The City of Fitchburg selected Weston & Sampson to provide preliminary assessment and final design engineering services related to the Arbor Way Retaining Wall repair project. Constructed in 1998, the mechanically stabilized earth (MSE) Arbor Way retaining walls are located on the east and west sides of Arbor Way. The walls are approximately 210 to 250 feet long and have retained earth heights up to 24 feet. The City observed failure of the north portion of the east wall around May 2006. Weston & Sampson completed a preliminary engineering assessment in 2014, which included observing the retaining walls and providing preliminary wall repair recommendations. The failure included backwards displacement due to loss of backfill, which likely occurred due to stormwater runoff directed towards this section on the wall. We evaluated several preliminary wall repair alternatives based on our observations and the fact that Arbor Way could not be closed during repair construction as it serves as the only access to two businesses at the end of Arbor Way. Based on these considerations, the recommended repair alternative included localized repair of the failed section of the wall and improved drainage along Arbor Way.
Our final design services, completed in July 2019, included additional wall observations. These observations indicated no change to the failed section of the east wall. However, we observed areas of new distress along the east and west walls. These areas included outward sliding of the wall blocks along sections of both the east and west walls, approximately 5 feet below the tops of the walls. The cause of the sliding was not evident from our observations but is likely due to internal instability.
Based on the new areas of wall distress, the preliminary wall repair recommended in 2014 was no longer considered practicable as construction for wall repair would require closure of Arbor Way. We reevaluated repair alternatives that considered maintaining access to the two businesses during construction, impacts to adjacent wetlands, ability to improve surface drainage, access to subsurface utilities that might need future repair, and costs. Based on these considerations, we recommended to the City that wall repair include construction of earthen embankments on each side of the walls to buttress the walls. This approach would limit impacts to adjacent wetlands, maintain access to the businesses during construction, allow grades to be modified to improve surface drainage, allow access to subsurface utilities, and eliminate maintenance on new structures. This alternative was also estimated as the lowest cost.