In geotechnical engineering, soil classification serves as a crucial framework for standardizing soil descriptions and grouping similar soils based on characteristics that profoundly influence their behaviour. These systems offer a systematic understanding of diverse soil types and their inherent properties, ultimately informing geotechnical engineering assessments and construction practices.

The primary determinant of a soil’s classification is the relative abundance of its constituent particle sizes: gravel, sand, silt, and clay. Additionally, specific attributes of the silt and clay fractions often come into play, particularly in distinguishing between these finer particle groups.

A key distinction arises from the definition of “clay” within soil classification. Unlike conventional size categorization, the term encompasses materials possessing specific mineralogical and behavioural characteristics. Clays are defined by the presence of clay minerals within the fines fraction, exhibiting distinct compositions and behaviours compared to silts and coarse-grained soils. Notably, clays inherently exhibit plasticity, the ability to remain deformed even after the removal of load. While finer particle sizes often correspond to clay minerals, some exceptions exist.

To quantify the plasticity characteristics of the fines fraction, laboratory Atterberg limit tests serve as the primary tool. These tests, including liquid limit and plasticity index measurements, provide essential data for classification purposes. However, in situations where laboratory testing is unavailable, simple “visual identification” tests can offer preliminary distinctions between clays and silts in the field.

The most popular methods of soil classification are the Unified Soil Classification System (USCS) and AASHTO Method. This article discusses the use of the USCS soil classification system and the typical range of engineering properties for different soil groups.

The Unified Soil Classification System (USCS)

The Unified Soil Classification System (USCS), as presented below, offers a widely adopted classification framework. Similar to the AASHTO system, it utilizes grain size distribution, liquid limit, and plasticity index as its primary classification criteria.

Soils are categorized into USCS groups designated by distinct symbols and corresponding names. Each symbol comprises two letters: the first indicating the dominant particle size fraction and the second serving as a descriptive modifier. In certain instances, dual symbols are employed to accurately represent the soil’s characteristics.

Coarse-grained soils are divided into two categories: gravel soils (symbol G) and sand soils (symbol S). Sands and gravels are further subdivided into four subcategories as follows.

symbol W: well-graded, fairly clean
symbol C: significant amounts of clay
symbol P: poorly graded, fairly clean
symbol M: significant amounts of silt

Fine-grained soils are divided into three categories: inorganic silts (symbol M), inorganic clays (symbol C), and organic silts and clays (symbol O). These three are subdivided into two subcategories as follows.

symbol L: low compressibilities (LL less than 50)
symbol H: high compressibilities (LL 50 or greater)

The most recognised and common classification of soils in engineering is shown in Table 1;

Typical Engineering Properties of Different Soil Groups

Compaction

Soil compaction is the process of mechanically increasing the soil’s density by reducing the air void space between its particles. This densification leads to several desirable outcomes, including higher bearing capacity, reduced permeability, and improved stability.

The basic laboratory test used to determine the maximum dry density of compacted soils is the Proctor test. In construction, the maximum dry density and its corresponding optimum moisture content are obtained from the proctor test. This is used as a guide in the field to check the effectiveness of the compaction achieved.

Several approaches exist for evaluating soil compaction in the field and laboratory. Here are a few prominent methods:

• Standard Proctor Compaction Test: This classic test involves compacting soil samples in a cylindrical mould at varying moisture contents and measuring the resulting dry density. The relationship between moisture content and dry density is plotted to determine the optimum moisture content (OMC) for achieving maximum density at a specified compaction effort.
• Modified Proctor Compaction Test: This method employs higher compaction energy compared to the standard test, simulating the harsher conditions encountered in certain construction projects. The OMC and maximum dry density for the modified test are typically higher than those obtained for the standard test.

Typical values of optimum moisture content and suggested relative compactions (based on the standard Proctor test) are shown in Table 2.

Permeability

Soil permeability describes the rate at which fluids flow through the porous matrix of soil, playing a critical role in numerous geotechnical and environmental applications. Measuring soil permeability accurately and efficiently is therefore essential for ensuring the stability and sustainability of constructed systems and mitigating potential environmental risks.

In the laboratory, the coefficient of permeability of soils is determined either through the falling head or constant head permeability tests. Typical values of the coefficient of permeability, K, are given in Table 3. Clays are considered relatively impervious, while sands and gravels are pervious. For comparison, the permeability of concrete is approximately 10^-10 cm/s.

Shear Strength

Shear strength describes the resistance of soil to deformation and failure under applied shear stresses, playing a pivotal role in the stability of slopes, foundations, and earth-retaining structures. Understanding and accurately measuring shear strength are therefore paramount for geotechnical engineers to ensure the safety and integrity of constructed systems within the intricate dance of forces acting upon the ground.

The equation for the shear strength failure envelope is given by Coulomb’s equation, which relates the strength of the soil, S, to the normal stress on the failure plane.
S = τ + c tanφ
φ is known as the angle of internal friction and c is the cohesion intercept, a characteristic of cohesive soils.

Representative values of typical strength characteristics φ and c are given in Table 4.

California Bearing Ratio (CBR)

California Bearing Ratio (CBR) plays a crucial role in pavement design and performance. This dimensionless index quantifies the relative strength of a soil compared to a standard crushed stone base material, serving as a critical indicator of its load-bearing capacity and susceptibility to deformation under traffic loads.

Table 5 gives typical CBR values.

Plate Bearing Value

The Plate Bearing Value (PBV) test offers insight into the soil’s ability to withstand applied loads. This critical in-situ technique sheds light on a soil’s bearing capacity, a fundamental property governing its suitability for supporting foundations, pavements, and other load-bearing structures.

The PBV test measures the load-deformation response of soil under a circular steel plate subjected to increasing pressure. The test quantifies the bearing capacity through the PBV itself, defined as the pressure at which the soil exhibits a predetermined, typically 12.5mm, deflection. Essentially, the PBV reflects the soil’s resistance to deformation under applied loads, providing a crucial indicator of its suitability for supporting structures.

The subgrade modulus (modulus of subgrade reaction), k, is the slope of the line (in psi per inch) in the loading range encountered by the soil. Typical values are shown in Table 6.

Clay minerals of natural soils are made up of silicon, aluminium, and/or iron and magnesium. They often have flat, layered structures with different shapes. Each clay particle in an expansive soil acts like a thin plate with negative charges on its surface and positive charges on its sides. The composition of clay can be thought of as different combinations of two basic building blocks. These building blocks are presented as simplified models for understanding the minerals’ structures.

Expansive soils, characterized by their significant volume changes (heave) upon wetting and shrinkage upon drying, constitute a major source of concern for civil engineering projects, impacting infrastructure stability and durability. These soils present complex geotechnical challenges due to their diverse mineralogical composition, sensitivity to moisture variations, and unpredictable behaviour under changing environmental conditions.

The primary clay mineral behind the expansive behaviour of these soils is the presence of smectite clays, particularly montmorillonite. These clays possess a 2:1 layered structure with interlayer spaces that readily accommodate water molecules, causing the layers to expand and the soil volume to increase. Upon drying, the interlayer water evaporates, resulting in layer contraction and soil shrinkage. This cyclic wetting and drying process can lead to significant differential movements, differential settlements, and the formation of desiccation cracks in expansive soils.

Therefore, clay minerals are an important microscopic factor affecting the engineering behaviour of soils. This article explores the common clay minerals and how their properties affect the expansive behaviour of soils.

Building Blocks of Clay Minerals

The building blocks have two basic elemental units: the silicon tetrahedron and the alumino-magnesium octahedron. They are shown in a simplified way in Figure 2. The silicon tetrahedron consists of silicon and oxygen atoms. Since silicon has a 4⁺ valence, it can bond with negative ions like oxygen (O^2-) or hydroxyl (OH^–), as seen in Figure 2(a). The relative sizes of the silicon and oxygen atoms make this structural unit take the shape of a tetrahedron.

The alumino-magnesium octahedron is made up of aluminium or magnesium atoms with hydroxyls around them, as seen in Figure 2(d). These atoms are placed so that they can be imagined as forming an octahedral shape.

In the silicon tetrahedron in Figure 2(a), the oxygen atoms have an unfulfilled chemical bond each. The oxygen atoms at the bottom of a tetrahedron are shared with nearby tetrahedra, and the resulting arrangement of tetrahedra makes sheets, as seen in Figure 2(b). Sharing of oxygen atoms among the tetrahedra satisfies the oxygen atoms at the bottoms of the tetrahedra, but the oxygen atoms at the tops still have unsatisfied bonds, as seen in Figure 2(b). So, the top face of the silica sheet can form chemical bonds with positive cations.

The octahedral units share hydroxyls to make a sheet structure, as seen in Figure 2(e). The placement of the octahedral units is such that the hydroxyls in the sheet structure have fulfilled chemical bonds. The central cation in the octahedral sheet can change.

The building blocks that are used to show the crystalline structure of the different clay minerals are represented by schematic symbols in Figures 2(c) and 2(f). By changing the way these two building blocks are arranged, different clay minerals can be formed.

Types of Clay Minerals

The different minerals are grouped according to the order of the sheets. For this article, we will only consider three basic clay minerals:

1. Kaolinite
2. Illite
3. Montmorillonite

Figure 3 shows simplified diagrams of the structures of these three minerals. The bonding between the different building blocks is very important for the behaviour of the different minerals.

The term bentonite is often used for expansive soils. This term means clays that have a lot of montmorillonite. Bentonite is a very plastic, swelling clay material that mainly has montmorillonite. It is mined for business and is used for many things like drilling fluids, slurry trenches, cosmetics, paint thickeners, and more. Not all expansive soils are bentonite, but they are often called bentonite, usually by people who are not engineers.

Kaolinite Clay Mineral

Kaolinite group minerals feature a 1:1 layered structure. Each layer consists of a single tetrahedral silica sheet directly bonded to an octahedral alumina sheet. Unlike the 2:1 structure of smectites like montmorillonite, the absence of an intervening octahedral sheet results in stronger interlayer bonding and minimal swelling behaviour.

The bonds between the silica and octahedral sheets affect the size of the mineral particles in a soil. The kaolinite structure in Figure 4 shows that there is a strong bond between the octahedral sheet and the top of the silica sheet. This is because of how the atoms are arranged and the fact that some of the hydroxyls in the octahedral sheets can be replaced by the oxygen atoms at the top of the silica tetrahedra. So, there is a strong chemical bond between the octahedral sheet and the silica sheet.

In Figure 3a, the chemical bonds at the bottom of the silica sheet are satisfied, and so, the bond between the octahedral sheet and the bottom of the silica sheet is mainly formed by weaker hydrogen bonds. That is called a weak bond. So, when the mineral breaks, the sheets will split at the weak bond. However, the bonds are strong enough that the clay particles can have a number of building blocks in each clay particle. Therefore, the kaolinite particles are relatively big with a lateral size of up to 1 micron (μm) or more, and a thickness of 1/3 to 1/10 of the lateral size.

Characteristics of Kaolin Clay Mineral

• High chemical stability: Kaolinite exhibits exceptional resistance to weathering and chemical attack due to its strong covalent bonds and low reactivity.
• Low cation exchange capacity (CEC): Compared to smectites, kaolinite possesses a significantly lower CEC due to the absence of exchangeable cations in the interlayer space.
• High whiteness and brightness: The pure aluminium silicate composition and minimal iron impurities endow kaolinite with excellent whiteness and brightness, making it desirable for applications requiring high optical qualities.
• Good rheological properties: The platy morphology and surface properties of kaolinite contribute to its ability to thicken and suspend particles in aqueous suspensions, essential for its use in various industrial processes

Montmorillonite Clay Mineral

Montmorillonite is a member of the smectite family, a group of 2:1 layered silicates characterized by a tetrahedral silica sheet sandwiched between two octahedral alumina sheets. This layered structure gives rise to several unique properties, including a large surface area, high cation exchange capacity (CEC), and the ability to intercalate guest molecules between the silicate layers.

The bond between the two silica sheets at the bottom is made by weaker van der Waals forces for the montmorillonite, and it is marked as “very weak” in Figure 2c, unlike just “weak.” Because of this, montmorillonite particles may have only one or two sets of building blocks in thickness. So, the clay particles may be very thin, about 10 Ångström or less.

The unit cell of montmorillonite consists of two silica tetrahedral sheets linked to a central octahedral alumina sheet via shared oxygen atoms. This basic structure forms repeating layers stacked upon each other with weak van der Waals forces holding them together. The negative charge on the basal oxygen atoms is balanced by exchangeable cations (commonly Na⁺, C^a2+, or Mg²⁺) residing in the interlayer space. These cations contribute to the high CEC of montmorillonite, enabling its selective adsorption of various cations and organic molecules.

Characteristics of Montmorillonite

1. Swelling: The presence of exchangeable cations and hydration of interlayer space allows montmorillonite to expand significantly upon contact with water, leading to increased volume and plasticity.
2. Adsorption: The high surface area and CEC of montmorillonite facilitate the adsorption of various pollutants, toxins, and organic molecules, making it useful for environmental remediation and water purification.
3. Cation exchange: The selectivity of montmorillonite towards specific cations can be exploited for soil amendment, nutrient retention, and catalysis.
4. Intercalation: Guest molecules, such as polymers and drug molecules, can be inserted into the interlayer space of montmorillonite, leading to the formation of nanocomposites with unique properties.

Illite Clay Mineral

Illite comprises a group of mixed-layer clay minerals with structural features mirroring both smectites and micas. Its basic 2:1 layer unit resembles that of smectites, featuring a single tetrahedral silica sheet sandwiched between two octahedral alumina sheets. However, unlike smectites, illite possesses interlayer potassium ions occupying vacant octahedral sites, resulting in stronger interlayer bonding and reduced swelling behaviour.

The illite particle has a similar structure to the montmorillonite particle. But in illite, the bond between the silica sheets at the bottom is made by potassium cations that are shared by nearby sheets. The potassium ions are the right size to fit into the gaps in the bottom of the silica sheet made by the tetrahedra. Sharing the potassium ions between the silica sheets makes a strong bond. So, the kaolinite and illite minerals are much less expansive than the montmorillonite.

Characteristics of illite clay minerals

• Intermediate swelling behaviour: Compared to highly swelling smectites, illite exhibits minimal swelling upon hydration due to the presence of fixed interlayer potassium ions and stronger layer bonding.
• Moderate cation exchange capacity (CEC): While lower than smectites, illite possesses a notable CEC due to the exchange of cations on the basal plane and edges, influencing its interactions with various cations and organic compounds.
• High thermal stability: Due to its strong Si-O-Al bonds and stable interlayer potassium ions, illite exhibits exceptional thermal stability, making it suitable for high-temperature applications.
• Excellent rheological properties: The platy morphology and surface properties of illite contribute to its ability to thicken and suspend particles in aqueous suspensions, valuable for its use in drilling fluids and other industrial processes.

The stress in the soil mass is affected by the previous loading history. When a new foundation is constructed, the new load can either increase or decrease the existing stresses in the soil. The response of the soil mass to the new load depends on the previous stress history. Therefore, it is important to understand the stress imprint of the soil mass before designing a foundation.

The term “stress imprint” refers to the state of stresses that are locked into the soil structure. These stresses are locked in because the soil particles have rearranged and formed bonds. The bonds prevent the stresses from being released, even when the load is removed. The stress imprint can be either positive or negative.

A positive stress imprint is created when the soil is loaded to a higher stress than it is currently experiencing (normally consolidated soil). A negative stress imprint is created when the soil is unloaded to a lower stress than it is currently experiencing (overconsolidated soil).

The stress imprint can affect the behaviour of the soil in a number of ways. For example, a positive stress imprint can make the soil more resistant to shearing, while a negative stress imprint can make the soil more susceptible to settlement.

The stress imprint can be difficult to measure, but there are a number of techniques that can be used. One common technique is to use a pressuremeter. A pressuremeter is a device that is inserted into the soil and used to measure the stresses in the soil.

In-Situ Stresses in Soil

In situ, the vertical stresses act on a horizontal plane at some depth z. These can be computed in any general case as the sum of contributions from n strata of unit weight γ_i and thickness z_i as;

p_o = ∑γ_iz_i —— (1)

In the context of the formation of soil deposits, the area of the land where soil accumulates is typically extensive, and the depth of the deposit keeps increasing until either the accumulation process or the internal weathering process halts. This transition leads to a gradual downward compression of the soil at any specific depth.

Likewise, the vertical stress also increases due to this compression, and in almost all instances, the unit weight of the soil is a function of depth. Given the substantial lateral extent of the deposit, there is limited justification for notable lateral compression to take place.

Consequently, it is reasonable to anticipate that the vertical locked-in effective stresses (p’_o) would surpass the effective lateral stresses (σ’_h) at the same location. This relationship between horizontal and vertical stresses can be defined as the ratio of the two.

K = σ_h/p_o ——– (2)

which is valid for any depth z at any time.

K₀ Conditions

Over geological time the stresses in a soil mass at a particular level stabilize into a steady state and strains become zero. When this occurs the vertical and lateral stresses become principal stresses acting on principal planes. This effective stress state is termed the at-rest or K₀ condition with K₀ defined as;

K₀ = σ’_h/p’_o ——– (3)

Therefore, K_o conditions refer to the state of stresses in a soil mass when there is no lateral strain. This means that the soil is not allowed to deform horizontally. K_o conditions are typically found in undisturbed soil, where the soil has not been subjected to any significant lateral stresses.

Figure 2 shows the range of K₀ and the relationship between p_o and σ_h in any homogeneous soil. The figure also shows the qualitative curves for preconsolidation in the upper zone of some soil from shrinkage/chemical effects. The figure clearly illustrates the anisotropic (σ_v ≠ σ_h) stress state in a soil mass.

In the figure above, although the linear vertical (also called geostatic) pressure profile is commonly used, the p’₀ effective pressure profile is more realistic of real soils since γ usually increases with depth. The lateral pressure profile range is for the geostatic pressure profile and would be curved similarly to the p’_o curve for real soils.

K₀ conditions are critical for:

• Retaining wall design.
• Excavation support.
• Earthquake-induced lateral pressures.
• Soil-structure interaction.

Because of the sampling limitations, it is an extremely difficult task to measure K₀ either in the laboratory or in situ. Some field methods are available, but note that they are very costly for the slight improvement—in most cases—over using one of the simple estimates following. In these equations use the effective angle of internal friction ‘ and not the total stress value.

Jaky (1948) presented a derived equation for K₀ that is applicable to both soil and agricultural grains (such as corn, wheat, oats, etc.) as;

K₀ = [(1 – sinφ’)/(1 + sinφ’)] × ( 1 + 2/3sinφ’) ——– (4)

which has been simplified—and erroneously called “Jaky’s equation”— to the following:

K₀ = 1 – sinφ’ ——— (5)

This equation is very widely used and has proved reasonably reliable in comparing initial to back-computed K₀ values in a number of cases and for normally consolidated materials. Kezdi (1972) suggests that for sloping ground Jaky’s equation can be used as follows:

K₀ = (1 – sinφ’)/(1 + sinβ) ——– (6)

where β is the angle with the horizontal (with sign) so that K₀ is either increased or reduced as site conditions dictate. This reference also gives a partial derivation of the Jaky equation for any interested user.

Brooker and Ireland (1965) (for normally consolidated clay) suggest;
K₀ = 0.95 – sinφ’ ——– (7)

Alpan (1967) (for normally consolidated clay) suggests;
K₀ = 0.19 + 0.233 log₁₀ Ip ——– (8)

An equation similar to Eq. (8) is given by Holtz and Kovacs (1981) as;
K₀ = 0.44 + 0.0042I_p ——– (9)

where I_p is the plasticity index of the soil in percent.

We can readily derive a value for K₀ in terms of Poisson’s ratio based on the definition of K₀ being an effective stress state at zero strain. From Hooke’s law, the lateral strain in terms of the effective horizontal (x, z) and vertical (y) stresses is;

ε_x = 0 = 1/E_s(σ_x – μσ_y – μσ_z) = ε_z ——– (10)

For a cohesionless, soil μ is often assumed as 0.3 to 0.4, which gives K₀ = 0.43 to 0.67, with a value of 0.5 often used. It is extremely difficult to obtain a reliable estimate of K₀ in a normally consolidated soil, and even more so in overconsolidated soils (OCR > 1). A number of empirical equations based on various correlations have been given in the literature. Several of the more promising ones are:

Alpan (1967) and others have suggested that the overconsolidated consolidation ratio K_0,OCR is related to the normally consolidated value K_0,nc in the following form;

K_0,OCR = K_0,nc × OCRⁿ (2-23) ——– (11)

where n = f(test, soil, locale) with a value range from about 0.25 to 1.25. For overconsolidated sand, n can be estimated from Figure 3.

For cohesive soil, Wroth and Houlsby (1985) suggest n as follows:

n = 0.42 (low plasticity — I_P < 40%)
n = 0.32 (high plasticity — I_P > 40%)

However, n ≈ 0.95 to 0.98 was obtained from in situ tests on several clays in eastern Canada. Mayne and Kulhawy (1982) suggest that a mean value of n = 0.5 is applicable for both sands and clays and that n = sinφ’ is also a good representation for sand. Their suggestions are based on a semi-statistical analysis of a very large number of soils reported in the
literature.

The exponent n for clays was also given by Alpan (1967) in graph format and uses the plasticity index I_P (in percent). The author modified the equation shown on that graph to obtain;

n = 0.54 × 10^(-I_P/281) ——– (12)

And, as previously suggested (for sands), we can use;

n = sinφ’ ——— (13)

Conclusion

Geostatic conditions refer to the stress state within a soil mass due to the weight of the overlying soil. These stresses are primarily vertical and compressive, but they also induce horizontal stresses due to the frictional resistance of the soil particles. The magnitude of these stresses depends on the depth of the soil layer, the unit weight of the soil, and the geometry of the soil mass.

The K₀ condition, also known as the at-rest earth pressure, represents a specific stress state within a soil mass where there is no lateral strain. This condition occurs when the soil is horizontally constrained, such as by a retaining wall or a layer of bedrock. In K₀ conditions, the horizontal effective stress (σ’_h) is typically about 0.5 to 0.7 times the vertical effective stress (σ’_v). The factors affecting K₀ values are the overconsolidation ratio, type of soil, confining pressure, stress history, and effective stress state.