Local effects of bonding on the strength of drystone masonry in Iron-age brochs

Collapses in drystone prehistoric Scottish broch towers probably started from localised failures at the supports of roofs or decks, settlements, ill-conceived modifications or gradual decay producing long-term accumulation of debris. Another source of instability is their precarious state during excavations. Some of these effects have been simulated as excessive lateral thrust or settlement and are studied here through experimental analysis on wallettes in 1/15 scale, expanding on earlier insight from complete broch models tested to settlement. Wallettes in typical bonds were tested following a parametric study on the effect of thickness, bond and architectural features (openings, corbelled chambers, uneven supports). Failure patterns at overturning and settlement were established triggered at a minimum lateral displacement of one-fifth of the wall thickness. The walls could resist a lateral earth pressure representing debris accumulation at least five times their active earth pressure and resistance to settlement was by arch formation at the base.