Reduced Modulus Action in U-Section Steel Sheet Pile Retaining Walls

U-section steel sheet piles are used for constructing retaining walls and they are connected together to form continuous walls using sliding joints located along their centerlines. Interpile movement along these joints can, in theory, reduce strength by 55% and stiffness by 70%, in comparison with the performance of piles in which no slip occurs (full composite action). This problem of interlock slippage is known as reduced modulus action (RMA). Despite the potential for this problem, it is common practice in many countries to ignore RMA in design, although the exact conditions governing when it becomes a design issue are not fully understood. This paper presents results from an investigation into this problem using experimental tests carried out using miniature piles. Unlike previous studies these tests were carried out using a similar load arrangement to that found in practice. The investigation indicates that the loading configuration affects the development of RMA and that friction between pile interlocks has the potential to mitigate much of the effect of RMA. A numerical model simulating the tests was developed and it has been used to model full-scale piles. The study indicates that many commonly occurring forms of steel sheet pile walls are unlikely to exhibit significant problems from RMA and this is relevant to pile design using Eurocode 3: Part 5.