Y. Shioi
Hachinohe Institute of Technology, Hachinohe, Aomori, Japan
T. Sakai
Applied Research Co. Ltd. Tsukuba, Ibaraki, Japan
value, the surface should remain intact. However, if there is excess traffic volume or vehicles with excessive load, the surface course will deteriorate and fine cracks, holes and corrugation will occur. After this, vehicles will impact the pavement and the breakages in the surface will progress rapidly. In particular, deterioration in the base course will severely affect the surface.
We know from daily experience that when a heavy vehicle passes over a hole in the surface, a strong impact generates and spreads. The impact transfers into the ground and may crush or powder weak aggregate of the base course. Photo 1 shows a com-parison between aggregate in an original state and crushed aggregate in a tri-axial compression test. As shown in Photo 1, crushed or powdered aggregate no longer has much bearing capacity and deformation and creep in it steadily becomes larger.
2 MEASUREMENT OF VIBRATION INCLUDING IMPACT
To examine the influence of impact at the edge of a road, we measured the vibration due to a vehicle passing over a stepped section. The sensor used was a part of the 46 DESIGN ANDCONSTRUCTION OFPAVEMENTS ANDRAILTRACKS
Figure 1. Standard composition of asphalt pavement.
Photo 1. Comparison of aggregate in an original and a crashed state.
seismograph newly developed to measure high cycle seismic waves below 500 Hz.
The step-up of the system is shown in Figure 2 and it has the following specifications.
Sensor: piezo-register type piezo-electric element Number of input channels: 3 ch
Measurement range: 5 g Frequency range: 500 Hz Interval of samples: 2 kHz A/D accuracy: 12 bits
Trigger: external (horizontal vibration sensors) Length of data: up to 300 sec
1/F: RS-232C
Records directly to a PC
The simplified one-way sensor shown in Figure 3 was taken from this system, Photos 2 and 3 show the sensor in position on the surface of a road. It has the fol-lowing specifications.
Accelerometer: PCB393A03
Components of measurement: Vertical component only Range of measurement: micro vibrometer 0.1 gal 100 gal
wide range vibrometer 10 gal 10 G
Length of time of measurement: 1 min. (maximum)/1 record
INFLUENCE OFIMPACTLOAD ONBASECOURSE ANDSUB-GRADE BYCIRCULATION 47
Figure 2. Diagram of seismograph for high frequency waves.
Accuracy of AD converter: 12 bit 2ch Trigger level: optional
Frequency of measurement: 1 kHz AD speed 2 kHz(0.5 ms)
A series of travel tests was performed on a road with a truck in the grounds of a research institute. The empty weight of the truck was 4 ton and the total laden weight was 8 ton. A board 25 mm thick, 2.5 m long and 200 mm wide was fixed on the road. The speed of the vehicle speed was set at 40 km/h.
Figures 4 and 5 show a record of the acceleration waves measured and the power spectrum of the density function of the waves for the 4 ton truck passing over the board. Figures 6 and 7 are the equivalent record and the spectrum for the same truck loaded to weigh 8 tons. The records and spectra for the 4 ton truck and the 8 ton truck are almost identical. The records for the loaded truck (8 ton) show slightly larger val-ues than the records for the empty truck (4 ton).
3 MEASURED RESULTS AND NOTES
The fact that the shapes of the two sets of waves and spectra are identical with two peaks near 20 Hz, means that the influence of loading is small probably due to the 48 DESIGN ANDCONSTRUCTION OFPAVEMENTS ANDRAILTRACKS
Figure 3. Simplified one-way seismograph.
Photo 2. The sensor in position. Photo 3. Close-up of sensor.
tires and suspension springs of the vehicle. The predominant peaks near 20 Hz at the side of the road are higher than the vibration of the vehicles itself and appear to indi-cate a different vibration. From 20 Hz to 100 Hz the values of acceleration decrease straightly and the waves at the range higher than 100 Hz maintain constant values larger than the micro tremor. Since the small values measured are disagreeable INFLUENCE OFIMPACTLOAD ONBASECOURSE ANDSUB-GRADE BYCIRCULATION 49
-40-20 0204060 0 1 2 3 4 5
Time (s)
Acc.(gal)
Figure 4. Acceleration of the empty truck (4 ton).
0.00010.001 0.01 0.1 110100100010000 1 10 100 1000
Frequency (Hz)
Power (gal2/Hz)
Figure 5. Spectrum of the power of the density function (4 ton truck).
with our actual sensation and it is questionable to promote crushing the pavement, we feel the necessity to perform tests of a wider range of cases using larger holes and steps.
However, it is clear from our past research that the type of force to break the pavement or structures is a sharp shear force. Figure 8 shows the impact wave from a test involving dropping a steel block of 100 N and Figure 9 shows the correspond-ing wave spectrum. The block fell to a reinforced concrete floor from a height of 50 DESIGN ANDCONSTRUCTION OFPAVEMENTS ANDRAILTRACKS
-40-20 0204060 0 1 2 3 4 5
Time (s)
Acc.(gal)
Figure 6. Acceleration of the loaded truck (8 ton).
0.00010.001 0.01 0.1 110100100010000 1 10 100 1000
Frequency (Hz)
Power (gal2/Hz)
Figure 7. Spectrum of the power of the density function (8 ton truck).
15 cm 30 cm away from the sensor. The three groups of waves indicate that the block rebounded. The maximum acceleration was about 300 gal and the largest values distributed at the high cycle range of more than 200 Hz. The level of these values was enough to cause damage to pavement if this action repeated. During pile driv-ing, by contrast, the maximum value through the pile is a few hundred G.
Although it was found that the impact from an automobile passing over a small step is not very large, mechanical impacts are large enough that to break and pow-der surface courses or the aggregate in the base course. The phenomenon of the mud pumping on track bed may be caused by repeated vibration with impact
INFLUENCE OFIMPACTLOAD ONBASECOURSE ANDSUB-GRADE BYCIRCULATION 51
-400-300-200-100 0100200300400 0 0.5 1 1.5 2 2.5
Time (s)
Acc.(gal)
Figure 8. Impact waves from a test involving dropping a 10 kg steel block.
10 kg L = 0.3 m H = 0.15 m
0.01 0.1 110 1 10 100 1000
Frequency (Hz)
Spectrum (gal)
Figure 9. Spectrum of impact waves from a 10 kg steel block.
4 CONCLUSIONS
The following information was obtained from a series of studies into the relation between impacts from vehicles and damage to a pavement with a newly developed seismograph.
(1) The impact waves from the measured vibrations caused by a vehicle of 4 ton passing over a step of 25 mm had a relatively small acceleration with the pre-dominant cycles being near 20 Hz.
(2) It was found that the load in the truck hardly influences the acceleration or spectrum of the vibration on the surface of road when it passes over a step, but this requires confirmation.
(3) The spectrum of the density function for the acceleration of vibration declines continuously between 20 Hz and 100 Hz and maintains a constant level larger than that of micro tremor at cycles higher than 100 Hz.
(4) Although the level of acceleration caused by the vehicle was not very high, given the other examples of impact measured, there is the possibility that repeated impact waves will damage the pavement.
(5) Since it is doubtful whether the values of acceleration measured in this paper will damage the pavement, tests of a wider range of cases will be required.
52 DESIGN ANDCONSTRUCTION OFPAVEMENTS ANDRAILTRACKS
ABSTRACT: This paper introduces a new testing method to examine the effects of the rotation of principal stress axes on cyclic plastic deformation of railroad bal-last. “Multi-ring shear apparatus” was newly developed as a kind of torsional simple shear test apparatus, and the applicability of the multi-ring shear test to an element test of railroad ballast subjected to moving wheel loads was examined by compar-ing test results of the multi-rcompar-ing shear tests with those of model tests. As the results, it was revealed that the multi-ring shear test is appropriate for simulating the actual stress states inside substructures under train loads.
1 INTRODUCTION
The study of “Track deterioration” is one of the principal assignments in railway engineering because track deterioration has serious consequences on the safety of train operation. Track deterioration observed mainly at ballasted tracks as shown in Figure 1 is a phenomenon such that the rail level at train passages is irregular toward the longitudinal direction of railway track with repeated train passages. In general, a dominant factor of track deterioration is supposed to be uneven subsidence of railroad ballast, a pile of well-compacted crushed stones, caused by cyclic wheel loading. The mechanism of track deterioration is being researched in many countries.
Nevertheless, there are a number of points uncertain in the mechanism of track deteri-oration at the present stage. One of the reasons for this seems to lie in the loading methods of conventional model tests and laboratory tests.
So far, a variety of loading tests with model track and model ballast have been performed by many researchers in order to elucidate the cyclic plastic deformation of railroad ballast (Raymond & Bathurst 1987, Kohata et al. 1999). For example, triaxial compression tests with constant amplitude of deviator stress while controlling confining pressure constant have been performed as the element test to elucidate