Piscataway Hills Landslide Stabilization

Fort Washington, Maryland

In late spring of 2014, heavy rains fell on already saturated soils, triggering a landslide in the Piscataway Hills neighborhood in Prince George’s County, southeast of Washington, D.C. Pavement cracks measuring between three to six inches wide and up to 100-feet in length began to appear along Piscataway Drive. The slide accelerated rapidly on May 4, leading to a major pavement collapse of nearly four feet in one location and a large crack extending 450-feet along the roadway. Earth and trees tumbled down the 65-foot high ridge, leaving several homes sitting precariously at the edge of the newly formed cliff. Piscataway Drive provides the only access to homes leading down to Piscataway Creek, a tributary to the Patuxent River. The damaged roadway, fractured water and sewer mains, downed power lines and a hazardous hillside led to an immediate road closure and evacuation of 28 homes.

Piscataway-Hills-Landslide-Stabilization-Construction-Road-ClosureKCI Technologies Inc. was called in to assist Prince George’s County DPW&T in investigating the cause of the failure and immediately mobilized on site. Engineers performed a thorough geotechnical exploration and forensic evaluation to establish conditions at the time of the event. It was critical to understand the resistant and driving forces that existed at the failure surface, when the movement occurred.

During the initial investigation, KCI’s design managers and engineers worked daily from the county’s on-site mobile emergency response center to monitor conditions, coordinate data and provide daily updates to the County Executive and communications staff. Our team installed inclinometers and piezometers to monitor slope and roadway movement and water levels throughout the project. Soil borings, cone penetrometer tests and dilatometer in-situ tests help identify major subsurface strata including problematic soils, as well as estimated depths of critical slope failures and ground movements. In just two weeks, the firm completed a comprehensive geotechnical study, surveys, and environmental site review, and submitted a draft report outlining findings that pointed to a 30-foot thick layer of clay beneath the soil as the cause of the slope failure. Found throughout Southern Maryland, Marlboro Clay is known to be highly susceptible to landslides due to the significant loss of shear strength under saturated conditions. In Fort Washington, intense rainfall of more than two inches in less than 24 hours infiltrated the soil above the Marlboro Clay, causing the interface between the two strata to lose frictional resistance and therefore become very slippery. Infiltrated water could not dissipate quickly in the clay, thus generating pore-pressure build up that resulted in a significant loss of shear strength. The draft report also included preliminary recommendations to stabilize the slope and reconstruct the roadway.

Following the landslide, the county worked diligently to inform and engage homeowners. Public meetings, which were held regularly during the investigation to discuss results and alternatives, were often challenging, as residents fought to regain access to their homes and in some cases retain their homes. Understandably, they were reluctant to accept the cause as a natural disaster, which may have allowed insurance companies to deny claims. As a result, the residents hired independent consultants to review the geotechnical report. Several agreed with the findings, but one engineer blamed the broken waterline. Instead of the landslide causing the WSSC main to break, this engineer suggested that the reverse was true. Despite this challenge, the KCI’s exhaustive investigation held true under extreme technical and legal scrutiny.

Temporary restoration of both above and below-ground utilities was critical in getting 22 of the 28 displaced families back into their homes as quickly as possible. Unfortunately, six homes were deemed uninhabitable. Based on the team’s recommendation, the county installed a temporary drainage diversion just above the slope failure to reduce additional infiltration into the hillside. County and utility crews worked to restore basic services, including water, sewer and roadway repairs. The team coordinated with Pepco to run temporary power around the site to ensure clearance for construction equipment. WSSC provided a temporary pumping station for the sanitary line and a temporary water line that was needed for possible emergency response.

With a goal of permanently returning residents to their homes as quickly as possible, the schedule drove the project. Engineers quickly began developing alternatives to stabilize the slope and reconstruct the road. During conceptual design, the team consulted with national geotechnical experts Richard Ortt, Maryland Geological Survey, and Francis Ashland, U.S. Geological Survey, in developing solutions. Slope stabilization approaches that were initially considered included drilled shafts, soil nailing, retaining walls with deep foundations, and driven steel piles. At the same time, preliminary engineering began to develop conceptual designs for the neighborhood roadway that accounted for efficient layout/location of all known utilities and identified impacts to streambanks and wetlands that would be affected by the slope stabilization system. Services included surveying, permit support, utility coordination, and structural and transportation engineering.

From the onset, the team realized that engineering and construction could not follow a typical project development process. Early in design, 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 immediately became a critical part of the design team.

Recognizing the need to keep the project moving forward, the county’s project manager and chief of construction management also agreed to weekly co-located coordination meetings held in KCI’s office. Attendees included design engineers, contractor staff, WSSC, and ALL permit review agencies.

As part of these meetings KCI’s environmental manager, coordinated input from the Maryland Department of the Environment to obtain a tidal wetland license and a Letter of Authorization for impacts to non-tidal wetlands and buffers within the Chesapeake Bay Critical Area, Maryland-National Capital Park and Planning Commission (M-NCPPC) to obtain approval for impacts to forest stands within the project corridor, as well as M-NCPPC, the Critical Area Commission for the Chesapeake and Atlantic Coastal Bays (CAC) and MD Department of Natural Resources to obtain a Roadside Tree Permit (RTP).

These working meetings facilitated over-the-shoulder review of and changes to the plans in real time as concerns and revisions were discussed. Submittal review times were virtually eliminated because comments were received weekly as the design progressed. The contractor’s input was critical in working through constructability issues that included equipment availability and material selection. Their early involvement and technical contributions to decision-making and design were key to developing an option that best fit their means and methods.

Having our engineers, environmental manager, contractor and the permit reviewers all participating in the design process with over-the-shoulder reviews helped facilitate design and construction and was fundamental to the project’s success.

Ken Briggs, PEProject Manager

Ken Briggs, PE

To stabilize the slope as well as protect the roadway and utilities, the team focused on preparing an environmentally-friendly solution that would not change the character of the area and minimized the construction footprint and inconvenience to the community. The county, engineering and contracting team continually revisited design alternatives in terms of cost, schedule and constructability. Collaborative discussions centered around possible solutions, cost saving opportunities, and efficiencies, including an evaluation of the merits of micro-piles versus H-piles in terms of constructability and cost. 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 the six condemned homes. This approach greatly simplified slope stabilization methods and types of equipment proposed, and significantly reduced costs.

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 at 5- to 7-foot spacing. The approach was selected based on considerations of site and legal constraints, including safety issues related to impacts on upslope buildings, existing water/sewer mains, access roadway, limited site disturbance, and accelerated constructability. The selected stabilization and ground improvement approach comprised innovative features, including optimized pile depth and spacing based on extensive instrumentation data and evaluation of ground movement. To ensure the materials were available for construction notice-to-proceed, the county approved an early payment to purchase the piles before roadway and utility plans were completed. A new Juntann rig was selected for its ability to quickly drive the H-piles (It peaked at 21 piles per day during construction). Its computerized controlled operation provided automated monitoring of the impact force on the piles to minimize vibration impacts and accelerate the installation process. While the design progressed, the contractor, engineer and county separately formulated costs and quantities for bid items. The team then compared prices, which lowered the overall cost of the project. The team also capitalized on an opportunity to simultaneously cut back and refine the design, generating an additional $1.1 million in savings.

Construction would take approximately six months and cost $11 million, making the Piscataway landslide the most costly natural disaster in county history. Identifying funding sources for both the construction and property acquisition was also a critical challenge since a capital planning allocation had not been set aside for an emergency of this magnitude. Officials had to balance dollars for the quick resolution of the Piscataway Hills landslide with long-term impacts to ongoing infrastructure expansion, rehabilitation and maintenance programs. The county also pursued grants to secure millions from MEMA and FEMA.

Notice-to-proceed was officially issued in July of 2015 in hopes of avoiding potential delays caused by winter weather. Mobilization of the large pile driving rig created concerns related to over-sized vehicle permits as well as access down the steep and narrow sections of Piscataway Drive. Bridges throughout the Southern Maryland region weren’t rated to carry the weight of the rig, and a circuitous route and expedited permit coordination were required to ensure that the equipment arrived on time. Engineers plotted a detailed course for the rig and pile delivery vehicles to navigate the winding neighborhood streets.

Piscataway-Hills-Landslide-Stabilization-Resident-StairwayWith their roadway access once again cut off, the community joined together to build two sets of lighted stairs up a steep cliff-side so they could bypass the construction work zone and continue to reside in their homes. Because Piscataway Drive is a secondary residential roadway that terminates at the last property, it was critical to establish a method whereby first responders could access the residences at the lower end of the roadway. The county, contractor and KCI worked with residents to develop a protocol that employed a locked cable across the roadway instead of traditional barricades. A key was given to neighborhood leader Daisy McClelland, who would open the cable in case of an emergency. Coordination between on-site inspectors and construction staff ensured a clear path when the contractor left each day.

The best-case scenario called for stabilizing the slope and restoring the utilities and roadway before dropping temperatures and seasonal weather required the road to remain closed throughout another winter. Thanks to fluid coordination, technical excellence, true partnering, and diligent management, along with favorable fall weather, the community celebrated when Piscataway Drive reopened on Christmas Eve. Later in May, County Executive Rushern L. Baker, III presented a proclamation to the community proclaiming May 2 as Piscataway Hills Day. As a safeguard, KCI continues to monitor the site regularly for movement and will do so for the year following stabilization.

Complex projects with demanding schedules that require constant communication between multiple disciplines and agencies are not uncommon in the transportation industry. What makes this project stand out was the emergency nature of the project, the early selection of the contractor, and the team’s approach to project management. Early coordination between the county, public, engineers and contractors enabled this complex problem to be solved in a timely fashion and below budget. Excluding funding and legal research, the entire analysis, design, permitting and construction life cycle was completed in less than 14 months. Teamwork also helped cut the cost of the emergency repairs nearly in half from a staggering original construction estimate of $20 million. In addition, the complex forensic investigation, design approach and construction methodology employed as part of the emergency response will set the precedent for responding to landslides where other problematic soil conditions (e.g., Marlboro Clay) are present. Although there have been similar slope failures in and around the Southern Maryland region, the Piscataway Drive project was by far the most challenging, expansive, and the most high profile.