Study On Dry-wet Cycle Deterioration Characteristics Of Mudstone In Badong Formation Under Unloading Effect

With the continuous improvement of the national economy and national strength,China’s infrastructure has been vigorously developed.Since many high-speed railways and expressways are built on mudstone formations,under the action of unloading-dry-wet cycle,the deterioration of the structural properties of mudstone makes the cuttings and tunnel bases of some high-speed railways have significant up-arch deformation,which has a great impact on the safe operation of driving,so it is urgent to study the deterioration mechanism of dry-wet cycles considering the unloading effect on mudstone foundations.Taking the mudstone of the Badong Formation of a high-speed railway in Enshibadong County,Hubei Province as the research object,this paper carried out the dry-wet cycle test,nuclear magnetic resonance test and XRD mineral analysis of mudstone under different unloading conditions,analyzed the expansion deformation and mechanical deterioration characteristics of mudstone under different unloading conditions,and revealed the evolution of internal pores and mineral components of mudstone under different unloading conditions.Based on the macroscopic deterioration characteristics of mudstone,a mudstone expansion constitutive model and strength deterioration model considering the unloading damage and the number of dry and wet cycles were established,and the micro-expansion deformation of the mudstone subgrade in the study area was studied by finite difference method.The following conclusions are drawn:(1)Through the 6th lateral limit dry and wet cycle test of mudstone,it is found that with the increase of the number of dry and wet cycles,the expansion rate and porosity of mudstone first increase and then decrease,and reach a maximum value in the fourth cycle.The mineral composition and content of mudstone do not change much with the number of dry and wet cycles,but the crystal structure of the internal mineral composition deteriorates,the cementation force between crystals weakens,and the crystal spacing increases,resulting in the peak strength and elastic modulus of mudstone decreasing with the increase of the number of dry and wet cycles of mudstone.(2)The dry and wet cycle test of mudstone under different initial unloading damage degrees shows that the expansion rate of mudstone under stress loading is greater than that in natural state,and the expansion rate of mudstone decreases with the increase of unloading capacity.The increase of unloading capacity and the number of dry and wet cycles will aggravate the internal damage degree of mudstone,resulting in a decrease in uniaxial compressive strength,but the porosity of the specimen decreases,and the influence of unloading is greater.(3)The dry and wet cycle test of unloading mudstone under different overlying loads shows that the increase of overlying load and initial unloading will aggravate the damage degree inside the mudstone after dry-wet cycle,resulting in a decrease in uniaxial compressive strength,and the influence of both is the same.With the increase of the number of dry and wet cycles,the porosity of unloaded mudstone gradually decreases without overlying load,while the porosity of unloaded mudstone increases after overlying load is applied.With the increase of overlying load,the small internal pores tended to decrease after the dry and wet cycles of the unloaded mudstone,while the middle pores and large pores showed an increasing trend,and the amplitude of each pore change became more obvious with the increase of the number of dry and wet cycles.(4)Firstly,a mudstone expansion rate-time relationship considering the amount of unloading and the number of dry and wet cycles is established,and its rationality is verified based on the mudstone dry and wet cycle expansion test,and a one-dimensional expansion constitutive model of unloaded mudstone is constructed by combining the modified Huder-Amberg model and the load expansion test results,and a three-dimensional expansion constitutive model of unloaded mudstone is established by combining Hooke’s law and Kinnick conditions.Based on the uniaxial compressive strength of mudstone after dry-wet cycle test,a mudstone UCS strength deterioration model considering the number of dry-wet cycles,unloading capacity and overlying load is established,and the verification shows that the model is suitable for characterizing the degree of dry-wet cycle strength deterioration of unloaded mudstone.(5)According to the tunnel engineering in the study area,the finite difference calculation model of tunnel subgrade was established by combining the modified expansion constitutive model of mudstone,and the deformation of tunnel subgrade under different dry and wet cycle times was carried out.By comparing the calculation results of the model considering the expansion of mudstone with the model without considering the expansion,it is found that with the increase of the number of dry and wet cycles,the area of the low-stress area around the tunnel without expansion is less than that of the expansion model.The expansion area of the expansion model increased,while the deformation of the non-expansion model continued to decrease.The expansion model will produce a new plastic zone after each dry and wet cycle,and the number of new plastic units decreases with the increase of the number of dry and wet cycles,while the non-expansion model has a large number of irreversible damage to the rock mass after only one cycle,and no new damage units are produced in the subsequent cycles.It is shown that the influence of mudstone expansion behavior cannot be ignored in the analysis of tunnel mudstone subgrade deformation.