Soil Classification. identify different types of soils

Notes of soil classifications

1. Soil Classification Lecture 3 (Week 1)

2. Description vs classification It is necessary to adopt a formal system of soil description and classification in order to describe the various materials found in ground investigation. Such a system must be meaningful and concise in an engineering context, so that engineers will be able to understand and interpret.

3. Soil description Description of soil is a statement that describes the physical nature and state of the soil. It can be a description of a sample, or a soil in situ . It is arrived at by using visual examination, simple tests, observation of site conditions, geological history, etc.

4. Soil classification Classification of soil is the separation of soil into classes or groups each having similar characteristics and potentially similar behavior. A classification for engineering purposes should be based mainly on mechanical properties: permeability, stiffness, strength. The aim of a classification system is to establish a set of conditions which will allow useful comparisons to be made between different soils. The system must be simple. The relevant criteria for classifying soils are the size-distribution of particles and the plasticity of the soil.

5. Specific Gravity (Gs) Specific gravity is defined as the ratio of the unit weight of a given material to the unit weight of water. The specific gravity of soil solids is often needed for various calculations in soil mechanics . It can be determined accurately in the laboratory. Table in the next slide shows the specific gravity of some common minerals found in soils. Most of the values fall within a range of 2.6 to 2.9. The specific gravity of solids of light-colored sand, which is mostly made of quartz, may be estimated to be about 2.65; for clayey and silty soils, it may vary from 2.6 to 2.9.

6. Mechanical Analysis of Soil Mechanical analysis is the determination of the size range of particles present in a soil, expressed as a percentage of the total dry weight. Two methods generally are used to find the particle-size distribution of soil: sieve analysis —for particle sizes larger than 0.075 mm in diameter, and hydrometer analysis —for particle sizes smaller than 0.075 mm in diameter .

7. Hydrometer Analysis Hydrometer analysis is based on the principle of sedimentation of soil grains in water. When a soil specimen is dispersed in water, the particles settle at different velocities, depending on their shape, size, weight, and the viscosity of the water. For simplicity, it is assumed that all the soil particles are spheres and that the velocity of soil particles can be expressed by Stokes’ law , according to which:

8. Sieve analysis Sieve analysis consists of shaking the soil sample through a set of sieves that have progressively smaller openings. U.S. standard sieve numbers and the sizes of openings are given in Table below.

9. Steps The sieves used for soil analysis are generally 203 mm in diameter. To conduct a sieve analysis, one must first oven-dry the soil and then break all lumps into small particles. The soil then is shaken through a stack of sieves with openings of decreasing size from top to bottom (a pan is placed below the stack). Figure in the next slide shows a set of sieves in a shaker used for conducting the test in the laboratory. The smallest-sized sieve that should be used for this type of test is the U.S. No. 200 sieve. After the soil is shaken, the mass of soil retained on each sieve is determined. When cohesive soils are analyzed, breaking the lumps into individual particles may be difficult. In this case, the soil may be mixed with water to make a slurry and then washed through the sieves. Portions retained on each sieve are collected separately and oven-dried before the mass retained on each sieve is measured.

10. Calculations Once the percent finer for each sieve is calculated (step 5), the calculations are plotted on semi-logarithmic graph paper with percent finer as the ordinate (arithmetic scale) and sieve opening size as the abscissa (logarithmic scale). This plot is referred to as the particle-size distribution curve.

11. Particle-Size Distribution Curve – how to use it?? A particle-size distribution curve can be used to determine the following four parameters for a given soil : Effective size ( D 10): This parameter is the diameter in the particle-size distribution curve corresponding to 10% finer. The effective size of a granular soil is a good measure to estimate the hydraulic conductivity and drainage through soil .

12. The particle-size distribution curve shows not only the range of particle sizes present in a soil , but also the type of distribution of various-size particles. Such types of distributions are demonstrated in Figure on the next slide. Curve I represents a type of soil in which most of the soil grains are the same size. This is called poorly graded soil. Curve II represents a soil in which the particle sizes are distributed over a wide range, termed well graded. A well-graded soil has a uniformity coefficient greater than about 4 for gravels and 6 for sands, and a coefficient of gradation between 1 and 3 (for gravels and sands). A soil might have a combination of two or more uniformly graded fractions. Curve III represents such a soil. This type of soil is termed gap graded.

13. Example

14. Particle Shape The shape of particles present in a soil mass is equally as important as the particle-size distribution because it has significant influence on the physical properties of a given soil. However , not much attention is paid to particle shape because it is more difficult to measure. The particle shape generally can be divided into three major categories: 1. Bulky 2. Flaky 3. Needle shaped

15. Bulky particles Bulky particles are formed mostly by mechanical weathering of rock and minerals. Geologists use such terms as angular , subangular , subrounded , and rounded to describe the shapes of bulky particles.

16. Flaky particles Flaky particles have very low sphericity —usually 0.01 or less. These particles are predominantly clay minerals. Needle-shaped particles Needle-shaped particles are much less common than the other two particle types. Examples of soils containing needle-shaped particles are some coral deposits and attapulgite clays .