Emergency Response, Technical Leadership, and Extensive Coordination Expedite Restoration after Catastrophic Landslide

In the late spring of 2014, heavy rains fell on already saturated soils, triggering a landslide in the Piscataway Hills neighborhood, located in Prince George’s County, Maryland, southeast of Washington, D.C. The slide opened large cracks along the roadway, caused trees to tumble down the 65-foot high ridge, and left several homes sitting precariously at the edge of the newly formed cliff. Piscataway Drive, which provides the only access into the neighborhood, dropped four feet in one location. That damage, along with fractured water and sewer lines and the unsafe nature of the hillside, led to an immediate road closure and subsequent evacuation of 28 homes. KCI assisted the Prince George’s County Department of Public Works and Transportation in investigating the cause of the failure, prepared engineering plans for repairs and provided construction oversight to restore service and access to the residents.

Piscataway-Hills-Landslide-Stabilization-Pre-Construction-Slope-Failure

Piscataway-Hills-Landslide-Stabilization-Pre-Construction-Roadway-Cracks
Large cracks split the asphalt along Piscataway Drive as sections of the roadway and surrounding land sloughed down the hillside toward the creek.
Piscataway-Hills-Landslide-Stabilization-Inclinometer-Monitoring
The KCI team installed six inclinometers and one piezometer to monitor the hillside for future movement, and completed 15 soil borings and 10 cone penetrometer tests to create a clear picture of the subsurface strata during the geotechnical study and then continued monitoring throughout design and construction.

The KCI team mobilized immediately on site to initiate a comprehensive geotechnical study. “Before you develop a design solution, you need to determine the cause of the problem,” said Geotechnical Practice Leader Kwabena Ofori-Awuah, PE. “Data collected during our subsurface investigation was used to perform complex calculations in determining the soil conditions and strength at the time of the slide.” Just two weeks after the collapse, his team delivered a report outlining their findings and identifying three distinct soil strata. The middle layer, which was composed of a 30-foot thick layer of Marlboro Clay, showed evidence of failure planes. Found throughout Southern Maryland, this type of soil stratum is known to be highly susceptible to landslides because it cannot absorb subsurface water. Instead water moves across the top of the layer, creating a slick surface.

Piscataway-Hills-Landslide-Stabilization-Marlboro-Clay-Slope-Failure-Cross-Section
The top layer of soil moved over the middle 30-foot-thick stratum of Marlboro Clay, bringing down trees and causing the roadway failure. The soil dropped as much as 12 feet in some locations. A bulge of material that was moved is evident further downslope towards Piscataway Creek.

FAST-TRACK DESIGN

With the cause defined, engineers quickly began developing alternatives to stabilize the area. The KCI team consulted with national geotechnical experts Richard A. Ortt, director of the Maryland Geological Survey, and Francis X. Ashland, a research geologist with the U.S. Geological Survey, in developing proposed solutions. Slope stabilization approaches that were considered included drilled shafts, soil nailing, retaining walls with deep foundations, and driven steel piles. Public meetings were held regularly to discuss alternatives and solicit input.

The county began investigating funding alternatives as well as legal responsibilities. “As we developed concept plans, we had to make sure we were pursuing something tangible that was defined and would pass scrutiny as legitimate costs and engineering effort,” said Vice President Mary C. Wiedorfer, PE, CCM, PMP, LEED AP. “The last thing we wanted was to be perceived as following leads that weren’t viable for the county or the taxpayers.”

Piscataway-Hills-Landslide-Stabilization-infographic-timeline

KCI worked with the county using their ‘Most Practical Source’ procurement method to identify and solicit proposals from several qualified contractors, bypassing the traditional design-bid-build and design-build approaches that could have delayed the schedule. Corman Construction was selected and became a critical part of the design team, attending weekly meetings and working through constructability issues that included equipment availability and material selection.

Piscataway-Hills-Landslide-Stabilization-Juntann-Pile-Driving-Rig-2
A new Junttan rig was selected for its ability to quickly drive the H-piles, offering the potential to further reduce schedule and reduce vibration. The roadway’s ability to support its weight as well as the complexity of maneuvering the equipment along the winding neighborhood street had to be considered.

The county, engineering and contracting team continued to revisit design alternatives for stabilizing the roadway and slope in terms of cost, schedule and constructability. One option even considered purchasing all 28 homes and turning that portion of the neighborhood into a permanent greenspace. Instead, the county focused on repairing the damage within their right-of-way and elected to purchase six homes that remained at risk and were deemed uninhabitable. Demolishing the homes greatly simplified slope stabilization methods and types of equipment proposed and significantly reduced construction and engineering costs.

Piscataway-Hills-Landslide-Stabilization-Neighborhood-Map
The landslide occurred along a 65-foot high ridge, with soil moving toward the creek below. Homes along the top (red), now sitting at the edge of the cliff, were deemed uninhabitable with the county planning to purchase the properties from the owners.

The final remediation design called for 411 steel H-piles in 60- and 70-foot lengths to be driven along either side of the roadway, with some areas requiring two rows of piles. “The contractor had been researching materials, sizes and the time frames for delivery on site,” said project engineer Dion K. Ho, PE. “The county approved an early payment to purchase the piles before roadway and utility plans were completed to ensure they were available for notice-to-proceed.” The construction would take approximately six months and cost $11 million, making the Piscataway landslide the most costly natural disaster in Prince George’s County history.

SUPPORTING A UNITED COMMUNITY

From the beginning, the county worked diligently to inform and engage residents. Public meetings were challenging, as residents fought to regain access to their homes and in some cases retain their homes. The goal of the entire team was to stabilize the slopes and restore the roadway so that residents could return to their homes as quickly as possible. Although homeowners had been allowed access to their property once basic services, including water, sewer and temporary roadway access, had been returned, construction of permanent repairs would again cut off their vehicular access, mail and trash removal services. In the months leading up to the pile driving that would completely block Piscataway Drive, the community joined together to build temporary mail boxes and garbage receptacles at the top of the roadway. They also erected two sets of stairs and a series of pathways that would allow residents egress and ingress to their properties without crossing the dangerous construction work zone.

Piscataway-Hills-Landslide-Stabilization-Resident-Stairway
The community built two sets of lighted stairs to bypass the construction site, which completely cut off vehicular access to the lower 22 homes. A zip line helped residents move heavy bags and larger items, like Christmas trees, up and down the steep slope.

KCI and the county worked with the contractor and the residents to ensure that first responders could service homeowners in case of a fire or medical crisis. A protocol was developed that employed a locked cable across the roadway width instead of traditional barricades. A key was given to neighborhood leader Daisy McClelland, who volunteered to coordinate with on-site inspectors to make sure nothing was in the way when the contractor left each day.

EXPEDITED CONSTRUCTION LEADS TO QUICK RESTORATION

Throughout the construction, KCI provided ongoing support and inspection, including regular observations of inclinometers for slope movement. Notice-to-proceed in June of 2015 kicked off an accelerated schedule aiming to stabilize the slope, and restore the utilities and roadway before winter weather could delay construction and turn the residents’ steep bypass into a treacherous icy crossing.

There were a lot of moving pieces—homeowner risk, how to mitigate their struggles, engineering studies, funding and county obligations—any and every delay could have been catastrophic.

Mary C. Wiedorfer, PE, CCM, PMP, LEED APVice President, Regional Practice Leader

Mary C. Wiedorfer, PE, CCM, PMP, LEED AP

Thanks in part to fluid coordination, technical excellence, and diligent management, along with favorable fall weather, the community celebrated when Piscataway Drive was reopened on Christmas Eve. KCI continues to monitor the site regularly for movement and will do so for the year following the stabilization efforts.

Piscataway Drive Post-Construction
The completed roadway features drainage and safety improvements.