Settlement of shallow foundations due to rise of water table in granular soils

Shallow foundations are designed to limit settlements within tolerable limits. Rise of water level due to seasonal changes produce additional settlements of footings resting on granular soils and can threaten the integrity of the structure. Effect of water table rise on shallow foundation settlement was addressed by various researchers. Terzaghi's intuitive suggestion was to double the predicted settlement in dry sand to get the settlement in submerged condition. Analytical, experimental and numerical works by others were aimed at developing a correction factor to account for the effect of water table rise on foundation settlement. The objective of this study is to investigate the effect of water level rise on shallow foundation settlement in granular soils through numerical, analytical and experimental studies. This study revisits Schmertmann's (1970) strain influence factor diagrams and modified influence factor diagrams for footings of various shapes (strip, circular, square, rectangular) are proposed using linear elastic models in FLAC and FLAC³ᴰ. Then a rational method is proposed, based on proposed strain influence factors, to predict the additional settlement produced by the rise of water table on a footing resting on sands. The proposed method is validated by extensive laboratory test data where model footings of five different shapes were loaded in sand placed at two relative densities, where water level was raised from the bottom while the additional settlements were measured. This study also investigates how the additional settlement due to submergence is affected by various soil parameters. Nine different granular soils with wide range of variety in grain size distribution, uniformity and void ratio range were used for laboratory model study, and the results were analysed to determine the effect of different soil properties on settlement in submerged condition. Effect of fines content on settlement increment was investigated and additional settlements in loose and dense sands were compared. Particle shape parameters of the nine soils were determined by analysing microscopic images of soil grains and effect of particle shapes on additional settlement due to submergence was studied. The study undertaken has also used explicit finite difference code FLAC and FLAC³ᴰ to simulate the rise of ground water table in granular soil and the resulting additional settlement was studied. The numerical results were compared with the laboratory test data and the proposed rational method for water table correction factor prediction. Elastic, nonlinear elastic and elasto-plastic constitutive models were used to investigate the variation of water table correction factor with water table depth. Effect of various parameters (footing embedment depth, Pois[s]on's ratio, finite layer thickness and layered soil system etc.) on additional settlement due to water level rise is also discussed in this study. The results obtained in this study will be valuable in understanding effect of different soil parameters and ground conditions on additional settlement that might occur as a result of water table rise. The rational method proposed herein will be useful for design engineers in predicting settlement correction factor for water table rise in granular soils.