This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
On account of the unsatisfactory nature of theoretical calculations, retaining walls are usually built by the application of purely empirical rules. Trautwine recommends that for a wall of cut stone or of first-class large ranged rubble in mortar, the thickness should be .35 of its vertical height. For a good common mortar rubble or brick, the thickness should be .4, and for a dry wall .5, of the height. Military engineers who have a very extensive experience in constructing retaining walls as a feature of fortification work, use a rule giving much less thickness than this, and make it depend on the batter of the wall. The thickness at the base in proportion to the height, is as follows:
Gen. Fanshawe's Rule for Thickness of Retaining Walls
Fig. 68. Variation of Intensity of Pressure on Base.
Base ÷ Height
The fact that experience has shown that the above proportions are usually safe, provided that the subsoil is sufficiently hard, is another proof that the assumptions made in the problem worked out above are excessively safe, since Fanshawe's rule would have required a ratio of base to height of only 24 per cent, while the ratio chosen for the problem was 40 per cent.