The importance of masonry arch bridges to the transport infrastructure throughout Europe is unquestionable. However with the ever increasing axle loads present on today's roads, and new European directives increasing the required load carrying capacity, the need for an accurate and reliable method of arch bridge assessment has never been more important. The current methods of arch bridge assessment have been shown to be conservative resulting in unnecessary and costly repair work or replacement of structures. The research described in this thesis is an element of an on-going study into soil-structure interaction, a very important factor for the accurate assessment of arch bridges. This overall investigation, continuing study into the effects of soil-structure interaction, is undertaken with a variety of methods and techniques. These include model bridge testing investigating different load types and fill heights, monitoring of full scale structures and subsequent analysis investigating thermal effects, comparison of the present arch bridge assessment methods linked with an actual arch bridge assessment program and the development of new numerical modelling models of arch assessment. Thus an investigation using discrete element analysis methods has been conducted, with comparison with finite element methods and physical models.