A Corduroy Road over Soft Ground: Load Spreading, a Log Sizing Rule, Service Life, and the Speed the Surface Allows

A corduroy road — logs laid crosswise, side by side — is one of the oldest answers to ground too soft to carry a wheel, and one of the least quantified. This paper computes what the log layer does, how large the logs must be, how long the road lasts, and how fast anything may travel on it. The load-spreading model gives a simple sizing rule: with logs touching, a wheel load is shared by the logs within four diameters along the road and spread along each log over 8.8 diameters, so mean soil pressure is the wheel load divided by 35.2 times the square of the log diameter, and the required diameter is the square root of the wheel load divided by 35.2 times the allowable soil pressure. For a 20 kN wheel load on soft peat with an allowable pressure of 22.5 kPa, the required diameter is 0.159 m; a standard 0.25 m log produces a soil pressure of only 9.1 kPa, a safety factor of 4.95 against the ultimate bearing capacity, and even a 35 kN wheel load needs only 0.210 m. Bending in the logs does not govern: stress is 1.0 MPa against a green-timber strength of 35 MPa. What governs service life is settlement. Of three stated life limits — loss of diameter until soil pressure reaches the allowable value, growth of surface undulation to a 45 mm serviceability limit, and bending failure under decaying strength — undulation arrives first in both climates modelled: the road must be re-laid after 15.0 years where the ground stays waterlogged and after only 6.0 years where it drains. The ride limit follows from the same undulation: 16.2 km/h for comfort when new, falling to 9.4 km/h at the end of life. On annualised cost, corduroy with on-site timber over wet ground, at 633 dollars per kilometre per year, beats a raised gravel causeway at 1,267 dollars; corduroy with hauled-in timber costs 1,467 dollars and corduroy over drained ground 1,583 dollars, and in both of those cases the causeway is the better purchase. Corduroy is therefore the right road in a narrow, statable set of conditions — wet ground that stays wet, timber at the roadside, light and slow traffic — and the wrong road outside them.