GPR Laboratory Tests For Railways Materials Dielectric Properties Assessment

Abstract

Abstract:In railways Ground Penetrating Radar (GPR) studies, the evaluation of materials dielectric properties is critical as they are sensitive to water content, to petrographic type of aggregates and to fouling condition of the ballast. Under the load traffic, maintenance actions and climatic effects, ballast condition change due to aggregate breakdown and to subgrade soils pumping, mainly on existing lines with no sub ballast layer. The main purpose of this study was to validate, under controlled conditions, the dielectric values of materials used in Portuguese railways, in order to improve the GPR interpretation using commercial software and consequently the management maintenance planning. Different materials were tested and a broad range of in situ conditions were simulated in laboratory, in physical models. GPR tests were performed with five antennas with frequencies between 400 and 1800 MHz. The variation of the dielectric properties was measured, and the range of values that can be obtained for different material condition was defined. Additionally, in situ GPR measurements and test pits were performed for validation of the dielectric constant of clean ballast. The results obtained are analyzed and the main conclusions are presented herein.

Railway Track Condition Assessment at Network Level by Frequency Domain Analysis of GPR Data

“…Both affect the GPR signal by reducing the wave speed propagation and attenuating the signal intensity. Nevertheless, they are difficult to dissociate from each other, as their effect is combined [2,25,38], unless in situ samples of material are collected and analysed in laboratory in order to characterise the grading, fouling level and water content. Nevertheless, this only characterises specific locations, as old tracks are quite heterogeneous in terms of materials and drainage conditions.…”

“…The GPR signal is most commonly processed in time domain [2,11,26,38,45] by: (i) calculating the layer thickness using an "default" dielectric value, generally assumed valid for the type of material tested; or (ii) by performing test pits at some locations, measuring the real thickness of the ballast and, based on this, calculating the real dielectric value and adopting this value for processing all the signal along the track. In case of homogeneous media, such as concrete or asphalt, these assumptions are close to the real situation.…”

“…Regarding the assessment of the moisture content in the ballast, this parameter was easily evaluated by GPR as the dielectric value of the ballast increases with the presence of water [36]. In [37,38], complementary laboratory tests were performed using different antennas (air-coupled antennas and ground-coupled antennas) with different frequencies between 400 MHz and 2 GHz, in order to evaluate the dielectric constant values for different levels of fouled ballast (from 0% to 55%) and different water contents (from 6% to 14%), which demonstrated that the increase of the dielectric value with water content is particularly more relevant in fouled ballast, as the fine soil particles decrease the drainage capabilities of the material. For fouling evaluation, particular studies were based on the development of new GPR signals processing and interpretation.…”

Ballast fouling evaluation with ground penetrating radar

“…Data were collected both in the laboratory and in situ, using antennas with different frequencies. The detailed conditions of these laboratory measurements are described in the documents [1,[9][10], where all the laboratory work involved is described, as well as the results obtained in these same evaluations. The data collected in situ comes from two distinct railway lines -one in Portugal (sections A, B, C, D and E) and another on the African Continent (sections F and G).…”

A Computer‐Aided Model for the Simulation of Railway Ballast by Random Sequential Adsorption Process

“…Over the last two decades, many studies investigated the effects of poor ballast conditions in both real‐life (Hugenschmidt, ; Olhoeft and Selig, ; Roberts et al., ) and experimental applications (Al‐Qadi et al., , ; Leng and Al‐Qadi, ; De Chiara et al., ). Nevertheless, the manufacturing of a full‐scale railway track‐bed in a laboratory environment is a logistic issue, due to the substantial amount of ballast aggregates to handle.…”