Mechanical Behaviour Of Granular Matter Under Gravity-induced Stress Gradient

Gravity is one of the primary sources of loading in geotechnical engineering,and in many cases the only source.Gravity-induced stress gradient exists widely in the area of terrestrial geotechnical practices,as well as in the area of deep-space planetary engineering which will be aimed at the extraterrestrial base construction and minerals mining.However,it has not been paid sufficient attention until now because of the lack of theoretical framework and corresponding experimental method.This work is performed from the aspects of the theoretical framework,numerical element tests,experimental method & related equipment,and modelling tests to investigate a related foundational scientific issue titled ‘behaviours of granular matter under gravity-induced gradient stress field'.The framework for a kind of stress gradient continuum theory is proposed to account for the gravity-induced stress gradient,which is thoroughly excluded from the classical continuum theory.A concept of representative volume element(RVE)is adopted at first,based on which the heterogeneous and discontinuous granular matter is conceptually equivalent to the stress gradient continuum by using Hill-Mandel law.The balance equation,geometric equation,boundary conditions,and related constitutive relation for the stress gradient continuum are then proposed.Finally,a method for obtaining the constitutive relationship of geomaterials by using the constitutive relationship of RVE is given.In order to investigate the gravity-induced stress gradient related constitutive behaviours of granular matter,a series of numerical triaxial tests for RVEs is performed under the theoretical framework of stress gradient continuum theory,in which only a simple mechanical state is considered that the major principal stress is applied in the direction of gravity.Macroscopically,the gravity-induced stress gradient has a significant influence on the isotropic compression,triaxial shear strength,elastic modulus,internal friction angle,volumetric deformation,and the critical state behaviour.Meanwhile,the effect of gravity on above behaviour shows a strong stress level dependence.Microscopically,the gravityrelated evolution of void cells is essentially caused by the gravity-sensitive particles,which lead to the force chains and micro slip bands show a gravity dependence.Those gravityrelated microscopic characteristics finally result in the macroscopic mechanical responses of granular matter demonstrate a gravity dependence.Further,a series of triaxial tests for RVEs with various element heights and particle sizes is performed under the same mechanical state.The influence of size effect and its origination on the macroscopic behaviour of granular matter,as well as its coupling effect with gravity,is studied.The size effect and its origination appear to have no obvious influence on isotropic compression.However,the shear strength,elastic modulus,internal friction angle,volumetric deformation,and the critical state behaviour all show size effect dependence,and the gravity-induced stress gradient leads to the significant differences between at different originations of size effect.Meanwhile,the above effect of gravity is closely related to the particle size,the bigger size leads to a stronger influence.Above macroscopic mechanical behaviour is essentially caused by both size effect and its origination related evolution of void cell,grain skeleton,and micro shear bands in the microscopic level under the gravitational field.Meanwhile,a geotechnical magnetic-similitude-gravity model testing method(GMMT method)is newly proposed to provide an alternative approach to current gravity simulation methods on the earth,especially to the extraterrestrial gravity simulation methods.The theoretical criteria for achieving a magnetic field to simulate the gravitational field are presented firstly,based on which an exact simulation of the gradient stress field is achieved between the physical models and its corresponding prototype.A kind of geotechnical magnetic-similitude-gravity equipment(GMMT equipment)is then designed and manufactured,which is capable of simulating 0-10 g gravity with a duration of 0-20 min inside a cylindrical experimental space with a size of ?95×160 mm.Accordingly,a kind of magnetically-sensitive granular media(MSG media)is developed and its magnetization & magnetic-similitude-gravity properties are studied.Finally,a series of trap-door experiments under various extraterrestrial gravities is performed,and the effect of gravity-induced stress gradient on the behaviour of granular matter is studied in the model scale.The results show that both the unloading rate for soil pressure and the reduction in soil pressure on the trap-door decrease with the increasing gravity;however,a gravity-independent stable state is finally reached.A theoretical solution for the soil pressure on the trap-door is proposed,and involves counting the effect of gravityinduced stress gradient and stress level on interparticle friction resistances.The predicted normaolized soil pressure demonstrates a good agreement with the experimental results.Meanwhile,the theoretical results show that the effect of gravity-induced stress gradient is significantly larger than the stress level.