Combined studies of creep and aging effects in granular materials

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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.