Soil mechanics is a core topic in geotechnical engineering, essential for US civil engineering jobs, GATE, and IES exams. This SciLitPulse guide features 250 soil mechanics interview questions and answers for easy preparation. Covering soil properties, shear strength, ASTM D698 compaction, and IS 2720 testing, it's tailored for US professionals (ASTM standards) and global aspirants. Use it to ace interviews at firms like AECOM or Bechtel, or score high in exams.
1. What is soil mechanics?
Soil mechanics is the branch of geotechnical engineering studying soil behavior under loads, including stress-strain, permeability, and failure mechanisms. It applies to foundations, slopes, and earth structures, foundational for civil engineering design and analysis.
2. Define soil in engineering terms.
Soil is a three-phase system (solids, water, air) with particles <2 mm, used as foundation material. Properties include void ratio e = Vv / Vs, influencing strength and settlement in geotechnical applications.
3. What is the difference between soil and rock?
Soil is unconsolidated, easily deformed particles <2 mm; rock is consolidated with RQD >50%, high strength. Soil relies on friction/cohesion; rock on intact mass per ASTM D6032.
4. Explain three-phase system in soil.
Soil three-phase system: solids (particles), liquids (water), gases (air). Total volume V = Vs + Vv, Vv = Vw + Va; water content w = Ww / Ws × 100%, critical for compaction and strength.
5. What is void ratio?
Void ratio e = Vv / Vs, where Vv is voids, Vs is solids; ranges 0.5–1.0 for sands, 0.5–2.0 for clays. High e indicates loose soil, low bearing capacity per IS 2720.
6. Define porosity.
Porosity n = Vv / V = e / (1 + e); measures void space percentage. n = 30–50% for sands, 50–70% for clays; affects permeability and settlement in geotechnical analysis.
7. What is degree of saturation?
Degree of saturation S = Vw / Vv × 100%; S = 0% dry, 100% saturated. Influences effective stress and shear strength; saturated soils prone to liquefaction.
8. Explain water content.
Water content w = Ww / Ws × 100%; Ww = water weight, Ws = solids weight. 10–20% for sands, 20–40% for clays; affects compaction and strength per ASTM D2216.
9. What is bulk density?
Bulk density γ = total weight / total volume = Gs γw (1 + w) / (1 + e); γw = 9.81 kN/m³. Used for earthwork calculations in foundation design.
10. Define dry density.
Dry density γd = Ws / V = Gs γw / (1 + e); key for compaction control. Target 95–98% of MDD per ASTM D1557 for highways.
11. What is relative density?
Relative density Dr = (emax - e) / (emax - emin) × 100%; emax from vibratory test, emin from densest state per ASTM D4253.
12. Explain Atterberg limits.
Atterberg limits define clay behavior: LL (liquid limit >50 for clays), PL (plastic limit), SL (shrinkage limit <20) per ASTM D4318.
13. What is plasticity index?
Plasticity index PI = LL - PL; measures clay plasticity. PI >17 indicates high plasticity clays; used for classification per USCS.
14. Define consistency index.
Consistency index CI = (LL - w) / PI; CI =1 liquid state, 0 plastic state. CI <0 over-consolidated, >1 under-consolidated clays.
15. What is specific gravity of soil?
Specific gravity Gs = density of solids / density of water; 2.65 for most soils, 2.7–2.8 for gravels per ASTM D854.
16. How to calculate dry unit weight?
Dry unit weight γd = Gs γw / (1 + e); γw = 9.81 kN/m³, e = void ratio. Used for compaction verification in earthworks.
17. What is permeability of soil?
Permeability k = rate of water flow through soil; k = C D2 / n, C = constant, D = grain size, n = porosity. k =10-2 cm/s for sands.
18. Explain Darcy's law.
Darcy's law q = k i A; q = discharge, i = hydraulic gradient, A = area. Valid for laminar flow (Re <1); used for seepage analysis.
19. What is hydraulic gradient?
Hydraulic gradient i = Δh / L; Δh = head loss, L = flow length. i =1 causes quicksand; critical for uplift in foundations.
20. Define seepage velocity.
Seepage velocity vs = k i / n; n = porosity. vs > va (actual velocity); governs erosion in filters per Terzaghi's criteria.
21. What is coefficient of permeability?
Coefficient of permeability k = D102; D10 = effective size. k =10-4 cm/s for silts, 100 cm/s for gravels per IS 2720.
22. Explain hydraulic conductivity.
Hydraulic conductivity K = k γw / η; η = viscosity. K ≈ k for water at 20°C; used in groundwater flow models.
23. What is capillarity in soil?
Capillarity hc = 2σ cosθ / (ρ g r); σ = surface tension, r = pore radius. hc =1 m for fine sands, causes capillary rise.
24. Define effective stress.
Effective stress σ' = σ - u; σ = total stress, u = pore pressure. Governs shear strength; σ' =0 in liquefaction.
25. What is total stress?
Total stress σ = γ z; γ = unit weight, z = depth. Ignores pore pressure; used in undrained analysis.
26. What is pore water pressure?
Pore water pressure u = γw hw; hw = head of water. u =0 in dry soil, hydrostatic below water table.
27. Explain hydrostatic pressure.
Hydrostatic pressure u = γw z; z = depth below water table. Linear increase; causes buoyancy in foundations.
28. What is seepage force?
Seepage force Fs = γw i V; i = hydraulic gradient, V = volume. Acts in flow direction, reduces effective stress.
29. Define critical hydraulic gradient.
Critical hydraulic gradient icr = (Gs -1) / (1 + e) ≈1; causes quicksand when i > icr.
30. Explain flow net.
Flow net grid of flow lines and equipotential drops; Δh = Δl × i, number of flow channels = equipotential drops for seepage calculation.
31. What is USCS classification?
Unified Soil Classification System (USCS) classifies based on particle size and plasticity; GW for well-graded gravel, CL for low-plasticity clay per ASTM D2487.
32. Explain AASHTO system.
AASHTO classifies for highway subgrades; A-1-a (best gravel) to A-7 (worst clay) based on sieve, LL, PI per ASTM D3282.
33. What is IS classification?
Indian Standard classification per IS 1498; coarse (sand/gravel >0.075 mm), fine (silt/clay <0.075 mm) based on Atterberg limits.
34. Define cohesive soil.
Cohesive soil (clay, silt) has PI >4, c >0; strength from cohesion; low permeability, high plasticity per ASTM D2487.
35. What is non-cohesive soil?
Non-cohesive soil (sand, gravel) has PI =0, strength from friction φ =30–45°; high permeability, drains quickly.
36. Explain granular soil.
Granular soil particles >0.075 mm, fines <5%; D10 <0.075 mm for classification; good for drainage per ASTM D422.
37. What is cohesive soil classification?
Cohesive soil classified by PI: CL (PI<50, low plasticity), CH (PI>50, high plasticity) per USCS; ML for silt.
38. Define well-graded soil.
Well-graded soil has Cu ≥4, Cc =1–3; Cu = D60/D10, Cc = (D302) / (D10 D60); stable, good compaction per ASTM D2487.
39. What is uniform soil?
Uniform soil Cu <4, poor grading; uniform particles, low stability under load, prone to piping.
40. Explain gap-graded soil.
Gap-graded soil missing sizes, Cc <1 or >3; unstable, segregates during compaction, used in filters.
41. What is organic soil?
Organic soil >30% organic content, LL >100%; compressible, low strength, high settlement per ASTM D2487.
42. Define peat.
Peat highly organic (>75%), LL >100%, low strength <10 kN/m², high compression; avoid for foundations.
43. What is expansive soil?
Expansive soil swells/shrinks with moisture, PI >35, LCLE <60; causes foundation damage; stabilize with lime per ASTM D4829.
44. Explain collapsible soil.
Collapsible soil loose, low density <16 kN/m³; collapses on wetting, e.g., loess; pre-wet or compact to 95% MDD per ASTM D5333.
45. What is compaction?
Compaction reduces voids in soil by mechanical energy, increasing dry density for stability; OMC 8–12%, MDD 18–22 kN/m³ per ASTM D1557.
46. Explain Proctor test.
Proctor test determines OMC and MDD by compacting soil in 3 layers, 25 blows/layer at varying water contents; standard Proctor for 2.5 kg rammer per ASTM D698.
47. What is OMC?
Optimum moisture content OMC is water content at maximum dry density; 10–15% for sands, 15–25% for clays; ensures lubrication for compaction.
48. Define MDD.
Maximum dry density MDD is highest γd from Proctor curve; 16–20 kN/m³ for clays, 18–22 kN/m³ for sands; target 95% for embankments.
49. What is relative compaction?
Relative compaction = field γd / lab MDD × 100%; ≥95% for highways, 90% for backfill per ASTM D1557.
50. Explain consolidation.
Consolidation is gradual volume reduction under load due to pore water expulsion; time-dependent, governed by Cv = k / (mv γw), Cv = consolidation coefficient.
51. What is consolidation coefficient?
Consolidation coefficient Cv = Tv H2 / t; Tv = time factor, H = drainage path; Cv =1–10 m²/year for clays.
52. Define primary consolidation.
Primary consolidation is volume change due to pore pressure dissipation; 80–90% of total settlement; secondary is creep under constant effective stress.
53. What is preconsolidation pressure?
Preconsolidation pressure σp = maximum past pressure; determined from Casagrande method on e-log σv' curve; σp > σv' indicates overconsolidated soil.
54. Explain normally consolidated soil.
Normally consolidated soil has σp = current overburden σv'; recent deposits, high compressibility, Cc =0.2–0.4 for clays per Terzaghi.
55. What is overconsolidated soil?
Overconsolidated soil has σp > σv'; OCR >1; stiffer, lower settlement, common in varved clays; OCR = σp / σv'.
56. Define OCR.
Overconsolidation ratio OCR = σp / σv'; OCR =1 normally consolidated, OCR >4 lightly overconsolidated; affects Cc in settlement calculation.
57. What is settlement calculation?
Settlement S = Cc H / (1 + e0) log (σf' / σ0'); Cc = compression index, H = layer thickness, e0 = initial void ratio.
58. Explain differential settlement.
Differential settlement ΔS >1/500 between footings causes tilting/cracks; limit <25 mm for buildings per IS 1904; rafts reduce by 50%.
59. What is shear strength?
Shear strength τ = c + σ tan φ; c = cohesion, σ = normal stress, φ = friction angle; Mohr-Coulomb criterion for soil failure.
60. Explain cohesion.
Cohesion c = shear strength at σ=0; 0 for sands, 20–50 kPa for clays; from inter-particle bonds in cohesive soils per ASTM D4767.
61. What is friction angle?
Friction angle φ = angle of internal friction; 30–35° for sands, 0–25° for clays; determines sliding resistance in granular soils.
62. Define undrained shear strength.
Undrained shear strength su = undrained cohesion for saturated clays; su = 0.25 σ' for φu=0; used in short-term analysis.
63. What is triaxial test?
Triaxial test applies confining pressure, then axial load; UU (undrained), CU (consolidated undrained), CD (consolidated drained) types per ASTM D4767.
64. Explain direct shear test.
Direct shear test shears soil along a plane under normal load; for drained conditions per ASTM D3080.
65. What is vane shear test?
Vane shear test for undrained strength in soft clays; su = T / (π D2 H / 2 + π D3 / 6) per ASTM D2573.
66. Define angle of repose.
Angle of repose θ = tan-1 φ; natural slope angle for granular soils, 30–35° for sands.
67. What is sensitivity of soil?
Sensitivity St = su undisturbed / su remolded; St =1 insensitive, >8 for quick clays.
68. Explain liquefaction.
Liquefaction loss of strength in saturated sands under cyclic loading; CRR < CSR per ASTM D5311.
69. What is residual strength?
Residual strength τres = cres + σ tan φres after peak; lower than peak for clays.
70. Define peak strength.
Peak strength at maximum shear resistance before failure; τpeak = c + σ tan φ.
71. What is shear box test?
Shear box test for granular soils; shear along plane per ASTM D3080.
72. Explain Mohr-Coulomb failure envelope.
Mohr-Coulomb failure envelope τ = c + σ tan φ; linear failure criterion in τ-σ plot, defining strength parameters.
73. What is permeability?
Permeability k = rate of water flow through soil; k = C D2 / n, C = constant, D = grain size, n = porosity. k =10-2 cm/s for sands.
74. Explain Darcy's law.
Darcy's law q = k i A; q = discharge, i = hydraulic gradient, A = area. Valid for laminar flow (Re <1); used for seepage analysis.
75. What is hydraulic gradient?
Hydraulic gradient i = Δh / L; Δh = head loss, L = flow length. i =1 causes quicksand; critical for uplift in foundations.
76. Define seepage velocity.
Seepage velocity vs = k i / n; n = porosity. vs > va (actual velocity); governs erosion in filters per Terzaghi's criteria.
77. What is coefficient of permeability?
Coefficient of permeability k = D102; D10 = effective size. k =10-4 cm/s for silts, 100 cm/s for gravels per IS 2720.
78. Explain hydraulic conductivity.
Hydraulic conductivity K = k γw / η; η = viscosity. K ≈ k for water at 20°C; used in groundwater flow models.
79. What is capillarity in soil?
Capillarity hc = 2σ cosθ / (ρ g r); σ = surface tension, r = pore radius. hc =1 m for fine sands, causes capillary rise.
80. Define effective stress.
Effective stress σ' = σ - u; σ = total stress, u = pore pressure. Governs shear strength; σ' =0 in liquefaction.
81. What is total stress?
Total stress σ = γ z; γ = unit weight, z = depth. Ignores pore pressure; used in undrained analysis.
82. What is pore water pressure?
Pore water pressure u = γw hw; hw = head of water. u =0 in dry soil, hydrostatic below water table.
83. Explain hydrostatic pressure.
Hydrostatic pressure u = γw z; z = depth below water table. Linear increase; causes buoyancy in foundations.
84. What is seepage force?
Seepage force Fs = γw i V; i = hydraulic gradient, V = volume. Acts in flow direction, reduces effective stress.
85. Define critical hydraulic gradient.
Critical hydraulic gradient icr = (Gs -1) / (1 + e) ≈1; causes quicksand when i > icr.
86. Explain flow net.
Flow net grid of flow lines and equipotential drops; Δh = Δl × i, number of flow channels = equipotential drops for seepage calculation.
87. What is USCS classification?
Unified Soil Classification System (USCS) classifies based on particle size and plasticity; GW for well-graded gravel, CL for low-plasticity clay per ASTM D2487.
88. Explain AASHTO system.
AASHTO classifies for highway subgrades; A-1-a (best gravel) to A-7 (worst clay) based on sieve, LL, PI per ASTM D3282.
89. What is IS classification?
Indian Standard classification per IS 1498; coarse (sand/gravel >0.075 mm), fine (silt/clay <0.075 mm) based on Atterberg limits.
90. Define cohesive soil.
Cohesive soil (clay, silt) has PI >4, c >0; strength from cohesion; low permeability, high plasticity per ASTM D2487.
91. What is non-cohesive soil?
Non-cohesive soil (sand, gravel) has PI =0, strength from friction φ =30–45°; high permeability, drains quickly.
92. Explain granular soil.
Granular soil particles >0.075 mm, fines <5%; D10 <0.075 mm for classification; good for drainage per ASTM D422.
93. What is cohesive soil classification?
Cohesive soil classified by PI: CL (PI<50, low plasticity), CH (PI>50, high plasticity) per USCS; ML for silt.
94. Define well-graded soil.
Well-graded soil has Cu ≥4, Cc =1–3; Cu = D60/D10, Cc = (D302) / (D10 D60); stable, good compaction per ASTM D2487.
95. What is uniform soil?
Uniform soil Cu <4, poor grading; uniform particles, low stability under load, prone to piping.
96. Explain gap-graded soil.
Gap-graded soil missing sizes, Cc <1 or >3; unstable, segregates during compaction, used in filters.
97. What is organic soil?
Organic soil >30% organic content, LL >100%; compressible, low strength, high settlement per ASTM D2487.
98. Define peat.
Peat highly organic (>75%), LL >100%, low strength <10 kN/m², high compression; avoid for foundations.
99. What is expansive soil?
Expansive soil swells/shrinks with moisture, PI >35, LCLE <60; causes foundation damage; stabilize with lime per ASTM D4829.
100. Explain collapsible soil.
Collapsible soil loose, low density <16 kN/m³; collapses on wetting, e.g., loess; pre-wet or compact to 95% MDD per ASTM D5333.
101. What is zero air void line?
Zero air void line theoretical maximum dry density at S=100%; γd = Gs γw / (1 + w Gs); lies above Proctor curve.
102. Define field compaction control.
Field compaction control uses sand cone or nuclear density gauge; ensures ≥95% MDD at OMC ±2% per ASTM D1556.
103. What is vibro-compaction?
Vibro-compaction densifies granular soils using vibrating probe; increases Dr >70%, reduces liquefaction risk.
104. Explain dynamic compaction.
Dynamic compaction drops 10–40 ton weights from 10–30 m; improves loose fills to 10 m depth, energy 100–400 t-m/m².
105. What is secondary consolidation?
Secondary consolidation creep under constant effective stress after primary; Cα = Δe / Δlog t, Cα = 0.01–0.05 for clays.
106. Define compression index.
Compression index Cc = Δe / Δlog σ'; slope of virgin compression line; Cc = 0.009 (LL - 10) for remolded clays.
107. What is swell index?
Swell index Cs = Δe / Δlog σ' on unloading; Cs ≈ Cc/5–10; used for overconsolidated clays.
108. Explain time factor.
Time factor Tv = Cv t / H2; dimensionless, Tv = 0.848 at U=90%; governs consolidation rate.
109. What is degree of consolidation?
Degree of consolidation U = settlement at time t / final settlement; U = 1 - (8/π²) e-π²Tv/4 for one-way drainage.
110. Define immediate settlement.
Immediate settlement elastic deformation under undrained loading; Si = q B (1-ν²)/Eu for footings, Eu = undrained modulus.
111. What is bearing capacity?
Bearing capacity qult = maximum soil pressure without shear failure; Terzaghi: qult = c Nc + γ D Nq + 0.5 γ B Nγ.
112. Explain Terzaghi's theory.
Terzaghi's theory assumes general shear failure; Nc, Nq, Nγ depend on φ; valid for shallow footings, D/B ≤1.
113. What is net safe bearing capacity?
Net safe bearing capacity qns = (qult - γ D)/FS; FS = 2.5–3; accounts for overburden removal.
114. Define factor of safety in bearing capacity.
Factor of safety FS = qult / qall; qall = allowable pressure; FS ≥3 for shear, ≥2 for settlement.
115. What is local shear failure?
Local shear failure occurs in loose sands (Dr <30%); punching with limited plastic flow; use reduced c' = 2/3 c, tan φ' = 2/3 tan φ.
116. Explain punching shear failure.
Punching shear failure in very loose soils or deep footings; vertical compression without wedge; common in D/B >4.
117. What is Meyerhof's theory?
Meyerhof's theory includes shape, depth, inclination factors; qult = c Nc sc dc + γ D Nq sq dq + 0.5 γ B Nγ sγ dγ.
118. Define shape factors.
Shape factors sc, sq, sγ adjust for non-strip footings; sc = 1 + 0.2 (B/L) tan(45+φ/2) for square/circular.
119. What is depth factor?
Depth factor dc = 1 + 0.4 (D/B); increases capacity with embedment; dc ≤1.5 typically.
120. Explain eccentric loading.
Eccentric loading reduces effective area B' = B - 2e; qmax ≤ 1.5 qavg; e < B/6 for no tension.
121. What is raft foundation?
Raft foundation spreads load over large area; reduces contact pressure <50 kPa; used on weak soils.
122. Define contact pressure.
Contact pressure q = P/A ± M y / I; uniform for centric, triangular for eccentric; max at edges.
123. What is presumptive bearing pressure?
Presumptive bearing pressure code values; 300 kPa for dense sand, 100 kPa for stiff clay per IS 1904.
124. Explain plate load test.
Plate load test applies load on 0.3–0.6 m plate; qult at 25 mm settlement; scale to footing using qf = qp (Bf/Bp).
125. What is SPT?
Standard Penetration Test drives 50 mm sampler with 63.5 kg hammer, 760 mm drop; N = blows for 300 mm penetration per ASTM D1586.
126. Define N-value.
N-value = blows for 2nd + 3rd 150 mm; N<10 loose sand, N>50 very dense; corrected N60 for energy.
127. What is CPT?
Cone Penetration Test pushes 10 cm² cone at 20 mm/s; qc = cone resistance, fs = sleeve friction per ASTM D5778.
128. Explain soil liquefaction.
Soil liquefaction occurs in saturated loose sands (Dr <50%) under cyclic loading; pore pressure = effective stress.
129. What is CSR?
Cyclic Stress Ratio CSR = τavg / σ'v = 0.65 (amax/g) (σv/σ'v) rd; rd = depth reduction factor.
130. Define CRR.
Cyclic Resistance Ratio CRR = resistance to liquefaction; CRR7.5 = f(N160, FC); FS = CRR/CSR >1.2.
131. What is dilatancy?
Dilatancy volume increase during shear in dense sands; causes negative pore pressure, increases strength.
132. Explain thixotropy.
Thixotropy strength regain in remolded clays at constant water content; common in sensitive clays.
133. What is quick sand?
Quick sand upward seepage makes i = icr ≈1; effective stress =0, soil behaves as liquid.
134. Define piping.
Piping erosion of fine particles by seepage; prevented by filters D15 filter < 4–5 D85 base.
135. What is frost heave?
Frost heave ice lens formation in frost-susceptible silts; lifts structures; mitigate with insulation or drainage.
136. Explain soil stabilization.
Soil stabilization improves engineering properties; mechanical (compaction), chemical (lime/cement), or geosynthetics.
137. What is lime stabilization?
Lime stabilization adds 3–8% lime to reactive clays; reduces PI, increases strength via pozzolanic reaction.
138. Define cement stabilization.
Cement stabilization mixes 3–10% cement with soil; forms cementitious bonds, UCS >1 MPa in 7 days.
139. What is geotextile?
Geotextile permeable fabric for separation, filtration, reinforcement; tensile strength 10–100 kN/m.
140. Explain earth pressure.
Earth pressure lateral force on retaining structures; at-rest K0, active Ka, passive Kp.
141. What is Rankine's theory?
Rankine's theory for cohesionless soils; Ka = (1-sinφ)/(1+sinφ), Kp = (1+sinφ)/(1-sinφ); assumes smooth wall.
142. Define Coulomb's theory.
Coulomb's theory includes wall friction δ; Ka reduced by cosδ; more accurate for rough walls.
143. What is at-rest pressure?
At-rest pressure σh = K0 σ'v; K0 = 1-sinφ for NC sands, K0 = 0.4–0.6 typically.
144. Explain active earth pressure.
Active earth pressure minimum when wall moves away; Pa = 0.5 Ka γ H²; triangular distribution.
145. What is passive earth pressure?
Passive earth pressure maximum when wall pushed into soil; Pp = 0.5 Kp γ H²; resistance in anchors.
146. Define surcharge effect.
Surcharge q adds ΔP = q Ka H; uniform for infinite, triangular for strip load.
147. What is slope stability?
Slope stability FS = resisting moment / driving moment; FS >1.5 for long-term, >1.2 short-term.
148. Explain infinite slope.
Infinite slope FS = (c + γ z cos²β tanφ) / (γ z sinβ cosβ); critical when β > φ for cohesionless.
149. What is Bishop's method?
Bishop's method circular slip analysis; FS = Σ [c b + (W - u b) tanφ] / (W sinα); satisfies moment equilibrium.
150. Define factor of safety in slope.
Factor of safety in slope FS = shear strength / shear stress along failure plane; FS = 1.3–1.5 typical.
151. What is pile foundation?
Pile foundation transfers load to deeper strata; end-bearing or friction piles, capacity Qu = qp Ap + fs As.
152. Explain negative skin friction.
Negative skin friction downdrag on pile due to settling soil; adds load, common in consolidating clays.
153. What is group efficiency?
Group efficiency η = Qgroup / (n Qsingle); η <1 for friction piles at s <3D, η >1 for end-bearing.
154. Define settlement of pile.
Settlement of pile S = Selastic + Spoint + Sshaft; elastic S = (Q L)/(A E) for compression.
155. What is under-reamed pile?
Under-reamed pile has bulbs at base; increases bearing in expansive soils, bulb diameter 2–3 times shaft.
156. Explain soil exploration.
Soil exploration determines subsurface profile; borings, SPT, CPT, lab tests per ASTM D420.
157. What is bore log?
Bore log records soil layers, N-values, water table; basis for foundation design.
158. Define disturbed sample.
Disturbed sample retains structure but not in-situ stress; used for classification, compaction tests.
159. What is undisturbed sample?
Undisturbed sample preserves structure and stress; thin-wall tube, used for strength, consolidation tests.
160. Explain vane shear test.
Vane shear test measures su in soft clays; torque T = π D² (H/2 su + D/12 su) per ASTM D2573.
161. What is unconfined compression test?
Unconfined compression test applies axial load without confinement; qu = 2 su for clays per ASTM D2166.
162. Define consolidation test.
Consolidation test loads soil in oedometer; measures Cc, Cv, σp per ASTM D2435.
163. What is hydrometer analysis?
Hydrometer analysis determines <0.075 mm particles by sedimentation; Stokes law per ASTM D422.
164. Explain sieve analysis.
Sieve analysis separates >0.075 mm particles; plots grain size distribution, D10, D30, D60 per ASTM D6913.
165. What is liquid limit test?
Liquid limit test Casagrande cup, 25 blows at LL; LL = w at 25 blows on flow curve per ASTM D4318.
166. Define plastic limit test.
Plastic limit test rolls soil to 3 mm thread; PL = w at crumbling; PI = LL - PL.
167. What is shrinkage limit?
Shrinkage limit SL = w when volume stops decreasing; SL = w - (Vi - Vf) ρw / md.
168. Explain core cutter method.
Core cutter method field density test; γ = mass / volume of 100 mm cutter per IS 2720.
169. What is sand replacement method?
Sand replacement method measures field density; volume = sand mass / density per ASTM D1556.
170. Define nuclear density gauge.
Nuclear density gauge non-destructive; gamma rays for density, neutrons for moisture per ASTM D6938.
171. What is CBR test?
California Bearing Ratio penetration resistance at 2.5 mm; CBR = (test load / standard) × 100 per ASTM D1883.
172. Explain Proctor needle.
Proctor needle field compaction control; penetration resistance correlates with density.
173. What is relative compaction?
Relative compaction RC = γd field / γd max × 100%; ≥95% for subgrade.
174. Define air voids.
Air voids Va = V - Vs - Vw; % air voids = Va/V × 100%; <5% at OMC.
175. What is specific yield?
Specific yield Sy = volume of water drained by gravity / total volume; Sy = n - Sr, Sr = specific retention.
176. Explain confined aquifer.
Confined aquifer between impervious layers; pressure > atmospheric, artesian flow possible.
177. What is unconfined aquifer?
Unconfined aquifer water table as upper boundary; recharge by infiltration, pumping causes drawdown.
178. Define drawdown.
Drawdown s = lowering of water table due to pumping; s = (Q/(4πT)) ln(2.25 T t / r² S) Theis equation.
179. What is transmissivity?
Transmissivity T = K b; K = permeability, b = aquifer thickness; T in m²/day.
180. Explain storativity.
Storativity S = volume of water released per unit area per unit head decline; S = 10-3–10-5 confined, 0.05–0.3 unconfined.
181. What is well hydraulics?
Well hydraulics studies flow to wells; steady state Q = 2π K b (H-h)/ln(R/r) Dupuit.
182. Define radius of influence.
Radius of influence R = distance where drawdown =0; R ≈ 3000 s √K for unconfined.
183. What is safe yield?
Safe yield maximum pumping without depletion; < annual recharge, typically 70–80%.
184. Explain seepage analysis.
Seepage analysis uses flow nets; q = k H Nf/Nd, uplift = γw i per unit area.
185. What is exit gradient?
Exit gradient ie = Δh / Δs at downstream; ie < icr/FS, FS = 4–5 to prevent piping.
186. Define filter design.
Filter design Terzaghi: D15f < 4 D85b, D15f > 4 D15b; prevents migration, allows drainage.
187. What is sheet pile?
Sheet pile interlocking steel/concrete for retaining water/soil; cantilever or anchored.
188. Explain cantilever sheet pile.
Cantilever sheet pile stability by embedment; D ≈ 1.5–2 H for granular, deeper in clay.
189. What is anchored sheet pile?
Anchored sheet pile tie rod + deadman; free earth support, D = 0.4–0.6 H.
190. Define braced excavation.
Braced excavation uses struts/wales; pressure diagram per Peck, strut load = 0.65 Ka γ H² s.
191. What is soil nailing?
Soil nailing reinforces in-situ soil with steel bars; 1–2 m grid, 6–12 m long, 20–30° inclination.
192. Explain reinforced earth.
Reinforced earth soil + strips/geogrids; friction mobilizes tensile strength, FS >1.5.
193. What is geogrid?
Geogrid polymer grid for reinforcement; aperture 25–200 mm, tensile strength 20–200 kN/m.
194. Define ground improvement.
Ground improvement enhances load-bearing, reduces settlement; vibro, grouting, stone columns.
195. What is stone column?
Stone column 0.6–1 m diameter gravel; improves soft clay, spacing 1.5–3 m, area ratio 10–30%.
196. Explain deep mixing.
Deep mixing blends soil with cement/lime in-situ; forms soil-cement columns, UCS 0.5–5 MPa.
197. What is jet grouting?
Jet grouting high-pressure cement slurry; forms 0.5–2 m columns, strength 1–20 MPa.
198. Define vacuum consolidation.
Vacuum consolidation applies -60 to -80 kPa suction; accelerates settlement, no surcharge needed.
199. What is prefabricated vertical drain?
Prefabricated vertical drain (PVD) 100 mm wide band; reduces drainage path, t ∝ H²/d².
200. Explain electro-osmosis.
Electro-osmosis water flows from anode to cathode in clays; ke = 10-9 m²/s V.
201. What is critical void ratio?
Critical void ratio ecr at constant volume shearing; dense soils dilate, loose contract to ecr.
202. Define steady state line.
Steady state line unique e vs log p' at large strain; defines liquefaction susceptibility.
203. What is state parameter?
State parameter ψ = e - ecs; ψ <0 dense, ψ >0 loose; correlates with peak φ.
204. Explain drained vs undrained.
Drained allows pore pressure dissipation (slow loading); undrained no drainage (rapid loading).
205. What is Skempton's parameters?
Skempton's parameters A = Δu / Δσ1, B = Δu / Δσ3; A = 0.5–1 for NC clay, B ≈1 saturated.
206. Define pore pressure coefficient.
Pore pressure coefficient Δu = B [Δσ3 + A (Δσ1 - Δσ3)]; predicts undrained response.
207. What is residual friction angle?
Residual friction angle φr after large displacement; φr < φpeak in clays due to particle alignment.
208. Explain progressive failure.
Progressive failure peak strength mobilizes sequentially; reduces overall FS in stiff clays.
209. What is strain softening?
Strain softening strength decreases after peak; common in dense sands, overconsolidated clays.
210. Define brittle vs ductile.
Brittle sudden failure post-peak (OC clay); ductile gradual (loose sand).
211. What is anisotropy in soil?
Anisotropy properties vary with direction; kh/kv = 1–10, strength higher horizontally.
212. Explain plane strain.
Plane strain εz =0 (long structures); φps ≈ 1.1 φds in triaxial.
213. What is stress path?
Stress path p'-q plot during loading; K0 line, CSL define behavior.
214. Define critical state.
Critical state simultaneous constant volume, stress, strain rate; q = M p' on CSL.
215. What is Cam Clay model?
Cam Clay model elastoplastic; yield surface f = q - M p' ln(p'/p'0) =0.
216. Explain Hvorslev parameters.
Hvorslev parameters true cohesion ce, true friction φe on dry side; for OC clays.
217. What is tension crack?
Tension crack depth z0 = 2c / γ √Ka; reduces active pressure in clay slopes.
218. Define factor of safety vs strength reduction.
Strength reduction divides c, tanφ by SRF; FS = SRF at collapse.
219. What is Morgenstern-Price method?
Morgenstern-Price method satisfies force and moment equilibrium; arbitrary interslice forces.
220. Explain Spencer method.
Spencer method assumes parallel interslice forces; accurate for non-circular slips.
221. What is block sliding?
Block sliding along planar weak layer; FS = (c L + W cosβ tanφ) / (W sinβ).
222. Define wedge failure.
Wedge failure two-plane slide; common in rock but applicable to soil with joints.
223. What is toppling failure?
Toppling failure overturning of blocks; steep slopes with vertical joints.
224. Explain pore pressure ratio.
Pore pressure ratio ru = u / γ h; ru = 0.3 reduces FS by 30–40% in slopes.
225. What is rapid drawdown?
Rapid drawdown sudden water level drop; undrained upstream, reduces FS.
226. Define seismic slope analysis.
Seismic slope analysis pseudo-static; kh = a/g, FS = (c + (γ h cos²β - kv γ h) tanφ) / (γ h sinβ cosβ + kh γ h cosβ).
227. What is Newmark displacement?
Newmark displacement permanent deformation under earthquake; integrates acceleration above yield.
228. Explain factor of safety vs probability.
Probability of failure uses Monte Carlo; variability in c, φ, γ.
229. What is reliability index?
Reliability index β = (μFS - 1) / σFS; β >3.5 for low risk.
230. Define observational method.
Observational method monitors performance; adjusts design during construction (Peck).
231. What is soil suction?
Soil suction ψ = ua - uw; matric suction in unsaturated soils, increases strength.
232. Explain SWCC.
Soil Water Characteristic Curve θ vs log ψ; defines air entry, residual saturation.
233. What is unsaturated shear strength?
Unsaturated shear strength τ = c' + (σ - ua) tanφ' + (ua - uw) tanφb; φb < φ'.
234. Define collapsing soil.
Collapsing soil metastable structure; large settlement on wetting at constant stress.
235. What is expansive clay mineral?
Expansive clay mineral montmorillonite; high CEC, swells 10–15 times on wetting.
236. Explain free swell index.
Free swell index FSI = (Vd - Vk) / Vk × 100%; FSI >100% highly expansive.
237. What is swell pressure?
Swell pressure ps = stress to prevent swelling; 50–500 kPa for black cotton soil.
238. Define CNS foundation.
Cohesive Non-Swelling layer under raft; 1–2 m thick to resist uplift from swelling.
239. What is under-reamed pile in expansive soil?
Under-reamed pile anchors in stable zone; bulbs resist uplift, spacing 2–3 m.
240. Explain belled pier.
Belled pier enlarged base; increases bearing, resists uplift in expansive soils.
241. What is soil structure interaction?
Soil structure interaction mutual influence; flexible raft settles more in center.
242. Define modulus of subgrade reaction.
Modulus of subgrade reaction ks = p / s; kN/m³, from plate load, ks ∝ 1/B.
243. What is raft-soil stiffness?
Raft-soil stiffness Kr/Ks >10 rigid, <0.01 flexible; affects moment distribution.
244. Explain machine foundation.
Machine foundation dynamic loading; amplitude <0.2 mm, resonant frequency avoided.
245. What is block resonance?
Block resonance when fn = operating frequency; fn = √(k/m)/(2π).
246. Define damping ratio.
Damping ratio ξ = c/ccr; ξ = 0.05–0.2 for soils, reduces amplitude.
247. What is liquefaction mitigation?
Liquefaction mitigation densification, drainage, grouting, stone columns.
248. Explain ground freezing.
Ground freezing circulates brine; forms ice wall, strength 2–10 MPa, temporary support.
249. What is bio-cementation?
Bio-cementation MICP; bacteria precipitate CaCO3, UCS 1–20 MPa.
250. Define sustainable geotechnics.
Sustainable geotechnics uses recycled materials, minimizes carbon; e.g., fly ash, tire chips.
Conclusion
Master these 250 soil mechanics interview questions to excel in geotechnical roles at firms like AECOM, Bechtel, or public sector exams (GATE/IES). Focus on effective stress, bearing capacity, ASTM/IS standards, and practical applications. Practice numericals on settlement, seepage, and stability. Stay updated with field techniques like CPT, PVD, and bio-remediation for modern interviews.
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