The Gautrain Rapid Rail Link (GRRL) is one of ten Spatial Development Initiative (SDI) projects implemented by the Gauteng Department of Public Transport, Roads and Works (Gautrans) to stimulate economic growth, development and employment opportunities in the Gauteng Province. As part of this development initiative, Viaduct 5c (a 3.2 km long viaduct) crosses highly variable and sinkhole prone dolomitic ground conditions in Centurion, Pretoria. The original design for piers 39 and 40 of Viaduct 5c consisted of four large diameter (1.2m) bored piles designed to be end-bearing onto the dolomite bedrock. Due to severe difficulties encountered with the installation of large diameter end-bearing piles and the highly variable rock head and ground conditions encountered, an alternative founding solution at piers 39 and 40 was proposed. The alternative solution consisted of a grid of small diameter piles (micropiles) installed as soil reinforcement elements overlain by a compacted soil mattress. As bearing capacity failure of the footing is unlikely, settlement or differential settlement of the footing was the main concern. Due to the complex and variable nature of dolomitic profiles in general, settlement of the foundation was estimated using a 3D finite element model. The analysis indicated that the micropile/soil mattress system was effective in reducing the settlement of the foundation to an acceptable value. This paper presents the design methodology and analysis that was carried out for the two Piers.