IJERD – International Journal of Environmental and Rural Development (2017) 8-1

Research article
erd

Development of a Device for Measuring the Load Bearing
Capacity of a Farm Road
KOJI WATANABE*
Graduated School of Agriculture, Tokyo University of Agriculture, Tokyo, Japan
Email: 45515008@nodai.ac.jp

FUTOSHI KAWANA
Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo, Japan

YASUSHI TAKEUCHI
Faculty of Regional Environmental Science, Tokyo University of Agriculture, Tokyo, Japan
Received 14 December 2016

Accepted 5 May 2017(*Corresponding Author)

Abstract The maintenance of infrastructure for agricultural production is essential tosustainable agricultural productivity. Farm roads are particularly important infrastructure fortransporting agricultural products. Load-bearing capacity is a performance index for evaluating
the soundness of a road, and recently, falling weight deflectometer (FWD) tests have been used
to measure such capacity. However, an FWD is a stationary device, so its use is time intensive.
This study aims to develop a moving wheel deflectometer (MWD) device that determines theload-bearing capacity continuously and economically, for the efficient management of farmroads. For that reason, the development of the MWD involved fabrication, testing and theverification of its practicality on a public road. The MWD was used to estimate the load-bearing
capacity of a public road at travel speeds of 10 to 60 km/h. The soundness evaluated based on
the MWD measurements were shown to be comparable to that as evaluated by the FWD at all
travel speeds. The results demonstrate that the MWD has the potential to be used for thecomprehensive measurement of load-bearing capacity.
Keywords pavement management, soundness, load-bearing capacity,
falling-weight deflectometer

INTRODUCTIONThe maintenance of agricultural infrastructure is essential for sustainable agricultural productivity, andmaintenance and repair should be implemented at times and scales that extend the service life.
Therefore, a sustainable, recycling-oriented society can’t be realized without preventive management
system for infrastructure. To apply that system to farm road management, the soundness ofinfrastructure must be quantitatively evaluated. In general, a road’s soundness is evaluated in terms of
its surface condition and load-bearing capacity. Road surface measuring vehicles are able to obtain
continuous data on road surface conditions, such as cracking, rutting and roughness, along a survey
road, so such devices enables the efficient determination of road conditions. In contrast, load-bearing
capacity is evaluated in terms of road surface deflection. The falling weight deflectometer (FWD) test
has been used to obtain road surface deflection data. This non-destructive testing device accurately
measures road surface deflection, however, its use is too time intensive for comprehensive roadmeasurement. Also, the FWD is a stationary device, which means that there is the risk of overlooking
localized damage between measurement points. Therefore, a method needs to be developed for
determining the load-bearing capacity continuously at high travel speed.
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Devices that continuously measure road surface deflection have been developed in other countries
since 1990 (Kano, 2008). The rolling wheel deflectometer (USA), road deflection tester (Sweden) andhigh-speed deflectometer (Denmark) have been put into practical use (Maruyama, 2012; Flintsch et al.,2012). These devices determine load-bearing capacity by road surface deflection measured by Laser
displacement sensors or assumed by deflection velocity measured by Doppler vibrometer (Van, 2009;
Elseifi 2012; Diefenderfer et al., 2008; Ferne et al., 2009). The Laser displacement sensor measuresdirectly the distance between the sensor and the road surface. This direct measurement enables toobtain road surface deflection easily without numerical calculation. Also, the deflection measurement
results are independent of running speed. However, the measurement accuracy is closely related toroad surface conditions, and measurement at curved sections is difficult. In contrast, the measurement
results by Doppler vibrometer are not affected by road surface conditions because the device measuresthe deformation velocity of the road surface. In addition, deflection can be assumed by integration ofdeflection velocity. However, the deflection velocity is zero in the stationary condition. This means
evaluation becomes difficult at slow running speed.
In general, Farm road has important roles of not only transporting products but also carrying inand out agricultural machines. Measurement at high travel speed makes ensuring safety difficult duringrunning test. In addition, there are many curved sections and narrow sections in farm roads. Currently
available moving devices for measuring load bearing capacity are used by heavy trailer vehicles thatgenerate large deflection, for example RWD consisting of a 16.3 m long semitrailer. It means thesedevices are not necessarily suitable for farm road management. Therefore, a device to be used bysmaller vehicles and which is capable of measuring load bearing capacity without effect on travel
speed should be developed. For that reason we developed a moving wheel deflectometer (MWD) todetermine which points of a deteriorating road sections require more detailed survey, such as FWD test
(Terada, 2012; Matui, 2013). The MWD performes measurements by using a laser displacement sensor
in combination with a Doppler vibrometer, and consists of 8.25 m vehicle. The Doppler vibrometer
have been used to measure deflection velocity at high travel speed, and laser displacement sensors havebeen used at low travel speeds. In the present work, we propose a method for evaluating the pavementload-bearing capacity on farm roads by using an MWD equipped with a laser displacement sensor, andgive the results of measurements on a public road.
METHODOLOGY
Outline of the MWD

Bearing
Deck for sensor
Odometer
450

Fig. 1 Installation state of deck for sensor

450

Laser displacement sensor

Unit: mmFig. 2 Sensor configuration

In this study, load-bearing capacity is evaluated in terms of the deflection under the rear wheel asshown in Fig. 1. The deflection is measured by laser displacement sensor. Because of the vehicle’s
suspension, the axle and the body frame do not move in unison. Hence, as shown in Fig. 1, in order toⒸ ISERD50IJERD – International Journal of Environmental and Rural Development (2017) 8-1

make the sensor and axle move in unison, a sensor deck was attached to the wheel axle by the axle
bearing. The sensor arrangement is shown in Fig. 2. One sensor is installed at the wheel axle, and othersensors are installed ±450 mm from the wheel axle. Additionally, running distance is measured byodometer installed in the rear body
Evaluation Method
The MWD test determines the load-bearing capacity based on road surface deflection generated by rear
wheel load. Fig. 3 shows the concept of the determination method proposed in this study. In Fig. 3, thex-axis indicates the vehicle travel direction and the y-axis indicates the deflection direction. The originof the x-axis is set at the wheel axle. Pavement deflection at the rear wheel is calculated by thefollowing equation in the case of the ideal condition without vibration or torsion of the vehicle body.

w0 =0(1)dWhere,𝑤0 : road deflection at the rear wheel (𝑥 = 0)
∆0 : distance between the wheel axle and the road surface at the rear wheel (𝑥 = 0)
∆𝑑 : distance between the wheel axle and the road surface without deflection (𝑥 = 𝑑)
The wheel radius and deformation are R and δ, respectively. If δ does not depend on pavement
rigidity, then the difference between R and δ is always constant.
0=R= Const(2)As mentioned above, the deflection immediately below the rear wheel can be calculated byequation (1) under ideal conditions. However, measurement at the position 𝑥 = 𝑑 is difficult, because
measurement value in that position is influenced by vibration of sensor and the tilt of vehicle body inpractical measurement. For that reason, as shown in Fig. 4, a deck for the sensors was attached to thewheel axle, and three sensors were attached so as to measure maximum deflection at the position 𝑥 = 0
and deflection at the position 𝑥 = 𝑎, −𝑎; also, the deck is designed to tilt around the wheel axle.
Therefore, in this study, the load bearing capacity was evaluated by the “deflection difference,” i.e., thedifference between deflection generated just under the rear wheel and deflection generated at any otherpoint (𝑥 = 𝑎). Assuming that the deflection curve is generated symmetrically with respect to the yaxis, the error of deflection measurement at the position 𝑥 = 𝑎 due to the tilt of the sensor deck can beeliminated by averaging the measurement value at the position 𝑥 = 𝑎 and measurement value at theposition 𝑥 = −𝑎. Deflection difference is defined by following equation.

w=a=0

12(3a)

a(f+b)(3b)

Where,𝑤 : deflection difference between deflection just under rear wheel and that at any other∆𝑓 : distance between the sensor and the road surface at the position 𝑥 = 𝑎∆𝑏 : distance between the sensor and the road surface at the position 𝑥 = −𝑎

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Deck for sensorWheel axle

x = －aR

x=dΔ0Δdw0

x=ax

Δbwyx

Δ0y

Fig. 3 Concept of deflection

ΔfΔa: Laser displacement sensor

Fig. 4 Correction of displacementfor the sensor deck tiltMWD Testing
The MWD test was carried out at the testing course of the Public Works Research Institute in Tukuba,Japan (Fig. 5). The length of the testing section was approximately 1400 m. To assess repeatability andthe effects of running speed, measurement was implemented three times for each with six runningspeed (10, 20, 30, 40, 50 and 60 km/h). Also, FWD test was conducted at the same test course todemonstrate accuracy of MWD. The pavement consists of an asphalt layer (14 cm) and a granulated
subbase course (31 cm), and there are many cracks at several points. The sampling frequency was 2000
Hz. The wheel load in a static state was 46.5 kN.

MWD test section(About 1400 m)

Fig. 5 Section for the MWD test
RESULTS AND DISCUSSION
Fig. 6 illustrates the measured pavement surface deflection difference, i.e., the difference between
deflection just under rear wheel ( 𝑥 = 0 ) and deflection at an arbitrary point ( 𝑒. 𝑔. , 𝑥 = 45cm )(hereinafter referred to as (D0−D45)) calculated by equation (3a). The MWD test was implemented
three times for each running speed. As shown in Fig. 6, the results at each running speed were similar,
which means that the measurements are repeatable. In addition, it is shown that the measurement
accuracy is not closely dependent on the running speed. The results of FWD testing for a given
measurement section is also presented in Fig. 6. In general, road surface deflection is affected bypavement temperature. Hence, temperature correction is necessary to get rid of effect of temperature on
the measured deflection, and the MWD and FWD can measure pavement temperature during operation.
The MWD and FWD tests were conducted in November and December respectively. As a result oftemperature measurement, there was not almost difference of the temperature. For that reason, it wasⒸ ISERD52IJERD – International Journal of Environmental and Rural Development (2017) 8-1

judged that there was no need for temperature correction. Also, the loads of MWD and FWD aredifferent. Hence, the results of FWD testing are those after linear correction of load. As shown in Fig.6,
it is showed that the results of MWD testing agree substantially with that of FWD. For example, thepart and number of the biggest (D0−D45) obtained by MWD test are consistent with that of FWD test.
It is possible that sensor deck designed to tilt around the wheel axle cleared measurement errors causedby tilt of sensor deck. In contrast, the results showed the scattering and uniformity of FWD and MWD
data in approximately 350 m. It is possible that this is attributed to greater vibration and bouncing
caused by road roughness during measurement. Also, these measurement results include effects onload change and impact load during measurement. Accordingly, the value of (D0−D45) does not always
need to be accurate. However, in comparison with RWD that have been put into practical use, theseresults indicate that the MWD have enough accuracy as the device to specify deteriorating roadsections which need detailed survey. Finally, the study determined from these results that the MWD
has applicability as moving device for measuring load bearing capacity.
2000

10kmph-1st
10kmph-2nd
10kmph-3rd
FWD(D0-D45)

D0-D45 [μm]

1500
1000
500
00200

400

600
800
Disance [m]

1000

1200

1400

400600800
Disance [m]

1000

1200

1400

400600800
Disance [m]100012001400400600800
Disance [m]100012001400(a) Running Speed: 10 km/h
D0-D45 [μm]2000

20kmph-1st
20kmph-2nd
20kmph-3rd
FWD(D0-D45)

1500
100050000200(b) Running Speed: 20 km/h
2000

30kmph-1st
30kmph-2nd
30kmph-3rd
FWD(D0-D45)

D0-D45 [μm]

1500
1000

50000200(c) Running Speed: 30 km/h
D0-D45 [μm]2000

40kmph-1st
40kmph-2nd
40kmph-3rd
FWD(D0-D45)

1500
1000

50000200(d) Running Speed: 40 km/h
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D0-D45 [μm]200050kmph-1st
50kmph-2nd
50kmph-3rd
FWD(D0-D45)

1500
1000
500
00200

400600800
Disance [m]

1000

1200

1400

400600800
Disance [m]

1000

1200

1400(e) Running Speed: 50 km/h

D0-D45 [μm]2000

60kmph-1st
60kmph-2nd
60kmph-3rd
FWD(D0-D45)

1500

1000
500
00200(f) Running Speed: 60 km/h
Fig. 6 The deflection difference (D0-D45)
CONCLUSION
This study aims at developing a device for measuring the load bearing capacity and at establishing anefficient method for evaluating the soundness of farm roads. In this paper, we proposed adetermination method that uses (D0−D45) obtained by MWD test. The results of MWD measurement
showed that MWD deflection measurements are repeatable. Also, MWD measurement is not closely
dependent on the running speed at travel speeds of 10 to 60 km/h. In addition, the results of MWD
testing were agreement with that of FWD. Therefore, it is showed that the applicability of the MWD
was demonstrated. A further direction of this study will be to accumulate test data under various
conditions in order to improve the proposed method.
ACKNOWLEDGEMENTS
This study was done in collaboration with the National Institute for Land and InfrastructureManagement. We are grateful to all those who provided assistance.
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