Basis of Soil Classification

In this article we will discuss about:- 1. Need for Soil Classification 2. Particle Size Classification 3. Scientific Basis 4. Textural Classification 5. AASHTO Classification 5. Unified Soil Classification System.

Need for Soil Classification:

Soil classification is a system of arrangement of soils into different groups so that soils within a group have similar characteristics and behavior. It is more convenient to study the behavior of groups than that of individual soil. The purpose of soil classification is to identify the group to which a given soil belongs. This enables the prediction of its behavior and its suitability as a foundation soil or construction material.

Soil classification provides standard terms to describe different groups of soils with distinct characteristics, among different engineers at the national and international level. Thus, soil classification serves as a language of communication to describe soils, thereby avoiding possible errors due to the use of local terminology.

Particle Size Classification:

The size of soil particles is found to have a significant influence on the engineering characteristics of the soil. As soils are formed by weathering, large soil particles are those that have sustained the disintegrating effects of weathering. Hence, the larger the particle size, the higher the stability and strength of the soil. The initial soil classification sys­tems were thus based on particle size.

The smallest particle which is visible to the naked eye has a particle size of 0.075 mm. Soils larger than this size are called coarse-grained soils and those finer than 0.075 mm are called fine-grained soils. Coarse-grained soils are generally formed by physical weathering and consist of electrically inert particles. Fine-grained soils are generally formed by chemical weathering and consist of clay minerals.

Clay minerals carry a negative charge on their surface, and forces of attraction and repulsion of electrical nature begin to exert their influence as the size of the particle goes on decreasing. Silts, which are also fine-grained soils, contain non-clay minerals and are electrically inert. Examples of coarse-grained particles are gravels and sands.

Soil classification systems based on particle size are discussed below:

1. US Bureau of Soil Classification:

This is one of the earliest soil classification systems and was developed by the US Bureau of Soils in 1895 (Fig. 6.1). As per this classification, soils that have a particle size > 2 mm are called gravels, those that have a particle size of 1—2 mm are called fine gravels, those that have a particle size of 0.05 —1 mm are called sands, those that have a par­ticle size of 0.005-0.05 mm are called silts, and those that have a particle size < 0.005 mm are called clays.

Sands are subdivided into four types:

i. Very fine sands with particle size 0.05-0.1 mm.

ii. Fine sands with particle size 0.10-0.25 mm.

iii. Medium sands with particle size 0.25-0.75 mm.

iv. Coarse sands with particle size 0.75-1 mm.

2. MIT Soil Classification:

The MIT soil classification system was developed by Prof. Gilboy of Massachusetts Institute of Technology as a simplification of the US Bureau of Soil Classification.

As can be seen from Fig. 6.2, this classification is very simple to remember as it consists of only two num­bers, 2 and 6. In it, soils are classified into four major categories. Sand and silt are each subdivided into three categories – fine, medium, and coarse sand (or silt).

3. International Classification:

The international classification was proposed at the 1927 International Soil Congress in Washington. As per this system, shown in Fig. 6.3, gravel has a particle size greater than 2 mm and has no subcategories. Sand, which is influenced more by particle size, is subdivided into four categories – fine, medium, coarse, and very coarse.

A new soil category, Mo, has been introduced that has a particle size of 0.02-0.1 mm and is finer than sand and coarser than silt. Silt and clay have the respective particle size range of 0.002-0.02 mm and 0.0002-0.002 mm.

Soil particles of size < 0.0002 mm were called ultra-clay or colloids. Mo, silt, and clay are subdivided each into two categories – fine and coarse.

4. Indian Standard Particle Size Classification:

The particle size classification recommended by the Bureau of Indian Standards (BIS; earlier called the Indian Standard Institution, ISI) is shown in Fig. 6.4. In this system, soil particles are classified into gravel, sand, silt, and clay.

As per the Indian standard particle size classification, clay has a particle size < 0.002 mm. Silt is coarser than clay, with a particle size of 0.002-0.075 mm. Sand, with a particle size of 0.075-4.75 mm, is subdivided into fine, medium, and coarse, as shown in Fig. 6.4. Gravel is coarser than sand and has a particle size of 4.75-80 mm. Particles coarser than gravel are called cobble, with a particle size of 80-300 mm, and boulder, which is coarser than cobble and has a particle size > 300 mm.

Scientific Basis for Soil Classification:

The following two criteria are found to be significant for soil classification after years of research and experience with wide-ranging soils, all over the world:

i. Particle size distribution of soils.

ii. Plasticity characteristics represented by the liquid limit and the plasticity index.

The soil in nature consists of a mixture of particles of different sizes rather than soil particles of single size. Thus, every soil consists of clay, silt, sand, and gravel in some proportion. Hence, to classify a given soil, it is necessary to determine its grain (or particle) size distribution. In addition to the grain size distribution, it is also necessary to consider the liquid limit and plasticity index of the soil for classifying the soil.

Textural Classification of Soil:

As soils occurring in the nature consist of a mixture of particles of various sizes, the textural classification [devel­oped by the US Public Roads Administration (PRA)] recommends classification of soils based on particle size distribution, instead of particle size.

The textural classification classifies soils on the basis of the proportion of clay, silt, and sand-sized particles in the soil as follows:

i. % Clay (< 0.005 mm).

ii. % Silt (0.005 – 0.05 mm).

iii. % Sand (0.05 – 2 mm).

The particle size to define sand, silt, and clay in the textural classification is the same as that used in the US Bureau of Soil Classification. Gravel particles of size greater than 2 mm are not considered in the textural classification.

The system consists of an equilateral triangular chart, as shown in Fig. 6.16. Each of the three sides of the triangle represents the % sand, % silt, and % clay from 0% to 100% in anticlockwise direction.

The triangular area of the chart is divided into a total of the following ten zones as specified in the following list:

i. Clay.

ii. Sandy clay and silty clay (two zones).

iii. Loam.

iv. Clayey loam, sandy loam, and silty loam (three zones).

v. Sand.

vi. Sandy clayey loam and silty clayey loam (two zones).

Any given soil with known percentages of sand, silt and clay, is plotted as a point on the classification chart, by drawing lines from each axis parallel to the lines shown in the key. The soil belongs to the zone in which the point falls on the chart.

For example, if a soil is composed of 45% sand, 40% silt, and 15% clay, the three lines are drawn from the respec­tive sides in the direction indicated by the key, as shown in Fig. 6.16. These lines intersect at point A situated in the zone “loam.”

A new term “loam” is used in the system, which is a soil type containing a mixture of sand, silt, and clay in the following proportion:

i. Sand = 30% – 50%.

ii. Silt = 30% – 50%.

iii. Clay = 0% – 20%.

The term “loam” is used in agricultural engineering to indicate the suitability of the soil for crops. As this term is not used in soil mechanics, the Mississippi River Commission (United States) proposed a modified triangular chart, removing the term “loam.” The modified triangular chart is shown in Fig. 6.17.

It consists of the following nine zones of soil:

i. Clay.

ii. Sandy clay and silty clay (two zones).

iii. Silt and sandy silt (two zones).

iv. Sand, silty sand, and clayey sand (three zones).

Following are the disadvantages of the textural classification:

i. Gravel and other coarser material are not considered in the classification.

ii. The system does not consider gradation characteristics of coarse-grained soils or plasticity characteristics of fine-­grained soils.

iii. Classification merely based on particle size does not help in predicting the behavior or properties of the soils.

With the development of the unified soil classification system, the textural classification system has become more or less obsolete.

AASHTO Classification of Soil:

The demerits of the textural classification system are overcome by the PRA classification system. This system is also known as the HRB (Highway Research Board) classification and the AASHTO (American Association of State Highway and Transportation Officials) classification. This classification system was first developed by Hogentogler and Terzaghi in 1929 and has been revised several times since then.

The system considers the grain size distribution as well as plasticity properties, that is, liquid limit and plasticity index, to classify soils. The system is useful to classify soils for construction of roads.

Group Index:

In AASHTO classification system, soil classification is done by computing an index called group index.

Group index is used to describe the relative performance of soils, when used for highway construction, and is defined by the following equation:

GI = 0.2a + 0.005ac + 0.01bd …(6.21)

where GI is the group index. Also, a is that portion of the percentage passing through the 75-µm sieve (ASTM no. 200 sieve) which is greater than 35 and does not exceed 75, expressed as a whole number (0 – 40) = F – 35; b is that portion of the percentage passing through the 75-µm sieve (ASTM no. 200 sieve), which is greater than 15 and does not exceed 55, expressed as a whole number (0 – 40) – F – 15; c is that portion of the liquid limit that is greater than 40 and does not exceed 60, expressed as a whole number (0 – 20) = LL –40; d is that portion of the plasticity index that is greater than 10 and does not exceed 30, expressed as a whole number (0 – 20) = PI – 10; and F is that percentage passing through the 75-µm or ASTM no. 200 sieve.

Substituting these parameters in the equation, we have –

GI = 0.2 (F – 35) + 0.005 (F – 35) (LL – 40) + 0.01 (F – 15) (PI – 10)

or GI = (F – 35) [0.2 + 0.005 (LL – 40)] + 0.01 (F – 15) (PI – 10) …(6.22)

From the definitions of a, b, c and d, their maximum possible values are 40, 40, 20, and 20. Substituting these values in Eq. (6.21), we get the maximum possible value of group index for any soil as 20. Similarly, the minimum possible values of a, b, c, and d are zero each and hence, the minimum possible value of group index for any soil is zero.

In calculating group index, any negative values, obtained within parentheses, should be taken as zero. Group index is rounded off to the nearest whole number. The AASHTO classification is useful to classify soils to check their suitability as subgrade material for highways, air fields, etc.

Classification Criteria and Soil Groups in AASHTO Classification:

As per the AASHTO classification, soils are divided into seven groups – A-1 to A-7. A-1 group consists of stone frag­ments, gravel, and sand with a maximum of 25% fines. A-3 group consists of fine sand with a maximum of 10% fines. Silty or clayey soil belongs to A-2, whereas pure silty soils with minimum 36% fines fall into A-4 and A-5 groups. A-6 and A-7 groups consist of clayey soils with minimum 36% fines.

The criteria used for classification in AASHTO soil classification are shown in Table 6.6. A soil is classified into a particular group based on %fines, plasticity index, and group index of the soil. For A-1-a group, the percentages of soil passing through the 2-mm and 425-µm sieves are the additional criteria. For classifying into A-1-b and A-3 groups, the percentage passing through the 425-µm sieve is the additional criterion. For all other groups, liquid limit of the soil is the additional criterion. The complete classification chart of AASHTO soil classification system is shown in Table 6.7.

Guide to Use AASHTO Soil Classification:

A simple guide to classify soils as per the AASHTO classification is given in Table 6.8.

Soil group A-7 is further divided into the following two subgroups:

1. A-7-5 if I_p < (LL-30).

2. A-7-6 if I_p> (LL-30).

The lower the number of the group, the more suitable is the soil as a highway material or subgrade. Thus, A-1 is the most suitable soil group, followed by A-2, A-3, A-4, A-5, and A-6 in decreasing order of their suitability as a highway material or as subgrade. A-7 is the least suitable of all soil groups.

The lower the group index, the more suitable is the soil as a highway material or subgrade. The minimum possible value of the group index is zero. The soil groups A-1-a, A-1-b, A-3, A-2-4 and A-2-5 have zero group index. Only the soil group A-7 can have maximum group index of up to 20. However, in the revised AASHTO classifica­tion, there is no specific maximum value for the group index. In showing the classified soil, the group index is to be mentioned within parentheses after the soil group. For example, when a soil is classified based on %fines, LL, and I_p as A-4, and if its group index is 3, the soil is finally designated as A-4 (3). The group index is to be specified even if its value is zero.

It is interesting to note that the group index is always zero for soil groups A-1, A-3, A-2-4, and A-2-5. Further, group index for soil groups A-2-6 and A-2-7 depends only on the plasticity index, and it increases by 1 for every 5% incre­ment in the plasticity index above 10. Similarly, the group index of soil group A-4 depends only on %fines, and it increases by 1 for every 5% increment in %fines above 35, as shown in Table 6.9.

Unified Soil Classification System:

The unified soil classification system was first developed in 1948 by Arthur Casagrande and was later modified in 1952 by the US Bureau of Reclamation (USBR) and the US Corps of Engineers. The system was also adopted by the American Society for Testing and Materials (ASTM).

The unified soil classification system is the most comprehensive of all the classification systems so far developed and is widely used today in most countries across the world, in all engineering problems involving soil.

The BIS has adopted the unified soil classification in framing IS – 1498 – 1970, “Code of practice for classification and identification of soils for general engineering purposes,” with only two minor changes to the original system.

The two changes are as follows:

i. The fine-grained soils are divided into three groups as low, medium, and high compressible soils in the IS soil classification based on values of liquid limit. However, in the unified soil classification, there are only two groups in fine-grained soils, that is, soils with low and high compressibility based on values of liquid limit.

ii. The upper limit line (U-line) in the plasticity chart defined by the equation PI = 0.9 (ω_L – 8), which is there m the unified soil classification, is omitted in the plasticity chart used in the IS soil classification. The U-line indicates the upper limit for the range of plasticity index and liquid limit so far found for any soils as per Casagrande. When the LL and PI for any soil plots above the U-line, they should be re-checked. Some highly active clays, such as bentonite, may plot high above the A-line close to the U-line.

Except for the above two differences, the unified soil classification is identical to the IS soil classification.