This paper presents an environmentally-friendly method, developed and validated by the authors, for economic design and optimisation of retaining walls in narrow trenches. The method considers the effects of overlapping passive zones and introduces an intensification factor as a design optimisation tool. First, a comprehensive review of the literature discussing different parameters affecting passive zone failure was conducted and briefly summarised. Then, numerical and analytical approaches were employed to validate the proposed method. Applying the proposed design factor, reduces the sheet pile length, the forces on retaining wall, the amount of steel, the settlement of adjacent structures, the additional charges as well as the required equipment and the environmental nuisance.