| For most families, owning a home is considered the major investment of their lifetime. Because this asset is so important to continued enrichment of their lives, maintenance requirements that result from home foundation problems should be addressed prior to their acquisition. Unfortunately, many house foundations are neither built to meet challenges of their expansive soil environment nor built to address moisture changes from various seasonal fluctuations. As a result of design and construction deficiencies, foundation failures may occur on a regular basis. In fact, the Department of Housing and Urban Development estimated foundation damage caused by expansive clay soil at {dollar}9 billion per year in 1981 (Jones 1981), which would make it more destructive than any other natural disasters including tornados, hurricanes and earthquakes. More recent estimates by Witherspoon (2000), based on data collected from foundation repair contractors, places this damage total at an approximate {dollar}13 billion per year. This cost is enormous and explains the severity of concerns for home owners situated in expansive clay zones.; Since these foundations must be repaired when they fail, a large industry has grown to address the needs of homeowners everywhere. Although a foundation may rise or fall, the dominant cause of problems is foundation settlement as the clay consolidates under the load of the foundation or the clay swells and shrinks after wetting and drying. To address these problems, a repair contractor frequently will install underpinning under the perimeter of the foundation and lift the low segment(s) back to a more desirable elevation position and thus prevent the distressed segment from dropping in the future. To address the interior, a foundation repair contractor will either place underpinning under interior grade beams or lift the slab with a process called mudjacking that first fills voids and then lifts the slab with injection pressure from the grout flow. By filling voids, the load of the slab will not shift to foundation contact areas where pliable clay soils may deform over time and allow settlement of the interior slab.; Underpinning techniques, using drilled shafts have received extensive testing and research to develop standards for design and practice, whereas other methods such as hydraulically pressed piles have not been researched to determine proper design or construction practices. Since underpinnings are predominantly used to lift the distressed foundations, it is important to understand their axial load transfer mechanism. Uplift issues are not focused in this research since these are deep foundation types and can go beyond the active depth of a test site. This research is an attempt to conduct comprehensive field studies on a variety of underpinning methods in order to understand axial load capacity of each underpinning technique at different weather conditions. Tests were conducted on each of the underpinning methods installed in expansive clays where skin friction transfer along the foundation length is the dominant load carrying mechanism.; Underpinnings including drilled straight shafts, drilled belled shafts, augercast piles, helical piers, pressed steel and pressed concrete piles were constructed utilizing the assistance of contractors who have been working with these underpinning methods. Standards of practice were followed to test the underpinning in a manner that would be fair, uniform, accurate and representative of actual site conditions. Final axial load capacity was measured by performing tests on foundations following ASTM D1143.; Actual practices for the helical anchors, pressed concrete and pressed steel pilings were reviewed to establish drive pressures available (pressed pilings) and torque drive (helical piers) for a normal residential underpinning project. In the case of drilled shafts, the normal active depth requirement for the local site expansive clay was determined, and the drill... |
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