Experimental Study On Air Permeability Of Malan Loess

The air permeation rate k_a and permeability coefficient K of soil are important parameters often used in engineering practice.As the permeability coefficient of remolded loess has a good correlation with the air permeation rate,therefore,the permeability coefficient of the remolded loess can be easily obtained through the rapid determination of the air permeation rate,and the permeation rules of the remolded loess can be revealed by the law of the air seepage in loess.Besides,the air permeation rate k_a can be applied to many environmental projects,such as the landfill site management and greenhouse gas emissions,the air purification of biofilter,the remediation of contaminated soil through gas drainage,the invasionof radon and other gas pollution in buildings due to pressure changes.In order to understand the factors that affect air permeation rate of loess throughly and deeply,we measured and analyzed the air permeability of loess samples of three typical regions which from Shaanxi Yanan,Xi'an and Gansu Heifangtai.The details are described as follows:?1?Both the law of Darcy seepage and the law of non Darcy seepage including the inertia effect and the slip effect were firstly summarized in this paper.And the transient pressure drop method and the corresponding calculation formula considering the above non Darcy seepage are given,on the combination of the indoor axial measurement device and the contact measuring probe used in the in-situ nondestructive permeability rate test.?2?Then,the effect of the inertia effect,under the high and low driving pressure difference,on air permeation rate is analyzed by changing the gas pressure difference applied at the two ends of the sample.The Waal model of the second considering inertia effect has better applicability to two driving pressures.Under the low initial pressure difference,the calculation results of different simplified forms are similar,and the maximum relative error is 7.5%.The results of the Waal model of the second under high initial driving pressure difference are much less than that of simplified form and the maximum relative error between them is 23.4%.?3?The availability of the contact probe in in-situ measurement is proved to be effective by comparing the results of both the in-situ measurement and the indoor original measurement.The results are similar and the maximum deviation between the two measurements increases with the increase of air permeation rate.The use of in-situ measurement probe have many advantages.It can reduce the measurement cost and preparation process,and causes less disturbance to the soil structure.It has wider applicability and stronger practicability.?4?The influence of parameters like water content and dry density on air permeation rate of loess is analyzed through the measurement of remolded and humidified loess sample of the study area.On this basis of above,the microscopic characteristics of the aggregates in the reconstruction process of the compacted loess are observed by the use of the intelligent ultrafield depth microscope and the scanning electron microscope.The mechanism of the inverse phenomenon between the water content of sample and the air permeation rate is analyzed,and the related structure model is established.When the density is low(1.35¹.45g·cm^-3),the air permeation rate is strongly influenced by molding water content and show the contrary rule with the conventional wetting test.When the density is in the range of1.50¹.65 g·cm^-3,the air-filled pores in the soil decrease significantly under compaction,the promoting effect of remolded loess to air permeability is gradually reduced when water content increase.And there is an obvious turning point between the gas permeability and the water content relationship curve.As the density increased to 1.70g·cm^-3,the number of macropores is further reduced under compaction,the few perforated pore and pore throat will also be blocked by water,the air permeation rate tends to be stable with the increase of water content.?5?The influence of site boundary effect and unfavorable geological environment on in-situ loess permeability is analyzed through site air permeation test.The unfavorable geological conditions such as cavitation cave and collapsible crack will increase the rate of permeability.In the intersections of multiple collapsing cracks,the interlacing of secondary fissures divide the soil layer into scattered soil blocks of different sizes,the permeation air flows rapidly through collapsible cracks and secondary fractures and the air permeation rate reaches 0.776?m².The effect of site boundary conditions on the gas permeation rate is about2m,and the gas permeation rate is the highest at the edge of the slots.Besides,the surface air permeability of shallow loess gradually decreases with the increase of depth.?6?Finally,the Poulsen model is introduced into loess,and the applicability and accuracy of the model to predict the permeability rate of the remolded loess are analyzed.According to the model,we can get the predicted value of loess permeability under specified water content and dry density quickly and accurately,and the linear regression fitting degree between predicted value and actual value is over 0.96.