Sensitivity

Schmertmann (1978b) suggested that sensitivity, S_t, could be estimated from where

(field vane) ^— Sensitivity

N_s = an empirical factor R_f = friction ratio

Schmertmann (1978b) suggested a value of N_s= 15 for R_t from a mechanical CPT and S_t obtained from Nikon or Geonor type field vane. Robertson & Campanella (1983b) suggested

N_s = 10 for electric cone data. Greig et al. (1986) and Robertson et al. (1986) suggested N_s = 6 for data collected from clays in the Vancouver area. Robertson (2009) suggested that sensitivity could be estimated from CPTU results using the normalized friction ratio as

where

If the sleeve friction can be taken as an estimate of the remolded strength, then sensitivity may also be obtained by combining sleeve resistance with an estimate of undisturbed strength from tip resistance or pore pressure.

Stress history – Preconsolidation Stress, Ϭ'p

The results of the CPT/CPTU may be used to make an estimate of the stress history of clays, by either estimating the preconsolidation stress, o'_p, or the overconsolidation ratio, OCR. Both empirical and analytical models have been suggested predicting o'_p and OCR using the corrected tip resistance or pore water pressure. More rigorous numerical models have also been used. Chen & Mayne (1994) presented a review of the various approaches suggested. In some cases, piezocones with multiple pore water pressure sensor locations are required. Table 4.8 summarizes the different approaches for estimating stress history for CPT/CPTU results.

Ϭ'P from qc

A direct relationship between q_c and the preconsolidation stress, o', has been noted for different clays. Tavenas 8c Leroueil (1987) showed that for sensitive Canadian clays

Mayne & Kemper (1988) suggested a general correlation between q_c and 0_P as

Table 4.8 Summary of empirical approaches for estimating stress history from CPT/CPTU

Parameter	General correlation	References
O'p		Tavenas & Leroueil (1987) Mayne & Kemper (1988)
		Mayne (1986) Tavenas & Leroueil (1987)
		Powell et al. (1989)
		Roy et al. (1981) Powell etal. (1989) Mayne (1995)
		Op =0.8(qt-U|)
OCR		Mayne & Kemper (1988)
		Sully et al. (1988)
		Chen & Mayne (1994)

Figure 4.33 Correlation between a'_? and CPT tip resistance. (After Mayne & Kemper 1988.)

As shown in Figure 4.33, values of k_c for both electric cones and for mechanical cones vary between about 2 and 8. A general trend to these data which may be useful for preliminary work may be taken as

It has also been suggested (e.g., Mayne 1986; Tavenas & Leroueil 1987) that an estimate of Op may be made using the net tip resistance, q_c - c_vo.

Ϭ'P from qt

Empirical correlations between the preconsolidation stress, o'_p, and the corrected cone tip resistance, q_t, have been suggested for a number of different clay deposits. These correlations have the general form

The value of a, typically is in the range of 0.20 to 0.40. Table 4.9 presents a summary of different reported values of a_t. The use of Equation 4.73 and a, requires that pore pressure be measured behind the cone base, i.e., in the u₂ position so that q_c may be corrected to q_t. Data collected by Mayne (1995) are shown in Figures 4.34 and 4.35.

c'p from Δu

Based on the results of pile tests, Roy et al. (1981) suggested that g'_p could be related directly to the pore pressure difference Au = u_x - u_D for sensitive clays in Canada. For Au measured at the pile tip, they found that

Table 4.9 Reported values of a_t

Figure 4.34 Estimation of o'_p from CPTU net tip resistance. (After Mayne 1995.) for Au measured on the shaft,

Since the pore water pressure obtained from the CPTU is dependent on the location of the piezoelement, specific correlations for predicting o'_p from Au would be needed for different cone designs. Mayne (1995) presented a summary of the available data shown in Figures 4.36 and 4.37, which shows that

and

Note that fissured clays do not follow the trend lines of these data since the measured pore water pressures are often dependent on other soil behavior. In highly overconsolidated clays, Equation 4.46b may not provide a realistic estimate of c'_p since Au may be zero or negative during penetration (Figure 4.37).

Figure 4.35 Global estimation of Op from CPTU net tip resistance. (From Mayne I995.J

Figure 4.36 Estimation of Op from Ди, (Type I). (After Mayne 2006.)

Figure 4.37 Estimation of Op from Ди₂ (Type 2). (After Mayne 2006.)

Ϭ'P from qt and u

Results presented by Mayne & Chen (1994) suggested that OCR may be determined from using both q_t and u. This approach may be used to make a direct estimate of o'_P as