RETAIN/SEAWALL

O.  RETAIN/SEAWALL: REPAIR/REBUILD/REPLACE
Retaining walls function to retain the earth against them.  When the earth is dry, particularly with heavy clay content soils, the load against the walls is slight and the retaining walls do quite well (Fig. O1, Item A).  When the earth behind walls becomes saturated with water (Fig. O1, Item B), the load against the wall can triple.  If the soil behind the wall is wet when a severe freeze occurs, the soil may expand due to the freezing and push excessively against the retaining wall.  With time, overloaded walls will usually tilt and fail (Fig. O1, Item C).

Warning: Walls that tilt more than 1/3 of their thickness are considered to be in immanent danger of collapse.

Tilting and collapse of walls tends to take a long time.  Ultimately, when the center of gravity of the wall passes over the outside edge of the footing, the wall will usually collapse.  WATER IS THE MAIN THREAT TO RETAINING WALLS: Walls that crack can often be pointed up and repaired rather than replaced.  The key to permanent repairs is usually handling the water around the wall.  Water should either drain through, away from, or drain over the top of the wall.  All retaining walls should have weep holes to handle any water that gets behind them.  The wall should be backed with gravel fill to reduce capillary attraction of water to the back of the wall, and to permit water to migrate to the weep holes.

Seawalls are essentially the same as retaining walls. The function of the seawall (commonly called a bulkhead) is to prevent erosion of the shore.  It must be able to withstand continuous wetting and drying, and the force against the wall from the water.  Unlike retaining walls, seawalls should not have any openings since soil may seep through them.  Sinkholes behind seawalls generally mean that water action is eroding the soil there.  Modern seawalls are often built with pressure treated pilings and horizontal beams bolted to the pilings.  Tongue and groove pressure treated planking is then driven into the ground and secured to the cross supports with this type of system.  The area behind the wall is then backfilled, often with dredgings.  Usually the tops of the pilings are covered with a continuous pressure treated plank system to reduce the exposure of the end grains.  Seawalls suffer wear and tear from water and the atmosphere.  Wooden ones rot and splinter while concrete seawalls spall, crumble, and get undermined.  Many will lean with time.

WHAT TO:    HOW TO:
Most minor damage to retaining walls can be repaired.  Often this may involve reinforcing from behind and through the walls.  Seriously damaged retaining walls must generally be rebuilt.  When rebuilding is considered, consider how well the land around the seawall drains water.  If the wall acts as a dam, the earth behind the wall will saturate and exert considerable pressure against the wall.  This is the time to consider installing earthen swales in the land’s surface to channel water on the surface.

Catches or catch basins are waterproof holes in the ground into which surface runoff water is fed.  Catch basins are drained by a pipe.  Combinations of swales and catches will often alleviate surface runoff water problems behind retaining walls.  Naturally, pipe installation should accompany wall rebuilding.  SIMPLY REBUILDING A FAILED WALL IN THE SAME MANNER AS IT WAS BUILT IN THE FIRST PLACE WILL ALMOST GUARANTEE A REPEAT FAILURE.

Walls can often be reinforced with steel placed within the wall or behind it.  This type of reinforcing does not detract from the cosmetic appeal of the front facade of the wall.  Shaping the footings and rigidly attaching the walls to the footings will often have a great impact on the longevity of the wall.  It is always wise to install “weep” holes through the face of retaining walls and back the wall with gravel filled trenches that lead to the weep holes.  The function of the gravel and weep hole combination is to quickly drain off water accumulating behind a wall.  Retaining walls are usually expensive and should be constructed correctly.  Consult with an architect, a professional engineer, or an experienced building analyst if you have doubts about proper construction techniques.

Sunken areas behind seawalls generally require excavation and repair of the breach in the seawall.  Always check the primary and secondary supports of the seawall for stability.  Expect that older wooden seawalls may have to be completely rebuilt.  Quite often extensive rotting is found.  Usually any planking replacement must be accompanied with replacement of cross beams and pilings.  It is wise to consider using water contact pressure treated lumber and stainless steel fasteners.  Never use less than hot dipped galvanized fasteners.  Check with the authorities prior to making any repairs.  In some areas repairing or rebuilding of a man made seawall and other protective devices, may be prohibited by law in.  Always check with the local authorities.

REFERENCES: B4, EX39, EX42