How Thick Must a Rubble Stone Wall Be? A Stability Bound by Height and Mortar

How thick a rubble stone wall must be is usually answered by habit: a fixed proportion of the height, the same for a mortared wall as for one laid dry. This paper replaces habit with a computation. Four stability checks — overturning, sliding, the position of the resultant force at the base, and the compressive stress at the toe — are applied to rubble walls from 1.0 m to 6.0 m high, under a stated wind pressure of 0.80 kN/m², in mortared and in unmortared (dry) construction, both free-standing and carrying a stated centred roof load of 12.0 kN per metre run. The governing check is not overturning or sliding but the position of the resultant: a dry wall, which can take no tension, must keep its resultant inside the middle third of its base, and that single condition sets almost the whole dry table. The rule originated here gives minimum thicknesses of 0.40 m at 1.0 m height, 0.64 m at 3.0 m, and 0.905 m at 6.0 m for a free-standing dry wall, against 0.30 m, 0.41 m, and 0.58 m for the same wall mortared. A centred roof load thins both, to 0.535 m (dry) and 0.33 m (mortared) at 3.0 m. Taking 0.90 m as the greatest thickness that remains a practical single wall, an unmortared free-standing rubble wall stops working at a height of 5.95 m; at the more common 0.60 m thickness it stops at 2.65 m. This study is simulated, not measured, and no wall described here has been load-tested; the thicknesses are computed bounds under the stated assumptions, with the safety factors used printed openly.