The computer program SETOFF analyzes foundation settlement of both, shallow and deep foundations, using commonly-accepted procedures. The total settlement of a foundation is generally composed of two parts, elastic and consolidation settlement. Elastic settlement occurs because of the pseudo-elastic nature of most soils and it occurs immediately on application of the foundation load. Consolidation settlement takes place as the pore space in the soil is reduced under the foundation loading and it may require a period of time to be fully developed. The elastic settlement may not be important because it takes place during construction as the structural loads are added. Because of this, some compensation for the elastic settlement may take place during construction. This does not mean, however, that elastic settlement should be overlooked.
For the above criteria, SETOFF computes the settlement under 100% consolidation (without elastic settlement that occurs during construction, which can be calculated by other programs like APILE and SHAFT), but will not provide the information for the percentage of consolidation versus time.
The computations in SETOFF follow a conventional settlement analysis in which the soil profile is divided into a number of layers, the average stress increase from all of the foundation-loaded area is determined for each layer, and the change in thickness for each layer is computed using the appropriate compressibility for the layer. The foundation settlement is the sum of the changes in the layer thickness.
Analyses of consolidation settlement may be divided into three parts. The first part is the determination of the soil stratigraphy and the representative properties of the soil in each stratum. The second part is the computation of the stress increase at pertinent points in the subsurface soils due to the foundation loading. The third part is the computation of settlement using the data from the first two parts. The computer program SETOFF will perform the last two parts.
Foundation settlement is always very specific. The actual settlement observed in the field will depend on actual foundation loads and soil conditions and not on values assumed for design. The foundation loading used should be the actual sustained loads and not the maximum design loads. If the rebound from an excavation is to be computed, the input soil compressibility should adequately represent the action of the soil under reducing stress as well as increasing stress; this may not be the unmodified results from consolidation tests.