JMT has assisted the City of Charleston, SC, since 2015, first with a structural assessment of the historic Low Battery seawall and subsequently with designing a multi-phase plan to address its deterioration. The project goals included improving the city’s frontline defense against sea level rise, stormwater, and flooding, as well as improving accessibility and safety for residents and visitors.

The Low Battery seawall was originally built in the decade between 1909 and 1919 as part of a larger land reclamation plan undertaken on the peninsula. The seawall has helped protect Charleston for more than 100 years, but by the time the city began running assessments on the wall in 2004, it was clear that the elements had taken their toll.

The original wall was made of concrete supported by a timber deck and battered timber pilings driven into the open water at an angle to resist the horizontal forces of the ocean and the Ashley River. These elements have naturally degraded across the seawall’s long life, and during early excavation and examination of the wall, the JMT team even found some pieces of the timber supports under the walkway were deteriorated and crumbling.

During the initial inspection phase, the project team discovered historical documents detailing the construction of the original wall. A thorough examination of the full length of the Low Battery seawall revealed that the as-built condition of the wall generally matched the documents, allowing the team to use those historical details to evaluate the wall’s current condition and explore rehabilitation strategies.

Pre-construction inspections of the wall also showed that the original timber pilings were more frequently spaced than anticipated. This discovery led to a redesign that ultimately worked in the project’s favor: with a more efficient installation of the new foundation, the redesign meant that the wall face was not under construction during the peak of hurricane season.

As the inspection progressed and team members met with residents to discuss the community’s needs, new details emerged regarding the wall itself and the area around it that suggested the project scope needed to be expanded. While the team planned to preserve the wall and improve the city’s defense against stormwater and flooding, they also incorporated improvements to the area’s drainage system and roadways and enhanced the seawall’s promenade to make it more accessible for tourists and locals alike.

Flood and sea-level rise protection

At the start of construction, a section of the wall called the Turn, which makes up 120 linear feet of the structure, was demolished and replaced. The Turn was prioritized due to its significant deterioration, and the construction process on this section used traditional wall construction methods.

The reconstruction of this section revealed the full extent of the wall’s deterioration, and the Turn served as a pivotal point for the project. This relatively small section of the wall cost $26 million to reconstruct, and at this point the design team began exploring alternative materials and methods.

JMT’s design team, along with a cadre of skilled sub-consultants, approached the project as both a preservation effort and an engineering challenge. To reduce the cost of the project, the team decided to creatively reuse portions of the existing wall as part of the construction process. This innovative approach had the added benefit of preserving a portion of the historic landmark in the new design.

In the following construction phases, the team cut away the top portion of the wall a little above the high tide point and reinforced the lower sections with shotcrete. The bottom portion of the wall then served as a cofferdam, keeping water out to maintain a dry environment for the construction of the new upper portion. This adaptive reuse of the wall helped to maintain the history of the wall and eliminated the need for temporary structures to keep the water at bay, reducing both the cost and time of construction and minimizing impacts on local marine life.

The project team also used an innovative micropile system to support the original base of the wall and connect it to the new upper section. This micropile system–which involves drilling a hole, placing a small diameter steel rod, and grouting it into place–allowed the team to reinforce the wall while minimizing impacts to the surrounding environment.

Because the micropile method does not involve large cranes, it requires less overall workspace than more traditional pilings. Further, the placing of micropiles creates less noise and vibration; the design team wanted to reduce the risk of potential damage to nearby historic homes and other structures due to significant vibrations from construction. The reduced noise also helped minimize disturbance to the neighborhoods near the seawall.

To complement the micropiles, the design team opted to use a non-traditional reinforcing material for the water-facing surfaces of the wall. In phases one and two, the sections of the wall that sit below grade were reinforced with steel and the upper sections were reinforced with glass fiber reinforced polymer (GFRP). By phases three and four, the contractor on the project had gained significant experience with the material and opted to use it as the primary reinforcing material for both the upper and lower portions of the wall.

GFRP was chosen over other materials because it is lightweight, strong, and easily handled. While the material itself is more expensive, it doesn’t require heavy machinery to move or place it, balancing out the cost in terms of equipment. GFRP doesn’t corrode or rust with exposure to the elements and requires little to no maintenance, allowing the wall to continue protecting the city for many years.

The team’s use of GFRP in this project is unique. GFRP is relatively new, and this is the first state-funded project to use GFRP as the primary reinforcing material in an infrastructure project of this kind.

These innovative solutions led to significant cost savings for the City of Charleston across later phases of the project. While the Turn alone had cost the city $26 million, phase one cost just $10 million and involved rehabilitation of approximately 800 linear feet of the wall.

Across four construction phases, the project team has reinforced the existing seawall and added one-and-a-half feet of additional height to protect against rising sea levels.

The new upper portion of the wall is also expandable: the top portion of the railing can, with minimal disturbance to local neighborhoods and marine life, be cut away and replaced with glass panels to provide additional protection against sea level rise. If needed, the supports between the panels can be extended and larger glass panels put into place. These considerations for the future mean that the new wall, while already providing enhanced protection against the sea, can accommodate another forty-two inches of height to meet the city’s changing needs.

Drainage and soil concerns

While the seawall itself prevents incoming water from overwhelming the Low Battery area during storm and flood events, the local drainage system helps to facilitate other movements of water.

The team replaced all the drainage and sanitary sewer pipes to meet modern standards, which mitigated concerns raised during the inspection phase. Additional high-capacity inlets were installed to better accommodate stormwater events, and water quality units were installed that remove debris and pollutants like fine metals, nitrate, and phosphates from stormwater to protect the local marine ecosystem.

The new drainage system also features tidal valves. While tidal valves are the standard now, their inclusion was relatively unusual when the new system was designed. The outfalls for the system sit below the high tide line, and the tidal valves allow stormwater to flow out but prevent seawater from flowing into the drainage system. In the event of a storm or flood event during high tide, the tidal valves ensure that the drainage system isn’t filled with seawater, allowing stormwater to drain into the system and remain contained until the tide goes out and the water can drain out into the sea.

Grading the area to direct stormwater into the updated drainage system presented a major challenge. Proper drainage typically requires a minimum of a 0.5% slope with 1% being the ideal. This means the land surface slopes downwards, providing a change in elevation that allows stormwater to flow off the road surface to the designated inlets, ensuring public safety by eliminating ponding water during storm events. However, due to the proliferation of driveways and the need to meet ADA compliance requirements for roadways and sidewalks, the longitudinal grading along the road was closer to 0.25%–about one quarter of an inch slope downwards per one hundred inches of distance.

To address the grading concern, the team utilized cross slopes—slopes running perpendicular to the road’s direction—to guide water to the curbs. Cross slopes are typically 1-2% slopes; this allows water to drain off the road to the curbs, which then guide the water into the drainage system.

The nearby roadways and sidewalks were also redesigned because they had settled over time, raising concerns about the soil conditions in the area and the potential loss of fill material under them. When the Low Battery was built, 47 acres of developable land were created along the peninsula by filling in wetlands with dredge material from the river. However, the land created using this method is not stable over time; some of the material erodes, leaving space between the remaining soil particles, which leads to instability and settling.

To replace the lost fill material and stabilize the soil, the project team injected an expanding polyurethane grout into the substrate behind and under the original wall. The grout expands to take up the space created by lost fill. This task was completed early in the construction process and had the added advantages of preventing tidal intrusion from disrupting construction and keeping fine particulates like soil and sand from migrating into the river. This grout helps to prevent the soil under and behind the wall from being washed away and undercutting the wall in the future.

Accessibility and the built environment

Besides acting as the city’s first defense against the sea, the seawall is also an important and popular local landmark. With the soil stabilized, the team reworked the roads and sidewalks around the Low Battery seawall to improve accessibility and safety for motorists, bicyclists, and pedestrians.

The team fully redesigned Murray Blvd, the road adjacent to the seawall, to feature better on-street parking alongside two 11-foot-wide travel lanes separated by a landscaped median. A ten-foot-wide sidewalk on the residential side of the street helps create a safe space for pedestrians; on the seawall side, a five-foot-wide sidewalk at street level runs alongside the elevated promenade. The new intersections along Murray Blvd include hardscape crosswalks and raised speed tables to control the speed and flow of traffic.

Given the seawall’s history, the city also wanted to ensure the promenade along the seawall remained an inviting public space. To this end, the project design includes a series of parklets along the wall that include landscaping and seating areas to encourage visitors to linger and enjoy the view–areas that JMT team member Jim O’Connor, who was instrumental in winning this project for JMT, calls the “front porches” of the Low Battery Seawall.

The future of the seawall

At the time of writing, three of the four construction phases are complete, and those areas of the project have been reopened to the public. The fourth phase is expected to be completed in April 2025.

The $64 million project has already made a major difference for the community.

In a statement for Charleston City Paper, City Councilman Mike Seekings praised the protection the updated seawall provided during storms and high tide events that took place in late 2023: “If you came down here to the Low Battery area that has not been refurbished–water was breaching, coming over the top,” but in the areas where the wall’s refurbishment was complete, Councilman Seekings noticed that the new wall “actually provided an incredible amount of protection against the oncoming tides” during those late fall and winter storms.

JMT’s dedication to innovation, sustainability, and safety throughout the inspection, design, and construction processes has helped to protect the Charleston community from rising tides and preserve this significant landmark for future generations to benefit from and enjoy. Here’s to 100 more years of the Low Battery seawall.