Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

Evaluating Soil Properties for Design The design of helical piles/anchors using the traditional soil mechanics approach described in the previous section requires evaluation of soil properties for input into the various bearing and side resistance capacity equations. Table 4-5 summarizes the required soil properties for different site conditions for design of single-helix and multi-helix helical piles/anchors. Geotechnical design of helical piles/anchors requires

4.3.1.2 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:

information on the shear strength of saturated fine-grained soils, i.e., undrained shear strength (s u ), and the drained friction angle of coarse-grained soils ( ’). The best approach to evaluating these properties for design is a thorough site investigation and laboratory testing program on high-quality, undisturbed samples. However, this is not always possible or practical, and engineers often rely on information obtained from field testing, such as the Standard Penetration Test (SPT). Whenever possible, other high-quality field tests, such as the Field Vane Test (FVT), Cone Penetration Test (CPT), Piezocone Test (CPTU), Dilatometer Test (DMT), Pressuremeter Test (PMT), or Borehole Shear Test (BST), are preferred. There is no substitute for a site-specific geotechnical investigation. The undrained shear strength of saturated clays, silty clays, and clayey silts is not an independent soil property like the liquid limit of clay content, but instead depends on the test method used for the measurement. Correlations are available for estimating undrained shear strength from the results obtained from several of the field tests noted above. The most common field results that may be available to engineers for design of helical piles/anchors are the SPT and CPT/CPTU. 4.3.1.1 s u from SPT A number of correlations exist for estimating the undrained shear strength and unconfined compressive strength (q u ) of fine-grained soils from SPT results. Several of these correlations are given in Tables 4-6 and 4-7. The undrained shear strength is generally considered to be one-half the unconfined compressive strength. Caution should be used when using these correlations since they have been developed for specific geologic deposits and the SPT field procedure used may not have been the same in all cases. 4.3.1 Estimating Undrained Shear Strength (s u ) in Clays

EQUATION 4-18

s u = (q c – σ vo )/N k

where q c = CPT tip resistance

σ 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 observations in which s u has been obtained from both laboratory triaxial tests and field vane tests suggest that a reasonable value 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 4-19

s u = (q t – σ 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 4-6 Correlation to Undrained Shear Strength Units of s u Soil Type Reference

Japanese cohesive soils

Hara et al. (1974)

s u = 29N 60

kPa

0.72

Insensitive overconsolidated clays in U.K.

Stroud (1974)

s u = 4.5N 60

tsf

Soil Properties Required for Helical Pile/Anchor Design for Various Site Conditions, Table 4-5

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

Required Soil Properties

Soil Property Category

Unsaturated Fine Grained Mixed

Saturated Fine Grained

Coarse Grained

Ajayi & Balogun (1988)

s u =1.39N 60 + 74.2

tsf

Tropical soil

Sao Paulo overconsolidated clay

Shear strength s u

’

c, ’

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

kPa tsf

Decourt (1989)

Unit weight

wet or buoy

sat

wet

Note: 1 kPa = 20.9 psf

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