Shrinkage-cracking Characteristics Of Lime Concretion Black Soil And Improvement With Biochar

Lime Concretion Black soil(in Chinese Genetic Soil Classification System)covering 3.13 million ha,is one of the major soil types with low and medium productivity in Huaibei Plain,a sub plain of Huang-huai-hai Plain(HHH)of China.Understanding its shrinkage and crack characteristic is of vital importance to improve the soil quality and productivity.Few researches have been reported on its shrinkage and crack behavior due to the complexity of cracking process.The dissertation investigated effects of different factors on shrinkage and crack characteristic of Lime Concretion Black soil.The effect of biochar on the soil shrinkage capacity after adding biochar was studied.The research provided scientific basis for the further study of shrinkage and cracking characteristic of Lime Concretion Black soil.The main research results are as follows:(1)The obstacls factors of Lime Concretion Black soil in the field were idenitified.The field survey on two typical Lime concertion Black soil cropland showed a consistent low nutrition status.The Lime concretion occurs with the descendent depth of soil profile,usually at 40 cm-80 cm.The Lime concretion increased saturated water content and decreased soil water holding capacity.The clay content at 0-40 cm is as high as 35%.With the occurrence of lime concrete,the clay content show a declining trend with the depth of soil profile.The bulk density(?b)of top soil in surveyed Lime Concretion Black soil fields varied from 1.161 to 1.561 g/cm3.Soil coefficient of Linear Extensibility(COLE)is all above 0.20 for the layer 0-40 cm,the extent of cracking area represented 9.12%of the total area of the surveyed fields.(2)The effects of initial bulk density(?b),initial water content,and lime concrete content on SSAC behavior.When the initial density were 1.10,1.22 and 1.32 g/cm3(B1,B2,B3)respectively:soil water retention capacity at same water suction was improved as the initial ?b increased,linear shrinkage ratio increased as the initial ?b decreased.The final ?b upon shrinkage was positively correlated with the initial ?b.The increased initial bulk density increased void ratio in soil structure shrinking phase,while decreasing void ratio in linear shrinking phase.When the initial water content were 0.49 g/g,0.43 g/g,0.36 g/g,0.31 g/g and 0.25 g/g(W1,W1,W3,W4,W5)respectively:the higher the initial water content the faster the water loss,and the increased linear shrinkage ratio.The void ratio in the structure shrinkage phase was highest when water content was 0.36 g/g,and 0.31 g/g.When the Lime Concrtion content were 0 g/g,0.04 g/g,0.06 g/g,0.08 g/g,0.10 g/g and 0.12 g/g(SJ1,SJ2,SJ3,SJ4,SJ5,SJ6)respectively:Lime Concrtion content had little effect on linear shrinkage ratio and bulk density after shrinkage,and increased Lime concretion decreased soil water content under same soil suction;Soil void ratio increased as Lime Concrtion content increased,but the soil moisture ratio decreased under the same void ratio.(3)The cracking characteristics of Lime concretion Black soil under different factors were studied.In view of characteristics of soil cracking,we chose the special denosing method,linear gray level transformation and biggest class methods to carry out image processing,edge detection,binarization.A model was designed to measure crack corner numbers bases on Harris algorithme.Such factors as the ambient temperature and humidity,soil thickness and diameter of samples may contribute to the morphology of crackings.The quantitative measurement revealed the complexity of the soil cracking decreased with the increase of ambient temperature and soil thickness,and it increased with the humidity and diameter.The range of crack area(Dc)was 11.27%-15.11%;the connectivity(CI)value was0.38-0.58;crack length(Lc)value was 0.25%-0.63%;cracking perimeter(P)value was 0.48%-1.33%;equivalent width(EW)was 17.78-51.50 pixels;the box dimension(FD)value was1.593-1.791.(4)The effect of soil thickness on soil cracking process was investigated.Under the same environmental factors,the finally cracking indeces of different thickness soil sample had good repeatability.Soil cracking index of mean and standard deviation for 4-5 mm soil samples are as following:Dc(12.55%±0.38%),CI(0.45±0.02),Lc(0.73%±0.07%),P(1.83%±0.07%),EW(13.72±0.54),FD(1.546±0.02).Besides,when the soil thickness is 4-5 mm,crack volume measured by image method and the iron powder method had the smallest difference,the max difference was 8.65%.During the drying process,soil crack area,volume and shrinkage volume had a good repeatability with the change of soil water content,the uneven water distribution may be responsible for the different cracking process for 9-10 and 15-16 soil.Therefore,when measure the soil shrinkage characteristic curve with the consideration of cracks,the soil thicknedd 4-5 mm was the highly recommended.(5)Adding biochar can reduce shrinkage and cracking characteristic of Lime concretion Black soil significantly and improve the water holding capacity.Biochar derived from mixed com straw and peanut shell mixture was added to the soil at rates of 0%,5%,10%and 15%by weight(T0,T5,T10 and T15).During the wetting and drying cycles,the pattern of cracks and characteristic of shrinking of unamended and amended soil samples were recorded.Biochar addition increased significantly the content of total carbon(TC)of the samples,and followed the order:T0<T5<T10<T15.During the desiccation of soil,biochar significantly reduced the rate of water loss.Cracks propagated slowly and stopped at relative higher water content in soil that was amended with biochar.Dc,Lc,P,EW,FD and CI all decreased with increasing dosage of biochar.The values of Dc of the amended samples decreased with the number of wetting and drying cycles increase.The COLE of unamended samples T0 was about 0.23.Adding 10%and 15%biochar significantly reduced the shrinkage capacity by 41.45%and 45.54%,respectively.The slope of shrinkage characteristic curve decreased with increasing biochar dosage.Moreover,the proportional shrinkage zone of the shrinkage characteristic curves of amended samples decreased.There were no differences in the moisture ratio where maximum curvature occurred on shrinkage curve under all treatments.Our results indicated that soil volume water content improved 5.90%,13.15%and 19.61%underT5,T10 and T15 treatments.