Experimental Study On The Factors Influencing Electromagnetic Parameters Of Frozen Water-rich Sandy Soils And GPR Detection

The Artificial ground freezing(AGF)method is an important construction technique for crossing complex formations and highly saturated soft soil layers,known for its excellent water isolation effect,strong adaptability,and environmental friendliness.The development of the frozen wall directly determines the success of freezing engineering projects,making early detection,process monitoring,and effectiveness evaluation of abnormal frozen wall development crucial.This paper aims to rapidly identify the development of frozen walls using Ground-Penetrating Radar(GPR)as the research objective.It combines theoretical analysis,numerical simulations,and physical experiments to conduct the research.The main research work of this paper is as follows:(1)Building on the analysis of the principles of GPR detecting geological formations,this study employs laboratory experiments to measure and analyze the evolution of the dielectric properties of sand and clay under different freezing temperatures,detection frequencies,initial water content,and salt content.Sensitivity analyses of the dielectric properties of frozen sand and clay under various influencing factors are conducted.The results indicate that salt content is the main factor causing frequency dispersion effects in unfrozen soil.At a detection frequency within2000 MHz,the relative permittivity of salt-contaminated soil decreases exponentially with increasing detection frequency.The relative permittivity of unfrozen soil significantly increases with the increase of water content.Under negative temperature conditions,the relative permittivity of salt-contaminated soil is similar to that of unfrozen soil,indicating the ability of GPR to distinguish between unfrozen and frozen soil.(2)A numerical model for identifying frozen walls using the Finite-Difference Time-Domain(FDTD)method is established.Experimental investigations are carried out to explore the imaging characteristics and electromagnetic wave response patterns of intersected and non-intersected frozen walls for different materials of freezing pipes,salt-bearing formations,and detection directions.(3)The development status of frozen walls is inferred through GPR physical detection experiments and field measurements,which are then cross-validated with temperature measurement data and numerical simulations.The research shows that within a detection depth of 800 mm,GPR with a frequency of 600 MHz has the capability to roughly identify the expansion of frozen walls.Differences in frozen soil properties,detection direction,and interference layers are identified as key factors influencing the accurate identification of frozen wall development by GPR.