Concentration of Wall Loads

Concentration of Wall Loads The drawings to the left and below illustrate how the loads on a wall can be systematically concentrated when going from a conventional spread footing foundation to straight shaft piers. In the first example (top-left), a load of 2,000 pounds per lineal foot of wall results in a load of 1,000 pounds per square foot on the soil using a conventional spread footing that is 24 inches wide. In the second example (left), the same load of 2,000 pounds per lineal foot of wall results in a load of 2,000 pounds per square foot on the soil using a conventional spread footing that is only 12 inches wide.

This same wall load (2,000 pounds per lineal foot of wall) results in a load of 30,000 pounds on the pier, when the piers are located 15 feet apart.

Similarly, the spread footings can be "partitioned" to form pads, thereby concentrating more of the house's weight on a smaller area. This concentrated loading is used to resist the potential uplift pressure due to swelling soil.

Typical Foundation Types

Conventional or Continuous Spread Footings Used when low to non-expansive soils extend from conventional foundation depths to depths of at least 5 or 6 feet below the bottom of foundation. Also, depends upon ground water conditions. This type of foundation consists of a single slab of concrete usually 12 to 20 inches wide upon which the foundation wall is placed. The footing has a relatively large base, which distributes the weight, rather than concentrating it, over a large area. This type of foundation design is most effective in non-expansive soils to reduce the amount of collapse or settlement. Generally, the spread footing design is not recommended when swelling soils are present. When used in these conditions, at the very least it should be used in conjunction with soil replacement.

Wall on Grade (Non-Voided) The most common type of foundation, the non-voided wall-on-grade design consists of a continuous wall that rests directly on the soil. The wall applies a moderate to high amount of pressure on the soil because it has a relatively small base. This foundation is only recommended in areas that have low swell potential. Usually continuous narrow footings at basement level Shallow walls may be intermittent footing pads Seldom used anymore, particularly where expansive soils are a problem.

Voided Wall On Intermittent Footing Pads This is an improved version of the common wall-on-grade design. Like the non-voided wall-on-grade design, it consists of a continuous wall that rests directly on the soil. Its integrity, however, is greatly enhanced through the use voids beneath the foundation wall, enabled through the use of the intermittent footing pads. Typically, this foundation is only recommended in areas that have low to moderate swell potential. Use: Low to moderate swelling soils, where piers are not readily constructible or not desired. Requirement for a minimum dead-load pressure to resist swelling soils: Footing pads Drilled pads Appropriate Voiding - Should be constructed with void material (WallVoid®) of appropriate strength and thickness

Straight Shaft Drilled Piers (Caissons) Used in moderate to high swelling soils. (This is one of the most effective foundation designs for use in sites that contain expansive soils.) Purpose is to attain required penetration into zone where there is little or no seasonal moisture variation. Current standard of care in the area is a minimum penetration of 6 feet into bedrock and minimum length of 16 feet. Dead loads should be as high as practical. This design requires relatively long spans between piers and more reinforcing in grade beam. Caissons into bedrock Friction Piers into stiff clays End Bearing Belled Piers Appropriate Voiding - Should be constructed with void material (WallVoid®) of appropriate strength and thickness

WallVoid® System The drawing above illustrates the use of SureVoid® WallVoid System™ to properly void a drilled pier foundation. The WallVoid System features the following products: A. Commercial SureTops™ to correctly shape the upper portion of cast-in-place concrete drilled piers B. ArcVoid™ to correctly void the circular edge of drilled piers C. WallVoid® (FormVoid™ (with panel flange), TrenchVoid™ or TrapVoid™) to create a temporary support for the placement of concrete walls and grade beams spanning between supporting element, where expansive soils are present below D. Seam Pads to cover exposed joints, eliminating concrete flow between void forms

 Construction Concerns

The use of straight shaft piers will not guarantee that you will not experience problems with the foundation. Generally, each pier is constructed in from 16 to 20 feet of expansive soil. If the soil surrounding a pier becomes wetted, it will swell and it will try to push that pier out of the ground. The potential force of the expanding soil can be very large. Resisting this force will be the weight of the house on the pier plus some anchorage provided by the embedment of the pier in drier, more stable bedrock. The reinforcing steel placed in the pier keeps the concrete from being pulled apart by these forces. As you can see from this, a pier foundation can be a dangerous one if not engineered and constructed properly. Adding another pier does not make a safer foundation. Some other construction concerns are:

• Piers - Drilling through wet and/or caving soils may require the use of temporary steel casing. May also require the use of a tremie and a pump to dewater the hole and place concrete. This is more expensive and usually requires a larger diameter hole (less safe).
• Piers - Reinforcing. It is difficult to get bars to the full depth of the pier with the proper concrete cover in deep holes. Use centralizes. Use larger diameter bars versus more bars.
• Piers - Don't leave holes open for any length of time even in dry conditions. Cuttings fall in, etc. Have concrete on site and fill right after drilling and cleaning.
• Spread Footings/Pads - Remove loose, disturbed soil before placing concrete. This is especially true if they are formed, but not poured, and there is a heavy rain overnight. In the winter, don't pour them on frozen ground.
• Don't pour a pad or footing that is close to the edge of a shallow excavation where it joins a deeper excavation.
• Pads and Piers - To assure that the appropriate strength of void material was selected for the job, inspect the void material after removal of forms. If properly selected, the void material will have sustained the weight of the concrete and will not have collapsed and/or filled with concrete.