UNDERSTANDING SEAWALLS

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 used in various methods in an attempt to provide a barrier between the water and the shoreline to prevent erosion of the land.  Seawalls, 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 seawall creating an excessive load, as well as wave action and/or currents creating erosion at the base or foundation of the seawall over time can lead to ultimate failure.  Soft seawall materials, such as wood and plastics, have shorter life spans.  Rodents, like muskrats and beaver, frequently reduce the seawall life by chewing through the wall material to gain access to voids behind the seawall, which provide safe haven.

 

Concrete structures, such as steel reinforced concrete block walls, formed and poured reinforced concrete walls, are much stronger and more durable than soft wall structures.  However, these seawalls create a dam-type structure whereby flood water or storm water can become trapped behind the seawall. This creates hydrostatic pressure thereby requiring the water to escape beneath the seawall 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. Another downside is that it 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 seawall 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 seawall is constructed.

 

8)      Life expectancy of your seawall.

 

9)      Aesthetics or the appearance of your seawall.

 

10)  Maintenance required.

 

11)  Cost.

 

 

 

To accurately plan your seawall, you need to understand that a retaining wall or seawall is a near vertical structure with varying degrees of loading or pressure being applied to one side (back or land side) of the seawall constantly trying to move the seawall forward or turn it over.  In the case of concrete or hard structure seawalls, these pressures are sustained by the use of a wide concrete footing and/or the use of tie backs or cables connecting the seawall structure to a stable area behind the seawall (referred to as “dead-men”).  When installed properly, these walls serve well as a permanent structure.

 

It is this author’s recommendation that any seawall structure, whether it be wood, concrete, etc., be backfilled with crushed rock to allow free drainage of water through the reinforced zone or fill zone behind the seawall.  Soils absorb water resulting in pressure exerted against the seawall.

 

           Where shoreline conditions allow, our preferred seawall 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. Prior to seawall construction, we encourage an examination and probing of the shoreline area to determine the soil conditions for load bearing capacity.  Typically you will find firm original ground which is ideal.  However, in some areas you may find deposits of soft, silty material or soft clays which must be removed from the wall construction area until you reach firm original ground then fill with compacted, crushed rock up to the wall foundation level.  In areas where soft gumbo clay is the prevailing soil material, wall foundation stability can usually be achieved by excavating the lake bed 2-3 feet deep by 4-6 feet in width (depending on wall height) and replacing with compacted, crushed rock and geogrid sandwiched in 6 inch layers.  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 as specified 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.

 

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 seawall for a moderate price.

 

            Over the past 10 years, we have performed a vast number of challenging seawall 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 many seawalls 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 seawalls are expected to be permanent with little or no maintenance.

 

If you have plans for a seawall 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 performed.  Check these references and decide for yourself if the contractor is right for you.

 

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 the  Alabama  area, the Regulatory Authority is the Alabama Power Company.

 

Should you attempt to build your own seawall?  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 seawall, 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 / mini-excavator 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.

 

            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 seawall 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

 

 

 

 

 

 

Check the level of your lake and plan the time for your seawall project.  Some bodies of water are regulated by the local power company.  Those companies should have a website that gives you the current lake levels as well as those of the past and predicted levels.  Alabama Power has just such a website: https://apcshorelines.com

The Next Generation of Modular Retaining Wall Systems