Soil Washed Away Under Foundation

Steve writes: A friend recently moved into a property where a downspout leaked for years causing soil under the footings to be washed away on one corner of property. Their builder said they need to fill the void with concrete to provide support in that area. Is this a good approach and what would you call this remedy?

Steve Bliss of BuildingAdivsor.com responds: This sounds like a reasonable solution, referred to, in general, as underpinning. Underpinning is required when the soil beneath a foundation cannot support the load. Examples include filled soil that has not been properly compacted, organic materials such as peat, other poor soils, or places where the soil under the foundation has been disturbed as in this case.

There are a variety of solutions depending on the specific situation. The soil can be stabilized and strengthened, or the foundation can be made deeper to reach stable, undisturbed soil — or wider at the base to spread the load over a larger area. Other more exotic (and expensive) solutions include special steel pins and helical piers that can support weight in poor soils.

Since it is difficult to replace and compact soil beneath an existing footing, filling the void with concrete may be the easiest and cheapest solution — and, of course, redirecting the downspout so the water drains away from the building without causing damage.

It’s surprising how many basement water leaks, erosion issues, and other drainage problems can be fixed by simply redirecting water from gutters and downspouts away from the house. Sometimes a splash block or a short run of horizontal pipe (drainpipe or pvc) along the ground is all that is needed. On a flat site with poor natural drainage, a dry well located away from the foundation may be needed to safely discharge the water back into the soil.

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  1. I bought a 1,300 sq.ft. house about 6 years ago. There are french drains along the west and north side of the foundation. The previous homeowner said one downspout (on the northwest corner of the foundation, draining east) had a black drain hose attached to the bottom to drain the water underground into the french drain.

    He said the contractor used black drainage pipe without holes in it to remove water that collects in the french drain during rain. The hose from the french drain is intended to drain water away from foundation and drain water from downspout. It stays underground and drains into a relatively small hole filled with river rock just outside the fence line.

    He wanted the drain pipe to do double work — drain water out of french drain AND drain water from a downspout. The contractor ended up punching holes in the french drain pipe with a simple knife. The contractor did NOT use a sock (filter fabric around the perforated pipe to keep out silt).

    On the northeast corner of home (the backyard) there is another downspout that empties into a splash block. This splash block was installed under the downspout with the back side LOWER than the front side. The combination of the french drain flowing from west to east on the north side of house AND the splash block being installed incorrectly has caused a lot of clay soil to be washed/moved/pushed away from underneath the corner of the foundation. It has also washed away clay underneath an area of river rock, on top of 3 mil plastic sheeting and soil, as well as under a flagstone walkway.

    The middle of flagstone walkway is bulging up, but the area between the flagstone walkway and house/foundation has almost completely washed away. Clay soil is also being washed away due to french drain installation under the northwest corner of foundation. I can see cracks in some interior walls, mortar between exterior bricks has fallen completely out, and some bricks have actually cracked through in some areas but mortar still intact. I can actually feel the floor sloping northwest AND northeast. I have a crack inside from a window also on the north side of home directly in the middle of house. Windows are hard IF possible to open. I’ve had the interior floor/foundation measured for unevenness and the findings are accurate with my explanations here.

    Here is my considered solution. Remove river rock and all hose from french drains on the west and north sides of house(no other french drains on property), fill them and any washed away clay(assuming not much more than a coupe of feet in spots especially at corners of foundation) with concrete but have a ‘trough looking’ finish on the surface(equal to or just below grade) for water from rain/downspouts to travel down to the street.

    Questions. Would this work? Be the cheapest method? Function as hoped for/secure foundation from sinking anymore? Will this prevent clay soil underneath foundation from being washed away to quickly and prevent more movement than concrete foundations are designed for?
    If so, I can then replace windows/doors, having the openings re-squared, not leak and open and close with little to no effort.

    If it makes any difference, I live roughly 10 miles NNW of Houston. Home was built in 1983.
    Thank you for addressing these issues as best you can.

    • buildingadvisor says:

      Based on your description, I can offer some general advice, but cannot provide specific recommendations. For that, you would need to have a qualified professional inspect the site, possibly do some soil testing, and provide a detailed plan for remediation. If it were my house, I would find a local consulting engineer (civil, soils, or geotechnical) with a lot of experience in foundation repair to develop a detailed plan. You are going to be spending a lot of money getting this house repaired so you want to make sure it is done correctly.

      Foundation-repair contractors in your area can also be a good source of information as they are familiar with local soils and building techniques, but a consulting engineer might be more objective.

      It’s quite possible that your house is built on expansive clay soil, which is fairly common in eastern Texas. If that’s the case, solving your problem will be more complicated (and expensive) than with a house built on normal soils. Expansive clay swells and shrinks much more than normal soils, often leading the kinds of problems you are facing. With expansive soils, you need to take extra steps to keep the soil dry around and under your foundation. You may also need to replace some of the soils and/or reinforce the foundation.

      The areas where the soil has washed away will need to be supported. Filling with properly compacted soil or concrete as you describe is one option. In some cases, it is more economical to “underpin” the foundation with mechanical supports such as helical piers or driven steel pins. For areas of the slab that have settled significantly, a process known as “slabjacking” might be useful. A wet concrete slurry is pumped under the slap to raise and support it.

      The steps you describe to improved drainage are all steps in the right direction. Regardless of the soil type, you always want to direct rain water away from foundation with gutters and downspouts. You can use concrete splashblocks or run solid piping run along the surface of the ground to get the water at least 5 ft. away from the house. Piping rain water into the footing drains (French drains) is never a good idea.

      There’s a good chance that your existing foundation drains have filled with silt. Ideally, you would excavate on all sides of the house and install new foundation drains with perforated pipe wrapped in filter fabric (sock) and covered with a layer of drainage material (crushed stone, course gravel, etc.). If you want to avoid this expense, you are taking the risk of water collecting under your slab. This can lead to excessive moisture in your house. With expansive soil, it can lead to swelling of the soil and damage to the slab.

      It sounds like the original builder drained the French drains to a sump pit. This is acceptable as long as the pit is far enough away from the house. Also the sump pit needs to be in soil that is capable of absorbing the water. Otherwise a sump pump may be needed to move the water to a suitable location. In some municipalities it can be pumped into the storm sewer. Unless you have a sloped site where you can “drain to daylight” you will need to find a suitable place for the water to drain to.

      In general, you want to get all runoff water away from the foundation. In a new home, you would grade the topsoil so it slopes away from the house – and use a layer of impermeable soil near the ground surface. In an existing home, it is often difficult or impossible to obtain the ideal grade. Depending on the drainage characteristics of your home site, you may need to take other steps to keep runoff away from your foundation. This might require drainage swales (shallow trenches), curtain drains, or catch basins to direct water away from the house.

      A well-designed system combines surface drainage and subsurface drainage to keep the soil dry around the foundation. This is more critical in areas with expansive clay soils (or in cold climates where frost heave is an issue).

      Once the foundation is stabilized, you can think about fixing the doors and windows. Sometimes these can be repaired by removing the interior or exterior moldings and re-shimming the racked window or door.

      Best of luck finding an economical solution. Related Links:
      Building on Expansive Clay Soil      Inspectapedia: Underpinning Foundations      Soil Issues in Texas Construction

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