Wednesday, September 5, 2007

Seawall and retaining wall Construction Information

Seawall News

With the recent hurricanes that have plagued us, many of our seawalls have been under water and damaged beyond repair. Some folks who had no seawall had their shorelines eroded and lost their back yard. A good seawall can help protect and beautify your valuable property. There are several wall systems to choose from. They are all priced about the same.

The following are some of the types on the market now-

Aluminum seawalls- This system allows you to either utilize a tieback system or you can install the wall in a cantilever type of configuration without tiebacks and dead men. The wall usually is installed with the panels, which are Z shaped, an equal distance of each panel embedded into the soil at the lake ward side (the mudline) and the same amount above the mudline. Then every 6’ to 10’ on center, tiebacks are placed with aluminum rods 10’ to 20’ behind the wall. The tieback rods attach from the face of the wall to a square aluminum plate (deadman) which is buried 10’ to 20’ landward of the wall. The threaded tieback rod is then attached to the wall though the aluminum wall cap which holds all of the wall straight and spreads the load of the wall to the tieback rods and deadmen. The length of the tiebacks, the size of the deadmen, the spacing of the tiebacks and the thickness of the wall panel is all engineered based on the site conditions.
When there is not enough room for tiebacks and deadmen such as the situation where there is a swimming pool very close to the wall, a cantilevered wall may be used. Instead of using tiebacks and deadmen, the wall is engineered to be self supporting. In normal soil conditions, this means that the panel will be thicker making the panel stiffer and it will be embedded deeper into the soil. This embedment is usually about twice the amount of wall above the mudline.
The aluminum wall cap is then installed and the wall is ready for backfill. You can also use a steel reinforced concrete cap on the aluminum panels although it requires a lot of extra labor and can be quite a mess. Concrete caps are normally used where the nature of the existing shoreline requires the wall to curve and meander. The aluminum caps cannot curve, but they can be mitered to make turns. Some people use wood caps on the wall, but it is a maintenance issue and I don’t recommend it. The aluminum seawall systems are available locally from Raven’s Manufacturing in Kissimmee. They also manufacture aluminum floating docks. Their phone number is 407-935-9799.

Vinyl seawalls- These walls can do the same thing as the above described aluminum walls. The caps for these walls can be aluminum, concrete or wood. The vinyl walls can even be used in a cantilever system but the depth of the panel corrugation is much deeper and the panel is thicker that the aluminum panel for the same application. You can purchase this system at Southern Pine in Pine Castle 407-251-1900.

Wood Seawall- An all wood wall can last a long time but has maintenance. They are constructed much in the same way as the aluminum and vinyl walls, but the deadmen are usually 6”x6” wood piles. The tiebacks are usually stainless steel cables but sometimes rods are used. The wood used must be suitable for water emersion. With the banning of CCA for most marine uses, the lumber you get now will be treated with ACQ or CA. Since these preservative treatments are new to us here in Florida, I believe a wood wall to be a poor choice. Southern Pine also has the materials to build a wood seawall.

MSE walls- The last seawall system I will discuss is the MSE wall which means “mechanically stabilized earth”. The wall is masonry and can be very beautiful. The choices of colors and designs are numerous. The first step in building the wall is to start with a firm, compacted gravel bedding layer of 6”. This gravel is placed approximately 12” below the mudline. Then the first row of the masonry block units is laid. Plastic pins are inserted between the rows of blocks and based on the engineer’s recommendation; plastic webbing is placed in the compacted backfill behind the wall and attached to the wall as a tieback system. This wall system cannot be installed in a cantilever design, but on shorter walls can be installed in a “batter” condition. While the wall is being constructed, each row of block is staggered landward by a very small amount. This results in the overall wall leaning back toward shore and thereby holding back the soil without the use of tiebacks.
This wall has the advantage of being able to be placed in a straight line as well as curved. Scour occurs at the front and bottom or toe of the seawalls when larger waves hit such as in a hurricane. I have seen where this scour caused the failure of one of these walls on Sandlake, but they had so much beach erosion, perhaps a wall with greater embedment may also have failed. Another benefit of the wall is that it can be repaired in such an instance without a great deal of work.
Paver Systems on Landstreet Road sells one of the many brands of MSE wall systems available. 407-859-9117

For beauty and longevity, I recommend the masonry MSE wall. For strength and durability and in cantilever systems I recommend the aluminum walls.

For those of you that have lost land during the last hurricane season, you need to move quickly to reclaim any lost land. I spoke to Wendy Meyer at DEP in the beginning of October with reference to this beach erosion issue on Lake Conway. She stated that there was a time constraint on how long you have to get the work done before the rules revert back to normal where you would not be allowed to recover you lost land. She said that there were three options. The first was to simply replace the soil that was lost. This soil down to the normal high water line would have to be sodded and staked or planted in some other manner and stabilized.
Another option is to replace the soil and then add a seawall to hold back the soil. The last solution is to replace the soil and then install a stone revetment along the shoreline. Although the stone revetment is not a vertical seawall, it can be a very beautiful and effective method to stop beach erosion. I recommend Coquina stone which is available on the East coast of Florida at Black Hawk Quarry-321-725-2400. First the beach is backfilled and graded to on a 2 to 1 slope. Then a filter fabric is placed over the graded soil. Then a 6” thick layer of gravel is placed over the fabric. Then the last step is to place large boulders over the gravel. Call DEP at 407-898-7864 for further information on the permitting.

For more information contact Rick Fender at Cloud 9 Services, Inc. 407.481-2750 or

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