1.5 CHARACTERIZATION OF SOILS BASED ON PARTICLE SIZE
The grading curve, which shows the distribution of different particle sizes in a soil sample, is a very important tool for classifying soils by their texture. Various classification systems have been developed over the years for specific engineering purposes. In the United States, popular systems include the Unified Soil Classification System (USCS), the ASTM system (a modification of USCS), and the AASHTO system (see Table 1.9). Other countries, such as those in Europe, use the Euro-Standards. For our educational purposes, we will use the USCS system.
In the USCS system, soils are divided into two main categories:
- Coarse-Grained Soils: More than 50% of the particles are larger than 0.075 mm. These soils are further subdivided into gravels and sands. The specific grain sizes that define these categories are provided in Table 1.1.
- Fine-Grained Soils: More than 50% of the particles are finer than 0.075 mm. These are further divided into silts and clays. In our modified system, clays are defined as having particles less than 0.002 mm.
Although sands (mainly composed of quartz) can sometimes be ground to sizes below 0.002 mm, they do not behave like clays. Clays, with their very high specific surface area, exhibit different water retention and mechanical behavior. The fines content—materials with particle sizes below 0.075 mm—in a soil greatly influences its behavior. Soils with more than about 35% fines tend to behave like fine-grained soils, while those with less than 5% fines are less affected.
To further differentiate soils, engineers use two important coefficients derived from the grading curve (see Fig. 1.7):
- Uniformity Coefficient (Cu): Defined as Cu = D60/D10, where D60 is the particle diameter at which 60% of the particles are finer and D10 is the diameter at which 10% are finer. A Cu value of 1 indicates that all particles are the same size.
- Coefficient of Curvature (CC): Defined as CC = (D30)²/(D10·D60), where D30 is the particle diameter at which 30% of the particles are finer. This coefficient describes the shape of the gradation curve.
The limits of these coefficients help characterize soils as well graded or poorly graded. The criteria are as follows:
-
Well Graded Soils:
- For gravelly soils (gravel content > sand content): Cu ≥ 4 and 1 ≤ CC ≤ 3.
- For sandy soils (sand content > gravel content): Cu ≥ 6 and 1 ≤ CC ≤ 3.
-
Poorly Graded Soils:
- For gravels: Cu < 4 and CC < 1 or CC > 3.
- For sands: Cu < 6 and CC < 1 or CC > 3.
Soils that do not meet these limits (i.e., have missing particle sizes) are classified as gap-graded.
Another key concept is the effective size (D10), originally described by Allen Hazen. This effective size represents the diameter of an “artificial sphere” that behaves similarly to the irregularly shaped soil particles in terms of water flow. It is especially important for determining the drainage characteristics and mechanical behavior of soils; a higher D10 generally indicates a coarser soil with better drainage.
Particle size analysis is crucial for many engineering applications, including selecting aggregates for concrete, designing dams and highways, creating drainage filters, and planning grouting or chemical injection projects. In Chapter 2, you will learn more about how particle size distribution, along with other physical properties, is used in a classification system to help choose the right soil for a specific construction purpose.
Below are the referenced diagrams and tables: Fig. 1.7, Table 1.9, and Table 1.1.
Table 1.9 Comparison of Four Systems Describing Soil Types Based on Particle Size
| Approx. Particle Size Range (mm) | European Standards | USCS | AASHTO | ASTM |
|---|---|---|---|---|
| < 0.002 | Clay | Fines (Silt/Clay) | Clay | Clay |
| 0.002 – 0.06 | Silt | Fines (Silt/Clay) | Silt | Silt |
| 0.06 – 2 | Sand (Fine to Coarse) | Sand (Fine to Coarse) | Sand (Fine to Coarse) | Sand (Fine to Coarse) |
| 2 – 63 | Gravel | Gravel | Gravel | Gravel |
| 63 – ~200 | Cobbles | Cobbles* | Cobbles* | Cobbles* |
| > ~200 | Boulders | Boulders* | Boulders* | Boulders* |
Notes:
1. This table is showing approximate ranges. Actual cutoff sizes may vary slightly between standards.
2. *In USCS, AASHTO, and ASTM, cobbles generally start around 75–76 mm, and boulders above 250–300 mm.
1. This table is showing approximate ranges. Actual cutoff sizes may vary slightly between standards.
2. *In USCS, AASHTO, and ASTM, cobbles generally start around 75–76 mm, and boulders above 250–300 mm.
Table 1.1 – Soil types, descriptions, and average grain sizes
| Category | Soil Type | Symbol | Description | Grain Size, D |
|---|---|---|---|---|
| Coarse-grained | Gravel | G | Rounded and/or angular bulky hard rock, coarsely divided |
Coarse: >75 mm Fine: 4.75 mm–19 mm |
| Coarse-grained | Sand | S | Rounded and/or angular hard rock, finely divided |
Coarse: 2.0 mm–4.75 mm Medium: 0.425 mm–2.0 mm Fine: 0.075 mm–0.425 mm |
| Fine-grained (also called fines) | Silt | M | Particle size between clay and sand; nonplastic or very slightly plastic; exhibits little or no strength when dried; easily brushed off when dried | 0.002 mm–0.075 mm |
| Fine-grained (also called fines) | Clay | C | Particles are smooth and mostly clay minerals; greasy and sticky when wet; exhibits plasticity and significant strength when dried; water reduces strength | <0.002 mm |
