io8 NOTES ON DOCK'S AND DOCK CONSTRUCTION. 



Water 1 naturally gravitates to the foundation of a wall, and 

 softening of the strata commences. Owing to the lateral thrust 

 of the backing, the pressure is not uniform, and instead of settling 

 regularly, the outer edge or toe of the wall settles quickest, and 

 the top of the wall is thrown forward. 



Water will act as an admirable lubricant in diminishing the 

 factional resistance obtaining between the bottom of a wall and 

 the ground on which it rests, and thus facilitate a forward 

 movement. The same softening, due to saturation, may reduce 

 the ground in front of the toe to a condition in which it is easily 

 ploughed up by the advancing wall. 



In designing dock walls there is a general tendency to make 

 them wider at the top than is actually necessary for stability, 

 and to reduce the width at the bottom as much as possible, a 

 course by which the centre of gravity is raised higher than it 

 should properly be, and the tendency to move outwards at the 

 top augmented. This widening of the top is due in some 

 measure to the necessity of providing a substantial coping to 

 carry fairleads, mooring-rings, etc., whilst the base is reduced 

 on account of the additional expense that a wide foundation 

 would entail. Generally, it would be better to narrow the wall 

 at the top as much as possible and to spread the base more, 

 so as to distribute the weight over a greater area, and thus 

 increase the resistance to sinking. Sir J. Rennie 2 adopted the 

 expedient of hollow walls, in order to secure a greater supporting 

 area without increasing the quality of material. 



Mr. A. Jacob, 3 B.A., M.I.C.E., gives the following as a good 

 general rule for the mean, top, and bottom proportions of a wall 

 designed to sustain water, or earth in a semi-fluid condition, the 

 height being represented by unity: top, (V3 ; middle, 0'5 ; base, 0'7. 4 



The use of counterforts at the back of a dock wall, unless 

 under exceptional circumstances, and when placed very close 

 together, is of very doubtful utility. In practice, it has been 

 found that they frequently separate from the body of the wall 

 owing to defective bond. When, however, Portland-cement 

 mortar, or concrete is used, this defect may be looked upon 

 as no longer possible, provided the work in the counterforts is 



1 M.P.I.C.E., vol. Ixv, pp. 167. 



2 *' Lives of the Engineers," vol. ii. chap, vii. p. 207. 



1 " Practical Designing of Retaiuing-walls," by A. Jacob, B.A., M.I.C.E. 

 1 Note these proportions apply more to duius than to retaining walls. 



