(ct - u) or a is the effective stress (intergranular pressure) at this point, and / is the 

 angle of internal friction or, more properly according to Terzaghi (1943, p. 7), angle 

 of shearing resistance. Bjerrum (1954b, p. 3-9) presents a more detailed discussion. 



B . ULTIMATE BEARING CAPACITY 



The sediments Investigated are water-saturated (Richards, in preparation) and 

 largely cohesive. Cohesive sediments contain significant amounts of fine-grained, 

 silty clay- and clay-size particles that possess plasticity. (Plasticity is the property 

 of sediment that allows it to be deformed beyond the point of recovery without 

 cracking or appreciable volume change — ASCE) . The problem of determining 

 bearing capacity of water-saturated, cohesive sediments has been considered rela- 

 tively simple, although recent investigations at the Massachusetts Institute of 

 Technology indicate that a number of simplifying assumptions are questionable 

 (Lambe, 1961; Whitman, 1961). Saturated clayey sediments stressed without change 

 In water content behave with respect to applied stress as if they were purely cohe- 

 sive materials with an angle of shearing resistance equal to zero (^ = 0), the shear 

 strength, as well as the effective stress, being independent of the total stress on the 

 failure plane. In this special Instance, equation 9 becomes 



s = c (10) 



Bjerrum and Kjaernsli (1957, p. 2-3) have presented a short summary of the develop- 

 ment of the/ = analysis by Skempton and others, which currently enjoys wide 

 usage in soil mechanics. 



Ultimate bearing capacity is reached just before sediment fails In shear under an 

 applied load. It is related to the product of the shear strength and one or more factors, 

 which are functions of the size, shape, and depth of loading. Bearing capacity of 

 plastic clays is nearly independent of the loaded surface area (Terzaghi, 1925, p. 1065), 



A formula for ultimate bearing capacity of sediment under strip loading (load 

 Infinitely long relative to the width), as developed from equations by Prandtl (1920) 

 and modified by Terzaghi (1943, p. 118-134), is 



q^ = c N_ + yd N + yb Ny (11) 



o c q 



where q is the ultimate bearing capacity (the average load per unit area required 

 to produce failure by rupture of a supporting sediment mass), y is the effective unit 

 weight, b is the half width of the strip load, d Is the depth of the load below the 

 surface, and N^, Nq, and Ny are dimenslonless constants dependent on/. 



48 



