The Effect Of Displacement Rate On Entrapped Air Saturation Of Quasi-saturated Soil And The Action Force Of Air Entrapment And Escape

The existence of entrapped air in aquifer has great influence on the permeability of medium.The Displacement rate is an important factor affecting the entrapped air saturation.The quantitative relationship between entrapped air saturation and displacement rate is of great significance to understanding the hydraulic properties of quasi-saturated zone.The study on the basis of coarse sand,medium sand and fine sand displacement test using capillary rise height of medium and corresponding rising rate as unified index displacement rate and type of medium respectively to depicted the relationship between Displacement rate and entrapped air saturation,also using dimensionless parameter such as the number of capillary,bond and weber to studies the main force of gas trap,by doing trap gas escaped experimental under different vacuum degree conditions,and discussed escape characteristics and conditions of the trap gas.The main conclusion as follows:(1)When the Displacement rate less than the rate(V_1/2Hmax)of half capillary rising height,the entrapped air saturation reached the maximum,and the Displacement process was dominated by snap-off mode.When the Displacement rate greater than the beginning rate of capillary rise(V_H=0),the entrapped air saturation reached the minimum,and the Displacement process was dominated by piston-like mode.When the Displacement rate is between V_1/2Hmaxand V_H=0,the entrapped air saturation has negatively correlation with Displacement rate,which was caused by snap-off and piston-like mode synthetic action.The signature of complete piston-like displacement was Displacement rate greater than V_H=0,and when Displacement rate less than V_1/2Hmax which depicted snap-off displacement.(2)Two new mathematical models of entrapped air saturation were established,one of them was statistical model based on Displacement rate(or capillary number),which has high fitting accuracy,the other one was a transformation function model based on the capillary height corresponding to the rate,which has the advantage convenient to use.(3)The result shows that the increases of capillary number,bond number,and weber number would delayed entrapped air saturation.Capillarity is the main force for the formation of entrapped air.(4)The entrapped air escape has to overcame resistance.When the medium more thicker or Displacement rate much greater than or equal to V_H=0 the entrapped air formed were more likely to escaping.Assuming restricted forces of bound entrapped air all came from capillary resistance,the throat radius of entrapped air escape critical pore was 1.54-2.67?m far less than the measured minimum pore throat radius(6.5?m)under conservative estimated circumstance.and also found that capillary resistance was 0.022 Mpa calculated by minimum measured radius of pore throat,which was smaller than the measured vacuum degree(0.03 Mpa)extracted from experiment.All this evidences shows that the binding force which prevents the trap gas from escaping was not only capillary resistance but also may have other forces.