Soil behaviour type from the CPT: an update

An initial application of CPT results is to evaluate soil type and soil stratigraphy. One of the more common CPT-based methods to estimate soil type is the chart suggested by Robertson et al (1986) based on cone resistance, qc and friction ratio, Rf. Although newer charts have been developed based on normalized parameters, the simple chart based on qc and Rf is still popular because it is easy to use and can be used in real-time during the CPT. This paper provides an update on the soil behaviour type charts based on basic CPT measurements of qt and fs. and proposes a new soil behaviour index for non-normalized CPT results. The chart by Robertson et al (1986) uses the basic CPT measurements of qc and fs and has 12 soil types, whereas the chart by Robertson (1990) uses normalized parameters and has 9 soil types. The different soil types in each chart have sometimes created some confusion when comparing results. The advantage of the early Robertson et al (1986) chart was that it could be used in real-time to evaluate soil type during and immediately after the CPT, since it only requires the basic CPT measurements. Although the normalized charts of Robertson (1990) are considered more reliable because they use CPT parameters normalized in terms of effective stress, they can only be applied after the CPT during post-processing, since they require information on soil unit weight and groundwater conditions that are not available during the CPT. This paper provides an update to the early Robertson et al (1986) chart using non-normalized CPT results. Zone Soil Behavior Type 1 2 3 4 5 6 7 8 9 10 11 12 Sensitive fine grained Organic material Clay Silty Clay to clay Clayey silt to silty clay Sandy silt to clayey silt Silty sand to sandy silt Sand to silty sand Sand Gravelly sand to sand Very stiff fine grained* Sand to clayey sand* * Overconsolidated or cemented Figure 1 SBT chart by Robertson et al (1986) based on CPT cone resistance, qt, and friction ratio, Rf (where Rf = (fs/qt)100%) 2 SOIL BEHAVIOUR TYPE (SBT) Robertson et al (1986) and Robertson (1990) stressed that the CPT-based charts were predictive of Soil Behaviour Type (SBT), since the cone responds to the in-situ mechanical behavior of the soil and not directly to soil classification criteria based on grain-size distribution and soil plasticity (e.g. Unified Soil Classification System, USCS). Grainsize distribution and Atterberg Limits are measured on disturbed soil samples. Fortunately, soil classification criteria based on grain-size distribution and plasticity often relate reasonably well to in-situ soil behaviour and hence, there is often good agreement between USCS-based classification and CPT-based SBT (e.g. Molle, 2005). However, several examples can be given when differences can arise between USCS-based soil types and CPT-based SBT. For example, a soil with 60% sand and 40% fines may be classified as ‘silty sand’ (sand-silt mixtures) or ‘clayey sand’ (sand-clay mixtures) using the USCS. If the fines have high clay content with high plasticity, the soil behaviour may be more controlled by the clay and the CPT-based SBT will reflect this behaviour and will predict a more clay-like behaviour, such as ‘clayey silt to silty clay’ (SBT zone 5, Fig. 1). If the fines were non-plastic, soil behaviour will be controlled more by the sand and the CPT-based SBT would predict a more sand-like soil type, such as ‘silty sand to sandy silt’ (SBT zone 7, Fig 1). Very stiff, heavily overconsolidated finegrained soils tend to behave more like a coarse-grained soil in that they tend to dilate under shear and can have high undrained shear strength compared to their drained strength and can have a CPT-based SBT in either zone 4 or 5 (Fig. 1). Soft saturated low plastic silts tend to behave more like clays in that they have low undrained shear strength and can have a CPT-based SBT in zone 3 (Fig. 1). These few examples illustrate that the CPT-based SBT may not always agree with traditional USCS-based soil types based on samples and that the biggest difference is likely to occur in the mixed soils region (i.e. sand-mixtures & silt-mixtures). Geotechnical engineers are often more interested in the in-situ soil behaviour than a classification based only on grain-size distribution and plasticity carried out on disturbed samples, although knowledge of both is helpful. In general, the normalized chart (Robertson, 1990) provides more reliable identification of SBT than the non-normalized charts, although when the in-situ vertical effective stress is between 50 kPa to 150 kPa there is often little difference between normalized and non-normalized SBT. The term SBTn will be used to distinguish between normalized and non-normalized SBT. 3 MODIFIED NON-NORMALIZED SBT CHART The early Robertson et al (1986) SBT chart was based on cone resistance, qt (or qc) on a log scale with friction ratio, Rf on a natural scale. Figure 2 provides an update of the chart in terms of dimensionless cone resistance, (qc/pa), where pa = atmospheric pressure (pa = 1 bar = 100 kPa = 0.1 MPa) and Rf (in percent), both on log scales to expand the portion where Rf < 1%. The number of soil behaviour types has also been reduced to 9 to match the Robertson (1990) chart. Table 1 summarizes the unification of the 12 (Robertson et al.1986) SBT zones to match the 9 (Robertson, 1990) SBTn zones. Reducing the number of SBT zones in the non-normalized chart allows easier comparison between the normalized and non-normalized SBT’s. Table 1 Proposed unification between 12 SBT zones (Robertson et al, 1986) and 9 SBTn zones (Robertson, 1990) SBT zone Robertson et al (1986) SBTn zone Robertson (1990) Proposed common SBT description 1 1 Sensitive fine-grained 2 2 Clay organic soil 3 3 Clays: clay to silty clay 4 & 5 4 Silt mixtures: clayey silt & silty clay 6 & 7 5 Sand mixtures: silty sand to sandy silt 8 6 Sands: clean sands to silty sands 9 & 10 7 Dense sand to gravelly sand 12 8 Stiff sand to clayey sand* 11 9 Stiff fine-grained* * Overconsolidated or cemented Jefferies and Davies (1993) identified that a Soil Behaviour Type Index, Ic, could represent the SBTn zones in the normalized chart where, Ic is essentially the radius of concentric circles that define the boundaries of soil type. Robertson and Wride, (1998) modified the definition of Ic to apply to the Robertson (1990) Qt – Fr chart. When the non-normalized SBT chart is presented on log-log scales (Figure 2) the boundaries are also essentially concentric circles and a non-normalized Soil Behaviour Type Index, ISBT can also be defined by: ISBT = [(3.47 log(qc/pa)) + (log Rf + 1.22)] (1) where: qc = CPT cone resistance (or corrected cone resistance, qt) Rf = friction ratio = (fs/qc)100% fs = CPT sleeve friction The non-normalized SBT index (ISBT) is essentially the same as the normalized SBTn index (Ic) but only uses the basic CPT measurements. In general, the normalized Ic provides more reliable identification of SBT than the non-normalized ISBT, but when the insitu vertical effective stress is between 50 kPa to 150 kPa there is often little difference between normalized and non-normalized SBT.