土石混合体宏细观力学特性和变形破坏机制的三维离散元精细模拟

为了更加精细的模拟土石混合体宏细观力学特性和变形破坏机制,针对已有三轴试验侧向柔性薄膜边界模拟方法进行改进,提出三维组合墙法实现了柔性薄膜边界的三维离散元模拟。结合已开发的不规则块石和土石混合体三维离散元建模方法建立土石混合体大型三轴数值试样,开展数值试验标定土石混合体数值模型的细观力学参数并与室内试验结果进行了对比验证。深入分析不同含石量下土石混合体的宏细观力学特性和变形破坏机制。结果表明:三维组合墙法原理简单、参数较少、易于实现且应用效果较好;大型三轴试验数值模拟能够较好地再现土石混合体宏观的应力–应变特征、强度特性和破坏模式;随含石量增加,土石混合体的骨架结构效应越来越显著;剪切带内土颗粒旋转量较大而块石颗粒旋转量较小,表明剪切面绕过了较大的块石颗粒。     

基于围压柔性加载的土石混合体大型三轴试验离散元模拟研究

综合运用计算机三维扫描与随机模拟技术,建立了不同块石含量和空间分布的土石混合体三维随机细观结构模型和离散元模型,考虑围压柔性加载,基于柔性黏结颗粒膜方法,采用颗粒流程序对不同土石混合体试样进行了不同围压下的大型离散元三轴试验模拟,研究了块石含量和空间分布对土石混合体力学特性和变形破坏规律的影响。数值模拟结果表明:土石混合体的强度和抵抗变形的能力随含量和围压的增大而增强,且在相同含石量下,受内部块石空间分布的影响,试样的内摩擦角和黏聚力虽会表现出一定的离散性,但总体上,内摩擦角随着含石量增加基本呈线性增加,而黏聚力却随着含石量增加逐渐减小;在围压柔性加载下,土石混合体试样表现为鼓胀变形破坏,破坏后形成的剪切带为一个曲折条带,形态上呈非对称的X形分布,厚度约为试样高度的1/3～1/2倍,且试样的破坏形态及内部剪切带大小和分布形态不仅受块石含量和空间分布影响,而且也取决于围压大小;土石混合体试样在破坏过程中内部剪切带的形成是伴随局部颗粒的转动开始的,在应变到达峰值应变时,局部发生转动的颗粒相互连接贯通,此时剪切带已基本形成,此后随着应变继续增加,受峰后鼓胀变形的影响,试样内部颗粒的转动仍会发生一定的变化,同时伴随着剪切带大小和分布形态也发生相应的变化。     

不规则颗粒及其集合体三维离散元建模方法的改进

以土石混合体中的不规则块石和含石量为40%的土石混合体室内大三轴试样为例,研究了如何针对当前岩土材料建模方法的不足进行改进以建立尽可能真实的不规则颗粒及其集合体的三维离散元模型。为了模拟块石的真实形态特征,明确了块石几何模型建模方法的控制参数及其确定方法,建立了与真实块石球度相同、棱角度相似的三维半真实离散元模型。为了计算模型的体积以用于模型密度优化及颗粒集合体孔隙率的计算,提出了一种三维离散元模型虚拟切片技术,结合数字图像处理技术可快速准确计算块石三维半真实离散元模型的体积。为了使得土石混合体大三轴试样三维离散元模型的密实度与室内试样保持一致并兼顾建模效率,提出了基于拟振动压实法和分层复制法的土石混合体大三轴试样三维离散元建模方法。研究结果表明:所提出的不规则块石几何模型建模方法控制参数较少且可分别对球度和棱角度进行单独控制;当块石离散元模型填充球体数较少时,其体积与对应几何模型体积相差较大,不能直接采用对应几何模型的体积;所建立的土石混合体数值试样与室内试样的细观结构特征基本相同,即块石随机散布于土体基质中。     

基于细观数值试验的非饱和土石混合体力学特性研究

 从土石混合体细观结构出发,融合细观结构模型生成技术、主–从接触面模型及非饱和土渗流与强度理论,建立非饱和土石混合体的细观数值模拟方法。通过与非饱和土石混合体室内试验结果进行对比,验证所建立的细观数值模拟方法的可行性和合理性。利用该细观模拟方法,分析土–石界面接触特性、含石量及饱和度等因素对非饱和土石混合体力学特性与破坏机制的影响。结果表明：(1) 非饱和土石混合体在低围压下表现出明显的剪胀性,且受含石量和饱和度影响显著;在较高围压下基本上表现为剪缩变形,随含石量的增大其剪缩变形减小,饱和度对剪缩性的影响较小。(2) 土石混合体的峰值强度和变形模量随土–石界面摩擦因数的增大呈非线性增长,在界面摩擦因数大于0.6以后,两者基本趋于稳定值。(3) 含石量越大,非饱和土石混合体的峰值强度和变形模量越大,应变硬化特征更为显著,在含石量增加到58%后峰值强度和变形模量趋于稳定值。在低围压下剪胀变形随含石量的增加而增大;在较高围压时,剪缩变形随含石量的增大而减小。(4) 饱和度越大,基质吸力越小,非饱和土石混合体的峰值强度越低,但变形模量变化不大。     

土石混合体三维细观结构随机重构及其力学特性颗粒流数值模拟研究

土石混合体是一种非连续、非均质、各向异性的土石混合多相介质,其力学性质极为复杂,与内部土石细观结构密切相关。从细观结构层次出发,运用计算机随机模拟技术,建立了一种基于不规则块石的土石混合体三维细观结构重构方法,并基于FORTRAN语言开发了相应的三维细观结构随机模拟系统(RMS3D),在此基础上,考虑块石的不规则形状,建立了土石混合体的离散元模型,并采用颗粒流程序对其开展了不同法向应力下三维直剪试验模拟,探究了块石空间分布对其力学特性的影响。研究结果表明:土石混合体的力学性质受内部块石空间分布影响显著,在相同级配和含石量下,不同块石空间分布的土石混合体试样的剪应力–剪切位移曲线和法向位移–剪切位移曲线均不相同,尤其是在峰后呈现出了明显的差异,且后者开始出现差异时相对于前者滞后;另外,受剪切面上块石阻碍的影响,由于试样内部块石空间石分布的不同,导致不同试样剪切破坏后所形成的剪切带的形态和厚度也存在一定的差异。     

基于大型直剪试验的土石混合体剪切带变形特征试验研究

土石混合体介质具有高度非均质性、显著的结构效应与尺寸效应等特点,这使其物理力学特性及其复杂。本文针对土石混合体在剪切过程中剪切带的变形性状与影响因素,采用自主研发的RSM–1000型电机伺服控制大型土工抗剪强度试验系统,考虑不同含石量(0,30%,50%,70%)、上覆压力(50,200,300,400 k Pa)、块石尺寸(L1,L2,L3)3个主要结构控制因素,进行土石混合体剪切变形试验,通过在试样内部钻孔设置铝丝与干灰的方法,监测剪切带特征变化规律。研究结果表明:当含石量小于30%时,块石对试样的变形影响较小,强度主要依赖于砂土强度;当含石量达到50%时,试样内已形成骨架结构,变形受块石的影响突显,强度由块石和砂土共同作用;当含石量达到70%时,试样内已形成块石架空结构。在高含石量与大粒径块石条件下,含贯穿剪切面的块石试样随剪切变形发展,块石发生挤压、翻转现象;剪切面附近分布块石的试样,随剪切变形发展,块石以剪胀作用为主,块石发生挤压、棱角剪断与错动重分布。试样的剪切变形现象可类比由后向前变形的推移式滑坡或由前后向中间变形的复合式滑坡的破坏特征,即后缘坡顶在主动土压力作用下产生裂隙,随之下沉挤密、失稳起滑;前缘坡脚蠕滑变形推移;坡中岩土体发生剪切错动至滑动面渐进扩展破坏,最终剪切面贯通,形成整体破坏。该研究成果对揭示土石混合体滑坡剪切带形成演化规律、破坏模式及土石混合体滑坡的防灾减灾具有重要意义。     

含石率对土石混合体–基岩界面剪切力学特性的影响

土石混合体与基岩接触界面是高填方体边坡和天然斜坡失稳不容忽视的潜在滑移面。通过室内大型直剪试验和离散元数值模拟探究了含石率对土石混合体—基岩界面剪切力学特性的影响及接触面剪切破坏机理。结果表明:土石混合体—基岩界面的剪应力–剪切位移曲线随法向压力的增大有由应变软化向应变硬化转变的趋势;剪应力–剪切位移曲线出现"V型跳跃"主要与颗粒破碎、转动和翻越有关;土石混合体—基岩界面抗剪强度和抗剪强度指标随含石率的增加先增大后减小,存在着最优含石率,但内摩擦角φ变化不大,在38°左右波动;剪切带的分布和形态受含石率和法向压力影响显著,法向压力和含石率越高,剪切带就越厚;剪切带内的块石破坏表现为表面研磨、局部破碎和完全破碎3种模式;法向压力和含石率都是影响相对破碎率B_r的主要原因,表现为随着含石率和法向压力的增加,块石相对破碎率不断增加。     

含超径颗粒土石混合体的大型三轴剪切试验研究

 为研究土石混合体在含超径颗粒情况下的力学响应,利用大型三轴剪切试验仪,分别对体积含石量为25%,35%的非常规土石混合体试样在3种不同围压条件下进行固结不排水剪切试验。试验结果显示：含超径颗粒土石混合体在不排水剪切条件下仍然存在体积变化;含石量35%的土石混合体在围压较高情况下,体应变表现为加载初期剪缩,随后剪胀,二次剪缩,再次剪胀的特征;含石量25%的土石混合体的应力–应变曲线较为平滑,但含石量在35%的情况下,应力–应变曲线则呈现锯齿状特征,且伴随着间接性的应力跳跃现象,相应的体应变、孔隙水压力也出现跳跃现象,且与试样的应力变化具有很好的对应关系。     

基于三维柔性薄膜边界的土石混合体大型三轴试验颗粒离散元模拟

为了有效模拟土石混合体室内大型三轴试验侧向柔性乳胶膜的力学行为,提出了一种可行的三轴试验侧向柔性薄膜边界的三维离散元模拟方法,即三维组合墙法。结合已开发的不规则块石及土石混合体三维离散元建模方法,建立了可考虑柔性薄膜边界的土石混合体大型三轴试样三维离散元模型。引入平行黏结模型以更好地模拟胶结土石混合体中颗粒间的胶结作用,通过开展大型三轴数值试验逐一全面地标定了无胶结土石混合体和胶结土石混合体数值模型的细观力学参数。分析了无胶结土石混合体和胶结土石混合体数值试样的变形破坏过程及特征,并与室内试验结果进行了对比。结果表明:所提出的三维柔性薄膜边界建模方法原理简单,参数较少,易于实现,且能节省计算资源;基于三维柔性薄膜边界的土石混合体大型三轴试验颗粒离散元模拟能较好地再现土石混合体的应力–应变特征、无胶结土石混合体的鼓肚变形破坏特征、胶结土石混合体变形局部化的过程及其剪切带的细观结构特征。     

含石量和压实度对格栅-土石混合体界面剪切特性的影响

为了探究含石量和压实度对格栅-土石混合体界面抗剪特性的影响，采用大型直剪仪对不同含石量的格栅-土石混合体界面进行了单调剪切试验。试验研究了5种含石量（0%,25%,50%,75%,100%）和3种压实度（88%,92%,96%）对格栅-土石混合体界面的抗剪强度和体变特性的影响，并基于室内直剪试验建立了离散元分析模型，对界面的作用机理进行了分析。结果表明，含石量由0%增加至100%，筋土界面的抗剪强度、内摩擦角及似黏聚力均表现出先增大后减小的趋势，含石量为75%时达到最高；试样在高含石量下表现为较明显的应变软化和剪胀现象。此外，试样的压实度越高，筋土界面的剪切应力在前期增长越快，抗剪强度也越高。数值研究结果表明，低含石量模型的力链细而密，高含石量试样的力链较粗，分布较稀疏，且两组模型在剪切破坏后均形成贯通的强力链；高含石量试样在剪切过程中会形成一个孔隙率较大的带状区域，剪切面上的孔隙自两端向中间发展直至贯通。     

An approach to predicting the shear strength of soil-rock mixture based on rock block proportion

Soil-rock mixture (SRM) shows complicated mechanical behaviors due to their complex compositions and structures, leading to challenging instability problems during the construction process. Typical SRM are composed of rocks with high strength and fine grained soils, and the mechanical characteristic is largely controlled by the rock block proportion (RBP) and component properties. It is noted that the rock sizes of natural SRM make it difficult for laboratory or in situ tests. There are few studies on empirical formulas to predict the mechanical characteristics of SRM. In this study, the nonlinear relationship between SRM shear strength and RBP was investigated, and an empirical formula predicting the shear strength of mixtures consisted of strong rocks and a weak soil matrix was proposed. For this purpose, a database of shear strength and uniaxial compressive strength (UCS) of SRM with different RBPs was built firstly on the basis of the laboratory test results from previous literatures. In order to focus on the interactions of rock blocks and soil matrix in SRM, a RBP range of 30–90% was set as the applicable range of the empirical formula and both of the compositions are held to provide shear resistance in the applicable range. Subsequently, a nonlinear equation to calculate the shear strength of SRM with RBP range of 30–90% was proposed using regression analysis considering the strengths of components and soil-rock contact faces. Several representative properties of rocks and soil matrix, such as RBP, UCS of the matrix (UCS_m), and the friction angle of the blocks (φ_block), were chosen as the input parameters based on the mechanical properties of SRM. An additional parameter “A” was used to describe the connect strengths of the soil-rock contact faces. In addition, uniaxial compression tests and large-scale direct shear tests were performed on the Taoyuan SRM samples. The test results and other measured data from the database were used to compare with the corresponding estimated values. The results demonstrated that the empirical approach could predict the shear strength with R² = 0.75 and can be considered a practical tool in engineering designs when mechanical tests are not available.

     

基于真实块石形态的土石混合体细观力学三维数值直剪试验研究

 块石形态是影响土石混合体细观结构及宏细观力学特性的一个重要因素。提出一种基于真实块石形态的土石混合体细观力学研究方法,首先采用三维扫描技术建立块石三维形态数据库,然后根据粒度组成生成试样内部块石三维空间分布模型,从而构建土石混合体的数字模型。利用所提出的基于多球颗粒均匀接触的复杂块体表征算法,实现任意复杂形态块体的多球颗粒均匀、紧密接触填充。基于上述方法,建立土石混合体离散元数值计算分析模型,并协同现场试验结果开展三维数值直剪试验研究。根据土体的现场试验结果反演得到离散元数值试验中土颗粒的细观接触力学参数,将其应用于土石混合体数值试验得到了较好的效果。通过数值试验研究表明,土石混合体在剪切过程中的块石效应,使剪切带变得更加宽厚和曲折,在宏观上造成了试样的内摩擦角有增高趋势;而由于块石含量增加,使得其黏结作用的土体含量减小,从而在宏观上表现为试样黏聚力呈略有降低趋势。     

A 3D synthetic rock mass numerical method for characterizations of rock mass and excavation damage zone near tunnels

In practice, a damage zone is generally formed after tunnel excavation in jointed rock mass. This damage zone is closely related to rock mass properties and requires careful examination in order for cost effective supporting designs. In this research, a synthetic rock mass (SRM) numerical method is applied for characterizations of the jointed rock mass and excavation damage zone (EDZ) near underground tunnels in 3D. The SRM model consists of bonded particles and simulates deformation and crack propagation of the rock mass through interactions between these particles. The effects of joint stiffness and distribution on the rock mass properties are systematically examined by comparing the numerical data with an empirical geological strength index (GSI) system and an associated Hoek-Brown strength criterion. The numerical results suggest that rock mass properties are comparable to the empirical GSI/Hoek-Brown system only when inclined joints are simulated in the rock mass subjected to axial loading. The rock mass is strengthened and the empirical GSI/Hoek-Brown characterization becomes inappropriate when the joints are less favorable to shear sliding. The SRM method is then applied for characterizations of tunnel EDZ. It appears that the depth and location of the EDZ are a function of the tunnel orientation, joints, and in situ stresses. The EDZ depth is expected to be higher when inclined joints are simulated. The EDZ area is reduced when the joints in the rock mass are horizontally and vertically distributed.

     

Interfacial shear strength of fiber reinforced soil

The interfacial mechanical interaction between the reinforcement and soil matrix is a key factor in controlling the engineering properties of reinforced soil. To evaluate the factors affecting the interfacial strength properties of polypropylene fiber (PP-fiber) reinforced soil, single fiber pull-out tests were performed by using a modified special apparatus. It has been found that the designed pull-out test is an efficient method to qualitatively obtain the interfacial peak strength (IPS) and interfacial residual strength (IRS) of fiber/soil. Both the IPS and IRS decrease with water content increase, while increase with increasing soil dry density. The cement inclusions dramatically improve the interfacial shear strength of fiber/soil, and the IPS and IRS increase with an increase in additives content and curing time. Finally, by using scanning electron microscopy (SEM), the micromechanical interaction behavior between soil particles and fiber reinforcement were discussed. The interfacial shear resistance of fiber/soil depends primarily on the rearrangement resistance of soil particles, effective interface contact area, fiber surface roughness and soil compositions, etc.     

深海能源土剪切带形成机理离散元分析

天然气水合物的分解开采过程将会劣化深海能源土的力学性能,从而引发一系列岩土工程问题。因此,要实现天然气水合物的安全开采,需要对能源土的强度和变形特性开展研究。结合深海能源土微观胶结模型,通过平面应变双轴试验的离散元模拟,研究了深海能源土剪切带形成机理以及剪切带内外的宏微观变量特征。结果表明：水合物胶结提升了深海能源土的强度,且使其呈现出明显的应变软化特性;剪切带在峰值应力后开始产生,伴随着胶结的大量破坏以及各宏微观变量的局部化;剪切带内外各宏微观变量差异明显,随着轴向应变的增加,土体微观结构也随之发生变化。     

Strength parameter identification and application of soil–rock mixture for steep-walled talus slopes in southwestern China

Soil–rock mixture (SRM) is a heterogeneous geomaterial which is widely used in geotechnical engineering projects. As a special engineering geological body, SRM is composed of many complex components and is a heterogeneous multiphase material with various structural characters, and, thus, exhibits complex mechanical characteristics. The mechanical and physical properties of SRM are major factors which lead to different developmental patterns and deformation characteristics for talus slides. The formation mechanism and mechanical parameters of SRM also play important roles in research regarding slope stability. Taking the Mahe talus slide of the Lenggu hydropower station located on the Yalong River in southwestern China as a study example, many methods, such as the analogy method used in engineering, as well as laboratory model tests, large in situ shear tests, the back analysis method and numerical experiments, are applied in the comprehensive analysis of SRM from a macroscopic–microscopic perspective. The SRM samples collected from the Mahe talus slide consist of various soil and rock contents. The parameters gained from the frontal methods are applied in the stability of the Mahe talus slide. The main contents of the study are as follows: (1) according to the special structure of SRM, ten groups of SRM samples collected from different slide parts are used to perform particle size analysis experiments. The grading combination of the ten groups of samples is analyzed and the gradation curves are obtained from laboratory tests; (2) based on the intensive considerations of different particle compositions, the ten SRM group samples collected from the talus slide are used to perform direct shear tests; (3) due to the fact that the samples containing large-sized particles cannot be simulated by means of indoor direct shear tests, large in situ SRM shear tests are performed in the field; (4) SRM containing large-size particles is used to carry out numerical experiments using the similarity ratio, which is determined by contrasting the results of the laboratory tests and numerical experiments for the same size samples containing the same particle combinations. The numerical experiments are then adopted to obtain the shear strength parameters of different large size samples containing different particle combinations from the perspectives of rock content, particle size, and particle graduation; (5) according to the terrain, geomorphology and stability of the talus slide, the shear strength parameters in the case of natural conditions and magnitude 6 earthquakes on the Richter Scale are obtained using the back analysis method from the perspective of the limit equilibrium of the talus slide; and (6) the shear strength parameters of the various methods listed above are contrast-analyzed. The general shear strength parameters of the SRM are attained properly by using the weighted superposition of the safety coefficients from the different calculation methods. The general strength parameters are used to calculate the stability factor of the Mahe talus slide.