Calvert Cliffs Slope Stabilization Naval Research Lab
Chesapeake Bay Detachment
2018 Outstanding Civil Engineering Achievement
Under $20 Million
United States Navy | Owner
Whitney Bailey Cox & Magnani, LLC | Consultant
Hayward Baker | Contractor
When the US Naval Research Laboratory (NRL) built its Chesapeake Bay Detachment (CBD) atop the Cliffs in the first part of the twentieth century, the edge of the Cliff was not as close to the facility as it is today. Significant failures along the CBD’s three-quarters of a mile of cliff frontage have brought the edge perilously close to the existing road, research structures, utilities, and buildings.
Aerial photograph of the existing conditions.
The Cliffs at the CBD are about 100-feet tall at the highest point with a near-vertical face at the upper 30 to 50 feet and milder slopes at mid-height and near the toe. Previous attempts have been made to stabilize the Cliffs. In 1996, a two-tiered soldier pile and timber lagging wall was installed after a 43-foot long slope failure resulted in the loss of approximately 10 horizontal feet of cliff top. After a slope failure in 2012 caused another 10-feet of cliff to slide, leadership realized that something had to be done to ensure the continued viability of the existing infrastructure and buildings.
The initial phase of the project began in 2012 when the Naval Facilities Engineering Command (NAVFAC) requested that Whitney Bailey Cox & Magnani, LLC (WBCM) investigate and stabilize a 700-foot long section of the 100-foot tall cliff. WBCM sub- contracted Schnabel Engineering to provide geotechnical engineering services for the investigation and design of the cliff stabilization measures as a supplement to WBCM’s survey and civil engineering services.
WBCM and Schnabel developed three conceptual design options and provided them to NAVFAC for consideration. After reviewing the options report, NAVFAC opted for the proposed soil nail stabilization solution. The soil nail stabilization option maintained the basic cliff geometry and allowed for a natural vegetated cliff surface. Maintaining the cliff’s features and the natural vegetated face was very important to NAVFAC, the Critical Area Commission for the Chesapeake Bay, and the Maryland Department of the Environment (MDE).
During the design phase, Schnabel performed a cliff stabilization analysis and created the design using both limit-equilibrium and finite element methods of analysis. Schnabel performed a reliability analysis of the existing cliff geometry and the soil nail stabilization design to estimate the probability that the design would be successful.
The soil nails were designed to be six-inches in diameter and 45-feet long. Each nail was drilled into the cliff and grouted in place. The stabilization of the lower slope required the design of a laid-back slope, resulting in a milder angle. Construction documents and permits for over 700 linear feet of stabilization were delivered to NAVFAC in late 2013. WBCM’s plans showed the matrix of 1,223 soil nails, the associated flexible facing for stabilization of the upper cliff face, and the seeding mix to promote vegetation on the cliff face.
Permitting for the project included coordination with the MDE, the Maryland Department of Natural Resources (DNR), the US Army Corps of Engineers, and the Calvert County Soil Conservation District (CCSCD).
In addition to the extensive slope stabilization, WBCM designed utility and infrastructure relocations to prevent infrastructure failures from causing damage to the restored slope. As a subtask, WBCM performed a hazard assessment of the cliff and outlined several immediate stabilization techniques that would allow for uninterrupted operation of the facility. One practice required design and permitting to relocate a piece of radar equipment that was in imminent danger of sliding down the cliff into the Bay.
Hayward Baker, Inc. was contracted by CCSCD to implement the cliff stabilization and associated civil works. The most challenging aspects of the construction were equipment access, personnel safety, and constructability of the soil nails. To safely access and install the soil nails, Hayward Baker constructed an access bench at mid-slope of the cliff. A drill mast attached to a long-reach excavator was used to install nails from the top of the cliff and a smaller excavator with a drill mast attachment was used to install the lower soil nails from the mid-slope access bench.
Aerial photograph of the completed conditions – early 2018.
The result of the $6.4 million project is a stable cliff face that will minimize future loss of land at the NRL facility. Construction was completed on time with zero change orders. The use of soil nails, flexible facing, hydroseed, and plantings provides a natural, vegetated face that created a suitable wildlife habitat, reduced erosion, maintained the basic cliff geometry, and provided a natural, unobscured view of the cliffs from the Chesapeake Bay.