DEM Study On The Creep Behavior Of Cemented Methane Hydrate-Bearing Sand

Methane hydrate,which is seen as a new type of green energy with great potential,has gained great attention in the research field.Methane hydrate is mainly distributed in the deep sea and permafrost areas,and the existence of methane hydrate may lead to significant changes in the soil properties.Researches have shown that some submarine landslides may be related to the dissociation or creep of methane hydrate,therefore,it is of great significance to study the mechanical properties of methane hydrate-bearing sand.For the energy utilization,achieving the long-term and commercial production of methane hydrate has been the goal of many countries,for which more research is needed of the time-dependent properties for methane hydrate-bearing sediments.In this paper,the creep behavior of cemented methane hydrate-bearing sand is simulated using the discrete element method(DEM).In the DEM model,soil particles are simplified as clumps made of 2 disks with unit thickness,and methane hydrate is simplified as contact bonds between clumps.In this paper,the methods of simulating both the time-dependent and time-independent behavior of methane hydrate-bearing sand are proposed.In the biaxial test simulation,the method that the index of bonding ratio is used to reflect the effect of methane hydrate saturation is proposed;and in the creep test simulation,the molecular mechanism for the creep of methane hydrate creep is analyzed by comparing with other materials including ice and frozen sand,the time-dependent bond strength degradation equation is proposed.Based on this,the time-dependent bond strength method is used to simulated the creep test of methane hydrate-bearing sand.The reliability of the DEM model and the simulation method was proved by comparing with the laboratory test.In the macro aspect,the different creep stages of methane hydrate-bearing sand are investigated,and the factors affecting the creep rate is studied.The results show that the creep of methane hydrate-bearing sand mainly exhibits two stages including primary creep and tertiary creep,and creep rupture finally occurs in the sample.For the methane hydrate-bearing sand,the higher deviator stress level,the higher temperature,the higher hydrate saturation and the lower confining pressure will lead to the higher creep rate.In the micro aspect,the shear band formation,bond breakage and the force chain evolution are investigated.According to the result,an “X”-shaped shear band is formed inside the sample during creep.Compared with outside the shear band,the sample inside the shear band has larger bond breakage ratio,larger particle rotation angle,larger local porosity and smaller local coordination number.During the creep,the number of bond breakage increases approximately linearly with the increase of creep strain,with the bond mainly broken in shear.In the early stage of creep,force chains are distributed vertically in the sample,and with the development of creep strain,the large number of bond breakage inside the shear band makes the particles lose restraint and slid e and rotate freely,which further leads to the bending of the force chain inside the shear band.A large proportion of force chain bending will eventually lead to the occurrence of creep rupture.