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Journal of Applied Geophysics 43 2000 147–155 www.elsevier.nlrlocaterjappgeo
Railway track inspection using GPR
J. Hugenschmidt
EMPA, Swiss Federal Laboratories for Materials Testing and Research, Ueberlandstrasse 129, CH-8600 Duebendorf Switzerland
Received 15 September 1998; received in revised form 23 February 1999; accepted 15 March 1999
Abstract
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Swiss Federal Railways SBB inspect their railway tracks at regular intervals. The first step of track renewal planning is a geotechnical study. Inspection is focused on the thickness of the ballast, on subsoil material penetrating upwards into the
ballast and on geotechnical properties of subgrade and subsoil materials. Up to now, the inspection has been done mainly by digging trenches at evenly spaced intervals and in locations of special interest. In order to evaluate the benefits and limits of
GPR railway track inspections, three GPR surveys were carried out on three different railway lines. Data were acquired using a mobile system travelling at 10 kmrh. Subsequent to radar data acquisition, trenches were dug. The positioning of
some of the trench locations was based on preliminary GPR results in order to support the interpretation of GPR data. Only those trenches were available during interpretation of radar data. In addition, SBB performed their usual investigation
programme. This provided an opportunity for checking the radar results in great detail. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: GPR; Railway track inspection; Ballast inspection; Non-destructive testing
1. Introduction
Up to now, in Switzerland, the inspection of railway ballast beds, subgrade and subsoil has
been done mainly by digging trenches at evenly spaced intervals and in locations of special in-
terest. By doing so, detailed information was obtained at the trench locations but only little
information was available in between. Also, digging trenches is expensive, destructive and
obstructive to railway traffic.
Tel.: q41-1-823-43-18; fax: q41-1-821-62-44; e-mail: johannes.hugenschmidtempa.ch
When searching for a method allowing for a reduction of trenches required and providing
information in between, GPR seems to be an obvious choice. GPR has been successfully ap-
plied for the investigation of other traffic-related structures such as road pavements and bridge
decks. If mobile acquisition systems are used, data can be recorded economically causing min-
Ž imal obstruction to traffic flow Davis et al.,
. 1994; Hugenschmidt et al., 1998 . However,
railway lines do not offer very favourable radar conditions. The problems caused by the pres-
ence of sleepers, tracks and numerous near- and subsurface installations have to be addressed by
0926-9851r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
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an appropriate approach to data acquisition and processing.
With the aim of evaluating the benefits and limits of GPR inspections, Swiss Federal Rail-
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ways SBB placed an order with EMPA for the investigation of three railway lines. The focus
of the radar inspections was on ballast thickness and on the detection of zones where subsoil
material had penetrated into the ballast. The total length of the inspected sections was 15.1
km. In addition to the 41 trenches based on preliminary GPR results that were available dur-
ing interpretation of radar data, SBB performed their usual investigation programme digging 77
trenches. This provided the opportunity for a quantitative evaluation of radar results.
2. Data acquisition
Data were acquired in summer 97 using a GSSI SIR-10A system and a 900-MHz antenna
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GSSI Model 3101D . Hanninnen et al. 1992
¨
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pp. 22 provide more information on the 900- MHz antenna. Fig. 1 shows the survey wheel
and the antenna mounted to a trailer which was pulled by a small diesel locomotive at 10 kmrh.
The acquisition parameters can be summa- rized as follows:
Ø acquisition speed: 10 kmrh, Ø horizontal sample rate: 15 scansrm,
Ø data word length: 16 bit, Ø samples per scan: 512,
Ø antenna height: 8 cm, top of sleeper to bot-
tom of antenna casing, Ø antenna orientation: at right angles to travel-
ling direction, Ø scan length: 25 ns.
In the weeks prior to the radar surveys and during data acquisition there was heavy rainfall.
Data were recorded and stored on tape with- out any processing. Also, no effort was made to
