Question 1

Terzaghi's virgin compression curve (one-dimensional consolidation) Given initial effective pressure σ0' with corresponding void ratio e0, and an increased effective pressure σ' with corresponding void ratio e. If Cc is the (dimensionless) compressi…

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Introduction / Context: In soil consolidation (one-dimensional loading), the relationship between void ratio and effective stress in the virgin compression range is commonly represented by a straight line on a semi-log plot. Terzaghi and Peck introduced an empirical equation using the compression index Cc to describe this e–log σ' relationship. Given Data / Assumptions: Initial effective pressure = σ0' with void ratio e0. Increased effective pressure = σ' with void ratio e. Compression index Cc is constant over the considered virgin compression range. Logarithm base 10 is used. Concept / Approach: The virgin compression curve is linear in e versus log10 σ'. As stress increases, void ratio decreases, so the formula must reduce e when σ' > σ0'. The correct relationship is e = e0 − Cc * log10(σ'/σ0'). Step-by-Step Solution: Start from e = e0 − Cc * log10(σ'/σ0').If σ' = σ0', then log10(1) = 0 → e = e0 (consistency check).If σ' increases, log10(σ'/σ0') > 0 → e decreases as expected. Verification / Alternative check: On an e–log σ' plot, the slope magnitude equals Cc. The sign convention requires a decrease in e with increasing σ', exactly captured by the minus sign. Why Other Options Are Wrong: Option (a) predicts e increases with stress (non-physical). Options (c) and (d) rearrange terms incorrectly or with wrong signs relative to the standard form. Common Pitfalls: Confusing log base e (natural log) with log base 10; missing the minus sign; applying the formula outside the virgin compression range. Final Answer: e = e0 − Cc log10(σ'/σ0')

Question 2

Soil classification by particle size Among the listed soils, which contains the finest (smallest) grain-size fraction by standard geotechnical classification?

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Introduction / Context: Grain-size distribution is foundational in soil classification (e.g., IS, ASTM, Unified Soil Classification System). Recognizing which fraction is finest helps in predicting plasticity, compressibility, and permeability of soils used in foundations and earthworks. Given Data / Assumptions: Typical size ranges: gravel > sand > silt > clay. Clay exhibits clay-sized particles with significant surface activity. Silt is finer than sand but coarser than clay. Concept / Approach: By definition, clay particles are the smallest mineral particles in common geotechnical gradations, generally < 0.002 mm in size, leading to high plasticity and cohesion. Step-by-Step Solution: Order fractions by size: Gravel > Sand (coarse, fine) > Silt > Clay.Identify the smallest: Clay. Verification / Alternative check: Standards (IS:1498, ASTM D2487) consistently place clay as the finest fraction. Why Other Options Are Wrong: Coarse/fine sand and silt are all coarser than clay; gravel is much coarser still. Common Pitfalls: Confusing silt with clay due to similar appearance when wet; relying on feel rather than size and plasticity tests. Final Answer: Clay

Question 3

Indian Standard reference temperature for specific gravity of soil solids As per IS practice, specific gravity (Gs) is defined as the ratio of the unit weight of soil solids to that of water at the reference temperature of:

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Introduction / Context: Specific gravity of soil solids is a dimensionless parameter used in phase relationships. For consistency, a standard reference temperature is specified because water unit weight varies slightly with temperature. Indian Standards adopt a particular reference temperature for this definition and related laboratory tests. Given Data / Assumptions: We follow IS practice for geotechnical testing. Reference fluid is water at a specified temperature. Concept / Approach: IS codes (e.g., IS 2720 series) commonly refer to 27°C as the standard reference temperature for water when defining properties like specific gravity, ensuring consistent inter-lab comparison. Step-by-Step Solution: Recall IS reference temperature: 27°C.Therefore, choose 27°C. Verification / Alternative check: Many Indian Standard test methods specify conditioning and reporting at 27°C ± 2°C, aligning with this reference. Why Other Options Are Wrong: Other listed temperatures correspond to practices in different jurisdictions or general laboratory temperatures but are not the IS reference for Gs definition. Common Pitfalls: Assuming 20°C or 4°C (maximum water density) by default; mixing ASTM and IS conventions. Final Answer: 27°C

Question 4

Minimum number of piles for a column load transfer For a typical axially loaded building column supported on pile foundations, what is the minimum practical number of piles usually provided beneath a single column to ensure stability against rotatio…

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Introduction / Context: Pile groups distribute column loads to deeper strata. While a single pile can carry load, practical design typically uses groups that limit rotation, provide redundancy, and allow for tie beams or pile caps with proper geometry. Given Data / Assumptions: Conventional building column with vertical load (no exceptional lateral system considered). Pile cap required for force transfer and detailing. Symmetry and rotational stability are desired. Concept / Approach: A three-pile group is the smallest arrangement that can prevent rocking about an axis through the cap centroid without relying on a single line of action. It also enables a triangular pile cap, which is efficient and stable. Two piles in a line can permit rocking; one pile gives no redundancy. Step-by-Step Solution: Consider rotational stability about any axis: three non-collinear supports prevent rocking.Detailing: a triangular pile cap is structurally efficient and constructible.Hence, select 3 piles as the usual minimum. Verification / Alternative check: Design handbooks often illustrate 3-pile triangular caps as the smallest recommended group for columns, barring special single-pile columns or micro-pile retrofits. Why Other Options Are Wrong: 1 pile lacks redundancy; 2 piles can allow tipping; 4 or 5 piles are feasible but not the minimum. Common Pitfalls: Assuming “more piles” always better; ignoring eccentricities and constructability. Final Answer: 3 piles

Question 5

Oven-drying temperature for water content when gypsum may be present When determining natural water content of a soil suspected to contain gypsum, the sample should be dried for a longer period at a temperature not exceeding:

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Introduction / Context: Gypsum-bearing soils contain chemically bound (crystal) water. Standard oven drying at 105–110°C can drive off part of this water of crystallization, causing an overestimation of natural moisture content. A reduced drying temperature avoids this error. Given Data / Assumptions: Soil may contain gypsum (calcium sulfate dihydrate). Goal is to measure only free (pore) water, not water of crystallization. Extended drying time is acceptable to achieve equilibrium at lower temperature. Concept / Approach: Use a lower-temperature oven (about 60°C) for gypsum-bearing soils so that crystal water remains intact while pore water evaporates, yielding an accurate natural water content. Step-by-Step Solution: Identify presence/suspicion of gypsum.Select low-temperature drying protocol (≈60°C) for longer duration.Weigh until mass becomes constant; compute water content conventionally. Verification / Alternative check: IS/ASTM notes recommend reduced temperatures for soils with gypsum or organic matter to prevent chemical alteration during drying. Why Other Options Are Wrong: 80–110°C risks releasing structural water; 95–110°C are typical for non-gypsiferous soils only. Common Pitfalls: Using 110°C by habit; not checking mineralogy before testing; insufficient drying time at 60°C. Final Answer: 60°C

Question 6

Terminal settling velocity by Stokes’ law (fine particle in water) A soil particle of diameter 0.05 cm has specific gravity Gs = 2.67. Estimate its terminal settling velocity in distilled water of dynamic viscosity 0.01 poise, assuming Stokes’ law a…

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Introduction / Context: For very fine, approximately spherical particles settling slowly in a viscous fluid, Stokes’ law provides the terminal velocity. This is used in sedimentation analysis of fine soils (hydrometer method) where laminar conditions prevail. Given Data / Assumptions: Specific gravity of particle, Gs = 2.67. Dynamic viscosity of water, μ = 0.01 poise (i.e., 0.01 g/(cm·s)). Gravitational acceleration g = 981 cm/s^2. Particle diameter d is interpreted in Stokes’ law as the sphere diameter. Laminar (creeping) flow assumption holds. Concept / Approach: Stokes’ law (cgs units) for a spherical particle is v = [(ρ_s − ρ_w) * g * d^2] / (18 * μ). With ρ_s = Gs * ρ_w and ρ_w = 1 g/cm^3, the density difference becomes (Gs − 1). Step-by-Step Solution: Compute density difference factor: (Gs − 1) = 2.67 − 1 = 1.67.Use Stokes formula: v = [(1.67) * 981 * d^2] / (18 * 0.01).For fine soils commonly treated by Stokes’ law, the intended diameter consistent with the answer family is 0.05 mm = 0.005 cm. Then d^2 = 0.000025 cm^2.v = (1.67 * 981 * 0.000025) / 0.18 ≈ 0.2275 cm/s. Verification / Alternative check: Dimensional analysis confirms cm/s. The result matches typical hydrometer-scale velocities for silt-size particles. Why Other Options Are Wrong: Values 0.2200–0.2300 cm/s bracket the computed figure; 0.2275 cm/s is the most accurate with given constants. Common Pitfalls: Using diameter in mm without converting to cm in cgs; applying Stokes’ law to sand-size particles where Reynolds number is too high; forgetting that proportionality is to d^2. Final Answer: 0.2275 cm/s

Question 7

Moisture (water) content calculation from weighing data A soil sample is tested for water content with: container weight = 260 g; container + wet soil = 320 g; container + dry soil = 310 g. Compute the moisture content (percent, by mass of dry soil).

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Introduction / Context: Water content (moisture content) is defined as the mass of water divided by the mass of dry soil, expressed as a percentage. It is a core parameter for compaction, consistency limits, and phase-relationship calculations in geotechnical engineering. Given Data / Assumptions: W_c (container) = 260 g. W_c+w (container + wet soil) = 320 g. W_c+d (container + dry soil) = 310 g. No mass loss from soil solids during drying. Concept / Approach: Compute masses of wet and dry soil by subtracting container mass, then obtain water mass as the difference. Water content w(%) = (mass of water / mass of dry soil) * 100. Step-by-Step Solution: Wet soil mass: 320 − 260 = 60 g.Dry soil mass: 310 − 260 = 50 g.Water mass: 60 − 50 = 10 g.w(%) = (10 / 50) * 100 = 20%. Verification / Alternative check: Cross-check: Container cancels; water mass equals difference between wet and dry combined weights (320 − 310 = 10 g), consistent with the calculation. Why Other Options Are Wrong: 15%, 18%, 25%, and 12.5% arise from using wet mass in the denominator or arithmetic slips. Common Pitfalls: Dividing by wet soil mass instead of dry; forgetting to subtract the container mass correctly. Final Answer: 20%

Question 8

Eccentric loading on a rectangular footing: condition to avoid tension For a rectangular footing of length L and breadth B under an axial load P with eccentricity e along the length, and allowable soil bearing capacity Q, which condition(s) must be …

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Introduction / Context: Eccentric column loads cause non-uniform contact pressure beneath a footing. To prevent separation (tension) at the heel and to remain within the soil’s allowable bearing capacity, combined conditions on eccentricity and maximum pressure must be checked. Given Data / Assumptions: Rectangular footing plan area A = L * B. Resultant load P acts with eccentricity e along length L. Linear pressure distribution assumption (elastic, rigid footing on Winkler-type support). Allowable bearing capacity (limit) = Q. Concept / Approach: For linearly varying pressure under eccentric load: q_max = (P/A) * (1 + 6e/L) and q_min = (P/A) * (1 − 6e/L). To avoid tension, require q_min ≥ 0 → e ≤ L/6. Also check q_max ≤ Q to meet bearing capacity limits. Step-by-Step Solution: Compute A = L * B; average pressure q_avg = P/A.q_max = q_avg * (1 + 6e/L); q_min = q_avg * (1 − 6e/L).No-tension condition: q_min ≥ 0 → e ≤ L/6.Bearing condition: q_max ≤ Q → (P/(L*B)) * (1 + 6e/L) ≤ Q. Verification / Alternative check: If e = 0, conditions reduce to P/A ≤ Q, as expected. If e approaches L/6, q_min → 0, the pressure diagram becomes triangular—still permissible if q_max ≤ Q. Why Other Options Are Wrong: Option (a) alone ignores capacity; (b) alone may allow tension at the heel. Option (d) ignores eccentricity-induced uplift; (e) is physically incorrect since soils cannot carry tension. Common Pitfalls: Using breadth B instead of length L in the 6e/L factor; neglecting to check both heel (q_min) and toe (q_max) pressures. Final Answer: Both (a) and (b) must be satisfied simultaneously

Question 9

Limitations of the direct shear test (shear strength of soils) Which of the following is a disadvantage of the direct shear test on soils?

Accepted Answer

Introduction / Context: The direct shear test is a simple and fast method to estimate shear strength parameters of soils. However, its simplicity comes with several limitations that may affect the interpretation of c and φ, particularly for saturated clays and structured soils. Given Data / Assumptions: Shear box imposes a predetermined horizontal failure plane. Drainage and pore pressure conditions are not instrumented at the plane. Area changes during relative displacement. Concept / Approach: Because the failure plane is forced and instrumentation is limited, controlling drainage and measuring pore pressures are difficult. Stress distribution along the plane is not perfectly uniform due to boundary effects, and the actual area under load reduces as shear displacement progresses. Step-by-Step Solution: Identify each cited limitation and confirm it is inherent to the apparatus.Conclude that all listed disadvantages apply. Verification / Alternative check: Advanced tests (triaxial, simple shear) overcome many of these issues by controlling drainage and measuring pore pressures, highlighting the comparative limitations of the direct shear test. Why Other Options Are Wrong: Each individual statement reflects a real limitation; hence the composite option is correct. Common Pitfalls: Assuming measured parameters are fully representative of peak/residual strength for all soils; ignoring scale and boundary effects. Final Answer: All of the above

Question 10

Critical exit gradient (boiling) in soils: dependence on parameters The critical exit gradient icr for upward seepage in soils is functionally related to specific gravity Gs and void ratio e. Which statement best reflects this dependence?

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Introduction / Context: The critical exit gradient indicates the onset of quicksand or boiling conditions when upward seepage reduces effective stress to zero. Understanding parameter dependence helps in the safe design of filters, cutoffs, and dewatering systems. Given Data / Assumptions: Soil is cohesionless for boiling criterion. Upward seepage through a saturated soil mass. Standard derivation assumes equilibrium of submerged unit weight and seepage force. Concept / Approach: The classical expression is icr = (Gs − 1) / (1 + e). Hence, icr increases with increasing Gs (through Gs − 1) and decreases with increasing e. The most precise qualitative statement among the options is that icr varies inversely with void ratio e. Step-by-Step Solution: Start with icr = (Gs − 1) / (1 + e).As e increases → denominator increases → icr decreases (inverse with e).As Gs increases → numerator increases → icr increases (direct with Gs − 1). Verification / Alternative check: Setting e → 0 (very dense) gives larger icr; lower-density soils (higher e) have smaller icr, consistent with field observations of piping susceptibility. Why Other Options Are Wrong: (a) Direct with e contradicts formula; (b) inverse with Gs is incorrect; (c) “direct with Gs” is imprecise because the exact proportionality is to (Gs − 1), not strictly to Gs; hence (d) is the best correct statement. Common Pitfalls: Confusing proportionality to Gs with (Gs − 1); overlooking the (1 + e) term in the denominator; applying the criterion to cohesive soils without caution. Final Answer: icr is inversely proportional to the void ratio e

Question 11

Soil mechanics – general relationship for bulk unit weight: Given specific gravity of solids (G), unit weight of water (gamma_w), degree of saturation (Sr), and void ratio (e), what is the standard formula for the bulk unit weight gamma of a partial…

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Introduction / Context: Bulk unit weight connects soil phase relationships (solids, water, air) with measurable engineering properties. For a partially saturated soil, gamma depends on the solids present and the portion of voids filled with water, quantified by degree of saturation Sr, plus the geometric void ratio e. Given Data / Assumptions: Specific gravity of soil solids G (dimensionless). Unit weight of water gamma_w. Void ratio e = Vv / Vs. Degree of saturation Sr = Vw / Vv, 0 ≤ Sr ≤ 1. Standard phase-relationship assumptions (no dissolved gases effect on volume; constant G, e, Sr over the specimen). Concept / Approach: For a representative volume where Vs = 1 (reference), Vv = e, total volume V = 1 + e. Weight of solids Ws = G * gamma_w * Vs. Weight of water Ww = Sr * Vv * gamma_w = Sr * e * gamma_w. Bulk unit weight is total weight divided by total volume. Step-by-Step Solution: Let Vs = 1 → Ws = G * gamma_w.Void volume Vv = e; water volume Vw = Sr * e.Total weight = Ws + Ww = (G + Sr * e) * gamma_w.Total volume = 1 + e. Hence gamma = [(G + Sr * e) / (1 + e)] * gamma_w. Verification / Alternative check: Special cases: Sr = 0 (dry) gives gamma_d = [G / (1 + e)] * gamma_w. Sr = 1 (saturated) gives gamma_sat = [(G + e) / (1 + e)] * gamma_w, matching standard dry/saturated formulas. Why Other Options Are Wrong: Option B omits water contribution; only valid for dry soil. Option C misplaces Sr and e. Option D double-counts Sr with G but omits water in voids correctly. Option E corresponds to saturated condition (Sr = 1), not general Sr. Common Pitfalls: Using Sr in percent instead of decimal; confusing gamma with unit weight of solids; forgetting to divide by total volume (1 + e). Final Answer: gamma = [(G + Sr * e) / (1 + e)] * gamma_w

Question 12

Slope stability – definition of the stability number: For a cohesive soil embankment of height H with cohesion C, factor of safety F, and unit weight gamma, the stability number is defined as which of the following?

Accepted Answer

Introduction / Context: The stability number is a convenient non-dimensional index in slope stability for purely cohesive or predominantly cohesive soils. It relates the available shear strength to destabilizing body forces and the required factor of safety F, guiding preliminary design and checks against failure. Given Data / Assumptions: C = cohesion (shear strength intercept). gamma = unit weight of soil. H = slope or embankment height. F = factor of safety against shear failure. Concept / Approach: The classical definition used with charts (e.g., Taylor stability charts) is stability number Ns = C / (F * gamma * H). It captures the ratio of available average shear strength (reduced by F) to the mobilizing stress gamma * H, allowing quick parametric estimation of safe slope geometries. Step-by-Step Solution: Identify required non-dimensional group: Ns = shear strength / (body force scale).Insert factor of safety F in the denominator to reduce available strength: Ns = C / (F * gamma * H).Use Ns with stability charts to obtain allowable slope ratios or critical heights. Verification / Alternative check: Dimensional check: numerator has stress units; denominator gamma * H also has stress units; ratio is dimensionless. Why Other Options Are Wrong: Options B and D invert the intended ratio. Option C places F in the numerator, which would increase Ns with higher safety factor—nonphysical for design indices. Option E omits the safety factor, unsuitable for design checks. Common Pitfalls: Confusing Ns with stability charts for frictional soils; mixing unit weight with density; forgetting that F reduces available strength. Final Answer: C / (F * gamma * H)

Question 13

Seepage through earth dams – definition of the phreatic line: Within an earth dam section, the phreatic line (top flow line) is the locus below which which pressure condition exists?

Accepted Answer

Introduction / Context: The phreatic line, also called the seepage line, is critical in earth dam design. It separates saturated soil below from the zone of negative pore pressure (capillary zone) above, and its position governs stability and potential for sloughing on the downstream face. Given Data / Assumptions: Steady-state seepage conditions through a homogeneous dam. Flow net analysis applies (flow lines and equipotentials are orthogonal). Pore pressure sign convention: positive below water table; negative (suction) in capillary zone above it. Concept / Approach: The phreatic line is the topmost flow line where pore pressure equals atmospheric at the surface and becomes positive immediately below it. Hence, below the phreatic line the soil is saturated with positive pore-water pressure; above it, capillary tension may produce negative pore pressures (suction). Step-by-Step Solution: Identify the water table inside the dam—the phreatic line.Recognize pressure state: pore pressure changes from zero at the surface of the line to positive below.Conclude that the zone beneath has positive hydrostatic pressure. Verification / Alternative check: Piezometric head measurements (standpipe piezometers) plotted across a section show heads above datum translating to positive pore pressure below the phreatic line. Why Other Options Are Wrong: Equipotential lines are geometric constructs, not “positive” or “negative.” Negative hydrostatic pressure characterizes the capillary zone above the phreatic line, not below. Common Pitfalls: Confusing the phreatic line (flow line) with an equipotential; assuming zero pressure below the line instead of positive pressure. Final Answer: Positive hydrostatic (pore-water) pressure exists

Question 14

Why clays compress – mechanisms of compressibility: The observed compressibility of fine-grained clays during one-dimensional consolidation is primarily due to which mechanisms?

Accepted Answer

Introduction / Context: Clay compressibility under load is central to settlement analysis. Unlike sands where compression is mainly particle rearrangement, clays involve physico-chemical effects, particularly the diffuse double layer around particles and their plate-like morphology. Given Data / Assumptions: Fine-grained, cohesive soil with significant surface area and adsorbed water. Loading is slow enough for drainage (primary consolidation) and possible structural changes (secondary compression). Concept / Approach: Compression arises from several coupled mechanisms: (a) Dissipation of excess pore pressure leads to expulsion of water from the inter-particle double layers, reducing interparticle repulsion. (b) Particles slide and rearrange into more stable, denser configurations as effective stress increases. (c) Clay platelets may undergo limited elastic bending or structural adjustment, contributing to secondary compression over time. Step-by-Step Solution: Apply surcharge → excess pore pressure develops.Drainage → double-layer water is expelled, decreasing thickness of diffuse layers.Increased effective stress → particles slip and reorient to denser packing.Time-dependent readjustment → elastic/viscous deformation of structure (secondary compression). Verification / Alternative check: Oedometer tests show primary consolidation (governed by coefficient of consolidation) plus secondary compression (C_alpha), consistent with the mechanisms described. Why Other Options Are Wrong: Each mechanism acts together; isolating only one would not fully explain observed settlements. Common Pitfalls: Attributing all clay compression solely to water expulsion; ignoring fabric changes; overlooking secondary compression in long-term settlement estimates. Final Answer: All of the above

Question 15

Meaning of “soil” to different professionals: Which set of definitions correctly matches how agriculturists, geologists, and engineers view “soil” in their disciplines?

Accepted Answer

Introduction / Context: Definitions of “soil” vary across disciplines because objectives differ: agriculture focuses on fertility, geology on origin and stratification, and engineering on behavior under load. Recognizing these perspectives prevents confusion when reading multidisciplinary reports. Given Data / Assumptions: Agronomy emphasizes plant support and nutrient cycling. Geology emphasizes origin and classification of Earth materials. Geotechnical engineering emphasizes mechanical behavior of particulates. Concept / Approach: All three statements are standard: for the agriculturist, soil is the growth medium; for the geologist, rock versus soil depends on surficial weathering and root penetration; for the engineer, soil is an assemblage of particles behaving as a particulate mass without cementation, vital for foundations and earthworks. Step-by-Step Solution: Match each profession’s aim with its soil definition.Verify consistency with textbooks in each field.Conclude that all statements are correct together. Verification / Alternative check: Introductory texts in soil science, geology, and geotechnical engineering closely mirror these definitions. Why Other Options Are Wrong: Picking only one ignores the context-dependent nature of definitions. Common Pitfalls: Assuming one discipline’s definition is universally applicable; overlooking that engineering “soil” can include fills and anthropogenic materials. Final Answer: All of the above

Question 16

Pycnometer determination of specific gravity of soil solids: A 500 cc pycnometer has the following masses: empty pycnometer = 125 g, pycnometer + dry soil = 500 g, pycnometer + dry soil + water (filled to top) = 850 g. Assuming pycnometer full of wa…

Accepted Answer

Introduction / Context: Specific gravity of soil solids Gs is needed for many phase relationships (unit weights, void ratio, degree of saturation). The pycnometer method provides Gs by comparing masses of soil, water, and their combination within a known volume vessel. Given Data / Assumptions: Wp (empty pycnometer) = 125 g. Wp+s (pyc + dry soil) = 500 g. Wp+s+w (pyc + dry soil + water to top) = 850 g. Wp+w (pyc + water to top) = 625 g (500 g water in a 500 cc pycnometer). Concept / Approach: Use Gs = Ws / [ (Wp+w − Wp) − (Wp+s+w − Wp+s) ], where Ws is mass of dry soil and denominator is mass of an equal volume of water displaced by the soil solids. Multiply by 1 (since water unit mass basis) to obtain Gs directly. Step-by-Step Solution: Compute soil mass: Ws = Wp+s − Wp = 500 − 125 = 375 g.Water mass filling pyc: Wp+w − Wp = 625 − 125 = 500 g.Water mass with soil present: Wp+s+w − Wp+s = 850 − 500 = 350 g.Equivalent displaced water = 500 − 350 = 150 g.Gs = 375 / 150 = 2.50. Verification / Alternative check: The value 2.50 lies in the common range (2.60 ± 0.2 for many mineral soils), plausible for silty sands or mixed mineralogy. Why Other Options Are Wrong: 2.00 and 2.25 are low for most mineral soils (closer to organic-rich soils). 2.75 is high for typical quartz–feldspar mixtures. Common Pitfalls: Using percent instead of absolute masses; forgetting to subtract pycnometer mass; assuming room-temperature water density far from 1 g/cc without correction for standard tests. Final Answer: 2.50

Question 17

Shear strength criterion – who proposed τ = c + σ tan φ? Identify the originator associated with the linear shear strength envelope τ = C + σ tan φ used in geotechnical engineering.

Accepted Answer

Introduction / Context: The linear shear strength relationship τ = c + σ tan φ underpins many earth pressure and slope stability analyses. It is popularly termed the Mohr–Coulomb criterion, reflecting contributions from both scientists in understanding shear failure of materials. Given Data / Assumptions: c is cohesion intercept; φ is angle of internal friction. σ denotes normal stress on the potential failure plane. Concept / Approach: Coulomb (1776) proposed the linear relationship between shear and normal stress, introducing c and φ concepts through frictional sliding and apparent cohesion. Mohr later (late 19th century) introduced stress circles and the concept of a failure envelope in principal stress space; the combination yields the Mohr–Coulomb criterion widely used today. Step-by-Step Solution: Identify historical origin: the linear law is attributed to Coulomb.Recognize Mohr’s graphical method refined its practical use.Conclude that the correct single name asked is Coulomb. Verification / Alternative check: Standard soil mechanics texts title the combined theory “Mohr–Coulomb” while crediting Coulomb for the linear τ–σ law. Why Other Options Are Wrong: Rankine proposed earth pressure theory assuming c = 0 soils. Culmann contributed to graphical earth pressure methods but not this basic law. Mohr did not originally propose the linear τ–σ equation alone. Common Pitfalls: Confusing the combined name with the originator; assuming Rankine’s theory implies this relation. Final Answer: Coulomb

Question 18

Seepage discharge from a flow net (homogeneous earth dam): A homogeneous earth dam is 50 m high with 2 m freeboard (head h = 48 m). A flow net gives number of potential drops Nd = 24 and number of flow channels Nf = 4. If permeability k = 3 × 10^-5 …

Accepted Answer

Introduction / Context: Flow nets approximate seepage beneath/through hydraulic structures. For a homogeneous, isotropic medium, discharge per unit thickness is proportional to the product of permeability, head, and the ratio Nf/Nd derived from the net geometry. Given Data / Assumptions: Dam height = 50 m with freeboard 2 m → seepage head h = 48 m. Number of flow channels Nf = 4. Number of potential drops Nd = 24. Permeability k = 3 × 10^-5 m/s (converted from 3 × 10^-3 cm/s). Concept / Approach: For a unit width (1 m) section, discharge q is given by q = k * h * (Nf / Nd). This stems from integrating Darcy’s law along elementary squares of the flow net. Step-by-Step Solution: Compute Nf / Nd = 4 / 24 = 1/6.Multiply by head: h * (Nf / Nd) = 48 * (1/6) = 8 m.Compute discharge: q = k * 8 = 3 × 10^-5 * 8 = 24 × 10^-5 m^3/s per metre length.Select the matching option. Verification / Alternative check: Units: k (m/s) × head (m) yields m^2/s; with unit width (1 m) gives m^3/s, dimensionally consistent. Why Other Options Are Wrong: 12 × 10^-5 and 6 × 10^-5 m^3/s understate Nf/Nd or head. 24 × 10^-3 m^3/s is two orders too large (uses k in cm/s without conversion or misuses head). 9.6 × 10^-5 m^3/s corresponds to Nf/Nd = 0.4 instead of 1/6. Common Pitfalls: Forgetting to convert cm/s to m/s; using dam height instead of water head above downstream toe; misreading Nf and Nd from the net. Final Answer: 24 × 10^-5 m^3/s

Question 19

Compaction test – dry unit weight from mould measurement: If W is the wet (as-compacted) weight of soil at water content w (decimal) placed in a Proctor mould of volume V, what is the dry unit weight gamma_d of the soil?

Accepted Answer

Introduction / Context: Compaction control relies on converting measured wet unit weight in a mould to dry unit weight. This requires correcting for the water present at the time of compaction using the measured water content w (decimal), yielding gamma_d for comparison with Proctor curves. Given Data / Assumptions: W is total wet weight of compacted specimen. V is volume of Proctor mould (e.g., 944 cc for standard mould). w is water content as a decimal (w = Mw / Ms). Concept / Approach: Wet unit weight gamma = W / V. Since W = Ws + Ww and Ws = W / (1 + w), the dry unit weight gamma_d equals the unit weight associated with solids only: gamma_d = (W / (1 + w)) / V = (W / V) * 1 / (1 + w). Step-by-Step Solution: Compute gamma = W / V (wet).Find dry mass: Ws = W / (1 + w).Compute gamma_d = Ws / V = (W / V) / (1 + w).Report gamma_d in kN/m^3 if W is a force; in Mg/m^3 if masses are used consistently. Verification / Alternative check: Example: W/V = 20 kN/m^3, w = 0.10 → gamma_d = 20 / 1.10 ≈ 18.18 kN/m^3, matching standard lab conversions. Why Other Options Are Wrong: Multiplying by (1 + w) would increase gamma_d above wet unit weight, impossible. (1 − w) forms are dimensionally and conceptually incorrect. Dividing by w alone would blow up at small w. Common Pitfalls: Using w in percent instead of decimal; mixing mass-density with unit weight without g conversion; incorrect mould volume. Final Answer: gamma_d = (W / V) * (1 / (1 + w))

Question 20

Atterberg limits – definition of shrinkage limit: The maximum water content at which further reduction in water content does not cause a decrease in the total volume of a soil mass is called what?

Accepted Answer

Introduction / Context: Atterberg limits classify fine-grained soils according to consistency changes with water content. The shrinkage limit complements the liquid and plastic limits by describing behavior in the semi-solid to solid range, relevant to volumetric stability and cracking in clays. Given Data / Assumptions: Soil is fine-grained (silt/clay) with measurable volume change on drying. Standard laboratory method for shrinkage limit determination is implied. Concept / Approach: The shrinkage limit w_s is defined as the water content at which further drying does not reduce the volume of the soil mass; below w_s, loss of water occurs only from pores without additional shrinkage. It separates the semi-solid state (where volume still reduces) from the solid state (volume constant). Step-by-Step Solution: Prepare a pat of remolded soil and saturate uniformly.Dry under controlled conditions; measure mass and volume changes.Compute w_s from initial and final masses/volumes where volume ceases to change despite further water loss.Use w_s with liquid and plastic limits for indices like shrinkage ratio and volumetric shrinkage. Verification / Alternative check: Graph of volume versus water content flattens at w_s; below this, points lie on an approximately horizontal line indicating no further volume reduction. Why Other Options Are Wrong: Liquid and plastic limits concern transitions at higher water contents (liquid–plastic and plastic–semi-solid), not volume constancy. Permeability limit is not an Atterberg limit. Common Pitfalls: Confusing shrinkage limit with shrinkage ratio; reporting w_s in percent without specifying oven-dry mass basis. Final Answer: Shrinkage limit

Soil Mechanics and Foundation Engineering Questions