In NTPC’s one hydro project, out of network of underground galleries, one Gallery#6 is constructed at 21 m below Dam’s deepest foundation level. It has circular (3.0 m dia, 161 m long) cross-section with sub-horizontal base and concrete lining. High inflow was anticipated inside despite exhaustive grouting campaigns. Therefore, to understand the impact on the level of water table, contour level map of water table in the rock foundation was done. Through water table mapping, deficiency of its water tightness is worked out for additional measures. Water pressure tests were carried out to observe efficacy of the grouting. To meet the design requirement of allowed seepage limits, grouting was necessitated under prevailing geological conditions. Various chemical grouting operations were explored but not found suitable under prevailing water and pressure conditions. Therefore, a quick setting time grout mix with appropriate grouting methodology was formulated, in order to be able to grout rock and concrete in artesian conditions. With adequate contact and consolidation grouting, seepage along the walls of gallery was reduced, however, still some seepage was remaining from invert of gallery. Considering condition of invert and construction joints, 250 mm thick second-stage concrete was provided at invert. With above measures, seepage reduced to 350 L/min and water level inside the gallery was significantly reduced to manageable level. Foundation monitoring equipment such as leak measurement weirs and piezometers were installed at suitable locations for monitoring of underground conditions in operating phase. With adopted methodology and techniques, posed challenges in construction including WPT test, grouting and seepage control in Gallery#6, were overcome without affecting the scheduled completion.