Masonry foundations and piers
Inspect stone, brick, concrete, or concrete block foundations for signs of the following problems (this may require some digging around the foundation.
Editor's note: This story is adapted from the U.S. Department of Housing and Urban Development's Residential Rehabilitation Inspection Guide, 2000.Click here for other stories in this series.
Parts of this story: Introduction ~~ Seismic and wind resistance ~~Cracking and deterioration of masonry, general issues ~~ Masonry foundations & piers~~Above ground masonry walls ~~Chimneys ~~Wood structural components ~~Iron and steel structural components ~~Concrete structural components
Problems associated with differential settlement
Uneven (differential) settlement can be a major structural problem in small residential buildings, although serious settlement problems are relatively uncommon. Many signs of masonry distress are incorrectly diagnosed as settlement-related when in fact they are due to moisture and thermal movements.
Indications of differential settlement are vertical distortion or cracking of masonry walls, warped interior and exterior openings, sloped floors, and sticking doors and windows. Settlement most often occurs early in the life of a building or when there is a dramatic change in underground conditions. Often such settlement is associated with improper foundation design, particularly inadequate footers and foundation walls.
- Soil consolidation under the footings
- Soil shrinkage due to the loss of moisture to nearby trees or large plants
- Soil swelling due to inadequate or blocked surface or house drainage
- Soil heaving due to frost or excessive root growth
- Gradual downward drift of clay soils on slopes
- Changes in water table level
- Soil erosion around footers from poor surface drainage, faulty drains, leaking water mains or other underground water movements (occasionally, underground water may scour away earth along only one side of a footer, causing its rotation and the subsequent buckling or displacement of the foundation wall above)
- Soil compaction or movement due to vibration from heavy equipment, vehicular traffic, or blasting, or from ground tremors (earthquakes).
Gradual differential settlement over a long period of time may produce no masonry cracking at all, particularly in walls with older and softer bricks and high lime mortars; the wall will elastically deform instead. More rapid settlements, however, produce cracks that taper, being largest at one end and diminishing to a hairline at the other, depending on the direction and location of settlement below the wall.
Cracking is most likely to occur at corners and adjacent to openings, and usually follows a rough diagonal along mortar joints (although individual masonry units may be split). Settlement cracks (as opposed to the similar-appearing shrinkage cracks that are especially prevalent in concrete block) may extend through contiguous building elements such as floor slabs, masonry walls above the foundation, and interior plaster work. Tapering cracks, or cracks that are nearly vertical and whose edges do not line up, may occur at the joints of projecting bay windows, porches, and additions. These cracks indicate differential settlement due to inadequate foundations or piers under the projecting element.
Often settlement slows a short time after construction and a point of equilibrium is reached in which movement no longer occurs. Minor settlement cracking is structurally harmful only if long-term moisture leakage through the cracks adversely affects building elements. Large differential settlements, particularly between foundation walls and interior columns or piers, are more serious because they will cause movements in contiguous structural elements such as beams, joists, floors, and roofs that must be evaluated for loss of bearing and, occasionally, fracture. Should strengthening of the foundation be required, it can be accomplished by the addition of new structural elements, such as pilasters, or by pressure-injecting concrete epoxy grout into the foundation wall. If movement continues and cracking is extensive, it is possible that the problem can only be rectified by underpinning.
Older buildings with severe settlement problems may be very costly to repair. Seek the advice of a structural or soils engineer in such cases.
Problems associated with masonry piers
|This pier has been overstressed by movement of the porch and column. The entire assembly should be rebuilt.|
Masonry piers are often used to support internal loads on small residential buildings or to support projecting building elements such as bay windows, porches, and additions. In some cases they support the entire structure. Piers often settle differentially and over a long period of time (particularly when they are exposed to the weather) they tend to deteriorate. Common problems are:
- Settlement or rotation of the pier footing, which causes a lowering or tilting of the pier and subsequent loss of bearing capacity. Wood frame structures adjust to this condition by flexing and redistributing their loads or by sagging. Masonry walls located over settled piers will crack.
- Frost heaving of the footing or pier, a condition caused by the lack of an adequate footing or one of insufficient depth. This will result in raising or tilting the pier, and in structural movement above it similar to that caused by settlement or rotation of the footing. Such a condition is most common under porches.
- Physical deterioration of the pier due to exposure, poor construction, or overstressing. Above-ground piers exposed to the weather are subject to freeze-thaw cycles and subsequent physical damage. Piers for many older residential structures are often of poorly constructed masonry that deteriorates over the years. A sign of overstressing of piers is vertical cracking or bulging.
- Loss of bearing of beams, joists, or floors due to the above conditions or due to movements of the structure itself.
Piers should be examined for plumbness, signs of settlement, condition, and their adequacy in accepting bearing loads. Check their width to height ratio, which should not exceed 1:10. Those that are deficient should be repaired or replaced. When appearance is not a factor (as is often the case), piers can be supplemented by the addition of adjacent supports.
Cracking associated with drying shrinkage in concrete block foundation walls.
The shrinkage of concrete block walls as they dry in place often results in patterns of cracking similar to that caused by differential settlement: tapering cracks that widen as they move diagonally upward. These cracks usually form during the buildings first year, and in existing buildings will appear as old cracks and exhibit no further movement. Although such cracks are often mistaken for settlement cracks, shrinkage cracks usually occur in the middle one-third of the wall and the footer beneath them remains intact. Shrinkage cracking is rarely serious, and in older building may have been repaired previously. If the wall is unsound, its structural integrity sometimes can be restored by pressure-injecting concrete epoxy grout into the cracks or by adding pilasters.
Sweeping or horizontal cracking of the foundation walls
The sweeping or horizontal cracking of brick or concrete block foundation walls may be caused by improper backfilling, vibration from the movement of heavy equipment or vehicles close to the wall, or by the swelling or freezing and heaving of water saturated soils adjacent to the wall. Like drying shrinkage, sweeping or horizontal cracking may have occurred during the original construction and been compensated for at that time.
Such distress, however, is potentially serious as it indicates that the vertical supporting member (the foundation wall) that is carrying a portion of the structure above is bent or broken. It may be possible to push the wall back into place by careful jacking, and then reinforcing it with the addition of interior buttresses or by pressure-injecting concrete epoxy grout into the wall. If outside ground conditions allow, the wall can be relieved of some lateral pressure by lowering the ground level around the building. When expansive soils are suspected as the cause of the cracking, examine the exterior for sources of water such as bro-ken leaders or poor surface drainage. Suspect frost heaving if the damage is above local frost depth or if it occurred during an especially cold period.
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