Transmission And Substation Foundations - Technical Design Manual (TD06088E)

TABLE 4-7. REPORTED CORRELATIONS BETWEEN SPT N-VALUE AND UNDRAINED SHEAR STRENGTH, S U Correlation to Undrained Shear Strength Units of s u Soil Type

Reference

s u = 29N 0.72

kPa

Japanese cohesive soils

Hara et al. (1974)

Insensitive Overconsolidated Clays in U.K.

s u = 4.5N

tsf

Stroud (1974)

s u = 8N N < 10 s u = 7N 10

Guabirotuba Clay

kPa

Tavares (1988)

tsf

tropical soil

Ajayi & Balogun (1988)

s u = 12.5N s u = 10.5N 60

kPa tsf

Sao Paulo overconsolidated clay

Decourt (1989)

Note: 1 kPa = 20.9 psf

DESIGN METHODOLOGY

TABLE 4-8. REPORTED CORRELATIONS BETWEEN SPT N-VALUE AND UNCONFINED COMPRESSIVE STRENGTH, QU Correlation to Unconfined Compressive Strength Units of q u Soil Type

Reference

q u = 12.5N q u = N/8 q u = 25N q u = 20N q u = 25N q u = 15N q u = 7.5N q u = 24N

kPa

Fine-Grained

Terzaghi & Peck (1967)

tsf

Clay

Golder (1961)

kPa kPa

Clay Silty Clay

Sanglerat (1972)

Highly Plastic Clay Medium Plastic Clay Low Plasticity Clay

kPa

Sowers (1979)

kPa

Clay

Nixon (1982)

q u = 62.5 (N-3.4)

kPa

Sarac & Popovic (1982)

Behpoor & Ghahramani (1989)

q u = 15N

kPa

CL and CL-ML

q u = 58N 0.72

kPa

Fine-Grained

Kulhawy & Mayne (1990)

q u = 13.6 N 60 q u = 9.8N 60 q u = 8.6N 60 q u = (0.19PI + 6.2)N 60

CH CL

kPa

Sivrikaya & Togrol (2002)

Fine-Grained Fine-Grained

The value of N kt also has been shown to vary for different soils but a reasonable conservative value for massive clays is on the order of 12. For very stiff, fissured clays, the value of N kt may be as high as 30. Other methods are available for estimating undrained shear strength from the pore pressure measurements from a CPTU or by first estimating the stress history from CPT/CPTU results and then converting to undrained shear strength, e.g., NCHRP (2007); Schnaid (2009), both of which are viable approaches.

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