Uncertainties in three dimensional group pile analysis are studied in this thesis. Firstly, sensitivity analysis is performed to find the uncertain parameters by which group piles are seriously affected. And secondly, uncertainties in group pile analysis are evaluated and overall error in group pile analysis is obtained by using Rosenblueth's Two-point estimates.
The Hybrid model, which uses p-y and f-z curves for single pile analysis and uses Mindlin equations for the consideration of pile-soil-pile interaction, is used for this study. The following five parameters are selected as uncertain items: (1) undrained shear strength ($C_u$), (2) constant of Young's modulus of soil, β in the equation $^uE=\beta C_u$, (3) p-y curve, (4) f-z curve, (5) interaction effect by Mindlin equation. Results of sensitivity analysis show that the parameters which greatly affect the group pile behavior are soil properties ($C_u,$β). Moreover, it is revealed from the uncertainty analysis of group piles that the variations of maximum shear force and bending moment of piles due to the above uncertain items are small; those of the displacements of pile cap are large. It is also found that the displacements are much more sensitive to the above items when the input values smaller than mean values are used in the analysis, causing larger diplacements than expected.