CPTu interpretation in fine grained soils

In geotechnical practice, the CPTu has three main applications: (i) stratigraphic profiling, (ii) estimation of the geotechnical parameters, and (iii) direct geotechnical design. By providing near-continuous data measurement, the CPTu is particularly suitable for the preliminary profiling of the investigated site and identification of the soil type. Then, additional information is collected from other field and laboratory

tests. However, it has been shown that a wide range of parameters can be derived from CPTu data, including strength, stiffness, and permeability (k) of the investigated soil. A unified approach in the interpretation of CPTu data is presented by Robertson (2009), where a variety of soil parameters is addressed. In particular, Robertson (2013b) investigated the perceived applicability of CPTu in deriving geotechnical parameters in different soil conditions (Table 3.3).

Table 3.3. Applicability of the CPT in deriving soil properties (Robertson 2013b) Soil type Initial state parameters Strength

parameters Deformation

Applicability: 1, high reliability; 2, high to moderate; 3, moderate; 4, moderate to low; 5, low reliability

The CPTu has been widely used for estimating the stress history and su of fine-grained soils. Indeed, the CPT is performed in clayey soils in undrained condition;

therefore, the prediction of the effective parameters (e.g., effective friction angle) is not accurate. In addition, the stiffness parameters are characterized by high uncertainty. However, improvements can be achieved using additional sensors, such as the seismic module, which allow for the measurements of vs.

Since its introduction in geotechnical practice, the CPT has been used for profiling and identifying the soil type. Several authors proposed classifications systems that link the cone parameters (qcand fs or Fr) to the soil type (Begemann 1965; Robertson et al. 1986; Robertson 1990). It was observed that fine-grained soils generally show high Fr and low qc, whereas coarse-grained soils are characterized by high qc and low Fr. These classification charts have been used over the past decades and subjected to modifications and improvements. Among them, the soil behavior type (SBT) chart proposed by Robertson et al. (1986) has become quite popular. The chart shown in Fig. 3.8 identifies 12 types of soil based on the qt and Fr values.

Robertson (1990) pointed out that this classification system depends on the in situ soil behavior and is based on the strength, stiffness, and soil compressibility. In contrast, the unified soil classification system (USCS) is based on the grain-size distribution and soil plasticity; therefore, it is not capable of considering the soil mechanical behavior. However, in most cases, there is a fairly good agreement between the USCS-based classification and the CPT-based SBT (Molle 2005).

Figure 3.8. Soil behavior type (SBT) chart (Robertson et al. 1986).

Recently, normalized SBT charts have been introduced to consider the influence of the increasing vertical stress on the CPT parameters. It is clear that the measured qc

tends to increase with the increasing overburden pressure. Similarly, when the cone is pushed at great depth, the measured excess pore pressure can be very high. The normalization should also account for the influence of the horizontal effective stress (V'h0), even though this aspect is generally neglected in common practice. Nowadays, these charts are widely used because the available pushing equipment allows CPT soundings to be performed at a depth of over 100 m. In particular, the differences between not normalized and normalized charts are evident when the value of V'v0

exceeds 150 kPa. Based on the theoretical work of Wroth (1984), Robertson (1990) developed two charts based on the normalized parameters Qt,Fr, and Bq, defined as follows.

Q_୲=^{୯౪ି஢౬బ}

஢_౬బ^ᇲ (3.4)

F୰= ቀ ^୤౩

୯_౪ି஢_౬బቁ x 100 (3.5)

B_୯ = ^{୳మି୳బ}

୯_౪ି஢_౬బ= ^ο୳

୯_౪ି஢_౬బ (3.6)

Here, Vv0is the in situ vertical total stress, V'v0 in the in situ effective vertical stress, u0 is the hydrostatic pore-water pressure, and 'u2 is the excess pore-water pressure.

The charts are illustrated in Fig. 3.9.

Figure 3.9. Normalized SBT chart (Robertson 1990).

Jeffries and Davies (1993) identified that the boundaries in the SBT chart can be assimilated to concentric circles. Therefore, they introduced an SBT index (Ic) by combining Qt and Fr, which represents the radius of these circles. The introduction of Ic allows for an easier representation of the soil type, which can be defined based on a single parameter. Robertson and Wride (1998) modified the definition of Ic to apply to the Qt–Fr chart proposed by Robertson (1990), thus obtaining the following equation.

Iୡ= ඥሾሺ3.47 െ logQ୲ሻ^ଶ+ ሺlog F୰+ 1.22ሻ^ଶሿ (3.7)

As shown in Fig. 3.10, small values of Ic indicate that the soil behaves as coarse-grained soil, whereas clayey soils are characterized by higher values of Ic. The transition between sands and clays is generally set by Ic= 2.60. Moreover, Jeffries and Davies (1993) pointed out that Ic is primarily controlled by soil compressibility, which can be linked to the soil plasticity; highly compressible plastic clays are characterized by Ic> 2.60.

Figure 3.10. Contours of Ic on a normalized soil behavior type (SBTn) chart (Robertson 1990).

The proposed normalization is observed to work fairly well in clays, whereas it is not suitable in coarse-grained soils. As an example, in clean sands, the qc tends to increase non-linearly with the increasing overburden pressure. Therefore, Robertson and Wride (1998) and the update by Zhang et al. (2002) suggested a normalized cone parameter with a variable stress exponent (n), defined as follows.

Q_୲୬= ቂ^{ሺ୯౪ି஢౬బሻ}

୮_౗ ቃ ቀ^୮౗

஢_౬బ^ᇲ ቁ^୬ (3.8)

Here, pa is the atmospheric pressure expressed in the same unit as qt and Vv0. This normalized chart is indicated hereinafter as SBTn. Note that for n = 1, Qtn= Qt.

Robertson (2009) proposed a methodology to estimate the stress exponent based on the Ic and effective overburden stress, defining the stress exponent as follows:

n = 0.38ሺI_ୡሻ + 0.05 ቀ^஢_୮^౬బᇲ

౗ቁ െ 0.15, (3.9)

where n” 1.0.The proposed updated contours are shown in Fig. 3.11. As discussed, the stress exponent ranges between 0.5 and 0.9 for most coarse-grained soils, whereas it is equal to 1.0 in the clay region.

Figure 3.11. Contours of the stress exponent (n) on normalized SBTn chart (Robertson 2009).