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Past Questions
Why do earthen dams require more maintenance compared to concrete dams?
A)
Because they are expensive to construct
B)
Because they are susceptible to seepage and erosion
C)
Because they do not store water efficiently
D)
Because they cannot withstand high pressure
Earthen dams are constructed using compacted soil, sand, and clay. Unlike concrete dams, which are rigid and impermeable, earthen dams are porous and subject to several natural degradation processes.
1. Seepage: Water naturally permeates through the soil body. If not properly controlled (via cores or filters), this can lead to 'piping,' where internal erosion creates tunnels through the dam, potentially leading to failure.
2. Erosion: The surface of an earthen dam is vulnerable to erosion from rainfall, wind, and wave action (overtopping). This requires constant maintenance of grass cover or rip-rap protection.
3. Biological Factors: Burrowing animals and vegetation growth can create pathways for water, further increasing seepage risks.
Comparing other options:
- Option (a) is incorrect because earthen dams are generally cheaper to construct than concrete dams due to the use of local materials.
- Option (c) is incorrect because earthen dams are highly efficient at storing water if designed correctly.
- Option (d) is incorrect because earthen dams are specifically designed to withstand high hydrostatic pressure through their wide base and massive weight (gravity principle).
What is the primary purpose of an abstract estimate?
A)
To provide a quick, approximate cost of the project
B)
To determine the detailed breakdown of each item
C)
To estimate only the labor cost
D)
To calculate the total area of the site
In civil engineering estimation and costing, an 'Abstract Estimate' (often associated with preliminary or rough cost estimates) is prepared to obtain a quick and approximate idea of the total expenditure required for a project. It is typically used during the initial planning stages to secure administrative approval or budget allocation before detailed drawings and specifications are finalized.
Reasoning for other options:
- Option (b) is incorrect because the detailed breakdown of each item is the purpose of a 'Detailed Estimate', which involves a complete quantity takeoff (Bill of Quantities).
- Option (c) is incorrect because an estimate covers all costs, including materials, labor, plant, and overheads, not just labor.
- Option (d) is incorrect because calculating the total area of the site is a surveying or measurement task, which is a prerequisite for estimation, not the purpose of the abstract estimate itself.
Which of the following is the primary binding material in concrete?
A)
Sand
B)
Cement
C)
Water
D)
Aggregates
Concrete is a composite material consisting of a binding medium and filler materials.
1. **Cement (The Binder):** Cement is a hydraulic binder. When mixed with water, it undergoes a chemical reaction called hydration, forming a calcium silicate hydrate (C-S-H) gel. This gel acts as the 'glue' that binds the aggregates together into a solid, rock-like mass.
2. **Sand (Fine Aggregate):** Sand acts as a filler material to occupy the voids between coarse aggregates and reduce shrinkage. It does not possess binding properties.
3. **Water:** Water is a reactant required to initiate the hydration of cement. While essential for the process, it is not the binding material itself; rather, it is the medium in which the binder reacts.
4. **Aggregates (Coarse Aggregate):** These provide the structural bulk and strength to the concrete but are inert and cannot bind to one another without a binder.
Therefore, cement is the primary binding material in concrete.
Why is river training necessary in hydraulic engineering?
A)
To increase the river’s flow velocity
B)
To control river meandering and prevent erosion
C)
To reduce the depth of the river channel
D)
To increase sediment deposition in floodplains
River training refers to the engineering works carried out to guide the flow of a river within a desired channel. The primary objectives of river training are: 1) To prevent the river from changing its course (controlling meandering), 2) To protect the banks from erosion (scouring), 3) To ensure a safe and efficient discharge of floodwaters, and 4) To protect hydraulic structures like bridges and barrages from being undermined.
Analysis of options:
- Option (a) is incorrect because increasing flow velocity often increases the erosive power of the water, which can destabilize banks.
- Option (b) is correct as it directly addresses the fundamental purpose of river training: stability and erosion control.
- Option (c) is incorrect because reducing the depth of the channel would decrease its carrying capacity, potentially increasing the risk of overbank flooding.
- Option (d) is incorrect because sediment deposition in floodplains is a natural process; river training usually aims to prevent excessive deposition within the main channel to maintain flow capacity.
Why is effective stress important in soil mechanics?
A)
It determines the soil’s permeability
B)
It governs the soil’s ability to sustain loads and resist shear failure
C)
It is only relevant for dry soils
D)
It depends solely on the total stress applied to the soil
Effective stress is the stress carried by the soil skeleton (total stress minus pore water pressure). It is important because it controls soil strength, compressibility, and shear resistance—therefore it governs the soil’s ability to sustain loads and resist shear failure. Option (b) is correct.
Option (a) is incorrect because permeability is mainly related to soil structure, grain size, and void ratio, not directly to effective stress. Option (c) is incorrect because effective stress is relevant in both dry and saturated soils; in saturated soils it is especially important due to pore water pressure. Option (d) is incorrect because effective stress does not depend solely on total stress; it also depends on pore water pressure.
Why are under-sluices provided in weirs and barrages?
A)
To divert excess water during floods
B)
To control the movement of fish in the river
C)
To remove sediment from the riverbed and prevent silting
D)
To allow boats and ships to pass through
Under-sluices are gated openings provided at the lower level of a weir or barrage, typically located adjacent to the canal head regulator. Their primary engineering purpose is to create a 'scouring' effect. By opening these gates, water is discharged at a high velocity along the riverbed, which flushes out accumulated silt and sediment (sediment exclusion). This prevents the riverbed from rising (aggradation) and ensures that the water entering the canal is relatively free of sediment.
Analysis of other options:
- Option (a) is incorrect because excess flood water is typically handled by the main spillway or crest gates, not specifically the under-sluices.
- Option (b) is incorrect because fish movement is managed by 'fish ladders' or 'fish ways'.
- Option (d) is incorrect because the passage of boats and ships is managed by 'navigation locks'.
What is the main difference between a gravity dam and an arch dam?
A)
Gravity dams rely on their weight, while arch dams transfer forces to the valley walls
B)
Gravity dams require strong abutments, while arch dams do not
C)
Gravity dams are more suitable for narrow valleys than arch dams
D)
Gravity dams require less construction material than arch dams
Step-by-step reasoning:
1. **Gravity Dam Analysis**: A gravity dam is a structure designed to resist the external forces (primarily the hydrostatic pressure of the water) by using its own massive weight. The stability is maintained by the friction between the dam base and the foundation, ensuring it does not slide or overturn.
2. **Arch Dam Analysis**: An arch dam is a curved structure that utilizes the 'arch effect.' Instead of relying solely on weight, it transfers the water pressure horizontally to the canyon walls (abutments) through compressive stress.
3. **Evaluating Option (a)**: This correctly identifies the fundamental structural difference: weight-based stability for gravity dams vs. load-transfer (arch action) for arch dams.
4. **Evaluating Option (b)**: This is incorrect. Arch dams require extremely strong, stable rock abutments to support the transferred load; gravity dams do not rely on abutments for stability in the same way.
5. **Evaluating Option (c)**: This is incorrect. Arch dams are specifically suited for narrow, deep valleys with strong walls, whereas gravity dams can be used in wider valleys.
6. **Evaluating Option (d)**: This is incorrect. Because arch dams use structural geometry rather than mass, they typically require significantly less concrete/material than gravity dams.
A road has a width of 6m and an average thickness of 20 cm. The length of the road is 1 km. What is the total volume of material required?
A)
1000 m3
B)
1200 m3
C)
1500 m3
D)
1800 m3
To find the total volume of material required for the road, we use the formula for the volume of a rectangular prism: Volume = Length × Width × Thickness.
1. **Identify and Convert Units to Meters (m):**
- Width = 6 m
- Thickness = 20 cm = 20 / 100 = 0.2 m
- Length = 1 km = 1000 m
2. **Calculate the Volume:**
- Volume = 1000 m (Length) × 6 m (Width) × 0.2 m (Thickness)
- Volume = 6000 × 0.2
- Volume = 1200 m³
Comparing this result to the given options:
- Option (a) 1000 m³ is incorrect.
- Option (b) 1200 m³ matches our calculation.
- Option (c) 1500 m³ is incorrect.
- Option (d) 1800 m³ is incorrect.
Why is seepage control essential in earthen dams?
A)
To maintain a stable water level in the reservoir
B)
To prevent water loss and structural failure due to internal erosion
C)
To increase the storage capacity of the dam
D)
To improve the water quality in the reservoir
In earthen dams, seepage is the movement of water through the dam body or its foundation. While some seepage is inevitable, uncontrolled seepage can lead to a phenomenon known as 'piping' (internal erosion). Piping occurs when water flowing through the dam carries soil particles with it, creating hollow tubes or 'pipes' within the structure. This significantly weakens the dam's integrity and can lead to a catastrophic structural failure. Therefore, seepage control measures (such as clay cores, grout curtains, or filters) are essential to prevent both the loss of stored water and the collapse of the dam.
Analysis of other options:
- Option (a): Water level is maintained by the dam's height and spillway management, not by seepage control.
- Option (c): Storage capacity is determined by the reservoir's geometry and the dam's height, not by the control of seepage.
- Option (d): Water quality is affected by environmental factors and pollution, not by the internal seepage mechanisms of the dam structure.
What is the major factor affecting the shear strength of soil?
A)
Permeability
B)
Cohesion and internal friction angle
C)
Soil color
D)
Moisture content only
The shear strength of soil is defined as the internal resistance per unit area that a soil mass can offer to resist failure along a plane. According to the Mohr-Coulomb failure criterion, the shear strength (τ) is expressed by the equation: τ = c + σ tan(φ), where 'c' represents cohesion and 'φ' represents the angle of internal friction.
1. Cohesion (c): This is the component of shear strength that is independent of the normal stress. It is dominant in cohesive soils like clays due to chemical bonding and electrostatic forces between particles.
2. Internal Friction Angle (φ): This represents the resistance due to the interlocking of soil grains and friction between them. It is the dominant factor in cohesionless soils like sands and gravels.
Analysis of other options:
- Permeability (a): This refers to the ease with which water flows through soil; while it affects the rate of consolidation, it is not a primary component of the shear strength formula.
- Soil color (c): This is a physical characteristic used for classification but has no direct impact on the mechanical strength of the soil.
- Moisture content only (d): While moisture content significantly influences cohesion and effective stress (and thus affects shear strength), it is not the 'major factor' in the sense of the fundamental parameters (c and φ) that define the strength of the soil matrix.
What is the primary chemical compound responsible for early strength gain in cement?
A)
Tricalcium silicate (C3S)
B)
Dicalcium silicate (C2S)
C)
Tricalcium aluminate (C3A)
D)
Tetracalcium aluminoferrite (C4AF)
Portland cement consists of four primary clinker compounds known as Bogue's compounds. Their roles in strength gain and hydration are as follows:
1. Tricalcium silicate (C3S): This compound hydrates rapidly and is primarily responsible for the early strength (initial set and strength gain within the first 7 to 28 days).
2. Dicalcium silicate (C2S): This compound hydrates much more slowly and contributes to the long-term strength gain (after 28 days).
3. Tricalcium aluminate (C3A): This is the most reactive compound; it hydrates very quickly and is responsible for the initial flash set, but it contributes very little to the structural strength of the concrete.
4. Tetracalcium aluminoferrite (C4AF): This compound acts mainly as a flux during manufacturing to lower the clinkering temperature and contributes minimally to the strength of the concrete.
Therefore, for early strength gain, C3S is the primary compound.
Which factor is the most critical in designing spillways for dams?
A)
Length of the dam
B)
Expected peak flood discharge
C)
Type of construction material
D)
Width of the river
A spillway is a structure designed to provide the controlled release of water from a dam or reservoir into a downstream area. Its primary purpose is to prevent the dam from being overtopped during extreme flood events, which could lead to catastrophic structural failure (especially in embankment dams).
1. Why 'Expected peak flood discharge' is correct: The capacity of the spillway must be sufficient to pass the Probable Maximum Flood (PMF) or the design flood without allowing the reservoir level to rise above the maximum permissible water level. If the spillway is undersized for the peak discharge, the dam may fail. Therefore, hydrology (the estimation of peak flood) is the most critical design parameter.
2. Why other options are incorrect:
- (a) Length of the dam: This is a geometric property of the dam body, not a governing factor for the hydraulic capacity of the spillway.
- (c) Type of construction material: While material affects the durability and cost of the spillway (e.g., concrete vs. rock), it does not dictate the required capacity or the fundamental design logic of the spillway.
- (d) Width of the river: The river width influences the site selection and the layout of the dam, but the spillway's design is driven by the volume of water that must be discharged, not the width of the channel it sits in.
What is the primary function of a head regulator in a canal system?
A)
To allow excess water to escape from the canal
B)
To regulate and control the entry of water into the canal from the river
C)
To store water for future use
D)
To measure the discharge of water entering the canal
A head regulator is a hydraulic structure constructed at the head (start) of a canal where it takes off from a river or a main reservoir. Its primary functions are: 1) To control the amount of water entering the canal system based on demand, and 2) To prevent excessive water or silt from entering the canal during floods.
Analysis of other options:
- Option (a) describes the function of an 'escape' or 'surplus weir'.
- Option (c) describes the function of a 'dam' or 'reservoir'.
- Option (d) describes the function of a 'flow meter' or 'gauging station' (though regulators can be used in conjunction with gauges, their primary purpose is control, not measurement).
Why are stepped spillways used in dams?
A)
They help dissipate kinetic energy and reduce erosion
B)
They increase the storage capacity of the dam
C)
They facilitate easy maintenance of the spillway
D)
They reduce the overall weight of the dam structure
A stepped spillway is a hydraulic structure designed to convey water from the reservoir to the downstream toe of the dam. The primary purpose of the 'steps' is to create turbulence and induce aeration as the water flows down the face. This process converts a significant portion of the water's potential energy into heat and sound through friction and turbulence, thereby dissipating the kinetic energy of the flowing water. By reducing the velocity and energy of the water before it reaches the bottom, the risk of scouring and erosion at the toe of the dam is significantly minimized, reducing the need for massive stilling basins.
Why other options are incorrect:
- Option b: Storage capacity is determined by the reservoir volume and dam height, not the design of the spillway surface.
- Option c: Stepped spillways are generally more complex to construct and maintain than smooth spillways due to the increased surface area and potential for cavitation at the step edges.
- Option d: Adding steps does not inherently reduce the overall weight of the dam; in many cases, it may require more concrete or specific reinforcement to handle the impact of the water on the step treads.
What is the main function of a cross regulator in an irrigation system?
A)
To regulate the flow from one river to another
B)
To control the water level in the main canal
C)
To divert water into different branch canals
D)
To measure the discharge in a canal
A cross regulator is a hydraulic structure constructed across a canal to control the water level (water surface elevation) in the main canal. This is primarily done to ensure that a sufficient head of water is maintained so that the required discharge can be diverted into off-taking branch canals, regardless of the flow rate in the main canal.
Analysis of options:
- Option (a) is incorrect because regulating flow between rivers is typically the function of a barrage or a diversion dam.
- Option (c) is incorrect because the diversion of water into branch canals is the function of a 'Canal Regulator' or 'Head Regulator' located at the start of the branch canal, not the cross regulator itself (though the cross regulator helps maintain the level for the head regulator to work).
- Option (d) is incorrect because measuring discharge is the function of measuring devices like flumes, weirs, or orifices.
Which test is used to determine the fineness of cement?
A)
Slump test
B)
Vicat apparatus test
C)
Sieve analysis
D)
Le Chatelier test
Fineness of cement refers to the size of the particles. Finer cement has a larger surface area, which increases the rate of hydration and strength gain. The two primary methods to determine fineness are: 1. Sieve Analysis (using a 90-micron IS sieve) and 2. Air Permeability Method (Blaine's test). Among the given options, Sieve analysis is the correct method for determining fineness.
Why other options are incorrect:
- a. Slump test: Used to determine the workability and consistency of fresh concrete, not the properties of cement powder.
- b. Vicat apparatus test: Used to determine the initial and final setting times and the consistency of cement paste.
- d. Le Chatelier test: Used to determine the soundness of cement (expansion due to free lime).
Why do we use bar bending schedules?
A)
To ensure accurate steel reinforcement placement
B)
To calculate the amount of concrete needed
C)
To determine the weight of the structure
D)
To estimate the cost of excavation
A Bar Bending Schedule (BBS) is a comprehensive list that provides detailed information about the reinforcement bars required for a reinforced concrete (RCC) structure. It includes the bar mark, diameter, shape, cutting length, and number of bars.
1. Why Option (a) is correct: The primary purpose of a BBS is to translate structural drawings into a precise list of instructions for the fabricator. By specifying the exact length and bending angles, it ensures that the steel is placed exactly as designed by the structural engineer, minimizing waste and ensuring structural integrity.
2. Why Option (b) is incorrect: The amount of concrete is calculated through volume calculations (Length x Width x Depth) based on the structural dimensions, not the bar bending schedule.
3. Why Option (c) is incorrect: While the BBS helps calculate the total weight of the steel, it does not determine the total weight of the entire structure (which includes concrete, masonry, and finishes).
4. Why Option (d) is incorrect: Excavation costs are estimated based on the volume of earthwork (area of foundation x depth), which is entirely independent of the reinforcement detailing.
Which factor does NOT significantly affect the stability of an earthen dam?
A)
Seepage control
B)
Gravity force
C)
Pore water pressure
D)
Soil compaction
In the context of earthen dam stability, we evaluate factors that can lead to failure (like slope instability or piping).
1. Seepage control (Option a): This is critical. Uncontrolled seepage can lead to 'piping' (internal erosion), which can cause the entire structure to collapse.
2. Pore water pressure (Option c): This is a primary factor in slope stability. High pore water pressure reduces the effective stress between soil particles, thereby reducing the shear strength of the soil and increasing the likelihood of a slope failure.
3. Soil compaction (Option d): The degree of compaction determines the density, permeability, and shear strength of the dam body. Poor compaction leads to instability and increased seepage.
4. Gravity force (Option b): While gravity provides the downward force that keeps the dam in place, it is a constant and inherent property of the mass. In stability analysis (such as the Method of Slices), gravity is the baseline force; it is not a 'variable factor' that typically threatens or significantly fluctuates to 'affect' stability in the way that seepage, pore pressure, or compaction quality do. Among the given choices, it is the least likely to be considered a 'risk factor' or a variable affecting the stability of the embankment.
How does a siphon aqueduct differ from a normal aqueduct?
A)
A siphon aqueduct allows the river to flow above the canal
B)
A siphon aqueduct allows the canal to pass underneath a river by siphon action
C)
A siphon aqueduct requires more energy to operate than a normal aqueduct
D)
A siphon aqueduct is only used in flat terrains
In hydraulic engineering, the distinction between an aqueduct and a siphon aqueduct depends on the relative levels of the canal and the drainage (river).
1. **Normal Aqueduct**: A canal is carried over a drainage (river) across a trough. In this case, the canal bed is always above the High Flood Level (HFL) of the river. The river flows freely underneath the canal.
2. **Siphon Aqueduct**: This is used when the canal bed is not high enough to pass over the river's HFL. Instead of raising the canal embankment excessively, the canal is taken under the river through a closed conduit (barrel). The water in the canal drops below the river bed and then rises back to its original level on the other side due to the principle of communicating vessels (siphon action).
Analysis of options:
- Option (a) is incorrect because in both cases, the canal is the primary structure being managed; in a normal aqueduct, the river flows below, and in a siphon aqueduct, the canal flows below the river.
- Option (b) is correct as it accurately describes the hydraulic mechanism of a siphon aqueduct.
- Option (c) is incorrect; while there are head losses due to friction in the siphon barrel, it does not 'require energy to operate' in the sense of external power; it operates by gravity.
- Option (d) is incorrect; the choice depends on the relative elevations of the canal and the river, not necessarily the flatness of the terrain.
Why are sluice gates installed in barrages?
A)
To control the sediment flow and maintain riverbed levels
B)
To increase the storage capacity of the barrage
C)
To divert water for hydroelectric power generation
D)
To regulate the velocity of water entering the canal
In hydraulic structures, a barrage is designed to raise the water level to divert it into a canal. However, this often leads to the accumulation of sediment (silt) upstream of the structure. Sluice gates (specifically undersluices) are installed at the bottom of the barrage to allow high-velocity water to flow through, which flushes out the accumulated sediment and prevents the riverbed from rising (aggradation). This process, known as 'scouring,' ensures that the riverbed levels are maintained and the canal head regulator remains free of silt.
Why other options are incorrect:
- Option b: Storage capacity is the primary function of a dam, not a barrage. Barrages are diversion structures with minimal storage.
- Option c: While water from a barrage may be used for power, the specific purpose of 'sluice gates' is sediment management, not power generation (which would use penstocks/turbines).
- Option d: The velocity of water entering the canal is regulated by the 'Canal Head Regulator,' not the sluice gates of the barrage itself.
Why are energy dissipaters necessary at the downstream of spillways?
A)
To increase the velocity of water flow
B)
To prevent erosion and structural damage caused by high-energy water
C)
To store additional water for irrigation purposes
D)
To divert water into different canals
Step-by-step reasoning:
1. **Understanding Spillways**: A spillway is a structure used to provide the controlled release of water from a dam or levee into a downstream area. As water flows down the spillway, potential energy is converted into kinetic energy, resulting in very high velocities.
2. **The Problem**: Water exiting a spillway possesses immense energy. If this high-velocity flow is allowed to enter the natural riverbed directly, it will cause severe 'scouring' (erosion of the bed and banks), which can undermine the foundation of the dam or the spillway structure itself, leading to structural failure.
3. **The Solution (Energy Dissipaters)**: Energy dissipaters (such as stilling basins, baffle blocks, or flip buckets) are engineered structures placed at the toe of the spillway. They use mechanisms like hydraulic jumps or impact to convert high-velocity kinetic energy into turbulence and heat, thereby reducing the flow velocity to a safe level before it reaches the natural channel.
Analysis of options:
- Option (a) is incorrect because the goal is to decrease, not increase, the velocity.
- Option (c) is incorrect because energy dissipaters are for flow control, not water storage.
- Option (d) is incorrect because diverting water is the function of headworks or regulators, not energy dissipaters.
Which type of dam is best suited for a narrow valley with strong rock foundations?
A)
Earthen dam
B)
Gravity dam
C)
Arch dam
D)
Buttress dam
The selection of a dam type depends primarily on the topography of the valley and the strength of the foundation.
1. **Arch Dam (Correct):** Arch dams are curved structures that transmit the water pressure to the canyon walls (abutments) through arch action. For this mechanism to work, the valley must be narrow (to maintain a reasonable arch ratio) and the side walls/foundations must be composed of strong, incompressible rock to resist the massive horizontal thrust. Therefore, it is the best choice for a narrow valley with strong rock foundations.
2. **Earthen Dam (Incorrect):** These are used in wide valleys and can be constructed on relatively weak foundations (like soil or gravel) because they distribute their weight over a large area. They are not suitable for narrow, rocky gorges where the footprint would be too small to maintain stability.
3. **Gravity Dam (Incorrect):** While gravity dams require strong foundations, they rely on their own weight to resist sliding and overturning. They are generally used in wider valleys than arch dams and require significantly more material (concrete/masonry).
4. **Buttress Dam (Incorrect):** These are a variation of gravity dams that use a thinner upstream face supported by buttresses. While they save material, they are not as efficient as arch dams in narrow, rocky canyons.
What does the void ratio (e) represent in a soil mass?
A)
The percentage of water present in the soil
B)
The volume of voids to the volume of solids
C)
The total volume of soil particles
D)
The weight of water in a soil sample
In soil mechanics, the void ratio (e) is a fundamental dimensionless parameter used to describe the relative volume of voids within a soil mass. It is mathematically defined as the ratio of the volume of voids (Vv) to the volume of solids (Vs), expressed as: e = Vv / Vs.
Analysis of options:
- Option (a) is incorrect because the percentage of water relative to the weight of solids is defined as the water content (w).
- Option (b) is correct as it matches the standard definition of the void ratio.
- Option (c) is incorrect because the total volume of soil particles refers only to the volume of solids (Vs), not a ratio.
- Option (d) is incorrect because the weight of water is a mass/weight measurement, whereas the void ratio is a volumetric ratio.
A canal has a duty of 2000 hectares per cumec and a base period of 120 days. What is the required delta (in meters)?
A)
1.04m
B)
0.52m
C)
2.07 m
D)
3.45m
Option B is correct: 0.52m.
Why are cutoff walls provided in hydraulic structures like dams and weirs?
A)
To increase the storage capacity of the reservoir
B)
To allow fish to migrate upstream
C)
To improve the aesthetic appearance of the structure
D)
To prevent seepage and reduce piping failure
Step-by-step reasoning:
1. **Definition of Cutoff Wall**: A cutoff wall is a vertical barrier (usually made of concrete, sheet piles, or slurry) constructed beneath the foundation of a hydraulic structure like a dam, weir, or levee.
2. **Function regarding Seepage**: The primary purpose of a cutoff wall is to increase the length of the flow path (creep length) that water must travel under the structure. According to Darcy's Law, increasing the flow path increases the resistance to flow, thereby significantly reducing the quantity of water seeping under the structure.
3. **Prevention of Piping**: Piping occurs when the exit gradient of the seeping water at the downstream end exceeds the critical hydraulic gradient, causing soil particles to be washed away. This creates 'pipes' or voids under the structure, which can lead to sudden collapse. By reducing the exit gradient and controlling seepage, cutoff walls prevent piping failure.
4. **Analysis of other options**:
- Option (a) is incorrect because storage capacity is determined by the height and location of the dam, not by subsurface walls.
- Option (b) is incorrect because fish migration is handled by 'fish ladders' or 'fish ways', not cutoff walls.
- Option (c) is incorrect because cutoff walls are buried underground and have no impact on the aesthetic appearance of the structure.