Nuclear Magnetic Resonance Test And Analysis On Pore Structure Of Expansive Soil Effected By Drying-Wetting Cycle

Expansive clay is widely distributed in more than 20 areas in China.Due to its high sensitivity to the change of water content,mechanical behavior of expansive soil in the shallow layer of subgrade significantly decreased after experiencing drying-wetting cycle.This decrease caused subgrade disease such as subsidence and fissure,and caused huge economic loss.The change of soil mechanical behavior is the macroscopic expression of many micro elements of soil.It is of great significance to study the change law of soil pore structure effected by drying-wetting cycle for the prevention and maintenance of subgrade disease in expansive soil.Assisted by the nuclear magnetic resonance(NMR),soil pore structure and pore water distribution under the influence of multiple drying-wetting cycles were tested in this paper.The influence of changes in pore structure on soil deformation and permeability was further evaluated.This paper provided theoretical reference for subgrade diseas treatment.The main work and conclusions in this paper were as follows.1.Drying-wetting cycle under different loads(0 k Pa,5 k Pa,15 k Pa,30 k Pa)was carried out to analyze the influence of these tests on the shear strength of soil.The fissure evolution of soil with different dry densities(?_d=1.4 g/cm³,1.6 g/cm³,1.8 g/cm³)during 0?3 cycle was explored.The conclusions were as follows.(1)The cohesion of soil under different loads decreased with an increasing cycle number,but the decay rates were discrepant.Vertical load effectively hindered the development of fissure and inhibited the attenuation of cohesion.However,this inhibition capacity was not directly proportional to the load.(2)Fissure rate of soil increased with the increase of cycle number.when the water content was the same,the fissure rate on drying path was lower than that of wetting path when the water content was the same,and it increased first and then decreased with the increase of water content.2.The T₂curves of three capillary models(r=0.25 mm,0.5 mm,2.0 mm)were measured,and the effects of pore size and pore volume on T₂curve were analyzed.The T₂curve of soil experienced different suction(0?1400 k Pa)was measured,and the conversion coefficient between T₂and pore radius was determined.The conclusions were as follows.(1)The integral area of T₂curve was directly proportional to the water content of capillary.The T₂was proportional to r.(2)The conversion function between T₂and pore radius,r_i=1.39T_2i,was obtained by a method named"saturation-suction combined measurement".The pore structure calculated by T₂curve was in good agreement with the result obtained by mercury intrusion porosimetry.3.Pore water distribution was quantitatively analyzed,which was employed to modify VG model.The conclusions were as follows.(1)T₂curves on drying/wetting path were almost coincident at the same water content.(2)The pore water distribution curve(PWDC)was proposed to describe the distribution characteristics of water in soil.(3)Damage potential,total damage potential,and relative damage potential were defined to quantify the PWDC for all pore greater than r_ccaused by a given suction,and a good functional relationship between the damage potential and suction was observed.(4)A modified VG model including the distribution information of pore water was obtained by introducing the damage potential.4.The T₂curves of soil subjected to 0?4 drying-wetting cycle were measured,and the relationship between pore structure change and deformation was analyzed.The conclusions were as follows.(1)The time-evolution curve of deformation was nonlinear and could be well fitted by exponential function model(equation(5-3)and(5-6));this exponential model showed an accurate prediction of soil deformation affected by dry density,load and cycle number.(2)Soil pore was divided into macropore(r>10?m),mesopore(3.2<r?10?m),small pore(1.0<r?3.2?m)and micropore(r?1.0?m).In wetting process of soil,the relationship between water content and the water volume in those four types of pore were fitted by an S-shaped curve.Macropore began to appear and quickly saturated when the water content exceeded 19%.Soil pore with different radius showed a unique variation law in the dry wet cycle.The pore(small pore and micropore)in aggregate were hardly affected by drying-wetting cycle,and the mesopore and macropore were the main contributors to the macro deformation of soil.(4)A reliable linear relationship between the pore index(H)and the pore increment was observed,which was a prediction model of soil deformation that reflected the contribution difference between mesopore and macropore.5.The variation law of pore-water form and average value of pore radius during drying-wetting cycle was studied,and their influence on soil permeability was further analyzed.The results obtained were as follows.(1)The pore water was divided into adsorbed water(T₂?0.37 ms)and movable water(T₂>0.37 ms).The adsorbed water was not affected by drying-wetting cycle,while the quantity of movable water increased linearly with the increase of cycle number.(2)According to the modified Coope permeability model based on pore-water form,the permeability of soil was directly proportional to the sixth power of cycle number.(3)The average pore size of soil increased linearly with the increase of cycle number.Known from the Poiseille equation,pore unit discharge(Q)was directly proportional to the fourth power of dry wet cycle number.6.The changes of solid-liquid contact angle,microstructure,mineral composition and chemical element of soil subjected to different cycles were measured,and relationship between micro-parameters and macro-mechanical behavior of soil was then analyzed.The conclusions are as follows.(1)The drying-wetting cycle did not change the hydrophilicity of soil(contact angle<90°);no reliable law was found between cycle number and the contact angle;the contact angle increased greatly(about 33.32%on average)after cycle 1,and then fluctuated in a range of 10.11°.(2)With an increasing cycle number,the proportion of clay sheets arranged in face-side and side-side increased,and the macro-mechanical behavior of soil tended to be isotropic.(3)The mineral composition of soil was not changed after a limited number of drying-wetting cycles,but the intensity of the main characteristic diffraction peak of mineral decreased after cycle 30.