SPT in Fine-Grained Soils

The SPT is used in fine-grained soils ranging from very sofr clays and silty clays to very dense over-consolidated clays or glacial tills. The use of the SPT in these materials has often been to give an estimate of the undrained strength or the undrained elastic properties of the soil. More recently, it has been shown that the SPT may also be used to indicate the stress history of fine-grained soils. Clearly, the SPT is not the most preferred technique for evaluating engineering properties of fine-grained soils, especially soft to very soft clays. Other tests, such as the CPT, CPTU, or DMT are preferred, however, the SPT may be helpful in certain situations for providing preliminary estimates of a number of pertinent parameters.

As previously noted, in soft clays, the spoon may advance after the drill string is lowered into the borehole, or after the hammer is placed on top of the drill string, referred to as “weight of hammer” (WOH). However, not all hammer assemblies have the same mass, which means that the term “weight of hammer” is not equal in all cases. Engineers should be careful to record the style, model, serial number, and manufacturer of the hammer in use.

Undrained Shear Strength

One of the practical uses of the SPT in fine-grained soils is to use N to make an estimate of the undrained shear strength. In soft to very soft normally consolidated clays, it might be expected that the results of the SPT would show a more or less linear increase in N values with increasing depth. This would be consistent with increasing effective stress and undrained strength. Calculation of the normalized N value, by dividing by the vertical effective stress, c'vo, would produce a constant value of N/o'o, indicative of a constant normalized undrained shear strength.

Such an observed trend in SPT results would be fine, except for the fact that in most soft clays, SPT values are typically very low and often result in Weight of Rod (WOR) or Weight of Hammer (WOH) penetration. Most correlations established between undrained shear strength and blow counts are for stiff over consolidated clays. Reference values of undrained strength are typically obtained from unconfined compression tests or unconsolidated- undrained triaxial compression tests. In general, local correlations are required to establish any reasonable degree of confidence. As DeMello (1971) and others have suggested, it appears that correlations between N and s_u should take into account the sensitivity of the clay to have any significant meaning.

Most soil mechanics or foundation engineering texts provide a simple chart correlating SPT N-values to the consistency (unconfined compressive strength) of fine-grained soils. Figure 2.14 shows a compilation of a number of correlations. Several correlations between SPT N and undrained shear strength are given in Table 2.11. The problem with many of these correlations is that they give no indication as to the type of SPT Hammer that was used to perform the test. Since many of them are from the 1970s and 1980s, it is reasonably safe to assume that they were not conducted using an automatic hammer which was not used until about 1995. Therefore, one may expect considerable scatter in the correlations.

An alternative is to use the SPT N-value to estimate the unconfined compressive strength (UCS) as noted in Table 2.12. Note that most of these correlations use the “uncorrected” SPT N-values and therefore are subject to some variability, depending on the system used in developing the correlation. Only limited work has been performed to correlate strength of clays from the energy-corrected SPT N-value; N_so.

The undrained strength of stiff intact clays can be related to N-values as proposed by Stroud (1974) using the relationship:

Figure 2.14 Comparison of historic reported correlations between SPT N-value and undrained shear strength. (After Kulhawy & Mayne 1990.)

Table 2.11 Reported correlations between SPT N and undrained shear strength

where

f, = an empirical factor

The SPT results from which the N-values were derived were based on the modern UK practice so that the parameter f, is more properly defined as:

f, = s„/N₆₀ (2.9)

In the U.K., the SPT split spoon sampler is not recessed for liners and therefore has a constant diameter of 34.9mm (1.375in.). In order to make use of Equation 2.8, an appropriate correction factor must be applied (to decrease N) if a standard U.S. split spoon sampler is used without liners. The value of f, with soil plasticity (P.I. 15-60) only varies from about 4 to 6. An average value of about f_t = 5 (for s_u in KN/m²) appears reasonable for most insensitive materials.

An interesting approach to estimating the unconfined compressive strength of soft and very soft clays was presented by Saiki (1983) for situations in which the split spoon may penetrate by either the weight of the hammer, or the blow counts may be very low (i.e., N<5). Two situations are considered: self-penetration and blow-penetration, and a corrected N value, N', is defined.

Spoon self-penetration

where

N' = corrected blow count

a = coefficient = ratio of a static weight to a standard amount of self-penetration (kN/m) S = total amount of self-penetration measured in the field (m)

W = total static weight applied to the test (kN)

(3 = modification factor

The value of the modification factor p was taken as 0 for c(,_o<50kPa and p = (o'_vo/50)-l for с'_о>50кРа. The value of a was taken for cases in which the weight was taken from the known mass of a standard hammer, standard spoon, and 40.5 mm (1.6 in.) drill rods for various test depths giving a value of a = 3.

Spoon blow penetration

where

N' = corrected blow count

N = measured blow count

С™ = effective vertical stress (kPa)

ctC_s = reference value of effective stress (kPa)

P = modification factor

In this case, the value of p = 0.406 was obtained and o'_vs = 91.4 kPa was determined.

Using both Equations 2.10 and 2.11, Saiki (1983) obtained excellent results compared with laboratory unconfined compression tests using the expression

where

q_u = unconfined compressive strength (kPa)

N'= corrected blow count (Equations 2.10 or 2.11)

More recently, there have been some attempts to develop theoretical models for estimating undrained shear strength in clays based on energy balance (Hettiarachchi &c Brown 2009) and energy transfer (Schnaid et al. 2009). These methods as based on separating the end bearing and side resistance acting on the spoon and then using the energy to predict the undrained shear strength. These approaches are fundamentally more sound than simple empirical correlations based on site-specific observations.