The Pinalito main hydroelectric tunnel, with a length of 11 km, is located in Cordillera Central mountain range, Dominican Republic. This mountain tunnel has been mined through extremely fractured andesitic-basaltic and rhyolitic tuffs. These rocks are been subjected to shear and collisional displacements between Caribbean and North America plates since middle Eocene. As a result, geological and hydrogeological conditions along the tunnel alignment are rather complicated. This paper presents the main aspects and results employed in order to overcome an unexpected water bearing incompetent rock zone, approximately eleven meters wide, that cross the tunnel alignment at Sta. 5+237.40. Ground treatment was well succeeded and took six months to be concluded. Procedures adopted took into account three steps, respectively: (1) geological drilling; (2) drainage; and (3) spilling and grouting injections