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Marine Clay and Foundation Movement in Hollin Hills

When speaking of settlement, it is important to differentiate between one-time settlement of a structure and seasonal movement.  One time settlement occurs after construction, as the ground under the structure adjusts to the new loads imposed on it.  Settlement should be negligible, but can be significant, and last for many years. Considering the age of Hollin Hills houses, most construction settlement should be complete.

The other type of settlement falls into the seasonal movement category.  Many houses in Hollin Hills, particularly in New Hollin Hills, sit on what is colloquially called “marine clay.”  The volume of this type of clay varies with moisture content.  Moisture content changes seasonally.  The clay shrinks in the summer as it dries out, and expands in the winter and spring as it is re-saturated with water.  As the soil changes volume, the foundation of the house supported by this clay will move up or down.  In slab-on-grade houses, the effects are manifested as sticking or stuck doors, wall and ceiling cracks, (particularly at doorways) foundation cracks, windows cracks, and out-of-level floors.  In some instances, the foundation, which supports the exterior walls and roof, moves differently from the slab, which is the floor.   I have seen cases where the floor has settled several inches, either leaving the interior walls hanging from the ceiling with a big gap between the wall and the floor, or the wall drops with the floor, leaving a gap between the wall and the ceiling.  Gaps also often occur between the chimney and the rest of the house.  As the chimney/fireplace mass is so heavy, it often settles more radically than the rest of the house.  In some instances, there is a gap big enough to put your hand through in summer and then, the gap closes to nothing in winter.

  • The severe drought this past summer caused the settling issues to be unusually noticeable.
  • There are two approaches to remediate the problem for a slab-on-grade house.
The less expensive, but perhaps less effective, approach is to try to keep the soil moisture content under the foundation constant throughout the year.  Large trees can remove up to 100 gallons of water per day from the soil.  Thus, trees should be kept away from the foundation.  The larger the tree, the further it should be from the house.  If the branches are close to the house, then so are the roots.  A large tree may remove much of the water under one area of the house, causing that section to settle, while the rest of the house settles comparatively less.  It is this differential settlement that causes the cracks and other problems.  If the whole house went up and down uniformly, there would be no problem.

Physically watering the foundation during dry periods can also be done, but gauging the right amount of water is difficult.  Running a soaker house around the perimeter and letting is run is probably the best way to get an even distribution of water.  Watering trees regularly that are near the foundation will also help replace the water removed by the trees.

Keeping excess water away from the foundation during the rainy season can also be beneficial.  Gutters and downspouts need to be kept clean and in working order, and the water needs to be carried away from the house far enough that it will not soak into the ground around the foundation.  Pipes that carry away downspout water should not be connected to the house foundation drain, but should be entirely separate.  Where practical, an impervious barrier can be installed at ground level or just below, sloping away from the house.  This will prevent rain soaking in during the winter, and prevent moisture from evaporating out of the soil in the summer.  A layer of plastic covered in mulch or gravel can serve, but may not be practical given landscaping considerations.

Houses with full or partial basements have a whole different set of marine clay problems and solutions.  Do not water your foundation if you have a basement, unless you dream of an indoor pool.

The other alternative is to install an engineered solution.  This means extending the footing supports deep enough to be below the layer of soil with seasonal moisture variation.  Generally, this depth is four to five feet.  In the old days, this “underpinning” involved digging down in sections below the footings and pouring new concrete footings below the existing footings.  This work is very labor intensive, disruptive to the landscaping, and quite expensive.  It also only supports the exterior walls, not the slab.

Newer technology uses helical piers.  These are basically big steel screws (6-10 feet long, a foot or so in diameter) that are driven into the ground around the outside of the foundation with a hydraulic driver (often a bobcat loader).  The piers are driven into stable soil, then the pier is mechanically attached to the concrete footing.  The piers usually are installed every 4 to 10 feet along the foundation depending on the situation.  A hole has to be dug for each pier/foundation connection, so again, this process is disruptive to the area around the house.  The piers can also be driven down to support the slab, but this means holes in the slab (and thus your floor) on a grid of every 4 to 6 feet.  So one ends up needing completely new floors.  It is important to note that underpinning part of a house may not work.  The underpinned part will cease to move, but the rest of the house may continue to move so cracks will still form at the joint between the underpinned area and the non-underpinned area. 

Underpinning a whole house is a project that is in the range of tens-of-thousands of dollars.
Thus, given the cost, attempting some of the easier remediation solutions might be a best first step.  Most Hollin Hillers have learned to live with some seasonal movement as part of the cachet of Hollin Hills.  


Questions and comments are welcome at 703-718-0804 or email us for more information.

     Call us at (703)718-0804 for more information,  or email us.