Some of the advantages of skirted foundations which have extended their applications in offshore are: improvement of bearing capacity of foundation against vertical, horizontal loads and moments, better performance under cyclic loads, more accurate, faster, easier, and cheaper installation in comparison with other choices like piles especially in deep water, and sometimes the capability of uninstallation in order to use in another place. However, the use of such structural skirts in conjunction with conventional shallow structural foundations has not been widely employed, nor have the improvements in bearing capacity occurring from their use been investigated in detail. However, there are few researches on skirted foundation for inshore structures and they are limited to some special type of soils. Therefore more extensive and accurate studies on bearing capacity of these foundations and the influencing parameters are necessary to make them applicable for conventional structures. In this dissertation, the results of numerical analysis of surface foundations reinforced with skirts under vertical loads are presented. The factors which influence the bearing capacity of these foundations including soil strength and stiffness parameters, shape and width of foundation, foundation base roughness, skirt depth, skirt roughness, skirt thickness, skirt stiffness, numerical behavior model of soil plug and skirt arrangement are investigated and analyzed and by comparing the bearing capacity of skirted foundation with the bearing capacity of surface foundation without skirts, effect of using skirts was determined and presented as the bearing capacity ratio. Furthermore, the bearing capacity of skirted foundation was compared with ones of the foundation at the skirt depth in different of skirt roughnesses. Also, a modified bearing capacity equation is proposed for skirted foundations. Numerical analysis was carried out using the ABAQUS FEM software by explicit method. Loading wa conducted by displacement controlled method. Three shapes of foundations including circle, square and strip with rigid bases and two type of skirts including rigid and flexible were modeled. By displacing the connection point of the skirts from edge of foundation base to the inner positions in circle and strip skirted foundations and by adding the skirts in square skirted foundations, the effect of skirt arrangement on bearing capacity of foundation was studied. Changing the numerical model behavior of soil plug with elastic and rigid model separately, the effect of soil plug on bearing capacity was investigated. The results of analysis showed that using the skirts in the conditions of this work, increases the bearing capacity of surface foundations from 1.1 to 2 times, depending on soil and foundation properties.
