PRESSURE OF EARTHWORK. 105 



wall has to resist, and tending to overturn or push it forward, 

 is due to the weight of the angular mass a b c, and varies 

 greatly with the character of the material forming the backing. 



The angle of repose, or natural slope, is the inclination which 

 any material will assume on being cast into a heap. The 

 following table * gives the angle which different materials may 

 be expected to take : 



It would, however, be unwise to trust too implicitly to these 

 figures; therefore, the material to be dealt with should be in all 

 cases carefully examined and, if necessary, tested under different 

 conditions before deciding upon the angle of repose to be adopted. 

 A material perfectly suitable for backing up a wall under favour- 

 able conditions may become saturated before a sufficient time 

 has elapsed for it to become thoroughly consolidated and, in 

 consequence, reduced to a condition by which the thrust on the 

 wall would be very greatly augmented. Clay is perhaps the 

 most treacherous material to deal with as backing, inasmuch as 

 on exposure it is liable to expand ; and under the influence of 

 wet, to slip away to almost any angle. 



Sir B. Baker, in a paper read before the Institute of Civil 

 Engineers, on "The Actual Lateral Pressure of Earthwork," 2 

 questions the accuracy of the generally accepted theory that 

 the mass to which the lateral thrust is due is found by bisecting 

 the angle of repose ; and demonstrates by direct experiment and 

 from practical experience and observation on work of both a 

 permanent and temporary character that the actual pressure to 

 which a wall is subjected is only about half that indicated by 

 theory. 



Experiments made at Chatham 3 many years ago by Lieu- 

 ton ant Hope, R.E., with coloured sand in a vessel with glass 



1 Ilankiue's " Manual of Civil Engineering." 



1 X.P.I.C.E., vol. Ixv. p. 140. Ibid. 



