Posted by Verve on March 01, 2002 at 06:09:24:
The foundation is the lowest division of a building - its substructure - constructed partly or wholely below the surface of the ground. The primary funtion being to support and anchor the structure above and apply its its loads safely into the earth. Thus serving a critical link in the distribution and resolution of the building load, the foundation must be designed to both accomodate the fom and layout of the structure above and respond to the varying conditions of soil, rock, and water below.
The principal loads on a foundation are the combination of dead and live loads acting vertically on the structure. In addition, a foundation system must anchor the structure against wind-induced sliding overturning, and uplift, withstand the sudden ground movements of an earthquake, and resist the pressure imposed by the surrounding soil mass and groundwater on basement walls and in some cases a foundation system may also have to counter the thrust from arched or tensile structures
Settlement is the gradual subsiding of a structure as the soil beneath its foundation consolidates under loading. As a building is constructed, some settlement is to be expected as the loead on the foundation increases and causes a reduction in the volume of voids containing air or water. This consolidation is usually slight and occurs rather quickly as loads are applied on dense, granular soils, such as coarse sand and gravel. When the foundation soil is moist, cohesive clay. Which has a scale like structure and a relitively large perccentage of voids, consolidation can be quite large and occur slowly over a longer period of time.
A properly designed and constructed foundation system should distribute loads so that whatever settlement occurs is minimal or is uniformly distributed undr all portions of the structure. This is accoplished by laying out and proportioning the foundation supports so that they transmit an equal load per unit area t othe supporting soil or rock without exceeding it's bearing capacity.
Differential settlement - the relitive movemet of different parts of the structure caused by uneven consolidation of the foundation soil - can cause a building to shift out of plumb and cracks occur in it's foundation, structure or finishes. In extreme circumstances differential settlement can result in the failure of the structural integritry of a building.
Underpinning refers to the process of rebuilding or strengthening the foundation of an existing building, or extending it when a new excavation in adjoining property is deeper than the existing foundation.
An alternative to extending a new foundation wall and placing new footings is to construct piles or caissons on either side of the existing foundation, remove a section of foundation wall,and then replace the section with a reinforced concrete pile cap.
When a building site is sufficiently large that the sides of an excavation can be bench terraced or sloped at an angle less than the angel or repose for the soil, no supporting structure is necessary. When the sides of a deep excavation exceeds the angel of repose from the soil, however, the earth must be temporarily braced or shored until the permanent construction is in place.
My question is what form of foundation is best for use with differential settlement? the existing building in question is of small size and it would be a personal shame to see this building go to ruin and in a state of disrepair with so much interior potential.