The 12 bored piles for the 2^# pier of the Dashu Dadu River Bridge in Hanyuan county have a diameter of 2500mm and a depth of 63~75m， posing some construction difficulty due to large diameter and great depth and drilling in massive sandy gravel formation below the river water level. There are multiple layers of sand and pebble soil overlying the bedrock at the 2^# pier， and the sand layer is very thick. Sandy soil prevails between 11.5~49.4m except for a layer of 5.5m thick gravel cobble with soil inclusion， which is very disadvantageous to drilling of pile boreholes. Percussion drilling was used to sink the holes with the mud purification device to remove cuttings； meanwhile， some technical measures were adopted to ensure successful completion of the works， such as control of the bit diameter， use of the inner casing， control of the tremie pipe buried depth during concreting. The relevant technical measures can provide reference for other similar projects.