Find out the answers to the most common foundation engineering questions

The foundation supports the house. The foundation includes the footings, foundation walls, columns, slab, and all other parts that provide the support for the house and transmit the load of the superstructure to the underlying earth. The safety and usability of a house is determined by its structural integrity. In many cases, foundation movement is minor and problems can be easily corrected. In other cases, the movement is significant and causes damage to the foundation and the superstructure.

There are many reasons for movement of both slab-on-grade and raised foundation type homes. No two buildings are identical. Each one is subject to an enormous number of variables ranging from the weather on the day construction began down to the last time it rained. Even in San Diego, where the average rainfall is less than 10" per year, moisture can have a significant impact on the performance of a foundation.

For the difference between slab and raised foundations, please see additional questions below.

Expansive Clay Soils

Clay soils have very small cohesive particles and can expand dramatically when the moisture content increases due to rain, plumbing leaks, excessive watering, poor drainage, etc. These conditions can cause upward pressure and stress to the foundation. Conversely, expansive soils can also "dry out" during droughts and other times without measurable moisture. When the amount of expansion / contraction of the soil is great enough, it puts tremendous stress on the foundation system. In order to minimize possible foundation stress, consistent soil moisture levels are required. It is the changes in the moisture content resulting in swelling and shrinkage of expansive soils which cause the foundation to heave or settle.

Soil Compaction and Fill Soils

Many homes in San Diego, particularly the older homes, were constructed before there were modern grading standards and rigid soil compaction testing requirements. Many of these homes were built on cut and fill lots, where the original native soil was removed and/or fill soil was placed on top of the native soil to make a flat building space. In many cases, the fill soils were not properly compacted, and over time these soils settled excesively, causing the foundation to settle along with the soil. Also, many of the fill soils were of the expansive type and when moisture was added to the fill soils, they expanded, causing the foundation to move upward along with soil. This effect is called upheaval. In addition to the primary structure, driveways, patios, and other concrete flatwork around the property can also experience settling, cracking, and other signs of distress.

Drainage and Water Leaks

Drainage issues can create low spots and allow water to pond and penetrate the soils under the foundation. This can cause heaving of expansive soils or settlement of poorly compacted soils. The principal of positive drainage dictates that water is diverted away from the foundation. Gutters with downspouts that discharge well away from the foundation are a good start. Broken plumbing lines or leaking irrigation pipes can easily cause damage because they are often difficult to detect until after they have cause visible damage to the structure.

Tree Roots

Trees can also have an impact on the foundation. When the tree root system extends into the soil beneath the foundation, the tree utilizes this soil as an additional water source during the drier seasons to supplement its moisture needs. As these roots grow, they can exert forces great enough to heave a stemwall or floor slab.

Workmanship Quality and Materials

The "what and how" at the time of construction can have a significant impact on the ability of the foundation to perform as designed. Was the reinforcing steel adequate? Was it placed too close to the surface of the concrete? Was the concrete mixed properly, placed evenly, and allowed to cure correctly? These considerations and more, when coupled with the soil and moisture issues addressed above, all influence how a foundation performs.

Slab foundation systems consist of perimeter footings with a concrete slab that is the floor of the home. Typically, the footings are a minimum of 12" deep and 12" wide with horizontal reinforcing steel (rebar) inside the footings for extra strength. The concrete slab portion of the foundation system is usually 4" thick and, especially in older homes, is usually not reinforced with rebar. In areas of very expansive soil, some builders have used post-tensioned foundations, which consist of a slab & footing system utilizing steel cables tightened through the slab. In San Diego County, slab-on-grade construction became widespread in the mid to late 1950's tract developments. This continues to be the prevalent foundation system used in new construction, except where the building sites are on hilly terrain, etc.

In San Diego County, raised foundations are mostly found in older homes built prior to the 1950's, although in newer construction this type of foundation is often used on slopes and hilly terrain. Raised foundations typically consist of stemwalls (foundation walls) and footings which support the roof and the perimeter exterior walls. The footings are buried below the surface of the soil. The middle portion of the house with its floor joist and girder system is typically supported by wood posts and concrete piers. Traditionally, the foundation stemwalls and footings are made of concrete. They can, however, be made with concrete blocks, etc.

Foundation movement indicators that may signify raised foundation damage:

• Horizontal or vertical cracks in stemwalls
• Stucco cracks, especially around windows and doors
• Deteriorated or crumbling concrete in foundation walls (due to age and moisture)
• Piers or posts leaning or damaged
• Unlevel or sagging floors
• Significant cracks in the interior walls and ceilings

Foundation movement indicators that may signify raised foundation damage:

• Epoxy repair of stemwalls (by high pressure injection)
• Steel reinforcement plates bolted across stemwall cracks
• Remove & replace part or all of perimeter stemwalls
• Post & pier repair or replacement, possibly with floor leveling
• Stabilization or underpinning of footings
• Seismic upgrades (bolting sill plates to stemwalls)