Abstract:
The Massachusetts Bay Transportation Authority's (MBTA) Charlestown Bus Facility was built adjacent to a deteriorated steel sheet pile bulkhead wall extending along 1, 000 ft of the tidally influenced Mystic River. The Charlestown Bus Facility is one of the largest MBTA maintenance facilities, which services buses that provide rides for over 99, 000 people daily. In the early 2000s, the bulkhead wall and associated steel drainage outfall pipes were failing due to corrosion. To mitigate this condition, after considering several alternatives, the MBTA and project’ s design team selected a stone embankment to buttress the existing wall as the preferred alternative. Simpson Gumpertz & Heger, Inc. (SGH) acted as the project's prime consultant for the design of the embankment and construction administration.
The embankment was designed to sit in the tidal zone of the Mystic River, where the subsurface conditions consisted of loose marine sands and sediments overlying a soft clay layer. The stone embankment is founded on a geogrid-reinforced stone load transfer platform supported on 4 to 7 ft diameter jet grout column struts extending into sound bearing material. The magnitude of the loads and a project team drive to reduce soil improvement costs by maximizing the spacing of the jet grout column struts (10 ft) required using high-strength geogrid reinforcements that were not available in the North American market at the time. During design, we identified sliding stability challenges with the embankment related to the interaction with the deteriorated bulkhead which required incorporation of a transverse geogrid reinforcement within the load transfer platform. Our presentation will discuss the embankment and geogrid design, as well as provide an overview of some of the challenges faced during construction, and recommendations for future geogrid reinforcement design, storage, placement, and possible repairs.