Intermediate Civil interview questions

150 questions with answers (CIV-151 to CIV-300). Try each one aloud before reading the answer.

Building Materials & Concrete Technology

CIV-151 · How does the water–cement ratio affect concrete?

By Abrams' law, strength falls as w/c rises. A lower w/c ratio gives higher strength, lower permeability and better durability, but less workability, so plasticizers are used to keep it workable.

CIV-152 · Why is gypsum added to cement clinker?

To control the rapid reaction of C₃A with water and prevent flash setting. About 3–5% is interground with clinker.

CIV-153 · What are the roles of C₃S and C₂S in strength gain?

C₃S hydrates quickly and gives early strength in the first 7–14 days with high heat. C₂S hydrates slowly and contributes to later strength with lower heat.

CIV-154 · What are the main steps of concrete mix design per IS 10262?

Find target mean strength, choose the w/c ratio (checking durability limits of IS 456), estimate water content for the slump and aggregate size, calculate cement content, find the coarse and fine aggregate proportions, then run trial mixes and adjust.

CIV-155 · What is the target mean strength for M25 with a standard deviation of 4 N/mm²?

f′ck = fck + 1.65s = 25 + 1.65 × 4 = 31.6 N/mm². The 1.65 factor means only 5% of results fall below fck.

CIV-156 · How do the compaction factor and Vee-Bee tests differ from the slump test?

The compaction factor test suits low-workability mixes and compares the density achieved by free fall with full compaction. The Vee-Bee test measures remoulding time under vibration and suits very stiff mixes. Slump is less sensitive at both extremes.

CIV-157 · How does fly ash improve concrete?

Its silica reacts with the calcium hydroxide released by hydration (pozzolanic reaction) to form extra C-S-H. This refines pores, lowers permeability and heat of hydration, and improves workability and long-term strength.

CIV-158 · Why is air entrainment used?

Tiny, well-distributed air bubbles relieve pressure from freezing water and greatly improve freeze–thaw resistance. They also improve workability, but strength drops about 5% for every 1% of air.

CIV-159 · What is the difference between creep and shrinkage of concrete?

Creep is time-dependent strain under sustained stress. Shrinkage is volume reduction from moisture loss (drying shrinkage) or hydration (autogenous shrinkage), regardless of load. Both increase long-term deflection and prestress loss.

CIV-160 · How does cylinder strength compare with cube strength?

Cylinder strength (150 × 300 mm) is roughly 0.8 times cube strength. The cube's lower height-to-width ratio gives more platen restraint, which raises its apparent strength.

CIV-161 · How is the flexural strength of concrete estimated per IS 456?

Modulus of rupture fcr = 0.7√fck N/mm². For M25, fcr = 0.7 × 5 = 3.5 N/mm².

CIV-162 · How is the short-term modulus of elasticity of concrete estimated per IS 456?

Ec = 5000√fck N/mm². For M25, Ec = 5000 × 5 = 25,000 N/mm².

CIV-163 · What is alkali–aggregate reaction?

A reaction between alkalis in cement and reactive silica in some aggregates. It forms an expansive gel that causes map cracking. It is controlled with non-reactive aggregates, low-alkali cement or pozzolans.

Structural Analysis

CIV-164 · Where is the bending moment maximum in a beam?

Where the shear force is zero or changes sign, because dM/dx = V. Moment can also peak at supports or point loads where shear jumps through zero.

CIV-165 · What are the relations between load, shear and bending moment?

dV/dx = −w (load intensity) and dM/dx = V. So the slope of the BMD equals the shear, and the change in moment between two points equals the area of the SFD between them.

CIV-166 · What is the flexure formula?

M / I = σ / y = E / R. Bending stress varies linearly from zero at the neutral axis to a maximum at the extreme fibre.

CIV-167 · What are standard deflection formulas for common beams?

Simply supported beam with UDL: δ = 5wL⁴/(384EI). Simply supported beam with a central point load: WL³/(48EI). Cantilever with an end point load: WL³/(3EI). Cantilever with UDL: wL⁴/(8EI).

CIV-168 · What are the moment-area theorems?

First theorem: the change in slope between two points equals the area of the M/EI diagram between them. Second theorem: the deflection of one point relative to the tangent at another equals the moment of that area about the first point.

CIV-169 · What is Castigliano's theorem?

The partial derivative of total strain energy with respect to a load gives the displacement in the direction of that load, δᵢ = ∂U/∂Pᵢ. It is used for deflections and redundant forces.

CIV-170 · What is Maxwell's reciprocal theorem?

In a linear elastic structure, the deflection at point A due to a unit load at B equals the deflection at B due to a unit load at A.

CIV-171 · What is an influence line?

A diagram showing how a response (reaction, shear or moment at a fixed section) varies as a unit load moves across the structure. It is used to place moving loads for maximum effect.

CIV-172 · What is Euler's buckling load?

Pcr = π²EI / Le². Effective length Le is L for both ends pinned, 0.5L for both fixed, 0.7L for fixed–pinned and 2L for a cantilever (theoretical values).

CIV-173 · What are distribution factor and carry-over factor in moment distribution?

The distribution factor at a joint is a member's stiffness divided by the total stiffness of members meeting there. The carry-over factor is the fraction of moment passed to the far end: 0.5 for a prismatic member with a fixed far end.

CIV-174 · What is the slope-deflection method?

A displacement method that writes member end moments in terms of joint rotations, chord rotation and fixed-end moments. Joint equilibrium equations then give the unknown rotations.

CIV-175 · What is the difference between the method of joints and the method of sections?

The method of joints solves equilibrium at each joint in turn and finds all member forces. The method of sections cuts through up to three unknown members and uses moment equilibrium to find specific member forces directly.

CIV-176 · What is the degree of static indeterminacy of a fixed–fixed beam under general loading?

Six reactions minus three equilibrium equations gives 3. Under vertical loads only, the axial reactions drop out and it is commonly treated as 2.

RCC Design

CIV-177 · Why is steel the preferred reinforcement for concrete?

Concrete is weak in tension. Steel has high tensile strength, bonds well with concrete, and has a similar coefficient of thermal expansion (about 10–12 × 10⁻⁶ /°C), so the two act together. The alkaline concrete also protects steel from corrosion.

CIV-178 · What are the limiting neutral axis depth ratios in IS 456?

xu,max / d = 0.53 for Fe250, 0.48 for Fe415 and 0.46 for Fe500. Keeping xu below this ensures steel yields before concrete crushes.

CIV-179 · What are the stress block parameters in IS 456 limit state design?

The compressive force is 0.36 fck b xu, acting at 0.42 xu from the extreme compression fibre. Maximum concrete strain is 0.0035 in flexure.

CIV-180 · What is the limiting moment of resistance for Fe415?

Mu,lim = 0.138 fck b d² for Fe415. The coefficient is 0.149 for Fe250 and 0.133 for Fe500.

CIV-181 · Find Mu,lim for a 230 × 450 mm effective section with M20 and Fe415.

Mu,lim = 0.138 × 20 × 230 × 450² = 128.5 × 10⁶ N·mm ≈ 128.5 kN·m.

CIV-182 · What basic span-to-effective-depth ratios does IS 456 give for deflection control?

7 for cantilevers, 20 for simply supported and 26 for continuous members, for spans up to 10 m. These are then modified by factors for tension and compression steel.

CIV-183 · Why is minimum shear reinforcement provided even when concrete alone can carry the shear?

To prevent sudden brittle shear failure if diagonal cracks form, to restrain crack growth, to tie the main bars and to give some ductility. IS 456 requires Asv / (b sv) ≥ 0.4 / (0.87 fy).

CIV-184 · What does the design shear strength τc of concrete depend on?

The tension steel percentage (100As / bd) and concrete grade, from IS 456 Table 19. The section must also satisfy τv ≤ τc,max (Table 20), otherwise it must be enlarged.

CIV-185 · What is the maximum spacing of vertical stirrups per IS 456?

The lesser of 0.75d and 300 mm. For inclined stirrups it is d.

CIV-186 · What are the limits on longitudinal reinforcement in RCC columns?

Minimum 0.8% and maximum 6% of the gross area, with about 4% as a practical limit for placement. Rectangular columns need at least 4 bars and circular columns at least 6 bars, minimum 12 mm diameter.

CIV-187 · What is lap length?

The overlap needed to transfer force between two bars. IS 456 requires the greater of Ld or 30φ in flexural tension, 2Ld or 30φ in direct tension, and Ld (compression) or 24φ in compression. Laps should be staggered and kept away from high-moment zones.

CIV-188 · What minimum eccentricity must columns be designed for per IS 456?

e_min = l/500 + D/30, but at least 20 mm, where l is unsupported length and D is lateral dimension. This covers construction imperfections and accidental eccentricity.

CIV-189 · How does IS 456 handle torsion in beams?

It converts torsion to an equivalent shear Ve = Vu + 1.6 Tu/b and an equivalent moment Me1 = Mu + Mt, where Mt = Tu(1 + D/b)/1.7. Closed stirrups and longitudinal steel are then designed for these values.

Steel Structures

CIV-190 · Why are HSFG bolts used in slip-critical joints?

They are pre-tensioned to clamp the plates, so load is carried by friction without slip. This suits joints under fatigue, vibration or load reversal, and where slip would affect alignment.

CIV-191 · What is the design strength of a tension member for gross section yielding per IS 800?

Tdg = Ag fy / γm0, with γm0 = 1.10.

CIV-192 · What is the design strength of a plate for net section rupture per IS 800?

Tdn = 0.9 An fu / γm1, with γm1 = 1.25. The 0.9 factor allows for stress concentration and uncertainty in net area.

CIV-193 · What is block shear failure?

Tearing out of a block of material at a connection end, with shear failure along one plane and tension failure along a perpendicular plane. IS 800 checks it along with yielding and rupture of tension members.

CIV-194 · What is lateral-torsional buckling?

Unbraced beams bent about their major axis can deflect sideways and twist before reaching full moment capacity. It depends on unbraced length, section shape and load position, and is prevented by lateral restraints.

CIV-195 · What is the effective throat thickness of a fillet weld?

For a standard 90° fillet weld, the throat t = 0.7 × weld size. The design strength of the weld is based on throat area × length.

CIV-196 · Find the yield strength of a tension member with Ag = 2000 mm², fy = 250 N/mm².

Tdg = 2000 × 250 / 1.10 = 454.5 kN. This must still be checked against net section rupture and block shear.

CIV-197 · What are the buckling curves in IS 800?

Curves a, b, c and d, with imperfection factors α = 0.21, 0.34, 0.49 and 0.76. The curve depends on section type, buckling axis and thickness.

CIV-198 · What is prying action?

Extra tension in bolts caused by flexible end plates or flanges bending under load, which creates contact forces at the plate edges. Thicker plates or stiffeners reduce it.

CIV-199 · What are web buckling and web crippling?

Web buckling is column-like buckling of the web under a concentrated load or reaction. Web crippling is local yielding near the flange-to-web junction. Both are checked under point loads and supports, and are prevented with bearing stiffeners.

CIV-200 · What is the difference between a slab base and a gusseted base?

A slab base is a thick plate under an axially loaded column. A gusseted base uses a thinner plate with gussets and cleats, and suits heavier loads or columns with moment.

CIV-201 · What is plastic section modulus?

Zp = sum of the first moments of area above and below the equal-area axis. The plastic moment is Mp = Zp × fy.

Geotechnical Engineering

CIV-202 · What is the difference between compaction and consolidation?

Compaction reduces air voids quickly by mechanical means such as rolling or tamping. Consolidation is slow expulsion of pore water from saturated clay under sustained load, and is time-dependent.

CIV-203 · What is the time factor in Terzaghi's one-dimensional consolidation theory?

Tv = cv t / d², where cv is the coefficient of consolidation and d is the drainage path (half the layer thickness for double drainage). Tv ≈ 0.197 for 50% and 0.848 for 90% consolidation.

CIV-204 · What are normally consolidated and overconsolidated clays?

A normally consolidated clay has never had a higher effective stress than its present one. An overconsolidated clay has, and OCR = σ′p / σ′0 > 1. OC clays are much less compressible.

CIV-205 · What is the critical hydraulic gradient?

The upward gradient at which effective stress becomes zero: ic = (G − 1)/(1 + e), about 1.0 for typical sands. At this gradient quick (boiling) conditions occur.

CIV-206 · What is a flow net used for?

A graphical net of flow lines and equipotential lines for 2D seepage. It gives seepage quantity q = k H (Nf/Nd), pore pressures, uplift and exit gradient.

CIV-207 · What is the difference between direct shear and triaxial tests?

The direct shear test forces failure on a predetermined plane and cannot control drainage well. The triaxial test applies controlled confining pressure, measures pore pressure and allows set drainage conditions, so it gives more reliable parameters.

CIV-208 · What are UU, CU and CD triaxial tests?

UU is unconsolidated-undrained, giving short-term undrained strength cu. CU is consolidated-undrained with pore pressure measurement, giving effective parameters. CD is consolidated-drained, sheared slowly, giving c′ and φ′ for long-term analysis.

CIV-209 · What is sensitivity of clay?

The ratio of undisturbed to remoulded unconfined compressive strength. Clays with values above about 8 are highly sensitive, and quick clays have very high sensitivity.

CIV-210 · Find the dry unit weight of soil with e = 0.6 and G = 2.7.

γd = G γw / (1 + e) = 2.7 × 9.81 / 1.6 = 16.55 kN/m³.

CIV-211 · What is the relation between e, S, w and G?

S e = w G. For saturated soil (S = 1), e = w G.

CIV-212 · How does the IS soil classification system work?

Coarse soils (more than 50% retained on the 75 μm sieve) are classified as G or S with gradation symbols W, P, M or C. Fine soils are M, C or O with low, intermediate or high compressibility (L, I, H) based on liquid limit and the plasticity chart.

CIV-213 · What is capillary rise in soils?

Water rising above the water table through small pores by surface tension, roughly h ∝ 1/D. It is negligible in gravels and can be several metres in clays. It causes frost heave and dampness.

CIV-214 · How is vertical stress below a loaded area estimated?

By Boussinesq's elastic solution for point and area loads, Westergaard's solution for layered soils, or the approximate 2:1 method, where load spreads over (B + z)(L + z) at depth z.

Foundation Engineering

CIV-215 · What is bearing capacity of soil?

The maximum pressure soil can carry from a foundation without shear failure. Design also limits settlement, so allowable bearing pressure is the lower of the shear-based safe value and the settlement-based value.

CIV-216 · What is Terzaghi's bearing capacity equation for a strip footing?

qu = c Nc + q Nq + 0.5 γ B Nγ, where q = γDf is overburden at founding level and Nc, Nq, Nγ depend on φ. Square and circular footings use shape factors such as 1.3c Nc and 0.4γBNγ.

CIV-217 · How does a rising water table affect bearing capacity?

It replaces γ with submerged γ′ (about half) in the affected terms. A water table at the base level can roughly halve the Nγ term; one at the surface affects the Nq term too.

CIV-218 · What are general, local and punching shear failure?

General shear occurs in dense soils, with a clear slip surface and heaving. Local shear occurs in medium soils, with partly developed slip surfaces. Punching shear occurs in loose or soft soils, with the footing sinking without much heave.

CIV-219 · What is pile group efficiency?

Group capacity divided by (number of piles × single pile capacity). In clays, the group capacity is the lesser of the sum of individual piles and block failure. The Converse–Labarre formula is one empirical estimate.

CIV-220 · What are initial and routine pile load tests?

Initial tests are done on test piles before construction, often to failure or about 2.5 times working load, to confirm design capacity. Routine tests are done on selected working piles, typically to 1.5 times working load, to verify performance.

CIV-221 · What are Rankine's active and passive earth pressure coefficients?

For a smooth vertical wall with horizontal backfill, Ka = (1 − sin φ)/(1 + sin φ) and Kp = 1/Ka. For φ = 30°, Ka = 1/3 and Kp = 3.

CIV-222 · Find the active thrust on a 5 m wall with dry backfill, γ = 18 kN/m³, φ = 30°.

Pa = ½ Ka γ H² = 0.5 × (1/3) × 18 × 25 = 75 kN/m, acting at H/3 = 1.67 m above the base.

CIV-223 · What are typical permissible total settlements for footings?

IS 1904 gives about 40 mm for isolated footings on sand and 65 mm on clay, with higher values for rafts. Differential settlement and angular distortion are often more critical.

CIV-224 · What is angular distortion?

Differential settlement between two points divided by the distance between them. Values around 1/300 cause cracking in panel walls, and about 1/150 can cause structural damage (Bjerrum).

CIV-225 · Why are under-reamed piles used in expansive soils?

Their enlarged bulbs anchor them below the zone of seasonal moisture change. This resists uplift from swelling and keeps the structure from heaving. IS 2911 (Part 3) covers their design.

CIV-226 · When is a combined footing provided?

When columns are close together and individual footings would overlap, or an exterior column is near a property line. The footing is proportioned so that its centroid coincides with the resultant of the column loads, giving uniform pressure.

Surveying

CIV-227 · What are the curvature and refraction corrections in levelling?

Curvature correction Cc = 0.0785 d² (subtractive), refraction correction Cr = 0.0112 d² (additive). The combined correction is −0.0673 d², in metres, with d in km.

CIV-228 · What is reciprocal levelling and why is it used?

Levelling across a wide river or valley from both banks, then averaging the two results. Averaging cancels collimation, curvature and mean refraction errors when equal sight lengths are not possible.

CIV-229 · A BM has RL 100.000 m. The BS on it is 1.500 m and the FS on point A is 2.250 m. Find the RL of A.

Height of instrument = 100.000 + 1.500 = 101.500 m. RL of A = 101.500 − 2.250 = 99.250 m.

CIV-230 · What is Bowditch's rule?

A method for balancing a traverse in which the closing error in latitudes and departures is distributed to each line in proportion to its length. It is used when angles and distances are measured with similar precision.

CIV-231 · What is the tacheometric distance equation?

D = (f/i) s + (f + d), where f/i = K is the multiplying constant (usually 100) and f + d = C is the additive constant (zero for anallactic lenses). So D = Ks for horizontal sights.

CIV-232 · What are the temporary adjustments of a theodolite?

Setting up over the station, centring with a plumb bob or optical plummet, levelling with the foot screws, and eliminating parallax by focusing the eyepiece and objective.

CIV-233 · What is the arithmetic check in levelling?

ΣBS − ΣFS = Last RL − First RL. In the rise and fall method, ΣRise − ΣFall must equal the same difference too.

CIV-234 · What is triangulation?

A control survey method in which a network of triangles is formed. A baseline is measured precisely, all angles are observed, and the other sides are computed with the sine rule.

CIV-235 · What corrections are applied to taped distances?

Corrections for standardization, temperature, pull (tension), sag, slope, and reduction to mean sea level. Sag and slope are always negative.

CIV-236 · What is the sag correction for a tape?

Cs = W² L / (24 P²), where W is the total weight of the tape between supports, L is the span and P is the applied pull. It is always subtracted.

CIV-237 · How are areas computed from offsets?

By the mid-ordinate, average ordinate, trapezoidal or Simpson's one-third rule. Simpson's rule needs an odd number of ordinates (even number of intervals) and is more accurate for curved boundaries.

CIV-238 · What is the relation between degree of curve and radius?

For a 30 m arc, R ≈ 1719/D m; for a 20 m arc, R ≈ 1146/D m, with D in degrees. A higher degree of curve means a sharper curve.

CIV-239 · What is the scale of a vertical aerial photograph?

S = f / (H − h), where f is the camera focal length, H is the flying height above datum and h is the ground elevation. The scale changes with terrain height.

Transportation Engineering

CIV-240 · What is the formula for stopping sight distance?

SSD = v t + v²/(2 g f), with v in m/s, perception–reaction time t (2.5 s per IRC) and longitudinal friction coefficient f. On a gradient, the braking term becomes v²/[2g(f ± 0.01n)].

CIV-241 · Find the SSD at 80 km/h with t = 2.5 s and f = 0.35.

v = 22.22 m/s. Lag distance = 22.22 × 2.5 = 55.6 m. Braking distance = 22.22² / (2 × 9.81 × 0.35) = 71.9 m. SSD ≈ 127.5 m, rounded to about 130 m.

CIV-242 · What is the equilibrium equation for superelevation?

e + f = V²/(127 R), with V in km/h and R in m. IRC limits e to 7% on plain and rolling terrain (10% in hill roads not bound by snow) and takes f = 0.15.

CIV-243 · What are the components of overtaking sight distance?

d₁ is the distance travelled during reaction time before starting to overtake. d₂ is the distance travelled while overtaking. d₃ is the distance travelled by the opposing vehicle in that time. OSD = d₁ + d₂ + d₃.

CIV-244 · Why is extra widening provided on horizontal curves?

For mechanical widening, because rear wheels track inside the front wheels (off-tracking, nl²/2R). For psychological widening, because drivers keep wider clearances on curves (V/(9.5√R)).

CIV-245 · How does IRC:37 design flexible pavements?

It uses a mechanistic–empirical approach. Subgrade strength (CBR to resilient modulus) and design traffic in million standard axles (msa) are used to select layer thicknesses that limit fatigue cracking (horizontal tensile strain at the bottom of the bituminous layer) and rutting (vertical strain on the subgrade).

CIV-246 · What does the Marshall test determine?

Stability (maximum load) and flow (deformation) of compacted bituminous specimens at 60 °C. Together with density and void analysis, it is used to fix the optimum binder content.

CIV-247 · What is the fundamental relation of traffic flow?

q = k × v (flow = density × speed). In Greenshields' linear model, maximum flow q_max = v_f k_j / 4 occurs at half the free-flow speed and half the jam density.

CIV-248 · What is Webster's optimum cycle length formula?

C₀ = (1.5L + 5)/(1 − Y), where L is the total lost time per cycle (s) and Y is the sum of critical flow ratios (q/s) of all phases.

CIV-249 · What is coning of wheels in railways?

Wheel treads are given an inward slope (about 1 in 20). On curves, the outer wheel rides on a larger diameter, reducing slip, keeping the wheelset centred and reducing flange wear.

CIV-250 · What is creep of rails?

Longitudinal movement of rails along the track, caused by wheel action, braking, acceleration and temperature changes. It is resisted with anti-creepers and proper fastenings.

CIV-251 · What is peak hour factor?

PHF = hourly volume / (4 × peak 15-minute volume). It ranges up to 1.0, and lower values mean more peaked traffic within the hour.

CIV-252 · What is grade compensation on curves?

A reduction in gradient on horizontal curves to offset the extra tractive resistance. IRC gives it as (30 + R)/R percent, limited to 75/R, and the compensated gradient need not go below 4%.

Hydraulics & Water Resources

CIV-253 · What is the Darcy–Weisbach equation?

hf = f L V² / (2 g D), where f is the Darcy friction factor. For laminar flow, f = 64/Re; for turbulent flow, f comes from the Moody chart or Colebrook equation.

CIV-254 · What are the conditions for the most economical rectangular channel section?

Width b = 2y, so hydraulic radius R = y/2. For a trapezoidal section, the most efficient shape is half a hexagon with sides at 60°.

CIV-255 · What is critical depth in a rectangular channel?

yc = (q²/g)^(1/3), where q is discharge per unit width. At critical depth, specific energy is minimum and Ec = 1.5 yc.

CIV-256 · Find the critical depth for q = 2 m²/s in a rectangular channel.

yc = (2² / 9.81)^(1/3) = (0.408)^(1/3) ≈ 0.742 m. Minimum specific energy Ec = 1.5 × 0.742 ≈ 1.11 m.

CIV-257 · What is the sequent depth ratio of a hydraulic jump in a rectangular channel?

y₂ / y₁ = ½ [√(1 + 8 Fr₁²) − 1], from momentum conservation. The energy loss is ΔE = (y₂ − y₁)³ / (4 y₁ y₂).

CIV-258 · What is water hammer?

A pressure surge in a pipeline when flow is suddenly stopped, for example by quick valve closure. The pressure rise is Δp = ρ c V for rapid closure, where c is the pressure wave speed. Surge tanks, slow valves and relief valves control it.

CIV-259 · What is the specific speed of a turbine?

Ns = N √P / H^(5/4), where N is rpm, P is power in kW and H is head in m. Low Ns suits Pelton wheels (high head), medium suits Francis turbines and high suits Kaplan turbines (low head).

CIV-260 · What is cavitation?

Formation and collapse of vapour bubbles where local pressure drops below vapour pressure. It pits and erodes turbine runners, pump impellers and spillway surfaces. It is prevented by enough NPSH and proper setting and design.

CIV-261 · What is the rational method for peak runoff?

Q = C I A / 360, with Q in m³/s, I in mm/h and A in hectares, where C is the runoff coefficient. Rainfall intensity is taken for a duration equal to the time of concentration. It suits small catchments.

CIV-262 · What is Lacey's regime theory?

A theory for stable alluvial channels that neither silt nor scour. It gives regime relations based on silt factor f = 1.76 √d (d in mm), for example wetted perimeter P = 4.75 √Q.

CIV-263 · What is the φ-index?

The constant rate of loss (infiltration) such that rainfall above it equals the observed direct runoff volume. It is used to separate effective rainfall from losses.

CIV-264 · What forces act on a gravity dam?

Self-weight, water pressure (upstream and any tailwater), uplift, silt pressure, wave pressure, ice pressure, and earthquake forces (inertia and hydrodynamic). Self-weight is the main stabilizing force.

Water Supply & Sanitation

CIV-265 · What are the main methods of population forecasting?

Arithmetic increase, geometric increase, incremental increase, decreasing rate of growth, simple graphical, comparative graphical, master plan and logistic curve methods.

CIV-266 · Populations were 80,000, 95,000, 108,000 and 125,000 in 1990, 2000, 2010 and 2020. Forecast 2040 by arithmetic increase.

Decadal increases are 15,000, 13,000 and 17,000, averaging 15,000. The 2040 population is 125,000 + 2 × 15,000 = 155,000.

CIV-267 · What is breakpoint chlorination?

Adding chlorine past the point where ammonia and other reducing compounds are oxidized and chloramines are destroyed. Beyond the breakpoint, added chlorine appears as free residual, giving reliable disinfection.

CIV-268 · What is the jar test?

A bench test in which raw water samples are dosed with different amounts of coagulant, rapid-mixed, flocculated and settled. The dose that gives the lowest residual turbidity at least cost is chosen.

CIV-269 · Why does surface overflow rate govern sedimentation tank design?

By Hazen's theory, an ideal tank removes all particles whose settling velocity exceeds Q/A, regardless of depth. Depth mainly sets detention time and sludge storage.

CIV-270 · What is the first-order BOD equation?

BODt = L₀ (1 − e^(−kt)), where L₀ is ultimate BOD and k is the rate constant. At 20 °C, BOD₅ is typically about 65–70% of ultimate BOD for domestic sewage.

CIV-271 · What are F/M ratio and SVI in the activated sludge process?

F/M is the food-to-microorganism ratio (BOD load per day per mass of MLSS). SVI is sludge volume index, the volume in mL of 1 g of sludge after 30 minutes of settling. SVI of about 50–150 mL/g means good settling; higher values suggest bulking.

CIV-272 · What is self-cleansing velocity?

The minimum flow velocity that prevents solids from settling in sewers, about 0.6–0.9 m/s for sanitary sewers. Maximum velocities are also limited to prevent scouring of the pipe.

CIV-273 · Find BOD₅ if initial DO = 8 mg/L, final DO = 4 mg/L and the sample is diluted to 2%.

BOD = (D₁ − D₂) / P = (8 − 4) / 0.02 = 200 mg/L.

CIV-274 · What is the Hardy Cross method?

An iterative method for analysing looped water distribution networks. Flows are assumed in each loop, then corrected by ΔQ = −Σ(r Qⁿ) / Σ(n r Qⁿ⁻¹) until head losses around every loop sum to zero.

CIV-275 · What are the fluoride limits in drinking water per IS 10500?

1.0 mg/L acceptable and 1.5 mg/L permissible in the absence of an alternative source. Excess fluoride causes dental and skeletal fluorosis.

CIV-276 · How does a trickling filter work?

Settled sewage is sprayed over a bed of stone or plastic media coated with a biological slime layer. Microorganisms in the film oxidize organic matter aerobically, and sloughed film is removed in secondary clarifiers.

Construction Management & Estimation

CIV-277 · How are expected time and standard deviation calculated in PERT?

te = (to + 4tm + tp) / 6 and σ = (tp − to) / 6, based on a beta distribution. The project variance is the sum of variances of critical activities.

CIV-278 · Find te and σ for an activity with to = 4, tm = 6 and tp = 14 days.

te = (4 + 24 + 14) / 6 = 7 days. σ = (14 − 4) / 6 ≈ 1.67 days.

CIV-279 · What is crashing in project scheduling?

Shortening activity durations by adding resources at extra cost. Critical activities with the lowest cost slope (crash cost − normal cost) / (normal time − crash time) are crashed first.

CIV-280 · What is the difference between resource levelling and resource smoothing?

Resource levelling adjusts start dates to stay within resource limits, and may extend the project. Resource smoothing reduces peaks and troughs in resource use within available float, without changing the project end date.

CIV-281 · What are CPI and SPI in earned value management?

CPI = EV/AC measures cost efficiency. SPI = EV/PV measures schedule efficiency. Values below 1 mean over budget or behind schedule.

CIV-282 · How is straight-line depreciation calculated for construction equipment?

Annual depreciation = (Initial cost − Salvage value) / Useful life. It is simple and gives a constant annual charge. Declining balance and sum-of-years methods front-load depreciation.

CIV-283 · How many modular bricks are needed per m³ of brickwork?

About 500 modular bricks (190 × 90 × 90 mm with 10 mm joints) per m³. Wastage is added separately.

CIV-284 · Estimate the cement needed for 1 m³ of 1:1.5:3 nominal mix concrete.

Dry volume ≈ 1.54 m³. Cement = 1.54 × 1/5.5 = 0.28 m³ × 1440 kg/m³ ≈ 403 kg, which is about 8 bags of 50 kg.

CIV-285 · What is a Work Breakdown Structure?

A hierarchical breakdown of the project scope into manageable work packages. It is the basis for scheduling, cost estimating, assigning responsibility and progress control.

CIV-286 · What is the hierarchy of controls in construction safety?

In order of effectiveness: elimination, substitution, engineering controls (such as guardrails and shoring), administrative controls (permits, training) and personal protective equipment.

CIV-287 · What is a mass haul diagram?

A cumulative plot of earthwork volume (cut positive, fill negative) along a road alignment. It shows balance points, haul distances, overhaul, borrow and waste for economical earthwork planning.

CIV-288 · What are owning and operating costs of equipment?

Owning costs include depreciation, interest, insurance, taxes and storage. Operating costs include fuel, lubricants, repairs, tyres and operator wages. They are combined into an hourly rate for rate analysis.

Earthquake Engineering

CIV-289 · What is the design horizontal seismic coefficient per IS 1893?

Ah = (Z/2) × (I/R) × (Sa/g). Z is the zone factor, I is the importance factor, R is the response reduction factor and Sa/g is the spectral acceleration coefficient for the soil type and period.

CIV-290 · How is the approximate fundamental period estimated per IS 1893 (Part 1):2016?

For bare RC moment frames, Ta = 0.075 h^0.75. For bare steel moment frames, Ta = 0.085 h^0.75. For other buildings, including RC frames with masonry infill, Ta = 0.09 h / √d, where d is base dimension in metres along the direction considered.

CIV-291 · Find the approximate period of a 30 m tall bare RC moment frame building.

Ta = 0.075 × 30^0.75 = 0.075 × 12.82 ≈ 0.96 s.

CIV-292 · What is the strong column–weak beam principle?

Columns are made stronger than beams at each joint, so plastic hinges form in beams rather than columns. This avoids storey collapse mechanisms. IS 13920 requires ΣMc ≥ 1.4 ΣMb at beam–column joints.

CIV-293 · What importance factors does IS 1893 (Part 1):2016 use?

1.5 for important and critical buildings such as hospitals, schools, emergency and lifeline facilities. 1.2 for residential or commercial buildings with occupancy above 200 persons. 1.0 for all others.

CIV-294 · What are typical response reduction factors in IS 1893?

R = 3 for ordinary RC moment-resisting frames and R = 5 for special (ductile) RC moment-resisting frames detailed to IS 13920. Higher R values reflect more ductility and overstrength.

CIV-295 · How is base shear distributed over height in the equivalent static method?

Qi = VB × Wi hi² / Σ(Wj hj²). The parabolic distribution approximates the first mode shape and puts larger forces at upper floors.

CIV-296 · What is torsional irregularity?

When the maximum storey drift at one end of a floor exceeds 1.2 times the average drift, due to eccentricity between the centre of mass and centre of rigidity. It amplifies demand on edge elements.

CIV-297 · What is the difference between SRSS and CQC modal combination?

SRSS takes the square root of the sum of squares of modal responses and is suitable when modes are well separated. CQC adds cross-terms for modal correlation and is needed when natural frequencies are closely spaced.

CIV-298 · What is the short column effect?

Columns restrained by partial-height infill walls, mezzanines or sloping ground have a reduced effective height. They become much stiffer, attract large shear and fail in brittle shear.

CIV-299 · What is seismic pounding and how is it prevented?

Collision between adjacent buildings that vibrate out of phase. It is prevented by providing a separation gap based on the combined design lateral displacements of the buildings, or by aligning floor levels.

CIV-300 · Why do ductile detailing codes require 135° hooks on ties?

135° hooks with long extensions stay anchored in the confined core after the cover spalls. 90° hooks open up under cyclic loading, so ties lose confinement and bars buckle.

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