What is the "Clay Bowl Effect"? - Expert Explain

September 6, 2023

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what is the clay bowl effect

When building a house, the construction process creates what is known as the "clay bowl effect" around the foundation. This refers to how excavating the ground and backfilling with looser soil can lead to drainage issues. 

So, exactly what is the clay bowl effect? Just like how a clay bowl holds liquid, the ground around the foundation ends up retaining more water. This saturation of the soil close to the house can cause moisture problems in basements over time.

How Home Foundations Create a "Bowl"

To build a house's foundation, the ground must first be excavated to create a large hole for the foundation walls and footings. This digging out of the earth results in an empty "bowl" shaped space.

Concrete is then poured to form the foundation walls and footings within the open hole. Once cured, backfill soil is used to fill the space between the foundation walls and the sides of the original hole.

This backfill earth is not as densely packed as the surrounding undisturbed soil. It typically contains a mix of the original soil, gravel, sand, and other materials. This looser fill is more porous and absorbent compared to the hard-packed ground outside the hole.

Just like an unglazed clay bowl, this backfill area retains more moisture when it rains or floods. The looser soil particles have gaps that allow water to soak in and penetrate easily. On the other hand, undisturbed soil is pressed together over years of pressure, reducing its absorption capacity.

If you require a foundation inspection due to issues with your property's soil, reach out now.

house being repaired by foundation contractors

How the Clay Bowl Effect Causes Wet Basements

When moisture builds up in the backfill area around a home's foundation, it creates pressure on the walls and saturates the ground. The water will naturally move from the higher-pressure backfill area into the lower-pressure of the basement interior.

Over time, this moisture can seep through tiny pores or cracks in the concrete walls and floors. Once inside, the water can create major problems like mold, floods, and structural issues. A perpetually damp basement also loses insulating value and becomes a poor storage space.

In extreme cases, a super-saturated backfill area can even push against the foundation walls and cause them to bow or crack under the pressure. This leads to very costly foundation repairs.

drain coming out of home for proper foundation drainage away from house

Risk Factors for Severe Clay Bowl Effect Issues

Certain conditions can exacerbate drainage issues related to the clay bowl effect:

  • Backfill Material - Clay-heavy backfill soil is especially problematic. Sand or gravel backfill provides better drainage.
  • Sloped Land - Downward sloping yards direct even more runoff toward the foundation.
  • High Water Table - A house built where the water table is close to the surface has far less buffer.
  • Poor Gutters - Missing or leaking gutters allow massive amounts of water to soak the ground near the house.
  • Settling Soil - If the backfill settles more than the yard over time, it can create a bowl shape that catches water.
  • Nearby Springs - Underground springs naturally saturate the nearby soil and can lead to very damp basements.
  • Cracks - Any cracks allow moisture easy access into the basement, especially when under pressure from saturated backfill.

wall crack needing repair in peoria

What is the Clay Bowl Effect and How It Got Its Name

While the bowl-like shape of backfill is universal, this issue is especially noticeable in regions with clay-heavy soils. Clay particles are flat and stack tightly together, forming a dense mass when compressed over long periods of time. However, clay backfill around a new home foundation has not yet compacted.

The combination of loose clay backfill and the high moisture retention of clay creates a problematic scenario. For this reason, the phenomenon became known as the "clay bowl effect" - particularly in locations like Kansas, where thick clay deposits are common. Even so, sandy and silty backfill still allows more absorption than undisturbed soils.

what is the clay bowl effect

Signs of Clay Bowl Effect Drainage Issues

Watch for these indicators that excess moisture is building up outside your foundation:

  • Wet spots or standing water in the yard near the house after rain
  • Downspouts and gutters frequently overflow and saturating the ground
  • Ground staying soggy for days after storms
  • Musty, moldy smell in the basement
  • White mineral deposits on basement walls or floors
  • Cracked or bowed foundation walls
  • Mushroom growth on wood in the basement

contractor inspecting flooded foundation

How to Reduce the Clay Bowl Effect Under Your Home

If you suspect your home's foundation is trapping water due to the clay bowl effect, several remedies can improve drainage:

  • Proper Downspouts - Direct downspouts at least 5 feet away from the house so water can fully permeate into the ground.
  • Working Gutters - Clean and maintain gutters so they don't overflow. Consider gutter guards.
  • Grading Yard - Regrade any settled areas to slope water away from your home's perimeter.
  • Backfill Improvements - Install a perforated drainage pipe along the foundation footer to divert water. Or excavate and replace several feet of backfill with gravel.
  • Waterproofing - Seal basement walls and floors with waterproof coatings to prevent seepage.
  • Sump Pump - A sump pump collects water pooling next to the foundation and pumps it away from your home.

sump pump installation

Conclusion

Left unchecked, the clay bowl effect can make basement moisture problems a recurring headache. But with vigilance and some drainage improvements, you can overcome this common challenge stemming from your home's construction. Pay attention to any signs of excess water accumulation around the foundation after rainstorms. 

Take proactive steps like extending downspouts and proper grading to keep the ground around your home's perimeter as dry as possible. Address minor seepage right away before it escalates into major leaks or cracks in the foundation walls. With smart water management, your basement will stay dry and healthy for decades to come.

About Peoria Foundation Repair & Waterproofing Pros

For over 20 years, Peoria Foundation Repair & Waterproofing Pros has provided expert basement and crawl space services to homeowners throughout the Peoria, IL, area. Our locally owned company specializes in solving foundation and moisture problems for houses of all ages and styles. At our company, we believe the foundation of your house deserves the very best maintenance. Contact us today at (309) 322-6979 to schedule your free inspection and estimate.

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