# (Aktu Btech) Geotechnical Engineering Important Unit-2 Soil Hydraulics

Aktu Btech Quantum Notes will help you be more prepared for the Geotechnical Engineering test. Your route to success lies in these vital, in-depth resources that address the most frequently asked issues. Improve your knowledge right now! Unit-2 Soil Hydraulics

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Important Questions For Geotechnical Engineering:
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## Q1. A granular soil deposit is 7 m deep over an impermeable layer. The ground water table is 4 m below the ground surface. The deposit has a zone of capillary rise of 1.2 m with a saturation of 50 %. Plot the variation of total stress, pore water pressure and effective stress with the depth of deposit, e = 0.6 and G = 2.65.

Ans. Given: Depth of soil deposit 7m, Depth of water table = 4 m, Specific gravity, G= 2.65, Degree of saturation, S = 50 * Void ratio, e = 0.6, Capillary rise = 1.2 m

To Find: Plot the variation of

with the depth of deposit.

1. Bulk unit weight,

4. At Level A-A :

5. At Level B-B (Upper Side):

6. At Level B-B (Bottom Side):

7. At Level C-C:

8. At Level D-D:

## Q2. Describe Darcy’s law and give its validity.

Ans. Darcy’s Law:

• i. Darcy’s law states that there is a linear relationship between flow velocity (v) and hydraulic gradient (i) for any given saturated soil under steady laminar flow conditions.

ii. If the rate of flow is q (volume/time) through cross-sectional area (A) of the soil mass, Darcy’s Law can be expressed as:

Validity of Darey’s Law:

• 1. If the flow through soils is laminar, Darcy’s law is true.
• 2. It is applicable to the flow of silts, clays, and fine sands. The flow may be turbulent in coarse sands, gravels, and boulders, making Darcy’s law inapplicable.
• 3. The relationship between velocity (v) and hydraulic gradient I must be linear for Darcy’s law to be true.
• 4. The interstices are exceedingly narrow in soils with extremely fine-grained particles, such as colloidal clay. As a result, the velocity is relatively low. The Darcy’s law is invalid on such soils.

## Q3. What are different methods for determination of the coefficient of permeability in a laboratory ? Discuss their limitations.

Ans. A. Permeability: It is a characteristic of a porous media that free or gravitational water (or other fluids) can travel slowly through its interconnecting spaces.

B. Laboratory Methods: The coefficient of permeability of a soil can be determined by using the following methods :

• i. For determining the coefficient of permeability of coarse-grained soils with high permeability, such as clean sands and gravel, constant-head experiments are used.
• ii. Fig. shows the arrangement in which the flow is one-dimensional and in downward direction.
• iii. The figure also indicates the head loss (h1) and the corresponding length of soil, L, over which the head loss occurs.
• iv. The experimental data consist of a measured quantitý of discharge Q during a time interval t, under steady state conditions of flow. The head loss hl is also noted.
• v. k can be computed from the formula,

Limitations: Following are the limitations of constant head permeability test:

• i. Equipment expensive.
• ii. Complicated to set up and to use.
• iii. Not suitable for most sites.
• iv. Test duration long.

2. Falling or Variable Head Permeability Test:

• i. Falling-head test are used for fine-grained soils with low permeability, such as silty or clayey fine sand, silts, and clays.
• ii. A typical set-up for falling head permeability test is shown in Fig. The water level in the stand pipe is observed from time to time.
• iii. Let A be the area of soil sample, a the area of stand pipe and L the length of soil sample.
• iv. If the head difference at time t1 is h1 and at time t2 is h2, then coefficient of permeability is calculated from the expression.

Limitations: Test specimen cannot be consolidated to expected in-situ effective stress.

## Q4. Discuss the factors that influence permeability of soil.

Ans. Following are the factors affecting the permeability of soils:

• 1. Properties of Pore Fluid:
• i. Fluids that fill pores in soil or rock are known as pore fluids.
• ii. Pore fluid viscosity and the unit weight of the pore fluid are inversely correlated with permeability.
• 2. Void Ratio: The area accessible for low thus increases with an increase in the void ratio, increasing permeability for crucial situations.
• 3. Entrapped Air and Organic Impurities: The flow is obstructed by organic contaminants and trapped air, and as a result, the coefficient of permeability is lessened.
• i. Adsorbed water refers to the microscopic water layer that envelops each soil particle.
• ii. Because this water cannot travel freely, it limits the effective pore space, which lowers the coefficient of permeability.
• 5. Shape of Particles: While angular soil has a greater specific surface area than round soil, it is less permeable than soil with rounded particles. Permeability is inversely proportional to specific surface area.

## Q5. Explain flownet. Describe its properties and its applications.

Ans. A. Flownet: Flownet is a type of grid created by drawing several equipotential and stream lines.

B. Characteristics / Properties of Flownet: Following are the characteristics of a flownet:

• 1. In a flownet, equipotential lines and stream lines cross one another orthogonally.
• 2. Flow cannot cross a flow line, and flow velocity always runs perpendicular to the equipotential line.
• 3. Equipotential drop is the name given to the loss of head between two equipotential lines, which is always the same.
• 4. Flow fields are the area between two equipotential lines and flow lines, and in an isotropic media, they are roughly square; in an anisotropic medium, they are roughly rectangular.

C. Application: Flownet is used to:

• 1. Estimation of seepage losses from reservoir.
• 2. Determination of seepage pressure.
• 3. Uplift pressure below dams.
• 4. To check against the possibility of piping and many others.

## Q6. What are the assumption and limitations of Dupuits’s theory ?

Ans. A. Assumptions: Following are the assumptions of Dupuits’s theory:

• 1. Darcy’s law is true because the flow is laminar.
• 2. The soil bulk is homogeneous and isotropic.
• 3. The well reaches all the way down into the aquifer.
• 4. The stream is constant.
• 5. Throughout, the permeability coefficient is constant.
• 6. The flow is horizontal and radial as it approaches the well.
• 7. The normal groundwater cycle is unaffected.

B. Limitations: Various assumptions have been made in the Dupuits theory formulae. n actual practice, however, none of these conditions may get fulfilled; say for example:

• 1. Aquifers are not entirely homogeneous, for one.
• 2. The aquifer may have been only partially penetrated by the well.
• 3. There may be variations in permeability.
• 4. The base of the cone might not be a circular because the ground water table could be sloped.
• 5. It’s possible that the equilibrium conditions weren’t entirely attained.