FAQs: 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. They consist of clayey and silty soils that
have poor drainage properties and low slope strengths. Unit 118 of
the 1990 Official County
Soils Map shows their approximate location.
Marine clay occurs in the Coastal Plain which, in Fairfax County is the
general area located primarily east of Interstates 95 and 395, in the Mt.
Vernon, Lee and parts of Mason tax districts. For specific information on
the location of marine clay in your area, look up the online soil
mapping based on the tax-map for the area, or contact the Northern Virginia Soil and
Water Conservation District at 703-324-1460, TTY 711
or the Site Development and Inspection Division at
703-324-1720, TTY 711.
Without adequate investigation, engineering design, review of engineering investigation and construction plan, and proper construction and inspection, several types of problems associated with these soils can exist or develop with time. When marine clays exists within wall backfills, large lateral pressures can be exerted on basements walls due to poor drainage and swelling and result in generally-horizontal structural crack/s and damage to the basement walls. When light structures (such as houses) are supported on shallow foundations bearing on shrink-swell susceptible soils (such as marine clays), the foundation walls can experience generally-vertical structural cracks and damage from differential upward and downward movements from swelling and shrinking, respectively. When shrink-swell susceptible soils exist immediately below ground-supported floor-slabs and pavements, heaving and cracking of the slab can occur. Slopes made of and/or underlain by marine clays can experience land slippage and progressive failure due to creep along pre-historic failure planes and/or new strains triggered as a result of steepened gradient of the slope (marine clay slopes typically need to be 5H:1V or flatter), new excavations near the toe of slope, new building and/or filling on top of the slope, large precipitation events, etc.; buildings, retaining walls and utility lines built on slopes experiencing such movements can eventually lead to undermining of their foundation support and, in some cases, can cause sections of the structure to separate or break apart.
Marine clay slopes and structures built on marine clays may not exhibit
signs of related problems immediately after construction or the
triggering event. It is not uncommon for the related problems to
surface well after (sometimes up to fifteen years) construction of the
slope the structure and/or the triggering event.
Not every crack in a house will be caused by marine clay. Many
hairline cracks on basement walls may be due to natural settlement of a
house and/or shrinkage of the concrete, and may not represent severe
structural damage. Many hairline cracks on ground-supported slabs
may be due to natural shrinkage of the concrete and/or poor construction
practices, and may not represent severe structural damage.
Settlement of improperly compacted non-clay fill under a foundation,
pressures exerted by water collecting next to the house, and sub-standard
building construction can also result in cracks in walls and floor slabs
and in foundation damage.
Weather patterns can provide clues to identify marine clay damage. For
instance, foundations on clays may shrink and settle during extended dry
periods; 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 results of the engineering investigation that identifies
the cause of the damages to the structure and/or slope. Depending
on the type of repair being proposed, you will need to obtain a
site-related plan and/or a building
permit before any repair work is undertaken. The
Fairfax County Code (Chapter 107 -- Problem Soils Ordinance)
currently requires a soil report prepared by a qualified geotechnical
engineer for all new construction and grading work in problem-soil
(including marine clay) areas. As a matter of practice, an engineering
report is required for repair work, unless the homeowner or contractor
prepares adequate recommendations and plans. Engineering designs should
be submitted with the application for a building permit. When the County
approves your designs, you will receive a building permit. The engineer
who prepared the repair designs must inspect the repair work and certify
that the designs are properly implemented.
A qualified geotechnical engineer with experience in marine clays should
be hired to properly investigate the root cause and extent of the
cracking being experienced on the building and/or the nearby ground
surface. An experienced geotechnical engineer can help identify the
source of the damage, and ensure that the proposed designs adequately
address the problem. An inadequate investigation of the problem may
result in an improper fix that will re-occur at a later time, making the
problem more costly from time and monetary standpoints. While the
causes of some types of damage are relatively easy to diagnose, the
causes of others can be baffling. An engineer can also make sure
that the repairs themselves will not create more damage to the existing
structure and potentially result in further problems.
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.
Patching may 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.
Not every structure that has been built in marine clay is or will be
experiencing problems. The house may be properly constructed to
withstand the soil conditions, or may be in a localized area of more
stable soils. While no design can provide a 100 percent guarantee
that problems will not occur, certain safeguards can be taken to reduce
the potential for damage by marine clay. Engineering designs that
have been properly incorporated into new house construction or in repairs
to existing houses can minimize the potential for later damage by marine
As a homeowner, you can take certain steps to minimize changes to the
soil moisture that could trigger future marine clay related problems and
damage to foundation, slab and wall. Maintain a positive grade, or
slope, away from the house. This encourages surface water to run off
away from the house rather than to soak into the ground next to the
house. Similarly, 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.
Fairfax County can help the homeowner determine the source, extent, and
potential solutions to damages that may be caused by marine clay. Contact
the Northern Virginia Soil
and Water Conservation District at 703-324-1460, TTY
711 for technical help. Information helpful in selecting a
geotechnical engineering firm and/or a contractor to investigate and
repair damages can be obtained by contacting the
Engineer of the Day, the Site Development and Inspection
Division at 703-324-1720, TTY 711 or the Northern Virginia Soil and
Water Conservation District, or by calling professional associations
such as WACEL, NSPE, ACEC of Metropolitan Washington, ASCE-NCS,
etc. Consumer information of engineering firms and contractors can
be obtained by contacting the
Virginia Department of Professional Regulation and the Fairfax County Department
of Consumer Affairs.
In certain cases, based on availability and criteria, low interest or deferred loans may be available for repair work from the Department of Housing and Community Development. Contact the Land Development Services of the Department of Public Works and Environmental Services for information on obtaining approval of a site-related plan and/or building permits.
Contact the Site Development and Inspection Division at 703-324-1720, TTY 711 or via email.