DO I NEED GEOTECHNICAL SOIL TESTING FOR MY OUTER BANKS HOMESITE?
I worked for an international engineering and consulting firm for 19 years and we rarely touched a job without first hiring a Geotechnical (soils) engineer to perform thorough geotechnical soil testing which most often included soil borings.
What is a Geo-Technical Engineer? A Geotechnical engineering is the branch of engineering concerned with the analysis, design and construction of foundations, slopes, retaining structures, embankments, tunnels, levees, wharves, landfills and other systems that are made of or are supported by soil or rock.
What is a Soil Boring? A soil boring is used to collect a soil sample for a geotechnical engineer to analyze. It typically consists of drilling exploratory holes in the earth from which a core sample can be taken. The core samples, as shown in the picture below, provide a visual analysis of the soil layers present beneath the grounds surface. These soil borings typically vary in number and depth depending on the type of project, structure and local geology. Based on the results of these borings, and other soil analysis, the geotechnical engineer would provide a report to the structural engineers. The reports often include recommendations like the soil’s expected bearing capacity (weight it could safely support), groundwater level, different types of soil strata at varying depths, likely amount of expected short term and long term settlement, soil behavior during an earthquake, etc.
The Structural Engineer. The structural engineer would then collaborate with the geotech to select the best foundation type such as a shallow slab on grade mat/raft foundation, continuous strip footing or a deeper piling, pier or caisson foundation. Oftentimes, the top portion of the soil would need to be removed (typically because it contained higher quantities of organic matter) and replaced with a more rigorous media (rock/gravel) capable of supporting larger loads with less settlement.
While geotechnical engineers are oftentimes not utilized for residential foundation soil analysis and recommendations, it’s never a bad idea to employ one. A geotechnical engineer can provide valuable information for a relatively low fee and save you lots of potential heartache and unnecessary expenses in the future.
Should I hire a Geo-technical Engineer for my Outer Banks property? Many areas along the Outer Banks have layers of peat or other organic matter hidden beneath the sand at varying depths and in varying thicknesses. Quantities of this organic matter vary by location. As the weight of a structure presses down on these organic strata over time, varying degrees of settlement are likely to occur. The settlement of these layers beneath the earth can wreak havoc on a homes foundation and in turn the home built upon the foundation. In addition, building on fill or piles partially embedded in fill or a peat layer can have the same deleterious effects. While not required, it is a good idea to have a geotechnical soil testing done on your property prior to new construction.
Take care when developing your foundation plan. Care should be taken to make sure that any fill placed on the lot is not considered part of the pile embedment depth. New or recently-placed fill is subject to increased settlement. Pile size is also an important factor. Eight inch diameter round pilings are common in the Outer Banks but vary in size, especially the tips. Eight inch pile tips should be close to eight inches. Anything less and the bearing capacity of the pile will be reduced.
–blog by Barrett Crook – Kitty Hawk Engineering. http://www.kittyhawkengineering.com