Seismic analysis and design of pile foundations in weak soil strata to support oil tanks are of great importance to geotechnical practitioners. This paper presents an actual field study and dynamic soil–structure interaction (DSSI) analysis for an oil tank foundation design in weak soil strata for the Kafza site in Iraq, which is located in a seismically active region. For the particular in situ soil profile, a pile group foundation is proposed comprising a 24·1 m dia. by 1·5 m thick pile cap connecting together 89 piles, each pile being 800 mm in diameter and 26 m long. The foundation system was modelled using the finite-difference-based geotechnical program Flac3D to carry out detailed DSSI analysis. Field pile load test data for a static analysis were used for validation of the Flac3D model. The outcome of a probabilistic seismic hazard assessment was used to estimate target input motions resulting in maximum bending moments of 114 kN.m and maximum settlement of 21·9 mm for piles under dynamic conditions. The results obtained, comprising total and differential settlements and bending moments, were used for the final design of the proposed pile foundation and can be used for similar practical applications.