2. A Beginner’s Guide to Soil Classification
SOIL CLASSIFICATION:
· Sorting the soil into groups and sub-groups which would show similar behaviour and common engineering properties.
· Done on the basis of simple index properties. Most commonly used properties are Grain Size Distribution and Plasticity (discussed in previous article).
· In any soil mass, the sizes of the grains vary greatly. To classify a soil properly, you must know its grain-size distribution. The grain-size distribution of coarse-grained soil is generally determined by means of sieve analysis. For a fine-grained soil, the grain-size distribution can be obtained by means of hydrometer analysis.
SIEVE ANALYSIS:
A sieve analysis is conducted by taking a measured amount of dry, well-pulverized soil and passing it through a stack of progressively finer sieves with a pan at the bottom. The amount of soil retained on each sieve is measured, and the cumulative percentage of soil passing through each is determined. This percentage is generally referred to as percent finer. These below mentioned sieves are commonly used for the analysis of soil for classification purposes.
· The percent finer for each sieve, determined by a sieve analysis, is plotted on semilogarithmic graph paper, as shown below. Note that the grain diameter, D, is plotted on the logarithmic scale and the percent finer is plotted on the arithmetic scale.
· Two parameters can be determined from the grain-size distribution curves of coarse grained soils:
(1) the uniformity coefficient (Indicates how well-graded or uniform the soil is, High value: wider range of particle sizes = well-graded and; Low value: particles are similar in size = uniform/poorly graded)
(2) the coefficient of gradation, or coefficient of curvature (
where D10, D30, D60 and are the diameters corresponding to percents finer than 10%, 30%, and 60%, respectively.
Few Results:
HYDROMETER ANALYSIS:
Hydrometer analysis is based on the principle of sedimentation of soil particles in water.
This test involves the use of 50 grams of dry, pulverized soil. A deflocculating agent is always added to the soil. The most common deflocculating agent used for hydrometer analysis is 125 cc of 4% solution of sodium hexametaphosphate. The soil is allowed to soak for at least 16 hours in the deflocculating agent. After the soaking period, distilled water is added, and the soil–deflocculating agent mixture is thoroughly agitated. The sample is then transferred to a 1000-ml glass cylinder. More distilled water is added to the cylinder to fill it to the 1000-ml mark, and then the mixture is again thoroughly agitated.
A hydrometer is placed in the cylinder to measure the specific gravity of the soil–water suspension in the vicinity of the instrument’s bulb, usually over a 24-hour period. Hydrometers are calibrated to show the amount of soil that is still in suspension at any given time t. The largest diameter of the soil particles still in suspension at time t can be determined by Stokes’ law,
SIZE LIMITS FOR SOIL:
Several classification systems are involved by different organisation having a specific purpose. They are:
o Unified Soil Classification System (USCS).
o American Association of State Highway and Transportation Official Classification System (AASHTO).
· The Unified Soil Classification System was originally proposed by A. Casagrande in 1948 and was later revised and adopted by the United States Bureau of Reclamation and the U.S. Army Corps of Engineers.
· It was slightly modified to make it applicable to foundations, dams and other the system in practically all geotechnical work.
· In the Unified System, the following symbols are used for identification:
· The AASHTO Soil Classification System was originally proposed by the Highway Research Board’s Committee on Classification of Materials for Subgrades and Granular Type Roads (1945).
· According to the present form of this system, soils can be classified according to eight major groups, A-1 through A-8, based on their grain-size distribution, liquid limit, and plasticity indices. Soils listed in groups A-1, A-2, and A-3 are coarse-grained materials, and those in groups A-4, A-5, A-6, and A-7 are fine-grained materials. Peat, muck, and other highly organic soils are classified under A-8. They are identified by visual inspection.
For qualitative evaluation of the desirability of a soil as a highway subgrade material, a number referred to as the group index has also been developed. The higher the value of the group index for a given soil, the weaker will be the soil’s performance as a subgrade.
A group index of 20 or more indicates a very poor subgrade material. The formula for the group index is
Example Sieve Analysis:
The various types of curves obtained in sieve analysis are classified as follows:
·
Curve (a): Well graded soil: mean soils of all sizes are present
· Curve (b and c): Poorly graded/uniformly graded soil of predominantly one size is only present
· Curve (d): Gap graded: some of the soil particles are missing
· Position of the curve indicates-type of soil, whereas shape of the curve indicates- gradation.
🧠 Test Your Understanding:
To wrap things up, here are some questions to help you reflect on what you’ve learned.
✅ Concept Check
1. Why is grain-size distribution important in soil classification?
2. What is the main difference between coarse-grained and fine-grained soils in terms of testing methods?
3. What do D₁₀, D₃₀, and D₆₀ represent in a sieve analysis?
📊 Apply Your Knowledge
4. How is the Uniformity Coefficient (Cu) calculated, and what does it indicate?
5. If D₁₀ = 0.1 mm and D₆₀ = 0.6 mm, calculate Cu. What does this tell you about the soil's gradation?
6. Why might an engineer combine both sieve and hydrometer analyses for one soil sample?
🔍 Explore Further
7. How does USCS differ from the AASHTO soil classification system?
8. Which AASHTO group represents better subgrade soil — A-1 or A-7? Why?
9. How could misclassifying soil type affect a construction project?
💬 Your Turn!
If you have thoughts, questions, or real-world experiences related to soil classification, feel free to share them in the comments below. Let’s dig deeper together!
1. The size of the grains of soil is important in soil classification. It determines the type of soil (sand, silt, clay, or gravel). It also affects how easily water can pass through the soil, how tightly packed the soil is, and how strong the soil is. It can predict if the soil will be damaged by frost heave and help assess the potential for erosion. Engineers use it to choose the right soils for construction projects. This helps to make sure that the projects are stable, durable, and manage water well.
ReplyDelete2. The main difference is that coarse-grained soils (like gravel and sand) are usually tested for their particle size distribution using sieve analysis, while fine-grained soils (like silt and clay) are tested for their particle size and plasticity using hydrometer analysis and Atterberg limits tests. This is because their tiny particles can't be accurately measured with sieves alone.
3. In a sieve analysis, D10, D30, and D60 are particle diameters (in mm) at which 10%, 30%, and 60% of the soil sample (by weight) passes through the sieve.
4. Cu= D60/D10
It indicates how well-graded a soil is — a higher Cu means a wider range of particle sizes (well-graded), while a low Cu means particles are similar in size (uniform/poorly graded).
5. Upon performing the calculations, we get a Cu=6, which means the soil is well-graded.
6. An engineer might combine both because sieve analysis measures coarse particles, while hydrometer analysis measures fine particles. Using both methods gives the full size of the particles in soil that has both coarse and fine particles.
7. The USCS mainly looks at how big the soil particles are and how easily they can be shaped. This is done for engineering and construction purposes. AASHTO looks at if soils are good for roads and pavement. They group soils based on how well they work as subgrade materials.
8. A-1 soils make better subgrades because they’re coarse, drain well, and stay strong. A-7 soils hold water, and weaken when wet.
9. Misclassifying soil can lead to using wrong foundation designs, poor drainage solutions, or inadequate compaction methods, causing structural failures and delays in the project.
Grain size matters in soil classification because it tells us if soil is gravel, sand, silt, or clay. It controls how water flows through the soil, how dense it can be packed, and how strong it is. It also helps predict frost damage and erosion. Engineers use this to pick the right soil so projects are safe, stable, and durable.
ReplyDeleteDifference in testing: Coarse soils (gravel, sand) are checked with sieve analysis, while fine soils (silt, clay) need hydrometer and Atterberg tests since sieves can’t measure very tiny particles well.
In sieve tests, D10, D30, D60 are the particle sizes at which 10%, 30%, and 60% of the sample weight passes the sieve.
Coefficient of uniformity (Cu = D60/D10): tells how varied particle sizes are. High Cu = wide mix of sizes (well-graded). Low Cu = mostly same size (poorly graded).
From calculation, Cu = 6 → soil is well-graded.
Engineers may use both sieve and hydrometer tests for soils that contain both coarse and fine particles, to get a complete picture of grain sizes.
USCS classifies soils by particle size and plasticity for construction use. AASHTO groups soils by how suitable they are for road subgrades.
A-1 soils are good for subgrades: strong, drain well. A-7 soils are weak when wet because they hold water.
Wrong soil classification can cause design mistakes—like weak foundations, poor drainage, or bad compaction—leading to failures and project delays.
Grain size is important in soil classification because it helps us figure out if the soil is gravel, sand, silt, or clay. This affects how water moves through the soil, how tightly it can be packed, and how strong it is. It also helps predict problems like frost damage or erosion. Engineers use this information to choose the right soil for safe and lasting construction projects.
ReplyDeleteTesting differences:
Coarse soils like gravel and sand are tested using sieve analysis, where the soil is passed through different-sized screens. Fine soils like silt and clay are tested with hydrometer and Atterberg tests, since their particles are too small for sieves to measure properly.
In sieve tests, D10, D30, and D60 are the particle sizes at which 10%, 30%, and 60% of the sample has passed through the sieve.
Coefficient of uniformity (Cu) is calculated using Cu = D60/D10. It tells us how mixed the particle sizes are:
A high Cu means there’s a good mix of sizes (well-graded soil).
A low Cu means most particles are about the same size (poorly graded soil).
For example, if Cu = 6, the soil has a good range of particle sizes, so it’s considered well-graded.
If the soil has both coarse and fine parts, engineers might use both sieve and hydrometer tests to understand the full range of grain sizes.
Soil classification systems:
The USCS system sorts soils by size and how plastic (moldable) they are, to help with building projects.
The AASHTO system is used for road construction. It ranks soils by how good they are for road bases.
For example:
A-1 soils are strong and drain well, making them good for roads.
A-7 soils hold water and become weak when wet, so they’re not ideal.
Misclassifying soil can lead to design problems—like weak foundations, poor water drainage, or bad compaction—which can cause construction failures and delays.
1. The size of soil particles plays a major role in soil classification. It helps determine whether the soil is gravel, sand, silt, or clay. Particle size also influences how water flows through the soil, how dense it can become, and how strong it will be. It can also indicate the soil’s susceptibility to frost damage and erosion. Engineers rely on this information to select suitable soils for construction, ensuring stability, durability, and proper drainage.
ReplyDelete2. Coarse-grained soils (such as sands and gravels) are mainly analyzed with sieve tests to determine particle size distribution. Fine-grained soils (like silts and clays) require hydrometer tests and Atterberg limit tests because their extremely small particles cannot be accurately measured with sieves alone.
3. In a sieve analysis, D10, D30, and D60 are the particle diameters (in millimeters) at which 10%, 30%, and 60% of the total soil sample has passed through the sieves.
4. The coefficient of uniformity is calculated using:
Cu = D60 / D10
It indicates how uniform or varied the soil’s particle sizes are. A higher Cu means the soil contains a broader range of particle sizes (well-graded), while a low Cu indicates mostly similar particle sizes (poorly graded).
5. From the calculations, Cu = 6. This value shows the soil is well-graded and contains a good variety of particle sizes.
6. An engineer may use both sieve analysis and hydrometer analysis when a soil contains a mix of coarse and fine particles. Sieve analysis captures the larger particles, while hydrometer analysis measures the fine particles, giving a complete picture of the soil's overall gradation.
7. The USCS classifies soils based mainly on particle size and plasticity, focusing on how the soil behaves for general engineering and construction purposes. The AASHTO system classifies soils based on how suitable they are as road and pavement subgrade materials.
8. A-1 soils are preferred for subgrades because they are coarse, drain efficiently, and maintain strength. A-7 soils tend to hold water, lose strength when wet, and therefore perform poorly as subgrade materials.
9. Incorrectly classifying a soil can result in choosing the wrong foundation type, poor drainage design, or inappropriate compaction methods. This can lead to structural issues, project delays, or even failure of the constructed facility.