Coastal Systems International, Inc. - Publications
To Repair or Replace a Bulkhead: That is the Question
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| New steel sheet pile bulkhead under construction,
Grove Harbor Marina (Dinner Key Boatyard Marina) Miami,
Florida |
The most common form of shoreline stabilization at marinas is
a bulkhead. The bulkhead provides an economical shoreline interface
between the upland marina area and wet slips. For some marinas,
the bulkhead also serves as a source of revenue, whether it
be parallel docking or perpendicular mooring. Marina management
is faced with the maintenance of marine structures, and a proper
assessment of the bulkhead with estimated remaining service
life is essential for budgeting repairs and/or replacement.
In addition, there are safety concerns relative to loads behind
the bulkhead such as forklifts, vehicles, and marina customers.
Assessing the condition of bulkheads on a regular basis can
extend the service life of a structure. Performing preventative
maintenance or minor repairs as a result of a bulkhead condition
study can be completed before they become major concerns. Often
times, bulkhead problems do not "surface" until it
is too late. Sinkholes or bulkhead failure can a have a substantial
economic impact on marina operations. Underwater bulkhead deterioration
or other problems such as undermining due to propeller scour
are not visible above water and cannot be monitored on a regular
basis.
This article will explain the difference between a seawall and
bulkhead, introduce elements of bulkhead design, present bulkhead
materials of construction, and discuss bulkhead evaluation methodology.
Seawall vs. bulkhead?
Many people refer to all vertical shoreline structures as
“seawalls,” but there is a difference between
a seawall and a bulkhead.
Seawall: structure that provides
shoreline protection from waves but also retains soil.
Bulkhead: vertical shoreline
stabilization structure that primarily retains soil, and provides
minimal protection from waves.
Seawalls are typically located on the coast fronting beaches,
and are subject to storm surges with pounding surf, eroding
shorelines and wave overtopping from coastal storm events.
Some localized waterfront properties may be subject to significant
wave activity, even though they are not exposed to ocean waves.
The “rule of thumb” in bulkhead design is to account
for wave impacts if the significant wave height at a project
site is expected to be in excess of three feet (1 meter).
Elements of bulkhead design
The following design considerations need to be addressed to
properly assess the condition of a bulkhead.
- Topography: elevations, grading,
etc.
- Embedment/Stability: depth of bulkhead
for stability
- Exposure: climate and saltwater vs.
freshwater
- Surcharge: live loads behind the
wall such as vehicles
- Prior Maintenance
- Soil Properties: unit weight of soil,
clay vs. sand, etc.
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- Water Table: differential water levels
behind and in front of walls can introduce additional
loading on the wall
- Material Properties: strength and
performance in the marine environment
- Ice Loads: in northern climates
- Changes in use
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Original or “as-built” plans can provide a wealth
of information including the age of the structure and many
of the design elements listed in the above paragraphs. The
deteriorated condition of a bulkhead be an indication that
the bulkhead is either in need of maintenance, or that it
has fulfilled its service life. In some cases, the bulkhead
may be damaged due to loads in excess of loads it was designed
to withstand.
Materials of construction
The material of the bulkhead must be properly identified during
the assessing. The condition. The following table presents
common wall construction materials with comments regarding
availability, construction issues, and general performance
in the marine environment:
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| Replacement steel sheet pile bulkhead under
construction, Bahia Mar Yachting Center, Ft. Lauderdale
, Florida |
Concrete
Pile/panel and sheet piling configurations provide a service
life of 30+ years if correct mix design and proper marine
structural design implemented. Sheet piling can be difficult
to install in hard substrates, and concrete pile/panel walls
can be subject to undermining.
Steel
Steel sheet piling commonly used for bulkheads. The material
provides excellent strength characteristics for high wall
exposure applications. has an interlocking seal, and is generally
easy to install, even in harder substrate. It must be properly
coated and maintained for long service life of 25+ years.
Aluminum
Sheet piling provides good corrosion resistance, but lighter
sections allow for minimal exposed wall height. It is important
to recognize corrosion potential of dissimilar metal hardware,
do not use in waters with low backfill with clay-mucky soils.
It is difficult to install in hard substrates.
Timber
A timber pile/wale/sheet system is a common structural configuration.
It is generally economical material, but has limited strength
characteristics for high wall heights. Preservative treatment
is essential for marine organisms. Service life is generally
less than 25 years. It is difficult to install in hard substrates.
Vinyl/F.R.P.
This is a relatively new, economical product with service
life of 50+ years (based on manufacturer warranties) It’s
available in different colors. Strength characteristics are
limited for high wall heights. It is difficult to install
in hard substrates.
Structural evaluation
The American Society of Civil Engineers (ASCE) Underwater
Investigations Standard Practice Manual was released in 2001
and provides guidance for the evaluation of marine structures
including bulkhead.
The following topics are covered related to structural bulkhead
evaluation:
- Qualifications of inspection personnel
- Typical forms of deterioration
- Condition Rating
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- Types and methods of inspections
- Frequency of Inspection
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Marina management should retain the services of a consulting
engineering firm that provides above/below water inspection
services.
A comprehensive report is essential to document a proper bulkhead
evaluation. The report should reference procedures and guidelines
from the associate manual. All of the above items should be
included along with photographs and sketches of the observed
configuration with notes regarding deterioration. Comparison
of previous reports provides an indication of the rate of deterioration.
Repair recommendations, along with construction cost estimates,
should be included to provide marina management with sound engineering
advice so they can plan of maintenance/repairs as necessary.
Even though a bulkhead is deteriorated, it may not require immediate
replacement. The report should be sealed by a registered professional
engineer experienced in the evaluation of in-service marine
structures. The engineer is required by law to provide independent
un-biased advice.
FAQ regarding bulkheads
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| Two examples of typical concrete
bulkhead: Eleuthera, Bahamas |
Can I raise the grade of my property with
the existing bulkhead?
The bulkhead must be evaluated by an engineer to determine
if the structure can withstand the additional loads from fill
and structural
modifications. Deterioration can severely weaken the structural
capacity of the bulkhead, and the bulkhead was most likely
designed for the existing conditions.
What is the best material for bulkheads?
Material selection is site-specific and dependents on design
conditions. Concrete (if designed appropriately) generally
provides a long service life, but it is not favorable from
a first-cost basis. Vinyl sheet piling and other composite
materials, where applicable, should be considered due to their
resistance to the harsh marine environment.
How long can I expect my wall to last?
Answers to this question are generally subjective. An experienced
marine structural engineer can also provide an indication
of material performance. Certain non-destructive and/or partially
destructive materials testing can provide additional information
for the assessment.
What regulatory permits are required?
Bulkheads typically require an environmental resource permit
from several agencies including local and state environmental
resource management agencies and U.S. Army Corps of Engineers.
The contractor must secure a permit from the building department.
How often should I inspect the wall?
The ASCE Manual provides guidance for the frequency of inspections.
Generally, seawalls and bulkheads should be evaluated every
5 to 6 years.
How are bulkheads repaired?
A qualified marine structural engineer should be consulted
to evaluate the existing wall and to determine if rehabilitation
or replacement is required. Options can generally be provided
to provide an economical approach to meet budget constraints.
Who can build a bulkhead?
Bulkheads are specialized structures, often requiring water-based
construction equipment and techniques. The costs for waterfront
construction are generally higher than for upland structures
such as buildings. Bulkhead or seawall work should be reserved
for a qualified and experienced marine contractor.
How much does Bulkhead construction cost?
Depending on the local construction market and bulkhead design
requirements, repairs may cost $100-$400 per linear foot of
wall. Bulkhead replacement may cost $500-$1,000 per linear
foot.
Conclusions
1. Bulkhead design is site-specific and the design elements
of a particular structure should be understood prior to evaluation.
2. Bulkhead materials of construction exhibit various forms
of deterioration in the marine environment. Proper material
identification is essential to assess structural performance.
3. ASCE recently released a standard practice manual to provide
guidance for the above/
below water assessment of marine structures.
4. Bulkhead evaluations should be conducted by qualified personnel
under the supervision of a licence professional engineer.
T.K. Blankenship P.E.-Department Head, CSI
Marina Dock Age-April 2004
Tim Blankenship is the Engineering Department Head at Coastal
Systems International, Inc., located in Coral Gables, Florida.
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