<p>The undrained bearing capacity of skirted shallow foundations is a key factor in the design and development of marine structures, such as offshore renewable energy installations, oil and gas platforms, and marine bridges. This study introduces an innovative approach by exploring the 3D failure envelopes for skirted foundations installed in non-homogeneous clay resting on a rigid base, subjected to combined loading conditions. These foundations experience horizontal, vertical, and moment loads under various marine conditions, making an accurate understanding of their behavior crucial. Using numerical analysis, this research applies the average of the upper and lower bound theorems to evaluate the bearing capacity of skirted strip foundations. To enhance precision, adaptive mesh refinement is employed to model the interaction between the heterogeneous clay and the rigid base. The study thoroughly investigates the influence of key factors, such as the depth of the rigid base, the degree of soil heterogeneity, and the skirt depth. Results are presented in terms of bearing capacity factors, including Nc′, Nc, and the inclination factor Ic. Most importantly, this research introduces novel 3D failure envelopes that provide a comprehensive visualization of the interactive effects of these variables, offering new insights into the complex behavior of skirted foundations in non-homogeneous clay under combined loads. This contribution fills a gap in the existing literature, paving the way for improved design practices in marine geotechnical engineering.</p>