REOLOGYCAL ANALYSIS
GEOTHECNICAL CHARACTERIZATION GRAIN SIZE ANALYSES
Grain size analyses confirmed the great variability of the analyzed samples (Table.6.10). For all sample shape curve parameters have also been evaluated:
uniformity coefficient (Cu) and coefficient of curvature (Cc).
Uniformity coefficient (Cu) has been evaluated by considering the following equation:
Eq 6.1
Where D60 is the grain diameter for which 60% of the sample is finer than D60 and D10 is the diameter for which the sample is finer than D10. The larger the Cu value the wider the size distribution and vice versa.
The coefficient of curvature (Cc) has been evaluated by considering the following equation;
Eq.6.2
Where D60 is the grain diameter for which 60% of the sample is finer than D60, D30
is the diameter for which the sample is finer than D30 and D10 is the diameter for which the sample is finer than D10. A soil is well graded for coefficient of curvature ranging between 1 and 3, with Cc>4 for gravels and Cc> 6 for sands.
Specifically, in the case of samples collected from Sc1 borehole, granulometric curves (Fig.6.31) are characterized by similar patterns, except PN5 sample, which is characterized by a not gradual transition from sand to silt.
Figure 6.31 Grain size curves, samples collected from Sc1 borehole.
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Figure 6.32 Grain size curves, samples collected from Sc2 borehole.
Figure 6.33 Grain size curves, samples collected from Sc3 borehole.
Looking at the data listed in Table 6.10 it is possible to see that all relative percentages tend to change without any correlation with depth.
The lowest uniformity coefficient characterizes the PN8 sample, whereas the higher one characterizes the MC4 sample.
No samples show a percentage of clay fraction higher than 22 wt%.
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Table 6.10 Grain size distributions.
Notes:CF=clay fraction; Cu= Uniformity coefficient; Cc= curvature coefficient.
SPECIFIC GRAVITY TEST
Despite the great lithological as well as mineralogical variability of the analyzed samples, their Gs values are quite similar, ranging between 2.56 and 2.72 (kN/m3) (Table. 6.11).
Specifically:
Specific gravity for all samples collected from Sc1 core ranges between 26.36 and 27.44 (kN/m3), for an average value of 26.85 (kN/m3).
Specific gravity for sample collected from Sc2 core ranges between 25.09 and 27.15 (kN/m3), for an average value of 26.29 (kN/m3).
Specific gravity for samples collected from Sc3 core ranges between 25.58 and 27.54 (kN/m3) for an average value of 26.61 (kN/m3).
ATTERBERG LIMITS
The WL, WP and IP values (Tab.6.12), obtained by the procedures described in the
“Methods” chapter range as here reported:
Sc1 32.51 ÷44.52 (WL); 18.38÷27.29 (WP); 11.80÷17.23 (IP)
Table 6.12 Atterbeerg limits values.
Notes: A= activity (Skempton 1953); WL=liquid limit; WP=plastic limit; IP=plastic index.
These data have been plotted into the Casagrande chart, which allows to futher characterize the soils by comparing the plastic index (IP) and the liquid limit (WL).
An empirical boundary called “A-line”, whose slope is expressed by the equation:
Eq. 6.3
separates inorganic clays from inorganic silts and organic soils.
Casagrande (1932) also defined the U-line which is considered the limit beyond which the plastic index data are too large for the measured liquid limit. The U-line is expressed by the equation:
Eq.6.4
None of the analyzed samples is located above this line.
Further vertical subdivisions of the chart allow to distinguish other engineering properties such as compressibility, permeability and toughness (Casagrande, 1932).
The analyzed data fall in the fields of the chart assigned to “inorganic silts and organic clay, mediumcompressibility” and “inorganic clays highcompressibility”.
Furthermore, one sample (PR5) is classified as “inorganic clays, low plasticity”, whereas the sample MC14 is classified as “inorganic silts, low compressibility”.
Skempton vs. Casagrande
The plastic properties of the clays are related to their ability to absorb water in the crystal structure; this aptitude depends on the mineral composition, specifically on the specific surface area (SSA) of the mineral phases. The smaller the clay particle size, the higher the SSA. The water molecules are electrostatically attracted to the surface of the clay crystal through hydrogen bonding. Consequentially the smaller the clay particle size, the higher the amount of adsorbed water. On this basis, several authors used the Atterbeg limits to obtain information about the mineralogical properties of soils, by using empirical correlations (Skempton, 1953; Holtz & Kovacs 1981; Cerato & Lutenegger, 2005):
Two tools have been used with this aim: Skempton diagram (Skempton, 1953) and Casagrande Chart (Holtz & Kovacs, 1981).
Skempton (1953) defined as “activity of a soil” the ratio between the plasticity index (Ip) and the clay fraction (CF) (Cerato & Lutenegger, 2005).
Eq. 6.6
On the basis of their Activity (A), clays have been subdivided into three groups:
Inactive, Normal and Active (Eq. 6.6). Each category is related to specific types of mineral phases, which are characterized by a specific SSA (Fig. 6.34).
Figure 6.34 Skempton diagram.
Inactive clays Normal clays Active clays
SC1 SC2 SC3
As regards the Casagrande chart, Holtz & Kovacs (1981) have shown the correlation between the liquid limit and the plastic index (Fig.6.35), and the mineralogical properties. For example samples located above the A-line, near the U-line, should contain great amount of montmorillonite. Samples located below the A-line should be mainly characterized by the presence of kaolinite. Finally illite should be the most representative mineral phase for all samples placed above the A-line.
By plotting the Termini-Nerano samples in the Skempton diagram (Fig.6.34) as well as in the Casagrande chart (Fig.6.35), it is possible to see that most of them are characterized by illite and kaolinite, and only a few samples should contain montmorillonite as main phase.
Figure 6.35 Casagrande chart: 1) Inorganic silts, low compressibility; 2) Inorganic silts and organic clays, medium compressibility;3) Inorganic silts and organic clays high compressibility; 4)Inorganic clays, low plasticity; 5) Inorganic clays, medium plasticity;
6) Inorganic clays, high plasticity.
Moreover, there is not perfect match between the two diagrams, as it is possible to verify by looking at data listed in Table 6.12. For example, the sample PN1 is characterized by active clays (montmorillonite) in the Skempton chart, but at the same time, it falls in the field of the Casagrande chart assigned to illite. The same happens to the PN7 and MC12 samples. The last sample contains only 4 wt% of clay fraction, suggesting that the relative activity is not reliable.
In a few cases, samples considered to be constituted by normal clays (e.g. illite), are mainly characterized by kaolinite (e.g. PN5 and MC5 samples).
All other samples are characterized by a good correlation between Casagrande chart and Skempton diagram.
Further correlations between geotechnical and mineralogical parameters are reported in the “Discussion” chapter.
USCS CLASSIFICATION
In the engineering and geology disciplines the most used soil classification method is the “Unified Soil Classification System” which allows to describe the
texture and the grain size of a soil. USCS can be applied to the unconsolidated materials (Fig. 6.36).
Figure 6.36 USCS classification
The USCS classification allows to subdivide soils in three major groups (Bhargavi
& Jyothi, 2009), on the basis of the major geotechnical properties described above:
Coarse grained soil (e.g. sands and gravels)
Fine grained soils (e.g. silts and clays)
Highly organic soils
The analyzed samples have been classified as follow (Table.6.13).
Table 6.13 Skempton data vs. Casagrande data.
ID
Samples Activity Skempton mineralogical classification
Notes: Active clay e.g. montmorillonite (Mnt); Normal clay e.g. illite (Ilt); Inactive clay e.g. Kaolinite (Kln);
DIRECT SHEAR TEST
Shear tests have been carried out to evaluated peak and residual shear strength of the collected undisturbed samples (Tab. 5.4).
Possible explanation of the obtained results will be shown in the next chapters. As shown in Figure 6.37, the cohesion for SC1 is null, whereas the shear strength as well as the friction angle decreases passing from peak to residual data (Fig. 6.37).
Figure 6.37 Shear test diagram SC1 sample.
In the case of the SC2 sample (Fig. 6.38), the transition from peak to residual values is characterized by the increase of the cohesion and the shear strength, whereas the friction angles remains stable.
Figure 6.38 Shear test diagram SC2 sample.
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These data will be explained in the Discussion and Conclusions.
The data obtained from direct shear strength tests have been used to evaluate another parameter called “Brittleness Index” (Bishop, 1967) useful to verify the existence of retrogressive movement (Cruden & Varnes, 1996):
was characterized by residual strength higher than peak strength. The obtained value (38.9%) indicates an area where a retrogressive movement is active, as it was also confirmed by the field survey.
FLOW BEHAVIOUR CHARACTERIZATION