Constrained Modulus

Other attempts have been made to correlate the results of the SPT to the constrained modulus of the soil (M) as a function of overburden stress (e.g., Schultze & Melzer 1965; D’Appolonia et al. 1970). Since the constrained modulus, M, is related to the elastic Young’s modulus, E_t, as

an estimate of Poisson’s ratio is required to estimate E_s from M. For most granular soils in drained loading conditions, the constrained modulus generally varies in the range of 1.2E_S-1.5E_S.

There is considerable scatter in suggested correlations between soil stiffness or modulus and SPT blow counts. Wroth (1984) considered correlations between N and soil compressibility as “secondary” and dependent on the relationship between strength and stiffness of granular soils. Since soil modulus is strain level-dependent, the correlations include comparisons at a range of strain levels.

Small-Strain Shear Modulus

The shear modulus of soils is also strain level-dependent and reaches a maximum value at strain levels in the order of 10-⁴%. The value at small strains is often referred to as G₀ or G_max and in recent years, it has become customary to use this value as a reference point. The value of G_max is often obtained from dynamic measurements, which produce small dynamic strains, by measuring the dynamic shear wave velocity, V_s. The shear modulus, G_max, may then be obtained as

where

G_max = shear modulus (MPa)

V_s = shear wave velocity (m/s) р = total soil unit weight (kg/m⁵) g = acceleration of gravity (9.8 m/s²)

Correlations between SPT blow counts and both shear wave velocity, V_s, and G_max have been suggested by a number of investigators (e.g., Sykora & Koester 1988; Lee 1992; Fabbrocino et al. 2015) and are summarized in Tables 2.9 and 2.10. Most empirical correlations between V_s and N are in the form of

Liquefaction Potential

The SPT has a long history of use for evaluating the liquefaction potential of sites composed predominantly of sands, sands and gravel, and silty sand. A detailed test procedure for performing the test is given in ASTM D6066 Standard Practice for Determining the Normalized Penetration Resistance of Sands for Evaluation of Liquefaction Potential. Seed et al. (1985) had recommended the following SPT procedure for use in liquefaction correlations of coarse-grained soils:

• 1. Borehole 10-15 cm (4-5 in.) diameter; rotary hole with bentonite drilling mud for stability;
• 2. Drill bit - upward deflection of drilling mud (tricone or baffled drag bit);
• 3. Sampler O.D. = 50.8 mm (2.0in.) I.D. = 34.9 mm (1.375 in.) constant, i.e., no room for liners;
• 4. Drill rod A or AW rods for depths less than 15.2 m (50 ft); N or NW rods for depths greater than 15.2m (50ft);
• 5. Energy delivered to sampler 60% theoretical;
• 6. Blow count rate = 30-40 blows/min;
• 7. Penetration resistance Count 0.15-0.46 m (6-18 in.).

While there has been a significant shift in this area of work from the use of the SPT to the use of the CPT, there is still considerable routine work being performed with the SPT.

Based on field performance, Seed et al. (1985), Seed and De Alba (1986) had presented charts representing the relationship between the cyclic stress ratio (CCR) causing liquefaction and (Н_г)₆₀ values for clean sands and sand with varying amounts of fines for M = 7.5 earthquakes. Based on field performance, three approximate ranges of liquefaction damage potential could be established as follows:

Figures 2.12 and 2.13 show typical charts for both clean sands and silty sands for earthquake magnitude M = 7.5. Note that the presence of fines increases the CSR needed to cause liquefaction for the same SPT (N,)₆₀ value. Adjustments to CSR have been suggested to account for factors such as high confining stress and nonlevel ground conditions and are available elsewhere (e.g., Kraemer 1996).

Table 2.9 Reported correlations between N and V_s for coarse-grained soils

(Continued)

Table 2.9 (Continued) Reported correlations between N and V_s for coarse-grained soils

^a V_s in m/s.

Table 2.10 Reported correlations between N and G_max for coarse-grained soils

^a G,_nJ„ in MPa except as noted.

Figure 2.12 Liquefaction potential based on SPT results for clean sands. (From Seed et al. I98S.)

Figure 2.13 Liquefaction potential based on SPT in silty sands. (From Seed et al. I98S.)