Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

Evaluating Soil Properties for Design

Reported Correlations between SPT N 60 Value and ’ for Coarse-Grained Soils, Table 4-4 Correlation Reference ’ = (0.3N 60 ) 0.5 + 27˚ Peck et al. (1953) ’ = (10N 60 )/35 + 27˚ Meyerhof (1956) ’ = (20N 60 ) 0.5 + 15˚ Kishida (1967) ’ = (N 60 /σ’ vo ) 0.5 +26.9˚ (σ’ vo in MN/m 2 ) Parry (1977) ’ = (15N 60 ) 0.5 +15˚ Shioi & Fukui (1982) ’ = (15.4(N 1 ) 60 ) 0.5 + 20˚ Hatanaka & Uchida (1996)

Empirical Values for Soil Consistency, Overconsolidation Ratio, and Undrained Shear Strength vs. SPT N 60 Value, Table 4-8

SPT N 60 Values

Undrained Shear Strength (skf) [kPa]

Comments

Stress History

Consistency Term

Normally consolidated OCR = 1 Normally consolidated OCR ≈ 1 to 1.2 Normally consolidated OCR = 1 to 2 Normally consolidated to OCR of 2 to 3

Runs through fingers Squeezes easily in fingers

Very soft

0-2 < 0.25 [< 12]

0.38 [18.2] to 0.63 [30.2]

Soft

3-5

EQUATION 4-21

Can be formed into a ball Hard to deform by hand squeezing Very hard to deform by hand Nearly impossible to deform by hand

0.75 [36] to 1.13 [54.1]

Medium

6-9

’ = arctan[0.1 + 0.38log(q t /σ’ vo )]

Equation 4-21 is shown in Figure 4-15. Additional test results from 24 different sands were compiled by Kulhawy and Mayne (1990) who proposed the following expression:

1.25 [59.9] to 2 [95.8]

Stiff

10-16

Very stiff

Overconsolidated OCR = 4 to 8

2.13 [102] to 3.75 [179.6]

17-30

EQUATION 4-22

Highly overconsolidated OCR > 8

’ = 17.70 + 11.0log(q t1 )

Hard

> 30 > 3.75 [> 179.6]

where q t1 = (q t /σ atm )/(σ’ vo /σ atm ) 0.5

σ atm = Atmospheric pressure (1 atm = 1 bar = 100 kPa = 1 tsf = 14.7 psi)

4.3.2.1 ’ from SPT Several correlations have been proposed to estimate the drained friction angle in sands from SPT results. A summary of several of the more popular correlations is given in Table 4-9. The correlation of Hatanaka & Uchida (1996) is shown in Figure 4-11, taken from the FHWA Reference Manual on Subsurface Investigations (2002). 4.3.2.2 ’ from CPT/CPTU An approach derived from bearing capacity theory, similar to the one used to estimate s u from the CPT/CPTU tip resistance in clays, may be used to estimate the friction angle of sands. Robertson and Campanella (1983) summarized a number of available calibration chamber tests on five sands and suggested a simple correlation between the normalized CPT tip resistance and a cone bearing capacity factor (N q ): EQUATION 4-20 N q = (q c / σ’ vo ) = 0.194exp[7.63tan( ’)] where σ’ vo = Vertical effective (corrected for pore water pressure) stress at cone tip This relationship is shown in Figure 4-13. The friction angle may also be estimated from the CPTU effective tip resistance. Early calibration chamber data suggested a simple empirical correlation:

Empirical Values for Relative Density, Friction Angle, and Unit Weight vs. SPT Blow Count (Assuming a 20-foot (6-meter) depth of overburden and 70% rod efficiency on hammer), Table 4-10 Description Very loose Loose Medium dense Dense Very dense Relative Density (D r ) (%) 0 15 35 65 85 4.3.2.3 Empirical Correlations The relative density of sands, gravels, and other granular soils is usually described as very loose, loose, medium dense, dense, very dense, or extremely dense. The Standard

Fine

1-2 3-6 7-15 16-30

?

SPT (N 70 )

Medium 2-3 4-6 8-20 21-40

40+

Coarse

3-6 5-9 10-25 26-45

45+

Fine

26-28 28-30 30-33 33-38

38+

Friction Angle ( ’)

Medium 27-29 29-32 32-36 36-42

50+

Coarse

28-30 30-34 34-40 40-50

50+

Total Unit Weight ( wet ) (pcf)

70-100 90-115 110-130 110-140 130-150

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