Which Test Gives a Better Estimation of the Friction Angle?
Usually, the economics of the project dictates the type of test you would use for the determination of the friction angle. Nonetheless, the best test to determine the friction angle of soil is the one that is more analogous to the problem at hand. For example, if you are to determine the bearing capacity of a square footing, the triaxial test is the best one.
Introduction of USCS ( Unified Soil Classification System )
The original form of this system was proposed by Casagrande in 1942 for use in the airfield construction works undertaken by the Army Corps of Engineers during World War II.
In cooperation with the U.S. Bureau of Reclamation, this system was revised in 1952. At present, it is used widely by engineers (ASTM Test Designation D-2487). The Unified classification system is presented in Table 2.
| Unit | Symbols | Description |
| Primary | G | Gravel |
| S | Sand | |
| M | Silt | |
| C | Clay | |
| O | Organic | |
| Pt | Peat | |
| Secondary | W | Well-Graded |
| P | Poorly Graded | |
| M | Non-Plastic Fines | |
| C | Plastic Fines | |
| L | Low Plasticity | |
| H | High Plasticity |
Table 1 – Symbols Used in USCS
a Gravels with 5 to 12% fine require dual symbols: GW-GM, GW-GC, GP-GM, GP-GC.
b Sands with 5 to 12% fines require dual symbols: SW-SM, SW-SC, SP-SM, SP-SC.
c Cu = D60 / D10 ; Cc = ( D 30 )2 / ( D60 x D10 )
d If 4 ≤ PI ≤ 7 and plots in the hatched area in below fig (Plasticity chart), use dual symbol GC-GM or SC-SM.
e If 4 ≤ PI ≤ 7 and plots in the hatched area in below fig (Plasticity chart), use dual symbol CL-ML.
Plasticity chart
For proper classification according to this system, some or all of the following information must be known:
1. Percent of gravel — That is, the fraction passing the 76.2-mm sieve and retained on the No. 4 sieve (4.75-mm opening)
2. Percent of sand — That is, the fraction passing the No. 4 sieve (4.75-mm opening) and retained on the No. 200 sieve (0.075-mm opening)
3. Percent of silt and clay — That is, the fraction finer than the No. 200 sieve (0.075-mm opening)
4. Uniformity coefficient (Cu) and the coefficient of gradation (Cc)
5. Liquid limit and plasticity index of the portion of soil passing the No. 40 sieve
The group symbols for coarse-grained gravelly soils are GW, GP, GM, GC, GC-GM, GW-GM, GW-GC, GP-GM, and GP-GC.
Similarly, the group symbols for fine-grained soils are CL, ML, OL, CH, MH, OH, CL-ML, and Pt.
Fine Fraction = Percent Passing No. 200 Sieve
Coarse Fraction = Percent Retained on No. 200 Sieve
Gravel Fraction = Percent Retained on No. 4 Sieve
Sand Fraction = (Percent Retained on No. 200 Sieve) – (Percent Retained on No. 4 Sieve)
USCS Classifies Soils into Two Broad Categories
Coarse-Grained Soils—If more than 50% of the soil is retained on No. 200 (0.075 mm) sieve, it is designated as coarse-grained soil. There are 8 groups of coarse-grained soils. Fine-Grained Soils—If more than 50% of the soil passes No. 200 sieve, it is called fine-grained soil. There are 6 groups of fine-grained soils.
Coarse-Grained Soils
The coarse-grained soils are designated as gravel (G) if 50% or more of a coarse fraction (plus 0.075 mm) is retained on No. 4 (4.75 mm) sieve; otherwise, it is termed sand (S). If the coarse-grained soils contain less than 5% fines and are well-graded (W), they are given the symbols GW and SW, and if poorly graded (P), symbols GP and SP.
The criteria for well-grading are given in the above table 1. If the coarse-grained soils contain more than 12% fines, these are designated as GM, GC, SM, or SC, as per the criteria given. If the percentage of fines is between 5 to 12% dual symbols such as GW-GM, SP-SM, are used.
Fine-Grained Soils
Fine-grained soils are further divided into two types :
1. Soils of low compressibility (L) if the liquid limit is 50% or less. These are given the symbols ML, CL, and OL.
2. Soils of high compressibility (H) if the liquid limit is more than 50%. These are given the symbols MH, CH, and OH. The exact type of soil is determined from the plasticity chart (above Fig). The A-line has the equation Ip = 0.73 (wt– 20). It separates the clays from silts. When the plasticity index and the liquid limit plot in the hatched portion of the plasticity chart, the soil is given double symbol CL- ML.
The inorganic soil ML and MH and the organic soils OL, OH plot in the same zones of the plasticity chart. The distinction between the inorganic and organic soils is made by oven-drying. If oven drying decreases the liquid limit by 30% or more, the soil is classified organic (OL or OH); otherwise, inorganic (ML or MH)
Highly Organic Soils
Highly organic soils are identified by visual inspection. These soils are termed peat (Pt).
Frequently Asked Questions (FAQ)
What Is the Best Test for Determining the Friction Angle of Soil?
The best test for determining the friction angle of soil often depends on the specific requirements of your project. Generally, the triaxial test is considered one of the most accurate methods for determining the friction angle, especially when assessing the bearing capacity of a square footing. Other tests, such as the direct shear test or vane shear test, might be more cost-effective for different applications.
How Does the Unified Soil Classification System (Uscs) Work?
The USCS classifies soils based on their particle size distribution and plasticity characteristics. It categorizes soils into two broad types: coarse-grained and fine-grained. Coarse-grained soils are further divided into gravel and sand, while fine-grained soils are divided into silts and clays. Soil classification is based on sieve analysis, plasticity index, and liquid limit, among other factors.
What Is the Significance of the Plasticity Chart in the Uscs?
The plasticity chart helps differentiate between types of fine-grained soils based on their plasticity characteristics. It plots the plasticity index against the liquid limit to categorize soils into different groups such as CL (low plasticity clay) or CH (high plasticity clay). This chart is crucial for determining the soil’s behavior under varying moisture conditions.
How Is the Unified Soil Classification System (Uscs) Applied in Practice?
The USCS is widely used for soil classification in geotechnical engineering, especially for construction projects. By classifying soils accurately, engineers can predict soil behavior, which informs foundation design, earthworks, and other construction practices. The system aids in selecting appropriate materials and methods for different soil types.
What Do the Symbols in the Uscs Represent?
In the USCS, symbols denote the soil type and its characteristics. For example, “GW” stands for well-graded gravel, “ML” for low plasticity silt, and “CH” for high plasticity clay. Dual symbols like “GW-GM” or “CL-ML” indicate soils with characteristics of two different types. The system uses these symbols to provide a concise description of soil properties.
What Are the Criteria for Classifying Coarse-Grained Soils?
Coarse-grained soils are classified based on their percentage of fines and grading. Soils are classified as gravel (G) if 50% or more of the coarse fraction is retained on the No. 4 sieve. If they are well-graded, they receive symbols like GW or SW, and if poorly graded, GP or SP. For coarse-grained soils with 5 to 12% fines, dual symbols are used.
How Are Fine-Grained Soils Categorized in the Uscs?
Fine-grained soils are categorized based on their liquid limit and plasticity index. Soils with a liquid limit of 50% or less are classified as low compressibility (ML, CL, OL), while those with a liquid limit greater than 50% are high compressibility (MH, CH, OH). The plasticity chart helps in distinguishing between clays and silts.
What Are Highly Organic Soils, and How Are They Classified?
Highly organic soils, known as peat (Pt), are identified by their visual appearance and organic content. These soils have a high percentage of decomposed plant material and are classified based on their organic content, which is different from mineral soils. Peat soils are generally not suitable for construction without significant treatment.
Why Might Different Tests Be Chosen for Soil Analysis?
The choice of test for soil analysis is influenced by factors such as the project’s requirements, cost considerations, and the soil’s specific characteristics. Tests like the triaxial test, direct shear test, and vane shear test offer different levels of detail and accuracy, making it important to select the appropriate test based on the needs of the project.
How Do You Determine the Liquid Limit and Plasticity Index of Soil?
The liquid limit is determined using the Casagrande cup method or a similar technique, while the plasticity index is calculated as the difference between the liquid limit and the plastic limit of the soil. These parameters are essential for classifying fine-grained soils and understanding their behavior.
