Canadian TSB Investigation: Subgrade Settlement. ⢠Led to NRC ... settlement and associated track geometry deviation development. 4. ... Joint bar overload.
Ted Sussmann VOLPE and Hugh Thompson FRA
FRA TSS § 213.103 Ballast: Unless it is otherwise structurally supported, all track shall be supported by material which will— (a) Transmit and distribute the load of the track and railroad rolling equipment to the subgrade; (b) Restrain the track laterally, longitudinally, and vertically under dynamic loads imposed by railroad rolling equipment and thermal stress exerted by the rails; (c) Provide adequate drainage for the track; and (d) Maintain proper track crosslevel, surface, and alignment.
Assumption: ballast support needed for geometric stability until next inspection 2
Photo: T. R. Sussmann
• Vertical Track Deflection is often related to the performance of the track. – Small deflections (0.10 inches) indicate stiff track with a slow degradation rates – Large deflections (0.50 inches or greater) indicate soft track with generally higher degradation rates
Source: W. W. Hay, Railroad Engineering, 1982
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Cox Landing, WV June 1998 (with Haz Mat Release) – NTSB Investigation: Unstable roadbed, Drainage •
•
Result: University of Nebraska Track Deflection Research
Westminster, VT April 2001 – Slope instability, salt train derailment •
•
Site assessment spurred interest in quantitative tools to assess track support
Canada Track Failure 2004 – Canadian TSB Investigation: Subgrade Settlement •
•
Led to NRC research on track deflection
3x- Powder River Basin, May 2005 – Blocked access to coal for energy producers (U.S. DOE Study) •
Surface Transportation Board: Coal Fouled Ballast
– Sparked interest in ballast fouling condition • Ground Penetrating Radar for coal dust
• Ellicott City, MD August 2012 (2 fatalities) – NTSB: Broken Rail with evidence of rail overstress – Vertical Track Deflection Report to Congress
Substructure Track Support Risks 1. Track Loads Vehicle applied track loads beyond design safety factored loads may cause deformation beyond that assumed in design that would reduce the useful life of the track substructure and result overloading track superstructure components.
2. CWR Stability Rail internal forces must be controlled and properly reacted to prevent buckling or track shift related alignment deviations.
3. Fouled Ballast Ballast must provide resilient support with minimal plastic deformation to limit track settlement and associated track geometry deviation development.
4. Subgrade Stability Subgrade must serve as a stable platform contributing to the stability and resilience of the entire track structure.
Track Support Proper track support (particularly 3 and 4) is required to assure rail infrastructure resilience.
• Track Support: key to analysis of track load capacity limitations and occurrence of critical stress/strain – Lateral: Buckling analysis. CWR Maintenance. Potential for better understanding the shift and buckling breakpoint. – Vertical: Poor support track failure, geometry deterioration. – Longitudinal: Rail stress, rail running, neutral temperature.
Figures adapted from Selig and Waters, Railway Geotechnology, 1994
Too stiff: Rail corrugation Tie abrasion Ballast crushing Crushed railhead Load
Too soft:
Void
Contact Deflection
Figure adapted from Sussmann et al., Fundamental Track Load Deflection Behavior, TRB, 2001.
Ballast instability Subgrade failure Joint bar overload
Photo courtesy Mike Coltman.
Large void: Track shift/buckling Track geometry Tie failure Fouled ballast Photo: T. R. Sussmann
Track Support Problems • Track Load Redistribution • Ballast Rearrangement • Track Geometry
Figure courtesy Mario Ruel, CN
Inspection: GPR (Ground Penetrating Radar) • Clean Ballast Depth, Moisture • Longitudinal Variations
Tie Bearing Capacity Problems • Tie and Rail Loads • Cross Level and Geometry • Track Position Movement
Figure courtesy FRA
Inspection: Seismic (SASW) • Resilient Modulus • Density
Squeeze/Heave
Ballast Pockets
Li et al., Railway Geotechnics, 2017
Photo: T. R. Sussmann
Photo: T. R. Sussmann
Inspection/Quality Control: Strength from Modulus Correlation
• FRA DOTX 218 Track Deflection System Installation
Photo courtesy John Choros, Volpe (ret.).
Photo courtesy Ensco, Inc.
Rough
Smooth
Rough
Smooth
Figures and photos from Stark et. al (2015): Evaluation of Tie Support, TRB, DOI: 10.3141/2476-08
0.01 in.
Tie-Ballast Gap
0.25 in.
Figures and photos from Stark et. al (2015): Evaluation of Tie Support, TRB, DOI: 10.3141/2476-08
VTD System Overview Uniform track with no geometry variations Deflection of track 4 feet aw ay from w heel
𝑌𝑌𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝐸𝐸𝐸𝐸𝐸𝐸 = 0 𝑉𝑉𝑉𝑉𝑉𝑉 ≈ 𝑌𝑌𝑅𝑅𝑅𝑅𝑅𝑅 − 𝐸𝐸𝐸𝐸𝐸𝐸
Unloaded track position
𝐻𝐻𝐿𝐿
Loaded spacecurve (w heel path)
yrel
𝑌𝑌𝑟𝑟𝑟𝑟𝑟𝑟 - references partially loaded rail position at 4 feet away from wheel
𝐸𝐸𝐸𝐸𝐸𝐸 - End Chord Offset references a loaded spacecurve calculated from track geometry measurements 𝑉𝑉𝑉𝑉𝑉𝑉 – Vertical Track Deflection is approximated by subtracting 𝑌𝑌𝑟𝑟𝑟𝑟𝑟𝑟 and 𝐸𝐸𝐸𝐸𝐸𝐸. Figure adaptation by Gary Carr, FRA.
VTD System Overview
𝒀𝒀𝒓𝒓𝒓𝒓𝒓𝒓 measurement with track geometry variations 𝐻𝐻𝐿𝐿 Loaded spacecurve (w heel path)
Unloaded track position
𝑉𝑉𝑉𝑉𝑉𝑉 ≈ 𝑌𝑌𝑅𝑅𝑅𝑅𝑅𝑅 − 𝐸𝐸𝐸𝐸𝐸𝐸
yrel ECO
𝐻𝐻𝐿𝐿 - Laser height measurement from UNL beam to top of rail Figure adaptation by Gary Carr, FRA.
Failure or rapid deterioration of track superstructure North Deflection (in.)
South Deflection (in.)
Failure or rapid deterioration of track superstructure
Six (6) failed concrete ties in a row - track immediately taken out of service Photos: T. R. Sussmann
Unrestrained track or rail due to settlement North Deflection (in.)
South Deflection (in.)
Unrestrained track or rail due to settlement
• Insulated joint with apparent signs of distress including loose huck bolt, which is not common • Spike lifted 3-4 in. at joint • Track deflection appears about ¾ in. of slack based on gap at bottom of tie. Photos: T. R. Sussmann
Goal: Foundation failure
- Settlement: 30 cm over 8 years - Failure due to sudden and significant settlement - Spurred Canadian research into VTD
Figures: Canadian Transportation Safety Board Railway Investigation Report: R04Q0040, 2004
• Track support is a critical element of a reliable transportation network – Poor track support slow orders and unpredictable maintenance
• Track deflection measurement is a challenge, but examples are emerging that demonstrate the criticality of track structural performance • Track settlement effects such as hanging ties and rail contribute to track deflection and dynamic load