Frictional Behavior 



Despite the fact that calcareous sands are composed of crushable 

 grains and have higher overall void ratios than noncalcareous sands, at 

 comparable state of compaction they exhibit considerably higher friction 

 angles (about 10 degrees higher) both in loose and dense conditions. 

 This result can be generalized to the fact that the friction angles of 

 calcareous sands are significantly higher than the values of most non- 

 calcareous terrestrial sands. The high values for loose sand are par- 

 ticularly surprising. Other information (Datta et al., 1979 and 1980; 

 Beringen et al. , 1982) confirms this result. 



High friction angles are often associated with dilational behavior 

 (volume increase during shear) , but the high friction angles of the cal- 

 careous soils tested were not due to dilational behavior during shearing. 

 In loose state, both soils (Guam and Florida calcareous sands) exhibited 

 volume decrease, yet they had friction angles in the range of 44 to 

 46 degrees. Surface roughness at grain contact points, and the rein- 

 forcing effects of elongated and/or flat particles in the soil matrix 

 might be factors contributing to high frictional resistance of calcar- 

 eous sands. 



The variety of friction angles of calcareous soils from loose to 

 dense condition is surprisingly narrow. For both soils tested, the in- 

 crease in friction angle form loose to dense condition was only 3 to 

 4 degrees. 



Some particle crushing occurred during shearing of these calcareous 

 sands in the triaxial compression tests. When tests were made on crushed 

 soil, further crushing during shear was reduced. 



Grain crushing also seemed to play a role in decreasing the friction 

 angle of calcareous sands observed with increases in confining pressure. 

 The reduction in friction angle (2 to 4 degrees) appeared to be the 

 result of this phenomenon, and is more intense at higher confining pres- 

 sures (Datta et al. , 1979 and 1980). Even partially crushed soil showed 

 some reduction in the angle of internal friction when tested under high 

 confining pressures. 



Although particle crushing that occurs during shearing caused some 

 reduction in frictional resistance, it cannot be concluded that individ- 

 ual crushed particles have lower frictional resistance. Tests on delib- 

 erately crushed and recompacted calcareous sands indicated that the 

 crushed soil was not weaker than the natural soil. This is probably 

 because of two compensating factors: (1) a tendency for soil strength 

 to decrease because at a given void ratio the net interparticle void 

 space in the crushed sand is higher than in natural soil; and (2) a 

 tendency for soil strength to increase because partially crushed soil 

 grains will break up less during shearing. 



The results of the isotropic compression tests indicated that the 

 calcareous sands tested were more compressible than the noncalcareous 

 sand (Figure 21). This could be due to the presence of intraparticle 

 voids, and crushing of highly irregular particles. 



The results of the soil-steel friction tests are presented in 

 Table 6. These data indicate that for the calcareous sands, the soil- 

 steel friction angle appears to be independent of soil density, and 

 seemed to increase slightly after being crushed. For the Ottawa sand, 



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