, average volume (area) and shear strain and the evolution of the shear band, we find that the traditional guidelines for direct shear tests are questionable. Scaling dependency of bulk friction on the property of granular materials is clearly present. Our current analysis points out that the scaling effects can vary significantly depending on the particle properties other than their sizes. Of all the parameters we observed, particle rotation appears to have a decisive correlation with the bulk friction. Formation of a shear band is universal. As the shearing progresses, particle rotation begins to concentrate near the shear plane. By defining the width of a shear band as the standard deviation of the distribution of translational gradient or the standard deviation of the distribution of particle rotation, quantitative evolutions of shear band are presented. Both measures of the shear band width dropped rapidly during pre-failure stage. After peak stress both measures begin to approach steady state as the bulk friction stabilizes to the residual stage. These observations suggest that structure formation inside the shear band controls the scaling effect.

Shear behavior of coarse aggregates for dam construction under varied stress paths

Coarse aggregates are the major infrastructure materials of concrete-faced rock-fill dams and are consolidated to bear upper and lateral loads. With the increase of dam height, high confining pressure and complex stress states complicate the shear behavfor of coarse aggregates, and thus impede the high dam＇s proper construction, operation and maintenance. An experimental program was conducted to study the shear behavior of dam coarse aggregates using a large-scale triaxial shear apparatus. Through triaxial shear tests, the strain-stress behaviors of aggregates were observed under constant confining pressures： 300 kPa, 600 kPa 900 kPa and 1200 kPa. Shear strengths and aggregate breakage characteristics associated with high pressure shear processes are discussed. Stress path tests were conducted to observe and analyze coarse aggregate response under complex stress states. In triaxial shear tests, it was found that peak deviator stresses increase along with confining pressures, whereas the peak principal stress ratios decrease as confining pressures increase With increasing confining pressures, the dilation decreases and the contraction eventually prevails. Initial strength parameters （Poisson＇s ratio and tangent modulus） show a nonlinear relationship with confining pressures when the pressures are relatively low. Shear strength parameters decrease with increasing confining pressures. The failure envelope lines are convex curves, with clear curvature under low confining pressures. Under moderate confining pressures, dilation is offset by particle breakage. Under high confining pressures, dilation disappears.

Effect of a filter cake on shear behavior of sand-concrete pile interface

A filter cake is often formed between soil and concrete during casting concrete in the ground,such as constructions of diaphragm walls and bored piles.The present study aims to investigate the effect of the filter cake on the shear behavior of the sand-concrete pile interface.A series of sand-concrete interface direct shear tests were performed with a large-direct shear apparatus while considering different roughness(I=0,10,20 and 30 mm)and filter cake thickness(Δh=0,5 and 10 mm).For a smooth interface without a filter cake,the shear stress-horizontal displacement curves showed a“softening”response.The peak shear strength and friction angle decreased exponentially with increasing theΔh.Whereas,for a rough interface withΔh=5 or 10 mm,the shear stress-horizontal displacement curves presented a“hardening”response.The peak strength,as well as friction angle,decreased linearly with increasing theΔh.Moreover,a critical roughness I_(cr)of 10 mm was observed in the tests without a filter cake.The interface shear strength initially increased with increasing I but gradually decreased when the I exceeded I_(cr).In addition,the filter cake could reduce the roughness sensitivity on shear strength.

砂土-混凝土接触面剪切带细观变形特征的试验研究

接触面剪切带的细观变形特征是研究结构物与土体相互作用的核心问题。为探明砂土-混凝土接触面剪切带变形特征,利用大型直剪仪开展砂土-混凝土接触面直剪试验,采用灌砂变形标记法实测砂土变形,依据标记线剪应变沿竖向的变化特征对砂土变形区域进行划分。在此基础上,研究粗糙度、法向应力和密实度对砂土-混凝土接触面剪切带细观变形特征的影响规律。研究结果表明:1)砂土可分为非剪切区、过渡区和应变集中区,应变集中区仅存在剪切面之上。2)规则型接触面的上剪切带厚度均值为33D_(50),远大于下剪切带9D_(50)和“光滑”接触面剪切带厚度20.4D_(50)。3)密实度对剪切带及其过渡区厚度的影响微弱,而粗糙度对其影响显著;法向应力、密实度对应变集中区厚度影响有限,应变集中区厚度随粗糙度的增大近似线性增大;应变集中区长度随粗糙度的增大先增大后减小,随密实度的增大近似线性增大,随法向应力增大而减小。4)随剪切位移增大,上剪切带厚度不断增大而下剪切带厚度先增大后保持基本不变,应变集中区的长度增大,但剪切位移对应变集中区厚度的影响有限。5)获得归一化剪切带厚度取值h_(t)/D_(50)与D_(50)的关系,在有限元模拟规则型结构与砂土的相互作用时,建议砂土体细化网格单元的范围不小于最大剪切带厚度37D_(50)。研究成果深化了砂土-规则型接触面剪切带变形特征的认识,可为大粗糙度、带肋桩的承载变形特性的精细化有限元模拟分析提供重要参考和依据。

珊瑚砂与土工格栅界面的剪切特性及本构模型

采用南海原位级配珊瑚砂和聚丙烯双向土工格栅,开展了不同法向应力下珊瑚砂和筋土界面大型直剪试验。试验结果表明,珊瑚砂和筋土界面剪应力—剪切位移曲线呈现明显的应变软化特征,峰值抗剪强度线呈现双折线形态,残余抗剪强度线则呈现较好的线性关系;各级法向应力下珊瑚砂与土工格栅界面摩擦比均值达到1.37左右,远高于石英砂与土工格栅界面摩擦比,说明在珊瑚砂中加筋能充分发挥加筋效果;珊瑚砂和筋土界面在剪切过程中总体上经历了相对剪缩—相对剪胀—相对剪缩的过程,对珊瑚砂加筋可显著减小其剪缩变形;珊瑚砂和筋土界面直剪试验试样的相对破碎率接近,均随法向应力的增大而增大,但增幅逐渐趋缓。采用邓肯—张模型和剪切软化模型分别描述南海原位级配珊瑚砂与聚丙烯双向土工格栅界面峰前和峰后剪应力—剪切位移曲线,所建立的筋土界面本构模型能够很好地反映其剪切特性。

循环荷载作用下钢桩-软黏土接触面剪切特性研究

目前,中国海上风电工程的建设已进入快速发展阶段。海上风电机常采用大直径钢管桩作为基础,然而现在的桩基承载力设计预测方法往往基于小直径柔性桩,不适用于大直径刚性桩。因此需要研究海上桩基桩-土界面的相互作用,来对海上大直径刚性桩基的设计预测方法进行优化。使用循环荷载作用下钢桩-软黏土大型接触面剪切试验数据,建立法向循环荷载作用下钢与黏性土接触面剪切试验的离散元模型,对各个细观参数进行标定,分别研究动荷载频率和幅值对剪切特性的影响,并进行了细观机理分析。数值模拟结果表明动荷载频率和幅值越大,钢与黏性土接触面强度越小,其中动荷载幅值的影响较为明显。通过细观机理分析可以发现:动荷载频率和幅值越大,接触面处黏性土的颗粒位移越小;静荷载下黏性土的颗粒间作用力大于动荷载下的颗粒间作用力;动荷载会导致颗粒间的扰动,从而使接触面强度减小。研究结果可以为海上风电桩基承载力的准确预测提供理论基础。

土工格室与沙漠沙界面特性大型直剪试验

为探明土工格室与沙漠沙的筋土界面特性,开展了多项大型直剪试验研究,包括不同土工格室位置条件下、不同剪切速率条件下、不同温度条件下的土工格室加固沙漠沙界面直剪试验。结果表明:在50,100,150 kPa的法向应力条件下,以土工格室加筋层上界面或下界面作为剪切面,不同剪切速率(0.5,1.0,2.0 mm/min),不同温度(30,15,0,-15℃)影响下的筋土界面剪应力-剪切位移曲线均呈应变软化特征;纯沙漠沙对剪切速率的变化不敏感,剪切速率由0.5 mm/min升至3 mm/min时沙漠沙抗剪强度基本持平,而土工格室加固沙漠沙的界面对剪切速率的变化敏感,界面抗剪强度随剪切速率的增加而减小;在较低剪切速率0.5 mm/min下,土工格室提供的似黏聚力对抗剪强度的贡献更大;土工格室加固沙漠沙在30℃与15℃下峰值剪应力变化不大,区别在于峰值过后30℃抗剪强度存在更快的衰减至平稳阶段的情况;随着温度的降低,土工格室-土复合体呈现出更大的刚性,界面剪应力峰值提高并前移;土工格室刚性的增加使土工格室壁不易受拉变形,造成紧箍作用削弱而水平摩擦力作用增强,综合体现在加筋土剪切面的似黏聚力降低,而内摩擦角增大。

灌木根系形态对土体强度影响的大型直剪试验研究

根系形态显著影响根土复合体强度,然而现有研究局限于草本根系根土复合体和室内常规直剪试验。为探究多年生灌木根系形态及空间分布对固土效果的影响,采用大型直剪试验装置(直径400 mm),对原状根土复合体进行了剪切试验。试验中测量了根系形态参数(根面积比RAR、根长密度、根体积密度和根表面积密度)、根土复合体抗剪强度和剪胀特性,探讨了根系沿深度的分布特征、根土复合体剪胀性以及根系形态参数与根土复合体抗剪强度的相关关系。结果表明,根系加剧了根土复合体的剪胀;在4个根系形态参数中,剪切带上的RAR和根表面积密度与根系对剪切强度的贡献之间相关性较好;Wu模型能反映根系固土的本质,却存在强度被高估或低估的问题。研究成果有助于推动植物根系在固土领域的应用。

块石破碎性质对土石混合体剪切带厚度的影响研究

土石混合体作为一种典型的浅表层堆积体,常引发不同规模的滑坡灾害。本文通过对比土石混合体在室内大型直剪和离散元(PFC2D)数值模拟直剪条件下剪切带厚度随含石量和上覆压力的变化特征,以期进一步揭示影响土石混合体剪切带厚度的主要控制因素。试验结果显示,室内试验中土石混合体的剪切带厚度主要受含石量的影响,其次为上覆压力,受块石尺寸的影响不明显。剪切带厚度随含石量从0增加到50%时表现为增大的趋势,随含石量的继续增大而表现为减小的趋势。但上覆应力小于200 kPa时,剪切带厚度随上覆应力的增大表现为增大的趋势,但上覆应力超过200 kPa时,其值表现为减小的趋势。然而,在数值模拟计算中,考虑块石不可破碎的情况下剪切带裂纹区域的宽度随含石量和上覆压力的增大表现为持续增大的趋势特征。分析认为,土石混合体剪切中大块石之间咬合后的齿轮带动效应是促使剪切带厚度增大的重要原因,而骨架结构含石量以及高垂直应力下块石咬合部位因高度的应力集中,随后导致的咬合棱角破碎,块石变形被限制在一定的区域内,却抑制了剪切带厚度的进一步增大。相反,在块石不可破碎的数值模拟计算中,高含石量试样内块石可持续的咬合滚动,促使周围的块石也发生变形,进而增大了剪切带的厚度。研究结果有利于揭示土石混合体的剪切变形破坏机制。

土石混合料剪切力学特性的精细化离散元分析

研究目的:土石混合料具有非均质、非均匀、非连续、显著的结构效应等特点,其物理力学性质十分复杂。为探究含石量对土石混合料剪切力学特性的影响,本文基于真实块石形状,提出了块石逆向重构的数字图像法,建立了土石混合料离散元数值模型,并在此基础上开展了6种含石量试样在4种不同法向应力下的三维离散元直剪试验。研究结论:(1)剪应力-剪切位移曲线主要经历弹性阶段、初始屈服阶段、峰值强度阶段、残余强度阶段四个阶段;(2)在剪切过程中,块石转动存在明显的“齿轮带动效应”,土颗粒运动具有明显的“绕石”现象,颗粒的运动呈明显的漩涡状;(3)力链主要沿着剪切力和法向力的夹角方向分布,随着含石量的增大,力链偏向水平剪切方向的角度越大;(4)剪切面演化主要经历微裂隙阶段、剪切面贯通阶段、剪切带扩宽阶段,剪切带宽度随含石量增加而增加,且逐渐向逆时针方向偏转;(5)土石混合料抗剪强度随含石量的增大而增大,通过拟合发现,黏聚力和内摩擦角均与含石量呈指数函数关系;(6)本研究成果可为土石混合料相关工程设计和施工中的力学参数取值提供参考。

含粗粒滑带土剪切带演化及空间展布规律研究

对含不同强度粗颗粒的滑带土进行了可视化中型直剪试验,通过竖直置入铝丝观察剪切后铝丝变形情况获取剪切破坏带的空间信息,并建立了剪切带空间曲面方程。通过粒子图像测速(particle image velocimetry,PIV)技术对可视面处二维剪切带信息进行提取,并与剪切试验得到的空间曲面方程的边界外推值进行对比,证明了空间曲面方程与PIV技术能够描述剪切带特征。然后通过PIV技术对不同剪切总位移与某一剪切位移下短时间内相对位移规律进行了分析。最后分析了预制损伤与粗颗粒强度对剪切带特征的影响。研究结果表明,剪切带形状为两端薄中间厚的带状,可采用高斯曲面方程对剪切带轮廓进行拟合。剪切带在发展过程中经历压密、自由损伤、损伤沿固定路径发展与剪切带贯通4个阶段,损伤使得剪切带发展各阶段前移,粗颗粒强度会对局部剪切带变形产生较大影响而对整体剪切带的变形影响较小。该研究成果对于揭示滑坡剪切带形成与演变规律从而设计合理的边坡防治方案具有重要意义。

泥石流物源土体标度分布参数与粘聚力的关系

颗粒级配影响土体力学性质(如粘聚力)。细颗粒(尤其是粘粒)含量对粘聚力起着决定性作用。颗粒级配可用土体特殊粒径(如D50等)来表征,但并不能避免同参异效等问题。采用土体标度分布μ和Dc两个参数,可准确地描述土体颗粒组成特征,但该分布参数的物理意义,以及特征参数与土体力学参数之间的关系尚不明确。本文实施室内试验测试,采用标度分布表征土体颗粒组成,研究在不同含水率和标度参数条件下土体粘聚力的变化特征,探讨土体颗粒组成、含水率和粘聚力之间的关系。结果表明:(1)标度分布参数μ随着细颗粒含量的增加而增大,并呈现指数函数关系;(2)随着细颗粒含量的增加,粘聚力先迅速增大后缓慢增加并趋于稳定,且粘聚力c与标度分布参数μ呈现对数函数关系;(3)粘聚力随着含水率的增大先增大后减小,即存在一个使得土体粘聚力达到最大值的含水率阈值,在达到此阈值之前,细颗粒含量对粘聚力的影响较为显著,达到该阈值之后,含水率成为更主要的影响因素;(4)基于试验结果,得到土体粘聚力随含水率及标度参数共同影响下的二元模型。研究可为明确标度分布参数的物理意义以及确定土体力学参数提供试验依据。

青藏高原雅江河谷地带碎块石土强度特性及颗粒破碎分析

碎块石土在西藏等高寒地区广泛分布,其力学性质受粗颗粒含量、干密度及含水率的影响较大,且对评价该区域的堆积层滑坡稳定性十分重要。为探究粗颗粒含量、干密度及含水率对青藏高原雅鲁藏布江河谷地带碎块石土强度和颗粒破碎率等参数的影响规律,开展了4种粗颗粒含量、3种干密度和2种含水工况下的碎块石土大直剪试验,分析了剪切过程中的应力变化规律、剪切后的颗粒破碎率及体变特征。结果表明:当试样干密度为1.4g/cm^(3)时,应力应变曲线在高粗颗粒含量(p_(5)≥40%)下没有出现峰值,试样未发生剪切破坏,随着干密度的增大,试样发生剪切破坏的临界粗颗粒含量和竖向压力不断降低;碎块石土的黏聚力c以p_(5)=20%为临界点,随着粗颗粒含量的增大先减小后增大,内摩擦角φ随p_(5)的增大逐渐变大;碎块石土的颗粒破碎率与粗颗粒含量、竖向压力和初始干密度均呈正相关;粗颗粒含量和干密度的增大均会提高试样的界面抗剪强度,降低试样的最终体变量。试样的剪胀性受碎块石相互咬合形成的骨架结构的强度和竖向压力的影响,当σ=100 kPa,p_(5)≥20%时,试样表现出剪胀特性,随着竖向压力的增大,试样由剪胀向剪缩转变;在饱和含水工况下,碎块石土的抗剪强度降低,最终体变量与剪缩量大幅增加。研究成果可为青藏高原碎块石土滑坡稳定性研究提供指导。

土体与混凝土界面剪切特性试验研究

为研究土体与混凝土两者之间的相互作用机制,通过大型直剪仪,对不同土体、法向应力及含水率条件下,土-混凝土界面的剪切特性进行了一系列试验研究。研究结果表明:对于砂土、淤泥及砂质泥岩,土-混凝土界面的抗剪强度呈现出随法向应力增大而增大、随含水率增大而降低的特性;砂-混凝土界面的内摩擦角和黏聚力受含水率影响不明显;随含水率提高,淤泥、砂质泥岩与混凝土界面的内摩擦角显著降低,黏聚力先升高再降低。研究成果可为不同土体的侧摩阻力分析提供参考。

单级配堆石料力学性质和变形特征的研究

颗粒级配是影响堆石料力学性质的主要因素之一,为研究单一级配堆石料的力学性质和变形特征,基于室内大型三轴试验,对单一级配的堆石料进行研究。结果表明:试样在密实状态下,剪切过程中不断发生剪缩现象,同时试样呈现应变硬化模型,与常规的级配良好的堆石料或者砂土材料呈现相反的规律;围压相同,且在轴向应变相同时,大粒径的堆石料剪缩效应弱于小粒径堆石料,而大粒径堆石料的偏应力大于小粒径堆石料的偏应力;单级配堆石料粒径大小对力学性质和变形特征会产生重要的影响。

蒋家沟砾石土不同粗粒含量直剪强度特征

蒋家沟流域内大量泥石流由滑坡引发,滑坡转化为泥石流的3个过程中,首先是在斜坡体内大范围剪切破坏。在其物源区与沉积区内分布大量砾石土,其强度特征对于蒋家沟泥石流的起动机制具有重要意义。由于砾石土颗粒粗大,粗细粒配比和试验尺寸对强度特征有一定的影响。为此在室内现场两用大型直剪仪下进行蒋家沟砾石土不同粗粒含量下的直剪强度试验,结果表明:在干密度为2.0g/cm3、含水率为7%时,砾石土除在粗粒含量为74.31%的低垂直压力(50,100 kPa)时剪应力-水平位移关系有峰值外,其余土样没有明显峰值;同一粗粒含量试样在水平剪切应变值为0.02时的剪切模量G0.02随垂直压力增加而增大;在相同垂直压力下,G0.02随粗粒含量增加而增大,且在较低垂直压力下的G0.02随粗粒含量增长更为显著。内摩擦角值随粗粒含量增加而增大;黏聚力在粗粒含量为48.7%时最小。粗粒含量小于48.7%时,试样黏聚力值随粗粒含量增大而减小;当粗粒含量大于48.7%时,黏聚力随粗粒含量增加而显著增大。

现场室内两用大型直剪仪研制(I):结构设计

直剪仪是测定岩土体抗剪强度参数应用最为普遍的一种试验仪器。从直剪仪的结构和大型直剪仪的研制两个方面对直剪仪的研究现状做了介绍,在此基础上,对新近研制的现场室内两用大型直剪仪的结构组成、工作机理及其特点进行了详细分析。该大型直剪仪与常规直剪仪相比,具有较大的优越性,其测试结果更加准确。适用范围广,测试精度高,可应用于现场和室内试验。

直剪剪切速率对粗粒土强度与变形特性的影响

土体的强度和变形对路基及边坡的稳定性具有控制作用,为分析剪切速率对粗粒土抗剪强度和变形特性的影响,基于THE-1000型室内大型直剪仪,对不同剪切速率下粗粒土的强度和变形特性进行试验研究。试验数据表明:不同剪切速率时的剪应力–剪切位移曲线均呈应变软化型;剪切速率为小于5 mm/min时,粗粒土强度理论公式拟合相关系数比较理想;随剪切速率的增加,内摩擦角有减小的趋势,约在27.8°～22.8°内变化,咬合力在90.3～112.2 kPa范围内振动变化;颗粒破碎率和最大垂直变形随着剪切速率减小或垂直压力增加而增大,即剪切速率越小越容易发生剪缩;剪切速率小而抗剪强度高的本质原因是:破碎后的细颗粒填充了由粗颗粒构成的土骨架孔隙,造成试样的密实度增加。