Sonic drilling has been carried out in the upstream support fill zone of the 101 m (332 ft) high and 1800 m (5900 ft) long Messaure embankment dam located in northern Sweden. The purpose was to investigate the upstream fill material and obtain samples to perform laboratory tests for further geotechnical characterization of the material. The support fill material zone consists of cobbles, gravel, and sand of hard rock types, and modification of the drilling equipment to suit the purpose of the drilling was required. A Sonic drilling rig was identified in Europe, which could drill holes inclined and recover samples in steps of up to 178 mm (7") diameter and 3 m (10 ft) length. Holes were by this method drilled, cored and cased by vibrations combined with rotation at high power resonant sonic frequencies, without adding fluid that disturbs the quality of the recovered material. To make the sampling of the coarse-grained friction material successful, a new catcher for the sampling tube (core barrel) was constructed. An anchor plate was developed, to prevent the casing from drifting and the process to mount and remove the casing was adapted. After a process of repeated trial drillings in a test area, during which the equipment was gradually developed to meet the requirements, the drill rig was moved to site. Drilling was conducted in the upstream direction from the crest at full reservoir level. Samples of the complete gradation of the supporting fill could successfully be recovered from up to 45 m (150 ft) long holes at an angle of 43-45° from the vertical axis. The retracted casing left a 279 mm (11") diameter hole that was backfilled during closely monitored conditions. The grout mix was balanced to avoid excess penetration and grout loss into the fill, at the same time as hydraulic fracturing had to be avoided in the drill holes close to the slope. This paper describes the drilling investigation and the used equipment, the drilling conditions and the experiences gained.