McComb 734.1: Replacing a double track concrete arch with two 104-foot concrete trestle bridges in ten days

Kevin R. Day, P.E. CN, Manager of Structures

2151 North Mill Street Jackson, Mississippi 39202 Telephone: (601)-914-2647 Facsimile: (601)-592-1753

Sandro Scola, P.Eng CN, Senior Manager of Structures

17641 South Ashland, Avenue Homewood, Illinois 60430 Telephone: (708)-332-4732 Facsimile: (708)-332-3514

ABSTRACT Over a ten-day period from September 14 through September 23, 2007, the CN

(Canadian National) engineering team successfully replaced a double track thirty-foot

arch with new concrete trestle bridges on the McComb Subdivision near Jackson,

Mississippi. On September 12th, the 1903 arch began to show signs of settlement.

Emergency work to stop the undermining of the arch floor was unsuccessful against

heavy rains from Hurricane Humberto. The arch separated longitudinally between the

tracks resulting in an out of service condition on September 14 at 01:30.

Immediately thereafter, the CN Bridge team began developing a strategy to get the

mainline back in service. As an interim measure, two corrugated steel pipes were placed

where the arch had collapsed, and track #2 was placed back in service at 08:00 on

September 15. With one track open for service, the CN Bridge team proceeded to

reestablish full service by constructing two 104-foot modified standard concrete trestle

bridges fifty feet north of the existing arch. The bridge for track #1 was built in 5 ½ days,

from September 15 through September 20, and the bridge for track #2 was built in four

days, from September 20 through September 23. Major construction activities included

placing a soldier pile retaining wall between the tracks, realigning the creek channel,

splicing and driving fifty H-piles, setting and welding ten precast caps, setting eight

twenty-six foot concrete spans, setting track panels, ballasting and surfacing. Double

track service was restored a mere ten days following the initial arch failure.

INTRODUCTION

On September 12, 2007, the track inspector on the McComb Subdivision notified the

bridge supervisor that the arch structure at Milepost 734.1 (Figure 1 and 2) was leaking

ballast. The bridge supervisor inspected the bridge, and the inspection revealed cracks

and signs of minor movement (Figure 3 and 4), indicating that the arch structure was in

danger of becoming unstable. The CN structures group placed a ten mile-per-hour (mph)

temporary slow order on the structure, and contacted a local contractor, Hill Brothers

Construction Company (HBCC) to make immediate plans to mobilize and begin repairs

to stabilize the arch. The plan was to place sheet piling at the four corners of the arch,

and pump concrete under the existing floor to prevent water from flowing underneath the

arch. The CN structures group felt that this would slow erosion at the base of the arch,

and help stabilize the arch from further separation. HBCC mobilized to the site using the

old Gulf, Mobile and Ohio (GM&O) right-of-way located approximately 300 feet east of

the existing arch. HBCC started pumping flowable fill concrete at approximately 19:30

on Wednesday, September 12. The pumping continued for approximately sixteen hours.

At around 16:30 on September 13, heavy rain from Tropical Storm Humberto began to

fall. Water began flowing through the arch at a steadily increasing rate. Given the

uncertain condition of the arch, the CN structures group decided to keep two employees

at the site to monitor the structure on a twenty-four hour, rotating basis.

ARCH FAILURE AND TEMPORARY REPAIR

At approximately 01:30 on Friday, September 14, the onsite CN bridge inspector noticed

rock dropping between the two tracks overtop of the arch structure. The bridge inspector

called the dispatcher and took the structure out of service for train operations.

Approximately ten minutes after calling the dispatcher, the east arch structure failed

(Figures 5 and 6). The arch cracked all the way through, and the north half of the east

arch shifted to the east (Figure 7).

With the mainline shut down to traffic, the CN structures group started reviewing options

to get the track back in service in the shortest time possible. Two options were

considered based on available material:

1) Tear down the remainder of the east and west arch structures, drive piles north

and south of the existing arch structures, place precast concrete caps and

either a second hand steel span or precast concrete spans.

2) Tear out the existing arch floor, set large corrugated steel pipes (CSP) in place

of the arch structures, place fill over the pipes, open one track for service, then

build a new bridge consisting of steel pile and precast concrete caps and spans

north of the existing arch, including realignment of the creek through the new

structure.

The CN structures group estimated that even though option one would be less costly to

build, it would require a few more days of total outage time on the mainline. Factoring in

the cost of having the mainline out of service and the urgency of the situation, it was

decided to proceed with option two. At 08:00 on September 14, the CN structures group

began securing materials to get track #2 back in service, and estimated a twenty four hour

duration to complete this first task.

Fortunately, corrugated steel pipe (CSP) were available from other projects. Two large

diameter pipes, a 72-inch and an 84-inch, were selected. HBCC mobilized two large

trackhoes with concrete busters to tear down the broken arch. Thirty seven rail cars of

crushed stone (CA-6), loaded at Greenwood, Mississippi for another project, were

brought to the site via a CN work train. The track #2 arch removal began at

approximately 09:00 on Friday, September 14. The arch structure for track #1 had

remained in place during the failure of the track #2 arch. The CN structures group

decided to use that track to off load the crushed fill material using a trackhoe from inside

the gondola cars (Figure 8). Due to the uncertain structural condition of the track #1

arch, the CN structures group decided to allow only empty gondola cars to cross over the

arch. The work train engine was placed on the north end of the cars, and the train was

pushed over the track #1 arch as the cars were unloaded. The pipes and fill were in place

by approximately 08:30 on Saturday, September 15, and track #2 was placed back in

service at 09:30 on Saturday, September 15 (Figure 9).

PERMANENT REPLACEMENT STRUCTURE

With the temporary culverts in place and track #2 open for train traffic, the CN structures

team turned their focus to building a permanent structure to replace the arch. The

temporary culverts provided only a fraction of the hydraulic capacity of the arch, and any

substantial rainfall would cause the mainline to wash out a second time. Understanding

the urgency to complete the new structure, the CN structures team devised a plan to build

two 104-foot standard concrete trestle bridges approximately fifty feet north of the

existing arch structure (Figure 10). This plan provided the following advantages:

1) It enabled rail traffic to continue to operate on track #2 during the

bridge construction for track #1.

2) It was clear of the old arch structure and newly installed CSP.

3) It allowed the creek channel to be diverted to a more natural path,

crossing the track at a ninety-degree angle.

The plan was to build the bridge under track #1 while traffic continued to operate on

track #2. Once the track #1 bridge was complete, rail traffic would be diverted to track

#1, and the bridge under track #2 would be constructed. The CN structures team

developed an aggressive 24-hour per day schedule to finish the track #1 bridge by 22:00

on September 20 (Figure 11). Every major activity was placed on a schedule, and a

material list was developed to ensure that all the necessary components would be onsite

to meet the critical path for the bridge construction.

Construction of the Track #1 Bridge

Mobilization of materials and pile driving equipment commenced immediately following

the completion of culvert installation. The first activity was to build a retaining wall

between the tracks to allow excavation of the bridge area under track #1. The retaining

wall was made of steel H-pile spaced on eight-foot centers with timber stringers placed

between the H-piles (Figure 12). The stringers were obtained from the old abandoned

GM&O bridge which was 300 ft downstream and torn down during the rebuild. The

excavation was kept to a minimum to allow for the installation of the concrete caps, and

was benched between bents to reduce the lateral pressure on the wall. The wall and track

were monitored after each train to immediately react to possible movement. This activity

started at 19:00 on Saturday, September 15, and was completed at 07:00 on Monday,

September 17.

Once the retaining wall was built, HBCC began driving the foundation piling for the new

bridge. Based on geotechnical information from other bridges rebuilt in the vicinity, the

CN structures team estimated 90-foot 14x73 H-piling would be necessary to achieve a

substructure factor of safety of two. The available H-pile sections came in 40 and 50-

foot lengths, so splicing of the piles was required (some 70-footers were available a day

later for the abutments). To save time during this activity, the CN structures and

operations team developed a method to allow trains to proceed underneath the boom of

the crawler crane while piles were being spliced and driven (Figure 13). A geotechnical

engineer from Burns Cooley Dennis (BCD) was brought to the site to perform a pile

driving analyzer (PDA) test to determine if the pile lengths could be shortened. The

results from the PDA showed that for the abutment piles, a safety factor of two could be

developed using a 70-foot pile. Twenty 70-foot piles were delivered (with a Mississippi

Department of Transportation escort) eliminating the need to splice the abutment piles.

This reduced the required pile driving time by almost a day. The total amount of time to

complete the twenty five piles for track #1, while operating on track #2, was sixty hours.

The next activity during the construction of bridge #1 was setting and welding the precast

concrete caps (Figure 14). Each substructure element consisted of five steel H-piles and

a standard Burlington Northern Santa Fe (BNSF) concrete cap. CN is grateful to our

colleagues at the BNSF for allowing us the use of ten concrete caps, which were ready

and available at their precast supplier. Piles were welded to the plates to complete the

substructure construction. Each cap took approximately six to eight hours to weld, and

this activity followed the completion of pile driving for one row of pier or abutment piles.

A majority of this activity ran concurrently with the pile driving. All the caps were

welded in place by 10:00 on Thursday, September 20.

Once the caps were welded in place, HBCC began setting the superstructure. The

superstructure consisted of four twenty six foot long, thirty inch deep precast concrete

spans. Each girder took approximately one hour to set in place. All the spans were set in

place by approximately 18:00 on September 20. Backfilling and compacting the

abutments was finished by 19:00. Track installation, dumping ballast and surfacing the

track took approximately three hours. Track #1 was returned to service at 22:00 on

Thursday, September 20, five days and three hours after beginning the project.

Construction of the Track #2 bridge

With traffic shifted from track #2 onto track #1, it was time to begin excavation work on

track #2. The temporary culverts were still handling the water flowing underneath the

mainline. With the threat of a washout at the temporary culvert still a distinct possibility

(with a heavy rainfall), the twenty-four hour operation continued during the bridge

construction for track #2. The first activity was to excavate the channel, including the

removal of the retaining wall that was required to support track #2 during the

construction of the bridge on track #1. The excavation work began at 00:00 on Friday,

September 21 and was complete by 12:00 on the same day.

Pile driving for the track #2 bridge commenced following the excavation of the new

creek channel. Seventy-foot piles were driven at the abutments, and ninety-foot piles

were driven at the piers. The design pile loads for the project were 100 kips in

compression and 25 kips uplift. The blow counts for the piles were consistent to those

obtained while building the bridge on track #1 ensuring a factor of safety greater than

two. Pile driving for the track #2 bridge started at 12:00 on Friday September 21 and was

completed thirty six hours later on Sunday, September 23.

The next activity during the reconstruction of the track #2 bridge was to weld the

concrete caps in place. Similar to the track #1 reconstruction, this activity required two

welders per cap to produce the 3/8 inch fillet weld required on all sides of each pile.

Once the piles were in place for one bent, the cap was set in place and welding began.

This activity was performed concurrently with the pile driving work. It started at 18:00

on Friday, September 21 and was complete by 08:00 on Sunday, September 23.

Once the caps were welded in place, the focus turned to setting the spans for the track #2

bridge. With the cranes set up on the east side of the mainline tracks, this task was

accomplished without reaching over an active mainline (track #2 is the east track). This

helped facilitate setting the spans in a timely manner (Figure 15). The four twenty six-

foot spans were set in place by 16:00 on Sunday, September 23. Backfilling and

compacting the abutments was completed by 18:00 and the track panel was set in place

(Figure 16) by 19:30 on Sunday, September 23. Dumping ballast, surfacing and signal

testing was completed by 02:30 on Monday, September 24. Double track service was

restored a mere ten days and one hour following the initial arch collapse (Figure 17).

Other Construction Activities

Once the two bridges were opened for train service, a variety of construction activities

still remained. Both structures were outfitted with a galvanized steel walkway. The two

CSP used as a temporary opening for water flow during construction were filled with

flowable fill concrete and covered with a crushed aggregate and riprap. This was done to

complete the rerouting of the creek to the new channel underneath the bridge.

Approximately thirty six rail cars of riprap were dumped to line the new creek channel

and help provide support for the new embankment where the old creek channel used to

be. Grass was planted on the embankments of the new creek channel to reduce the

potential for erosion, and pile bracing (4”x4”x1/2” angle) was placed on the middle

bents. HBCC completed all of these activities on a normal 07:00 to 17:00 work schedule

within twelve days after opening both CN tracks for service (Figure 18).

CONCLUSIONS

This project was challenging from the start. Anytime the mainline is out of service, there

is an increased urgency to restore service as soon as possible. The most challenging part

of completing a rush job such as this is to ensure that the result to the problem is not just

a quick fix, but a sustainable solution that will provide service well into the future. As

with all construction activities, it is wise to review the successes and shortcomings to

determine areas for improvement on future projects. Some of the lessons learned prior to

and during this project include:

• Ensure that personnel entering and exiting the site are given a thorough job

briefing. Once the CN structures team developed the plan for reconstruction, a

twenty four hour command post was set up at the entrance to the site to ensure

each person entering the site was provided with the type of track protection and

the current construction activities. This helped ensure that the project was

completed in a safe manner, and it also allowed the project team to control the

amount of personnel on the site at any given time.

• The full extent of the scour around the arch structure was not readily visible as the

drift that had accumulated over the years on the downstream GM&O bridge kept

a few feet of water stagnant through the arch. If a spread footing structure is

permanently under water, even for a few feet, it must be part of a regular

underwater inspection program.

• When developing a solution with the mainline out of service, make sure that all

angles are covered. In this situation, our first thought was to build a bridge

overtop of the existing channel where the arch had collapsed. Taking an hour or

two to determine what materials were available and their location allowed us to

develop a plan to place pipes temporarily to restore mainline service. This bought

the CN structures team extra time to create a final solution that will provide

service for many years.

This emergency project was a success due to the effort and dedication put forth by all

railroad and contractor employees. CN and HBCC pulled in the necessary resources to

provide around the clock supervision and workforce to proceed uninterrupted while the

mainline was out of service. Building two 104-foot bridges in ten days was no small feat.

Building it ahead of schedule and injury-free is a source of satisfaction and pride for all

involved.

ACKNOWLEDGEMENTS

I would like to thank all the people who contributed to making this project a success

including:

• The CN structures team on the Memphis and Gulf Zones – Allen Dunn, Terry

Ellington, Jack Burrell, Shane Crain, Ray Neu, Greg Roberts, and the bridge crews

that assisted with the project.

• The CN design team for quickly producing construction drawings, sourcing precast

materials and answering field questions – George Nowak, Bin Zhang, Po Sun, Trina

Kuzik, Christophe Deniaud.

• Jim McLeod and Nigel Peters for helping to determine the options and establishing

the schedule.

• The CN track department – Bryant McCuan, Eddie Pickett and the track forces that

supported the work.

• CN transportation – Rich Miller and his team.

• Coreslab Structures for assisting with the precast concrete material.

• The BNSF and Union Pacific structures departments for allowing us to use their

available precast concrete material.

• Paul Leonards and Hill Brothers Construction Company for their quick response to

our out of service condition.

This venture was truly a team project. The hard work and dedication of these individuals,

and all personnel involved with the project, made it an engineering success.

LIST OF FIGURES

Figure 1 – Map showing the structure location ................................................................ 14

Figure 2 – Photograph of thirty-foot double track arch structure ..................................... 15

Figure 3 – Photograph showing undermining of arch structure after draining................. 15

Figure 4 – Photograph showing undermining under northwest wing wall....................... 16

Figure 5 – Photograph showing sheet pile installation on southeast corner of arch......... 16

Figure 6 – Photograph showing a track view of the failed arch ....................................... 17

Figure 7 – Photograph showing the east side of the failed arch cracked through ............ 17

Figure 8 – Photograph showing the use of track #1 to unload crushed rock.................... 18

Figure 9 – Photograph showing the two CSP used to support water flow temporarily.... 18

Figure 10 – General layout drawing for the new bridge structure.................................... 19

Figure 11 – Schedule of major construction activities...................................................... 20

Figure 12 – Photograph showing the retaining wall used to support track #2.................. 21

Figure 13 – Photograph of pile driving for track #1 bridge.............................................. 21

Figure 14 – Photograph of setting the abutment cap for the track #1 bridge.................... 22

Figure 15 – Photograph of HBCC setting a span for the track #2 bridge......................... 23

Figure 16 – Photograph showing the track panel for the track #2 bridge being placed .. 23

Figure 17 – Photograph showing a train crossing the new track #2 bridge ..................... 24

Figure 18 – Photograph showing the as-constructed bridge............................................. 24

Arch Location

Arch Location

Figure 1 – Map showing the structure location

Figure 2 – Photograph of thirty-foot double track arch structure

Figure 3 – Photograph showing undermining of arch structure after draining

Figure 4 – Photograph showing undermining under northwest wing wall

Figure 5 – Photograph showing the elevation view of the east side of the failed arch

Figure 6 – Photograph showing a track view of the failed arch

Figure 7 – Photograph showing the east side of the failed arch cracked through

Figure 8 – Photograph showing the use of track #1 to unload crushed rock

Figure 9 – Photograph showing the two CSP used to support water flow temporarily

Figure 10 – General layout drawing for the new bridge structure

Figure 11 – Schedule of major construction activities

Figure 12 – Photograph showing the retaining wall used to support track #2

Figure 13 – Photograph of pile driving for track #1 bridge

Figure 14 – Photograph of setting the abutment cap for the track #1 bridge

Figure 15 – Photograph of HBCC setting a span for the track #2 bridge

Figure 16 – Photograph showing the track panel for the track #2 bridge being placed

Figure 17 – Photograph showing a train crossing the new bridge under track #2

Figure 18 – Photograph showing the as-constructed bridge