From corrosion to site drainage, coastal regions present a host of unique and challenging issues for homebuilders and residential property owners alike. In this article, issues specific to coastal residential foundation installation and in-service function will be addressed. “Coastal residential” will refer to single-family homes within AE or VE tidal flood zones for the purposes herein. These locations typically exist in areas with close proximity to saltmarsh or beachfront, or on barrier sea islands. Per FEMA, in AE zones, the elevation of the first finished floor must be above the flood elevation for the building site. In VE zones, the elevation of the lowest horizontal structural member must be above the flood elevation for the site. Most building jurisdictions also require a minimum of (1) foot of freeboard, or additional space, between the flood elevation and these building features. As a result of these requirements, coastal residential structures must be supported by either crawl-space or elevated “drive-under” style foundations. It should be noted that wall-type foundations are prohibited in VE zones, and structures within these zones are required to be supported by monolithic elements such as columns, piers, or piles. Slab-on-grade type foundations are not permitted in flood zones where the foundation will exist within the flood plain.
There are two basic building foundation base structure types: shallow and deep. In coastal residential construction, shallow foundation bases consist of reinforced concrete continuous spread footings or isolated pad footings, or a combination of both. Deep foundations typically consist of driven timber piles. Elevated foundations may be of either type. They may have cast-in-place concrete or concrete masonry elements supported by the shallow footings, or deep driven pile supports. A subsurface geotechnical exploration of the building site will dictate which type of foundation base is mandated. If the soil type and allowable bearing pressure is sufficient for the application, shallow foundations are generally preferred due to decreased cost and installation cycle time. However, oftentimes deep foundations are required due to loamy, sandy coastal soils, or due to design considerations such as erosion and scour caused by wave action. Such is the case with most beachfront sites. In either case, each foundation type presents its own set of challenging installation and in-service issues, many of which are identified herein.
One of the most prominent issues with shallow foundations near the coast occurs during the installation phase. Due to the close proximity of the groundwater table with respect to grade at most coastal sites, in many cases the excavations for the footings require dewatering to allow placement of concrete. Dewatering is typically achieved by installing French drains and/or using sump pumps.
However, due to water table fluctuations caused by tides and weather, it can be difficult to keep the excavations dry for a long enough period of time to cast fresh concrete. Even after thorough and successful dewatering, oftentimes a layer of gravel must be placed in the bottoms of the excavations to address the somewhat muddy surface which remains.
Another issue with shallow foundations occurs in-service and involves differential or non-uniform settlement of the footings caused by the soft soils found at most coastal residential sites. While some settlement is expected, it is typically uniform, and poses no issues. Differential and excessive foundation settlement can be an expensive problem to address, however. Prior to the commencement of any foundation excavation activities, it is important to thoroughly compact the soil and any imported structural fill using proof-rolling equipment as well as testing the soil density to ensure that it is within design specifications. Performing this step properly prior to beginning any excavation activities can greatly alleviate the potential for future settlement problems.
With respect to deep driven timber pile foundations, one installation issue is the cost of simply mobilizing the pile driving equipment to the jobsite, which can be quite expensive, depending on availability and demand. The cost of driving the individual piles, and demobilizing the equipment when installation is complete, can also be substantial.
An additional installation issue with piles can potentially be inducing vibration of any surrounding structures during driving, which is a nuisance at best, but can be significant enough to cause damage. Therefore, vibration monitoring of adjacent structures close to the driving operation is a prudent consideration. Pile driver hammer energy may need to be reduced if the possibility of collateral damage exists.
A prevalent in-service issue with timber pile foundations is the necessity of X-bracing. X-bracing between adjacent piles typically takes the form of two treated approx. 2×10’s crisscrossed, mounted to one side of adjacent piles, and through bolted to the tops and bottoms of the piles. Another type consists of two approx. (1) inch diameter galvanized threaded steel rods with turnbuckles crisscrossed and through-bolted to the tops and bottoms of the adjacent piles at the pile centerlines. X-bracing is necessary to minimize wind-induced vibration or oscillation of the piles which can adversely affect the comfort of the inhabitants and the stability of the structure. Such undesirable pile movement can also be caused by the motions of persons inside the dwelling, or when a washing machine is on the spin cycle, for example. On the other hand, X-bracing poses a dilemma structurally, due the fact that during a flood event, the bracing can hang flood-borne debris, and increase the hydrodynamic drag on the entire foundation. So, while X-bracing is necessary for occupancy, it can be a burden structurally. Therefore, strategic placement of X-bracing, and minimization of the overall quantity of braces, are important considerations.
An additional installation issue which can be common for any elevated type foundation, either shallow or deeply supported, is maximum ridge and/or eave height restrictions. Depending on how high the structure must be elevated to get the required elements above the flood zone, including the required freeboard, it can sometimes be difficult to fit the desired amount of living space, combined with the desired roof pitch, within the maximum ridge and/or eave heights allowed by the building jurisdiction. Architectural creativity and flexibility are necessary to hopefully minimize the sacrifices that this hurdle may require.
As is apparent, many issues arise with coastal residential foundations during both the installation and in-service phases. Whether soil conditions mandate a deep foundation, or a shallow foundation is suitable, it is typical that any or all of the challenges mentioned herein must be overcome. The key is mitigating these obstacles to ultimately ensure a structurally sound foundation supporting the dwelling, while maintaining cost effectiveness and livability.
George Sanford, PE, holds a Bachelor of Science in Mechanical Engineering from North Carolina State University in Raleigh, North Carolina. George has more than 20 years of applied structural engineering experience specializing in residential, commercial, and industrial structures and foundations. Throughout his career, George has designed and analyzed structures, supervised engineers, prepared construction documents (drawings and specifications). He has an in-depth knowledge of many building codes, standards, rules, and regulations including the agencies that govern and provide guidance to building designers such as the International Code Council (ICC) American Society of Civil Engineers (ASCI), Steel Joist Institute (SJI) and the American Iron and Steel Institute (AISI).