Study Of Thermal Deterioration And Detachment Of Superficial Layer On Rammed Earthen Ruins In Arid Area

The centralized and heavy rainfall not only erode the superficial layer of the rammed earthen sites,but also lead to the formation of surface crust on rammed earthen sites in the northwest arid region of China.As is known to all,the detachment of superficial layer is a important deterioration on the earthen sites.Dramatic changes of daily and seasonal temperature differences in large sites will result in significant changes of thermal field and thermal stress field in sites.Furthermore,The significant changes of thermal field and thermal stress field will intensify the process of superficial layer's deterioration and detachment on the rammed earthen sites.It is the thermal deterioration and detachment process of superficial layer on rammed earthen that mentioned aboveField survey,systematic sampling and laboratory tests are conducted to study the thermal deterioration and detachment process in superficial layer on rammed earthen sites taking the Wuwei Ming Great Wall,Gansu,China for example.The field survey find that the sites is kept intact and the superficial layer deterioration is developed widely.laboratory X-ray diffraction,SEM observation and laser particle size analysis are used to research the mineral composition difference,microstructure difference and the particle composition difference between superficial layer and host soil.X-ray diffraction result shows that the mineral composition difference between superficial layer and host soil is very little,and the content of quartz,micas and feldspar exceeds75%.SEM observation shows that the particles in superficial layer are shown as lamelliform and imbricate arrangement,but rough elliptical shape,unordered close arrangement and obvious rammed vestige are shown in host soil.Laser particle size analysis result that host soil is marked by higher sand and lower silt and superficial layer soil is marked by higher silt and lower sand.In addition,the volume distribution curve of superficial layer is single peak and host soil is double peak.Based on the simulated rainfall test on remolded specimens,the superficial crust is formed.Then the thermal deterioration accelerated simulation test was taken to research the variation of superficial layer's thermophysical properties of specimens.The results show that the superficial layer's thermal conductivity coefficient,thermal diffusion coefficient and specific heat of volume are lower after the superficial crust formed.And superficial layer's conductivity coefficient,the thermal diffusion coefficient and specific heat of volume keep relatively stable during the thermal deterioration accelerated simulation process.The test proved that there are obvious thermophysical properties differences between superficial layer and the host soil.meanwhile,The obvious differences keep stable during the thermal deterioration and detachment process.Therefore,rammed earthen sites are similar to double layer structure,and obvious thermal field difference and thermal stress field difference between the superficial layer and the host soil will be generated under the cyclic action of temperature.The differences will lead to fatigue deterioration of contact zone which between the superficial layer and host soil.Furthermore,it will lead to the detachment of superficial layer.Based on the property that there are obvious differences between superficial layer and host soil in rammed earthen sites.In order to semi-quantitative research the superficial layer's deterioration and detachment of rammed sites in arid region,some necessary assumption are used to build two models which are named homogeneous medium crack propagation model and two-layered medium interface fatigue deterioration model.