Chance Technical Design Manual

REPORTED CORRELATIONS BETWEEN SPT N 60 VALUE AND UNCONFINED COMPRESSIVE STRENGTH (q u ), TABLE 5-7 CORRELATION TO UNCONFINED COMPRESSIVE STRENGTH UNITS OF q u SOIL TYPE REFERENCE

are given in Tables 5-6 and 5-7. The undrained shear strength is generally considered to be one-half the unconfined compres sive strength. Caution should be used when using these cor relations since they have been developed for specific geologic deposits and the SPT field procedure used may not have been the same in all cases. 5.3.1.2 s u FROM CPT/CPTU The undrained shear strength may also be estimated from the CPT tip resistance or from the CPTU effective (net) tip resistance (e.g., Lunne et al. 1995). An estimate of s u can be found from the CPT tip resistance by using an equation derived from the bearing capacity equation: EQUATION 5-24 s u = (q c – s vo )/N k where q c = CPT tip resistance s vo = Total vertical stress at the cone tip = depth x total soil unit weight N k = Empirical cone factor The value of N k varies somewhat with soil stiffness, plasticity, stress history and other factors. However, many reported ob servations in which s u has been obtained from both laboratory triaxial tests and field vane tests suggest that a reasonable val ue of N k for a wide range of soils is on the order of 16. Estimating s u from the CPTU effective tip resistance uses a modified approach since the tip resistance is corrected for pore pressure effects to give the effective tip resistance (q t ). The undrained shear strength is estimated from: EQUATION 5-25 s u = (q t – s vo )/N kt where q t = CPTU effective tip resistance N kt = Empirical cone factor REPORTED CORRELATIONS BETWEEN SPT N 60 VALUE AND UNDRAINED SHEAR STRENGTH (s u ), TABLE 5-6

Terzaghi & Peck (1967)

q u = 12.5N 60

kPa

Fine grained

Golder (1961)

q u = N 60 /8

tsf

Clay

q u = 25N 60 q u = 20N 60 q u = 25N 60 q u = 15N 60 q u = 7.5N 60

kPa kPa

Clay Silty clay

Sanglerat (1972)

Highly plastic clay Medium-plasticity clay Low-plasticity clay

Sowers (1979)

kPa

q u = 24N 60

kPa

Clay

Nixon (1982)

Sarac & Popovic (1982)

q u = 62.5(N 60 - 3.4)

kPa

Behpoor & Ghahramani (1989)

q u = 15N 60

kPa

CL and CL-ML

Kulhawy & Mayne (1990) Sivrikaya & Togrol (2002)

0.72

q u = 58N 60

kPa

Fine grained

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

CH CL

kPa

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 CPTU pore pressure measurements or by first estimating the stress history from CPT/CPTU results and then converting to undrained shear strength, e.g., NCHRP (2007) and Schnaid (2009), both of which are viable approaches. 5.3.1.3 ESTIMATING SHEAR STRENGTH OF FINE-GRAINED SOILS—OTHER METHODS 5.3.1.3.a Vane Shear Test Shear strength of fine-grained soils may be measured in the field and in the laboratory. One of the most versatile devices for investigating undrained shear strength and sensitivity of soft clays is the vane shear test. The test device generally consists of a four-bladed rectangular vane fastened to the bottom of a vertical rod. The blades are pressed their full depth into the clay surface and then rotated at a constant rate by a crank han dle. The torque required to rotate the vane is measured. The shear resistance of the soil can be computed from the torque and dimensions of the vane. One type of portable vane shear tester is the Torvane. It is a con venient hand-held device useful for investigating the strength of clays in the walls of test pits in the field or for rapid scan-

DESIGN METHODOLOGY

CORRELATION TO UNDRAINED SHEAR STRENGTH

UNITS OF s u

SOIL TYPE REFERENCE

Japanese cohesive soils

Hara et al. (1974)

0.72

s u = 29N 60

kPa

Insensitive overconsolidated clays in U.K.

Stroud (1974)

s u = 4.5N 60

tsf

s u = 8N 60 for N 60 < 10 s u = 7N 60 for 10 < N 60 < 20 s u = 6N 60 for 20 < N 60 < 30 s u = 5N 60 for 30 < N 60 < 40

Guabirotuba clay

Tavares (1988)

kPa

Ajayi & Balogun (1988)

s u =1.39N 60 + 74.2

tsf

Tropical soil

Sao Paulo overconsolidated clay

s u = 12.5N 60 s u = 10.5N 60

kPa tsf

Decourt (1989)

Note: 1 kPa = 20.9 psf

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