Mocnik A.(1,2), Dossi M.(1), Forte E.(1,2), Zambrini R. (2), Zamariolo A. (2), and Pipan M. (1,2)

(1) Università degli Studi di Trieste, Dipartimento di Matematica e Geoscienze (2) Esplora srl, Academic Spin-off-University of Trieste

GROUND PENETRATING RADAR APPLICATIONS FOR ROADS AND AIRPORT PAVEMENTS INVESTIGATIONS

Fig 3. Depth map of H4 horizon

from the topographic surface

de

pth

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lor

sc

ale

of

H4

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cto

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ravel ti

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ravel

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ns)

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100 200 300H1-concrete interface

H2-conglomerate interface

H4-sedimentary contact

H3b-inner subgrade horizon

Core1: 348mmtracenumber

Core2: 342mm

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1. INTRODUCTION

In this work we present some examples of application of Ground Penetrating Radar (GPR) method for

pavement thickness evaluation and stratigraphic analysis. We used R.Ex. (Road Explorer), a new

system developed by Esplora srl, an academic spin-off of the University of Trieste, in collaboration with

the geophysical group of the Department of Mathematics and Geosciences of the same University and

a material testing laboratory.

Direct investigations are necessary for the local characterization of asphalts, but they require material

removal, they have logistical constrains and their lateral spacing can be very high. GPR represents an

indirect method that can provide a continuous and accurate survey of a road segment (Saarenketo,

2009). This is a very important factor since both the moisture content and thickness may have strong

lateral variations. GPR measures can be joined with FWD and TDR (Briggs et al., 1991; Topp, 1980)

techniques, which are very accurate but furnish only punctual information.

We describe the GPR data acquisition by using the R.Ex. system, which combines two channels,

showing applications in:

1) airport runways: for the analysis of the thickness and the shallow stratigraphy in an airport transit area

and thickness evaluation in a newly constructed aircraft apron;

2) new paved roads for thickness evaluation and analysis of its lateral changes.

2. METHOD: R.Ex. Road Explorer

- Prototype of GPR equipment support in fiberglass for roads investigations (Fig.1).

- Homologated for transiting on all kind of roads without the need vehicles stopping, or

limitations or support vehicles.

- Composed by anti-shock boxes for two ground coupled antennas with different

frequencies, a positioning system made up of an odometer combined with a RTK

mode GPS allowing centimetric precision.

- The double frequency acquisition consists in the symultaneous use of medium

frequency antennas (500MHz or 800MHz) with high frequency antennas (1.6GHz or

2.3GHz).

- Dragged by a van, it could reach speed up to 50-60km/h; it can be dragged also by

hand as a function of the lenght of the survey and of logistical constrains.

5. ConclusionsThe R.Ex. system is a GPR acquisition device aimed at obtaining a huge amount of high quality GPR data in a relatively short time. The integration of multy-frequency antennas is essential to obtain high resolution in the shallower portion (0.-0.3m) of paved roads and detailed information down to 2-3 meters below the topographic surface.

The examples described in this work report some applications on airports and highways that allowed traffic infrastructure companies and airport managers to achieve useful information for maintenance operations planning and for the evaluation of the conditions of new roads.

Future developments have to be addressed to the improvement in the precision (and the speed) of picking horizons. In this regard, a picking method was recently implemented and tested in different fields of application (Forte et al., 2013; Dossi et al.,2015a) including pavements (Dossi et al., 2015b, c). The obtained results are encouraging allowing to achieve a faster and more accurate picking, diminishing at the same time the subjectivity level of the interpretation.

3. CASE -1: AIRPORT APPLICATIONS

4. CASE-2: NEW PAVED ROADS APPLICATIONS

Test Site 2 is a new aircraft apron where we

used the 1.6 GHz and 800 MHz antennas

couple.

Due to its relative small thickness the base of

the concrete is evidenced by a weak reflector

only on the 1.6 GHz data (Fig.5). We

hypothesize that the high overall attenuation

can be related to metallic mineral compound

within the concrete mixture.

For the depth conversion, we used a variable

lateral velocity field, fixing the constraints in

correspondence of the cores (generally 2/3

cores for each profile) and calculating the

gradient of the velocity values between them.

Test Site 1 is an airport transit area, where we used the combination of 500 MHz and 1.6 GHz

antennas to reach a depth of about 2m.

The depth conversion has been obtained by diffraction hyperbola analysis and by the available drill

cores and penetrometric surveys.

The lateral contrast between the concrete plates (left of profile) and the bituminous aggregate (right of

profile in Fig.2) is clear. In particular, it was observed that the sedimentary contact between granular

sub base and clay-silty levels (H4) deeps below the concrete reaching a depth close to 2.5 m and it

disappears in the central sector of longitudinal profiles (maps in Fig.3 and Fig.4).

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0.9

0m 100m 200m 300m 400m

me

ters

[v=

11

.33

cm/n

s.]

GPS

High frequency GPR antennas

odometer

We tested R.Ex. system on two different sites on an airport pavement in order to:

1) give information on the stratigraphic sequence up to about 2m of depth from the surface, recognize possible

failures or subsidences structures, and highlight possible lateral variations of the geotechnical properties

(Test Site1);

2) verify the thickness of new constructed FRC (Fiber Reinforced Concrete) plates (Test Site 2).

Test Site 1 The example belongs to a survey of 149Km of recently paved roads where we used

800 and 2300 MHz GPR antennas. We recorded a total of 351 Km of data, composed by the

acquisition along each road lane in the two track directions.

In Fig. 6 the GPR profile is peculiar because it evidences strong lateral variations of thickness of

the aggregates layers (here composed only by binder and base). We interpreted the trend of

the conglomerate layers (binder and base interfaces, in green and yellow) mainly using the high

frequency antennas, while within the 800 MHz data we interpreted the horizons associated to

the old pavement (in pink) and subgrade (in light blue).

The imaged strong lateral variations, have been observed also by the FWD data response,

associated to strong inhomogeneities of the road embankment, subsidence structures, finally

ascribed to water seepages.

The depth conversion has been obtained using a variable velocity field, applying a lower

velocity to the subgrade respect that applied to bituminous conglomerate.

Test Site 2 The example belongs to a survey performed on a new paved road.

Stratification is composed by the three aggregate layers well evidenced by GPR data shown in Fig.7. Here the thickness is almost regular and

homogeneous demonstrating the high accuracy during the road buiding and asphalting phases.

In this case we used a constant lateral velocity field calibrated by the available cores.

Dep

th (

m)

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TRACE 2500 200050m

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z

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concrete interface of aircraft apron

conglomerate interface of external sector of the apron

probably saturated material

Fig5. a-1.6GHz profile in CASE1-Test site 2 and b- detail of the contact between the different pavements of the aircraft apron and the external area of transit.

b

b

AcknowledgmentsWe gratefully acknowledge Pavenco srl (Eng. F. Picariello), Emaprice srl (Dr. Eng. M. Grippo Belfi), Elletipi srl (Eng. C. Bratti), Acmar scpa (Eng. M. Lenzi) for the availability of using data for research purpose. All the dataset shown have been acquired by Esplora srl.

800M

Hz

500M

Hz

800M

Hz

Fig.1 : R.Ex. System and its components

We report two examples of R.Ex. application on new paved roads to evaluate the roads condition and verify the thickness of the layers, locating possible defects and lateral variations (Test site-1 and 2).

Fig 5. GPR profile acquired in a new paved road. This picture is composed by 2.3GHz data from 0 to 0.25m and 800MHz from 0.25 to 0.8m from the topographic surface. The strong lateral variation in the layers thickness is apparent.

Fig 7. GPR profile acquired in new paved road; the image is composed by 2.3GHz data from 0 to 0.3m and 800MHz from 0.3 to 1.0m from the topographic surface.

Binder interface

Base interface Subgrade interface

old pavement

wearing course interface binder interface base interface subgrade interface

References Briggs R.C., Scuillon T., and Maser K.; 1991: Asphalt thickness variation an effect of backcalculation. 2nd International Conference on Backcalculation. Nashville, USA Dossi M., Forte E., Pipan M.; 2015a: Automated reflection picking and polarity assessment through attribute analysis: theory and application to synthetic and real GPR data. Geophysics, 80 (5),

H23-H35 Dossi M., Forte E. and Pipan M.; 2015b: Auto-picking and phase assessment by means of attribute analysis applied to GPR pavement inspection. Proceedings of IWAGPR 2015, 3-7 July 2015,

Florence, Italy, in press. Dossi M., Forte E., Pipan M.; 2015c: Application of attribute-based automated picking to GPR and seismic surveys. Proceedings of the 2015 GNGTS, Trieste, 17-19 November 2015. Forte E., Dossi M., Colucci R.R., Pipan M.: 2013: A new fast methodology to estimate the density of frozen materials by means of common offset GPR data. Special issue of the J. of Applied

Geophysics: Forward and Inverse Problems in GPR Research, 99, 135-145. Saarenketo, T.; 2009: NDT Transportation. Chapter 13 in text book ―Ground Penetrating Radar: Theoryand Applications. Ed- Harry M. Jol. Publisher Elsevier, 524 p. Topp G.C., Davis J.L., Annan A.P.; 1980: Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resources Research, 16, 574–582.

H1H3a

H3b

H4

Fig 4.: 500MHz GPR profile showing: the inner stratigraphy down to 3m of depth; the sedimentary contact H4 which stops close to the midpoint of the profile; another sedimentary contact in violet showing variable depth below horizon H3b that deeps in the same position. H3a (pink) is almost constant below the concrete interface (H1).

Trieste, 17-19 November 2015

H3a-inner subgrade horizon

Fig 2.: GPR profile composed by 1.6GHz data from 0 to10ns and 500MHz data from 10 to 60ns TWT; the arrow indicates the transition from concrete to conglomerate.

0m

0m

78m

490m