The paper presents a numerical analysis of piled rafts containing one defective pile. First part of the paper focus on the influence of interaction factors. Attention is paid to the influence of the rigid base depth and other factors (relative spacing of the piles, slenderness ratio, etc.) on two commonly used methodologies. The impact of the interaction factors change on the foundation system is discussed on the examples of the total and differential settlement. Second part of the article discusses the impacts of having a defective pile in the group to the behaviour of the raft and potentially to the entire upper construction. As the possible defects of piles have been already described in past publications, the major aim of the first part is to show the actual changes in settlement and pile load for two of the most common defects, i.e. piles with a lower both stiffness and length than designed. The analysis assumptions already take into account partial conclusions of the firs part.