MSE wall rebuild

	Herzog's MPM lowers sections of the reconstructed MSE wall into place along light rail tracks in Littleton, Colo.

	The MPM bucket attachment was used to place backfill material around the new MSE wall.

	The MPM performed a variety of tasks while working on the MSE wall project, including excavating waste soil, removing the damaged wall, as shown above, bringing in fill material, transporting new wall sections and track materials.

In the early hours of Jan. 17, 2009, 18 freight cars carrying molten sulfur derailed on a main line running beside the Regional Transportation District’s light rail tracks in Littleton, Colo. Not only did the derailment damage two of three main lines, but also the mechanically stabilized earth wall supporting one of RTD’s two light rail tracks, located six feet above the main line tracks.

Herzog Contracting Corp. sent two of its MPMs to aid in the clean up and begin repairs. One came from just north of Denver, while the second was rerouted to Littleton on its way to Iowa. According to Herzog, the machines had experience working in tight quarters and performing trackway wash-out work, but never before had they been used to perform wall replacement.

“We recommended the use of the MPMs, after we assessed the damage that occurred,” said John West, project manager with Denver Transit Construction Group, a joint venture between Herzog and Stacy & Witbeck, Inc. “By using the MPMs, we were able to leave the three main line tracks in service and did not have to remove or replace the other light rail line or remove the overhead catenary wire.”

“The MPMs worked very well,” said Henry Stoppelcamp, engineering technical services manager at RTD. “We had 400 plus feet of wall that needed to be rebuilt and [the MPMs] pulled out all the debris from the derailment and brought the materials in.”

A derailment contractor was hired to pick up the tank cars and then the MPMs picked up the rest of the debris and began the wall work, which started the afternoon of January 18, a little more than 24 hours after the derailment.

“The nature of an MSE wall is that there are straps that are embedded in the backfill,” said West. “In order to construct a new wall, you have to remove all the backfill, as well as the straps and panels for the wall. There was a mass of soil, as well as all the wall panels that had to be removed by the MPMs, which were then hauled off and dumped at a staging area. We were fortunate that we are constructing MSE walls on the West Corridor and we were able to go to our supplier and coordinate with them to incorporate some of the wall panels that were designated for that job into the design for the replacement wall.”

In addition to the more than 400 feet of wall that was replaced, 4,000 yards of backfill material was hauled in and set in place, 420 feet of coping on top of the wall was also set and then the sub-ballast and ballast materials were placed by the MPMs.

West points out that it took two weeks from the time of the derailment to the time of project completion and believes the MPM’s versatility was key to the project’s timeline.

“We could use the rig as a shovel front to dig out the wall, as a backhoe to place and help grade the materials and then as a crane to hoist the panels and coping,” said West.

“This was a perfect storm from the standpoint of what these machines can do,” said Joe Knieb, vice president at Herzog, “Various types of material can be put on the MPM at the same time, plus they cleaned up debris such as metal axles, excavated the soil in order to take out the damaged wall panels, brought in new fill material and even brought in the new wall panels.”

Challenges

“We were in a restricted area working adjacent to two live lines that needed to be operational while work was performed,” said West.

The derailment occurred on tracks that handle 30 to 40 trains a day, primarily transporting coal from Wyoming to southern Colorado and into Texas. In addition, the light rail handles in excess of 150 trains a day.

“We had three freight main lines and two passenger lines [located] in a depression that is 150 feet wide,” said Stoppelcamp. “We did not have a whole lot of room to work in and we had to work from the bottom up. It was a challenge to build the wall and leave the tracks in service, but it would have been an operational challenge to have those tracks out of service.”

Crews worked around the clock to finish the project quickly, but performing trackwork during January in Colorado can be its own challenge.

“The backfill of the wall required moisture density control,” said West. “We did have to shut down operations a couple of nights when the temperature dropped below zero and the water in the water trucks began freezing. January wasn’t the best time of year to be doing this, but I think it enhanced the success of being able to get this [track] open in two weeks.”

Safety

Knieb points out that using the MPM on the project added an extra element of safety.

“Safety was a significant portion of this project,” said West. “Because of the nature of the derailment being right in the middle of the City of Littleton, there was a lot of public interest and by being able to do the work from the track way there was no way the public could access the work zone or be affected by the work zone.”

West pointed out that the alternative to getting the work done had the MPMs not been available would have involved tearing out a section of the light rail line, building an access road down from a nearby parking lot and removing the overhead catenary system, which would have taken much longer.

Stopplecamp added, “If we did not use the MPMs, we would have had to pull one of the tracks out of service and the flexibility to do the work would have been greatly reduced.”

“Not only from a safety aspect, but with limiting machinery and the work zone, you minimize fugitive dust and limit damage done to existing streets. I guess I could say it was like using a magnifying glass and some tweezers,” said Knieb.