The assessment of masonry arches has gained significant importance due to the substantial reliance of transportation infrastructure on historic structures. Over the past few decades, there has been a discernible shift in the focus towards understanding the interaction between the soil fill and the arch, thus forming a composite system. While much attention has been devoted to advances in faster numerical methods, it remains crucial to elucidate the mechanics of the problem from fundamental principles. The current paper concentrates on the ultimate limit state of the soil-arch system under plain-strain conditions. Insights have been gleaned from recent experimental endeavours involving full-scale physical models with backfill under laboratory conditions, complemented by numerical modelling work. These findings have been synthesised into a series of modelling assumptions, along with equilibrium conditions, to establish an analytical framework and procedure. This framework offers a means to assess the ultimate limit state capacity of the system based on well-understood mechanical principles.