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Understanding the Technology of Sea Walls

For ages efforts have been made to prevent the erosion of shoreline property surrounding bodies of water everywhere.  Materials including wood, rock, block, concrete in many forms, steel and even plastics have been using various methods in an attempt to provide a barrier between the water and the shoreline to prevent erosion of the land.  Sea walls, as we will refer to these structures as a whole, have experienced a relatively high failure rate and short life span when compared to inland retaining walls.  Hydrostatic pressure (water pressure) behind a wall creating an excessive load upon the wall, as well as wave action and/or currents creating erosion at the base or foundation of the wall over time can lead to ultimate failure.  Soft wall materials, such as wood and plastics, have shorter life spans.  Rodents, such as muskrats and beaver, frequently reduce the wall life by chewing through the wall material to gain access to voids behind the wall, which provide safe haven.

Concrete structures, such as steel reinforced concrete block walls, formed and poured reinforced concrete walls, are much stronger and durable than soft wall structures; however, these walls create a dam-type structure whereby flood water or storm water can become trapped behind the wall thereby requiring the water to escape beneath the wall causing erosion below the foundation which can lead to ultimate failure.

An alternate method of controlling erosion is to place rip-rap rock along the shoreline from the lake bed to above the high-pool water level.  This is an excellent method of erosion control; however, it is certainly not attractive and will not allow access from the shoreline to the water, and the large rocks provide an ideal environment for snakes.

When selecting the type of sea wall for your property, you should carefully consider the following factors:

1)      Ease of access to construction site (shoreline).

2)      Fluctuation of water level from low pool to flood levels.

3)      Amount of currents or wave action at the shoreline.

4)      Degree of slope to the land in the area behind the shoreline.

5)      Height of bank at shoreline.

6)      Depth of water near shoreline at low pool.

7)      Your intended use of the shoreline area after the wall is constructed.

8)      Life expectancy of your wall.

9)      Aesthetics or the appearance of your wall.

10)  Maintenance required.

11)  Cost.

To accurately plan your sea wall, you need to understand that a retaining wall or sea wall is a near vertical structure with varying degrees of loading or pressure being applied to one side (back or land side) of the wall constantly trying to move the wall forward or turn it over.  In the case of concrete or hard structure walls, these pressures are held intact by the use of a large, wide concrete footing and/or the use of tie backs or cables connecting the wall structure to a stable area behind the wall (referred to as “dead men”).  When installed properly, these walls serve well as a permanent structure.  It is this author’s recommendation that any sea wall structure, whether it be wood, concrete, etc., be backfilled with crushed rock to allow free drainage of water through the reinforced zone behind the wall.  Soils absorb water resulting in pressure exerted against the wall.

           Where shoreline conditions allow, our preferred sea wall structure is a geogrid reinforced modular wall using an all crushed rock backfill.  This structure needs to be constructed at low pool, preferably in a dry lake bed several feet in front of the shoreline. 

This area is usually gained by the cutting and dressing of the bank during shoreline preparations.  The foundation of this wall is placed below the lakebed surface on a footing of compacted crushed stone or concrete.  The wall modules interlock by design thus eliminating the need for mortar between modules.  Geogrid reinforcing mats are placed between courses of block and extend back into the compacted crushed rock backfill, which is placed between the wall and the earth bank at the shoreline.  This wall system allows any water located behind the wall to move freely through the rock backfill and the interlocking modular wall, thus eliminating any hydrostatic pressures that can overload the solid wall systems.

If you have plans for a sea wall in the future, this article has given you enough information to ask the right questions and judge for yourself the value of the answers you are given.  We strongly suggest that you ask your prospective installer for references on prior jobs he has performed.  Call the references and visit the job sites to see the quality of his work.

During your planning stages, you must contact the designated shoreline management representative for the Regulatory Authority having jurisdiction over the body of water you are interested in to obtain a permit.  For most rivers and large lakes in our area, the Regulatory Authority is the Alabama Power Company.

Should you attempt to build your own sea wall?  Perhaps yes.  If your shoreline conditions are right, and you are a “hands-on” type of person and want to accomplish this task yourself, then “Go for it.”  If you choose to build a modular wall, you will need a wall design, a construction plan, the wall modules, geogrid reinforcing and crushed rock.  All of the above can be obtained through the GeoStone office on Highway 280 in Westover.  The machinery you will need will be a skid steer loader or backhoe and a vibrating plate tamp for compacting the stone leveling pad in the footing and the stone behind the wall.  Labor can range from family and friends to hired help.  Experience helps but it is not essential if you follow your construction plan.

            Our module of choice for years has been the GeoStone module, which offers the maximum flexibility, connection strength and durability.  In addition, GeoStone is the most installer-friendly structural module on the market.  The split face of the GeoStone block gives the elegant appearance of an upscale sea wall for a moderate price.

            Over the past 10 years, we have performed a vast number of challenging sea wall installations in the many lakes and rivers in Alabama, including the Coosa River chain from Mitchell Lake to Weis Lake, as well as Smith Lake and Lake Martin.  We have designed and installed several walls along the shoreline of the main Tennessee River near Wheeler Dam, which must sustain the pounding of 10-foot waves during severe gale-type storms.  These walls are expected to be permanent with little or no maintenance.

            You may call the GeoStone office and arrange for someone to visit your property, assess your shoreline conditions, advise you and make recommendations.  We can suggest qualified installers for you to talk with.  Additional information on sea wall and retaining wall construction with photographs can be seen on the GeoStone website at www.geostone.com or contact the GeoStone office at 11321 Highway 280 East, Westover, AL 35185 or phone us at 205-678-9969.

Contributed by Fred Agee, owner of GeoStone Retaining Wall Systems, Member of the Board of Directors, Lay Lake Home Owners and Boat Owners Association

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