Applied Research On Measuring Water Content Of Subgrade In Seasonal Frozen Area Based On Dielectric Properties

As the road is in service for a long time,the changes of temperature will lead an increasment of the water content of subgrade.Due to the freezing and thawing cycle,the high water content will result in a decrease in strength,the occurrence of surface subsidence,cracking and other disease,especially in the seasonal frozen area.Therefore,it is important to comprehensively measure the water content of subgrade.The traditional measurement method has to destroy the road structure and the work efficiency is low.Meanwhile,the single point data can not represent the changes of water content over the full scale of road.Ground penetrating radar(GPR)has been widely used in road engineering for surface layer thickness and internal defect detection,with advantages of non-destructive,simple and efficient.However,there are few studies on the application of GPR to detect the water content of subgrade.The existing research has many problems.Some research is based on the soil's dielectric properties,lacking consideration of factors such as compaction and temperature.Exsting research focus more on theoretical and laboratory test,which lacks the verificaton in field scale.Meanwhile,frequency domain analysis is less with more concentration on time domain analysis.In view of above problems,research on measuring water content of subgrade in seasonal frozen area based on dielectric properties were carried out by theoretical analysis,laboratory experiment and field experiment.Firstly,the water content and dielectric constant relationship model of sand and clay was established through laboratory experiments.Test pieces were made with different water content and compaction degree.The dielectric constant under normal and negative temperature were measured.Then,the influencing factors of dielectric constant were analyzed,and the water content-dielectric constant relationship model was obtained by fitting.Secondly,through the analysis based on the time domain finite difference algorithm,the index for judging the dielectric constant is determined.The theory related time domain finite difference algorithm and forward modeling software are expounded,which lays a founfation for numerical simulation of radar wave propagating in the model.Next,the single-layer,double-layer and inhomogeneous media structure models were established.Different time domain radar signals were obtained by changing the electromagnetic parameters of models.Direct wave amplitude,reflected wave amplitude,two-way travel time,attenuation were extracted,then analyzed the correlation between dielectric constant and those electromagnetic parameters.Finally,the outdoor tests under different model size and different antenna center frequencies were carried out,obtaining the measured data under various conditions.Next,the dielectric constant were calculate by using the two-way traval time and compared with the data obtained by FOM2-mst.Thus,the influence of the center frequency on the detection effect were analyzed.On the other hand,the fast Fourier transform of the radar signal was used to obtain the frequency domain signal.Peak frequency and band energy percentage were proposed as indicators for evaluation of water content.The theoretical analysis,laboratory experiments and field tests above provide support for data processing of Ground penetrating radar detection of subgrade' water content on the eve of spring thawing and winter freezing,which has important theoretical significance and practical value.