Research On Correlation Between Shear Strength And Cementing Material Of Collapsing Gully

Collapsed gully is the most serious composite soil erosion type in red soil area of South China,the reduction of soli shear strength(SS)is one of major causes to gully collapsing.Shear strength is determined by the form and amount of cementing materials in soil,however there have been few researches about relationship between shear strength of collapsed wall soil and cementing materials fornow.Therefore,this research chose Anxi County in Fujian Province as study area,two pedons with severrly collapsed gullies and another two pedons without gully erosion were collected repectively from Longmen Town and Gande Town.Soil samples from different depth were subjected to basic physicochemical properties,mineralogical and shear resistance characteristics(cohesive force and internal friction angle)analysis.The mineralogical characteristics of typical collapsing gully soils and principal influencing factors to their shear strength were preliminarily revealed.Inadditon red soil,sandy soil and detritus soil developed from typical collapsed gully were collected,which were treated with different sequential removal or addition of cementing maerials.The variation of soil shear strength and effect of cementing material content on soil texture,particle size distribution and fractal dimension was studied and analysed.The results are as follows,(1)Noncollapsible soils had superior physicochemical characteristics to collapsded soil,especially contents of total iron and different forms of iron-oxides existed significant differences.Structural properties and water binding capacity of noncollapsible soils were better than collapsed ones.Furthermore,noncollapsible soils were able to resist shearing due to their greater cohesive force,and the internal friction angle did not differ considerably from that of the collapsed soils.Kaolinite,illite,hydroxy-interlayered vermiculite(HIV)and gibbsite were the dominant clay minerals in both soils.However,in the collapsed soils,kaolinite accounted for more than 85%of the total clay mineral composition,which meant the weathering of collapsed soils were more fast and easily.The cohesive force and internal friction angle of all pedons were significantly correlated with content of Fe-,Al-,Mn-oxides,Ca,Mg and Fe.(2)Concentration of Fed decreased due to its activation across the process of sequential removing free and amorphous oxides.Soil texture,particle size distribution and fractal dimension changed after removal of oxide colloid,relationship between which and shear strength were also affected by wet-dry cycling.In general,removal of oxide colloid increased the amount of fine particles,which resulting in the rise of fractal dimension(D).Under different concentrations of free oxides,the correlation between soil fractal dimension and cohesive force(c)and internal friction angle(ψ)could be expressed by c =D18.78725.c = D15.93285 andψ=0.1148D5.62174、ψ=0.9718D3.29803 respectively.Moreover removal of oxide colloid would weaken the stability of soil structure hence lower the cohesive force.Equation of In c=0.02641d+94.549 could be used to express the correlation between Fed content and cohesive force of red soil.With sequential removing treatments of amorphous oxides,the correlation between FeO concentration and sandy soil’s cohesion and detritus soil’s internal friction angle could be simulated by c = 64.9636o 0 19487 and ψ = 21.8360+0.04189o 4.3626.(3)Due to the adsorption and particle size characteristics,goethite and kaolin had larger impact on particle size distribution and fractal dimension of soil,consequently influencing the shear strength.With addition treatment of goethite,correlation between fractal dimension and cohesion of sandy soil before and after wet-dry cycling could be expressed by c =-776.98D2 + 3538.51D—3960.24 and c =-355.61D2 +1622.99D-1793.58.Cementation of goethite would strengthen the soil shear strength,before and after wet-dry cycling,the correlation between Fed concentration and cohesion of sandy and detritus soil could be expressed by c = 68.7044-5.3366×106×0.1463d,57.9026-1.2047×106×0.1609d and c = 35.3880×1.1206d,c =33.1300+5.7010d respectively.Though cohesive force and internal fricition angle of collapsed soil increased because of kaolin addition,cohesion of red soil decresed.Equations of c = 224.5571g-0.3166,c =-2.4570g2 + 17.6144g-28.7447 and c = 62.9967-19.4204×0.5733g could express the relationship between kaolin addition proportion and cohesion of red,sandy and detritus soil.(4)The sequential addition of organic material(humic acid and fulvic acid)increased the contents of organic components in varying degrees,which changed the original cementing state and structural composition of soil.Amout of aggregates and fine particles rose with addition of organic materials.After addition of humic acid,the correlation between fractal dimension and cohesive force of sandy and detritus soil could be expressed by c = 1.11 ×D25.0981 and c ＝ 76.45691-6.06×1071×1.13×1029D.Cohesive force of red soil reduced with increase of organic material and those of sandy and detritus soil were opposite.Equations of c = 304.7311×H-0.1548,c = 73.6231-40.44246×0.67748H and c = 56.6089×H0.1125 could express the correlation between carbon content of humic acid and cohesion in red,sandy and detritus soil.And with addition of fulvic acid,the relationship between arbon content of humic acid and cohesion in red and sandy soil could be expressed by c ＝297.7895H-0.07351 and c =-0.63481H2+11.80794H+ 35.67314.Owing to complexity of organic colloid’s molecular weight and structure,except the significant relationship between carbon content of fulvic acid and internal friction angle of sandy soil,no significant relationships were found,which merit further study.