| Under the action of natural and human factors for a long time,the earthen sites have formed weathering diseases of different degrees.Rainfall is one of the most important factors leading to soil erosion.Rainfall erosion is affected by rainfall intensity,rainfall duration,site surface slope,site soil properties and other factors.Therefore,different weathering prevention measures should be selected according to the difference of the environment.In recent years,weathering prevention technology of earthen sites has achieved remarkable results.However,due to the unclear mechanism of site weathering disease and the unknown failure mechanism of reinforcement materials,the environmental adaptability of materials has not been determined,which seriously restricts the further research and application of weathering prevention technology on the surface of earthen sites.Based on the current situation of earthen site weathering technology,literature review,environmental data analysis,reinforcement mechanism and failure mechanism research,this study simulated rainfall experiment with the help of multi-field coupling laboratory,and evaluated the environmental adaptability of PS,sacrificial layer and soft cover.This study also proposes the applicability range of different measures.The main research conclusions are as follows:(1)Through the analysis and investigation of the distribution of soil environment at earthen sites,the distribution characteristics of soil environment at the earthen sites were found out.Combined with temperature,rainfall,evaporation and other indicators,the climate and environment of the earthen sites were classified in detail,and the climate and environment of the earthen sites were divided into extreme arid,arid,semiarid,semi-humid and humid regions.In this study,the boundary conditions of temperature and rainfall were defined.(2)Experimental study on soil weathering shows that rainfall has the greatest influence on site weathering.The higher the clay content is,the stronger the rainerosion resistance is.In the absence of other factors' intervention,the influence of temperature on the weathering of earthen sites is not obvious.In the experiment of high and low temperature alternating cycle,the volume expansion rate of soil is less than1‰.The compressive strength,tensile strength,hardness,wave velocity and other parameters of soil have no obvious changes.However,the dry and wet cycle showed obvious differences,and the expansion and contraction rates of different soils ranged from 1.7-7.0%.The changes mainly occurred in the first two dry and wet cycles.(3)The earthen site models with different slopes show that 45° is the critical breaking point of rainfall erosion in erosion experiments with different rainfall intensities.The erosion degree of 0-45° slope increases with the increase of slope.The erosion degree of 45-90° slope decreases with the increase of slope.For the same rainfall,the smaller the conditional slope is,the shorter the time of runoff generation is,and the erosion at the foot of slope is the most serious.Other conditions being the same,the greater the rainfall intensity,the shorter the stabilization time of runoff,flow velocity,sediment yield and particle size.At the initial stage of flow generation,the grain size of sand production is mainly clayey and silt.As the sediment yield gradually stabilized,the clay and silt particles gradually decreased,and the gravel increased until it was close to the original particle size composition.The smaller the slope,the higher the content of clay and silt in the initial stage.The larger the slope is,the faster the particle size of sand will approach the original particle size composition.The degree of weathering is closely related to rainfall penetration depth and slope.For the same rainfall,the smaller the slope,the deeper the penetration depth and the deeper the weathering thickness.(4)The chemical reinforcement materials have a good reinforcement effect on the shallow surface of the earthen sites.The adhesion effect of reinforcement material not only improves the strength of soil,but also restricts the activity of clay minerals.Thus,restrains the expansion and contraction capacity of reinforcement layer.The effect of water on soil expansion and contraction is much greater than that of temperature.Differential shrinkage caused by dry and wet cycle is the main reason for the failure of chemically reinforced layer.According to rainfall simulation experiment,PS material has good applicability in extreme arid area.In arid areas,PS materials can be used when the site slope is greater than 75°.PS materials are also suitable when the site is preserved indoors or is not affected by capillary water.(5)The sacrificial layer containing plant fiber has the characteristics of high strength and good ductility.At the same time,compared with the unreinforced site,the plant fiber can improve the rain erosion resistance(more than 60%),and has a good compatibility with the main layer interface.Rainfall simulation experiments show that the sacrificial layer has better durability under the rainfall intensity of 50mm/h,and is less affected by the slope.Taking the size effect into account,the rainfall intensity was reduced by 50% as the safety value,and the 25mm/h rainfall intensity was taken as the boundary condition.In this case,the sacrificial layer can have better durability in the extremely arid,arid and semi-arid regions,and can be used on a small slope.(6)In the case of rainfall,soft mulching can prolong the initial runoff generation time and significantly reduce the overland velocity and sediment yield.In the area of heavy rainfall,the sites covered by soft mulching have very good resistance to rain erosion.And rainfall erosion resistance is related to vegetation density.The durability of soft mulching depends on the environmental adaptability of the selected vegetation.Based on the NDVI index,soft mulching can be used in areas with NDVI greater than0.65(summer),including humid and semi-humid areas.In moist areas,soft mulching can cover all year round.However,in the semi-humid areas,NDVI will decrease in winter and recover to more than 0.65 in summer.From the above conclusions,the thickness of weathered layer of earthen sites in arid and extremely arid areas is thin.Currently,it is suitable for the shallow reinforcement depth of chemical reinforcement.With the increase of rainfall,the weathering layer in semi-arid and semi-humid areas gradually thickens,so it is necessary to reduce the weathering rate of the weathering layer,and the sacrificial layer is an ideal reinforcement method.Rainfall intensity and rainfall frequency are both large in humid areas,and slope erosion is the most important weathering mode.In this case,soft mulching can effectively improve the rain-erosion resistance.The above results revealed the failure mechanism of chemical reinforcement,sacrificial layer and soft cover,and proposed the applicable scope of weathering prevention measures of PS material,sacrificial layer and soft cover through rainfall simulation experiments,which provided technical support for the research and engineering practice of weathering prevention protection of earthen ruins. |
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