Well Hydraulics MCQ Quiz - Objective Question with Answer for Well Hydraulics - Download Free PDF

Explanation:

Centrifugal pump

A well-point system is a widely used dewatering technique to lower the groundwater table during excavation. It is especially effective in permeable soils like sandy or gravelly strata.

• The system consists of multiple well points (small-diameter tubes with strainers at the bottom) installed at close intervals along the excavation.

• These well points are connected through a header pipe to a vacuum pump—typically a centrifugal pump equipped with a priming tank.

• When the pump operates, it creates a vacuum that draws water through the well points, lowering the groundwater level around the excavation site.

Additional Information

Reciprocating Pump:

• Operates in a pulsating manner, not suitable for high-flow, continuous applications.

• Higher maintenance and lower efficiency in such scenarios.

Jet Pump:

• Often used in deep wells where high lift is needed.

• Not typically used in the well-point system due to lower efficiency and complexity.

Sump Pump:

• Used to remove water that has already accumulated in a pit (sump).

• Not effective for drawing groundwater over a large area like a well-point system requires.

 

Concept:

Discharge velocity:

It is the volume of water flowing in unit time across the unit cross-section perpendicular to the direction of flow.

V_d  = K i

where

K = Coefficient of permeability 

i = hydraulic gradient

Seepage velocity:

It is the real velocity of water through the soil.

seepage velocity = \(Distance travelled\over time \)

Further, the relation between discharge velocity and seepage velocity is given as

V_d = n V_s 

Calculation:

Given

n = 0.25

i = 0.04

V_s = 6 m/h = \(600\over 3600\) cm/sec = \(1\over 6\) cm/sec

The permeability of the aquifer is

k i = n V_s

k× 0.04 = 0.25 × \(1\over 6\)

k = 1.042 cm/sec

Concept:

Specific yield (S_y) is the ratio of the volume of water that drains from a saturated rock (due to gravity) to the total volume of the rock.

The Specific Retention (S_r) of a rock or soil is the ratio of the volume of water a rock can retain against gravity to the total volume of the rock.

Therefore, the total porosity is equal to the volume or water that a rock will yield by gravity drainage (S_y) and the volume held by surface tension (S_r) and is given as:

\({S_y} + {S_r} = \eta\)

Calculations:

Given:

S_y = 30%

η = 50%

\({S_y} + {S_r} = \eta\)

\(30\% + {S_r} = 50\%\)

\({S_r} = 50\% - 30\%\)

\({S_r} = 20\%\)

Explanation:

Ashford Strainer

• It consists of a perforated tube with a wire mesh surrounding it.

• To keep the space between the mesh and the tube a wire is wound around the tube. The wire-mesh is soldered to this wire.

• The wire mesh can be protected by a wire net above all. This type is delicate because there are many loose parts.

 Cook strainer

• It consists of a solid drawn brass tube. The wedge shaped slots are cut on the surface of the tube. The slots are horizontal.

• The slots are cut from inside of the tube. Thus the openings are wide inside of the tube.

• The width of slot varies from 15/100 to 40/100 of a mm depending upon the coarseness of sand.

• This strainer is quite good but it is very expensive as the slot cutting is a difficult process.

• Only difference between the Tej and Cook strainer is in former the brass sheet is first slotted and then bent to form a tube.
• The tube is then brazed. As there are joints this strainer is weaker.
• It is cheap and hence widely used.

Explanation:

Pumping Out Test

The Pumping Out Test is a method used to measure the permeability of soil by pumping out water from a well and observing the effects. It is often utilized in larger engineering projects where understanding the permeability of extensive soil deposits is crucial. The test influences soil over a large area, giving a comprehensive measure of soil permeability.

Analyzing the Given Options

1. "It is the most accurate test." (Correct)

   • The Pumping Out Test is generally considered accurate as it assesses the permeability of a large volume of soil.

2. "Pumping out tests are expensive." (Correct)

   • Due to the extensive setup and equipment required, the test is costly.

3. "It is used to measure permeability of soils for small engineering projects." (Incorrect)

   • The Pumping Out Test is not typically used for small projects due to its extensive nature and cost.

   • This statement is incorrect as the test is more suitable for larger projects.

4. "In a pumping out test, soil deposit over a large area is influenced and represents an overall coefficient of permeability of a large mass of soil." (Correct)

   • This statement correctly describes the test's ability to measure permeability over a large area.

Explanation:

Different types of geological formations:

Aquitard: These are porous but less permeable geological formations from which water cannot flow but instead sieves through it. A good example of aquitard is silty clay material. 

Aquifers: These are porous and permeable geological formations from which sufficient discharge can be extracted. It generally comprises layers of sand and gravel and fractured bedrock.

Aquiclude: These are porous but impermeable geological formations from which discharge cannot be extracted. A good example of aquiclude is clay.

Aquifuge: These are neither porous nor permeable geological formations. A good example of it is a massive compact rock.

Classification of tube wells:

(a) Strainer type tube well:

The strainer type tube well is generally unsuitable for very fine sandy strata, because in that case the size of mesh opening will have to be reduced considerably which may result in choking of the strainer, and if the screen openings are kept bigger, the well will start discharging sandy water.

(b) Cavity type tube well

If a suitable strong clay stratum is not available then cavity type tube well cannot be provided because it does not have strainers and hence, it draws its supplies from the bottom and not from the sides.

(c) Slotted type tube well

In this type of tube well, the gravel and coarse sand shrouding prevents the fine particles from entering the well pipe.  If sufficient depth of water bearing stratum is not available, then it is not possible to provide strainer or cavity type tube wells, hence, slotted type tube wells provided in that case.

Concept:

Available water in m3 = (Specific Yield) × (Area) × (Change in water level)

Calculation:

Given, 
Specific Yield = 10% = 0.1, area = 1 km2 = 1 × 106 m2, and change in water level = 1.0 m

∴ Available water in m3 = (0.1) × (1 × 106 m2) × (1.0 m) = 100000 m3

Explanation:

• Aquifuge is neither porous, not permeable, these are not inter-connected opening, so it does not contain water and does not allow it to pass through it.
• A good example of it is a massive compact rock without any fracture.

Aquifer:

• An aquifer is a saturated formation of earth material from which water is yield in sufficient quantity, due to the high permeability of earth material.
• Unconsolidated deposits of sand and gravel are good for aquifer formation.
• Groundwater is generally extracted from aquifers, so it is of much importance to us.
• These are generally of 3 types.
  • Unconfined aquifer
  • Confined aquifer
  • Perched aquifer

Aquiclude:

• It contains a large amount of water in pores, but extraction of water is very difficult. It may be considered as close to water movement.
• A good example of aquiclude is clay.

Aquitard:

• Aquitard form by that material through which the only seepage is possible but extraction of water is not so easy as in aquifer.
• A good example of aquitard is silty clay material. 

Concept:

Capillary potential:

It is also known as soil matric potential. Capillary potential can be defined as, The force per unit area that must be exerted in order to extract water from the soil. Capillary forces are the main force moving water in the soil and it typically will move water into smaller pores and into the drier regions of soil. Water potential is highly texture-dependent. Clay particles have a larger surface area and thus will have a higher affinity for water than that of silt or sandy soils. Tensiometer is used for the measurement of capillary potential.

Soil moisture deficiency:

The water required to bring the soil moisture content of a given soil to its field capacity Is called the field moisture deficiency or soil moisture deficiency. The infiltrated water first meets the soil moisture deficiency if any and excess water moves vertically downwards to reach the groundwater table.

Moisture equivalent:

It is defined as the water content expressed as a percentage of the dry weight that a soil can retain against a centrifugal force one thousand times the force of gravity and used as a convenient laboratory measure of soil moisture conditions.

Concept:

(i) Darcy velocity V_D is a fictitious velocity since it assumes that flow occurs across the entire cross-section of the sediment sample. Flow actually takes place only through interconnected pore channels (voids), at the seepage velocity V_S.

From the Continuity Equation,  Q = constant

Q = A x V_D = A_v x Vs

Where, Q = flow rate, A = total cross-sectional area of material

A_V = area of voids, Vs = seepage velocity, V_D  = Darcy velocity

Since A > A_V, and Q = constant, Vs > V_D

Calculation:

Discharge through the aquifer is 0.25 m² / s

Porosity = 25%

Depth = 5 m and Length = 100 m

Area of voids (Assuming unit width) = 5 × .25 = 1.25 m

Seepage discharge = Seepage velocity x area of voids 

0.25 = V_s x 1.25 

V_s = 1/5 m / s

So time (t) = 100 x 5 = 500 s

Concept:

Discharge velocity:

It is the volume of water flowing in unit time across the unit cross-section perpendicular to the direction of flow.

V_d  = K i

where

K = Coefficient of permeability 

i = hydraulic gradient

Seepage velocity:

It is the real velocity of water through the soil.

seepage velocity = \(Distance travelled\over time \)

Further, the relation between discharge velocity and seepage velocity is given as

V_d = n V_s 

Calculation:

Given

n = 0.25

i = 0.04

V_s = 6 m/h = \(600\over 3600\) cm/sec = \(1\over 6\) cm/sec

The permeability of the aquifer is

k i = n V_s

k× 0.04 = 0.25 × \(1\over 6\)

k = 1.042 cm/sec

Explanation:

Wells

• It is defined as an opening or hole dug or drilled into an aquifer with the view of withdrawing water for drinking, agriculture, or other uses.
• Mostly these are vertical holes drilled or dug into the ground up to the aquifer. Water may flow through these wells either due to natural hydrostatic pressure or may have to be pumped out.
• These may be quite shallow or deep depending on the depth at which the water-bearing strata are encountered.

Gravity Wells

• It is also called a water table well and is a vertical or nearly vertical hole penetrating the zone of saturation below the ground.
• The essential characteristic of such a well is atmospheric pressure and represents when at rest, the water table of the area around the well.
• Water will not normally flow out of such a well on its own; it has to be pumped out or taken out.
• Most wells driven in the aquifer for withdrawal of water are actually gravity well. When water has to be pumped out of such wells it forms the location of the tube wells.

Artesian Wells

• These are the holes drilled through the confined or artesian aquifer.
• In such wells water generally flows out at the ground surface, and even may gush out to some height.

Concepts:

Storage coefficient:

• The water yielding capacity of the confined aquifer can be expressed in terms of its storage coefficient.
• Storage coefficient or storativity is the volume of water released from the storage per unit decline in Piezometric head, per unit area of the aquifer(not from the entire aquifer). 
• It is a dimensionless parameter. 

Coefficient of transmissibility (T):

• The rate of flow of water through the entire medium of unit width under a unit hydraulic gradient is termed the coefficient of transmissibility.

Specific Retention (Sr):

• Specific retention of a rock or soil is the ratio of the volume of water a rock can retain against gravity to the total volume of the rock.
• In coarse gravels, the size of the pores is large, due to the large size of pores, water holding capacity against gravity will be less. Hence the specific retention will also be less.

\(\left( {{S_R}} \right) = \frac{{{Volume\;of\;water\;retain\;against\;gravity\;(V_{RG})}}}{Volume\;of\;soil} \times 100\)

Explanation:

Specific Capacity:

(i) Specific Capacity of a well is defined as the rate of flow through the well per unit drawdown.

(ii) The specific capacity is not the same for all the unit drawdown, but it is computed only for the initial first-meter fall of the height of water in the well.

\({Q_{Specific\;Capacity}} = \frac{{Discharge\;through\;the\;well\;\left( Q \right)}}{{Unit\;Drawdown\;\left( {{S_T}} \right)}}\)

Specific storage: 

It is the amount of water that a portion of an aquifer releases from storage, per unit mass or volume of the aquifer, per unit change in hydraulic head, while remaining fully saturated.

Specific yield: 

It is defined as the volume of water released from storage by an unconfined aquifer per unit surface area of the aquifer per unit decline of the water table.