A segmental arch constructed from concrete blocks is tested to failure as validation of a discrete automatic limit analysis technique. A mesh description of geometric compatibility, based on Kirchhoff's network laws, provides the model on which collapse is defined. A minimisation of the work rate of the parametric loads provides a prediction of the failure load and configuration of collapse through the use of a Linear Program. However, the predicted strength of the test structure is significantly overestimated. A detailed set of geometric measurements were seen to reveal small imperfections in the concrete blocks which when assembled create a geometrically flawed structure. The flexibility of the procedure allows for such imperfections to be incorporated into the model by manipulation of the limit conditions defined for each of the interfaces between blocks. The re-analysis gives good agreement with the observed results, highlighting the effect imperfections have on the strength of such a geometrically defined structure. The numerical analysis procedure is seen to be practical, and to provide an accurate prediction of strength.