Overcoming Problems with Marine Clays
Marine clays are soils that were deposited by rivers flowing into the Atlantic Ocean millions of years ago. In Fairfax County, marine clays can be found in widespread areas east of Interstate 95, mostly in the Lee and Mount Vernon Districts.
While Fairfax County maintains soils maps which can locate marine clays, the only definitive way to tell if your house is located in marine clay is to have a soils investigation conducted on your property.
Marine clays shrink during dry periods of the year and swell during wet periods. The pressures exerted by the swelling action can far exceed the strength of your foundation. Damage may develop slowly and become more serious over time. However, many houses located in marine clays may never experience problems.
Learn how to diagnose a potential problem and simple maintenance measure that may prevent damage to your home. For more information, go to our marine clay FAQs and Soil Ratings. You may also contact Land Development Services at 703-222-0801, TTY 711.
The most common problem in marine clay areas is the settlement and
heave of house footings that are three feet deep or less. During dry
periods, the soil loses moisture and shrinks which causing a gap under
the footings. The house then settles resulting in cracked masonry
walls, interior cracks in plaster, and warped door and window frames.
See Figure 1.
Deep-rooted trees contribute to the problem by drawing water from the
soil through the root system causing further shrinkage.
- Foundations that have settled during dry periods will often return to near the original position after rainfall replenishes the soil moisture causing the soils to swell again. After several cycles, the rebound of the foundation may become progressively less resulting in larger cracks.
Footing support must extend to deeper and more stable soils; do this by one of the three methods below.
- In new construction footings can be placed four to six feet or more below the ground surface; the exact depth is determined by the soils investigation.
- Trees should be planted at least 20 feet away from the house.
- On existing construction, underpinning or helical piers are often necessary to reinforce a foundation damaged by settlement. Underpinning a house consists of new footings that extend below the original footings throughout the damaged area; see Figure 2. Helical piers are steel anchors that are drilled into the ground to a deep depth and attached to the existing footings.
Underpinning and installation of helical piers require a building permit.
Basement Wall Damage
Damage to foundation walls is caused when marine clays are used as
backfill. Now prohibited, this practice was common prior
Damage is caused from the yearly cycles of pressure exerted
by shrinking and swelling marine clay in the backfill; see Figure
- Swelling pressure of the soil can be compounded by improper drainage around the house from rainwater.
Depending on the extent of damage, foundation walls may need to be replaced while others can be repaired. To prevent future damage, the clay must be removed and replaced with sandy or gravelly soils; see Figure 4. Your engineer or contractor should follow the steps below when repairing or replacing a damaged basement wall:
The house should be temporarily shored if the basement walls are
severely damaged or are to be replaced.
The marine clays should be removed from against the basement wall for a
four- to eight-foot. The actual distance for your house will be
approximately equal to the depth of the footing.
The exterior side of the wall should be waterproofed after the wall is
repaired or replaced. Waterproofing must meet the requirements of the
Foundation drains, encased in gravel, should be placed at the
footing level on the outside of the house. The foundation drains should
lead to an outlet on a slope away from the house or to a sump pump. A
filter fabric should be placed on top of the gravel surrounding the
The new backfill soils should consist predominantly of coarse-grained
sand and gravel soils which minimize soil pressures against the walls
and allow good drainage.
- A one- to two-foot thick capping layer of low shrink-swell silty or clayey soils should be placed at the ground surface to reduce the infiltration of rainwater. The slope next to the house should be at least five percent (a six-inch drop for each 10 feet in distance away from the house). See Figure 4.
Foundation wall design and repair require a building permit.
As a homeowner, your observations and records can help evaluate the problem Use the following as a guide.
If you see foundation wall cracks, note the date and width and length
of the cracks. Small settlement cracks (smaller than 1/16 inch) are not
uncommon in new construction within the first year. However, cracks
that appear abruptly after 5, 10, or 30 years are probably caused by
If the cracks open up during the summer months and close up during the
winter months, they are probably caused by shrinking and swelling of
marine clays below the footings.
Note the location of trees with respect to damaged walls.
Cracks that extend through masonry walls and follow block and brick
mortar joints may indicate advanced settlement due to shrink-swell
If one or more basement walls is cracked and bowing inward, you may
have marine clays in the backfill. Concrete block walls that are bowing
inward usually have one or more horizontal cracks about mid-height on
the wall. Vertical cracks often develop near the corners.
- Bowed walls may exert pressure against sewer, water and natural gas pipes. Call the gas company immediately if you think that your natural gas pipe or gas meter is under pressure from a damaged wall.
There are several steps you can take if you think that your house is located in marine clay.
Provide positive grades (at least five percent) away from the
Make sure the gutters and downspouts carry all roof water at least 10
feet away from the house.
Large trees should be at least 20 feet away from the house.
If you have a swimming pool, make sure there are no leaks.
- Basement sump pumps should be in good working condition.