Genesis And Engineering Geological Characteristics Of Deep Gravel Layers In Jiuxiang Area,Hanyuan,Sichuan Province

Quaternary developed in Jiuxiang area,Hanyuan,Sichuan Province.According to a large number of drilling data,the Quaternary sediments in this area are mixed with a layer of gravel with a grain size of 2⁶0mm as the main sediment.The gravel layers are widely distributed and thick,and their formation causes are complex.The gravel layers of different origins are overlapped each other and have complex structural characteristics,which have caused the construction of the project difficulties.With the gradual development of infrastructure construction in Western China,the geological environment in this region has become a major factor that must be considered in human engineering construction.Therefore,it is particularly important to grasp the genesis and engineering properties of the deep gravel layer in this area.In this paper,through field investigation and laboratory tests,the grain composition,grain size characteristics,sedimentary characteristics,sporopollen composition,physical properties,permeability characteristics and strength characteristics of deep gravel beds in Jiuxiang area are analyzed,and the following conclusions are drawn:?1?The grain composition of deep gravel layer in Jiuxiang area is mainly composed of coarse grains with diameter of 2-60 mm,which are mixed with giant grains.The content of giant grains in gravel layer varies with elevation at different locations.There are many kinds of particle size-frequency curves with particle size less than 2 mm,including single-peak,double-peak and wide-state curves.In terms of particle size parameters smaller than 2 mm,the gravel layer in the study area is obviously different from the typical glacial sediments,but close to the loess,river and lake sediments.?2?Sedimentary characteristics are mainly gravel and breccia in natural sections,which are usually mixed with stones;the content of gravel is different in different locations,and the degree of enrichment is different;there are obvious layers in gravel layers,or muddy interlayers developed;gravel layers are also found to be overhead;from the point of view of roundness,the grinding roundness of high-rise gravel and boulder is poor,and the compactness changes greatly.The compactness of gravel layers in different valleys ranges from loose to compact.At lower elevations,the particles are mostly rounded better,and there are also gravel layers with poor roundness as a whole.Almost all gravel layers are shaly cemented with little calcareous cementation.According to drilling data,there are many kinds of lithology in gravel beds,dolomite and limestone are the main ones.?3?In terms of sporopollen species,the Jiuxiang gravel layer is mainly divided into three sporopollen zones?I,II,III?.The average sporopollen concentration in zone I is the lowest,mainly woody plant pollen,among which the cold-tolerant plants have more sporopollen.In zone II,the average sporopollen concentration was the highest,the pollen content of woody plants decreased,the pollen content of herbaceous plants increased,and the pollen content of warm and moist plants increased.In zone III,herbaceous pollen is the main pollen,while woody pollen decreases and warm-wet plant pollen increases.?4?Generally speaking,during the interglacial period,the temperature rises rapidly,glaciers melt,and melted water carries debris accumulate.In the later period,the temperature in the study area is relatively stable,and the melted water carries debris deposition continuously.Because the gravel layer has undergone a long period of evolution,during which it experienced heavy rainfall,rainwater gathering triggered floods,forming multi-stage sediments of complex origin.The compactness of gravel layer soil is generally slightly compact to very compact state by dynamic penetration test,and the ultimate bearing capacity of soil is high,up to 1490 Kpa;the bearing capacity of coarse and giant particles is 500-950 Kpa in medium-compact to compact state.Indoor permeability experiments show that KT is1.04×10^-2 and 3.4×10^-3.Combining with the results of in-situ permeability experiments,it is concluded that the permeability coefficient of gravel layer in the study area is generally floating within 10^-2 cm/s¹0^-3 cm/s,with medium permeability.The parameters of shear strength of gravel layer are:C=96.4Kpa,?=27 degrees,and the stress-strain curve is of softening type.