Rammed Earth by Layers: Lift Thickness, Compaction Energy, and the Slenderness Bound

A rammed earth wall is built one shallow layer at a time, and its quality is decided by choices made hundreds of times a day: how thick each loose layer is spread, how much work is beaten into it, and how wet the soil is when the rammer falls. This paper originates a wall rule connecting those choices to the finished wall. Density is modelled as a function of compaction energy and of a depth-decay factor for each lift, strength follows density by a stated fourth-power relation, and a slenderness bound then converts strength and wall load into a maximum unsupported height for each wall thickness. The study is simulated, not measured, and no wall described here has been load-tested. For the reference soil at a standard hand-ramming energy of 350 kJ/m3, raising lift thickness from 100 mm to 200 mm lowers computed dry density from 1940 kg/m3 to 1801 kg/m3 and compressive strength from 2.12 MPa to 1.58 MPa; at 300 mm lifts strength falls to 1.30 MPa. Moisture has an optimum at 10.0%: at 5.0% or 15.0% moisture the same wall computes to 1.47 MPa instead of 1.98 MPa. The practical rule that emerges is lifts of 100 mm to 120 mm, a pour height of 0.60 m for the recommended 120 mm lift, and a moisture window of 8% to 12%. Crushing does not govern the height of the finished wall under the stated loads — the slenderness bound does: maximum unsupported height is 3.6 m at 300 mm thickness, 5.4 m at 450 mm, and 7.2 m at 600 mm, in each case set by the stated slenderness cap of 12 rather than by stress, so that a two-storey wall of 5.4 m requires at least 450 mm of thickness even though a 300 mm wall at that height would be stressed to only 0.61 of its reduced allowable stress.