The foundation of a building transfers the dead and live loads of the superstructure to the soil that has enough bearing capacity to support the structure in a permanent, stable position. Footings are used under foundation components, such as columns and piers, to spread concentrated loads over enough soil area to bring unit pressure within allowable limits. Foundation design is determined not only by the weight of the superstructure but also by occupancy or use of the building or structure and by the load-bearing capacity of the soil at the site. The latter conditions may change and introduce maintenance and repair problems even in initially well-designed foundations.
Foundations should be inspected at least annually and more often where climate, soil conditions, or changes in building occupancy or structural use present special problems. Evidence of initial foundation failure may be found during routine inspection of other structural components.
A foundation should be checked regularly for proper elevation and alignment. Complete failure in a foundation is rare; however, some settling or horizontal displacement may occur.
Some common causes of foundation movement include the following:
The structure is overloaded.
Excessive groundwater that reduces the bearing capacity of the soil.
Inadequate soil cover that fails to protect against frost heaving.
Adjacent excavations that allow unprotected bearing soil to shift from under foundations to the excavated area.
Some indications of localized foundation displacement are:
Damaged framing connections
Leakage through a displaced roof
Corrective actions that can be taken to alleviate foundation displacement include the following:
Replace any missing or dislodged part of the foundation immediately.
Repair cracks or open joints in concrete or masonry foundation walls.
Replace defective wood members.
Replace unstable fill around the foundation with clean properly compacted fill.
Remove growing roots of trees or shrubs that may dislodge footings or foundations.
Increase bearing area of inadequate footings.
Maintain enough soil cover to keep footings below the freezing zone.
Prohibit loads from exceeding the design loads of buildings and structures.
Isolate foundations from heavy machine operations by providing independent footings and foundations for heavy machines.
Air-conditioning equipment, cooling towers, and compressors should be provided with cork or rubber isolation mounts to prevent transmission of vibrations to the structural frame of the building.
When excavations are made near the footings of buildings, care must be taken in removing bearing soil under existing structures. Temporary stabilization can be gained by shoring, underpinning, or needling to relieve pressure of the footings on the soil. Sheetpiling may be driven and supported laterally to contain the bearing stress in the soil under the footings.
When water erosion removes soil from around and under footings some means of erosion prevention, such as ditching or the use of splash blocks, must be used.
Footings that fail because of insufficient bearing area must have their bearing area increased. The amount of movement in the wall dictates the repairs necessary. Minor settlement, especially when uniform, may require no repair. If serious settlement occurs, the wall may have to be jacked back to its original elevation, a new footing provided, and repairs made to the wall.
Improved drainage is the basic solution to the most common groundwater problems (fig. 7-1). Moisture in structures caused by a high water table can be drained away from a foundation by the installation of open-joint drain tiles surrounded by loose gravel fill. The drains should be laid so as to drain the water away from the footings and into a sump with aContinue Reading