The conventional design methods for sheet-pile cellular cofferdams were developed in the 1940's and 1950's based on field and limited experimental observations. The analytical techniques of the day were unable to account for the complexities involved. The procedures used only rudimentary concepts of soil-structure interaction which do not exhibit the true response of the cofferdam for most circumstances. During the past decade it has been demonstrated that with proper consideration of the soil-structure interaction effects, the two-dimensional finite element models can be powerful tools in the investigation of cellular cofferdam behavior. However, universal implementation of the findings of these analyses was difficult to justify, since uncertainties remain about the assumptions made in arriving at the two-dimensional models. The only way to address these uncertainties was to perform a three-dimensional analysis. This investigation has focused on the study of the three-dimensional behavior of Lock and Dam No. 26 (R) sheet—pile cellular cofferdam. The work involved the development of a new three-dimensional soil-structure interaction finite element code for cellular cofferdam modeling, and the application of the new code to the study of the behavior of the first- and second-stage cofferdam at Lock and Dam No. 26 (R). The new code was used to study the cell filling process where the main cell is filled first with the subsequent filling of the arc cell. The finite element results show that interlock forces in the common wall were 29 to 35 percent higher than those in the main cell which are less than those calculated by conventional methods and compare well with the observed values. After cell filling, the new code was used to model the cofferdam under differential loading due to initial dewatering of the interior of the cofferdam and changes in river levels. The finite element analysis results show that increasing differential water loads cause the confining stresses in the cell fill to increase which results in a decrease in the level of mobilized shear strength in the cell fill. This explains why the cellular cofferdam can withstand extremely high lateral loads and lateral deformations without collapsing. / Ph. D.