Transmission And Substation Foundations - Technical Design Manual (TD06088E)

DESIGN METHODOLOGY The undrained shear strength may also be estimated from the tip resistance obtained from the total cone tip resistance from a CPT or the effective (net) cone tip resistance from a CPTU (e.g., Lunne et al. 1995). Estimating s u from the CPT total tip resistance is from a form of the bearing capacity equation as: 4.4 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 friction capacity equations. Table 6-6 summarizes the soil properties for different site conditions for design of both single-helix and multi-helix helical piles/anchors. Geotechnical design of helical piles/anchors requires 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, φ ’. 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 Field Vane Tests (FVT), Cone Penetration Tests (CPT), Piezocone Tests (CPTU), Dilatometer Tests (DMT), Pressuremeter Tests (PMT) or Borehole Shear Tests (BST) are preferred. THERE IS NO SUBSTITUTE FOR A SITE SPECIFIC GEOTECHNICAL INVESTIGATION. Estimating Undrained Shear Strength, s u , in clays: The undrained shear strength of saturated clays, silty clays and clayey silts is not a unique soil property, like Liquid Limit of clay content, but 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. s u from SPT A number of correlations exist for estimating both 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 4-7 and 4-8. The undrained shear strength is generally taken as 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. s u from CPT/CPTU

s u = (q c – σ vo )/N k

Equation 4-23

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 where 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 , as the undrained shear strength

is obtained from: s u = (q t – σ vo )/N kt

Equation 4-24

where: q t = CPTU effective tip resistance N kt = empirical cone factor

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