Study On The Relationship Between The Dielectric Properties And Water Content Of Soil Subgrade

The water content of subgrade soil is an important factor which influence theroadbed strength and stability. The drilling and sampling methods commonly used totest the water content of subgrade,the results of which are accurate and reliable, butthe breakage detection method with high cost、low efficiency and poor representationis actually quite difficult to achieve at the existing roads, especially in terms of theoperating-road.Ground penetrating radar method is the detection technology that based on thetheory of electromagnetic wave propagation, the prerequisite of which is thedifference of media electrical (conductivity and dielectric) The greater water contentdifference lead to the obvious discrepancy of medium electrical. Therefore, thismethod can be used to study the water content of the subsurface. Based on GPRtheory and methods to study the relationship between water content and the dielectricconstant of the subgrade soil, trying to establish empirical formula, to providetheoretical and methodological support for the research of using GPR methods todetect the water content of subgrade.In this paper, using GPRMax2D software based on finite difference time domainmethod, to establish a different water content, different dielectric constant andtypical types of diseases in the typical single-layer、multi-media model,to simulateand analyze the phase of the electromagnetic wave、amplitude、 vertical and delayresponse characteristics and discipline, concluded that:the addition of water contentin subgrade soil will increase the dielectric constant and conductivity and decrease thespeed of electromagnetic waves, the specific performance is that the two-way traveltime of radar echoes at the same depth is increased; when the electromagnetic wave isincident from the lager dielectric constant medium to the small dielectric permittivity,the reflection coefficient is negative, the phase of the reflected wave is opposite tothe excitation electromagnetic wave, and vice versa; When the difference ofdielectric constant between upper and lower interface is large, the absolute value ofreflection coefficient is bigger also, it will cause a strong reflection of electromagnetic waves;The greater conductivity is, the more absorption ofelectromagnetic waves obvious, which make amplitude to be smaller;But thedifferences of conductivity do not affect the velocity of the electromagnetic wave.In order to determine the water content of subgrade soil by using GPR, we designphysical model on the basis of numerical simulation, and change the water content ofthe medium model, to detect several times to obtain the electromagnetic wavepropagation velocity in different water content medium by using GPR, and then obtainthe dielectric constant based on the relationship between the dielectric constant withthe electromagnetic wave velocity at low-loss dielectric permittivity, then analyze、count、 fit the mathematical relationship of dielectric constant and water content ofsubgrade that can be used as GPR reference to measure the water content of the soilsubgrade. Compared with Topp, Alharathi and other common empirical formulamodel,the results have a consistent tendency, and simply count on the causes of thedifferences. Combined with the radar detecting results of the actual, analyze theroadbed of water and its impact on strength and stability of the pavement subgrade.Studies have shown that the GPR response characteristics of different watercontent of the soil subgrade is not the same, the two-way travel time of the reflectedsignal will be increased,amplitude and low frequency will let down as the watercontent increased. Analysis by simulating multiple models can deepen understandingof radar reflection profiles, and help to improve the precision and accuracy ofinterpretation.