Marine clay is a type of soil found in parts of Fairfax County and northern Virginia that has a high potential to swell upon wetting and shrink upon drying. It consists of clayey and silty soils that have poor drainage properties, low bearing strength and can cause slope instability/landslides. 

Marine clay is found in the Coastal Plain, which in Fairfax County is the general area located primarily east of Interstates 95 and 395, in the Mount Vernon, Lee and portions of Mason districts. Marine clays are labeled on the county soils map as either "Marumsco soil" or "Previously Mapped Marine Clay." For specific mapping information for your area, search the soils map by address using the County's Digital Map Viewer. The most recent soils information is found on the 2011 soils map. For further information contact the Northern Virginia Soil and Water Conservation District at 703-324-1460, TTY 711, or the Engineer of the Day at 703-324-1575, TTY 711.

Without adequate engineering design and construction of a foundation, several types of problems associated with these soils can exist or develop with time:

• Large lateral pressures can be exerted on basement walls due to poor drainage and swelling soil pushing against the wall, resulting in structural cracks and shifting. 
• Shallow marine clays beneath the foundation can shrink and swell, causing uneven settling and cracks in floors and walls. 
• Any change to the geometry and/or loading of the slope or large amounts of rain or snow can trigger down-slope movement of the ground that could eventually undermine the foundations of buildings, retaining walls and utility lines.

Not every crack in a house will be caused by marine clay. Weather patterns can provide clues to identify marine clay damage. For instance, foundations on clays may settle during extended dry periods, especially during the summer; wet periods may increase swelling pressures on basement walls; slope movement typically occurs during wet periods of the year. Cracks caused by marine clays tend to open and close with changing moisture conditions. Often, several houses will be affected in areas which are experiencing marine clay problems. Check to see if any of your neighbors are experiencing similar problems.

Most types of structural damage resulting from marine clay can be repaired. However, the extent and cost of the repair will vary, depending on the extent of damages and/or slope.

• Patching wall, ceiling or floor cracks caused by marine clay is only a cosmetic repair and is not a long-term solution to the problem since it does not address the source of the underlying problem. Damage and cracks may reappear, resulting in more extensive and costly repairs. If you patch up cracks and later sell your house without disclosing the damage, you may be sued by the purchaser.
• A qualified geotechnical engineer with experience in marine clays should be hired to properly investigate the root cause and extent of damage and to recommend possible solution(s).
• Depending on the type of repair being proposed, a grading plan and/or a building permit will be required before any repair work is started.
  • If the solution requires site work (land disturbance) greater than 2500 square feet, the geotechnical (soil) report must be submitted to SDID prior to the associated site plan being submitted pursuant to geotechnical-related regulations.
  • If the solution is mostly structural, a copy of the geotechnial report must be submitted with the building plans and permit application pursuant to requirements for building in a problem soil.
• The engineer who prepared the building plans should inspect the repair work and certify that the designs are properly implemented, per Soil-Related Design and Inspection Requirements.

Not every structure that is built on marine clay will experience problems. As a homeowner, you can take certain steps to minimize changes to the soil moisture that often trigger future marine clay related problems.

• Maintain a positive grade, or slope, away from the house. This encourages surface water to flow away from the house rather than to soak into the ground next to the house.
• Make sure that gutters are in good repair and divert all downspouts away from the house. 
• Do not leave outside water hoses running unattended or over-irrigate, which can add large quantities of water to the soil next to the house. 
• Keep all deeply-rooted trees and shrubs well away from the house. Roots growing around and underneath the foundation can take up large amounts of soil moisture during the growing season, dry out the clays, and lead to soil shrinkage and loss of foundation support.

• Northern Virginia Soil and Water Conservation District can help a homeowner determine the source, extent and potential solutions to damages that may be caused by marine clay, as well as provide a list of local geotechnical engineering firms. Contact them at 703-324-1460, TTY 711.
• Virginia Department of Professional Regulation can confirm if a geotechnical engineering firm is licensed in Virginia.
• Search the complaint history of a company.