Flow Nets

Q. Consider the following statements:
Phreatic line in an earth dam is
1. elliptical in shape
2. an equipotential line
3. the top most flow line with zero water pressure
4. approximately a parabola
Which of these statements is/are correct?

1. 1, 2 and 3
2. 2, 3 and 4
3. 3 and 4
4. 1 alone

Q. The coefficient of permeability of a soil in horizontal and vertical directions are 3.46 and 1.5 m/day respectively. The base length of a concrete dam resting in this soil is 100 m. When the flow net is developed for this soil with 1:25 scale factor in the vertical direction, the reduced base length of the dam will be

1. 2.63 m
2. 4.00 m
3. 6.08 m
4. 5.43 m

Q. Consider the following statements:
1. Quantity of seepage in each flow channel of a flow-net is independent of the size of field.
2. Drop in head between adjacent equipotential lines of a flow-net is dependent on upstream head.
3. With increase in the length of a flow path, the corresponding exit gradient will decrease.
Which of these statements are correct?

1. 1, 2 and 3
2. 1 and 2 only
3. 1 and 3 only
4. 2 and 3 only

Q. The flow net around a sheet pile wall is shown in the sketch. The properties of the soil are : permeability coefficient = 0.09 m/day (isotropic), specific gravity = 2.70 and void ratio = 0.85. The sheet pile wall and the bottom of the soil are impermeable.

The seepage loss (in $m^3$ per day per unit length of the wall) of water is

1. 0.33
2. 0.38
3. 0.43
4. 0.54

Q. Piping in soil occurs when

1. the soil is highly porous
2. sudden change in permeability occurs
3. effective pressure becomes zero
4. the soil is highly stratified

Q. A flownet constructed to determine the seepage through an earth dam which is homogeneous but anisotropic, gave four flow channels and sixteen equipotential drops. The coefficients of permeability in the horizontal and vertical directions are $4.0\times {10}^{-7}m/sand1\times {10}^{-7}m/s$ respectively. If the storage head was 20 m, then the seepage per unit length of the dam (in m3/s) would be

1. $5\times {10}^{-7}$
2. $10\times {10}^{-7}$
3. $20\times {10}^{-7}$
4. $40\times {10}^{-7}$

Q. The seepage occuring through an earthen dam is represented by a flow net comprising of 10 equipotential drops and 20 flow channels. The coefficient of permeability of the soil is 3 mm/min and the head loss is 5 m. The rate of seepage (expressed in $c{m}^3/s$ per m length of the dam) through the earthen dam is .

1. 500

Q.

The flow net constructed for a dam is shown in the figure below. Taking coefficient of permeability as $3.8\times {10}^{-6}m/s$, the quantity of flow $\left(inc{m}^3/sec\right)$ under the dam per m is

1. 7.182

Q. The ratio $N_f/N_d$ is known as shape factor, where $N_f$ is the number of flow lines and $N_d$ is the number of equipotential drops. Flow net is always drawn with a constant b/a ratio, where b and a are distance between two consecutive flow lines and equipotential lines, respectively. Assuming that b/a ratio remains the same, the shape factor of a flow net will change if the

1. upstream and downstream heads are interchanged
2. soil in the flow space is changed
3. dimensions of the flow space are changed
4. head difference causing the flow is changed

Q. A flownet of a coffer dam foundation has 6 flow channels and 18 equipotential drops. The head of water lost during seepage is 6 m. If the coefficient of permeability of foundation is $4\times {10}^{-5}$ m/min, then the seepage loss per metre length of dam will be

1. $2.16\times {10}^{-2}{m}^3/day$
2. $6.48\times {10}^{-2}{m}^3/day$
3. $11.52\times {10}^{-2}{m}^3/day$
4. $34.56\times {10}^{-2}{m}^3/day$

Q. For a sheet pile wall constructed in a soil having effective grain size = 0.1 mm, the difference of the upstream and downstream water levels is 3 m. If the flow net drawn for the problem yields 2 as the ratio of number of head drops to number of flow channels, then what is the discharge in unit of m${}^3$/s /m length of sheet pile wall?

1. $3.0\times {10}^{-4}$
2. $3.0\times {10}^{-2}$
3. $1.5\times {10}^{-4}$
4. $1.5\times {10}^{-2}$

Q. A flow net below a dam consists of 24 equipotential drops and 7 flow channels. The difference between the upstream and downstream water level is 6m. the length of the flow line adjacent to the toe of the dam at exit is 1 m. The specific gravity and void ratio of the soil below the dam are 2.70 and 0.70, respectively. The factor of safety against piping is

1. 1.67
2. 2.5
3. 3.4
4. 4

Q. The proposed dam shown in the figure is 90 m long and the coefficient of permeability of the soil is 0.0013 mm/second. The quantity of water $\left(m^3\right)$ that will be lost per day by seepage is (rounded to the nearest number):

1. 55
2. 57
3. 59
4. 61

Q. The configuration of flow nets depends upon

1. the permeability of the soil
2. the difference in the head between upstream and downstream sides
3. the boundary condition of flow
4. the amount of seepage that takes place

Q. A flownet is drawn to obtain

1. seepage, coefficient of permiability and uplift pressure
2. coefficient of permeability, uplift pressure and exit gradient
3. exit gradient, uplift pressure and seepage quantity
4. exit gradient, seepage and coefficient of permeability

Q.

Moving of body in water is called?

Q. From a flow net which of the following information can be obtained?
1. Rate of flow 2. Pore water pressure
3. Exit gradient 4. Permeability
Select the correct answer using the codes given below:

1. 1, 2, 3 and 4
2. 1, 2 and 3 only
3. 2, 3 and 4 only
4. 1 only

Q. Consider the following statements regarding flow net:
1. It helps determine the quantity of seepage.
2. It helps determine the upward lift below a hydraulic structure.
3. It is applicable to rotational flow only.
Which of the above statements are correct?

1. 1 and 2
2. 1 and 3
3. 2 and 3
4. 1, 2 and 3

Q. Statement-I : In a flownet, each field must be a (curvilinear) square:
Statement-II: Each flow channel in a flownet has the same rate of flow.

1. Both Statement (I) and Statement (II) are individually true and Statement {II) is the correct explanation of Statement (I)
2. Both Statement {I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I)
3. Statement (I) is true but Statement (II) is false
4. Statement {I) is false but Statement (II) is true

Q. A sheet pile has an embankment depth of 12 m in a homogeneous soil stratum. The coefficient of permeability of soil is ${10}^{-6}m/s$. Difference in the water levels between the two sides of the sheet pile is 4m. The flow net is constructed with five number of flow lines and eleven number of equipotential lines. The quantity of seepage (in $c{m}^3/s$ per m. up to one decimal place) under the sheet pile is

1. 1.6

Q. The flow net below a dam consists of 25 equipotential lines and 9 flow lines. The available head is 6 m. Taking coefficient of permeability as $3.8\times {10}^{-6}m/s$, the seepage $\left(m^3/sperm\right)$ under the dam is

1. $3.8\times {10}^{-6}$
2. $45.6\times {10}^{-6}$
3. $7.6\times {10}^{-6}$
4. $8.21\times {10}^{-6}$

Q.

The number of flow channels and head drops is 4 and 12 respectively. If the difference in the up-stream and down-stream water levels is 3 m, what is the discharge per meter width of a sheet pile, if k = 0.1 m/s .

1. 0.1

Q. The flow net around a sheet pile wall is shown in the sketch. The properties of the soil are : permeability coefficient = 0.09 m/day (isotropic), specific gravity = 2.70 and void ratio = 0.85. The sheet pile wall and the bottom of the soil are impermeable.

The factor of safety against the occurrence of piping failure is

1. 3.55
2. 2.93
3. 2.60
4. 0.39