X-ray radiography imaging has been used to investigate the formation of the grain structure during directional solidification of multi-crystalline silicon (mc-Si). A unique experimental apparatus has been developed and allows following the evolution of the solid/liquid interface and describing its features accurately. In the present paper, grain boundary groove evolution mechanisms during solidification have been studied. Their shape and evolution revealed grain competition phenomena and were drastically modified by the occurrence of new grains or twins. Moreover, the grain boundary groove repeatedly deepens and shrunks during the experiments, with different behaviours depending on the groove type. We studied two groove types in detail: facetted on both sides, or rough on one side and facetted on the other. In the case of grain boundary grooves facetted on both sides, two mechanisms were observed. In the first situation, both facets that form the grain boundary groove grow at the same rate. The grain boundary followed the bisector of the angle of this groove during the progression of the interface during solidification. In the second situation, a small grain or a twin nucleated inside the groove. This new grain entered into competition with previously existing grains and modified the evolution of the grain boundary groove in a manner depending on the crystallographic orientation of the new grain. In the case of a rough/facetted grain boundary groove, the grain boundary followed the facet. (c) 2013 Elsevier B.V. All rights reserved.