Seawalls perform important functions to property owners and waterfront residents; albeit the most important function is to protect property from loss of land due to erosion and wave action. Seawalls also help maintain proper water depth in canal systems. A properly maintained seawall helps the permanence of neighboring properties by providing structural support hence enhancing value of properties.
SEAWALL VS. SEAWALL CAP
A seawall is a structure made of concrete, rock, metal, or plastic panels, which separates water from adjoining land guarding waterfront property from erosion. In yesteryears, seawalls were built from coral rock. These walls, made from once-living organisms, were made to breath. Once sealed however, the coral rock seawall is much less porous and requires weep holes to be installed to allow water flow in and out, which reduces hydro-static pressure. Coral rock seawalls, while naturally beautiful, require quite a bit of maintenance. These seawalls are made from coral rocks stacked in a pyramid that move naturally due to vibration and other natural forces. Cracks that form must be mudded and maintained to conserve soil. Today’s seawalls are composed of distinct portions: a series of interlocked panels, vertically from the land elevation to below the water floor, a “concrete cap” which ties the panels together and tie rods which anchor the vertical structure to an upright position and prevent it from falling into the water. The seawall is provided with weep holes to allow water collecting behind the panels to drain and relieve pressure on the structure. The ends of the tie rods are secured to concrete blocks or aluminum panels called dead-men.
WHERE SEAWALLS FAIL
Seawall construction methods have improved throughout the years. The original seawalls were made from rock piled in a pyramid shape. Afterwards, concrete panels were used and constructed with unprotected metal rebar. Today, seawall panels are constructed within protected rebar and high strength concrete to extend life span. The failure of seawalls is generally classified into any of the following four categories: Joint Separation; Tie-Back or Seawall Cap Failure; Toe and Berm Failure; and Breakage at the Water Line.
WHERE JOINTS SEPARATE
Reasons: Age, settling, structural failure or insufficient berm at the slab toe line. Joint separation is Reasond when slabs or panels move apart vertically, allowing soil to migrate through separations into the water body. Differential hydrostatic pressure is exerted on the panels or slabs, as tide migrates from high to low. Differential hydrostatic pressure is exacerbated by heavy rain and low tide conditions. Indications: Holes behind the wall, visible seawall backfill in the water on the canal side seawall joints (most visible at low tide). Solutions: Repairs may be as simple as patching the joint with hydraulic cement or more detailed work involving wall replacement and major excavation.
TIE-BACK VS. SEAWALL CAP FAILURE
Reason: Corrosion in the cap reinforcing or tieback rods. This condition is often aggravated by movement of the structure, resulting in cracking or crumbling of the concrete cap and its ability to keep the panels or slabs aligned; often manifested by slab tilting toward the water. Indications: A deteriorating cap and wavy or sagging panels. Often these indications occur together. Solutions: Replacement of the seawall cap with a new concrete cap. Panels or slabs may have to be replaced. If failure is due to corroded tieback rods, excavation further into the property is necessary for replacement of the tie-backs.
FAILURE AT TOE AND BERM
Reason: Deficiency of berm at the bottom of the slabs or panels in the water. The panels tilt out, and sometimes crack or Reason the cap to rotate or fracture. Loss of berm is usually associated with canal dredging, wave action, or fast currents. Indications: Cap rotation, movement or cracking, a gap opening between seawall and dock (if present) and support pilings (if present) tight against the seawall meaning pressure on the structure from the failure. A good way to determine berm loss is to measure the height of the wall from the cap to the berm. Originally, panels or slabs may have less than two or three feet of berm holding them in place and may be the reason for existing toe-out or future toe-out. Solutions: Placement of additional rip-rap to stabilize the bottom of the structure if the toe- out is not too severe. In bad cases, the panels may be pulled and replaced. Repairs to the cap will depend on the amount of damage.
Reason: Aging concrete, corrosion of reinforcing rods, and uneven hydrostatic pressure. Slabs or panels develop horizontal cracks usually along the waterline and the panels eventually break along these lines. Indications: The principal symptom is the cracks along the top of edge of the water line. Solutions: The remedy for an advanced failure will usually mean new panels, cap tie-back rods and deadmen; or a complete new seawall. Seawalls with minor cracks should be monitored for progression. Attention will need to be given to any existing voids or holes landward of wall panels to reduce uneven hydrostatic pressure from water behind these walls.
REPAIR AND REPLACEMENT MEASURES
Inspections: It is essential that owners and their representatives continually monitor seawalls and or inspect them to correct defects before developing into major issues. Owners should be on vigil and take notice of sinkholes or newly formed cracks. Voids: Sinkholes or voids should be filled regularly. Caps: Caps can be placed into two categories. They can be repaired or replaced depending on the severity. If the cap can be repaired by patching, this can be performed as part of a regular maintenance routine. Seawall replacement work is performed by licensed contractors under the direction of licensed engineers through properly inspected and permitted procedures. Walls: A seawall is deemed for replacement if it has already fallen over or broken. On inspections, if a wall has severe cracks along the water line, it is suggested the wall panels be replaced before failing. It should be pointed out a wall or cap will be replaced for the portion to be considered bad which may or may not extend the entire width of the property. Good engineering judgment should be employed in conjunction with budget constraints issues in determining the extent of serviceable life when considering the replacement along the remainder of wall or bulkhead.
About the Author: Mark Weber, PE, and MW Engineering Inc., specialize in Marine Engineering, Inspection, and Consulting. Our General Engineering Services Include: Specialty Foundation Design and Inspection, Underpinning, Repair and Mitigation; Seawall Inspection, Design and Permit Plans; and Dock Inspection, Design and Permit Plans. Please visit www.MwEngineering.net for more.