for normally consolidated sands. For overconsol i dated sands, he sug- 

 gests a method of calculating an equivalent normal consolidated sand 

 cone pressure from the measured overconsol idated sand cone pressure for 

 use in Figure 26. 



q cOC a + _3 K oOC (1) 



q cNC = 4 K oNC " 



where: q _ = cone pressure for overconsol idated sand 



q N _ = cone pressure for normally consolidated sand 



K' r = coefficient of lateral pressure for overconsol idated 

 sand 



K' ,_ = coefficient of lateral pressure for normally consolidated 

 oNC j 

 sand 



^ £>0L B (0CR) 0.42 (2) 



oNC 



Villet and Mitchell have shown that this relationship can be 

 improved by performing one or two triaxial tests to define the relative 

 density-friction angle relationship. With this data a procedure by 

 Durgunoglu and Mitchell (1975) can be used to develop a complete rela- 

 tive density, overburden pressure, cone pressure relationship. This 

 procedure provides better accuracy but is more complex and, therefore, 

 is not presented here. With this procedure, relative density relation- 

 ships can be tailored for a particular sand. 



Friction Angle . Friction angle for a given sand is proportional to 

 relative density and can, therefore, be estimated from a relative den- 

 sity determined as described previously. An example of this type of 

 relationship is shown in Figure 27 (Schmertmann, 1978). The relation- 

 ship can be developed from one triaxial test by establishing a single 

 point on the graph and drawing a line that follows the trend shown. 

 Another method is shown in Figure 28 (Mitchell et al., 1978). With this 

 graph, the friction angle can be estimated directly from the cone pres- 

 sure and the overburden pressure. As with the relative density rela- 

 tionship of Figure 26, the curves given in Figures 27 and 28 must be 

 applied with engineering judgment. In addition to these simple 

 approaches for estimating friction angle, the Norwegian Institute of 

 Technology (Senneset et al., 1982) has developed a method in which pore 

 pressure is measured. The cone used must be a piezocone. The pore 

 pressure information is used to convert total stress data to effective 

 stress data, thereby eliminating some of the empiricism of previous 

 methods. 



43 



