NEW LOW SEAWALL IN FRONT OF EXISTING SEAWALL
The new low seawall is installed directly in front of an existing seawall. The top elevation of the new seawall is lower than the existing, normally 2 to 3 feet below top of the existing seawall cap.
The design requires that aluminum sheet piles or precast concrete sheet piles be installed directly in front of the existing seawall. After pile installation is complete, a new reinforced concrete cap and new anchor system is installed.
This system can be installed prior to failure or where moderate movement has occurred.
A new low seawall can be installed at a lesser cost than a complete new seawall.
If this system is installed properly, it can be expected to last 50 years.
This is an excellent system, unless it is installed incorrectly, or the design is inferior. It should be noted that designs and specifications differ from contractor to contractor. It is important that before choosing a contractor, that the owner compare more than the price. Many inferior designs have no anchor system or reinforced concrete cap, and rely on bolting the sheet piles to the existing seawall by using either wood or aluminum whalers. In most cases, the lower price is due to lesser specifications and will not be the better deal.
The new seawall sheet piles should penetrate the existing ground a minimum of 40% of their length.
The new sheet piles used should be structurally designed to meet the required loads.
When choosing aluminum seawall sheet piles, the thickness of the metal is very important and can affect the cost drastically. Sheet piles less than .125" should not be used.
Concrete sheet piles up to 12' long should be a minimum of 6" thick. Size of the horizontal and vertical reinforcing steel in sheet piles up to 12' long should have #5 vertical bars spaced 6" on center and #3 horizontal bars spaced 24" on center.
Choosing alternative sheet piles such as vinyl, plastic, fiberglass and PVC are not recommended for seawalls 5' in height. NOTE: These products have only been in use in this area for only a short period of time. Use of these products should be done cautiously.
Filling the void between the old and new seawall sheet piles with concrete is very important and should be insisted upon by the owner, no matter which sheet piles are to be installed.
If aluminum sheet piles are used, contact of dissimilar metals should not be allowed.
Contractor should provide a 10 year structural warranty for the new seawall.
See final notes in New Seawall in Front of Existing Seawall