Mechanical Analysis
It is defined as the determination of mass of particles in fraction containing particles of only certain size.
The sieve analysis is suitable for coarse grained soil i.e. the soil having particle size greater than 75 microns. Similarly, the sedimentation analysis is suitable for fine soil i.e. the soil having particle size less than 75 microns.
If the soil contains both fine and coarse fraction, sedimentation analysis as well as sieve analysis are required to be conducted.
It is defined as the determination of mass of particles in fraction containing particles of only certain size.
- Sieve Analysis: suitable for coarse soil (greater than 75 micron particles) e.g. sand, gravel
- Sedimentation Analysis: suitable for fine soil (fines - particle size less than 75 microns) e.g. clay silt
The sieve analysis is suitable for coarse grained soil i.e. the soil having particle size greater than 75 microns. Similarly, the sedimentation analysis is suitable for fine soil i.e. the soil having particle size less than 75 microns.
If the soil contains both fine and coarse fraction, sedimentation analysis as well as sieve analysis are required to be conducted.
Sieve Analysis
In the method of sieve analysis, the soil is sieved through a set of sieves. The sieves are usually made of brass. The sieves are generally circular in shape and range from 15 to 20 cm in diameter. Sieves of various sizes ranging from 80 mm to 75 microns are available. 10 mm sieve means each square of sieve cloth has a dimension of 10 mm.
Above 4.75 mm sieve ---> Gravel fraction
Below 4.75 mm sieve ---> Sand fraction
The sieves are stacked one over the other, with decreasing size from the top to the bottom. A receiver, known as pan, which has no opening, is placed at the bottom of the smallest sieve.
In the method of sieve analysis, the soil is sieved through a set of sieves. The sieves are usually made of brass. The sieves are generally circular in shape and range from 15 to 20 cm in diameter. Sieves of various sizes ranging from 80 mm to 75 microns are available. 10 mm sieve means each square of sieve cloth has a dimension of 10 mm.
Above 4.75 mm sieve ---> Gravel fraction
Below 4.75 mm sieve ---> Sand fraction
The sieves are stacked one over the other, with decreasing size from the top to the bottom. A receiver, known as pan, which has no opening, is placed at the bottom of the smallest sieve.
Dry Sieve Analysis
80 mm 60 kg
20 mm 6.5 kg
4.75 mm 0.5 kg
The mass of the soil lost during the practical should not be greater than 2%.
- Take the dry soil. The mass of the soil is taken as per the size of particles.
80 mm 60 kg
20 mm 6.5 kg
4.75 mm 0.5 kg
- Sieve the soil sample through 4.75 mm sieve. The portion retained on the sieve is the gravel fraction; the portion passing through is the sand fraction.
- For the gravel fraction, the following sieves are arranged in descending order from the top which is placed on the vibrator and the soil is vibrated at least for 10 minutes. The mass of the soil retained on each sieve is determined.
- Similarly, for the sand fraction, the soil is vibrated arranging the following sieves as before.
The mass of the soil lost during the practical should not be greater than 2%.
Wet Sieve Analysis
If the soil contains more than 5% fines, then wet sieve analysis is required.
- Take the dry soil. The mass of the soil is taken as discussed above.
- The material passing though 4.75 mm sieve is sieved through a 75μ sieve. The material is washed until the was water becomes clear.
- The portion retained on a 4.75 mm sieve is the gravel fraction. It is dried and a dry sieve analysis is conducted.
- The wash water is kept on a 75μ sieve and is sieved as before. The portion retained is the sand fraction for which dry sieve analysis can be done. For the remaining portion, sedimentation analysis is required.
Grading of Soil
- A curve with a hump, such as curve A, represents the soil which has an excess or deficiency of certain particle sizes or a soil that has at least one particle size missing. Such a soil is known as gap-graded or skip-graded.
- A flat S-curve, such as curve B, represents a soil that contains particles of wide range of sizes and has a good representation of all sizes. This type of soil is known as a well-graded or uniformly graded soil.
- A steep curve, like C, has most of its particles of almost at same size. This type of soil is uniform soil.
Calculating the coefficients of uniformity and curvature
D60 = particle size corresponding to 60% finer
D30 = particles size corresponding to 30% finer
D10 = particle size corresponding to 10% finer
D30 = particles size corresponding to 30% finer
D10 = particle size corresponding to 10% finer
Importance
Soil gradation is very important to geotechnical engineering. It is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity.
In a design, the gradation of the in situ or on site soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil because there are more void spaces in a poorly graded soil.
When a fill material is being selected for a project such as a highway embankment or earthen dam, the soil gradation is considered. A well graded soil is able to be compacted more than a poorly graded soil. These types of projects may also have gradation requirements that must be met before the soil to be used is accepted.
When options for ground remediation techniques are being selected, the soil gradation is a controlling factor.
Soil gradation is very important to geotechnical engineering. It is an indicator of other engineering properties such as compressibility, shear strength, and hydraulic conductivity.
In a design, the gradation of the in situ or on site soil often controls the design and ground water drainage of the site. A poorly graded soil will have better drainage than a well graded soil because there are more void spaces in a poorly graded soil.
When a fill material is being selected for a project such as a highway embankment or earthen dam, the soil gradation is considered. A well graded soil is able to be compacted more than a poorly graded soil. These types of projects may also have gradation requirements that must be met before the soil to be used is accepted.
When options for ground remediation techniques are being selected, the soil gradation is a controlling factor.