Non-Traditional Noise and Vibration Mitigation Strategies Christopher Layman, Ph.D. Shannon McKenna Judy Rochat, Ph.D. ATS Consulting Pasadena, CA
Non-Traditional Mitigation Measures We can take into account the condition of the rail and vehicles operating on the system Maintenance (rail grinding and wheel truing) Low-noise vehicles We can capitalize on elements already included in the project: Extending the width of the ballast to reduce noise Vibration reduction due to noise barrier foundation
Key Presentation Take-Aways There could be noise and vibration reduction measures already included in the project that are not being accounted for.
Maintenance as a Mitigation Measure Noise and vibration levels can rise by 10-15 db for severely corrugated rail or wheels Acoustic rail grinding and wheel truing can be used to reduce noise levels
Maintenance as a Mitigation Measure Mitigation recommendations will depend on the reference noise level used in the analysis Important to document conditions of the existing system, including rail roughness System Lmax *, dba FTA Reference Level 81 Sacramento 82 San Diego 75 * Lmax for a 2-car train at 50 ft, 40 mph on ballast-and-tie track Maintenance was recommended as a mitigation measure Less mitigation is required
Maintenance as a Mitigation Measure Maintenance could also be used as a vibration mitigation measure Up to 15 db difference in Force Density Level before and after re-profiling Source: Wilson, Ihrig & Associates Northgate Link Extension Final Design About 10 db difference in vibration level before and after grinding Source: Ben Lawrence, proceedings of Acoustics 2004
Some systems, like San Diego Trolley, have low-noise vehicles If you are procuring a vehicle, you can request a low-noise vehicle. Features may include: Wheel skirts Under-car absorption Others? Specify a Low-Noise Vehicle
Extending Ballast Ballast is a sound absorbing material Can use a simple reflection model to determine the region of influence affected by ballast Region of Influence Source Absorptive ground
Extending Ballast: Predicting Effects Can measure effective flow resistivity to quantify absorption Can model ground absorption in TNM, CadnaA, or SoundPlan Width of ballast beyond ties (ft) Estimated extent of influence* Reduction* 3 52 ft ~1 db 5 64 ft ~1.5 db 10 94 ft ~2 db *Assumes receiver is 5 ft above ground level
Extending Ballast: Example Project Source Extra Ballast Existing noise (dba) Allowable increase due to project (db) Predicted noise (dba) Predicted increase (db) Predicted increase including ballast effect (db) 58 2.4 61.8 3.8 2.3* *Assumes 1.5 db decrease due to ballast
Ballast with Berm Ballast with small berm next to tracks can provide 10 db reduction (Attenborough, Inter-Noise 2005) Note: 38 cm = 1.2 ft, 110 cm = 3.6 ft, 530 cm = 17.4 ft
Vibration Mitigation with Sound Walls Train vibrations measured on BART at the Walnut Creek/Pleasant Hill region. Wayside vibration measured with (2012) and without (2005) sound wall in place. B&T Track on Embankment
db Difference in Measured Vibration Measured Train Vibration at Two Similar Locations 20.0 Sound Wall Insertion Loss 15.0 10.0 5.0 0.0-5.0-10.0 Site 4/5 Site 3/6 Site 3/5 Site 4/6 6.3 8 10 12.5 16 20 25 31.5 40 50 63 80 100 125 160 1/3 Octave Band Center Frequency, Hz
What Have Others Found? Stiff Foundations Block Waves[ usually] Measured Insertion Loss (Red Curve) 10m 14m Track 50 ft El Realengo, Spain 24m 32m Jet Grouting Wall cement bentonite mixtures Soil Dynamics and Earthquake Engineering 77 (2015) 238 253
Can We Simulate Our Measurements? Modeling Approach 3D Finite Element Solid Mechanics Ignores the track and moving vehicle Random point sources are located at axle locations of a typical 3-car consists. Frequency domain linear analysis
Modeling Geometry and Details Concrete Sound Wall Soil PML (grey regions) Vertical Insertion Loss 20*log10( u_zref / u_zwall ) Moderately Stiff Soil Geometry and mesh scales with wavelength Hysteretic damping throughout Given measurement locations and frequencies of interest, model is limited to f <= 70 Hz This entire face is Symmetry Soil Probe
Insertion Loss Results Data Model (115ft) Spatial Patterns of Vertical Displacement Sources Wall+Embankment Embankment 50 Hz Model qualitatively agrees with measurements. Agrees better with farther measurement(?) Does embankment area have softer soil? Is there a coincidence frequency effect? We don t see significant amplification. General trend: moderate distances from track have highest IL
Sensitivity To Wall and Embankment Geometry Reducing Embankment Height Reducing Foundation Depth Embankment has little effect on IL Stronger dependence on foundation depth
Conclusions Take into account the noise and vibration conditions of the existing system during environmental assessments Identify any noise and vibration reduction measures already included in the project: Extending the width of ballast could provide a noise benefit Properly designed sound walls may reduce vibration impacts significantly. However, the optimal range appears narrow.
END THANK YOU!
Modeling Parameters
Background Noise and vibration can be mitigated at the: Source Path Receiver Noise propagation through air: Vibration propagation through soil:
Rail Dampers Rail dampers are tuned to absorb specific vibration frequencies which reduces the amount of noise radiated by the rail Measurements at SacRT show reduction in wayside noise levels by 2-3 db Rail dampers can be used where sound walls are not feasible
12th OCt. Corrugation (dba) Rail Dampers A study at BART show rail dampers reduce noise over time and limit corrugation growth Rail dampers are easy to add to existing track Date Difference * Apr 2013 5.8 Oct 2013 2.7 Apr 2014 1.7 Sept 2014 3.0 Apr 2015 3.3 Oct 2015 2.1 * Difference in noise level for a track section without dampers minus a section with dampers 104.0 102.0 100.0 98.0 96.0 94.0 92.0 90.0 88.0 86.0 Growth of Corrugation Noise around 450 Hz y = 1.49x + 88.2 y = 0.25x + 86.5 R² = 0.96 0 10 20 30 Months after Grinding No Rail Dampers Rail Dampers Linear (No Rail Dampers) Linear (Rail Dampers)