final hill, colwell - arema...the cn case study’s limitations included work within short windows...

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Rapid Subgrade Stabilization Method for High Volume Mainline Track

Jeffrey Hill, PE, Hayward Baker Inc. – Rail Services Division, 1530 South Second Street, St. Louis, MO 63104, 847-343-2023, [email protected]

Daniel Colwell, CN Southern Region Homewood Administration Building, 17641 South Ashland Avenue Homewood, IL 60430. Telephone: 708-332-4713; Cell: 312-909-5908, [email protected]

Word Count: 2435

Abstract This paper describes the technology, implementation, and performance of rapid subgrade stabilization by railroad subgrade injection, and recommends procedures based upon observations. Heavy winter snow accumulation combined with spring rain during 2014 created soft subgrade conditions in north eastern Minnesota. The geotechnical report classified the subsurface soils as peat, organic silt, and soft clay with high moisture content. These soils made upkeep of track geometry challenging and required the owner to implement Transportation slow order limitations. The resulting Engineering Action Item from Transportation was to restore track speed across north eastern Minnesota while working between operating train traffic. Eight sections between Superior, Wisconsin and International Falls, Minnesota required subgrade improvement. This paper focuses on the 5 locations in Minnesota. Time and physical constraints at each location limited which soil improvement method could be used. In addition, each section had different subgrade conditions. In each of the 5 locations the subsurface soil conditions did not allow for conventional construction; instead the subgrade was constructed on corduroy at the turn of the century. For these locations CN implemented driven piles to reduce the slow orders. Initially, a pile driving contractor drove timber piles at the edge of the ties on 3-foot centers to refusal. This program heaved the track. The injection contractor then injected slag cement from the top of the corduroy at approximately 12 feet below track to the bottom of the ballast. In areas absent corduroy the injection program extended to 20 feet below track. Inspection of the subgrade verified firming within a few days of the injection. Historical Background

Logging railroads constructed in Minnesota were almost exclusively standard-gauge railroads (4' 8-½" wide inside the rails). Most were well-graded and graveled, but some were used only during the winter and consisted of ties and rails laid across the frozen ground. Typical construction costs were around $5,000/mile.

The rolling stock consisted of a few locomotives, typically in the 50 to 75 ton range, and a few dozen to a few hundred logging cars, depending on the size of the operation. Railroad logging often operated in conjunction with river transportation. After unloading logs at river landings the logging companies sent them to mills downriver.

In 1886 Minnesota saw completion of its first logging railroad. The growth of lumbering across northern Minnesota from the 1890s to the end of the white pine era in the mid-1930s was in great part due to the logging railroads. The industry peeked at the turn of the 20th century. At that time the rail industry only performed work in the winter months using the frozen ground to support its steam engines, horses, and workers. As spring brought warmer temperatures to the area, the use of the railroad also trailed off. The line construction began with a corduroy layer of mature trees hewn from the right-of-way. The workers laid the trees perpendicular to the tracks, on top of the existing organic soils. They then built up the grade using cinders, site organic soils, saw dust, and other easily had material. This type of

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construction was more than sufficient for the train loads of the time and the frequency at which the trains ran.

The Minnesota and International (M & I) railroad, a subsidiary of the Northern Pacific, originating at Brainerd and terminating at International Falls, and the Duluth, Rainy Lake and Winnipeg, which allowed rail access as far as Rainier from Duluth, went operational in 1907. The five injection locations discussed in this paper are from this railroad.

In approximately 2000, the freight along this route was substantially increased. The increase in volume combined with the original construction techniques and the subsurface soils result in a significant amount of annual maintenance. In 2014, five sections of CN-owned track in Minnesota required ground improvement because of newly created soft subsurface conditions. Ground Instability Following Record Precipitation Several large precipitation events inundated the region during the spring and summer months of 2014 (Figure 1). Heavy rain over winter snow accumulations caused flooding which triggered service disruptions for many areas on CN’s ROW and road closures for the Minnesota Department of Transportation. Two events in particular resulted in much damage to the railways, highways and dams in the area. Landslides, wash outs, frost heave, bearing capacity, cross level, and other flood damage occurred along railway and roadway systems within the area requiring subgrade stabilization (Figures 1, 2, 3, 4, & 5).

Figure 1. June 2014 Rainfall Departure from Normal for Minnesota. Numbers on northern part of

the state indicate areas requiring injection work.

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The soils in the areas consist of peat deposits generally within a few feet of the surface. In some areas these deposits can extend to depths greater than 50 feet. As the peat becomes more mature the soils grade into organic clays. Some areas feature rock outcroppings. The peat soils in the region can see moisture contents approaching 1,000% with nearly zero shear strength. High precipitation events increase the deterioration of these soils, resulting in unstable subgrade conditions.

Figure 2. Roadbed infiltration schematic (courtesy of Mario Ruel, CN).

Figure 3. Frost boils during thaw with underlying saturated silty soils.

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Figure 4. Bearing capacity failure after significant rainfall with underlying saturated clayey soils.

Figure 5. Cross level measurements after significant rainfall with underlying saturated organic soils.

After the record precipitation events CN experienced a notable increase in slow orders and other warning signs of track instability. Inspectors frequently observed movement of the adjacent ground (Figure 6) as trains passed through various sections. These sections required stabilization (Figure 7).

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Figure 6. Adjacent ground movement under load.

Figure 7. Typical injection location with fouled ballast just south of the crossing. Typical Subgrade Improvement Techniques for Track Roadbed Track remediation to correct poor organic soil conditions has taken on many forms over the last century. Techniques have included mass soil excavation and replacement, ground improvement techniques, driven timber piling, displacement of organic soils with shot rock, and various other techniques. Table 1 summarizes some of the classic remediation techniques in addition to advantages and disadvantages of each system. Figure 8 demonstrates historic asphalt injection into the subgrade. In trying to do things rapidly, those who aren’t geotechnical engineers have done a variety of things without geotechnical evaluation. This lack of geotechnical evaluation results in a lack of ability to verify and improve efficacy. Therefore many techniques have been tried regardless of the long term viability or the economics of the technique.

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TABLE 1. Summary of Classic Remediation Techniques.

Technique Advantages Disadvantages Cost

Factor Typically Performed by Railroad MOW Personnel

Tamping Short term Increases depth of fouled ballast; creates pinnacle; lowers lateral stability

Low

Undercutting Removal of fouled ballast Not as effective when fines are not removed

Medium

Lateral Drainage Improved sub drainage by removing water from ballast pockets.

Does not improve deeper soil issues Low

Typically Performed by General Earthwork Contractor

Removal & Replacement

Readily available equipment and material

Not feasible in most Cases; Time; Cost; Accessibility; Out of Service until completed

Medium

Corduroy with select fill above


Not feasible in most Cases; Destabilizes over time; track taken out of service for installation

Medium

Rock Fill Displacement

Readily available equipment and material, inexpensive

Will likely cause settlement of existing track, track taken out of service

Medium to High

Geogrid with rock fill


Track taken out of service Medium to High

Typically Performed by Structural Contractor

Timber Soldier Piles

Readily available equipment Question effectiveness; Expensive; Induced heave / settlement of track roadbed

Medium

Steel Piles Quick Installation

Expensive; Induced heave / settlement of track roadbed (needs design based on geotechnical information to be cost effective and efficient)

High

Typically Performed by Ground Improvement Contractor

Lime Injection Quick reduction in water content

Destabilizes over time; soil type dependent

Medium

Cement Slag Injection

On track equipment Not readily available; Moderately expensive

Medium

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Figure 8. Historic photograph of injection of asphalt into rail road subgrade, demonstrating that injection techniques are nothing new.

Ground Improvement Program

Considerations must be taken into account to determine which ground improvement technique is most appropriate for railroad subgrade stabilization. The CN case study’s limitations included work within short windows between trains, specialized mobilization for equipment in remote locations, and rapid results with limited slow orders. Additionally, the ground improvement needed to be effective for a long period of time, reducing future maintenance. Table 2 includes the selection consideration for various techniques on the project. CN chose timber soldier piles and slag-cement injections for its ground improvement program based on the constraints included above. The primary deciding factors were implementation time and effectiveness. While timber soldier piles were traditionally used in this region, the injection technique added the benefit of completing the work with a high-rail vehicle at a faster rate. The structural contractor drove the piles on both sides of the track 4 feet from the tie edge on 3-foot centers, to depths ranging from 20 to 30 feet below grade (Figure 9). The contractor used conventional equipment off track with an average pile installation time of 14 days for a 100-foot section. Inspections noted ground heave after pile installation. This required additional surfacing as a remediation technique. The surfacing frequency has reduced significantly in the year since initial remediation.

Figure 9. Driving timber piles along the shoulders.

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TABLE 2: Ground Improvement Selection Consideration.

The ground improvement contractor used a track-mounted injection rig to inject cement slag slurry as deep as 20 to 30 feet deep below the track to within 2 feet from the top of rail (Figure 10). During injection, inspectors observed water flowing out of the subgrade. Injecting on a primary-secondary pattern and over a 2-day period allowed initial set of the primary grout locations prior to the secondary injection (Figures 11 and 12). The primary injection locations also may fill subgrade void features, allowing the secondary pass to provide greater effectiveness at treating subgrade mud pockets. The primary secondary sequencing also results in injection at every other tie crib. The contractor achieved an average 1-day injection time per 100 feet of track. Following a 48 hour post-work cure period track inspectors noted a significant improvement with respect to previously reported defects. Qualitatively, sections where the injection treatment was performed displayed a “smoother ride” per transportation employees. Quantitatively, performance increased by a reduction in the surfacing frequency as demonstrated in Table 3. It is important to note the grout requires some cure time immediately following the injection procedure. The authors recommend slow orders from the initial injection to a period of at least 24 to 48 hours following the final injection application to allow the material to cure. This project followed these recommendations.

Technique Work

between Trains

Selection Consideration

Typically Performed by Railroad MOW Personnel

Tamping Yes Not effective for treatment of deeper organic and soft soils

Undercutting No Not a ballast issue

Typically Performed by General Earthwork Contractor

Removal & Replacement

No Time requirement for work block exceeds transportation demands.

Corduroy with select fill above

No Time requirement for work block exceeds transportation demands. Deteriorates over time with groundwater fluctuations.

Rock Fill Displacement No Time requirement for work block exceeds transportation demands. Quantity depends on depth of organics.

Geogrid with rock fill No Time requirement for work block exceeds transportation demands.

Typically Performed by Structural Contractor

Timber Soldier Piles Yes Mobilization; ability to work between live track, ability to quickly get on and off track; Question effectiveness; Expensive; Potential heave / settlement of track roadbed.

Typically Performed by Ground Improvement Contractor

Lime Injection N/A Not allowed on CN Track; Inconsistent long term stability.

Cement Slag Injection Yes

Mobilization; ability to work between live track, ability to quickly get on and off track; ability to treat subgrade to depth required for project. Proven track record for 30+ years in North America; Recent successful use on other CN Subdivisions.

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Figure 10. High Rail injection rig performing the work.

Figure 11. Parallel section view with initial and secondary injection points.

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Figure 12. Perpendicular section view with subgrade conditions.

Track surfacing before the injection program is an important consideration for a successful injection program. Track surfacing after injection completion is possible; however, this is more difficult and generally harder on the surfacing equipment. The authors recommend surfacing the track just prior to injection. A before-and-after injection comparison of surfacing frequency provides a good measure of the success of the injection program. As can be seen in Table 3, the work dramatically reduced the surfacing frequency.

TABLE 3. Surfacing Frequency for Each Injection Area.

Injection Area Surfacing One Year before Injection

Surfacing One Year After Injection

Notes

1 8 0 2 8 0 3 7 0 4 10 1 Profile issue. 5 5 0

The Importance of Comprehensive Ground Improvement Approach While injection provides improvement, it does have limitations. It should not be used to solve all subgrade defects. For this reason, a qualified geotechnical engineer should be involved to evaluate site and soil conditions. Once evaluation occurs an appropriate ground improvement program can be selected. The evaluation’s consideration of area geotechnical investigations will result in a more effective and economical program. USGS soil maps or other commonly available information augments this soil information. The selection of the ground improvement technique, as well as its effectiveness is based on both quantitative and qualitative data. This comprehensive approach and the use of slag-cement

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injection is a significant change over the past quarter of a century. Another significant change in injection techniques is that lime and fly ash are no longer being injected under active railway lines in North America. Conclusion and Summary

The injection areas discussed in this paper have seen a hard winter freeze, a spring thaw and another wet summer. The areas are still performing as anticipated. To date there have been no schedule surfacing corrections for cross level or warp defect. Injection corrects issues such as pumping attributed to mud pockets, perpetual cross level concerns, generally soft subgrade as a result of saturated clays, and as observed in the subject work areas, treatment of some organic soils. Track performance continues to become more demanding as freight loading increases. Therefore, the need for a quick method for subgrade stabilization is even more critical than in years past. Injection of grout into the rail subgrade has been ongoing for well over half a century. However, the rail industry does not widely understand injection techniques and material. Subgrade injection provides a method of keeping the track in service while providing improvement for poor cross level performance. The likelihood of this stabilization technique’s use for a wider variety of subgrade issues increases as the industry learns more about injection. Tables and Figures

TABLE 1. Summary of Classic Remediation Techniques. TABLE 2: Ground Improvement Selection Consideration.

TABLE 3. Surfacing Frequency for Each Injection Area. Figure 1. June 2014 Rainfall Departure from Normal for Minnesota. Numbers on northern part of the state indicate areas requiring injection work. Figure 2. Roadbed infiltration schematic (courtesy of Mario Ruel, CN). Figure 3. Frost boils during thaw with underlying saturated silty soils. Figure 4. Bearing capacity failure after significant rainfall with underlying saturated clayey soils.

Figure 5. Cross level measurements after significant rainfall with underlying saturated organic soils. Figure 6. Adjacent ground movement under load. Figure 7. Typical injection location with fouled ballast just south of the crossing. Figure 8. Historic photograph of injection of asphalt into rail road subgrade, demonstrating that injection techniques are nothing new. Figure 9. Driving timber piles along the shoulders.

Figure 10. High Rail injection rig performing the work. Figure 11. Parallel section view with initial and secondary injection points. Figure 12. Perpendicular section view with subgrade conditions.

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A R E M A 2 0 1 5 A N N U A L C O N F E R E N C E

Minneapolis, MN | October 4-7, 2015

Presentation Outline

History of the line

Background and subgrade conditions

Ground Improvement Options

Post Improvement Performance



CN Rainy Sub

RR from Turn of the century, constructed as a logging RR

Heavy Traffic Line, Single Main

Abnormal wet season results in maintenance issues and slow orders



Early Roadbed Construction Technology



Transcontinental Railroads in North America

US Completion of TCRR 1869

Canadian TCRR 1885



Minnesota Logging Railroads



Fall 2013

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CN Rainy Sub heavy rain fall, note injection locations at north end of mapCN Rainy Sub 2014 heavy snow fall



Rainy Sub, Annual Precipitation Graph



Rainy Sub, maintenance concerns following heavy precipitation



Slurry Grouting for Railroads



The troubled rail cross section!

Mud filled ballast pocket,High moisture, low shear

Squeeze into ditch

Squeeze into ditch




Squeeze forming due to low subgrade strength caused by ballast pocket ‐ results in loss of track geometry

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Squeeze forming due to low subgrade strength caused by ballast pocket ‐ results in loss of track geometry

High Water Table

Saturated clay



Stabilization techniques typically performed by Railroad MOW Personnel



Stabilization techniques typically performed by General Earthwork Contractor



Stabilization techniques typically performed by General Earthwork Contractor



Stabilization techniques typically performed by Structural Contractor



Stabilization techniques typically performed by Structural Contractor

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Stabilization techniques typically performed by Ground Improvement Contractor



CN Rainy Sub, injection location near a crossing, note mud in ballast



CN Rainy Sub

Variety of areas with different subgrade soils

Heave and cost of driving piling were no longer acceptable

Variety of subgrade issues

CN requested an alternative to driven piling





The use of slurry injection to improve RR subgrades has been in practice for over 70 years.

Alternative maintenance - Injection - Then



…. And now. Injection technique showing the surrounding organic soils

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Profile of track subgrade and the injection technique



Cross section of the roadbed and the injection technique showing the corduroy material




Parameters of slurry grouting a RR subgrade

Slurry grouting is performed in two stages resulting in the injection of every other crib

The grout material used should always be a cementitous material capable of achieving a set irrespective of a reaction with the soil

Application rate of material should be on the order of 300 – 400 lbs per track ft.f 300 lbs. per track foot




Rule of thumb for rate of treatment is 400 track feet every three days

Injection depths of up to 30’ are possible, however, 15’ to 20’ is more typical

Re‐tamping and reestablishing the track geometry should be done prior to slurry grouting



Frequency of surfacing after completion of the injection work

Injection Area Surfacing One Year

before Injection

Surfacing One

Year After

Injection

Notes

1 8 0

2 8 0

3 7 0

4 10 1 profile issue at

culvert, culvert not

injected

5 5 0



Conclusions

RR Subgrade injection is an effective and quick means of subgrade stabilization in a variety of soil conditions

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Conclusions Continued

RR Subggrade injection can be an effective quick stabilization technique

More cost effective, less heave, better technical solution than driven piling

Know your soils, select the correct technique

Treat the problem not the symptoms

final hill, colwell - arema...the cn case study’s limitations included work within short windows...

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