Q: Primary treatment of wastewater — typical BOD removal efficiency Under normal operating conditions, what is the approximate Biochemical Oxygen Demand (BOD) removal (%) achieved in primary treatment (sedimentation + skimming)?

Introduction / Context: Primary treatment removes settleable and floatable solids through sedimentation and skimming. Because most dissolved organics are not removed at this stage, the percentage reduction in BOD is modest. This question checks recall of the typical BOD removal in primary treatment alone. Given Data / Assumptions: Conventional primary clarifier operation with normal detention time and surface overflow rate. No chemical coagulants unless specified. Influent is typical municipal wastewater. Concept / Approach: Primary clarifiers mainly remove suspended solids (about 50–70%) associated with particulate organics. The corresponding BOD removal generally falls around 25–35%, commonly remembered as about 30%. Much higher BOD removals (e.g., 65–85%) require biological treatment (secondary) or chemically enhanced primary treatment. Step-by-Step Solution: 1) Identify stage: only primary settling → limited removal of dissolved organics.2) Recall typical performance: BOD removal ≈ 30% for municipal wastewater.3) Compare options and select the closest standard value.4) Conclude that 30% is the representative figure. Verification / Alternative check: Design texts and exam references consistently quote BOD removal around 25–35% in primary clarifiers under normal conditions. Why Other Options Are Wrong: 65% and 85% reflect secondary/tertiary treatment levels; “Zero” is unrealistic because particulate-associated BOD is partially removed in settling. Common Pitfalls: Confusing TSS removal percentages with BOD removal; assuming chemical addition without being stated. Final Answer: 30%

Q: Equivalent parallel pipes — replacing one pipe by three identical pipes A single pipe of length L and diameter D is to be replaced by three pipes of the same material, each of length L and equal diameter d (d < D), connected in parallel between the …

Introduction / Context: When multiple equal pipes are laid in parallel between the same heads, each pipe experiences the same head loss, and the total discharge is the sum of individual discharges. For turbulent flow using Darcy–Weisbach with the same roughness and length, discharge scales with diameter to the power 2.5 for a given head loss. This yields a clean diameter relation. Given Data / Assumptions: Three identical pipes in parallel, each length L, diameter d. Original single pipe: length L, diameter D. Same material → same friction factor regime. Same head loss between reservoirs. Concept / Approach: For Darcy–Weisbach, with head loss h_f fixed and length L, velocity V ∝ sqrt(h_f * D / (f L)). Discharge Q = A V ∝ (D^2) * sqrt(D) = D^(2.5). For n equal parallel pipes at the same head loss, Q_total = n * k * d^(2.5), where k is a constant for given h_f and L. Equate to the original single-pipe discharge k * D^(2.5). Step-by-Step Solution: 1) Write Q_single = k * D^(2.5).2) Write Q_parallel = 3 * k * d^(2.5).3) Equate: D^(2.5) = 3 * d^(2.5).4) Solve: d = D / 3^(2/5). Verification / Alternative check: Numerically, 3^(2/5) ≈ 1.552 → d ≈ 0.644 D, a plausible reduction when splitting into three parallel branches. Why Other Options Are Wrong: Exponents 1/4, 1/2, 3/5 do not arise from the D^(2.5) scaling; they correspond to incorrect flow–diameter relationships. Common Pitfalls: Using laminar (D^4) or Hazen–Williams exponents indiscriminately; forgetting that each parallel pipe has the same length and head loss. Final Answer: d = D / 3^(2/5)

Q: Aquifer storage change due to water-table decline An unconfined aquifer under an area of 200 hectares experiences a water-table drop of 4 m. Porosity n = 0.35 and specific retention Sr = 0.15. Compute the change in volume of water stored (m^3).

Introduction / Context: In an unconfined aquifer, the change in stored water due to a water-table movement equals the product of the area, the change in head, and the specific yield. Specific yield Sy is the drainable portion of porosity after accounting for water retained by capillarity (specific retention). Given Data / Assumptions: Area A = 200 ha = 200 × 10^4 m^2 = 2.0 × 10^6 m^2. Water-table decline Δh = 4 m. Porosity n = 0.35; specific retention Sr = 0.15 → specific yield Sy = n − Sr = 0.20. Gravity drainage dominates; compressibility effects neglected. Concept / Approach: For unconfined storage change: ΔV = A * Δh * SySpecific yield represents the fraction of pore water that actually drains when the water table falls. Step-by-Step Solution: 1) Compute Sy = 0.35 − 0.15 = 0.20.2) Convert area: A = 200 ha = 2.0 × 10^6 m^2.3) Multiply: ΔV = 2.0 × 10^6 * 4 * 0.20.4) ΔV = 1.6 × 10^6 m^3. Verification / Alternative check: Order-of-magnitude: A * Δh = 8 × 10^6 m^3 of aquifer volume; with 20% drainable yield → 1.6 × 10^6 m^3, consistent. Why Other Options Are Wrong: 8 × 10^6 m^3 ignores Sy; 160 m^3 and 1.6 × 10^3 m^3 are far too small for the stated area and drawdown. Common Pitfalls: Using porosity instead of specific yield; mis-converting hectares to square metres; forgetting to apply the water-table change. Final Answer: 1.6 × 10^6 m^3

Question 1

Cantilever with end point load – free-end deflection A prismatic cantilever beam of span L with flexural rigidity E I carries a concentrated load P at its free end. Determine the vertical deflection at the free end (use small-deflection Euler–Bernou…

Accepted Answer

Introduction: Standard beam formulas are essential tools for quick checks in structural analysis. The cantilever with a tip load is among the most frequently referenced cases, providing a simple expression that relates load, stiffness, and span to tip deflection. Given Data / Assumptions: Cantilever beam of length L. Flexural rigidity constant along the span: E I. Concentrated load P acting vertically at the free end. Small-deflection Euler–Bernoulli beam theory; shear deformation neglected. Concept / Approach: The curvature–moment relationship is d^2y/dx^2 = M(x) / (E I). For a cantilever with a tip load, the moment diagram is linear with maximum at the fixed end. Integrating twice with appropriate boundary conditions (zero deflection and slope at the fixed support) yields closed-form expressions for slope and deflection; evaluating at the free end provides the desired tip deflection. Step-by-Step Solution: 1) Bending moment at a section x from the fixed end: M(x) = P * (L − x).2) Curvature: d^2y/dx^2 = M(x) / (E I) = P * (L − x) / (E I).3) Integrate once to get slope and again to get deflection; apply y(0) = 0 and dy/dx(0) = 0.4) Evaluate at x = L: δ_free = P * L^3 / (3 * E * I). Verification / Alternative check: Tabulated beam tables list tip slope θ = P * L^2 / (2 * E * I) and tip deflection δ = P * L^3 / (3 * E * I), confirming the derived result. Why Other Options Are Wrong: Options B and C correspond to slope or different load cases; D doubles the correct value; E is the UDL deflection form, not tip load. Common Pitfalls: Confusing slope and deflection expressions; applying the simply supported formula; ignoring shear deformation in very deep beams (not significant for slender beams). Final Answer: δ = P * L^3 / (3 * E * I)

Question 2

Working Stress Design – Permissible Bending Tensile Stress for HYSD Fe 415 Bars in Beams According to classic working-stress practice (IS 456, WSM context), what is the permissible bending tensile stress in high yield strength deformed (HYSD) bars o…

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Introduction / Context: The working stress method (WSM) limits the service stresses in steel and concrete to values below yield or crushing, providing a margin of safety against long-term effects. In Indian practice for reinforced concrete, permissible bending tensile stresses for different steel grades are tabulated. This question checks recall of the allowable bending tensile stress for HYSD Fe 415 bars in beams under WSM. Given Data / Assumptions: Steel type: HYSD (high yield strength deformed) bars. Grade: Fe 415 (characteristic yield strength ≈ 415 N/mm^2). Design philosophy: working stress method (service stress limits). Member: reinforced concrete beam (flexure). Concept / Approach: In WSM, the permissible bending tensile stress in steel is taken as a fraction of the yield strength to ensure adequate factor of safety and control of serviceability. For Fe 415 HYSD bars, the widely taught and referenced allowable bending tensile stress in beams is about 230 N/mm^2. This value reflects both material properties and the conservative nature of WSM compared with limit state design (LSD), where stresses can reach closer to yield under factored loads. Step-by-Step Solution: Identify steel grade: Fe 415 ⇒ fy ≈ 415 N/mm^2.Recall WSM permissible bending tensile stress for HYSD Fe 415 in beams: approximately 230 N/mm^2.Compare with options and select the matching value. Verification / Alternative check: Classic Indian RCC design handbooks and solved examples using WSM consistently adopt 230 N/mm^2 for bending tension in Fe 415 bars. This sits comfortably below fy, in line with the WSM philosophy of allowable stress design. Why Other Options Are Wrong: 190 N/mm^2: Too low for Fe 415 in bending tension; more aligned with lower strength steels or conservative contexts not typical for HYSD Fe 415. 140 N/mm^2: Characteristic of mild steel (Fe 250) bending tension in WSM, not Fe 415. None of the above: Incorrect since 230 N/mm^2 is listed. Common Pitfalls: Mixing WSM values with limit state partial-safety-factor stresses; confusing values for bars in direct tension vs bending; using Fe 250 permissible values for HYSD bars by mistake. Final Answer: 230 N/mm^2

Question 3

Earth Pressure with Cohesion – Effect of c on Active and Passive States In classical Rankine/Coulomb earth pressure theory for cohesive backfill (c ≠ 0), how does soil cohesion affect the magnitudes of active earth pressure and passive earth resista…

Accepted Answer

Introduction / Context: Earth pressure theories predict lateral stresses on retaining structures. When soils possess apparent cohesion c (in addition to friction), the lateral pressure expressions include cohesive terms that alter both active and passive magnitudes. Understanding the direction of this influence is vital for safe and economical wall design. Given Data / Assumptions: Backfill has shear strength parameters c (cohesion) and φ (friction angle). Rankine framework for simplicity; similar trends apply in Coulomb theory. Unit weight γ and wall height H are otherwise unchanged. Concept / Approach: For cohesive-frictional soils, the active pressure at depth z is reduced by a cohesion term, while the passive pressure is enhanced by a cohesion term. In Rankine form for a vertical wall with horizontal backfill, the resultant forces include ± 2c√K terms (per unit height) multiplied by H, where the sign is negative for active (reducing it) and positive for passive (increasing it). Step-by-Step Solution: Active resultant (conceptually): Pa ≈ Ka * (γ H^2 / 2) − 2 c √Ka * H.Passive resultant (conceptually): Pp ≈ Kp * (γ H^2 / 2) + 2 c √Kp * H.Therefore cohesion c reduces Pa and increases Pp compared to the purely frictional case. Verification / Alternative check: Mohr–Coulomb envelopes show that cohesion shifts the failure line upward. In the active case, less lateral stress is needed to reach failure (so pressure reduces). In the passive case, higher lateral stress can be mobilized before failure (so resistance increases). Why Other Options Are Wrong: Decreasing both or increasing both: Contradict the opposite-sense effect of c on active vs passive. Increasing active and decreasing passive: Opposite to the effect predicted by the equations. Common Pitfalls: Using short-term undrained c values in long-term drained analyses; neglecting tension cut-off in the upper zone of active pressure where cohesion may predict unrealistic tensile stresses. Final Answer: Decreases active pressure and increases passive resistance

Question 4

Pavement Distress – What Is Mud Pumping Commonly Associated With? In highway engineering, “mud pumping” is a distress characterized by ejection of water-silt slurry through joints or cracks under wheel loads. It is most commonly associated with whic…

Accepted Answer

Introduction / Context: Mud pumping is a classic distress in rigid pavements. Under repeated traffic, pore water pressure builds up in fine-grained subgrade soils beneath slab corners and joints. When the slab deflects, water mixed with fines is expelled upward through joints and cracks, creating voids and loss of support, which accelerates faulting and cracking. Given Data / Assumptions: Rigid pavement (cement concrete slab) with load transfer at joints. Fine-grained, moisture-susceptible subgrade (e.g., clays or silty clays). Traffic repetitions providing dynamic loading and slab curling/warping movements. Concept / Approach: Pumping requires: (1) free water, (2) erodible fines, and (3) a mechanism to apply repeated pressure and rapid relief (wheel loads). Cement concrete slabs distribute load but still develop edge and corner deflections. On clayey subgrades with poor drainage, this environment is ideal for pumping; contrast this with well-drained, coarse granular subgrades where fines are minimal and pumping risk is lower. Step-by-Step Solution: Recognize mechanism: slab deflection + trapped water + erodible fines ⇒ slurry expulsion.Identify the riskiest combination: rigid concrete slab over clay subgrade (low permeability, high fines).Conclude: mud pumping is commonly associated with cement concrete pavement on clay sub-grade. Verification / Alternative check: Case histories and design guides recommend subgrade stabilization, edge drains, filter layers, and tight joint sealing to mitigate pumping under rigid pavements, especially over fine-grained soils. Why Other Options Are Wrong: Bituminous penetration macadam and premixed bituminous: Flexible systems with lower slab action; pumping is less characteristic. Cement concrete on granular subgrade: Coarse grains and drainage lower the risk. Common Pitfalls: Ignoring drainage; omitting filters between subbase and subgrade; poor joint sealing allowing surface water ingress. Final Answer: Cement concrete pavement on clay sub-grade

Question 5

CBR Method – Governing Layer for Total Pavement Thickness In the California Bearing Ratio (CBR) method for flexible pavement design, the required total pavement thickness is primarily governed by the CBR value of which layer?

Accepted Answer

Introduction / Context: The CBR method relates the pavement thickness to the bearing capacity of the supporting soil. The weakest supporting medium dictates how thick the overlying pavement must be to distribute loads and keep subgrade stresses within permissible limits. Understanding which layer’s CBR controls the thickness is fundamental in preliminary design and evaluation. Given Data / Assumptions: Pavement type: flexible pavement. Design approach: empirical CBR charts or equations. Traffic category and reliability handled separately by charts/adjustments. Concept / Approach: CBR design charts index total thickness against the CBR of the subgrade soil because the pavement structure’s role is to protect the subgrade from excessive stresses. While base and surface layers have their own strength/CBR, these values are used to apportion the total thickness into layer-wise requirements. The total thickness, however, is set primarily by the subgrade support level. Step-by-Step Solution: Determine subgrade CBR from laboratory soaked CBR tests.Enter design chart with subgrade CBR and traffic repetitions to read total pavement thickness.Distribute total thickness into surface, base, and sub-base as per layer quality and minimums. Verification / Alternative check: Poor subgrades (low CBR) produce large total thicknesses on charts; improving subgrade CBR through stabilization or thicker sub-base demonstrably reduces the required total thickness. Why Other Options Are Wrong: Base or surface course: Their CBR affects internal layer distribution, not the total thickness driver. All layers: While layer qualities matter, the controlling parameter for total thickness in CBR design is the subgrade CBR. Common Pitfalls: Using unsoaked CBR where soaked values are required; ignoring seasonal moisture variation; failing to apply minimum layer thickness constraints. Final Answer: Subgrade

Question 6

Vertical Curve Design – Deviation Angle Between Two Ascending Grades Two ascending gradients, 1 in 50 and 1 in 30, meet at a summit. What is the deviation angle A (algebraic difference of grades) to be used in vertical curve computations? Provide th…

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Introduction / Context: In geometric design of highways, the deviation angle A between two grades governs the required length of a vertical curve for comfort and sight distance. A is defined as the algebraic difference of grades, taking sign into account (ascending positive, descending negative). This question deals with computing A for two ascending grades meeting at a summit point. Given Data / Assumptions: Grade 1: +1 in 50. Grade 2: +1 in 30. Both are ascending (same sign). Summit curve design context. Concept / Approach: Express each grade as a percentage: g = rise/run × 100. For 1 in N ascending, g = (1/N) × 100 %. The deviation angle A (for vertical curves) equals the absolute algebraic difference |g2 − g1| when both grades have the same sign. This A then enters formulas for curve length based on sight distance or comfort criteria. Step-by-Step Solution: Convert 1 in 50 to percent: g1 = (1/50) * 100 = 2.00%.Convert 1 in 30 to percent: g2 = (1/30) * 100 ≈ 3.333…%.Compute algebraic difference: A = |g2 − g1| = |3.333… − 2.00| ≈ 1.333…%.Express in a convenient equivalent: A ≈ 1.33%, which corresponds roughly to 1 in 75 (since 1/75 ≈ 0.0133). Verification / Alternative check: If one grade were descending, A would be the sum of absolute values. Here, since both are ascending, only the difference is taken, giving a relatively small A and therefore a shorter summit curve than if grades opposed each other. Why Other Options Are Wrong: 3.33% and 2.00%: These are the individual grades, not the deviation angle. 5.33%: Sum of 3.33% and 2.00%, applicable only if grades had opposite signs (one up, one down). Common Pitfalls: Forgetting to convert to percentage; adding instead of subtracting when grades have the same sign; mixing up algebraic difference (A) with the actual curve gradient at any point. Final Answer: A = 1.33% (equivalently, about 1 in 75)

Question 7

Soil Consistency (Cohesive Soils) – Primary Factor Affecting Consistency in Saturated State For a saturated cohesive soil, which parameter most directly controls its consistency (soft, medium, stiff) under usual geotechnical classification?

Accepted Answer

Introduction / Context: Consistency of cohesive soils describes their relative firmness or resistance to deformation and penetration. In classification and site description, terms such as soft, medium, stiff, and hard are used. For saturated clays and silts, consistency is closely tied to their moisture state relative to Atterberg limits, which ultimately govern undrained shear strength at field water contents. Given Data / Assumptions: Soil type: cohesive (clayey/silty) and saturated. Objective: identify the dominant control on consistency in practice. Framework: Atterberg limits and undrained strength correlations. Concept / Approach: For saturated clays, undrained shear strength su and penetration resistance vary systematically with liquidity index LI = (w − PL) / (LL − PL), where w is water content, LL is liquid limit, and PL is plastic limit. As water content increases (higher LI), soils become softer; as it decreases, soils become stiffer. Thus, water content is the primary direct factor affecting consistency for saturated cohesive soils. Step-by-Step Solution: Relate consistency terms (soft/stiff) to undrained strength su.Recognize su correlates strongly with water content through liquidity index.Conclude that changes in water content directly change consistency for saturated cohesive soils. Verification / Alternative check: Field tests such as hand penetrometer, vane shear, or SPT N-values in cohesive strata show clear sensitivity to water content; drying increases strength (stiffer), while wetting decreases strength (softer), holding fabric and mineralogy fixed. Why Other Options Are Wrong: Particle size distribution: Influences plasticity limits but does not control short-term consistency as directly as water content variations. Density index: Applicable to cohesionless soils (relative density), not cohesive soil consistency. Coefficient of permeability: A hydraulic property; not a direct indicator of consistency descriptors. Common Pitfalls: Confusing consistency with compaction state; interpreting particle size as the immediate cause rather than a background influence via plasticity. Final Answer: Water content

Question 8

Sewer Design Practice – Matching Crown (Soffit) Levels at Junctions Assertion [A]: The crown (soffit) of the outgoing larger-diameter sewer is matched with the crown of the incoming smaller-diameter sewer at a junction. Reason [R]: Matching crowns p…

Accepted Answer

Introduction / Context: At sewer junctions where a smaller-diameter line discharges into a larger one, detailing the relative soffit (crown) and invert levels is essential for smooth flow. A common rule is to match the crown of the larger outgoing sewer with the crown of the smaller incoming sewer. This practice influences hydraulic grade line behavior and maintenance performance. Given Data / Assumptions: Incoming sewer: smaller diameter. Outgoing sewer: larger diameter. Design objective: avoid hydraulic discontinuities that could cause surcharge or sediment deposition. Concept / Approach: Matching crowns ensures that the incoming stream does not encounter an upward step at the junction. If inverts were matched instead, the larger sewer’s crown would be lower, creating a hydraulic drop and a pocket where flow from the smaller sewer could back up or pond, increasing the chance of surcharge and grit deposition. Aligning crowns provides a continuous soffit, reducing turbulence zones and improving flow transition into the larger conduit. Step-by-Step Solution: State [A]: Crown of larger outgoing sewer aligned with crown of smaller incoming sewer ⇒ accepted good practice.State [R]: Doing so prevents backing up due to eliminating an adverse hydraulic step at the junction.Link: The reason directly explains the asserted practice. Verification / Alternative check: Standard sewer design manuals illustrate soffit alignment as a measure to prevent ponding at manholes and to maintain self-cleansing velocities, especially at low flows. Why Other Options Are Wrong: [B]: True/true but unrelated is incorrect; the reason is causally correct here. [C]: Both are not false; they reflect standard practice. [D]: The reason is not false; it is the technical basis for crown matching. Common Pitfalls: Matching inverts instead of crowns; ignoring energy losses at junctions; failing to consider drop connections when depth differences are large. Final Answer: Both [A] and [R] are true and [R] is the correct reason for [A]

Question 9

Water Supply Engineering – Sizing of water distribution systems For municipal water networks, the design diameter of distribution mains is typically chosen to satisfy which combination of demand conditions?

Accepted Answer

Introduction / Context: Water distribution systems are sized so that pressures and velocities remain within permissible limits while meeting critical demand scenarios. Designers consider diurnal variations, peak factors, and contingency flows such as firefighting. This question asks which demand combination is commonly used as a governing condition for sizing distribution mains in urban networks. Given Data / Assumptions: Public water supply system with storage and pumping arranged to meet variable demand. Fire demand may occur concurrently with normal consumption. Hydraulic design must respect minimum residual pressure at hydrants and consumer taps. Concept / Approach: Distribution mains convey water from service reservoirs to consumers and hydrants. The “maximum daily demand” represents the higher end of routine consumption over a day. Fire demand (a short-duration but high-rate withdrawal) can coincide with peak-day consumption. Sizing for “maximum daily demand plus fire demand” ensures that, even during a fire event on a peak day, adequate residual pressures and flows are available without unacceptable headloss or velocities. Transmission mains and storage are sometimes checked for maximum hourly demand, but distribution main diameters are usually governed by the fire-plus-peak-day criterion in many design manuals and exam standards. Step-by-Step Solution: Identify governing scenarios: maximum day, maximum hour, and fire.Recognize that firefighting demand can be concurrent with peak-day usage.Adopt the composite criterion: design for maximum daily demand + fire demand with required residual head at hydrants.Verify by hydraulic modeling that velocities and headlosses stay within limits (e.g., v ≤ about 2–3 m/s; adequate node pressures). Verification / Alternative check: Many standard texts specify checking both “maximum hourly demand” and “maximum daily + fire.” In distribution design, the latter commonly governs because fire flows are large and local headlosses are significant in reticulation pipes. Why Other Options Are Wrong: (a) Ignores simultaneous fire flow; (b) is too low for design; (d) uses average day instead of maximum day; (e) suggests the smaller of two peaks, which is unsafe—design should envelope the more severe condition. Common Pitfalls: Using peak-hour factors without superimposing fire flow, neglecting residual pressure criteria at hydrants, or overlooking looping benefits that reduce headloss. Final Answer: Maximum daily demand plus simultaneous fire demand

Question 10

Pavement Materials – Rapid-curing (RC) cutback bitumen Rapid-curing cutback bitumen is obtained by blending penetration-grade bitumen with which volatile diluent?

Accepted Answer

Introduction / Context: Cutback bitumen is produced by dissolving bitumen in a volatile petroleum solvent so that the mixture has temporarily reduced viscosity at ambient temperature. Depending on solvent volatility, cutbacks are classified as rapid-curing (RC), medium-curing (MC), or slow-curing (SC). Selecting the correct solvent is important for setting time and early strength in road works such as surface dressing and patch repairs. Given Data / Assumptions: Base binder is penetration-grade bitumen. Field application may require quick set and early opening to traffic. Solvent must evaporate readily to restore binder viscosity. Concept / Approach: Solvent volatility controls cure rate: RC uses highly volatile solvents, MC uses moderately volatile solvents, and SC uses low-volatility diluents. Petrol (gasoline) has a low boiling range and evaporates quickly, producing the “rapid-curing” behavior. Kerosene (for MC) and diesel/fuel oil (for SC) evaporate more slowly, which extends curing time. Step-by-Step Solution: Identify needed setting time: rapid.Match to solvent class: highly volatile → petrol/gasoline.Blend bitumen with petrol to form RC cutback; upon spraying, solvent flashes off quickly.Result: rapid viscosity build-up and early chip retention in surface dressing. Verification / Alternative check: Standard highway specifications list RC binders (e.g., RC-70, RC-250) that use gasoline-range solvents; MC grades (MC-30, MC-70, etc.) use kerosene-range distillates; SC grades use diesel/fuel oil fractions. Why Other Options Are Wrong: Kerosene and diesel correspond to MC and SC cutbacks, respectively; benzene is not used for standard cutbacks due to toxicity and unsuitable properties; fuel oil is too heavy and slow evaporating. Common Pitfalls: Using inappropriate cutback in cold weather (leading to delayed set) or in hot weather (risk of bleeding), and not accounting for environmental restrictions on cutback use. Final Answer: Petrol (gasoline)

Question 11

In-situ Shear Strength of Soft Clay – Most suitable field test Which field test is most appropriate for directly determining the in-situ undrained shear strength of soft, saturated clays?

Accepted Answer

Introduction / Context: Soft cohesive soils require careful evaluation of undrained shear strength for stability analyses of excavations, embankments, and foundations. A rapid, disturbance-minimizing, and directionally appropriate in-situ test is preferred to capture the intact structure of sensitive clays. Given Data / Assumptions: Soil: soft, saturated clay with low effective stress strength during rapid loading. Objective: undrained shear strength su in situ (short-term conditions). Minimal sample disturbance desired; rapid test acceptable. Concept / Approach: The field vane shear test uses a cruciform vane inserted into the clay and rotated at a controlled rate. The peak torque at failure gives the undrained shear strength su based on cylindrical shear along the failure surface. The method is direct, quick, and especially well-suited to very soft to soft clays where other tests (e.g., SPT) are unreliable and cause significant disturbance. Step-by-Step Solution: Push the vane to the desired depth with minimal disturbance.Rotate the vane at a standard rate; record torque vs time.Compute su from peak torque using the calibrated geometric factor.Optionally, remould and re-test to obtain remoulded strength and sensitivity. Verification / Alternative check: Comparisons with high-quality UU triaxial tests on block samples and with CPT correlations confirm that vane shear gives reliable su in soft clays where sampling is difficult. Why Other Options Are Wrong: Plate load test yields bearing/settlement parameters, not direct su. CPT provides correlations (indirect). SPT is unsuitable in very soft clays (N-values very low/erratic). Pressuremeter gives undrained moduli/limits but is more involved and not the simplest choice for su alone. Common Pitfalls: Not correcting for rate effects or anisotropy, ignoring thixotropic recovery between tests, and misapplying correlations outside soft clay range. Final Answer: Vane shear test

Question 12

Plate Bearing Test – Standard plate size for modulus of subgrade reaction (k) What is the standard circular plate diameter commonly used to determine k in highway/airfield plate load tests?

Accepted Answer

Introduction / Context: The plate bearing test is used to estimate the modulus of subgrade reaction k, an input for rigid pavement design and for evaluating foundation support characteristics. A standard plate size is specified so that test results are comparable and representative of slab behavior over a larger area than very small plates would sample. Given Data / Assumptions: Load is applied through a rigid, circular steel plate. Deflections are measured under incremental loading. Subgrade is reasonably uniform over the test influence zone. Concept / Approach: For pavement applications, a 75 cm diameter plate (approximately 30 in) is widely adopted to derive the k-value. The subgrade reaction modulus relates contact pressure to surface deflection under the plate, approximating slab-on-grade behavior. Using the standard plate size helps align field measurements with analytical models and code-based design procedures (e.g., Westergaard-type formulations) that assume a certain load distribution footprint. Step-by-Step Solution: Prepare test pit and level the subgrade at the desired density and moisture.Place the 75 cm diameter plate; ensure seating and alignment.Apply load increments; measure elastic deflection after stabilization.Compute k = pressure / deflection at service-level load; adjust as per specification if needed. Verification / Alternative check: Other diameters (e.g., 30 cm) are used for foundation tests, but for pavement k-value determination, standards and many specifications call for the 75 cm plate to represent slab action adequately. Why Other Options Are Wrong: 25–50 cm plates under-sample the subgrade area and may overstate k; 100 cm is heavy and uncommon for routine fieldwork; 30 cm is typical for small footing studies, not pavement k. Common Pitfalls: Poor seating of the plate, measuring plastic settlement instead of elastic rebound, or not correcting for layering and moisture variability. Final Answer: 75 cm diameter

Question 13

Bitumen Testing – Standard penetration test temperature At what temperature is the standard needle penetration test for bitumen conducted?

Accepted Answer

Introduction / Context: The needle penetration test classifies paving bitumen grades by measuring the depth (in 0.1 mm units) that a standard needle penetrates a bitumen sample under specified conditions. Consistent test temperature is essential because bitumen viscosity is highly temperature dependent. Given Data / Assumptions: Standard load and time: 100 g for 5 seconds. Temperature is controlled. Bitumen sample prepared per standard procedure (no air bubbles, smooth surface). Concept / Approach: International and national standards (e.g., ASTM D5, IS 1203) specify 25 °C as the standard temperature for the penetration test. At this temperature, penetration values are comparable across laboratories and are used to designate grades (e.g., 60/70, 80/100), which influence binder selection for climatic and traffic conditions. Step-by-Step Solution: Condition the sample and water bath to 25 °C.Load the needle with 100 g and place gently on the sample surface.Allow 5 seconds of penetration; record depth in 0.1 mm units (dmm).Repeat and average multiple readings for the reported value. Verification / Alternative check: Viscosity tests at 60 °C are also common (for paving viscosity grading), but they are different tests. The softening point (Ring and Ball) uses yet another temperature regime, confirming that 25 °C is specific to the penetration classification method. Why Other Options Are Wrong: 60 °C and 50 °C correspond to viscosity/softening tests, not standard penetration; 37 °C and 20 °C are nonstandard for this test and would give noncomparable results. Common Pitfalls: Not maintaining temperature, testing oxidized surface layers, or applying the needle off-vertical, all of which skew penetration readings. Final Answer: 25 °C

Question 14

Steel Design – Net effective area in angle tension members In the design of a single-angle tension member, the net effective cross-sectional area used for strength checks explicitly accounts for which factor(s)?

Accepted Answer

Introduction / Context: For tension members composed of angles, especially when only one leg is connected, the stress distribution is nonuniform near the connection. This leads to the phenomenon of shear lag, which reduces the effectiveness of the outstanding leg. Codes therefore distinguish between net area and net effective area when computing tensile capacity. Given Data / Assumptions: Member type: single-angle or unequal leg angle connected through one leg. Connection uses bolts or welds at the end (gusset). Strength check performed in limit state of tension yielding or rupture. Concept / Approach: The net area An is obtained by deducting holes from the connected cross-section along the critical path. However, due to shear lag, not all of An is effective in transmitting axial force at the critical section. The net effective area Ae is computed by multiplying An (or gross area of outstanding leg, as applicable) by a reduction factor that accounts for connection geometry (e.g., distance from the connected leg to centroid). Thus, Ae reflects both hole deductions and the end-connection effectiveness in force transfer, ensuring that rupture capacity is not overestimated. Step-by-Step Solution: Determine An by deducting bolt holes (plus allowances) along the weakest section.Evaluate eccentricity/Shear lag effect via codal factor (e.g., k or beta factor) based on connection length and spacing.Compute Ae = k * An (or as per specific provisions).Use Ae for rupture capacity and Ag for yield capacity as required. Verification / Alternative check: Design standards (e.g., IS 800 LRFD/WSD analogues) explicitly include shear lag reduction for angle members; examples show Ae < An when only one leg is connected, validating the need to include connection effectiveness. Why Other Options Are Wrong: (a) Ignores shear lag; (b) mentions eccentricity but the relevant codal concept is shear lag effectiveness, not adding direct bending; (c) tack welds/bolts along the length are not relied upon for net effective area; (e) gross area overestimates strength. Common Pitfalls: Using An instead of Ae for rupture checks, neglecting stagger effects in hole deduction, or assuming full effectiveness of the outstanding leg without verifying connection details. Final Answer: Effectiveness of the end connection (shear lag) in addition to bolt-hole deductions

Question 15

Fluid Mechanics – Applicability of Bernoulli’s equation along a streamline Bernoulli’s energy equation written along a streamline is strictly valid for which flow condition?

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Introduction / Context: Bernoulli’s equation expresses conservation of mechanical energy per unit weight between two points on the same streamline. It is a cornerstone of ideal fluid flow analysis and provides intuition about how pressure, velocity, and elevation trade off in ducts, jets, and around bodies. Given Data / Assumptions: Equation form: p/(gamma) + V^2/(2g) + z = constant along a streamline. Gravity field uniform; body forces limited to weight. No pump/turbine shaft work between sections unless explicitly added. Concept / Approach: The strict derivation assumes steady flow, negligible viscosity (inviscid or frictionless), incompressible fluid, and flow along a streamline. Under these conditions, mechanical energy is conserved and the sum of pressure head, velocity head, and elevation head remains constant. In real flows with friction, energy grade lines drop due to head losses; in compressible flows with significant density changes, compressible-flow forms must be used; in unsteady flows, local acceleration terms violate the steady assumption. Step-by-Step Solution: Recognize governing assumptions: steady, inviscid, incompressible, along a streamline.Apply between sections 1 and 2: p1/γ + V1^2/(2g) + z1 = p2/γ + V2^2/(2g) + z2.Add pump head or subtract turbine/head loss terms when moving to real-flow extended Bernoulli models.Check limitations: significant viscous effects or compressibility require corrections. Verification / Alternative check: Experimental pressure and velocity measurements in low-Re flows with short, smooth passages often approximate Bernoulli predictions closely, validating applicability under idealized conditions. Why Other Options Are Wrong: (a) Compressibility invalidates the simple form; (b) includes friction losses that must be accounted for; (d) unsteady behavior violates steady assumption; (e) strong viscous dissipation contradicts the frictionless premise. Common Pitfalls: Applying Bernoulli across streamlines where swirl exists, ignoring pump/turbine work or head losses, and using it in high Mach number flows without compressibility corrections. Final Answer: Steady, frictionless, incompressible fluid

Question 16

Lateral–torsional buckling in steel beams The problem of lateral buckling can arise only in those beams where the compression flange is not adequately restrained against lateral movement and twist during bending. Which statement best reflects this c…

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Introduction / Context: Lateral–torsional buckling (LTB) is a stability limit state in flexural members. It occurs when a beam bent about its strong axis deflects laterally and twists because the compression flange is insufficiently restrained. This question probes the correct qualitative condition that leads to LTB. Given Data / Assumptions: Steel beam in bending about its strong axis. Compression flange may or may not be laterally braced. No special warping restraints unless stated. Concept / Approach: LTB is governed by restraint of the compression flange and torsional stiffness (J, Cw), not by a simple comparison of the two principal second moments of area alone. A fully restrained (continuously braced) compression flange prevents LTB. Merely stating that one moment of inertia is larger or smaller does not establish the presence of lateral restraint, so such statements do not by themselves cause or prevent LTB. Step-by-Step Solution: 1) Identify the key trigger: unbraced compression flange → risk of combined lateral deflection and twist.2) Recognize that inertia magnitudes (I_major vs I_minor) influence susceptibility, but are not the definitive condition.3) Note that a fully supported compression flange eliminates LTB (hence option c is not a trigger condition).4) Therefore, none of the statements (a–c) correctly isolate the true cause; choose “None of the above”. Verification / Alternative check: Design codes compute LTB capacity using unbraced length between lateral restraints, section properties (I_y, J, Cw), and loading position. The decisive parameter is bracing of the compression flange. Why Other Options Are Wrong: Option (a) and (b): Comparing principal inertias does not define LTB onset; both are true for many beams with or without LTB. Option (c): A fully supported compression flange prevents, rather than causes, LTB. Common Pitfalls: Assuming large I_major alone prevents LTB; ignoring load height relative to shear center; overlooking intermittent lateral bracing effectiveness. Final Answer: None of the above

Question 17

Primary treatment of wastewater — typical BOD removal efficiency Under normal operating conditions, what is the approximate Biochemical Oxygen Demand (BOD) removal (%) achieved in primary treatment (sedimentation + skimming)?

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Introduction / Context: Primary treatment removes settleable and floatable solids through sedimentation and skimming. Because most dissolved organics are not removed at this stage, the percentage reduction in BOD is modest. This question checks recall of the typical BOD removal in primary treatment alone. Given Data / Assumptions: Conventional primary clarifier operation with normal detention time and surface overflow rate. No chemical coagulants unless specified. Influent is typical municipal wastewater. Concept / Approach: Primary clarifiers mainly remove suspended solids (about 50–70%) associated with particulate organics. The corresponding BOD removal generally falls around 25–35%, commonly remembered as about 30%. Much higher BOD removals (e.g., 65–85%) require biological treatment (secondary) or chemically enhanced primary treatment. Step-by-Step Solution: 1) Identify stage: only primary settling → limited removal of dissolved organics.2) Recall typical performance: BOD removal ≈ 30% for municipal wastewater.3) Compare options and select the closest standard value.4) Conclude that 30% is the representative figure. Verification / Alternative check: Design texts and exam references consistently quote BOD removal around 25–35% in primary clarifiers under normal conditions. Why Other Options Are Wrong: 65% and 85% reflect secondary/tertiary treatment levels; “Zero” is unrealistic because particulate-associated BOD is partially removed in settling. Common Pitfalls: Confusing TSS removal percentages with BOD removal; assuming chemical addition without being stated. Final Answer: 30%

Question 18

Equivalent parallel pipes — replacing one pipe by three identical pipes A single pipe of length L and diameter D is to be replaced by three pipes of the same material, each of length L and equal diameter d (d < D), connected in parallel between the …

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Introduction / Context: When multiple equal pipes are laid in parallel between the same heads, each pipe experiences the same head loss, and the total discharge is the sum of individual discharges. For turbulent flow using Darcy–Weisbach with the same roughness and length, discharge scales with diameter to the power 2.5 for a given head loss. This yields a clean diameter relation. Given Data / Assumptions: Three identical pipes in parallel, each length L, diameter d. Original single pipe: length L, diameter D. Same material → same friction factor regime. Same head loss between reservoirs. Concept / Approach: For Darcy–Weisbach, with head loss h_f fixed and length L, velocity V ∝ sqrt(h_f * D / (f L)). Discharge Q = A V ∝ (D^2) * sqrt(D) = D^(2.5). For n equal parallel pipes at the same head loss, Q_total = n * k * d^(2.5), where k is a constant for given h_f and L. Equate to the original single-pipe discharge k * D^(2.5). Step-by-Step Solution: 1) Write Q_single = k * D^(2.5).2) Write Q_parallel = 3 * k * d^(2.5).3) Equate: D^(2.5) = 3 * d^(2.5).4) Solve: d = D / 3^(2/5). Verification / Alternative check: Numerically, 3^(2/5) ≈ 1.552 → d ≈ 0.644 D, a plausible reduction when splitting into three parallel branches. Why Other Options Are Wrong: Exponents 1/4, 1/2, 3/5 do not arise from the D^(2.5) scaling; they correspond to incorrect flow–diameter relationships. Common Pitfalls: Using laminar (D^4) or Hazen–Williams exponents indiscriminately; forgetting that each parallel pipe has the same length and head loss. Final Answer: d = D / 3^(2/5)

Question 19

Aquifer storage change due to water-table decline An unconfined aquifer under an area of 200 hectares experiences a water-table drop of 4 m. Porosity n = 0.35 and specific retention Sr = 0.15. Compute the change in volume of water stored (m^3).

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Introduction / Context: In an unconfined aquifer, the change in stored water due to a water-table movement equals the product of the area, the change in head, and the specific yield. Specific yield Sy is the drainable portion of porosity after accounting for water retained by capillarity (specific retention). Given Data / Assumptions: Area A = 200 ha = 200 × 10^4 m^2 = 2.0 × 10^6 m^2. Water-table decline Δh = 4 m. Porosity n = 0.35; specific retention Sr = 0.15 → specific yield Sy = n − Sr = 0.20. Gravity drainage dominates; compressibility effects neglected. Concept / Approach: For unconfined storage change: ΔV = A * Δh * SySpecific yield represents the fraction of pore water that actually drains when the water table falls. Step-by-Step Solution: 1) Compute Sy = 0.35 − 0.15 = 0.20.2) Convert area: A = 200 ha = 2.0 × 10^6 m^2.3) Multiply: ΔV = 2.0 × 10^6 * 4 * 0.20.4) ΔV = 1.6 × 10^6 m^3. Verification / Alternative check: Order-of-magnitude: A * Δh = 8 × 10^6 m^3 of aquifer volume; with 20% drainable yield → 1.6 × 10^6 m^3, consistent. Why Other Options Are Wrong: 8 × 10^6 m^3 ignores Sy; 160 m^3 and 1.6 × 10^3 m^3 are far too small for the stated area and drawdown. Common Pitfalls: Using porosity instead of specific yield; mis-converting hectares to square metres; forgetting to apply the water-table change. Final Answer: 1.6 × 10^6 m^3

Question 20

Fatigue life comparison — welded versus riveted steel structures In general, the ratio (fatigue life of a welded steel structure) / (fatigue life of a riveted steel structure) is:

Accepted Answer

Introduction / Context: Fatigue performance in steel structures is governed by stress ranges and detail categories. Welded details often introduce higher stress concentrations and potential defects (undercuts, lack of fusion), which lower endurance limits compared to mechanically fastened (riveted/bolted) details. Given Data / Assumptions: Comparison on similar geometry and loading, differing mainly by connection type. Welds considered are standard structural welds (fillet/butt) with typical workmanship quality. Riveted joints provide alternate load paths and lower notch severity at the net section. Concept / Approach: Fatigue life N typically follows S–N curves. Welded details are assigned lower detail categories (higher notch sensitivity), so for the same stress range, N_welded < N_riveted. Therefore, the ratio N_welded / N_riveted is generally < 1, unless special fatigue-rated welds or post-weld treatments are applied. Step-by-Step Solution: 1) Identify controlling phenomenon: crack initiation and growth from stress raisers.2) Recognize welded joints have sharper notches and residual stresses.3) Conclude lower fatigue category → shorter life → ratio < 1.4) Select “smaller than 1”. Verification / Alternative check: Design codes assign lower allowable stress ranges for welded categories compared to comparable riveted/bolted details, confirming the trend. Why Other Options Are Wrong: =1 or >1 imply equal or better life for welded structures, contrary to typical S–N classifications; “> 2.1” is arbitrary and unjustified. Common Pitfalls: Ignoring improvement methods like peening/TIG dressing; assuming weld quality is perfect; neglecting residual stresses from welding. Final Answer: smaller than 1

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