A Treatise On The Principles And Practice Of Dock Engineering

174 DOCK ENGINEERING. Empirical Formulæ. —General Fanshawe’s rule was to inake the thickness of rectangular revetment walls of brickwork, sustaining ordinary earth, the following percentages of the height :— For a batter of y 24 per cent. 1 „ K 25 1 » 8 26 1 » 10 27 „ _1 ” 12 28 1 » -24 30 „ For a vertical wall 32 A rule sometimes adopted for perpendicular retaining walls on railways is to divide the height into three equal parts and make the thicknesses g, 4, and I respectively of the total height. The following general observations on the subject are given on the authority of Sir Benjamin Baker* :— “ Expérience has shown that a wall | of the height in thickness and battering 1" or 2" per foot on the face possesses sufficient stability when the backing and foundation are both favourable. It has been similarly proved by experience that under no conditions of surcharge or heavy backing is it necessary to make a retaining wall on a solid foundation more than double the above, or J of the height in thickness. Within these limits the engineer must vary the strength in accordance with the conditions affect- ing the particular case.” As the result of his own experience Sir Benjamin Baker “makes the thickness of retaining walls in ground of an average character equal to | of the height from the top of the footings.” Conditions of Stability.—Having duly selected a provisional sectional profile for a dock wall, and having defined in magnitude and line of action the overturning and restraining forces, it now remains to take the resultant of the latter and consider its effect upon the wall as a whole. The possi- bilities of failure are threefold— 1. The wall may fail by overturning about the outer edge of its base or of any bed joint. To achieve such a result the overturning moment about these points must exceed the moment due to the restraining force. When the moments are equal there is theoretical equilibrium ; but, in order to ensure a sufficient margin of safety, the axis of overturning should be assumed to lie some little distance within the wall—say, at least, | of the width of the base. 2. The outer edge of the wall at any horizontal section may be crushed in consequence of excessive compression. This is not likely to arise so much from the actual total weight upon any section as from the unequal distribution of stress. Unless the resultant thrust pass exactly through the centre of gravity of each horizontal plane the stress intensity is not uniform throughout. Uniformity of stress is possible in revetment walls Min. Proc. Inst. C.E., vol. Ixv., p. 181.