Heavy vibratory plate compaction is an efficient technique for improvement of granular soils. However, its practical application is limited by insufficient knowledge about the compaction process and the optimum procedure of execution. In this study, compaction tests were conducted in the field with a 40.5-ton heavy vibratory plate compactor. The site consisted of partially saturated, overconsolidated medium-dense silty sand and sandy silt. Such conditions can generally be characterized as difficult for vibratory compaction. The objective of the tests was to study the compaction depth and the influence of treatment layout, duration of compaction, and vibration frequency. In-situ measurements conducted before, during and after treatment included static cone penetration tests, flat dilatometer, and heavy dynamic probing. The in-situ tests showed a statistically significant compaction after treatment. The main conclusions highlight the importance of a sufficient centrifugal force of the vibratory hammer to cause disconnection between the plate and the soil. Other important parameters are the vibration frequency and the duration of treatment, which can be optimized by field monitoring. Full coverage of the ground surface is required for compaction of the entire soil volume. It was also found that the compaction depth corresponds to the side length of the plate, in line with previous studies.