io6 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



sides, did not uniformly confirm the usual theory that the angle 

 of pressure of maximum thrust is half that contained between 

 the natural slope and the back of the wall. Thus the line of 

 separation was found to be at an angle of 24 with the vertical, 

 instead of 28. Again, with a gravel bank 10 feet high, the line 

 of separation ranged from 3 feet 8 inches to 5 feet 8 inches from 

 the back of the wall; whilst, as the natural slope was 1J to 1, 

 the distance should have been 5 feet in all instances, if Coulomb's 

 theory applied strictly to even such exceptionally favourable 

 materials as dry sand and shingle. 



Experience has shown 1 that a wall a quarter of the height 

 in thickness, and battering from 1 to 2 inches per foot on the 

 face, possesses sufficient stability when the foundations and 

 backing material are both satisfactory. Sir B. Baker prefers, 

 however, to say that experiment has shown that when the 

 backing and foundations are favourable, the actual thrust of 

 good filling is equivalent to that of a fluid weighing about 

 10 Ibs. per cubic foot ; but to allow for variations in the ground, 

 vibration, and contingencies, a factor of safety of 2 should be 

 adopted. Therefore, any wall under ordinary favourable condi- 

 tions should be able to sustain a 20 Ibs. fluid pressure, or in 

 other words, a pressure due to a fluid weighing 20 Ibs. per cubic 

 foot, which will be the case if the thickness is made a quarter 

 of the height. 



The results of experiments carried out by Lieutenant Hope, 

 RE., 2 and General Pasley, RE., at Chatham, justify the conclu- 

 sion that a properly built wall, in mortar or cement, just balancing 

 the theoretical pressure, would really have a factor of safety in 

 the ratio of 2 to 1. 



Experiments made by Sir B. Baker 8 prove that a wall which, 

 according to the ordinary theory, would be on the point of being 

 overturned by the thrust of a bank of large boulders would in 

 fact have a factor of safety of 2*5 to 1. 



Sir B. Baker 4 draws attention to the fact, which must also 

 have been observed by many who have had to deal with con- 

 struction, that the methods adopted in temporarily stacking and 

 storing materials, coal, quarry rubbish, etc., where rough-and- 

 ready retaining- walls are run up with the larger portions of the 

 same materials, built up dry, and with the least expenditure 



1 M.P.l.C.R, vol. Ixv. p. 183. 2 Hid., p. 149. 



2Md. t p. 15G. 4 Ibid., p. 141. 



