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Southeastern Pennsylvania Transportation Authority Serving Bucks, Chester, Delaware, Montgomery, and Philadelphia Counties

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NHSL Update | Emergency Embankment Repair

Project Update | 2/27/2013

Normal NHSL service was restored to the area between Roberts Road and Garrett Hill stations in January 2012 and the project was completed on March 30. In February 2013, FEMA announced that it had awarded SEPTA almost $3 million in funding for the emergency slope repair project. For more information about the award, click here.

SEPTA and HNTB Corporation received a 2013 Diamond Award Certificate from the American Council of Engineering Companies of Pennsylvania (ACEC/PA) for this project. The ACEC/PA Excellence Awards program honors the engineering achievements of ACEC/PA members and their clients. SEPTA retained HNTB to inspect and survey the entire slope before performing a subsurface investigation at the affected area. HNTB also designed the permanent slope repair and provided construction support throughout the project.

BACKGROUND INFORMATION

Even before Hurricane Irene and Tropical Storm Lee passed through the region, rainfall levels during August and September 2011 were at an all-time high. These storms caused flooding, track washouts, and slope erosion at a number of locations throughout the SEPTA system. The Norristown High Speed Line was particularly hard hit during Tropical Storm Lee. In the area between Roberts Road and Garrett Hill Stations, a layer of saturated soil lost its strength, which resulted in about 150 feet of the 40-foot tall embankment supporting the tracks to drop vertically and push out the side of the slope. The track in this area dropped about 12 inches and moved horizontally by about 16 inches. In the photograph below, showing SEPTA inspectors assessing the track displacement, you can see some of the damage, including:

1. Displaced soil from embankment
2. Leaning SEPTA and PECO power poles

SEPTA'S IMMEDIATE RESPONSE

Service on the affected track was discontinued until the damage could be assessed and the cause of the slope failure determined. SEPTA retained a geotechnical engineering consultant to inspect and survey the entire slope before performing a subsurface investigation at the affected area. After performing soil borings, the engineers installed inclinometer casings at the top and toe of the slope. Inclinometer casings are steel tubes which permit the lowering of a probe to a depth of 65 feet below the ground surface to measure movement in the embankment. With the soil boring and inclinometer data, a permanent fix for the embankment failure was designed. Our engineers continue to monitor the track area and embankment on a daily basis. These inspections will continue while the repair work takes place, along with close observation of weather forecasts to make certain that additional, excessive rain does not further compromise the embankment.

SEPTA'S LONG-TERM SOLUTION

There were several challenges that had to be considered in designing the permanent slope repair. There was no direct vehicle access to the site and a temporary access road had to be built. There is a PECO distribution line that must be raised prior to construction and protected and a PECO high voltage transmission line that must be protected during construction.

The site repairs will consist of:

Protecting adjacent properties and stream from erosion and runoff

Constructing a 245-foot-long post and plank retaining wall to support the new embankment

Removing portions of the saturated soil from the slope

Installing drainage features so that water will be able to infiltrate out of the slope

Rebuilding the slope at a less steep angle, which will be more stable

Planting native grasses to protect the surface of the new embankment from erosion.

SEPTA issued an emergency construction contract to perform permanent repairs to the slope. Alan A. Myers of Worchester, PA was awarded the construction contract. Work commenced on October 10, 2011.

UPDATE ON WORK

SEPTA and its contractors are continuing to expedite the work necessary to permanently correct the slope stability issue. The access road to the work site along the Right of Way is complete. The contractor has fabricated all the precast concrete retaining wall panels and structural steel posts, and last week started installing the caissons to support the new retaining wall. The caissons with the steel posts are embedded up to 25 feet into the earth and rock to support the toe of the embankment and the new wall.

Due to the complexity of the permanent repairs, crews will be working six days per week. The work is scheduled to be completed in February 2012. In order to stabilize the tracks on an interim basis, SEPTA issued a second construction contract for the installation of a soil nail earth retention system along the top of the embankment. This will allow for return of normal service on the line around the first of the year, and will be integrated into the permanent repairs to increase the overall stability of the reconstructed slope. SEPTA will closely monitor the slope to confirm that the interim stabilization method is performing as expected.