A contact model for rough crushable sand

Sand roughness is now accessible to measurement. Incorporating this parameter into sand models using the discrete element method (DEM) is known to improve bulk small strain response. In this work we explore the effect on problems where particle crushing takes place. A well-established DEM particle crushing model and a rough Hertzian contact model are here combined to incorporate both effects in a single contact model. Including contact roughness results in stronger particles whilst all other material parameters being equal. The model is then used to simulate high pressure oedometric compression tests on a strong silica sand. It is shown that including realistic values of surface roughness enables to correctly capture both load-unload behaviour and particle size distribution evolution while using realistic values of elastic bulk properties for the sand grains. Roughness is then a model refinement that may result in simpler, more objective DEM calibrations.     

考虑砂土颗粒破碎的圆孔扩张半解析分析

圆孔扩张理论在岩土工程中应用较广。采用修正的临界状态模型来描述砂土三种不同压缩变形机制:颗粒重分布,颗粒破碎和拟弹性变形,结合空间滑动面(SMP)屈服准则和不相关联流动法则,通过编写Matlab微分方程求解程序,获得考虑砂土颗粒破碎的圆孔(柱孔和球孔)扩张问题的半解析解答。以加拿大Quebec砂土为例,将该圆孔扩张解答结果与基于常规临界状态模型的解答进行比较分析,结果表明:颗粒破碎对砂土在高应力及临界状态的应力解答及变形特征影响显著,在采用圆孔扩张理论解释静力触探试验及旁压试验时应考虑颗粒破碎的影响。     

A critical state based viscoplastic model for crushable granular materials

?Experimental studies have confirmed that the critical state of a granular material varies with alteration in granular fabric, particle shape and grain size. On the other hand, granular materials demonstrate significant strain rate dependency in the presence of particle crushing. While the first feature is well explored, the strain rate effects on the crushability of granular material and consequent critical state alteration are less ventured. This study highlights the strain rate dependence of the critical state of crushable granular materials like sand. A rate-dependent model is proposed bridging the macro and microscopic understanding. The model follows a consistent viscoplastic formulation without using any overstress function. The proposed model considers various loading rate effects at different porosities, confinements and pore water drainage conditions. Further, it can predict the strain rate-dependent particle crushing and dilation features that affect the critical state of granular materials. The model has been validated by comparing its responses with both the experimental and discrete particle simulations for drained and undrained triaxial conditions. An implicit stress return integration scheme is devised to enable accurate numerical response from the model. ? Finally, a parametric study is presented that envisages the evolution of critical state due to coupled strain rate and particle crushing effect.     

A bounding surface plasticity model for highly crushable granular materials

A bounding surface plasticity model is presented for crushable rockfills in the framework of the critical state soil mechanics which includes translation of the critical state line due to particle crushing. A translating limiting isotropic compression line is also introduced and incorporated in the model to describe the position and evolution of the bounding surface. A particle breakage index is introduced as a function of stress invariants which controls the translation of the critical state and limiting isotropic compression lines. The performance of the model is demonstrated using the results of experimental tests on different types of rockfill materials conducted under both monotonic and cyclic loading conditions. The study shows the capability of the model in capturing the characteristic features of the behavior of rockfill and other crushable materials such as ballast and coarse gravel under both conventional and complex loading paths.     

饱和砂土液化后流体本构模型研究

利用空心圆柱扭剪仪开展了饱和南京细砂液化后常速率加载试验,考虑了有效固结压力和加载速率对饱和南京细砂液化后流体特性的影响,结果表明:液化后静加载过程中剪应力与孔压比的发展具有明显的二阶段特性,且剪应力与孔压比的发展有着良好的线性相关关系,有效围压及加载速率对二者的关系有着明显影响;据此提出一种符合液化后静加载过程的率相关性及孔压相关性流体本构模型,该模型将构成饱和砂土强度的土颗粒摩阻力与土–水黏滞阻力分别表示为时变型和非时变型剪切稀化流体;最后进行了该模型的验证性试验,并将模型预测结果与其他学者研究成果对比,验证试验及对比结果均表明该模型具有较好的适用性。     

考虑颗粒破碎的堆石体本构模型

对于高堆石坝而言,在高应力场作用下堆石颗粒将明显破碎,从而导致坝体变形率增加,因此,在高坝数值计算中必须考虑颗粒破碎的影响。传统的堆石体本构模型由于不能合理反映颗粒破碎的影响,导致计算变形与实测变形差别较大。在基于岩土破损力学二元介质模型概念的基础上,将堆石体视为结构体和破损带组成的二元介质,建立了颗粒破碎率和破损参数之间的关系,提出了可以考虑颗粒破碎的堆石体本构模型。并通过一个大型三轴剪切试验的算例,说明了该模型的合理性。     

黏土和砂土简单的三维本构模型(英文)

把修正剑桥模型推广为适合于黏土和砂土的简单、统一三维本构模型是基于以下两点提出的 :第一点 ,把由笔者已提出的基于SMP准则的一个变换应力张量用于修正剑桥模型使其简单实现了三维化 ,改进的模型对于黏土实现了从剪切屈服到剪切破坏的统一及临界状态理论同SMP准则的结合 ;第二点 ,为了建立对于黏土和砂土简单、统一的本构模型 ,推导出了一个新的硬化参数 ,新硬化参数不仅能描述砂土不同程度的剪胀性 ,对于正常固结黏土又能退化成塑性体积应变 ,所提硬化参数的合理性也被各种路径下分别在三轴压缩和三轴伸长下的试验结果所证实。所提模型的计算参数仅为 5个常规试验参数 ,易于确定     

考虑颗粒破碎影响的珊瑚砂临界状态与本构模型研究

临界状态是土力学中一个非常重要的概念,是许多土体本构模型建立的基础。对于珊瑚砂,由于显著的颗粒破碎,其变形特性和临界状态值得深入探讨。通过不同密度和不同围压组合的一系列三轴试验,研究了颗粒破碎对土体变形特性和临界状态的影响,建立了考虑颗粒破碎影响的珊瑚砂临界状态数学表达式,并将其引入砂土状态相关剪胀方程,提出了考虑颗粒破碎影响的珊瑚砂状态相关剪胀方程和本构模型。该模型仅采用一套模型参数就能较好地反映珊瑚砂在不同密度、不同围压条件下的应力变形特性,并且可以考虑珊瑚砂的颗粒破碎特性。     

砂雨法制备砂土地基模型控制要素试验研究

为掌握砂雨法制备砂土地基模型相对密实度D_r的主要影响因素,设计加工了包括点式和线式出砂头的成套砂雨装置,讨论了D_r随落距、出砂孔尺寸、出砂头总流量、移动速度的变化规律,分析了D_r空间均匀性分布特征。试验表明：①随落距增大,D_r随之提高,但增速逐渐减缓,可将D_r相对增速K介于±3%所对应落距范围内的D_r均值为稳定相对密实度;②稳定相对密实度与出砂孔尺寸呈线性负相关关系,同一流量下随出砂头移动速度表现出非线性变化特性,出砂头总流量对稳定相对密实度影响不明显;③无论是在水平层面还是沿深度的不同位置,模型箱中部附近的D_r分布均匀性都优于四边和四角处;④一线点式出砂头所制备的砂土D_r与目标值误差较筛孔点式小,具有较好的可控性。     

A constitutive model for evaluation of mechanical behavior of fiber-reinforced cemented sand

The aim of this study is to present a constitutive model for prediction of the mechanical behavior of fiber-reinforced cemented sand. For this purpose, a generalized plasticity constitutive model of sandy soil is selected and the parameters of the model are determined for three types of sandy soils using the results of triaxial tests. Next, the proposed model is developed using the existing models based on the physico-mechanical characteristics of fiber-reinforced cemented sand. The elastic parameters, flow rule and hardening law of the base model are modified for fiber-reinforced cemented sand. To verify the proposed model, the predicted results are compared with those of triaxial tests performed on fiber-reinforced cemented sand. Finally, the efficiency of the proposed model is studied at different confining pressures, and cement and fiber contents.     

Constitutive model for monotonic and cyclic responses of loosely cemented sand formations

This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands. An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict the mechanical behaviour of soft sandstone under monotonic and cyclic loadings. In this model, the loading surface always passes through the current stress state regardless of the type of loading. Destruction of the cementation bonds by plastic deformation in the model is considered as the primary mechanism responsible for the mechanical degradation of loosely cemented sands/weak rock. To model cyclic response, the unloading plastic and elastic moduli are formulated based on the loading/reloading plastic and elastic moduli. The proposed model was implemented in FLAC2D and evaluated against laboratory triaxial tests under monotonic and cyclic loadings, and the model results agreed well with the experimental observations. For cyclic tests, hysteresis loops are captured with reasonable accuracy.     

基于统一硬化参数的砂土临界状态本构模型

在临界状态理论的基础上,通过引入状态参数来调整硬化参数和剪胀方程,从而更好地反映砂土的剪胀、剪缩特性以及砂土对密度、有效主应力的双重依赖性,通过引入修正屈服函数的参数,能更好地描述砂土的塑性变形特性。同时,模型采用砂土等向固结线和临界状态线在孔隙比与有效应力幂函数平面内呈线性关系,并且只用一组材料参数即可描述在较大密度和较大有效应力范围内砂土的应力应变响应。通过模拟结果与试验结果对比,验证了模型的有效性。     

砂土液化大变形的弹塑性循环本构模型

循环剪切过程中饱和砂土的3个体积应变分量(有效球应力变化引起的体变、剪切引起的可逆性体变和不可逆性体变)的变化规律决定了液化后剪应变的发展。基于上述机理、对剪切引起的可逆性和不可逆性体变的数学描述、体积相容性条件以及边界面本构理论框架,建立了一个可描述饱和砂土液化后大变形的弹塑性循环本构模型。通过对饱和砂土排水和不排水循环扭剪试验结果的模拟表明,该模型不仅可以合理地模拟饱和砂土循环加载条件下从液化前到液化后、从小剪应变到大剪应变的变形发展过程,而且可以合理地模拟饱和砂土液化后再固结大体变的累积特性。本文的研究为定量描述砂土液化后大变形提供了一条合理而有效的途径。     

考虑颗粒破碎的动力UH模型

在已发展的考虑颗粒破碎效应的静力UH本构模型基础上,采用变换应力方法,将原模型拓展为适用于无黏性土的动力统一硬化模型（DUH模型）。具体做法：①针对静力模型中屈服面形状随当前应力状态变化而改变的现状,采取含有 p 变换的变换应力方法,将原真实应力空间中变化的屈服面变换为变换应力空间中的椭圆屈服面;②采用旋转硬化规则,在变换应力空间中椭圆的位置以及形状由旋转轴来确定,并建立了一并考虑等向硬化以及旋转硬化的混合硬化参数;③在变换应力空间中,参考屈服面与当前屈服面之间通过应力比参数 R 来建立联系。模型参数较少。通过3种不同岩土材料在高围压下的单调以及循环加载试验结果的比较,验证了所提DUH模型的合理性。     

冻结砂土三轴试验中颗粒破碎研究

压力作用下颗粒发生破碎是引起砂土力学特性变化的重要因素之一,冻结砂土也是如此。对冻结砂土进行了不同温度和围压下的三轴剪切试验,并筛分得到三轴试验前后的颗粒大小分布曲线。通过引入Hardin定义的颗粒破碎率Br,分析了围压与颗粒破碎的关系及颗粒破碎对冻土抗剪强度的影响。结果表明:在温度为-0.5℃,-1℃,-2℃,-5℃和围压为0.5,2,5,10 MPa的条件下,三轴剪切过程中会产生较为可观的颗粒破碎;颗粒破碎率Br随围压增大,到达一定围压后Br不再随着围压的增大发生明显变化,即存在一个颗粒不再发生明显破碎的临界围压σr。结合前人研究发现,-5℃下一般工程关心的围压范围内压融对冻土力学特性没有显著影响,而颗粒破碎起控制性作用。分析表明:-5℃条件下在不同的围压范围颗粒破碎对抗剪强度具有不同的影响。试验所采用的围压范围内,随着围压的增大,颗粒破碎率增大使得冻土的抗剪强度降低;破碎率达到极限以后,由于破碎的颗粒重排列又导致抗剪强度有所提高。     

沙盘模型在建设领域的功用初探

从具体实例分析入手,阐释和强调沙盘模型的深度利用,以期引起各界的共识和重视,提高沙盘模型的整体制作水平,达到指导实物建筑,助推总体规划的提升,最大限度地发挥土地和其他物质资源的利用。     

砂性土中桩基模型试验研究

根据在砂性土中的模型试验，介绍了其试验模型和试验方法，对桩筏基础的荷载-沉降和桩轴力特性进行了分析、研究，并得出一些基本的规律，为进一步的研究提供参考依据。     

高压气藏地层三维出砂预测模型及方法

根据有效应力定律和气体非线性达西定律,建立了应力场和压力场相互作用的三维弹塑性高压气藏地层出砂模型,并运用弹塑性有限元方法进行求解。根据编制的计算程序,研究了射孔井筒附近有效塑性应变、压力和流速的分布规律:最大有效塑性应变位置是在射孔起始部位的上下端;稳定后的压力由远至井筒呈不规则漏斗状,并且气体流速急剧增加,最大值位置是射孔底部。高压气藏地层出砂是由受压力影响的有效应力场控制的,而由于气体的压力敏感性造成的流速急剧增加是出砂的重要诱因。地层压力越高临界生产压力越低,因而越不易出砂。对于高压气藏地层,尽量稳定井底生产压力、保持合理的生产压差、维持较高的地层压力、采用合理的气井工作制度,有助于减小出砂量。     

砂土的剪胀理论及其本构模型的发展

经典的应力剪胀理论被广泛应用于土体弹塑性本构模型的建立,较好地模拟了黏性土的变形特性。最新的研究表明:由于应力剪胀理论中忽略了土体内部状态参量对剪胀的影响,而使得该理论应用于砂土时发生了困难。近来一些学者纷纷将土的密度作为变量引入剪胀方程,提出了状态相关剪胀理论,成功地模拟了砂土的各种变形特性。本文就砂土的状态及其描述、剪胀理论的发展、砂土本构模拟等方面进行介绍。