Look here http://www.fairfaxcounty.gov/gis/DMV/soln/2008/82-4
that whole area is built on clay (Marumsco soil). This had to have been disclosed or discussed in the engineer's report.
What is marine clay?
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.
Where is marine clay found?
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.
How can marine clay affect my property?
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.
How can I be sure my problem is due to marine clay?
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.