Mud Brick and Stabilised Earth: Strength by Mix, and the Least Cement or Lime That Does the Work

Earth dug on site is the cheapest wall material available in much of the world, and also the least predictable, because two soils that look alike can differ by a factor of two in strength and by far more once rain reaches them. This paper originates a mix rule that predicts, from a soil's clay fraction alone, its unstabilised dry and wet compressive strength, its erosion rate under a stated spray test, and the least dose of cement or of lime — scanned in half-percent steps up to a stated practical cap of 8.0% by dry mass — that brings it past stated thresholds for one-storey walls (1.50 MPa dry, 0.65 MPa wet) and for two-storey walls (2.80 MPa dry, 1.10 MPa wet). The study is simulated, not measured, and no wall or brick described here has been load-tested. Under the stated assumption curves, drawn from published ranges for earth bricks, wet strength rather than dry strength decides almost every dose: unstabilised mixes with 0.8% straw fibre reach 0.96–1.92 MPa dry yet retain only a tenth of that when wet, 0.10–0.19 MPa. Cement is the cheaper stabiliser in sandy and loamy soils, needing 3.0% for two-storey loam against 5.5% for lime, while lime closes much of the gap in clay soils, needing 4.0% in clay loam against 3.0% for cement there. One soil class fails outright: expansive heavy clay at a clay fraction of 0.60 reaches only 2.30 MPa dry with cement and 2.57 MPa dry with lime at the 8.0% cap, below the 2.80 MPa two-storey threshold under either stabiliser, although it passes the one-storey thresholds at 3.5%. A second, quieter finding is that the dose which satisfies strength usually leaves the wall face eroding too fast for exposed use: loam at its two-storey cement dose of 3.0% still erodes at 23.6 mm per hour against a stated exposed-wall limit of 20 mm per hour, so strength and surface protection must be specified separately.