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Foundation Failure
 

The Causes of Foundation Failure

Foundation movement may result from a wide range of factors, which can include:

•  Shrinking or swelling of clays caused by changes in moisture content
•  Compression of a soft layer in the ground as a result of the applied foundation loads
•  Soil softening
•  Frost heave
•  Improper back filling
•  Variation in groundwater levels
•  Erosion
•  Vibration from nearby construction
•  Hydrostatic Pressure

Inadequate design of basement walls and footings, before construction, traditionally account for 75 to 85 percent of all problems in residential structures built upon expansive clay soils. These failures are generally divided between two broad classes – lateral pressure and differential settlement. Both classes of failure generally have few primary causative factors.

Lateral pressures on basement walls have four likely sources:
•  Pressure from soil weight
•  Pressure from soil swell
•  Hydrostatic pressure
•  Pressure from frost

Identifying lateral pressure damage is not difficult, but accurately quantifying the contributing sources is very difficult and should only be handled by a qualified engineer. The inward bowing of a basement wall is the simplest indication of lateral pressure. The bowing generally occurs when the external forces exceed the wall strength. The maximum bowing will often occur near the center of the wall because the adjoining perpendicular walls provide support in the corners. If bowing becomes severe, these walls can collapse inward.

 
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Cracking can also occur when lateral pressure exceeds the strength of the concrete or block wall. The most common crack pattern begins in the corners and move up or down at 45 degree angles in concrete walls. For block walls, the cracks move along the mortar joints in a stair step pattern. Often these cracks end at a long horizontal fracture that parallels the basement floor.

 

Lateral pressure can affect the overall integrity of a house. Severe damage results in a visible opening between the top of the basement wall and the structure. Since water is one of the main causes of these cracks, water infiltration becomes significant in the largest of the cracks. Filling these cracks with epoxy, without solving the water problem, only moves the lateral pressure to another section of the wall.

The difference of the outside ground level and the basement floor creates a mass of soil that must be retained thus causing a lateral pressure. The pressure of soil weight is typically considered during the design of an engineered wall using theoretical earth pressures.

Clay soils undergo a change in volume when the moisture content of the soil changes. When expansive clays are placed against basement walls, the swelling of these soils can induce lateral pressures not accounted for in the original design. Cyclic shrink/swell can also reduce the shear strength of the backfill and thus increase the lateral pressures. The solution to this problem can be as easy as replacing clay backfill with gravel or other non-swelling material. When used in conjunction with a footing drain, gravel will prevent increased lateral pressure.

Hydrostatic pressure is pressure exerted by a fluid due to its weight. Hydrostatic pressure against a basement wall develops when water fills voids or “ponds” within backfill immediately adjacent to the wall. This water buildup can cause dripping, seepage, dampness or efflorescence (salt residual). Leakage during heavy rains or poorly designed/maintained drainage increase hydrostatic pressure. Like soil swell, hydrostatic pressure is not typically considered during design and construction of basement walls.

Water that accumulates in backfill and then freezes may cause large lateral pressures on basement walls. Severe damage can result from frost causing lateral pressures much greater than even hydrostatic pressure. The expansive natures of water crystals have been know to create catastrophic structural damage.

Backfill, that is heavily clay laden, present long term lateral soil pressure problems. Their cohesive nature makes it practically impossible to re-compact them to a uniform moisture content and density. Clay backfills require significantly stronger basement walls to withstand the larger horizontal pressures. The obvious solution is to backfill with non-cohesive aggregate with proper drainage.

Structural settlement is characterized as either total and /or differential settlement. Total settlement is a complete structure downward movement. Differential settlement is the difference in vertical movement between various locations causing structure distortion. Generally, total settlement is not a critical factor as long as it is uniform. Utility connections are affected to the greatest degree by total settlement. Even relatively small differential settlements can cause cracks in floor slabs, brick walls and drywall.

 
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Settlement can be tolerated in most homes provided it is within specified limits. Small amounts of settlements are anticipated in most design work. When homes experience excessive settlement special procedures must be employed to stop or limit the amount of settlement. These special procedures usually employ the use of resistance piers or helical anchors. A foundation engineer is recommended when implementing underpinning procedures.

 
 
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