Identification of type and degree of railway ballast fouling using ground coupled GPR antennas P. Anbazhagan ⁎, P.S. Naresh Dixit, T.P. Bharatha Department of Civil Engineering, Indian Institute of Science, Bangalore, India abstract article info Article history: Received 19 May 2015 Received in revised form 21 January 2016 Accepted 22 January 2016 Available online 23 January 2016 GPR is widely used for ballast fouling identification, however, there are no robust guidelines to find the degree and type of fouling quantitatively. In this study, GPR studies were carried out on model and actual railway tracks using three ground coupled antennas and considering three fouling materials. Three ground coupled antennas viz., 100 MHz, 500 MHz and 800 MHz antennas were used for the initial survey and it was found that the 800 MHz ground coupled antenna is an optimum one to get quality results. Three major fouling materials viz., screened/broken ballast, coal and iron ore were used to construct prototype model sections, which were 1/2 of the actual Indian broad-gauge railway track. A separate model section has been created for each degree and type of fouling and GPR surveys were carried out. GPR study shows that increasing the fouling content results in a decrease in the Electromagnetic Wave (EMW) velocity and an increase in the dielectric constant. EMW ve- locity of ballast fouled with screened ballast was found to be more than coal fouled ballast and iron ore fouled ballast at any degree of fouling and EMW velocity of iron ore fouled ballast was found to be less than coal and screen ballast fouled ballast. Dielectric constant of iron ore fouled ballast was found to be higher than coal and screen ballast fouled ballast for all degrees of fouling. Average slope of the trend line of screen ballast fouled section is low (25.6°), coal fouled ballast is medium (27.8°) and iron ore fouled ballast is high (47.6°). © 2016 Elsevier B.V. All rights reserved. Keywords: Railway ballast Fouling GPR Dielectric constant Fouling percentage 1. Introduction In a demographically large country like India, railways play a major role in the movement of passengers and freight from one place to anoth- er. Railroad system effectively connects all major ports and refineries to the major cities of India, which plays a pivotal role in the transportation of goods. Ballast railroad plays an important role keeping the rail in po- sition, supporting the heavy cyclic load from train wheels and transforming/dissipating train load with less track deformation. It also provides free drainage of water from the railway track formation and prevents the growth of vegetation that might interfere with the rail movement. Ballast is usually made of crushed stone from materials like granite, dolomite or quartzite; sometimes it may consist of undesir- able material though. The mechanical and physical properties desirable for a good ballast are they must be strong, stable, drainable, workable, easily available locally and most of all cheap to purchase (William, 1982). When the railways were first introduced, engineers did not know the importance of the railway ballast formation and locally avail- able soft materials like limestone, ashes, chalk and cinders from locomo- tives were used as ballast. Soon with time, they came to know that good quality ballast rock is important for good foundation stability and good drainage for the railway tracks (Solomon, 2001; Bonnet, 2005). Size smaller or larger than 9.5 mm to 65 mm is not desirable as smaller sizes can reduce the drainage properties and larger size may dissipate the track loads in a nonuniform manner. Railway ballast layer is the most important component of the railway track foundation. It is sub- jected to both traffic loads and exposure to environmental changes. Due to this the ballast bed deforms and degrades, hence adversely af- fecting the performance of the railway track. Ballast gets fouled due to train load, spillage of goods and weak subsurface soil (William, 1982). Ballast contamination or the filling of voids due to ballast breakdown and infiltration of other materials from the ballast surface or infiltration from the base of the ballast layer is called ballast fouling. The fouling of ballast decreases the performance for which it has been designed and results in the deformation of the track section. The decrease in track per- formance is controlled by the type of fouling and also the amount of fouling (Selig and Waters, 1994). At moderate degrees of fouling, softer fouling material such as fine soil can create an adverse slippery effect (mud pumping) when compared to harder fouling materials such as iron ore and broken blast. Degraded ballast in non-screened track has large amount of fine particles accumulated within the voids (i.e., fouling) thus impeding drainage. When fouling becomes extreme, then excess pore water pressure is generated under fast moving trains (i.e., high cyclic loading), thereby reducing the track resiliency and sta- bility in undrained conditions (Indraratna et al., 2010). For this reason, it Journal of Applied Geophysics 126 (2016) 183–190 ⁎ Corresponding author. E-mail addresses: anbazhagan@civil.iisc.ernet.in, anbazhagan2005@gmail.com (P. Anbazhagan), nareshdixit82@gmail.com (P.S.N. Dixit), tpbharath02@gmail.com (T.P. Bharatha). http://dx.doi.org/10.1016/j.jappgeo.2016.01.018 0926-9851/© 2016 Elsevier B.V. All rights reserved. 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