Proposal for a New Classification of Common Inorganic Soils for Engineering Purposes

Abstract

This study proposes a new soil classification based on the different behaviour of two fractions that can generally be distinguished in clayey soils: the “active, binder” fraction composed of clay minerals (conventionally constituted of particles less than 0.002 mm, termed clay fraction) and the “inert” fraction composed of non-clay mineral particles (greater than 0.002 mm). Apart from the soil stress history, the clay percentage (and the type of clay minerals it contains) is of primary importance for the mode of soil behaviour. Hence, inorganic soils are assigned into soil groups based on their clay percentages. Each group includes soils with similar intrinsic geotechnical properties. In describing the soil groups, the terms “grainy” and “fine” are used to distinguish soils with behaviour dominated by the characteristics of their granular and clay phase constituents, respectively. Between these, lie soils (semi-grainy and semi-fine) that are “transitional” between grainy and fine soils, with an intermediate behaviour determined by the increase in the clay fraction. Particularly for common inorganic soils with behaviour dominated by the clay phase, both the soil liquid limit and the clay percentage allow to estimate some soil properties such as the plasticity index, clay activity, intrinsic compressibility and residual shear angle.

Appendix: Example of the New Soil Classification

Figure 2 shows an example of possible grading curves. For each soil, A and B, two clay contents (see curves A–A₁, A–A₂ and B–B₁, B–B₂) are shown in order to highlight the importance of CF in soil behaviour. First, each soil is classified (see Table 3) on the basis of the grading curve. Then, applying either the grainy soil or fine soil procedures as appropriate, more information can be obtained as follows:

Fig. 2

Example of possible grading curves. For each soil, A and B, two clay contents (curves A–A₁, A–A₂ and B–B₁, B–B₂) are shown in order to highlight the importance of CF in soil behaviour (rather than the content of silt and clay adopted by the ASTM and ISO standards). This soil data are also shown in Fig. 1 and Table 3. More information on the soil classification is given in the Appendix. According to ASTM and ISO standards, soils with granulometric curves that lie above and below the respective bold points plotted, are classified as fine soils and coarse soils, respectively

Table 3 Classification of soils shown in Fig. 2 using Table 2 (or Fig. 1)

Soil A ₁ —Soil group: grainy soil (G); Soil name: sandy silt (saSi), (from the two predominant soil fractions).

It can be characterized following the grainy soil procedure:

Calculation of the uniformity coefficient, with Eq. (2)

$$ C_{\text{U}} = d_{60} /d_{10} = 0.024/0.006 = 4 $$

Calculation of the coefficient of curvature, using Eq. (3)

$$ C_{\text{C}} = \left( {d_{30} } \right)^{2} /\left( {d_{10} \times d_{60} } \right) = (0.011)^{2} /(0.006 \times 0.024) = 0.84\, ({\text{soil}}\;{\text{even-graded}}) $$

Soil A ₂ —Soil group: fine soil (F); Soil name: clay (Cl), (derived from the principal soil fraction).

It can be characterized following the fine soil procedure. The soil properties (in addition to CF = 62 %) will be affected by the W _L value, for example

• (a) (assuming) W _L = 50 %

  Calculation of the plasticity index, using Eq. (5)

  $$ I_{\text{p}} = 0.96W_{\text{L}} - (0.26CF + 10) = 0.96 \times 50-(0.26 \times 62 + 10) = 22\,\% $$

  Calculation of the clay activity, using Eq. (6)

  $$ A = I_{\text{p}} /CF = 22/62 = 0.35\,({\text{soil}}\;{\text{of}}\;{\text{low}}\;{\text{activity}}) $$

  Calculation of the intrinsic water compression index, using Eq. (7)

  $$ \begin{aligned} W_{{C_{\text{c}} }}^{*} =\, & 0.35W_{\text{L}} -(0.10CF + 4) \\ = \,& 0.35 \times 50-(0.10 \times 62 + 4) \\ = \,& 7.3\,\% \,({\text{soil}}\;{\text{of}}\;{\text{low}}\;{\text{compressibility}}) \\ \end{aligned} $$

  In terms of the void ratio (assuming the density of soil particles, e.g., ρ = 2.65 g/cm³), the intrinsic compression index using Eq. (8) is: $C_{\text{c}}^{*} = \left( {W_{{C_{\text{c}} }}^{*} /100} \right)\rho = 0.19$

  The expected value of the residual shear friction angle, φ′_r, should lie in the 12°–15° range.

• (b) (assuming) W _L = 120 %

  Calculation of the plasticity index

  $$ I_{\text{p}} = 0.96W_{\text{L}} -(0.26CF + 10) = 0.96 \times 120-(0.26 \times 62 + 10) = 89\,\% $$

  Calculation of the clay activity

  $$ A = I_{\text{p}} /CF = 89/62 = 1.43\,({\text{soil}}\;{\text{of}}\;{\text{high}}\;{\text{activity}}) $$

  Calculation of the intrinsic water compression index

  $$ \begin{aligned} W_{{C_{\text{c}} }}^{*} =\, & 0.35W_{\text{L}} -(0.10CF + 4) \\ =\, & 0.35 \times 120-(0.10 \times 62 + 4) \\ =\, & 31.8\,\% \,({\text{soil}}\;{\text{of}}\;{\text{high}}\;{\text{compressibility}}) \\ \end{aligned} $$

  In terms of the void ratio (assuming the density of soil particles, e.g., ρ = 2.73 g/cm³), the intrinsic compression index is $C_{\text{c}}^{*} = \left( {W_{{C_{\text{c}} }}^{*} /100} \right)\rho = 0.87 .$

  The value of the residual shear friction angle, φ′_r, is expected to fall in the 4°–10° range.

Soil B ₁ —Soil group: grainy soil (G); Soil name: silty sand (siSa).

It can be characterized following the grainy soil procedure:

Calculation of the uniformity coefficient

$$ C_{\text{U}} = d_{60} /d_{10} = 0.148/0.010 = 14.8 $$

Calculation of the coefficient of curvature

$$ \begin{aligned} C_{\text{C}} =\, & \left( {d_{30} } \right)^{2} /\left( {d_{10} \times d_{60} } \right) = (0.030)^{2} /(0.010 \times 0.148) \\ =\, & 0.61\,({\text{soil}}\;{\text{medium-graded}}) \\ \end{aligned} $$

Soil B ₂ —Soil group: semi-fine soil (S-F); Soil name: clayey sand (clSa).

It can be characterized following the fine soil procedure:

$$ \begin{aligned} CF =\, & 36\,\% ;\;CF({\text{of}}\;{\text{the}}\;{\text{soil}}\;{\text{fraction}} < 0.425\,{\text{mm}}) \\ = \,& 36/0.82 = {\text{ca}}\;44\,\% ;({\text{assuming, e}} . {\text{g}} .)W_{\text{L}} = 88\,\% \\ \end{aligned} $$

Calculation of the slope k

$$ {\text{k}} = W_{\text{L}} /CF = 88/44 = 2 $$

Calculation of the W _L corresponding to the entire soil

$$ W_{\text{L}} = {\text{k}}CF = 2 \times 36 = 72\,\% \,({\text{or}}\;{\text{alternatively}},W_{\text{L}} = 88 \times 0.82 = 72\,\% ) $$

Calculation of the soil plasticity index

$$ I_{\text{p}} = 0.96W_{\text{L}} -(0.26CF + 10) = 0.96 \times 72-(0.26 \times 36 + 10) = {\text{ca }}50\,\% $$

Calculation of the clay activity

$$ {\text{A}} = I_{\text{p}} /CF = 50/36 = 1.38\,({\text{soil}}\;{\text{of}}\;{\text{high}}\;{\text{activity}}) $$

Calculation of the intrinsic water compression index

$$ \begin{aligned} W_{{C_{\text{c}} }}^{*} =\, & 0.35W_{\text{L}} -(0.10CF + 4) \\=\,& 0.35 \times 72-(0.10 \times 36 + 4) \\ =\, & 17.6\,\% \,({\text{soil}}\;{\text{of}}\;{\text{high}}\;{\text{compressibility}}) \\ \end{aligned} $$

In terms of the void ratio (assuming, e.g., ρ = 2.73 g/cm³), the soil intrinsic compression index is

$$ C_{\text{c}}^{*} = \left( {W_{{C_{\text{c}} }}^{*} /100} \right)\rho = 0.48 $$

Table 4 reports the parameters calculated based on the soil W _L and CF, according to the new classification. The same soils classified according to the ASTM standard (D 2487-10) are shown in Table 5 to facilitate the comparison between the two methods.

Table 4 Summary of soil properties (Fig. 2) according to the proposed classification

Table 5 Classification of soils (Fig. 2) according to the current ASTM standard

It is worth noting that (apart from the two fundamentally different criteria to distinguish the soil groups) the ASTM standard classifies the <0.425 mm soil fraction (clay, silt and fine sand) as “silt” or “clay” using the Casagrande plasticity chart. However, it has been demonstrated that on the empirical chart of Casagrande, the position of the clay and silt zones are not accurate (the silt zone should lie above the clay zone).