Undrained Shear Strength of Clay . The undrained shear strength of 

 clayey soils can be determined from the formula 



q c ' °vo (3) 



N k 



where: s = undrained shear strength 

 u 



q = cone pressure 

 x 



a = total overburden pressure 

 vo 



N, = cone factor 

 k 



The average value of N is 15 with a variation of about ±5. The recom- 

 mendations on how to select the best N for reducing data are confusing. 

 In his paper, deRuiter (1982) recommends a value of 10 to 15 for nor- 

 mally consolidated clays and 15 to 20 for overconsoli dated clays. 

 However, Schmertmann (1978) indicates the N varies according to cone 

 type and clay strength. He says data suggest that weaker clays have 

 higher N 's and stronger clays have lower N 's. However, each author 

 suggests that caution be used and that a local correlation be made, 

 preferably using a value backfigured from a failure. Further research 

 with piezocone data may narrow the range of N. . The work of Senneset 

 et al. (1982) indicates that estimates of s can be made from cone data 

 that include the effect of pore water pressures. By subtracting the 

 pore water pressure piezocone data from the cone pressure some of the 

 scatter can be reduced. 



q c " M c (4) 



u N' 

 k 



where: p = pore pressure near the cone 



N,' = effective cone factor (9±3) 

 k 



The likely variation of N' is ±3. 



Other Properties 



Estimates of the remolded strength and sensitivity of clays can be 

 made when a friction cone is used. Schmertmann (1978) has presented 

 these methods and states that they represent one measure of these prop- 

 erties. Schmertmann also indicates overconsolidation of clays can be 

 estimated, but large errors may be involved. Compression moduli for 

 sands and clays can be estimated with empirical correlations (Senneset 

 et al., 1982). When a piezocone is used, the coefficient of consolida- 

 tion of a clay can be roughly determined (Senneset et al . , 1982). 



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