Foundation walls support weight (load) of a building and distribute that load to the ground. For shallow foundation walls on firm soil, design is typically simple, especially for light-weight buildings.
However, for foundation walls along full basement, proper design tends to be more complex, even for relatively lightweight wood-framed buildings.
The following report discusses problems with defective design of basement foundation walls for lateral earth pressure. Although examples discussed are from New Jersey, the basic principles are of course applicable anywhere.
|NJ DCA 041912 Foundation walls vpdf.pdf (PDF — 122 KB)|
Design For Settlement
Prevention of excessive vertical movement ("settlement") is the best understood purpose of foundation walls and footings under the wall.
For all structures supported on soil, some settlement will occur since soil is compressible. For most single-family homes, almost all of the eventual settlement will occur during construction and during the first few years after construction. Of course major problems with excessive settlement have occurred. Primary reasons are compressible soil underlying the site along with lack of adequate investigation of underlying soils (before construction) and lack of proper design.
Basic types of soil are sand, clay, silt and organic. However, even within each general soil type there is a very wide range of characteristics, including compressibility.
Weight ("load") applied to soil causes stress in soil far below the elevation where the load is applied. Soils tend to occur in layers, with each layter having different soil properties. A layer of very compressible ("soft") soil can be below a layer of competent, firm soil. This condition can easily result in excessive settlement, especially if the compressible layer is only several feet below base of footing.
IBC /IRC building codes include "presumptive" bearing capacities based on visual identification of soil at base of footing. Presumptive bearing capacity is usually conservative for lightweight building such as a wood-framed house. However, there is always the possibility that a layer of different soil with much greater compressibility is under the visible soil.
Shallow excavations ("test pits") can sometimes be useful to obtain information about underlying soils. However, deeper soil borings are required to obtain detailed information about all underlying soils that could influence foundation settlement.
Soils can have unexpected properties that must be carefully considered. In various areas of the US, the effect of "swelling" clay soils must be addressed during design.
Protection For Frost Heave
In areas with cold winter climate, soil near the surface can freeze. Depending on various factors, such frozen soil can move upward or "heave", causing damage to just about any foundation built on the soil.
To prevent such damage, building codes require that base of footing be a minimum distance below finished grade (ground surface). So-called "frost depth" varies with location, as shown on standard frost-depth maps.
Pure sand and pure clay are generally not susceptible to frost heave. However, most soils are mixtures of sand, clay and silt, such that protection against frost heave is necessary.
Basic Design Requirements for Foundation Walls
It is generally understood that a foundation wall must support vertical load (weight) from the building. The wall must therefore have adequate capacity to resist compressive force. Masonry (stone, brick, concrete block) and concrete have more than adequate compressive strength.
Foundation walls around basements must also act as retaining walls, resisting lateral (horizontal) earth pressure from soil against outside face of wall. This important requirement is often neglected by the designer, resulting in major damage; see the report noted above for examples.
In areas subject to high wind speeds, foundation walls must resist uplift forces from tierods and anchor bolts. Careful design is necessary to ensure adequate capacity.
Design for Lateral Earth Pressure
Design of basement foundation walls to resist lateral earth pressure must take into account the following;
1-Type of soil backfill, against foundation wall
2-Risk of high groundwater, above base of wall
3-Potential for vertical surcharge, on top of finished grade
4-Lateral support along top of wall
5-Bending strength of wall
6-How wall resists lateral pressure
Amount of lateral pressure depends on the type of soil. Clay and silt soils exert greater lateral pressure than sandy soil.
Total lateral force from soil backfill varies with the the square of soil height (measured above base of wall). Lateral force due to soil height of 6 feet is 4 times greater than lateral force due to soil height of 3 feet (6/3)^2.
Weight of vehicles on garage slab or driveway causes increased lateral force against foundation wall.
Weight of additional soil placed on original grade can greatly increase lateral force. This condition often occurs when a raised patio is installed along back wall of house; severe damage to adjacent foundation wall can easily occur.
Contractors mistakenly persuade owners that installation of a narrow gap between raised patio and foundation wall will prevent any adverse effect on the foundation wall due to weight of raised patio. The "gap" scheme has no validity. Weight of new soil (for raised patio) and the increased pressure against foundation wall remains essentially the same. One way (perhaps) to understand this is by considering the weight of a large dumptruck filled with soil, parked near a basement foundation wall. This weight causes increased pressure against the foundation wall, even though there is no contact between truck and wall. The same is true of weight of soil for a raised patio.
Risk of high groundwater is most often controlled using "perimeter" drain pipes. However, when high groundwater is known to occur frequently, it may be most appropriate to design the foundation wall for hydrostatic (water) pressure, along with soil pressure (which is then reduced to the "buoyant" value).
Adequate lateral support along top of wall is essential if the design is based on the wall spanning vertically, which is the usual condition. When floor joists are parallel to the wall, lines of wood blocking should be installed between joists, with plywood floor sheathing nailed to this blocking.
Often overlooked is the need to properly design connections of floor joists and blocking to sill plate on top of wall. These connections must have design capacity to resist lateral force from soil backfill.
Anchor bolts connecting sill plate to top of wall must also be designed to resist lateral force from soil bacfill. Minimum anchor bolt spacings specified by building codes are not intended to provide necessary capacity for resisting lateral earth pressure. Proper design is especially important when height of soil backfill is 6 feet or more.
Lack of lateral support along basement stair opening should be carefully considered. Upper part of wall must then span horizontally, between adjacent segments of wall that can span vertically.
Lateral support along top of wall is not required when propertly designed pilasters or piers are installed against inside face of foundation wall. To be effective, such piers (spaced about 6 to 8 feet for residential) must be reinforced with steel reinforcing bars extending down into a concrete base (footing) that is larger than the pier.
Away from corners, other intersecting walls or piers, tension stress occurs on inside face of wall as the wall bends (vertically) in response to lateral earth pressure. Horizontal mortar joints of concrete block walls are especially weak in tension, such that cracks occur frequently if the wall does not have adequate bending strength, which is a function of wall thickness.
Near corners and other vertical lateral support elements (intersecting wall, piers) the wall resists bending in the horizontal direction, as well as vertical direction. This behavior causes "step-cracks" to occur in horizontal and vertical mortar joints of block wall, on inside face of wall.
Also near corners and other vertical lateral supports, tension stress occurs on outside face of wall, causing vertical cracks in outside face.
Vertical reinforcing bars can be installed in block walls (and concrete walls) to greatly increase bending strength. Reinforcing will also minimize width of any cracks that do occur.
Concrete Foundation Walls
Plain concrete walls (without reinforcing bars) have much greater bending strength than concrete block. However, proper design remains important.
Concrete has the well-known property of cracking due to "shrinkage". Most often, many narrow ("fine") and very-fine cracks (vertical or near-vertical) will occur in basement foundation walls. Shrinkage cracks often extend into a wall from corners at window openings. In general, narrow shrinkage cracks do not adversely effect structural capacity of the wall.
Relatively wide shrinkage cracks can also occur. However, a wide crack indicates potential problem with excessive lateral earth pressure or settlement. Such crack may have started as a narrow shrinkage crack and then expanded due to force resisted by the wall.
American Concrete Institute (ACI) provides detailed specifications and standards for design of concrete foundation walls.
Use of precast concrete foundation walls has been increasing due to benefits including reduced construction time at the site. However, the building designer remains responsible for ensuring that the precast wall has adequate lateral support, especially along top of wall. These walls are designed (by the manufacturer) to span in the vertical direction only.
Precast wall manufacturers tend to recommend that the wall be placed on a "footing" of crushed stone only. Although this method of support has been shown to be adequate for lightweight buildings, careful consideration must be given to providing adequate width and thickness of stone to ensure that load from foundation wall is well-distributed to soil under the stone.