FAQ Frequently Asked Questions

DESIGN STANDARDS

What every owner should know about Seawalls

 

Florida is a unique environment in its own right, but this is especially true for coastal, bay, and riverfront areas. It is relatively low in elevation and has little vertical relief in topography. We are subject to very heavy rains, both in normal summer thunderstorms and tropical events. Our soils are mostly sand, often underlain by rock or layers of very weak muck, the presence of both of which have significant impact on design.

Additionally, Florida is a very salty environment. Salt is present in spray, oceans and bays, in the air, and even in freshwater. The presence of salt leads to the corrosion of metals and the deterioration of other building materials such as concrete. Furthermore, in ocean front environments, waves batter the coastline during storm events, generating high dynamic loads.

We consider each of these items in our designs. We are shocked to see numerous designs that could not be put on paper without misunderstanding these conditions. Your wall must perform in the MOST SEVERE conditions it will experience in its service life, often multiple times over. We regularly see walls designed for NOMINAL (average, everyday) conditions, which eventually leads to failure as the wall was not designed for maximum loads.

A seawall must support the loads of both the earth and the water that is behind the wall, similar in concept to a dam. Normally, these loads are resisted by soil pressures on the outside of the wall and typically a tieback that anchors the wall to the rear or a brace support on the front. The wall itself must be stiff enough to resist bending outward, which is where material selection is key.

Here is a diagram of the basic forces on a seawall. Working towards the sea are water pressure from groundwater and soil pressure. Resisting these are toe soil pressure, seawater pressure and the tieback or braced (not shown) resisting forces. So long as the landside pressures don’t exceed the available resistive forces available, everything is just fine!

Now lets look at things that can go wrong and things are ‘not fine’

 

RAINS

During the service life of your wall, your property will experience rains from a hurricane or summer thunderstorm several times. This causes the soil behind the wall to become saturated and the water level to rise to the top of the wall. This results in a massive increase in loading on the wall due to the increase in water pressure, known as the maximum ‘hydrostatic’ loading. 

We can’t understand why so many designs ignore this simple yet crucial load case. Normal water tables in coastal areas are 1-4 ft below ground. A 4’ high dry column of soil can accommodate about 10” of rain (SFWMD Vol 4 Design Aids). In most South Florida, a regular large rain event can easily drop 12”-20” of rain. The soils become saturated. Suddenly, the groundwater pressure becomes very high, often doubling the water pressure impact from nominal conditions. This condition is often further exacerbated by passing low pressure offshore, LOWERING the water on the seaward side of the wall, decreasing the resistive force.

This sudden loading, coupled with the loss of resistance to the loading, is why most seawall failures occur during these events. There are several modes of failure caused by these conditions, including tieback failure (top of the wall leans out), toe failure (bottom of the wall kicks out), or panel failure (panels get ‘belly’ pressure and either warp or crack).

LESSON: If you don’t select us to design your wall, make sure your engineer designs your wall to maximum hydrostatic pressure loading. Otherwise, the wall may experience an overload or potential failure years down the road.

POTENTIAL COST: Severe wall deformation (bellies and bowing), failure, and loss of land, resulting in unnecessary costs for repairs and replacement that otherwise could have been avoided.

SOILS

Typically, soils in coastal Florida are fine sands. However, some parts are solid rock basements of sandstone, coquina, and limestone. There are also gravels, clays, and muck present in some areas. Soils can also vary significantly with depth, as often there are different layers of different soil types present depending on where you are in the soil column. Knowledge of the soil parameters is quite important to understand both the loads induced on, and that support a wall. The primary method to find these parameters is through a comprehensive in-situ soil boring study provided by a qualified geotechnical engineer. The effects of different soil materials are as follows:

Rock or Gravelly Layers – These provide excellent support and lower loads on a wall. However, they are not easily penetrated by vinyl or composites; therefore, other materials or construction techniques must be used. Finding rock where it is not anticipated can be pretty costly to the construction process. Soil borings can also often miss chunks of rock, which affects construction.

Dense Sands – Dense sands also have lower loads and more excellent resistance, which are great for seawall construction. Sands are also non-cohesive materials, avoiding the complications (but also the benefits) associated with cohesive materials such as clay or silty clay. However, very dense sands can be difficult to penetrate with all wall materials and special construction methods must be considered in this case.

Loose Sands and Moderate Sands – These are the most typical soils encountered, and are also non-cohesive. They provide less support than dense sands and sometimes higher loads. Usually these indicate the need for stronger and longer panels.

Mucks and Organic Soils – These are usually old coastal wetlands or mangrove forests. The soils produce high loads and offer very little support. If they are in the soil profile, typically they are found between 4 and 12 ft below land surface and can be one to several feet thick. Finding these soils means care must be taken to assure the lack of support or increased loading are accounted for in the
design.

Clays – These are not frequently encountered on the coast but become a little more common as one moves inland. Working with clay layers is complex but their presence is great because their cohesion provides excellent support and they provide relatively low loads. However, the complexity lies in that care must be taken to understand if the clays swell when saturated and shrink on drying, which can
greatly affect the loading on the wall.

Lack of understanding of the soil layers can result in under design, which provides a threat of wall failure or severe deformations. It can also result in overdesign, which means an owner is buying much more wall strength than needed. We have seen walls installed that cost over $150,000 where a $90,000 wall would have been more than sufficient.

LESSON: If you don’t select us to design your wall, make sure your engineer has the adequate studies conducted to understand the soil parameters of your property and how to adjust the design to the given parameters. Otherwise, you could end up with a wall that is either inadequate or over designed.

POTENTIAL COST: Repairs or replacement due to under design, paying way more than you should due to over design. 

WAVE IMPACT (Oceanfront Environments)

One of the most often overlooked design parameters is that of wave impact loading on the wall in oceanfront areas. This consideration is often not made because many of the engineers that design walls are structural engineers rather than coastal engineers, and thus lack the understanding of this concept. Wave impact loading occurs during major storm events. As a major storm approaches prior to landfall, much larger waves than normal are generated off the coast and begin to set up. These fetch and duration limited waves are exacerbated by storm surge as the storm continues its approach. As the water levels rise due to surge conditions and waves set up quickly due to the energy provided by the storm, they begin breaking far further upland than normal with much more energy than normal.

This results in massive waves breaking directly onto your seawall, which is the equivalent of a semi truck driving over the face of the wall each time a wave breaks onto it. We have seen several walls that have collapsed as a result of this phenomenon, as waves will break through a portion of the wall, allowing floodwaters to pour in and overload the backside of the wall, causing it to collapse forward onto the beach.

LESSON: If you don’t select us to design your wall and you’re on the oceanfront, make sure your engineer understands wave impact loading and how to compensate for it, as well as which materials are optimum for these conditions.

POTENTIAL COST: Catastrophic failure during a major storm event, replacement of your seawall.

Before you search for a list of contractors, contact us!

 

Our value to you is to be your RELENTLESSLY PROFESSIONAL guide through the whole process.  We maintain and continually update a network of quality contractors who specialize in solutions, products, and applications.  When you speak with a Terragone Engineer, he or she will match you with a contractor in your area who is trained specifically for your project. We’re ready to get started. Are you?

Questions to Ask A Contractor

How long has my contractor been in business?
How long has my contractor been installing seawalls in my area?
Who will obtain state, local, and county permits?
When can the contractor start my job? When will it be finished?
Does the contractor have insurance?
Does the contractor have references I can contact?

 

What You Need to Know to Build a Seawall Correctly

What sheet piling materials/brands does my contractor recommend?
How many years has the manufacturer been making sheet piling?
Has the US Army Corps of Engineers recommended the manufacturer’s sheet piling?
Will the sheet piling for my seawall be a Co-Extruded material?
How many linear feet is my seawall?
How tall will the seawall be from the firm mudline, and how much of the wall will be embedded?
What materials do my contractor recommend for the anchor rods, wale, and cap?
What type of backfill will be used?
Can I see design options, plans, and samples of materials?

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