A seawall is a major investment, costing $30,000 to $100,000+ or up depending on the property. A well constructed seawall provides the foundation for your property so assurance it will serve you for decades it should have a 50-100 year design service life. Therefore, you want to make sure it is designed and built by competent professional engineering and contracting.
In preparation of designing a new or replacement seawall, waterfront property owners should consider the following:
A good engineer will ask you these questions and be able to advise on appropriate approaches,
How long did the previous wall serve?
How long do you want the new wall to last?
What traditional materials and new technologies should be put into the project?
What will the wall serve to do, hold land, provide docking be a front for buildings, pools or patios?
How high do you want the wall to be?
How deep of water do you ultimately want?
Your seawall professional engineer and contractor will need the following information,
Recent survey – required now by most building departments
Soil’s information – for weak layers or rock layers for installation
Aesthetic considerations – wall face, top of wall, docks etc.
Also, make sure both the engineer and contractor carry the appropriate certifications and insurance. Both should carry Errors and Omissions Insurance to safeguard your investment. We have seen many wall failures within the first few years of service and homeowners having no recourse for shoddy design and construction.
Marine contractors also need solid general liability, specific licensing, and Longshoreman’s insurance.
Seawalls Consist or Panel’s and Support Systems
There are normally three types of Seawalls utilized:
Tie – Back system – A tieback supported wall utilizes ground anchors embedded into the cap or into a drop wale (beam across the face of the wall) These systems are very efficient design. Ground anchors can be excavated and placed, can be driven in by a jackhammer, or screwed in like big soil screws. New technologies are frequently being introduced to minimize surface disruptions and still achieve superior performance. We urge care be taken in material selection as corrosion of tieback tendons can be a concern. But normally these are the preferred option
Batter Piles – Forward support pushing from the waterside of the wall. These are concrete piles set at an angle. These are often used in site-constrained walls where tiebacks are not an option because of other buried infrastructure behind the wall. A typical pile is supposed to be end-loaded, or axial-loaded. Battering forces, a lateral load across the tipoff the pile. We often see cracking at the top of the battered pile allowing seawater to get to the steel and the pile corrodes from the inside out. Also, improper embedment of batter piles is a common installation shortfall. Overall, we avoid using batter piles if possible as they seem to last only 10-15 years before they start to fail. We are careful to design them closely and oversee installation as the failure rate of batters is higher and sooner than we like to see.
Cantilever Wall – These walls are very stiff material, concrete, hybrid, or steel embedded deep into the earth. They stand alone without exterior support. These are very nice to use where batter piles and tiebacks are not an option. They require robust material and deep embedment so are rarely utilized in residential situations.
Seawall Panel Materials
Wood – can be effectively used in short walls provided they are robustly designed, have proper preservation treatment, are protected from marine borers, and metals are protected from the corrosive effects of wood preservatives. It is very difficult to get aluminum to not quickly corrode in the presence of many wood preservatives.
Vinyl – Has excellent design life when properly designed and will last 50+ years. It doesn’t corrode or lose strength. It has limitations in where a lot of hydrostatic pressure (think hurricane rains saturating the ground) so practical maximum wall height is 7-9 foot before other products are more practical. Beware contractors and engineers specifying too light of a panel or too short. Vinyl is a flexible material and will deform if corners are cut in order to make a project cheaper. The most common and costly construction practice is to cut project pricing and utilize short panels or weak panel sections.
Fiber Reinforced Plastic (FRP) or “Composite” – A very strong material utilized in situations where vinyl leaves off. It has better stiffness with the same non-corrosive properties. It has a strength between vinyl and steel and provides a good ‘gap’ filler in wall size. There have been instances where it doesn’t hold up well on direct ocean breaking wave impacts but is superb in the bay and canal applications. It is also limited to soils without rock as they can split on installation if hard items are encountered.
Aluminum – Aluminum is a great material in freshwater with low corrosion chemistry. Aluminum is a terrible long-term solution in saltwater. Most aluminum walls we see have corrosion holes forming in the tidal zone within 15 years, with ultimate failure within 20. We rarely design aluminum in our coastal market and never in saltwater.
Concrete – Most seawalls built from 1920-1980 were reinforced concrete. USACOE standards suggest a 30–50-year design life. Reinforced concrete contains steel which rusts over time inside the concrete resulting in ultimate failure. While many of the walls built from 1955-1980 are still in service, they have exceeded their service life. Concrete is relatively inexpensive as a material but requires heavy machinery to handle and install. Panels are often short which can result sometimes in soil leakage under the wall.
Hybrid Vinyl-Steel-concrete – These walls, introduced around 2000, are essentially vinyl forms that are left in place and reinforced concrete walls are poured in place. They have the advantage of stiff reinforced concrete and vinyl coating corrosion protection. They are price competitive to slightly higher, but their robustness and service life are some of the best in the industry. We have been a leader in engineering many of these walls with great success.
Steel – Has a 30–75-year design life. It is very robust and very high wall exposures require steel because it is the king of robust materials. Its major downfall is corrosion. We compute corrosion loss into the design of the wall for its design life, anticipating annualize loss of strength. Steel, therefore, gets expensive for most residential applications. It can be coated with coal tar to slow down corrosion. We also have coated steel with concrete resulting in a concrete core with a steel core for strength resulting in for very long design life.