Combined studies of creep and aging effects in granular materials

土的力学特性是随时间和速率变化的。时间和速率依赖性通常被称为蠕变（在恒定有效应力下应变的时间依赖性发展）和应力-应变行为依赖于应变速率。恒定应变下应力随时间的变化，即所谓的松弛，是应变速率为零时速率依赖的一种特殊情况。蠕变、速率依赖和松弛是土壤粘度的一种表现。土壤时间依赖性的一个重要现象是老化。它描述了在恒定有效应力下，土的力学和水力特性，如压缩性、抗剪强度和水导率随时间的变化。在粗粒土中，在岩土工程设计中考虑土体随时间和速率变化的特性的必要性近年来变得很明显。大多数情况下，时间效应不显著，可以忽略不计。但在某些情况下，它们是不容忽视的。颗粒材料在时间和速率效应方面表现出非常丰富的行为。尽管潜在的微观力学机制已经广为人知，但目前还没有数学模型能够基于颗粒特性（晶粒形状、晶粒尺寸分布、矿物学）和状态变量（应力、密度）来预测蠕变、速率依赖性和老化。因此，本研究计划的最高目标是为建立模拟干燥和饱和颗粒土随时间变化行为的本构模型提供实验和数学基础。这包括以下目标：1 .建立颗粒材料在各向同性压缩和圆柱形剪切过程中蠕变、老化和速率依赖性的综合研究方法。结合实验方法，分析了颗粒特性和状态变量对蠕变、老化和速率依赖性的影响。单调加载对蠕变和老化影响的试验研究。发展本构方程，以模拟颗粒材料的时间依赖性行为，基于文献中的两个现有模型，并将其实现到一个元素测试软件中，以模拟实验室实验结果。制定确定本构模型参数的程序，并通过实验室实验对模型进行验证。

The mechanical behavior of soil is time- and rate-dependent. Time- and rate-dependency is usually referred to as creep (time-dependent development of strains under constant effective stress) and dependency of the stress-strain-behavior from the strain-rate. The change of stress with time under constant strain - so called relaxation - is a special case of rate dependency at strain rate zero. Creep, rate-dependency and relaxation are an expression of the viscosity of the soil. An important phenomenon of time dependency of soils is ageing. It describes the change of mechanical and hydraulic properties, like compressibility, shear strength and hydraulic conductivity of the soil with time under constant effective stress. In coarse grained soils the necessity to account for the time and rate-dependent behavior of the soil body in geotechnical designs has just in recent years become obvious. Mostly the time effects are insignificant and can be neglected. But in some cases they cannot be disregarded.Granular Materials show very abundant behaviors regarding the time- and rate-effects. Despite the fact that the underlying micromechanical mechanisms are widely known, no mathematical model exists that allows the prediction of creep, rate-dependency and ageing based on granulometric properties (grain shape, grain size distribution, mineralogy) and state variables (stress, density). Therefore the superordinate goal of this research proposal is to create the experimental and mathematical basis for the development of a constitutive model for the simulation of time-dependent behavior of dry and saturated granular soils. This includes the following objectives:1. Development of experimental laboratory methods for the combined study of creep, ageing and rate-dependency of granular materials during isotropic compression and cylindrical shearing.2. Combined analysis of the influence of granulometric properties and state variables on creep, ageing and rate-dependency with the developed experimental methods.3. Experimental study on the influence of monotonic loading on creep and ageing.4. Development of constitutive equations to simulate the time-dependent behavior of granular materials based on two existing models from literature and implementation into an element test software to simulate the results of laboratory experiments.5. Development of procedures for the determination of the constitutive model parameters and verification of the model by means of laboratory experiments.