Investigations on Traffic Vibration around Telecommunication Facilities: Maintenance Holes as an Example

1. Investigate the vibration state around the maintenance hole by measuring vibration levels.

2. Investigate the presence or absence of resonance with the maintenance hole by measuring the frequency of traffic vibration.

2.1 Investigation 1: Vibration levels

(1) Overview of investigation

The spaces in which people live are subject to numerous vibrations such as those caused by factory machinery, by civil engineering and construction work, and by trains and large vehicles.

To protect the living environment of local residents, the Vibration Regulation Act [1] was enacted in 1976 to set limits on the vibration level (i.e., an index, measured in decibels, expressing the magnitude of vibration). Since then, the government has taken vibration-prevention measures such as providing guidelines to businesses and repairing road surfaces. The required limits on vibration levels are set by the enforcement regulations of the Vibration Regulation Act as listed (as standard values) in Table 1. The vibration level for each class of vibration is measured and evaluated on the basis of whether it exceeds the standard values stipulated in Table 1. In investigation 1, the vibration level around a target maintenance hole installed in a road passing through a residential area was measured.

Table 1. Standard values of vibration levels stipulated by the Vibration Regulation Act.

(2) Method of investigation

Vibration levels were measured using the measurement equipment specified in JIS (Japanese Industrial Standards) C 1510. As shown in Fig. 1, the measurement points were near the target maintenance hole in a road (point B) and at two nearby (30 m from B) locations where maintenance holes are not buried in the road (points A and C). To ensure measurement reproducibility, a bucket truck (weight approximately 6 t) was driven past points A, B, and C multiple times (at speed of 50 km/h), and the vibrations generated by the moving truck were measured at each point. The vibration direction measured was taken as vertical in accordance with JIS Z 8735.

Fig. 1. Location of measurement points in vibration investigation.

The three general patterns of the measured vibration waveforms are shown in Fig. 2. According to the guidelines of the Ministry of the Environment [2], it is recommended that the method of measuring the vibration level be changed according to the vibration pattern being measured. For example, when the vibration-level meter readings do not fluctuate or fluctuate slightly (Fig. 2(a)), each reading can be used as the measured level as is; in contrast, when the readings fluctuate intermittently or periodically (Fig. 2(b)), the average of the maximum measured values for each fluctuation is taken as the measured level. Traffic vibration, which occurs only when vehicles are passing, corresponds to the pattern shown in Fig. 2(c); namely, the vibration-level meter reading fluctuates irregularly and significantly. In this case, the upper limit of the 80% range is taken as the measured vibration level and its magnitude is evaluated.

Fig. 2. General patterns of vibration waveforms.

The 80% range refers to the remaining 80% of multiple measured vibration levels arranged in order of magnitude, after the maximum 10% and minimum 10% have been excluded, as shown in Fig. 3. The maximum vibration level within the 80% range is taken as the upper limit of the 80% range (Lv₁₀).

Fig. 3. Example of 80% range and its upper limit.

(3) Results of investigation

The measured vibration levels (Lv₁₀) are listed in Table 2. According to these investigation results, none of the vibration levels measured at the three measurement points exceeded the standard values stipulated in the Vibration Regulation Act. Also, Lv₁₀ values measured near the maintenance hole (point B) and at the other two points (A and C) did not significantly differ, indicating no particularly large (or small) vibrations occurred near the maintenance hole. It can be concluded from these results that when the vehicle passed the target maintenance hole, no vibrations that exceeded the legal standard values were generated, and regardless of whether a maintenance hole was buried nearby, no significant differences in the vibration levels at each measurement point were observed.

Table 2. Upper limit of 80% range of vibration level at each point.

2.2 Investigation 2: Vibration frequency

(1) Overview of investigation

We investigated vibration frequency to ascertain any effect of the maintenance hole on traffic vibration. It is thought that the effect of a maintenance hole (buried underground) on traffic vibration is one of resonance between the traffic vibrations and the maintenance hole that amplifies the vibrations. We therefore verified whether the maintenance hole resonated by comparing the frequency of vibrations observed on site with the resonance frequency of the maintenance-hole casing.

(2) Method of investigation

First, we created a three-dimensional (3D) model (see Fig. 4) from a drawing of a maintenance-hole casing and calculated the resonance frequency by displacing the 3D model by simulation with the finite element method.

Fig. 4. 3D model of maintenance hole and example of simulation related to resonance frequency.

Resonance frequency is unique to the object in question, and when vibrations with that frequency are applied externally to the object, resonance occurs and the vibrations are amplified. According to the results of the simulation, the natural (resonance) frequency of the target maintenance hole was estimated to be in the range of 228.1 to 359.6 Hz.

The vibration frequency on site was obtained by measuring vibration acceleration by using acceleration sensors installed at the site, and the obtained acceleration data were converted to frequency by using a Fourier transform. The acquisition interval for the acceleration data was set to 0.1 ms, and the measurement points were the three points used in investigation 1, as shown in Fig. 1.

(3) Results of investigation

Vibration frequency measured on site when the truck passed by is plotted against vibration acceleration in Fig. 5. According to this plot, vibration frequency is prominent at around 20 Hz at all measurement points (A to C), and almost no vibration was detected in the maintenance hole’s resonance-frequency range (shown in orange in the figure). These measurement results are consistent with the results of similar investigations conducted by TASC as well as with other published results [3, 4].

Fig. 5. Vibration frequencies measured at each measurement point.

The above results of investigation 2 suggest that the target maintenance hole does not resonate with traffic vibrations; in other words, it does not affect (amplify) traffic vibrations.