Chapter 4 — Failure Mechanisms and Triggers

Reading the substructure through maintenance demand

Profillidis uses a maintenance coefficient k = I / I_m, where I is the annual number of maintenance sessions on a section and I_m the average for track of the same age, UIC group and axle load. k = 1 is average; k = 0.5 is good; on poor subgrade k can reach 10. Maintenance hours rise steeply with k and with years since renewal (Figure 4-1). He uses k to judge whether the track bed is adequately sized:

When I passes a threshold, geometry can no longer be held by surfacing alone and the remedy is to reduce k, mainly by thickening the track bed. Applied with Dormon’s rule, subgrade stress also depends on maintenance: traffic between sessions scales with T/k, so tracks with different k values load the subgrade differently. For a ground-hazard program the practical point is that a segment needing repeated spot surfacing (high k) is evidence of a substructure mechanism, and that evidence lives in maintenance records.

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After Profillidis (2014), §9.9, Table 9.5, Fig. 9.6. Full reference under Supplemental Sources.

Ballast or subgrade: which one sets the maintenance cycle?

Short-term versus long-term settlement. On most track, ballast causes most of the settlement between surfacings, both average and differential. This is the short-term component. The subgrade adds a long-term component that builds slowly over decades of traffic. Ballast governs only when three conditions hold:

• A filter or separation layer is present.
• The subgrade is strong enough.
• Surface drainage is good.

If any of these is missing, the subgrade dictates the maintenance cycle.

Why track subgrade fails by creep, not by sudden collapse. Train loading on the subgrade does not reverse the direction of shear stress. Under that kind of loading, pore pressure rises to a limit and failure comes as permanent strain accumulating at an increasing rate. Cyclic strain stays small. Two factors make it worse:

• Static shear stress, such as beneath an embankment shoulder or slope, magnifies the plastic strain caused by traffic.
• Subgrade saturation changes with the seasons, so the same section can perform differently in spring and in autumn.

After Selig & Waters (1994), chs. 3, 4 and 8. Full reference under Supplemental Sources.

Mass-movement types and internal-erosion collapse

Mass movements are classified by rate against water content, from extremely slow dry creep through earthflows and rotational or translational slides to extremely rapid rockfalls, debris flows and debris avalanches in saturated ground. An earthflow has a characteristic hourglass shape. Creep is spotted by its effects: curved trunks, bent fences and tilted poles.

Embankment collapse by internal erosion (rated Severe) is the hidden mode. Water moves through the fill or overflows through buried rock drains, carrying soil out and leaving voids until the surface collapses. In one case a culvert roof failed about 45 ft inside the fill and produced a sink hole up to 70 ft deep. In another, a 6.5-ft multi-plate culvert with a rusted-out invert let water erode beneath it, causing outlet and toe scour, an over-steepened slope, head-scarp cracks, 14 in. of mainline settlement and 2 in. of lateral deflection.

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After Geotechnology for Railroaders Program, Module 5; §05, §09, §14. Full reference under Supplemental Sources.

Underestimated peak flows

Undersized drainage is a hydraulic mechanism in its own right. Profillidis gives the U.S. Soil Conservation Service method based on Fuller’s formula for the peak flow from a catchment of A km² for a return period of T years:

His worked values: A = 10 km², T = 10 years gives Q₁ = 11.6 and Q_max = 47.7 m³/s; A = 15 km², T = 20 years gives 15.7 and 69.9 m³/s. He cautions that such semi-empirical formulas must be checked against measured data. On the TGV Méditerranée, observed extreme runoff was about twice the top of the predicted range (Chapter 3, §3.5). He argues that increasing rainfall intensity calls for return periods far longer than the traditional 50–100 years, up to 1,000 years, with the probable maximum flood as the reference event.

After Profillidis (2014), §9.14.2–9.14.3, Eq. 9.11. Full reference under Supplemental Sources.

Scour: why it is invisible from the hi-rail

Local scour forms horseshoe and wake vortices around piers and abutments; contraction scour deepens the channel through a constricted opening. Both can undermine a foundation well below the water surface. Away from bridges, any bend near the track is scour-critical: the outer radius scours while the inner radius deposits. Banks migrate as the stream undercuts and collapses them. Attack migration before it passes a 2:1 intercept slope with the track, because cost rises steeply with delay, and get engineering help, since riprap alone may not be enough.

Signals stay green when track is undermined by side scour or washed out. Peak flow can arrive after the rain stops, and a long moderate storm can do more damage than a short intense one. Get off the hi-rail and walk the bank at scour-critical locations. AREMA countermeasures (after FHWA) are riprap at piers and abutments, guide banks, spurs and revetments, with riprap always placed from the toe upward.

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After Geotechnology for Railroaders Program, Module 6.3; §02–05. Full reference under Supplemental Sources.

Differential head, beaver dams and rapid drawdown

When the water level on one side of an embankment exceeds the other by more than about 6 ft, flow is forced beneath the track. Within days, depending on the fill, wet spots appear at the downhill toe. Dirty or milky seepage and sand fans at the outlet mean soil is being carried out and cavities are forming. In spring, a culvert plugged with ice is the most frequent cause.

Beaver dams in culverts and ditches cause about 80% of beaver trouble but less than 10% of the risk. Dams far uphill, sometimes 100 ft or more above the track and outside railway property, are about 20% of problems but carry the highest risk. The 1992 Nakina derailment, in which two employees were killed, followed a rapid drawdown of a lake after a dam failure and led to the formal Beaver Activity Hazard Assessment (BAHA) program. The program rates watersheds and dams 1–5 and combines them into a global score:

Never use explosives on a beaver dam or culvert plug. In one case a dynamited dam overloaded a 3-ft culvert and more than 300 ft of a 60-ft-high embankment failed, with the water travelling through old permeable fill from a buried timber bridge. After any high water is drawn down, the embankment is least stable for about 12–24 hours, while the confining water is gone but the fill is still saturated and seepage reverses. Beaver ponds are protected fish habitat under the Canadian Fisheries Act; permits are required, and penalties can reach $1,000,000 and imprisonment for individuals.

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After Geotechnology for Railroaders Program, Module 6.5, Module 7.1, Module 4; Module 6.5 §02–06; Module 7.1 §04, §06. Full reference under Supplemental Sources.

Seismic train-protection thresholds

Profillidis treats earthquakes in two parts: designing structures, track and subgrade for the regional design acceleration (up to about 0.3 g in his text), and protecting trains during an event. For the second, accelerometer sensors in protected housings along the line report to an alarm center:

• measured acceleration above 0.066 g (2.1 ft/s²): stop traffic immediately;
• between 1.3 and 2.1 ft/s² (≈ 0.04–0.066 g): reduce permitted speeds;
• an alarm is issued when three consecutive sensors report within 5 seconds;
• reliability target: no more than one false alarm in 30 years.

These values describe one European system and can be compared with client post-earthquake inspection protocols (Chapter 14).

After Profillidis (2014), §9.17. Full reference under Supplemental Sources.

What earthquakes have done to track

More than 1,500 seismic events are recorded in Canada each year, with a significant one every two to five years. Energy travels farther through dense eastern Precambrian rock. The 1935 Temiskaming M6.2 earthquake caused an embankment failure more than 220 mi from the epicenter. The 1988 Saguenay M~6.3 event temporarily liquefied a muskeg zone and triggered nine slope failures; derailments were avoided only because the track was closed. In a 2010 event, straight rail was left kinked into S-curves, buckled at ties and disturbed at culverts, damage not visible from a moving train. Post-event inspection must cover track and roadbed, drainage, culverts, retaining walls, tunnels, overpasses, bridges and approaches, adjacent buildings, fiber and utilities, and signals. Response criteria are in Chapter 14.

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After Geotechnology for Railroaders Program, Module 7.3; §02, §05–06. Full reference under Supplemental Sources.

Triggers and the four don’ts

A trigger is an external stimulus that causes a near-immediate landslide or washout by raising stress, reducing strength or removing soil: intense or persistent rain, floods, high stream flow, rapid snowmelt, earthquakes, elevated seepage, water-level change, freeze–thaw and rapid drawdown. Weather is the most common trigger, and antecedent wetness matters as much as the storm. Underlying causes are geological (weak, sensitive, weathered or jointed materials, adverse bedding, permeability contrasts), morphological (toe erosion, piping, crest loading, vegetation loss, thaw) and human (toe excavation, crest loading, reservoir drawdown, deforestation, irrigation, vibration, leaking utilities).

Water weakens slopes in five ways: it adds weight in the voids, is absorbed by clay minerals with loss of strength, dissolves cement between grains, erodes loose material, and becomes trapped under pressure. A failure surface balances driving and resisting mass, and four actions tip it the wrong way: steepening the slope, removing support at the toe, adding mass high on the slope, and allowing bad drainage to continue.

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After Geotechnology for Railroaders Program, Module 5, Module 6.4; Module 5 §10, §14; Module 6.4 §07. Full reference under Supplemental Sources.

Declaring an extreme weather event and scaling the response

In the course’s Class I practice, the Regional Chief Engineer declares an extreme weather event for a defined subdivision and mile range. The declaration draws on weather-service forecasts and warnings and on reports from train crews and engineering forces, and is passed to Rail Traffic Control, the General Manager and the Regional Senior VP. Restrictions are decided jointly. They weigh the nature of the event, pre-existing ground conditions, topography and incident history, the quality of the weather information, the recency of patrols (a daylight inspection within four hours counts for more than a night inspection twelve hours old), train frequency, and wayside protection such as slide fences and high-water detectors.

• Increased patrol frequency.
• Patrols immediately ahead of each train.
• Trains on lookout, prepared to stop within one-half the range of vision.
• Daylight operations only.
• No new trains into the territory; clear trains to safety.
• Cessation of operations; move crews to safety.

The flash-flood trigger used across the Prairies and East is more than 1 in./h totaling over 3 in.; elsewhere, 2 in. in 1 h, 3 in. in 2 h, 4 in. in 4 h or 6 in. in 12 h. A double precipitation index plots short-term intensity against cumulative rainfall to define low-risk, speed-restriction and suspension zones. An antecedent rainfall index plots 2-day against 28-day totals over the history of past landslides at a station. In spring, bearing capacity can drop 30–50%.

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After Geotechnology for Railroaders Program, Module 7.2, Module 7.1, Module 4; Module 7.2 §03–06; Module 7.1 §07. Full reference under Supplemental Sources.

Geometry defect types and what drives them

The book defines geometry defects against a theoretical track position averaged over a chord: longitudinal defect (surface or profile), transverse defect (cross-level relative to design cant), horizontal defect (alignment), gauge, and twist (deviation of one of four points on two ties from the plane of the other three, i.e. the change in cross-level per unit length).

• Longitudinal defect is the most reliable indicator of vertical loading on the track and, with transverse defect, the main driver of maintenance cost. This is where subgrade settlement and pumping appear first.
• Horizontal defect depends mostly on lateral track effects and rolling-stock behavior, which points toward ballast-section and lateral-resistance issues (Chapter 2, §2.3).
• Gauge deviations depend mainly on the subgrade and the type of rolling stock.
• Twist is one of the most frequent causes of derailment, and at speeds below 87 mph it is the main critical safety parameter. Localized differential settlement under one rail, for example at a culvert, a soft spot or a shoulder failure, shows up as twist.

Geometric parameters degrade about 5–15 times faster than mechanical ones (rail, ties, fastenings). On a typical UIC group 4 line, geometry is restored after about 40–50 million tons, while rail lasts 500–600 million tons.

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After Profillidis (2014), §16.2–16.4, Figs. 16.3–16.4. Full reference under Supplemental Sources.

Match the remedy to the cause

Mud on the ballast surface can come from three sources, and each needs a different fix:

• Subgrade pumping: renew the ballast and add a blanket.
• Worn ballast: renew the ballast.
• Spillage from cars: clean the ballast.

Before committing to cleaning or renewal, examine the layers down into the subgrade with trenches, pits or trackbed borings (Chapter 5).

Condensed from the book’s failure-condition and remedy table, which is partly illegible in the source scan. Benesch wording.

Clues in the geometry record.

• A sharp rise in the deterioration rate from one cycle to the next flags fouling.
• Track that settles back to the same profile after each tamp is showing its inherent shape (Chapter 15). Low-lift tamping will not change that shape.

After Selig & Waters (1994), chs. 14 and 15. Full reference under Supplemental Sources.

Track-recording data as an early-warning stream

Recording cars run at fixed intervals: about 3 times a year on main routes, twice a year on intermediate routes and once a year on branch lines. They measure longitudinal, transverse and horizontal defects on a base of about 33 ft and twist on 8–10 ft. At low and medium speeds the absolute (peak and mean) values govern. At higher speeds the standard deviation over 660–980 ft is used as the quality index. Results are ranked by severity and compared with earlier runs and with the limits.

Defects grow quickly until about 2 million tons after maintenance, then slowly and roughly with log T:

Here a₁ is the mean settlement at T_r and a₀ is the settlement rate per log cycle, 0.079–0.24 in. per decade, depending mainly on subgrade quality (a₀/a₁ = 0.25–0.70). The standard deviation of longitudinal defects follows the same form with c₀ ≈ 0.0039–0.0079 in. per decade. A segment whose fitted a₀ or c₀ drifts above its neighbors is a candidate for substructure investigation. The sensitive period (about 1–4 months on a UIC group 4 line, 15–40 days on groups 1–3) is when monitoring after a repair matters most.

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After Profillidis (2014), §16.5, §16.7, §16.10, Eqs. 16.3–16.5, Figs. 16.5, 16.6, 16.8. Full reference under Supplemental Sources.

Signs of instability job aid and field reporting

Warning signs appear in a predictable order: unusual high water on one side, seepage at the downhill toe, tension cracks along the shoulder (often hidden in ballast), a bulging toe and a small sand fan, increasingly dirty or milky seepage, trees leaning in different directions, and finally settlement or cross-level change in the track. Test hidden cracks with a lining bar; it meets little resistance over a fissure. Paint reference marks to see whether cracks widen. Walk down to the base of the fill when checking culverts, because much of this cannot be seen from the track. If movement is large and accelerating, take the track out of service and restore drainage urgently.

Moisture clues: groundwater discharge in depressions, wet zones or salt stains in cut ditches, frost boils, ice at culvert sides, moisture-loving plants (cattails, willows), and sudden lush vegetation on a slope. Report every natural-hazard incident, including rock found in a ditch, on the Natural Hazard Incident Report. It records site and weather (at the time and over the prior 24 h), a rockfall section, a mass-movement and washout section, and consequences (damage, delays, slow orders). The data is used to deploy rock gangs and maintenance crews.

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After Geotechnology for Railroaders Program, Module 5, Module 7.1, Module 4, Module 6.4; Module 5 §11; Module 7.1 §05; Module 4 §10; Module 6.4 §05. Full reference under Supplemental Sources.