A Treatise On The Principles And Practice Of Dock Engineering

FOUNDATIONS. *83 are very liable to slide forward on their bases, owing to insufficient weight, as actually happened in this instance. A trench or gallery, for hydraulic supply pipes and water and gas mains, may often be managed within the body of the wall, at a short depth below the coping level. Access to this will be obtained by man- holes placed at convenient distances apart, say, 75 to 100 feet. In setting out the line of a dock wall, it is by no means desirable to make it absolutely straight, even if intended to be so. Apart from the possibility of some slight forward movement producing an appréciable and unsigbtly bulge, there is the effect of an optical illusion which causes a perfectly straight coping to appear curved outwards. This latter can be counteracted by giving the wall an almost infinitesimal curvature in the opposite direction. A versed sine of 6 inches in 1,000 feet will generally be found sufficient. Foundations—The foundation constitutes so important a feature in connection with the construction of dock walls as to call for some detailed observations. Care should be taken to see that in each case certain essential conditions are fulfilled. These conditions may be stated as follows :_ 1. The inclination to the vertical of the resultant pressure upon the surface of the foundation should not exceed the angle of repose of the earth in question. This ensures what is termed stability of friction ___i.e., there will be no likelihood of the wall sliding bodily forward upon its base. The condition can always be met by giving a suitable bevel to the surface, so that it slopes downward from the front of the wall to the back. 2. The deviation of the resultant pressure, from the centre of symmetry of the foundation, should not be more than one-sixth of the width. This condition is necessary to maintain absence of tension at the back. 3. The maximum intensity of pressure, at any point, should not exceed a certain lirait, dependent upon the nature of the ground. The safe intensity of pressure on natural foundations has been determined as follows :— On hard rock, . . 9 or 10 tons per sq. ft. On soft rock and hard clay, 2 to 3 ,, ,, On sand and gravel, . IJ,, 2 ,, ,, On compact earth, . 1 ,, 1J ,, On soft, uncertain ground, . -i,, „ Where an artificial foundation has been prepared, the following intensities should not be exceeded :— For Portland cement concrète, . . . 10 to 12 tons per sq. ft. For rubble masonry in hydraulic mortar, . 4 „ 5,, In the case of natural foundations, care must be taken that there is no possibility of lateral escape, and, in the case of artificial foundations, the pupaied bed must have sufficient depth to prevent transverse fracture, as indicated in fig. 109. The depth of the bed, cl, will depend upon the