Drystone retaining walls: Ductile engineering structures with tensile strength

Drystone retaining walls are sustainable engineering structures constructed with locally obtained natural stone. They were commonly built with very slender profiles compared with modern mass-masonry structures, leading to a common belief among engineers that they have very low margins of safety. These structures remain critical to the transport infrastructure in many parts of the world, and have proven to be very durable, yet very few new drystone retaining walls are built, and walls which do fail are usually replaced with concrete constructions. We show that these walls are ductile even though their components are brittle, and have tensile strength through the interlocking of their stones, even though they are assembled without any cohesive material such as mortar. These properties are critical to a proper understanding of their behaviour and durability. Full-scale testing of five drystone retaining walls has shown that bulging, most commonly regarded as a sign of incipient failure, begins as a ductile adaptation of the geometry to the loads imposed on it. Localised bulging can be a consequence of small defects in construction or foundation conditions, or concentrated loading, and may be sustained indefinitely in a wall which is in general well-constructed. These insights into the behaviour of walls allow the design of new walls which use materials efficiently, and enable existing walls to be kept in service, and may inspire new ways of achieving ductility in engineering materials.

► Five full-scale drystone retaining walls were built and tested to destruction.
► We show that such walls are ductile even though their components are brittle.
► They have tensile strength through frictional interlocking.
► Bulging begins as a ductile adaptation of the geometry to the imposed loading.
► This work provides further insight into the behaviour of drystone walls.