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Foundation

 

Foundation Basics

A home's foundation is in direct contact with the ground and joins the building's structure with the underlying zone of soil or rock. The foundation's job is to transfer the structure's load to the underlying soil or rock, without excessive settlement or movement.

Movement of the soil beneath a home can severely damage the foundation. That’s why the soil beneath your home is often referred to as your second foundation.

Slab-on-grade foundations are shallow foundations that are most often constructed of reinforced concrete. Slab-on-grade foundations can be built quickly and are relatively inexpensive to build. The function of a slab-on-grade foundation is not to resist or limit the amount of heave that might occur beneath a slab foundation, but to move up and down with the shrink and heave. Slab-on-grade houses do not have basements.

Shallow foundations are susceptible to seasonal movement from rainfall, drought, freeze/thaw cycles, and temperature changes and transpiration of moisture thru the roots of large plants and trees.

Pier and beam foundations Pier and beam foundations, as the name suggests, are a concrete footing and pier which supports wood beams and floor joists. These foundations usually have crawl spaces underneath the home.

Basements are most often constructed in northern climates where freeze/thaw conditions occur and the footing depth must extend beneath the frost line—often four or more feet below the surface. In many of these cases, builders will go ahead and excavate for a basement and build basement walls that provide the support for the house. The bottom of the basement is typically below the depth over which the majority of the soil’s shrink or swell due to climate occurs. Basements can suffer basement floor heave and lateral wall movement, however.

Deep foundations reach depths that are not normally affected by seasonal environmental changes and are considered to be out of the zone of influence.

There are a multitude of other types of both shallow and deep foundations.

Maintaining a Foundation

In dry periods the soil adjacent to the foundation should be watered to maintain constant moisture. Proper watering is critical as the purpose is to keep the soil water content next to and under the foundation at approximately the same moisture content.

The use of a soaker hose is most often the best solution and the soaker hose should be used 24" to 36" from the house. Ron Davidson at Ram Jack Distribution, a foundation repair product distributor in Ada, Oklahoma notes that in many cases homeowners have waited too long before watering and cracks have already occurred in the soil. In many cases starting a watering program at this time is too late.

July, August, and September is when cracks in the soil often appear. You want to have initiated a watering program long before this so the cracks do not appear.

Some of the other maintenance issues:

Drainage- it is very important that ground surface water drains away from the foundation. Surface water should never be allowed to collect around the foundation. Annually inspect the ground from the foundation out at least five feet immediately following a rainstorm. If there is water ponding against the foundation this situation must be corrected by regrading the area.

Downspouts should be directed away from the house and the water should discharge 3-4’ away from the house at a minimum.

Vegetation- Understand the types of trees you have planted near your home. Certain trees have extensive shallow root systems that remove water from the soil. The Department of Housing and Urban Development suggests trees be planted no closer than their ultimate height. Plants with large, shallow root systems can grow under a shallow foundation and, as the roots grow in diameter, produce an upheaval in the foundation beam.

Observation- As Tom Witherspoon notes in Residential Foundation Performance, during times of excessive moisture changes you must inspect your foundation and note movement signs that signal a problem. See Identifying a Foundation Problem. Don’t let early warning signs go unnoticed. Call an expert and find out if you have a problem that should be corrected. Not all problems are disastrous. They may only be a signal of something that can be corrected with increased watering or root barriers at a tree. Acting early can save you money.

Soil Conditions

Soils with the potential to shrink or swell are found throughout the United States. Soils with this shrink/swell potential create difficult performance problems for buildings constructed on these soils. As the soil water content increases, the soil swells and heaves upward. As the soil water content decreases, the soil shrinks and the ground surface recedes and pulls away from the foundation. These problems are of particular concern in homes with shallow foundations. See Foundation Basics.

Homes are normally not built in areas where the soil conditions are perfect. The developer selects land for various reasons, which may include availability, cost, proximity to industrial areas, and proximity to schools. The main reason for selecting a parcel to develop, of course, is that there is a strong demand of people who want to buy homes in that area and a profit can be made from selling the homes.

The best way to find out if the soil beneath your house is expansive is to ask a geotechnical engineer. In many housing developments a Soils Report will have been prepared, but this requirement varies depending on the region of the country.

A geotechnical engineer will make soil borings on your site and take samples so these soil samples can be tested for expansiveness. These samples will show how expansive the soil is and at what depths. The geotechnical engineer will provide a written report on his findings.

What is the "active zone?"

From the ground surface downward, there is a depth over which expansive soils experience a change in moisture conditions as the climate (or seasons) change. This results in the soils shrinking or heaving. This zone is an average of 18’ deep.

A shallow foundation will be more impacted by soil and climate considerations than a deep foundation (see Foundation Basics for more information about shallow foundations).

Soil Types:

Here is an overview of soil types:

Expansive Clay Soils

Expansive clays will swell/ heave when wet and contract/consolidate when dry. If the foundation system is in the active zone (a shallow foundation), the foundation will move as moisture conditions change in the active zone.

Select Fill/ Loam

Select fill is normally defined as a sandy loam that shows little change with moisture variations. A building pad properly built with select fill/loam will support the foundation. Problems could occur if erosion occurs that changes the bearing capacity of the soil.

Sand

Sand will not change as moisture conditions change. However, sand can erode if drainage around the lot allows water to work its way under the foundation. Sand can also fall in a crack created by drying soils and cause the foundation to drift (move horizontally).

Rock

Rock can erode and expand slightly only if it is a low density of shale. In some slope conditions, fractures/ faults in the rock can allow sliding and failure if not properly pinned with tie back anchors.

When a structure is supported by various soil conditions, the house may move differentially. As an example, if one half of the foundation sits upon expansive clay and the other half bears on select fill and/or rock, the amount of seasonal movement will vary from one half to the other half. If the foundation system is not properly designed, the differential movement may cause damage to the foundation and structure.

Many times building pads will be cut and/or filled so the bearing soil is all of the same type.

Other Resources

The USDA Natural Resources Conservation Service, formerly the Soil Conservation Service, has been publishing soil surveys for 100 years. A soil survey contains maps and a description of each major soil in the survey area.

USDA Service Centers are designed to be a single location where customers can access the services provided by the Farm Service Agency, Natural Resources Conservation Service, and the Rural Development agencies. This web site will provide the address of a USDA Service Center and other Agency offices in your area along with information on how to contact them.

You can find the center nearest you by clicking here:

Having a home built? You may anticipate a few things going wrong, but you'd expect your builder to erect your house on solid ground, right? Don't be so sure.

Read this excerpt from Ten Things Your Home Builder Won't Tell You, by Terrance Noland:

Population growth and urban sprawl mean there's not much residential land left in many areas. "What's left is not very good," says Daniel G. Knowler, a senior engagement manager at Navigant Consulting, which specializes in construction disputes. A lot of homes are being built on expansive soil — earth that swells when it rains — without adequate safeguards. In mid-1994, shortly after John Duffy and his family moved into their $234,000 home in Highlands Ranch, Colo., long cracks started showing up in the walls, and the porch started pulling away from the house. After badgering his builder for the soil report, Duffy learned his lot was a hot spot for potential swell. Writer Homes, the builder, was ordered to pay Duffy $544,000. John Palmeri, Writer's attorney, says the company offered to fix the Duffys' house, but "they were bent on going to court."

Swelling soil isn't the only problem. In March 1998, four hillside homes built on the site of an ancient landslide in Laguna Niguel, Calif., toppled after the unstable soil gave way. Early in 1999, Capital Pacific Homes (which had bought the builder, J.M. Peters), the lot developer, the grading company and the engineering company that checked the soil agreed to pay about $35 million to the homeowners, the homeowners association and the people whose condos at the bottom of the slope were also destroyed, according to Andrew Kurz, the association's attorney. Capital Pacific declined to comment.

Causes of Foundation Problems

Water is the main enemy in expansive soils problems. There is either too much water, causing the soil to swell, or not enough, causing the soil to shrink.

If all soil beneath a foundation swells uniformly or shrinks uniformly it is unlikely to cause a problem. But when only part of the foundation heaves or settles, differential movement causes cracks and other damage.

Most differential movement is caused by differences in soil moisture. After construction, soil beneath part of the foundation becomes wetter or drier than the rest of the soil.

Here is why this happens:

When there is a gain in soil moisture:

This is the most serious threat since the swelling potential of expansive soils is much greater than the shrinkage potential. Moisture gain can come from plumbing leaks, subsurface water like wet weather or a high water table, or surface water. Surface water is improper drainage of landscape water or rainwater.

Poor drainage can be a major contributor to soil moisture gains. Roof runoff should be directed away from the house through the use of gutters. Gutter downspouts should not be permitted to discharge the water next to the foundation. Surface drainage next to the foundation should slope away from the house approximately ¼ " per foot.

When there is a loss of soil moisture:

The soil may be at or near its optimum moisture content when the foundation is built, but it may lose enough moisture during a drought to cause the foundation to settle. Settlement is usually greatest near the perimeter of the foundation where the soil dries most quickly.

Extremely low or high soil moisture during construction

If the soil content is very low when a slab-on-grade foundation is poured, soil to the slab edges regains moisture first because it is directly exposed to rain water or irrigation water.

If the soil moisture is extremely high during construction, the slab will hold in the moisture except at the perimeter, where it is exposed to more wind and heat. In cases like this the slab edge loses moisture at a different rate than the soils under the house and the house will settle.

Poor Pre-Construction Compaction of the building pad

Slab-on-grade foundations depend on the uppermost soil layers to provide bearing capacity to support the structure and keep the foundation stable. If the bearing soil was not compacted properly during grading, the foundation is subject to settlement as the supporting soil consolidates.

Also of concern is when a structure is supported by various soil conditions. In this case the house may settle differentially. As an example, if one half of the foundation sits upon expansive clay and the other half bears on select fill and/or rock, the amount of seasonal movement will vary from one half to the other half. If the foundation system is not properly designed, the differential movement may cause damage to the foundation and structure.

Identifying a Foundation Problem

Exterior indications of foundation problems

- Doors that will not open or close properly

- Gaps at the corner of fascia trim

- Diagonal cracks in the wall at corners of doors and windows

- Gaps between the garage door and the pavement at either side of the garage door

- Windows that are hard to open and close

- Cracks in the exposed grade beam of the house

- Cracks in bricks and mortar

- Cracks in brick and mortar

Interior indications of foundation problems

- Unlevel Floors

- Large cracks in the concrete slab

- Gaps above kitchen cabinets

- Cabinet doors that will not stay shut

- Diagonal cracks in the wall at corners of doors and windows

- Curling and tearing of existing sheetrock repairs

- Leaks and cracks in and around the fireplace

Exterior or interior indications of foundation problems could be caused by either settlement or upheaval. Settlement means a portion of the house foundation has dropped below the original foundation elevation. This occurs due to a loss of soil bearing capacity caused by compaction of fill, loss of moisture in the supporting soil, or due to the erosion of the supporting soil. In many cases, settlement originates and is more pronounced at the perimeter of the slab since the slab perimeter is the most susceptible to loss of moisture and differential moisture conditions.

Upheaval means the slab has risen above the original foundation elevation. This happens most often due to the introduction of moisture under the foundation. The most frequent cause of this moisture is a plumbing leak under the slab. The most common leak is on the drain side of the plumbing system.

Note that some damage is strictly "cosmetic" damage that does not interfere with the foundation's load bearing capacity. These "cosmetic" items could be cracks in floor tiles, cracks in sheetrock, etc. A qualified foundation repair contractor or a structural engineer can help you determine if the problems you are faced with are cosmetic or will require foundation repair.

Post Foundation Repair Tips

To make repairs last, the property owner must change the conditions that caused the problem. Soil swelling can usually be stopped by cutting off the moisture supply. Plumbing leaks are major causes of soil swelling. If water leaks do occur, they must be repaired immediately to prevent damage to the foundation.

Poor drainage near the foundation is another cause of soil heaving. The owner may have to regrade his lot to make sure rainwater drains away from the foundation. Downspouts for gutters should carry water well away from the foundation and there should be no places where water ponds near the house during rains.

Regular lawn watering during dry periods helps to prevent edge settlement caused by soil shrinkage.

Post foundation repair tips are just like the items mentioned in Maintaining a Foundation. Follow these suggestions to minimize the chance of a reoccurrence of your foundation problem or for new problems developing. Also, discuss these items with your foundation repair contractor who makes your repairs.

From Maintaining a Foundation:

In dry periods the soil adjacent to the foundation should be watered to maintain constant moisture. Proper watering is vital, the purpose is to keep the soil water content next to and under the foundation at approximately the same moisture content.

The use of a soaker hose is most often the best solution and the soaker hose should be used 24"-36" from the house. Ron Davidson at Ram Jack Distribution, a foundation repair product distributor in Ada, Oklahoma notes that in many cases homeowners have waited too long before watering and cracks have already occurred in the soil.

July, August, and September is when cracks in the soil often appear. You want to have initiated a watering program long before this so the cracks do not appear.

Some of the other maintenance issues:

Drainage- it is very important that ground surface water drains away from the foundation. Surface water should never be allowed to collect around the foundation. Annually inspect the ground from the foundation out at least five feet immediately following a rainstorm. If there is water ponding against the foundation this situation must be corrected by regrading the area.

Downspouts should be directed away form the house and the water should discharge 3-4’ away from the house at a minimum.

Vegetation- Trees such as Cottonwood, Weeping Willow, and Mesquite have extensive shallow root systems that remove water from the soil. The Department of Housing and Urban Development suggests trees be planted no closer than their ultimate height. Plants with large, shallow root systems can grow under a shallow foundation and, as the roots grow in diameter, produce an upheaval in the foundation beam.

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