LOW FREQUENCY VIBRATION FROM LIGHT RAIL …docs.trb.org/prp/17-00536.pdf · 1 LOW FREQUENCY VIBRATION FROM LIGHT RAIL VEHICLES ... 15 vibration monitoring system ... 19 The MIA limits

Saurenman, H.J., Nelson, J.T., Rajaram, S. Page 1

LOW FREQUENCY VIBRATION FROM LIGHT RAIL VEHICLES 1

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Hugh Saurenman (Corresponding author) 3

ATS Consulting 4

215 N Marengo Avenue, Suite 100 5

Pasadena, CA 91101 6

1-626-710-4400 7

[email protected] 8

9

Shankar Rajaram 10

Sound Transit 11

425 S Jackson Street 12

Seattle, WA 98109 13

1-206-903-7336 14

[email protected] 15

16

James T. Nelson 17

Wilson, Ihrig & Associates, Inc. 18

6001 Shellmound Street, Suite 400 19

Emeryville, CA 94608 20

[email protected] 21

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Submitted: August 1, 2016 38

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Word count: 5241 words text + 9 figures x 250 words (each) = 7491 total 40

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ABSTRACT 1

The question of how much low-frequency vibration is generated by operations of typical light 2

rail vehicles is an important issue when a proposed light rail system will be in close proximity to 3

vibration sensitive research facilities. In this paper we define low-frequency vibration to be 4

vibration radiated at frequencies below 25 Hz because rail transit vibration often is lower than 5

the vibration from buses and trucks at frequencies below 25 Hz. This paper is largely based on 6

vibration measurements vibration performed in support of the vibration monitoring system that 7

was installed in the University-Link extension of the Seattle Link LRT system. The monitoring 8

system is designed to protect vibration sensitive research facilities that are relatively close to the 9

subway. The testing has shown that there is substantial low-frequency vibration at the vibration 10

monitors from traffic on nearby roadways and that this vibration is often greater than the 11

vibration generated by train operations. In other words, at low frequencies, the vibration from 12

trains passing about 10 ft from the monitors is often less than the vibration created by traffic that 13

is at least 200 ft from the monitors. This is a non-intuitive result, and the primary purpose of this 14

paper is to examine possible reasons for this result and what factors contribute to the low-15

frequency traffic vibration being greater than the low-frequency train vibration. The results of 16

this analysis are applicable to other rail transit projects where low frequency vibration interfering 17

with research equipment is a concern. 18

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INTRODUCTION 1

This paper is basically a case history of a detailed exploration of low-frequency vibration from 2

train operations in a subway, the University Link (U-Link) extension of the Sound Transit Link 3

light rail system. Researchers at the University of Washington (UW) Seattle campus have been 4

concerned that vibration from the light rail extension through the campus would cause problems 5

with various vibration sensitive research facilities that are distributed throughout the campus. 6

The departments concerned about the low frequency vibration from rail operations affecting their 7

research equipment ranged from the Fisheries Center to the Forest Sciences Laboratory to 8

various physics, engineering, and materials science laboratories. 9

Negotiations between UW and Sound Transit led to several accommodations by Sound Transit to 10

minimize the potential for vibration and electromagnetic fields (EMF) to interfere with the 11

research equipment. This paper focuses on vibration and does not cover the EMF issue. The 12

negotiations led to a Master Implementation Agreement (MIA) signed by the CEO of Sound 13

Transit and the President of the University. This agreement includes specific requirements for a 14

vibration monitoring system located in the subway that provides real-time alerts whenever the 15

vibration levels in nearby buildings may exceed the maximum allowed vibration levels. The 16

MIA provides vibration limits in terms of 1/3 octave band velocity levels over the 2 to 100 Hz 17

range for 24 buildings. 18

The MIA limits are based on measurements of ambient vibration that were performed prior to the 19

start of construction of the U-Link extension. The test results presented in this report were 20

performed as part of the effort to estimate the relationships between vibration at the monitoring 21

system stations in the subway and the train vibration inside the 24 buildings identified in the 22

MIA. The buildings covered by the MIA limits are labeled in red in Figure 1. The four buildings 23

where vibration measurements were performed are the Center for Human Development and 24

Disability (CHDD), the University of Washington Medical Center Cyclotron facility (UWMC), 25

Winkenwerder Forest Sciences Laboratory, and Wilcox Hall that is used by a variety of 26

engineering departments. As seen in Figure 1, the horizontal distances from the subway to these 27

buildings ranged from approximately 600 ft to 1,000 ft. The subway is approximately 100 ft 28

deep. 29

The vibration monitoring system was supplied by International Electronic Machines, Corp. 30

Relevant to this paper are the nine monitoring stations in the subway segment near the UW 31

campus. The monitors are at 150 ft intervals alternating between the northbound and southbound 32

tunnels. Monitors 4-1 through 4-3 are in the bored tunnels, Monitors 5-1 through 5-3 are adjacent 33

to the crossover structure south of the UW Station, and Monitors 5-4 through 5-6 are in the UW 34

Station. Each of the nine monitors are programmed to collect data every time a train is within a 35

set distance of the monitor and to report the maximum 8-second RMS vibration velocity level 36

(Lmax) during the period that the train is within the set distance of the monitor. The data are 37

collected at 1-second intervals and the 8-second RMS levels are calculated from the 1-second 38

data. The 1/3 octave band Lmax spectra of are provided by the monitor software for each train 39

event. 40

The system allows specifying adjustment factors that are used to estimate the building vibration 41

levels from the Lmax levels for each train event. In this report the adjustment factors are referred 42

to as Vibration Adjustment Estimates (VAEs). The monitoring system software adds the 43


appropriate VAEs at each building to the measured Lmax by each monitor to determine if any of 1

the building vibration levels may exceed an MIA limit. 2

The remainder of this paper outlines the measurement programs, discusses the results of the 3

measurement programs, and presents a discussion of the theoretical considerations relative to 4

when and at what amplitudes vibration in the 2 to 6 Hz range will occur. 5

6

Figure 1. University of Washington Campus Map. 7

Buildings identified by UW as sensitive buildings are labeled in red. 8

Measurements were performed at CHDD, the UW Medical Center Cyclotron, 9

Winkenwerder, and Wilcox. The distance to the subway is shown 10

adjacent to each of the measurement buildings. 11

CHALLENGES TO ESTIMATING VAES 12

The program to develop valid estimates of the VAEs began in July 2015 and needed to be 13

completed in time for the U-Link extension to open in March 2016. Before the system could 14

open the VAE values had to be uploaded to the monitoring system software and tested to confirm 15

that ambient vibration did not generate alarms and that the sum of the measured Lmax at the 16

monitors and the VAEs did not underestimate the train vibration levels at the sensitive buildings. 17

Some of the challenges to developing reasonable estimates for the VAEs were: 18

740 ft

916 ft

840 ft

593 ft


As seen in Figure 1, the closest sensitive buildings were 600 ft to 1,000 ft from the 1

subway. Ground vibration tends to attenuate relatively rapidly, which meant that 2

relatively low vibration levels at the buildings could be expected. 3

There are a number of structures in the paths between the subway and the buildings. The 4

structures include an underground parking structure, and various buildings, utilities, and 5

roadways. 6

There was considerable interest in the results of the measurements and analysis. The 7

stakeholders included the managers of the vibration sensitive facilities, the UW 8

executives and department heads who wanted to maintain the University’s reputation as a 9

world class research facility that could accommodate modern nanotechnology research, 10

and Sound Transit that wanted to both avoid false alarms and avoid exceeding the MIA 11

vibration limits. Another consideration is that this exercise was basically a dry run for the 12

analysis that will be required for the Northgate Link that is currently under construction. 13

The Northgate Link route will be considerably closer to a number of vibration sensitive 14

laboratories and will include a low-frequency floating slab system to control the vibration 15

levels. 16

In many cases, the train vibration was lower than the ambient inside buildings. The 17

ambient was due to equipment and activities within the buildings, traffic on nearby 18

streets, and various unidentified sources. The low levels of train vibration were due to the 19

distances of the buildings from the subway and the various vibration mitigation measures 20

that were used (e.g. soft fasteners and low impact frogs at the crossover). 21

Limited time to perform measurements and provide results. 22

A key issue is that UW and Sound Transit needed to mutually agree that the approach to and the 23

results were reasonable before the U-Link extension could open, which it did in March 2016. 24

MEASUREMENT PROGRAMS 25

The data available for evaluating ambient vibration at the U-Link subway monitors includes: 26

1. Measurements of train vibration and ambient vibration performed in August 2015. The 27

results of these measurements are documented in Reference 1. 28

2. Certification measurements that were performed by Wilson, Ihrig & Associates (WIA) on 29

August 18 and 19, 2015. These measurements are documented in Ref. 2. 30

3. Continuous monitoring at three VMS monitors from Monday November 23 through 31

Friday November 27, 2015. Thanksgiving Day was Thursday November 26, 2015 and a 32

football game was played at Husky Stadium on Friday November 27, 2015. The data 33

were provided by IEM. 34

4. Measurements performed on February 2016 to further investigate and document the 35

ambient vibration at the monitor sites in the subway and the train vibration levels at the 36

buildings. The results from those measurements are documented in Ref. 4. 37

The locations where train vibration measurements were performed in August 2015 are shown in 38

Figure 2. The measurements were performed on two mornings between midnight and 4 AM. The 39

first morning was focused on locations west of the subway and the second morning focused on 40


sites northwest of the subway. The measurements were performed with up to 20 seismic 1

accelerometers, four RION DA-20 four channel data recorders, plus a National Instruments 2

LabVIEW recording system with 12-channels. 3

The measurements west of the subway during the first morning of testing consisted of: 4

Four channels between the subway and Montlake Boulevard. 5

Three channels west of Montlake Boulevard. 6

Inside and outside measurements at CHDD and UWMC. 7

Vibration at each of the monitors. 8

The measurements the second morning of testing consisted of: 9

Four channels parallel to the north end of the station and Montlake Boulevard. 10

Two channels in a UW utility tunnel that was approximately halfway between the station 11

and Winkenwerder Hall. 12

Inside and outside measurements at Winkenwerder and Wilcox. 13

14

Figure 2: Surface Measurement Locations for Train and Ambient Testing (August 15

2015) 16

The supplementary measurements performed in February 2016 were just a few weeks prior to the 17

opening in March. Those measurements consisted of two phases: 18


(1) Measuring ambient vibration at three of the monitors, at the ground surface above the 1

monitors and close to Montlake Boulevard, and on one of the footings of the Montlake 2

Draw Bridge. These measurements were performed during the daytime to obtain 3

vibration from normal traffic. Videotaping of traffic also was performed to allow accurate 4

identification the trucks and buses that caused vibration events. 5

(2) Supplementary train vibration measurements using a test train that was purposely 6

selected because it had one or more wheel flats. 7

The purpose of the February measurements was to identify the source of the ambient vibration at 8

the monitors and to pin down the levels of train vibration in two of the buildings (CHDD and 9

UWMC). 10

A key feature of the test programs was time synching the recording systems. This was achieved 11

by assembling all of the systems in the same location and then triggering record simultaneously 12

on all of the devices using an electronic signal. The recording systems were then run 13

continuously until the end of the measurements, which ranged from 30 minutes to several hours. 14

There was some drift in the time synching between the five systems, but it was on the order of 15

10 msec or less, which was small enough to ignore. 16

Accelerometers were used for the measurements performed in August 2015 and geophones were 17

used for the February 2016 measurements. The accelerometers used had sensitivities of 1 V/g 18

and 10 V/g. In general, the 10 V/g accelerometers were used at the more distant locations. The 19

accelerometers used in the subway were all 1 V/g units. Due to instrumentation noise, the results 20

of the tunnel are not valid at frequencies below 10 to 20 Hz, which is the frequency range 21

focused on in this report. Therefore, the analysis was based on data from the VMS monitors and 22

the surface measurement positions. The geophones used for the February 2016 measurements 23

were similar to those used by IEM at the monitoring stations. 24

The accelerometer and geophone data were processed to obtain 1/3 octave band spectra at 1/2 25

second intervals. The acceleration data were then integrated in the frequency domain to obtain 26

vibration velocity levels. 27

MEASUREMENT RESULTS 28

Figure 3 illustrates typical data from the August 2016 measurements. On all five spectrograms 29

the horizontal axis is time from 1:10:30 to 1:20:30, the vertical axis is frequency from 2 to 40 30

Hz. Color indicates vibration amplitude. The vertical lines show when the test train passed 31

Monitor 5-1. Figure 3(a) shows the vibration at Monitor 5-1. The measurement at Monitor 5-1 32

was performed with an accelerometer that was not capable of accurately characterizing vibration 33

below 10 to 20 Hz and is included to show when the test train passed Monitor 5-1. In general, we 34

used the 40 Hz 1/3 octave band to identify when trains passed the monitors. The odd numbers 35

(T1-T7) are the test train operating in the northbound direction at speeds of 20 to 30 mph. The 36

even numbers (T2-T8) are trains operating in the southbound direction speed of 40 mph. 37

Observations from the results shown in Figure 3 are: 38

Figure 3(b): This position was directly above the tunnel. There are peaks at 40 Hz and 10 Hz for 39

all of the trains and a weaker peak at 20 Hz for trains in the southbound direction. At 40

frequencies below 8 Hz, the vibration appears to be largely uncorrelated to the train 41

operations. 42


Figure 3(c): This measurement position was on the east sidewalk of Montlake approximately 1

centered on the crossover box. Now the vibration shows substantially less correlation to the 2

train events although there is still an indication that train vibration caused a measurable 3

increase in the vibration levels in the 8 to 40 Hz range, although the variations between the 4

test trains suggests that traffic contributed to the vibration during train events. 5

Figure 3(d): This measurement position was west of Montlake approximately 30 ft from the 6

northbound traffic lane. The 10 volts/g accelerometer was used at this position compared to 7

the 1 volt/g accelerometers used east of Montlake. There is very little change in the vibration 8

levels during trains T1, T4, and T6. T1 was a northbound run at 20 to 30 mph and T4 and T6 9

were southbound runs at 40 mph. The other trains have vibration level increases that are 10

almost simultaneous with the train events, but offset by 10 to 30 seconds. Because these 11

vibration events are not synchronized with the train events and similar vibration signals are 12

not evident during the other train events, it is unlikely that these events are associated with 13

train operations. 14

Figure 3(e): This is the most distant surface accelerometer for this series of vibration tests. There 15

is very little indication of vibration increases during the train events although there are other 16

vibration events that generally are not synchronized with the trains. One interesting 17

observation in Figures 3(d) and 3(e) is about 3 minutes of vibration in the 2 and 3.15 Hz 18

bands starting at 01:13. This vibration fluctuates between about 25 to 40 dB and is not seen 19

for the remainder of the period shown in Figure 3. Similar periods of intermittent low-20

frequency vibration occurred during most of the measurements. We have not identified the 21

source of this vibration. 22

The key conclusion drawn from this close inspection of the vibration data at the ground surface 23

is that there is no indication that the train operations caused any vibration at frequencies of 8 Hz 24

and lower that was greater than the normal, middle of the night, background vibration. 25


1

Figure 3. Example Spectrograms of Surface Train Vibration 2 The vertical lines indicate when the test train passed Monitor 5-1. Odd numbers are northbound 3 runs at 20 to 30 mph. Even numbers are northbound runs at a speed of 40 mph. 4

(a)

(b)

(c)

(d)

(e)


Figure 4 shows the vibration from the two-car test train measured at Monitor 4-2. Figure 4a shows the vibration 1 from all the test runs that did not have simultaneous vibration from traffic (blue lines), the average train vibration in 2 red, and the L1, L5, and L10 exceedance levels during the train testing. The periods of the train operations were 3 removed before determining the exceedance levels. Train testing was performed between 1 AM and 4 AM when 4 traffic on Montlake was at a minimum. Figure 4a shows that even in the early morning hours when traffic was at a 5

minimum, train vibration was often lower than the ambient vibration at frequencies below about 16 Hz. Figure 4b 6 compares the average level of train vibration at Monitor 4-2 with the measured levels of truck and bus vibration 7 measured at Monitor 4-2 as the vehicles passed over the Montlake Bridge. From Figure 4b, it is evident that 8 vibration from trucks and buses passing over the Montlake Bridge usually exceed the train vibration at Monitor 4-2 9 over the 2 to 25 Hz 1/3 octave bands. 10

Because the vibration from traffic on Montlake Boulevard is lower than the vibration from vehicles passing over the 11 Montlake Bridge, the ambient vibration at the other monitors is lower than at Monitor 4-2. As a result, train 12 vibration in the 12 Hz and higher 1/3 octave bands could be determined without subtracting out the effects of 13 ambient vibration. 14

Figure 4. SB train vibration at Monitor 4-2 vs. ambient vibration 15

COMPARISON OF BUS AND LRV VIBRATION 16

In past projects when potential for interference with vibration sensitive equipment has been a 17

concern, the vibration from existing automobiles, buses, and trucks often has been overlooked. 18

Figure 5 shows a comparison of the force density levels derived from vibration testing that was 19

performed for the Central Corridor LRT project in Minneapolis (Ref. 3). The measurements were 20

performed in a corridor where both light rail trains and buses operated in parallel. Figure 5 shows 21

that bus vibration can be expected to be equivalent to and frequently greater than LRT vibration 22

at lower frequencies. The testing in Minneapolis shows that the crossover is in the 20 to 30 Hz 23

range. 24

Of course there are a number of factors that will affect the relationship between vibration 25

generated by buses and LRVs. These include speed, roadway condition, roadway construction, 26

track construction, track condition, wheel condition, and localized geology. 27


1

Figure 5. Comparison of Bus and LRV Force Density Curves, Minneapolis 2

3

THEORETICAL CONSIDERATIONS, MOVING LOAD IN TUNNEL 4

This section presents some theoretical considerations relative to the vibration in the tunnel and 5

when the quasi-static deflection of the tunnel by the train would cause measurable low-frequency 6

tunnel motion at the U-Link monitors. 7

The ground vibration displacement response to a sinusoidal load is essentially uniform or slightly 8

increasing with frequency for an infinite half-space. Thus, the velocity response of the ground 9

increases with increasing frequency at the rate of at least 6 dB per octave. Ignoring the effects of 10

soil layering, the displacement response of the ground is roughly inversely proportional to the 11

shear stiffness. For sources below grade, the dominant wave types are shear waves, compression 12

waves, and non-radiating near-field responses. Rayleigh surface waves form at a horizontal 13

distance of roughly five times the tunnel depth. Because the U-Link tunnel is approximately 14

100 ft deep, Rayleigh surface waves are not expected to be significant for the vibration in the 15

tunnel due to surface traffic along Montlake Blvd. Most of the vibration energy is carried in 16

shear. Thus, the ground vibration propagates through increasing depths of the ground with 17

decreasing frequency. The shear stiffness of the ground tends to increase with increasing depth 18

due to confining stress, further decreasing the response with decreasing frequency. 19

At the UW campus, the geologic materials consist of over-consolidated sands and gravels, 20

glacial tills, and silts that were deposited by glaciers. These overlay sediments at depth that also 21

have high shear stiffness, though slightly less than the overlying glacial deposits, as indicated by 22

shear wave velocity data for the Sound Transit alignment on the UW campus illustrated in Figure 23

6. The shear stiffness increases with depth down to about 50 feet below grade, below which the 24

shear wave velocity lessens slightly. At some depth, an increase of shear stiffness with increasing 25

depth is expected due to increasing confining stress. 26

The shear stiffness of the ground is proportional to the product of soil density and the square of 27

the shear wave velocity. As shown in Figure 6, the shear wave velocity on the UW campus is 28

about 1,500 feet per second, roughly twice the shear wave velocity of alluvial soils where over-29

consolidation has not occurred. Thus, shear stiffness is roughly four times that of typical alluvial 30


soils, which means that the response of the ground at low frequencies is perhaps 12 dB lower 1

than the response of alluvial materials in locations such as the Central Valley of California 2

assuming that all other factors are unchanged. The increased density of over-consolidated soils 3

also increases the shear stiffness and reduces the response. 4

The compression wave velocities shown in Figure 6 are generally much higher than the shear 5

wave velocities, reaching 7,500 feet per second, which is about 60% greater than the speed of 6

sound in water (4,700 feet per second). The high compression wave velocity relative to shear 7

wave velocity is typical of saturated soils. The soil is virtually incompressible. As a result, the 8

amplitude of the compression wave is very low compared to the shear wave amplitude. 9

These over-consolidated materials exhibit low material loss factor, with the result that dissipation 10

of propagating wave energy on the UW campus is low compared with alluvial or sandy materials 11

not subject to over-consolidation. This is one of the reasons that the vibration velocity at 12

frequencies above 30 Hz is more apparent than at frequencies below 20 Hz at the UW campus. 13

Observed vibration energy dissipation at low frequencies was negligible due to masking by 14

vibration from traffic on Montlake Boulevard and other sources within several hundred feet of 15

the tunnel. 16

The response at the ground surface is dominated by the non-propagating near-field at low 17

frequencies, where the source-receiver distance is roughly less than a wavelength. At 10 Hz, the 18

shear wavelength is about 150 feet, so that vibration at receivers at the ground surface above the 19

tunnel or at the tunnel bench adjacent to the track are significantly affected by the near-field. The 20

near-field is inversely proportional to the square or cube of the source receiver distance, and thus 21

attenuates rapidly with distance, and is non-propagating. The static deflection field of the moving 22

loads of the vehicle also produces a very low frequency non-radiating ground motion at very low 23

frequencies (see below). 24


1

Figure 6. Seismic velocities at various borings on the UW campus 2

Figure 7 illustrate the line source responses (LSR) measured at the tangent track south of the UW 3

cross-over box. A shaker was used at frequencies below 20 Hz and impact forces above 10 Hz. 4

The shaker data were taken at 1/3 octave band center frequencies, while the impact data involve 5

a continuous spectrum over the entire frequency range. Impact test results below 12.5 Hz are not 6

shown, due to poor coherence in the impact data. These data indicate that the LSR decreases 7

rapidly with decreasing frequency below 50 Hz, for the reasons cited above. At frequencies 8

below 8 Hz, the response is much less uniform, possibly due to near field effects, but also due to 9

the response of the ground, which may involve nodes in the response function related to layering, 10

or tunnel/soil dynamics. Background vibration may have also affected the data at some 11

geophones. 12

13

0 1000 2000 3000 4000 5000 6000 7000 8000

0

50

100

150

200

250

Seismic Velocity Summary

NB-123-P

NB-123-S

NB253-P

NB253-S

NB-255-P

NB-255-S

NB-259-P

NB-259-S

NB-354-P

NB-354-S

NB-356-P

NB-356-S

VELOCITY - FT/S

DE

PT

H -

FT


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Figure 7. Comparison of LSTM’s Measured at Southbound Tangent Track South of UW 2

Station 3

Vibration forces produced by transit vehicles are roughly proportional to the acceleration of the 4

mass of the vehicle truck at frequencies between the primary resonance frequency of about 8 to 5

10 Hz and secondary suspension resonance frequency of around 2 Hz. Above the primary 6

resonance frequency, the un-sprung mass of the wheel set is expected to dominate the load up to 7

the resonance frequency of the resilient wheels, axle and direct fixation of the rail, which is about 8

40 to 60 Hz depending on rail fastener stiffness. The track forces due to a sinusoidal roughness 9

between the wheel and rail will thus decrease at a rate of 12 dB per octave with decreasing 10

frequency at frequencies between the primary resonance frequency and the wheelset/track 11

resonance frequency. 12

A peak occurs at the primary resonance frequency, which is controlled by the truck mass and the 13

parallel combination of the primary and secondary suspension stiffnesses. The frequency appears 14

to be about 8 to 10 Hz for the Kinkysharyo vehicle. Below this frequency, the track loading 15

again decreases with decreasing frequency at 12 dB per octave decrease in frequency until the 16

secondary suspension resonance frequency is encountered at about 2 Hz. These characteristics 17

are illustrated in Figure 8. In this figure of the theoretical invert force estimate for a one micro-in 18

amplitude sinusoidal roughness, the secondary, primary, and wheelset/track resonances are at 2, 19

9, and 36 Hz, respectively. 20

The rail roughness tends to decrease with increasing frequency at a rate of 3 to 6 dB or more per 21

doubling of frequency. This is partially compensated by the increasing bandwidth of one-third 22

octave band filters. The bandwidth is approximately 23% of the nominal center frequency, so 23

that the response of the third-octave filter increases by 3 dB per doubling of frequency. 24

-10

0

10

20

30

4 8 16 31.5 63 125 250

LSR

(D

B)

FREQUENCY (HZ)UWT LSR (TRAP RULE)

SHAKER & IMPACTS

SHAKER LSR (SB 1199+50)

SHAKER LSR (SB 1199+66)

IMPACT LSR (SB 1199+50)

IMPACT LSR (SB 1199+66)


In view of the above, one may expect rates of attenuation of the vibration velocity at the ground 1

surface to be of the order of 18 dB per halving of frequency, notwithstanding peaks in the 2

spectrum at the wheelset/track resonance, primary resonance, and secondary resonance 3

frequencies. 4

The roughness of asphalt or concrete road surfaces is generally much greater than the roughness 5

of high quality steel rails used on transit systems. The static load of the LRV at AW4 loading is 6

about 160,000 lbs, which may be compared with the static load of a heavily loaded tractor-trailer 7

of around 80,000 lbs. If the LRV were operated on rails with roughness equal to that of asphalt 8

or concrete pavement surfaces, the low frequency vibration produced by the LRV would likely 9

be substantially greater than that produced by trucks and buses. The smoothness of the wheels 10

and rails relative to that of the road surface is a major factor in the difference between vibration 11

produced by road vehicles and transit vehicles. 12

Tunnel motion due to the moving static loads of the vehicle trucks may be very apparent within 13

the tunnel in which the train travels, with spectral peaks at frequencies below 4 Hz. The 14

estimated 1/3 octave velocity levels for a 2-car train with three trucks per vehicle traveling at 40 15

mph are shown in Figure 9 along with the estimated train vibration levels observed at monitor 4-16

2. The peak at 1.6 Hz is due to the static loads of the individual trucks passing by. The estimated 17

levels of train vibration at monitor 4-2 that are included in Figure 9 are in the same general range 18

as the predictions based on static loading analysis over the 2 to 5 Hz range. At higher 19

frequencies, the estimated values from the measurements diverge sharply from the theoretical 20

value. This is the frequency range that the vibration shifts from non-radiating vibration related to 21

the moving static load to radiating vibration that is caused by the interaction of the rolling wheels 22

with the rail roughness. The ground motion due to moving static loads attenuate rapidly with 23

distance and are unobservable beyond roughly 50 ft. 24

Discrepancies between the measured and predicted vibration due to a moving static load may be 25

related to the distances between the trucks, here assumed to be 30 feet, and the actual distances 26

and various other factors. The secondary and primary suspension resonances should not be 27

factors for moving static loads. One may also expect the vibration response of an actual tunnel 28

wall to differ from the response of a semi-infinite half space with a vertical load as modeled here. 29

The moving loads are assumed to be at 100 feet below the surface. The receiver is assumed to be 30

10 feet from the track center and at mid tunnel height, 10 feet above the load application point. 31

The load of each truck is assumed to be 50,000 lbs. The soil parameters are assumed to be 32

similar to those found at the UW station. The calculations are based on an exact theory (R. D. 33

Mindlin, Physics, v7. 1936, pg. 195) for a uniform half-space. 34


1

Figure 8 Transmitted Force for Relative Roughness of 10−6 in. 2

3

4

1.00E-03

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1 10 100

TRA

NSF

ER F

OR

CE

-LB

/MIC

RO

-IN

HCDF-MAGNITUDE

HCDF-MAGNITUDE


1

Figure 9. Estimated Vibration Velocity Level at Tunnel Wall for 2-car train traveling at 40 2

mph in the same tunnel 3

4

CONCLUSIONS 5

1. There is not much low frequency train vibration evident in the U-Link measurements. The 6

results show that ambient vibration from traffic and other sources at the monitor locations on 7

the tunnel safety walk often exceeded the low frequency vibration generated by test trains. 8

This non-intuitive result indicates that the Sound Transit LRVs do not generate much low-9


frequency vibration and trucks and buses tend to generate higher levels of low-frequency 1

vibration. Similar results have been observed at other transit systems. 2

2. The limited low-frequency vibration energy observed at the subway monitoring stations in 3

and south of the UW Station are consistent with other measurements of train vibration 4

measured in locations with overly consolidated soils. 5

3. Based on the soil properties in UW area, this result is consistent with the models that would 6

be used to predict the vibration properties. 7

4. Although this report focuses on when low-frequency vibration is not observed, it is important 8

to recognize that there are situations when low-frequency energy will be generated by LRV 9

operations. This report does not include specific examples of this because the focus is on why 10

low-frequency vibration from LRV operations has not been observed at the ground surface at 11

the north end of the U-Link extension. 12

5. For the U-Link extension in the University of Washington area, at frequencies below 10 to 13

20 Hz, we expect the ground vibration from buses and trucks to exceed the vibration 14

generated by LRVs. The observation includes the following qualifiers: (1) LRVs with good 15

condition wheels, (2) similar distances between the measurement positions and the vehicles, 16

and (3) the buses/trucks and LRVs operate at similar speeds. 17

6. The trigger thresholds for the U-Link vibration monitoring system should be set so that they 18

not are triggered by the vibration caused by the moving static loads of the LRVs, as these are 19

non-radiating vibration sources, nor by background vibration from external sources. 20

REFERENCES 21

1. ATS Consulting, “Final Report: Derivation of Transfer Functions for IEM Vibration 22

Monitoring System,” December 18, 2015. 23

2. Wilson, Ihrig & Associates, “Vibration Threshold Compliance Testing at the University 24

of Washington,” September 18, 2015 25

3. ATS Consulting, Central Corridor Light Rail Transit Project Final Environmental Impact 26

Statement, Appendix J4, “Vibration Measurements and Predictions for Central Corridor 27

LRT,” December 19, 2009. 28

4. ATS Consulting, “Updated Vibration Attenuation Estimates for IEM Vibration 29

Monitoring System,” July 8, 2016. 30

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