Pile Design for Coastal Forces
©RACE Coastal Engineering
By Matthew Rakowski, Project Manager, RACE Coastal Engineering
RACE Coastal Engineering (RACE) provides specialty consultation and design for waterfront structures and shorelines. As coastlines and the marine environment are constantly changing, we must maintain and integrate state-of-the-art methods to incorporate coastal, structural and geotechnical engineering into our projects in order to design site appropriate, resilient solutions.
Most coastal infrastructure, such as buildings, piers, and wharfs, include deep foundation elements which require a number of factors be taken into account for proper design. Local zoning regulations and state building codes dictate specific standards that need to be considered as well as design methods that need to be met for the design of structures. In the coastal environment, standards are prescribed in ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) and ASCE 24 (Flood Resistant Design and Construction). Designs are also guided by the Federal Emergency Management Agency (FEMA), National Flood Insurance Program (NFIP) and the NFIP’s technical bulletins.
Along the coast, there are flood zones that need to be accounted for which include specific elevation requirements based on the Base Flood Elevations (BFE) published by FEMA. The type of flood zone and proposed use determines methods that can be used for designing the structure and its foundation. Along any waterfront site, it is important that property owners, designers and contractors are aware of what flood zone they are working in as each zone requires different design considerations for the structure type and/or foundations.
As most of RACE’s projects are in flood zones, RACE assesses the project with respect to the requirements of the NFIP, State Building Codes, and local zoning regulations. Based on this assessment, RACE develops the loading on the structure and its associated foundation. These loads include wind, wave, flooding, ice and impact forces. RACE develops these loads based on site specific conditions, FEMA Flood Insurance Rate Maps (FIRM) and regulatory requirements.
The zones are assigned based upon the height of wave impact and include base flood elevations which include stillwater levels, storm surge, and wave heights associated with the “100-yr” storm event. The 100-year storm event equates to a storm that has a 1% chance of occurring within a given year. Therefore, structures within these zones are designed to withstand the “100-yr” storm risk.
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Source FEMA Coastal Construction Manual
The Velocity Zone, or “VE” zone, is typically directly adjacent to the waterward edge and extends out into the body of water. In these areas you can expect wave heights greater than 3’ and higher flood water velocities. These zones typically require deep pile foundations and structures to be elevated such that they are above the specified base flood elevation for the site.
Other zones that typically require special designs are the Coastal AE Zone, which is the next zone landward from the VE zone, followed by the AE zone. Coastal AE Zones can expect to see wave heights between 3’ and 1.5’. Many areas impose the same construction requirements in the Coastal AE Zone as they do in the VE Zone.
The AE Zone can expect to see waves less than 1.5’ in height. The construction requirements in this area are less than the VE and Coastal AE Zone requirements, but structures in this area still require special consideration.
RACE’s Coastal and Structural Engineers utilize coastal modeling programs to design coastal structures within VE and AE zones. The modeling is first used to determine site specific wave conditions which are based on: site grades, water depths, wind speed, wind duration, wind direction, and site exposure. With the wave modeling and empirical equation tools, physical impacts of waves, storms, tides, currents and sediment transport at specific sites are then evaluated for the lateral and vertical loading on the structure foundation.
Site specific numerical wave modeling determines parameters and load conditions on the structure. ©RACE Coastal Engineering
Combining the sediment transport modeling with the site’s material grain size, potential local and global scour around the foundation or structure is able to be determined. Scour is the loosening and removal of granular bed material by hydrodynamic forces in the vicinity of coastal structures. This natural process can result in a significant height change of a structure or foundation as well as reduce the embedment depths.
The previously at grade bulkhead had significant scour along the shoreline which resulted in rotation of the bulkhead. ©RACE Coastal Engineering
Elements that are specifically vulnerable to scour impacts are deep foundation elements, such as piles, cassions, sheet piles, etc. This grade reduction can impact the overall lateral and vertical stability of the deep foundation element and can have significant impacts on the structure that they are supporting.
RACE uses structural modeling tools to assist with determining pile designs. Typical structural loads (dead, live, seismic, wind, snow, etc.) in addition to flood and wave loads are input into structural modeling programs. In conjunction with site specific geotechnical properties, building code load combinations, and safety factors, a pile design is developed. Pile material (timber, steel, concrete, etc.) is selected based on the pile stresses calculated. Additionally, pile embedment is determined. In some cases, pile embedment is not able to be met due to underlaying soils and, therefore, special anchoring methods, such as tension anchors, are required.
When bedrock prevents ultimate pile embedment, tension anchors prevent pile uplift. ©RACE Coastal Engineering
The ever-changing coastal environment and site conditions create a difficult environment not only to design in but also to construct in. The various codes, standards and guidelines that need to be taken into consideration tend to create deep foundation elements that need to be driven to significant embedments in order to provide proper resistance to the applied loads within coastal zones. Depending upon the location, wave conditions can add significant loads to the structure and its foundation as well as scour impacts can affect the final design of the foundation elements. It is important that these conditions are accounted for in order to provide sustainable and storm resilient structures in these zones. One size does not fit all when working in the coastal environment.
About the Author: Matthew Rakowski, P.E., a Civil Engineering graduate of the University of Rhode Island and Advanced Coastal Engineering Degree Certificate from Old Dominion University, has 15 years experience analyzing and designing waterfront structures, foundations and buildings under a broad range of load conditions. Mr. Rakowski recently managed the design and construction of a 700 ton pile supported travel lift rail system in Bridgeport, CT, for which the project was awarded the 2022 Achievement in Civil Engineering (ACE) Award – Coastal Structural Engineering.