Study On Engineering Geological Properties Of The Ming Rammed Earth Wall Under Irrigation Condition

‘Time is silent,only cultural heritage can tell about the true story',this sentence fully explains the necessity and urgency of strengthening the protection of cultural heritage.The Great Wall in Yongdeng County is an important part of the defense system of the Ming Great Wall.It has a long time span and a wide range of construction.Therefore,the Great Wall in this section has a high value of witnessing the historical productivity and the evolution of topography.However,with the expansion of contemporary agricultural production,the boundary of farmland is gradually approaching the Great Wall.On the one hand,the period of farmland irrigation is long and the irrigation amount is large,so the irrigation water frequently acts on the root of the wall for a long time;on the other hand,the northwest region in the summer rain heat over the same period.The above reasons induce significant wetting-drying cycles,which leads to the deterioration of soil at the bottom of the wall,resulting in the development of large-scale erosion disease,and even serious wall collapse.Taking the Wangjiawan and Wulidun sections of the Ming Great Wall in Yongdeng County as the research objects,this study explored the adverse effects of farmland irrigation on the engineering geological properties of Ming Great Wall by means of field investigation,indoor experiments and numerical simulation.By investigating the existing environment and preservation status of the site,and comparing the development scale of the erosion disease on both sides of the wall,this paper explored the correlation between irrigation and the scale of the erosion disease.Through the indoor test,the changes of physical properties,hydraulic properties,mechanical properties and microstructure of the site soil with the increase of dry-wet cycle times were explored.Combined with the obtained parameters and FLAC^3Dsoftware for numerical simulation,the stress and strain cloud of the site wall was obtained,and the influence of irrigation on the Great wall was further explained.The main conclusions are as follows:(1)The Wangjiawan and Wulidun sections of the Ming Great Wall in Yongdeng County are close to farmland.Irrigation mainly acts on the west facade of the wall(the facade of the wall near the farmland).Therefore,the scale of erosion on the west facade of the wall is significantly larger than that on the east facade,and other diseases such as wall collapse are more serious.(2)Both the wetting-drying cycles and the interface of ramming layer will affect the physical,mechanical and hydraulic properties of samples.With the increase of wetting-drying cycles,the degradation of the bottom surface of the sample is more obvious.Meanwhile,the longitudinal and transverse cracks appear on the side surface,and the mass loss rate also increases gradually;the dry density of the sample first decreases and then remains stable.The disintegration coefficient is mainly affected by the number of wetting-drying cycles and the location of the test points.With the increase of dry-wetting cycles,the surface hardness of the lower part of the sample decreases gradually,and the compressional velocity,compressive strength,shear strength and shear strength index also show a decreasing trend.(3)In the process of upward migration of capillary water,the salt also migrates,so the salt content in the bottom of the sample decreases,while the salt accumulates in the upper part of the sample.The loss of cement between soil particles at the bottom leads to the increase of soil particle spacing.As a result,the mechanical properties of the soil at the bottom of the wall become worse.Then the local falling blocks,collapse and other diseases gradually develop,and the bottom cavity forms and threatens the stability of the wall.(4)There are medium porosity,small porosity and micro porosity in the soil,and the existence of compaction layer interface increases the number of macro porosity,medium porosity and small porosity.After the wetting-drying cycles,both the number of pores and pore diameter increase;the shape of soil particles gradually changes from irregular shape with many uneven edges to ellipsoid shape with smooth edges,and the contact mode of particles gradually changes from surface contact to point contact.(5)Irrigation and irrigation-induced erosion will affect the stress-strain distribution of the wall.On the one hand,the wall affected by irrigation forms larger scale of displacement,strain and tensile stress area,which will reduce the stability coefficient of rammed soil layer.