Smith's Elements of Soil Mechanics. Ian Smith

Чтение книги онлайн.

СКАЧАТЬ in 1947 was the basis of the soil classification system used in the UK for many years. With the widespread adoption of BS EN ISO 14688:2018 (BSI, 2018a,c), the classification of soils across Europe now follows a standardised approach.

In order to classify soils, the system considers natural soils as falling into one of the following categories: very coarse, coarse, fine and organic. The majority of soils are inorganic, and particle sizes and plasticity characteristics of these soils are used to identify the primary soil fraction. Developing the notion of fraction sizes listed in Table 1.1, BS EN ISO 14688‐2:2018 (BSI, 2018c) offers the principles of classification listed in Table 1.3.

      Since soils are usually composite (i.e. contain various amounts of different particle sizes) it will be the case that secondary and tertiary fractions will also exist in the soil. In soil classification and description, these additional fractions are used as adjectives and the primary fraction is the noun. The primary fraction is written in capitals: e.g. sandy, silty CLAY; gravelly, coarse SAND; clayey SILT.

For fine soils, the results of the liquid and plastic limits tests are used to classify these soils. Classification is done through the use of a plasticity chart (Fig. 1.9). To use the plasticity chart, a point is plotted whose coordinates are the liquid limit and the plasticity index of the soil. The soil is then classified by observing the position of the point relative to the sloping straight line drawn across the diagram. This line, known as the A‐line, is an empirical boundary between inorganic clays, whose points lie above the line, and organic silts and clays whose points lie below. The A‐line goes through the base line at (w_L = 20%, I_P = 0) so that its equation is:

Table 1.3 Principles of soil classification.

Soil group	Primary function	Criteria
Very coarse	Boulders (Bo)	50% of particles >200 mm in size
	Cobbles (Co)	200 mm > 50% > 63 mm
Coarse	Gravel (Gr)	63 mm > 50% > 2 mm
	Sand (Sa)	2 mm > 50% > 0.06 mm
Fine	Silt (Si)	Low plasticity or non‐plastic
	Clay (Cl)	Plastic

Fig. 1.9 (a) Plasticity chart based on BS 5030 (BSI, 2015) (b) Plasticity chart based on BS EN ISO 14688‐2 (BSI, 2018c).

      Two versions of the plasticity chart exist (BS 5930 and BS EN ISO 14688‐2) and either may be used: the former in the UK, the latter elsewhere in Europe. The boundaries between the degrees of plasticity are the same from both charts. It is only the symbols used to indicate the plasticity group that differ between the two.

      The U‐line in Fig. 1.9b indicates the approximate upper limit for natural soils and has equation,

      Example 1.5 Soil classification (i)

      1 Classify the soil of Example 1.2 whose particle size distribution curve is shown in Fig. 1.3.

      2 If the soil tested in Example 1.3 experienced 100% passing the 63 mm sieve, 96% passing the 2 mm sieve and 85% passing the 63 μm sieve, classify the soil.

       Solution:

      1 Using Tables 1.2 and 1.3, the soil is classified a medium graded gravelly SAND.

      2 The PSD may be plotted to visualise the results (see spreadsheet example_1.3_and_1.5b.xls) or the classification may be made by reviewing the data directly:Particle fractions:Gravel: 100 – 96 = 4%Sand: 96 − 85 = 11%Silt: 85 − 58 ^* = 27%Clay: 58*%

^*In the pipette analysis, 68.7% of the sample was found to be clay. That test was performed only on the fraction passing the 63 μm sieve. To establish the percentage of clay in the whole soil, we must consider the proportion passing the 63 μm sieve as a fraction of the whole soil sample.

Based on the percentages of each fraction, the soil may therefore be classified as a slightly sandy, silty CLAY.

      Since it is a clay, it must also be classified in terms of its plasticity. To do this we use the results of Example 1.4 together with the plasticity chart (Fig. 1.9). It is seen that the soil therefore is a slightly sandy, silty CLAY of high plasticity.

Example 1.6 Soil classification (ii)

      A set of particle size distribution analyses on three soils, A, B and C, gave the following results:

СКАЧАТЬ

---

• В начало
• <
• 23
• 24
• 25
• 26
• 27
• 28
• 29
• 30
• 31
• 32
• >
• В конец

	Percentage passing
Sieve size (mm)	Soil A	Soil B	Soil C
20	90	–	–
10	56	–	–
6.3	47	–	–