Soil Classification and Typical Engineering Properties of Soils

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.

The Unified Soil Classification System (USCS)
The Unified Soil Classification System (USCS)
image 11
Plasticity Chart for fine-grained soils

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;

Class group SymbolDescription
GWwell-graded, clean gravels, gravel-sand mixtures
GPpoorly graded clean gravels, gravel-sand mixtures
GMsilty gravels, poorly graded gravel-sand silt
GCclayey gravels, poorly graded gravel-sand-clay
SWwell-graded clean sands, gravelly sands
SPpoorly graded clean sands, sand-gravel mix
SMsilty sands, poorly graded sand-silt mix
SM-SCsand-silt-clay mix with slightly plastic fines
SCclayey sands, poorly graded sand-clay mix
MLinorganic silts and clayey silts
ML-CLmixture of organic silt and clay
CLinorganic clays of low-to-medium plasticity
OLorganic silts and silt-clays, low plasticity
MHinorganic clayey silts, elastic silts
CHinorganic clays of high plasticity
OHorganic and silty clays
Table 1: General classification of soils according to USCS

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.

Soil Class Group SymbolDescriptionOptimum Moisture Content for Compaction (Range in %)Range of maximum dry density (kN/m3)
GWwell-graded, clean gravels, gravel-sand mixtures11–819.6 – 21.2
GPpoorly graded clean gravels, gravel-sand mixtures14–1118.0 – 19.6
GMsilty gravels, poorly graded gravel-sand silt12–818.85 – 21.2
GCclayey gravels, poorly graded gravel-sand-clay14–918.0 – 20.4
SWwell-graded clean sands, gravelly sands16–917.3 – 20.42
SPpoorly graded clean sands, sand-gravel mix21–1215.7 – 18.85
SMsilty sands, poorly graded sand-silt mix16–1117.3 – 19.63
SM – SCsand-silt-clay mix with slightly plastic fines15-1117.3 – 20.4
SCclayey sands, poorly graded sand-clay mix19-1116.5 – 19.63
MLinorganic silts and clayey silts24-1215.0 – 18.85
ML – CLmixture of organic silt and clay22-1215.7 – 18.85
CLinorganic clays of low-to-medium plasticity24-1215.0 – 18.85
OLorganic silts and silt-clays, low plasticity33-2112.57 – 15.7
MHinorganic clayey silts, elastic silts40-2411.0 – 14.92
CHinorganic clays of high plasticity36-1911.78 – 16.49
OHorganic and silty clays45-2110.21 – 15.71
Table 2: Typical Values of Optimum Moisture Content and Suggested Relative Compactions (based on standard Proctor test)

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.

Class group SymbolDescriptionTypical coefficient
of permeability
(cm/s)
GWwell-graded, clean gravels, gravel-sand mixtures2.5 × 10-2
GPpoorly graded clean gravels, gravel-sand mixtures5 × 10-2
GMsilty gravels, poorly graded gravel-sand silt> 5 × 10-7
GCclayey gravels, poorly graded gravel-sand-clay> 5 × 10-8
SWwell-graded clean sands, gravelly sands> 5 × 10-4
SPpoorly graded clean sands, sand-gravel mix> 5 × 10-4
SMsilty sands, poorly graded sand-silt mix> 2.5 × 10-5
SM-SCsand-silt-clay mix with slightly plastic fines> 10-6
SCclayey sands, poorly graded sand-clay mix> 2.5 × 10-7
MLinorganic silts and clayey silts> 5 × 10-6
ML-CLmixture of organic silt and clay> 2.5 × 10-7
CLinorganic clays of low-to-medium plasticity> 5 × 10-8
OLorganic silts and silt-clays, low plasticity
MHinorganic clayey silts, elastic silts> 2.5 × 10-7
CHinorganic clays of high plasticity> 5 × 10-8
OHorganic and silty clays
Table 3: Typical values of the coefficient of permeability

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.

Soil Class Group SymbolDescriptionCohesion (as compacted), C (lbf/ft2(kPa))Cohesion (saturated), C (lbf/ft2(kPa))Effective Stress friction angle φ (degrees)
GWwell-graded, clean gravels, gravel-sand mixtures00 > 38°
GPpoorly graded clean gravels, gravel-sand mixtures00> 37°
GMsilty gravels, poorly graded gravel-sand silt> 34°
GCclayey gravels, poorly graded gravel-sand-clay> 31°
SWwell-graded clean sands, gravelly sands0038°
SPpoorly graded clean sands, sand-gravel mix0037°
SMsilty sands, poorly graded sand-silt mix1050 (50)420 (20)34°
SM – SCsand-silt-clay mix with slightly plastic fines1050 (50)300 (14)33°
SCclayey sands, poorly graded sand-clay mix1550 (74)230 (11)31°
MLinorganic silts and clayey silts1400 (67)190 (9)32°
ML – CLmixture of organic silt and clay1350 (65)460 (22)32°
CLinorganic clays of low-to-medium plasticity1800 (86)270 (13)28°
OLorganic silts and silt-clays, low plasticity
MHinorganic clayey silts, elastic silts1500 (72)420 (20)25°
CHinorganic clays of high plasticity2150 (100)230 (11)19°
OHorganic and silty clays
Table 4: Typical shear strength parameters

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.

Class group SymbolDescriptionCBR values
(%)
GWwell-graded, clean gravels, gravel-sand mixtures40-80
GPpoorly graded clean gravels, gravel-sand mixtures30-60
GMsilty gravels, poorly graded gravel-sand silt20-60
GCclayey gravels, poorly graded gravel-sand-clay20-40
SWwell-graded clean sands, gravelly sands20-40
SPpoorly graded clean sands, sand-gravel mix10-40
SMsilty sands, poorly graded sand-silt mix10-40
SM-SCsand-silt-clay mix with slightly plastic fines5-30
SCclayey sands, poorly graded sand-clay mix5-20
MLinorganic silts and clayey silts≤15
ML-CLmixture of organic silt and clay
CLinorganic clays of low-to-medium plasticity≤15
OLorganic silts and silt-clays, low plasticity≤5
MHinorganic clayey silts, elastic silts≤10
CHinorganic clays of high plasticity≤15
OHorganic and silty clays≤5
Table 5: 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.

Class group SymbolDescriptionModulus of subgrade reaction k
(psi/in (kPa/mm)))
GWwell-graded, clean gravels, gravel-sand mixtures300–500 (80–140)
GPpoorly graded clean gravels, gravel-sand mixtures250–400 (68–110)
GMsilty gravels, poorly graded gravel-sand silt100–400 (27–110)
GCclayey gravels, poorly graded gravel-sand-clay100–300 (27–80)
SWwell-graded clean sands, gravelly sands200–300 (54–80)
SPpoorly graded clean sands, sand-gravel mix200–300 (54–80)
SMsilty sands, poorly graded sand-silt mix100–300 (27–80)
SM-SCsand-silt-clay mix with slightly plastic fines100–300 (27–80)
SCclayey sands, poorly graded sand-clay mix100–300 (27–80)
MLinorganic silts and clayey silts100–200 (27–54)
ML-CLmixture of organic silt and clay
CLinorganic clays of low-to-medium plasticity50–200 (14–54)
OLorganic silts and silt-clays, low plasticity50–100 (14–27)
MHinorganic clayey silts, elastic silts50–100 (14–27)
CHinorganic clays of high plasticity50–150 (14–41)
OHorganic and silty clays25–100 (6.8–27)
Table 6: Typical Values of the Subgrade Modulus

LEAVE A REPLY

Please enter your comment!
Please enter your name here