Experimental investigation and development of a theoretical model for the vibrofluidisation of dry granular materials

拟议研究项目的目的是根据岩土实验室试验的结果，开发一种理论模型，用于描述砂子等干燥颗粒材料的振动流态化。为了解决以后复杂的岩土边值问题，将该理论模型转化为适合于有限元程序实现的材料模型。在这种情况下，振动流态化是指颗粒材料在受到大加速度振动的情况下出现类似流体的性质。例如，类似流体的性质是剪切强度降低或完全缺乏、粘性应力分量和流体静应力状态或对流运动的演变。虽然这一现象的存在在岩土工程界早已为人所知，但对其解释的研究却很少，方法也不完善。由于最近的研究结果表明，振动流态化与振动打桩和振动压实等施工技术具有相关性，因此可以清楚地说明进一步研究的必要性。该研究项目集中于颗粒材料在振动作用下的变形行为和抗剪强度的降低。后者归因于颗粒摆动引起的碰撞应力的出现、惯性力引起的有效应力的降低和颗粒稳定运动引起的动摩擦的减少。实验工作将包括研究压实行为的固结试验和研究振动时剪切强度的三轴试验。为此，传统的电池将安装在振动台上并进行改装，以便可以气动施加载荷。作为验证材料模型的一种手段，将进行模拟砂土上基础在振动下的沉降的模型试验。建立该材料模型将是理论工作的主题，并将其应用到有限元程序中，并利用所执行的实验室试验的结果进行验证。

The aim of the proposed research project is the development of a theoretical model for describing the vibrofluidisation of dry granular materials like sand, based on the results of geotechnical laboratory tests. This theoretical model is then supposed to be transferred into a material model suitable for implementation into a finite element program in order to solve later complex geotechnical boundary value problems.In this context, vibrofluidisation means the emergence of fluid-like properties within a granular material subject to vibrations with large acceleration amplitudes. Fluid-like properties are, for example, a reduced shear strength or complete lack thereof, viscous stress components and hydrostatic stress states or the evolution of convective motions. Although the existence of this phenomenon has long been known in the world of geotechnical engineering, only few studies and incomplete approaches toward it’s explanation exist. Since recent findings suggest the relevance vibrofluidisation has for construction techniques like vibratory pile driving and vibrocompaction, a need for further research can clearly be stated. The research project focusses on the deformation behavior and shear strength reduction of granular materials, while being subjected to vibrations. The latter is attributed to the emergence of a collisional stress caused by oscillating grains, the reduction of effective stresses due to inertial forces and the reduction of mobilized friction due to steady grain movements.The experimental work will consist of oedometric tests for investigating the compactive behavior, as well as triaxial tests for investigating the shear strength during vibration. For this purpose, conventional cells will be mounted on shaking tables and modified, so that the loading can be applied pneumatically. Model tests simulating the settlements of a foundation on sand under vibrations will be performed as a means of validating the material model.Developing this material model will be the subject of the theoretical work, as well as the implementation into a finite element program and the validation with the results of the executed laboratory tests.