Month: July 2018

Mohr Stress Circle When an Element is Subjected To Normal Stress Only

Mohr Stress Circle When an Element is Subjected To Normal Stress Only

For drawing the Mohr stress circle, the normal stresses are represented along the x-axis and shear stresses along the y-axis. The Mohr stress circle is drawn, as shown in the figure, with its centre at a distance of ( 𝞼1 + 𝞼3)/2 and ( 𝞼1 – 𝞼3)/2.

Mohr Stress Circle When an Element is Subjected To Normal Stress Only

Where 𝞼1 &  𝞼3 are the major and minor principal stresses respectively. A point M on this circle, subtending an angle 2α at the center C, will represent the stresses on the plane which was making an angle α with the major principal plane.  

In other words, the coordinates of this point would be equal to shear stress (𝞃 )and normal stress (𝞼).

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Mohr-Coulomb Failure Theory(Criterion)

Two & Three Phase Diagram For Dry, Moist & Saturated Soil

Normal & Shear Stress, Principal Planes & Stresses

5 Ways To Improve Soil Fertility -Mixed Cropping, Crop Rotation, Deep Taproot Plants, Land Fallow

5 Ways To Improve Soil Fertility

How To Improve Soil Fertility? We all know that we get all types of food directly or indirectly from the plant, but we need soil to grow plants, but not all soil types are suitable for plant growth, so there is some way to improve soil fertility.

5 Ways To Improve Soil Fertility -Mixed Cropping, Crop Rotation, Deep Taproot Plants, Land Fallow

There are various strategies that are usually adopted to hold the fertility of the soil. Following are the 5 ways to improve soil fertility:

1. Mixed Cropping

Mixed Cropping means that two or more crops are cultivated together in the same field during the same crop season. For example, wheat and mustard are cultivated together and they grow simultaneously on the same ground. In mixed cropping, the ideal utilization of the nutrients present in the soil is made.

2. Crop Rotation

Crop rotation refers to the cultivation of different crops each year in the same field. such as, if wheat is cultivated for this running year on certain land, the next year, the land will be cultivated in another crop instead of wheat. 

If the same crop is repeatedly cultivated in the same field, then the fertility strength of the land is reduced, because of the cultivation of the same nutrients consumed by soil every year.

On the other hand, If different crops are cultivated in the same field each year, then since different crops require different nutrients and in different proportions, balanced utilization of the nutrients results, and the soil does not become deficient in a particular type of nutrients. 

3. Deep Taproot Plants

Different crops have different depths of the root zone. The taproot plants are advantageous as they can absorb more water and nutrients from the soil. In this way, water, as well as nutrient deficiency can be eliminated for shallow-rooted plants. Thus, by combining shallow-rooted crops and deep-rooted crops in the rotation of crops the ideal utilization of the nutrients available in the soil is made.

4. Applying Fertilizer

If there is a deficiency of some nutrients in the soil, we must think about it first. If there is any possibility of replenishing such nutrients naturally, it should be implemented first. If not, you have to think about applying fertilizers. The optimum use of fertilizers can increase nutrients in the soil needed for crops to grow efficiently.

There are several advantages of applying fertilizer to the land, like – it can improve the metabolism of plants, it is easily absorbed by plants, it helps in faster growth of crops, etc. But, before applying fertilizer to your land, you should take care of the quantity. Overuse of fertilizers can damage your crops.

Applying Fertilizer

5. Keeping The Land Fallow

If the land is left fallow (if some land is left uncultivated), then it allows the soil of that land to regain its natural nutrient capacity.

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Core Cutter Method For Determination of Field Density

Mohr-Coulomb Failure Theory(Criterion) 

13 Advantages and Disadvantages of Drip Irrigation System

Methods and Facilities of Providing Drainage

The process of removing excess water from the underground soil mass is called drainage.  

Methods and Facilities of Providing Drainage

Facilities of Providing Drainage

For the construction and operation of foundation work, drainage facilitates the following:

1) It intercepts the seepage flow.

2)It lowers down the groundwater table.

3)It gives dry working condition.

4)It increases the effective stress and thereby helps to improve the shear strength of soil mass.

5)It increases the stability of excavated slopes.

6)It quickens the process of consolidation.

7) It reduces the lateral stresses on sheeting and bracing

Methods of Providing Drainage

The following methods are adopted for drainage of excess water from the underground soil mass:

a) Slump and ditches.
b) Sheeting and open pumping.
c) Wellpoint system.
d) Deep well system.
e) Vacuum dewatering system.
f) Electro-osmosis system.
g) By providing a sand drain.
h) By the sumps in the well.
i) By side drain.

Read Also: 

5 Way To Improve Soil Fertility 

Mohr-Coulomb Failure Theory(Criterion) -Soil

Determine the Coefficient of Permeability of Soil by Constant Head Permeameter

The Coefficient of Permeability of Soil by Constant Head Permeameter

This test is suitable for coarse-grained soils which are highly permeable. The apparatus used for this test is called a constant head permeameter. Fig (a,b) shows the diagrammatic representation of this test.

Determine the Coefficient of Permeability of Soil by Constant Head Permeameter
Determine the Coefficient of Permeability of Soil by Constant Head Permeameter

The sample of length ‘L’ & cross-sectional area ‘A’ is subjected to a head ‘h’ which is kept constant (by overflows) during the progress of the test.

The test is performed by allowing the flow of water through the sample and measuring the quantity of discharge (Q) in time ‘t’ with the help of a measuring jar.

The value of the coefficient of permeability (K) can be computed directly from Darcy’s law.   We have from Darcy’s law, Discharge per unit time-
(q) = Q/t = K.i.A

Or, K = Q/t.i.A

Or, K = QL/A.h.t
[where, i (Hydraulic gradient) =h/L]
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Coefficient of Permeability of Soil by Falling Head Permeameter

Relation Between Porosity(n) & Void Ratio(e) -Soil

Permeability of Soil, Darcy’s law-Limitation, Coefficient of Permeability, Importance of Study of Seepage Analysis

Determine the Coefficient of Permeability of Soil by Falling Head Permeameter

Co-efficient of Permeability by Falling Head Permeameter

This test is suitable for fine-grained soils. The apparatus used for this test is called a falling head permeameter.  

Determine the Coefficient of Permeability of Soil by Falling Head Permeameter

The figure shows the diagrammatic representation of this test. The soil sample is kept in a vertical cylinder of cross-sectional area ‘A’ and length ‘L’.

A transparent standpipe sectional area ‘a’ is attached to the test cylinder. The test cylinder is kept in a container filled with water of constant level. The soil sample is saturated by allowing the water to flow through the sample continuously. 

After the saturation is complete, the standpipe is filled with water and the water is allowed to run down and a stopwatch is started.  

The head at any time instant ‘t’ is equal to the difference in the water level in the standpipe and the bottom tank. Let h1 and h2 be the heads at time interval t1 and t2 (t2>t1)respectively.   

Let ‘h’ be the head at any intermediate time interval ‘t’ and dh be the change in the head in ‘a’ smaller time interval dt. Hence, from Darcy’s law, the rate of flow q is given by.              

q = (- dh.a )/dt = K.i.A  [minus sign has been used since ‘h’ decrease as ‘t’ increase] where i = Hydraulic gradient at a time ‘t’ = h/L  

K.h.A/L = -dh.a /dt
or, A.K.dt/a.L = -dh/h

Integrating between two-time limits, we get

Coefficient of Permeability of Soil

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Coefficient of Permeability of Soil by Constant Head Permeameter  

Permeability of Soil, Darcy’s law-Limitation, Coefficient of Permeability, Importance of Study of Seepage Analysis

Relation Between Porosity(n) and Void Ratio(e) -Soil

Relation Between Porosity(n) & Void Ratio(e) 

By the definition of porosity, we have, n = Vv/V  

Where, Vv = Volume of voids in a given soil mass.  

V= Total volume of the soil mass = Volume of voids in the given soil mass(Vv) + Volume of soil solids in the given soil mass(Vs).  

Thus, n = Vv / (Vv + Vs)

Or, n =  1 / (1 + Vs/Vv) [Both sides diveded by Vv]

Or, n = 1 / (1 + 1/e) [By the definition of void ratio e=Vv/Vs]

Or, n = e / (1 + e)

Read Also:

Relation Between Dry Unit Weight, Bulk Unit Weight and Water Content

Coefficient of Permeability of Soil by Constant Head Permeameter  

Permeability of Soil, Darcy’s law-Limitation, Coefficient of Permeability, Importance of Study of Seepage Analysis

Phase Diagram of Soil – Two & Three Phase Diagram

Phase diagram of soil 

A soil mass consists of solid particles, containing void space between them. These voids may be filled either by water or by air or both. 

Phase Diagram of Soil- Two and Three Phase Diagram For Dry, Moist and Saturated Soil

Two-Phase Diagram of Soil

The soil will behave as a two-phase system when its void space is filled by either water or air alone. Such condition is possible when the soil is either fully saturated or fully dry. 

Three-Phase Diagram of Soil

But, when the soil mass is partially saturated, its void space will be filled up by water as well as air. Under such condition, soil mass will behave a three-phase system.

This three-phase system can be easily represented by the block diagram as shown in the figure. The total volume of soil mass (V) consists of the following:

V = Vv + Vs
V = (Va  + Vw ) + Vs

Where,

  • Vv = Volume of voids in the soil = Volume of air in the soil (Va)+ Volume of water in the soil (Vw).
  • Vs = Volume of solids.

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Particle Size Classification System Of Soil

Particle size classification of soil

In this system, soils are classified according to their grain size. Terms such as gravel, sand, silt, and clay are used to indicate grain sizes. The following systems are commonly used to classify soil according to their grain size.

  1. U.S Bureau of soil and Public Road Administration (PRA) System.
  2. International Soil Classification.
  3. MIT Classification.
  4. IS Classification. 

IS Classification

The IS classification system (IS: 1498-1970) was first developed in 1959 and revised in 1970. This system has been derived from the unified soil classification system. In this system soils are classified as follows: 

Particle Size Classification System Of Soil - IS Classification

MIT Classification

The MIT classification system was proposed by Pro. Gilboy of U.S.A. In this system soil is classified as follows: 

Particle Size Classification System Of Soil - MIT Classification

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 Unified Soil Classification

Textural Classification of Soil

Types of Soil Samples

What is Unified Soil Classification

Unified Soil Classification

This system was originally developed by A.Casagrande (1940) and was used for airfield construction. Later it was slightly modified to make it applicable to other constructions like foundation, earth dams, etc.  

In this system, coarse-grained soils are classified on the basis of their grain size distribution and the fine-grained soils are classified on the basis of their plasticity.  

This system classifies various soils into four major groups and 15 subgroups. The major group is:

  1. Coarse-grained
  2. Fine-grained
  3. Organic soils
  4. Peat

The type o soil is indicated by a suitable combination of symbols such as GW, SP, SC, CL, SM, etc. 

For example, SC indicates clayey sand, SM indicates silty sand.

Read Also:

 Particle Size Classification System Of Soil

Textural Classification of Soil

Types of Soil Samples

Textural Classification of Soil

Textural classification of soil

Soil occurring in nature are composed of different percentage of sand, silt, and clay. Classification of composite soil exclusively based on the particle size distribution is known as textural classification of soil.  

In this system of classification, a soil is given some textural names on the basis of the percentages o sand, silt and clay making up the soil.  

To use this classification, the percentage of sand, silt, and clay in the soil sample are determined by sieve analysis. They are then plotted in a triangular chart and a point is located. The designation of the area in which the point falls indicates the soil type.  

This system of classification is more suitable for describing coarse-grained soils.

Read Also:

Unified Soil Classification

Particle Size Classification System Of Soil

Types of Soil Samples