Experimental Study On Swelling Characteristics And Swelling Constitutive Model Of Red Sandstone

Taking Zhuzhou red sandstone as the research object,this paper comprehensively reveals the microstructure,mineral composition and swelling characteristics of red sandstone and influencing factors of red sandstone swelling through scanning electron microscopy experiment,X-ray diffraction experiment,free swelling ratio experiment,swelling ratio experiment under lateral restraint without load,swelling stress experiment and swelling ratio experiment under lateral restraint loading,and then establishes red sandstone swelling constitutive model.The main contents and conclusions of this paper are as follows:(1)Through scanning electron microscopy(SEM)experiment,the micro-structure image of red sandstone was obtained,and then its microstructure characteristics were analyzed.The element distribution in the microarea and at a certain point of red sandstone were measured in order to determine the main element composition of red sandstone.Then,the mineral composition of red sandstone was determined by X-ray diffraction experiment.The results show that there are many microfissures and pores in red sandstone,and their structures are mainly granular and flaky.The former has obvious characteristics and complete structure,while the latter is uneven and fragmented.The main elements of red sandstone are O,Mg,Al,Si,K,Ca and Fe,and some of them contain Na.The minerals of red sandstone include quartz,calcite and kaolin.Stone,feldspar,chlorite,mica,montmorillonite and illite.Based on the characteristics of these microstructures,elements and mineral compositions,the main factors affecting the swelling of red sandstone are studied.(2)Through free swelling ratio experiments,lateral constraint unloaded swelling ratio experiments and swelling stress experiments,the axial strain-time curve of free swelling ratio experiments,the axial strain-time curve of lateral constraint swelling ratio experiments without load and stress-time curve of lateral constraint swelling stress experiments are obtained.It is found that the swelling exhibits rheological and creep characteristics,and the process can be roughly divided into three stages,namely,isovelocity swelling stage,deceleration swelling stage and stable swelling stage.At the beginning of swelling,the swelling ratio reaches the maximum,and it gradually decreases over time.Eventually,the swelling deformation tends to be stable and no longer increases,and the swelling process shows discontinuous changes.Before reaching stabilization,the deformation grows sometimes fast and sometimes slow,and its deformation presents obvious attenuation characteristics.Then,the lateral restraint swelling ratio experiments under loading of Zhuzhou red sandstone sample was conducted by high pressure soil consolidation apparatus.Then,the axial strain-time curves of the lateral constraint swelling ratio experiments under loading and the axial stress-strain relation curves of lateral constraint swelling experiments under loading were obtained.Under the first grade load,the axial straintime curve of sample A of loading group presents the stages of constant swelling,deceleration swelling and stable swelling.Moreover,axial strain reaches maximun at the initial stage,then it decreases gradually and finally remains constant.In addition,the axial strain of the progressive loading group decreases with the applying of each load,and there is instantaneous deformation.The axial strain of each unloading group increases with the applying of each load,and there is instantaneous deformation.(3)From the experimental point of view: According to the experiment data of the lateral constraint swelling ratio experiment under loading,the data are fitted by using the software of matlab,and the axial strain-time relationship suitable for the lateral constraint swelling ratio experiment under loading of red sandstone in this paper is obtained.Subsequently,its validity is verified and compared with the same type of swelling constitutive model.The results prove the superiority of the relationship.Then,after analysis,the well-known Huder-Amberg swelling model is selected,and the onedimensional swelling constitutive model considering time effect is established by paralleling the axial strain-time relationship.By using the generalized Hooke's law introducing the volume swelling ratio and the first stress invariant into the model,the one-dimensional model is successfully extended to the three-dimensional swelling constitutive model.The model is no longer constrained by side restraints and has a wider application range.This is the swelling constitutive model for the red sandstone proposed in this paper.The model is not only applicable to the experiment data of stepby-step loading specimen A of lateral constraint swelling exeperiment of red sandstone,but also to the experiment data of step-by-step unloading specimen D,which can better simulate the swelling characteristics of swelling rock under various working conditions.