Standard Penetration Test
Although correlations have been developed between N and the undrained shear strength, Su, of
cohesive soils, the correlations do not account for an N-value standardized to a given energy value, any particular reference strength for Su, or the sensitivity of the clay. Recall that different drilling
rigs, hammer arrangements and general test practices can impart significantly different amounts of energy to the split spoon and result in very different N-values for the same soil. Therefore, unless calibrated for a given geologic setting, energy level and reference strength, the SPT does not provide a useful quantitative correlation with undrained shear strength.
Figure 6-5
Correlation of Triaxial Compression Effective Stress Friction Angle with Cone Penetration Tip Resistance
(after Kulhawy and Mayne, 1990)
Cone Penetration Test
6-10 σvo u k c=N S + q (Eq. 6-1) where:
qc = Cone tip resistance (kPa)
σvo = Total overburden stress (kPa)
Nk = Cone bearing factor (dim)
Theoretical predictions of Nk range from about 7 to 18, and empirical correlation for specific sites
range from about 5 to 75. The reliability of Eq. 6-1 can be improved by substituting a corrected tip resistance accounting for pore pressure effects when using a piezocone (CPTU) by:
u a) - (1 + q = qt c bt (Eq. 6-2) where:
qt = Corrected cone tip resistance (kPa)
a = Net area ratio (dim). Defined as the ratio of the square of the cone diameter within the pore pressure sensing filter to the square of the cone diameter at the base of the conical point
ubt = Pore pressure measured behind the cone tip (kPa). Note Eq. 6-2 applies only
to piezocones in which the pore pressure transducer is located immediately behind the tip of the cone.
Robertson, et al. (1986) have concluded that no unique relationship exists between CPTU data and Su for any soil type. The CPTU still remains a potentially valuable tool for characterizing Su due to
its relatively economic rate of data collection. In practice then, the use of the CPT or CPTU to predict Su should include site specific calibration of the Nk factor through use of some reference
strength such as might be determined with the field VST.
Field Vane Shear Test
The shear strength measured with the VST should not be used directly in design without prior correction to account for strain rate and anisotropy effects. A correction factor, µ, was first proposed by Bjerrum (1972) based on a two-dimensional, plane strain back analyses of 14 case histories of embankment failures. This correction factor has since been modified by Azzouz, et al. (1983) as shown in Figure 6-6 to include three-dimensional end effects of these failures using a database of 18 case histories.
6-11
Figure 6-6
Field Vane Shear Correction Factor
(after Azzouz, et al., 1983)
6.3.1.2 Compressibility
Pressuremeter Test
The PMT provides a direct measure of a modulus value which is typically assumed to be equal to an undrained modulus, Eu, in cohesive soils and Young's modulus, E, in cohesionless soils.
Standard Penetration Test
Prior to the introduction of probability based design concepts, numerous non-statistical correlations between the SPT N-value and the drained modulus for cohesionless soils were proposed (e.g. D'Appolonia, et al., 1970, Mitchell and Gardner, 1975). Due to the lack of a quantitative estimate of the reliability associated with these early methods, only some more recent correlations are described herein. Figure 6-7 shows a correlation between the uncorrected SPT N value and a normalized pressuremeter modulus, EPMT/pa.
The wide spread in data about the trend line in Figure 6-7 illustrates the high degree of uncertainty associated with trying to predict the deformation properties of a cohesionless soil based on SPTs. The COV for this model ranges from a low of about 1.5 to greater than 2.0. A similar relationship is shown in Figure 6-8 for cohesive soils, for which the COV ranges from approximately 1.5 to 2.5. Considering the uncertainty associated with the relationships in Figures 6-6 and 6-7, SPTs should only be used for preliminary estimation of the modulus of soils.
6-12
Figure 6-7
Pressuremeter Modulus versus N Value for Cohesionless Soil
(after Kulhawy and Mayne, 1990)
Figure 6-8
Pressuremeter Modulus versus N Value for Cohesive Soils
6-13
Cone Penetration Test
Calibration chamber testing has provided a correlation between the tangent constrained drained modulus, Mdt, relative density, Dr, and the CPT tip resistance, qc. Correlations for NC and OC sands
are shown in Figures 6-9 and 6-10, respectively.
Figures 6-9 and 6-10 illustrate that the relative uncertainty (0.05 <COV< 0.63) associated with modulus predictions is significantly smaller using the CPT than using the SPT. However, it is important to note that the relationships shown in Figures 6-9 and 6-10 are based on a data set consisting of calibration chamber tests which do not account for effects such as aging and the wider range of in-situ variability encountered in practice.
Figure 6-9
Correlation of CPT Tip Resistance with Constrained Modulus and Relative Density for NC Sands
6-14
Figure 6-10
Correlation of CPT Tip Resistance with Constrained Modulus and Relative Density for OC Sands
(after Kulhawy and Mayne, 1990)
6.3.2 Laboratory Tests
The primary laboratory strength and compressibility tests consist of the triaxial compression test and the consolidation test. Results from each of these tests exhibit some measurement error and error due to inherent variability of the sample tested. As each test provides a direct measure of a particular engineering property, a transformation model is generally not required to utilize the test results. However, test results may be correlated to other parameters where the potential failure modes in the field do not match the laboratory boundary conditions in the triaxial test (e.g. plane strain vs. triaxial failure modes).
Laboratory index tests can be used to provide cost-effective preliminary estimates of some common engineering properties. For cohesive soils, Atterberg limits and their relationship to the in-situ water content are most commonly used for such correlations. Figure 6-11 shows one correlation between the Liquidity Index, LI, and the ratio of undrained shear strength to vertical effective stress level in triaxial compression, Su/σ'vo.
6-15
Figure 6-11
Correlation Between Normalized Undrained Shear Strength and Liquidity Index for NC Clays
(after Kulhawy and Mayne, 1990)
Correlations of index tests with some deformation properties are also available, as shown in Figure 6-12 which depicts relationships between the Compression Index, Cc, and the Plasticity Index, PI,
and between the Recompression Index, Cur, and the PI, for cohesive soils.
Figure 6-12
Compression and Unload-Reload Indices versus Plasticity Index (after Kulhawy and Mayne, 1990)
6-16