Q: Time-dependent volume change: The compression of a saturated soil mass under a sustained static load, caused by gradual expulsion of pore water, is called:

Introduction / Context: Two different processes reduce soil volume: compaction (mechanical densification at nearly constant water content) and consolidation (time-dependent volume decrease in saturated soils due to drainage of pore water). Distinguishing them is crucial for predicting settlements of structures and embankments. Given Data / Assumptions: Saturated soil subjected to sustained static load. Drainage path allows pore water to exit over time. Result is a gradual decrease in void ratio and settlement. Concept / Approach: Consolidation is governed by Terzaghi’s one-dimensional theory in many practical cases. Effective stress increases as pore pressure dissipates: sigma′ = sigma − u. As u decreases with time, soil skeleton carries more load, leading to primary consolidation; secondary compression may follow due to creep within the soil skeleton. Step-by-Step Solution: Apply load → generate excess pore pressure in saturated soil.Excess pore pressure dissipates via drainage over time.Effective stress increases, void ratio decreases, settlement occurs.Completion of primary consolidation when excess pore pressure ~ 0. Verification / Alternative check: Oedometer tests exhibit time–deformation curves with primary consolidation and secondary compression phases, matching field settlement observations. Why Other Options Are Wrong: Compaction: rapid densification by mechanical effort in unsaturated soils. Swelling: volume increase due to wetting or unloading. Creep: time-dependent strain at approximately constant effective stress; not primarily water expulsion. Common Pitfalls: Assuming all time-dependent settlement is consolidation; in organic soils, secondary compression can dominate long-term behavior. Final Answer: Consolidation

Q: Air-voids line relationship: Select the correct fundamental relationship between dry unit weight (γd), specific gravity of solids (G), water content (w), and percentage air voids (na) for a soil (use γw for unit weight of water).

Introduction / Context: Compaction charts often include constant air-voids lines. A useful closed-form expression relates dry unit weight γd to water content w, specific gravity G, and the percentage of air voids na. This formula enables quick plotting and consistency checks in the laboratory and field. Given Data / Assumptions: na is in percent (%), so na/100 is the air-voids fraction. γw is the unit weight of water. G is the specific gravity of solids. w is water content on a decimal basis (use w%/100 if starting with percent). Concept / Approach: Use phase relations: e = (G * γw / γd) − 1, n = e/(1 + e) = 1 − γd/(G * γw), Sr = (w * G) / e. Percentage air voids is na = 100 * (1 − Sr) * n. Eliminating e and Sr yields the compact form: na = 100 * [1 − γd/(G * γw) − w * γd/γw]Rearranging for γd gives γd = [γw * (1 − na/100)] / (w + 1/G) This is the straight-line relation used for air-voids lines on γd–w plots. Step-by-Step Solution: Write e = G * γw / γd − 1 and n = 1 − γd/(G * γw).Use Sr = (w * G) / e to express saturation in terms of γd, w, and G.Form na = 100 * (1 − Sr) * n and simplify.Solve the linear equation for γd to obtain γd = [γw * (1 − na/100)] / (w + 1/G). Verification / Alternative check: When na = 0 (zero air voids), the formula reduces to γd(ZAV) = γw / (w + 1/G), which is the well-known zero-air-voids line. When w = 0, γd = γw * (1 − na/100) * G, consistent with dry packing at given air voids. Why Other Options Are Wrong: They either invert signs, omit G or γw properly, or fail to reduce to the zero-air-voids line at na = 0. Common Pitfalls: Forgetting to convert na from percent to fraction; using w in percent without dividing by 100; mixing units of γd and γw. Final Answer: γd = [γw * (1 − na/100)] / (w + 1/G)

Q: Agents of transportation and redeposition: Which of the following natural agents can transport and redeposit soils?

Introduction / Context: Soil deposits are frequently classified by their mode of transport and deposition, which controls grading, fabric, and engineering behavior. Recognizing depositional agents helps anticipate properties like density, permeability, and variability. Given Data / Assumptions: Water (fluvial, lacustrine, marine) can carry particles in suspension and bed load. Glaciers move and grind materials, producing tills and outwash. Wind (aeolian) transports fine sands and silts over long distances. Gravity yields colluvium and talus by downslope movement. Concept / Approach: Each listed agent is a recognized transporter: alluvial sands/silts (water), glacial tills (ice), dune sands and loess (wind), and colluvial slopes (gravity). The agent leaves a signature in sorting, particle shape, and layering that guides geotechnical investigation and sampling strategies. Step-by-Step Solution: Match agent to deposit: water → alluvium/delta; glacier → till/outwash; wind → dunes/loess; gravity → colluvium.Note typical traits: water-borne → better sorting; glacial tills → poorly sorted; wind-blown → very uniform fine sands/silts; gravity → angular coarse fragments.Infer likely engineering implications (e.g., density, compressibility, anisotropy).Select the combined option acknowledging all agents. Verification / Alternative check: Site reconnaissance and stratigraphic logs typically reveal such depositional signatures, confirming the correct classification. Why Other Options Are Wrong: Each single agent is correct but incomplete; the question asks which agents can transport and redeposit soils—hence all listed apply. Common Pitfalls: Assuming only water acts at meaningful scales; in arid and glaciated terrains, wind and ice dominate. Final Answer: All of the above

Q: Compute void ratio from mass–volume data: A moist soil sample has total mass 108 g and volume 60 cm³. Its water content is 25% and specific gravity of solids G = 2.52. Determine the void ratio e.

Introduction / Context: This numerical problem uses basic phase relationships to compute void ratio e from measured mass, volume, water content, and specific gravity. It illustrates how laboratory data translate to structure-defining parameters like e that control compressibility and permeability. Given Data / Assumptions: Total mass M = 108 g, total volume V = 60 cm³. Water content w = 25% = 0.25 (decimal). Specific gravity of solids G = 2.52; take density of water = 1 g/cm³. Concept / Approach: Water content w = Mw / Ms ⇒ M = Ms + Mw = Ms (1 + w). Solve for solids mass Ms, then compute volume of solids Vs = Ms / (G * ρw). With total volume V known, void volume is Vv = V − Vs. Finally, e = Vv / Vs. Step-by-Step Solution: Compute Ms: Ms = M / (1 + w) = 108 / 1.25 = 86.4 g.Compute Vs: Vs = Ms / (G * ρw) = 86.4 / (2.52 * 1) ≈ 34.2857 cm³.Compute Vv: Vv = V − Vs = 60 − 34.2857 ≈ 25.7143 cm³.Compute e: e = Vv / Vs ≈ 25.7143 / 34.2857 ≈ 0.75. Verification / Alternative check: Check porosity n = e / (1 + e) ≈ 0.75 / 1.75 ≈ 0.429 (about 43%), a reasonable value for a moist, medium-density fine-grained soil. Why Other Options Are Wrong: 0.55, 0.65, 0.80, 0.95: Do not match the computed ratio based on mass–volume consistency. Common Pitfalls: Using w in percent without converting to decimal; forgetting that total volume V = Vs + Vv even in partially saturated states; using G incorrectly (it multiplies ρw). Final Answer: 0.75

Q: Critical Hydraulic Gradient for Quicksand – Compute Using Gs and Porosity For a sand with specific gravity of soil solids Gs = 2.696 and porosity n = 44% (loose state), determine the critical hydraulic gradient i_c for the onset of quick (boiling) c…

Introduction / Context: The critical hydraulic gradient i_c marks the threshold for boiling (quicksand) in cohesionless soils under upward seepage. Designers use i_c to check against piping at exits beneath hydraulic structures and to plan safe dewatering around excavations below the water table. Given Data / Assumptions: Specific gravity of solids Gs = 2.696. Porosity n = 44% = 0.44. Water unit weight is reference; formula based on submerged weight balance. Concept / Approach: For upward seepage, the critical gradient is i_c = (Gs − 1) / (1 + e), where e is void ratio. Convert porosity to void ratio by e = n / (1 − n). If the applied gradient equals or exceeds i_c, the seepage force cancels the submerged unit weight, making σ′ → 0 (boiling condition). Step-by-Step Solution: Compute void ratio: e = 0.44 / 0.56 = 0.785714.Compute numerator: Gs − 1 = 2.696 − 1 = 1.696.Compute denominator: 1 + e = 1 + 0.785714 = 1.785714.i_c = 1.696 / 1.785714 ≈ 0.9496 ≈ 0.95. Verification / Alternative check: Check reasonableness: For typical sands with e around 0.7–0.9 and Gs near 2.65, i_c usually lies near 1.0; the computed value 0.95 fits expectations. Why Other Options Are Wrong: 0.91–0.94 underestimate the ratio slightly given the stated porosity and Gs; 0.95 best matches the computed i_c. Common Pitfalls: Using n instead of e directly in the denominator; forgetting to convert percent to fraction; confusing Gs with unit weight ratio γ_s/γ_w (they are consistent here since γ_w is the reference). Final Answer: 0.95

Question 1

Soil phase relationships: What is the fundamental equation linking air content (ac), degree of saturation (Sr), and void ratio (e) for a soil mass? (Note: air content ac is defined as Va / Vv, where Va is volume of air and Vv is volume of voids.)

Accepted Answer

Introduction / Context: This question checks core soil mechanics terminology. Air content (ac), degree of saturation (Sr), and void ratio (e) are often used together in phase-relationship problems. Knowing how they relate prevents confusion when switching between porosity-based and saturation-based descriptions of a soil’s state. Given Data / Assumptions: Air content ac = Va / Vv (fraction of void space occupied by air). Degree of saturation Sr = Vw / Vv (fraction of void space occupied by water). Void ratio e = Vv / Vs (voids to solids). It is not needed to determine ac once Sr is known, but is commonly present in phase relations. Concept / Approach: Within the void space, the air volume and water volume together equal total void volume. Hence the fractions must sum to 1. Therefore, ac + Sr = 1, which immediately gives ac = 1 − Sr. Although e is a common variable, it cancels out in this particular relation because both ac and Sr are referenced to Vv, not to total volume or solids volume. Step-by-Step Solution: Start with Vv = Va + Vw.Divide by Vv: 1 = Va/Vv + Vw/Vv.Recognize Va/Vv = ac and Vw/Vv = Sr.Therefore, ac + Sr = 1 ⇒ ac = 1 − Sr. Verification / Alternative check: If the soil is fully saturated, Sr = 1 and ac = 0, which is physically correct because no air remains in the voids. If Sr = 0.7, ac must be 0.3, confirming the complementarity. Why Other Options Are Wrong: ac = e(1 − Sr): Adds an unnecessary factor e; ac is defined on void space itself, so e is irrelevant here. ac = Sr / e or ac = Sr + e: Mixes definitions with inconsistent denominators. ac = (1 + e) / Sr: Dimensionally and conceptually incorrect. Common Pitfalls: Confusing air content with percentage air voids (na = Va/V), which is referenced to total volume and does involve e. Air content is strictly within void space, so it is simply the complement of Sr. Final Answer: ac = 1 − Sr

Question 2

Compaction testing concepts (Proctor): Identify the correct statements about compaction curves, air-void lines, hammer specifications, and drop height, and choose the best combined answer.

Accepted Answer

Introduction / Context: Laboratory compaction (Standard/Modified Proctor) establishes the relationship between dry density and water content to find the Optimum Moisture Content (OMC). Understanding the shape of the curve, the meaning of the air-voids line, and the standard hammer specifications is essential for correct interpretation. Given Data / Assumptions: A: OMC corresponds to the peak of the dry density–water content curve. B: An air-void line shows combinations of water content and dry density for a constant percentage of air voids. C: Hammer weight stated as 25 kg (this is incorrect for standard tests). D: Standard Proctor drop height is 30.5 cm (305 mm). Concept / Approach: Standard Proctor uses a 2.5 kg rammer with 305 mm drop; Modified Proctor uses a 4.5 kg rammer with 457 mm drop. Therefore, 25 kg is not a standard Proctor hammer weight. The OMC definition and the concept of the air-void line are correct, and 30.5 cm drop matches Standard Proctor. Step-by-Step Solution: Confirm A: Peak of curve = OMC → correct.Confirm B: Constant air-voids line = locus at fixed percentage of air voids → correct.Check C: 25 kg rammer → not a recognized Proctor standard → incorrect.Check D: Free fall 30.5 cm → matches Standard Proctor → correct. Verification / Alternative check: Refer to standard specifications (e.g., ASTM D698 for Standard Proctor): rammer 2.5 kg, drop 305 mm; ASTM D1557 for Modified Proctor: rammer 4.5 kg, drop 457 mm. Why Other Options Are Wrong: All A–D correct: false because C is incorrect. Other subsets exclude one or more correct statements (A, B, D), so they are incomplete. Common Pitfalls: Confusing Standard with Modified Proctor; or thinking the air-void line is the zero air-voids line—these are different concepts. Final Answer: Only A, B and D are correct

Question 3

Negative skin friction on piles: Which of the following statements about negative skin friction (downdrag) is/are correct?

Accepted Answer

Introduction / Context: Negative skin friction (downdrag) occurs when the surrounding soil moves downward relative to the pile shaft. The interface shear then acts downward, opposite to positive shaft resistance, effectively increasing the load the pile must carry and reducing available capacity for superstructure loads. Given Data / Assumptions: Soft, compressible clays or recently placed fills that consolidate under their own weight. Groundwater changes or surcharge that trigger settlement of the surrounding soil mass. Pile installed to a deeper, relatively stable stratum. Concept / Approach: When soil settles relative to the pile, the shear stress mobilized at the pile–soil interface acts downward. This is termed negative skin friction and contributes an additional downdrag load. It must be added to the structural load when checking axial capacity and settlement. The design may include bitumen sleeves, coatings, or ground improvement to reduce downdrag. Step-by-Step Solution: Identify scenarios: consolidating soft clays, new embankments, or drawdown.Recognize that relative downward movement of soil induces downward shear on the shaft.Account for downdrag load in design as an additional action.Verify capacity and settlement with both shaft and base resistances after including downdrag. Verification / Alternative check: Load test data often show larger apparent loads under the same jack pressure when negative skin friction is present due to the extra drag along the shaft. Why Other Options Are Wrong: Each of A, B, and C correctly describes negative skin friction; hence “All of the above” is the most accurate single choice. Common Pitfalls: Ignoring seasonal groundwater changes; assuming downdrag is negligible for bored piles—this depends on soil and construction sequence. Final Answer: All of the above

Question 4

Soil behavior under load — recovery: A soil specimen changes shape and volume under an applied load but immediately and completely recovers both shape and volume upon unloading. This property is called:

Accepted Answer

Introduction / Context: Engineering design relies on understanding whether soil deformations are recoverable or permanent. Terms like elasticity, plasticity, compressibility, and resilience have specific meanings and consequences for settlement prediction and cyclic loading response. Given Data / Assumptions: Under load, the soil deforms (shape and volume change). Upon load removal, the soil recovers fully and immediately. No time-dependent drainage effects are implied in the description. Concept / Approach: Elasticity means strains are recoverable upon unloading and the stress–strain relation is reversible (within the elastic limit). Compressibility describes the propensity to decrease in volume under load but does not imply recovery. Resilience is the elastic energy stored per unit volume (area under the elastic portion of the curve) and is not itself the descriptive property for full recovery. Plasticity implies permanent deformation after unloading. Step-by-Step Solution: Match behavior: full, immediate recovery → elastic.Exclude: compressibility (volume decrease without implying recovery).Exclude: resilience (energy measure, not the deformation mode).Exclude: plasticity (permanent set remains after unloading). Verification / Alternative check: In a typical unloading–reloading path on an elastic material, the curve retraces, confirming full recovery. Soils often show partial elasticity within small strains; the description here fits ideal elasticity. Why Other Options Are Wrong: They either describe different concepts (energy storage, permanent deformation) or are incomplete. Common Pitfalls: Confusing “elastic recovery” with “resilient modulus” used in pavement design; resilience is a parameter derived from elastic behavior, not the behavior itself. Final Answer: Elasticity of soils

Question 5

Particle-size limits: According to standard soil classification, the maximum size for particles classified as clay is:

Accepted Answer

Introduction / Context: Soil classification systems (such as IS, BS, and ASTM) define particle-size boundaries for clays, silts, sands, and gravels. These thresholds underpin laboratory testing, specification compliance, and interpretation of index properties like plasticity and activity. Given Data / Assumptions: “Clay-size” refers to particle-size limits used in standard classifications. We are concerned with the upper boundary for clay-sized particles. Concept / Approach: Clay-size particles are those finer than 0.002 mm. Particles between about 0.002 mm and 0.075 mm are typically silt and fine sand ranges depending on the standard; but for the clay threshold the widely adopted upper limit is 0.002 mm. Step-by-Step Solution: Recall standard limit: clay-size < 0.002 mm.Compare with options: only 0.002 mm matches the recognized boundary.Select 0.002 mm as the maximum size for clay-size fraction. Verification / Alternative check: Laboratory hydrometer analysis and pipette tests reference this 0.002 mm boundary for settling calculations based on Stokes’ law. Why Other Options Are Wrong: Values 0.04–1.0 mm lie in silt/sand ranges; they exceed the clay threshold. Common Pitfalls: Confusing mineralogy (e.g., “clay minerals”) with particle-size terms (“clay-size”); a soil can contain clay-size quartz that is not a clay mineral. Final Answer: 0.002 mm

Question 6

Time-dependent volume change: The compression of a saturated soil mass under a sustained static load, caused by gradual expulsion of pore water, is called:

Accepted Answer

Introduction / Context: Two different processes reduce soil volume: compaction (mechanical densification at nearly constant water content) and consolidation (time-dependent volume decrease in saturated soils due to drainage of pore water). Distinguishing them is crucial for predicting settlements of structures and embankments. Given Data / Assumptions: Saturated soil subjected to sustained static load. Drainage path allows pore water to exit over time. Result is a gradual decrease in void ratio and settlement. Concept / Approach: Consolidation is governed by Terzaghi’s one-dimensional theory in many practical cases. Effective stress increases as pore pressure dissipates: sigma′ = sigma − u. As u decreases with time, soil skeleton carries more load, leading to primary consolidation; secondary compression may follow due to creep within the soil skeleton. Step-by-Step Solution: Apply load → generate excess pore pressure in saturated soil.Excess pore pressure dissipates via drainage over time.Effective stress increases, void ratio decreases, settlement occurs.Completion of primary consolidation when excess pore pressure ~ 0. Verification / Alternative check: Oedometer tests exhibit time–deformation curves with primary consolidation and secondary compression phases, matching field settlement observations. Why Other Options Are Wrong: Compaction: rapid densification by mechanical effort in unsaturated soils. Swelling: volume increase due to wetting or unloading. Creep: time-dependent strain at approximately constant effective stress; not primarily water expulsion. Common Pitfalls: Assuming all time-dependent settlement is consolidation; in organic soils, secondary compression can dominate long-term behavior. Final Answer: Consolidation

Question 7

Air-voids line relationship: Select the correct fundamental relationship between dry unit weight (γd), specific gravity of solids (G), water content (w), and percentage air voids (na) for a soil (use γw for unit weight of water).

Accepted Answer

Introduction / Context: Compaction charts often include constant air-voids lines. A useful closed-form expression relates dry unit weight γd to water content w, specific gravity G, and the percentage of air voids na. This formula enables quick plotting and consistency checks in the laboratory and field. Given Data / Assumptions: na is in percent (%), so na/100 is the air-voids fraction. γw is the unit weight of water. G is the specific gravity of solids. w is water content on a decimal basis (use w%/100 if starting with percent). Concept / Approach: Use phase relations: e = (G * γw / γd) − 1, n = e/(1 + e) = 1 − γd/(G * γw), Sr = (w * G) / e. Percentage air voids is na = 100 * (1 − Sr) * n. Eliminating e and Sr yields the compact form: na = 100 * [1 − γd/(G * γw) − w * γd/γw]Rearranging for γd gives γd = [γw * (1 − na/100)] / (w + 1/G) This is the straight-line relation used for air-voids lines on γd–w plots. Step-by-Step Solution: Write e = G * γw / γd − 1 and n = 1 − γd/(G * γw).Use Sr = (w * G) / e to express saturation in terms of γd, w, and G.Form na = 100 * (1 − Sr) * n and simplify.Solve the linear equation for γd to obtain γd = [γw * (1 − na/100)] / (w + 1/G). Verification / Alternative check: When na = 0 (zero air voids), the formula reduces to γd(ZAV) = γw / (w + 1/G), which is the well-known zero-air-voids line. When w = 0, γd = γw * (1 − na/100) * G, consistent with dry packing at given air voids. Why Other Options Are Wrong: They either invert signs, omit G or γw properly, or fail to reduce to the zero-air-voids line at na = 0. Common Pitfalls: Forgetting to convert na from percent to fraction; using w in percent without dividing by 100; mixing units of γd and γw. Final Answer: γd = [γw * (1 − na/100)] / (w + 1/G)

Question 8

Agents of transportation and redeposition: Which of the following natural agents can transport and redeposit soils?

Accepted Answer

Introduction / Context: Soil deposits are frequently classified by their mode of transport and deposition, which controls grading, fabric, and engineering behavior. Recognizing depositional agents helps anticipate properties like density, permeability, and variability. Given Data / Assumptions: Water (fluvial, lacustrine, marine) can carry particles in suspension and bed load. Glaciers move and grind materials, producing tills and outwash. Wind (aeolian) transports fine sands and silts over long distances. Gravity yields colluvium and talus by downslope movement. Concept / Approach: Each listed agent is a recognized transporter: alluvial sands/silts (water), glacial tills (ice), dune sands and loess (wind), and colluvial slopes (gravity). The agent leaves a signature in sorting, particle shape, and layering that guides geotechnical investigation and sampling strategies. Step-by-Step Solution: Match agent to deposit: water → alluvium/delta; glacier → till/outwash; wind → dunes/loess; gravity → colluvium.Note typical traits: water-borne → better sorting; glacial tills → poorly sorted; wind-blown → very uniform fine sands/silts; gravity → angular coarse fragments.Infer likely engineering implications (e.g., density, compressibility, anisotropy).Select the combined option acknowledging all agents. Verification / Alternative check: Site reconnaissance and stratigraphic logs typically reveal such depositional signatures, confirming the correct classification. Why Other Options Are Wrong: Each single agent is correct but incomplete; the question asks which agents can transport and redeposit soils—hence all listed apply. Common Pitfalls: Assuming only water acts at meaningful scales; in arid and glaciated terrains, wind and ice dominate. Final Answer: All of the above

Question 9

Compute void ratio from mass–volume data: A moist soil sample has total mass 108 g and volume 60 cm³. Its water content is 25% and specific gravity of solids G = 2.52. Determine the void ratio e.

Accepted Answer

Introduction / Context: This numerical problem uses basic phase relationships to compute void ratio e from measured mass, volume, water content, and specific gravity. It illustrates how laboratory data translate to structure-defining parameters like e that control compressibility and permeability. Given Data / Assumptions: Total mass M = 108 g, total volume V = 60 cm³. Water content w = 25% = 0.25 (decimal). Specific gravity of solids G = 2.52; take density of water = 1 g/cm³. Concept / Approach: Water content w = Mw / Ms ⇒ M = Ms + Mw = Ms (1 + w). Solve for solids mass Ms, then compute volume of solids Vs = Ms / (G * ρw). With total volume V known, void volume is Vv = V − Vs. Finally, e = Vv / Vs. Step-by-Step Solution: Compute Ms: Ms = M / (1 + w) = 108 / 1.25 = 86.4 g.Compute Vs: Vs = Ms / (G * ρw) = 86.4 / (2.52 * 1) ≈ 34.2857 cm³.Compute Vv: Vv = V − Vs = 60 − 34.2857 ≈ 25.7143 cm³.Compute e: e = Vv / Vs ≈ 25.7143 / 34.2857 ≈ 0.75. Verification / Alternative check: Check porosity n = e / (1 + e) ≈ 0.75 / 1.75 ≈ 0.429 (about 43%), a reasonable value for a moist, medium-density fine-grained soil. Why Other Options Are Wrong: 0.55, 0.65, 0.80, 0.95: Do not match the computed ratio based on mass–volume consistency. Common Pitfalls: Using w in percent without converting to decimal; forgetting that total volume V = Vs + Vv even in partially saturated states; using G incorrectly (it multiplies ρw). Final Answer: 0.75

Question 10

Infinite slope — stability number: For an infinite slope of inclination i with soil internal friction angle φ (cohesion may be considered separately), the stability number Sa defined as c / (γ z) required for limiting equilibrium is:

Accepted Answer

Introduction / Context: For an infinite slope, the shear stress on a plane parallel to the slope and the corresponding normal stress vary linearly with depth. In a frictional–cohesive soil, cohesion provides additional resistance when the gravitational shear demand exceeds frictional resistance. The required cohesion per unit overburden is often expressed as a stability number, Sa = c / (γ z). Given Data / Assumptions: Infinite slope with inclination i. Soil has internal friction angle φ and cohesion c (to be determined for limiting equilibrium). Unit weight γ and depth to the plane of interest z. Dry or drained condition with no pore pressure; seepage effects are neglected here. Concept / Approach: On a plane at depth z parallel to the slope: shear stress τ = γ z sin i cos i and normal stress σ = γ z cos^2 i. Frictional resistance available = σ tan φ = γ z cos^2 i tan φ. The cohesion required to exactly satisfy equilibrium at limiting state must balance the excess of τ over frictional resistance: c_required = τ − σ tan φ = γ z [sin i cos i − tan φ cos^2 i]= γ z cos^2 i [tan i − tan φ] Step-by-Step Solution: Compute τ = γ z sin i cos i.Compute σ tan φ = γ z cos^2 i tan φ.Take difference to find c_required.Divide by γ z to obtain Sa = c / (γ z) = (tan i − tan φ) cos^2 i. Verification / Alternative check: If i = φ (pure friction slope at limiting state), Sa = 0, which is consistent: no cohesion is needed. If i < φ, Sa is negative (no cohesion required), again consistent with stability. Why Other Options Are Wrong: Forms with sin^2 i do not arise from τ − σ tan φ when using τ = γ z sin i cos i and σ = γ z cos^2 i. tan i / tan φ is the inverse of a friction-only factor of safety, not the stability number defined here. Common Pitfalls: Confusing friction-only factor of safety FS = (tan φ / tan i) with cohesion requirement; neglecting pore pressure which would modify σ and hence Sa. Final Answer: (tan i − tan φ) cos^2 i

Question 11

Quick Sand (Boiling) Condition – Identify the Correct Statements In geotechnical engineering practice, the term “quicksand” (a boiling condition under upward seepage) refers to a phenomenon in cohesionless soils when the effective stress becomes zer…

Accepted Answer

Introduction / Context: “Quicksand” is not a special type of sand; it is a hydraulic condition that develops in cohesionless soils under upward seepage when the seepage force balances the submerged unit weight and the effective stress approaches zero. Understanding when this occurs is crucial for safe dewatering, excavation below the water table, and evaluating piping/boiling risks near cutoffs or sheet piles. Given Data / Assumptions: We are dealing with saturated cohesionless soils. Upward seepage exists (e.g., under a hydraulic gradient beneath a structure). Critical condition occurs when effective stress → 0 at some point. Concept / Approach: The critical hydraulic gradient i_c is approximated by i_c = (Gs − 1) / (1 + e), which depends on the specific gravity of solids Gs and void ratio e (or porosity n). When i ≥ i_c, seepage force equals the submerged weight; the soil skeleton loses intergranular contact, leading to a “boiling” appearance and loss of shear strength in sands/silts. Cohesive soils do not exhibit classic quick conditions because their undrained shear strength resists boiling. Step-by-Step Solution: Recognize that quick condition is an effective stress issue: σ′ = σ − u → 0 under upward seepage.Note that i_c depends on packing (e) and mineral density (Gs), not on grain size alone.Finer cohesionless soils (fine sand/silt) are more susceptible due to lower permeability and higher upward gradients under the same head loss.Thus each individual statement aligns with theory; collectively, 'All of the above' is correct. Verification / Alternative check: Design checks against piping at exits of dams/cutoffs use exit-gradient limits consistent with the same i_c concept. Field boiling tests near excavations confirm the dependence on Gs and e. Why Other Options Are Wrong: Each of (a)–(d) is correct; choosing any single one ignores the rest, so only (e) fully captures the truth. Common Pitfalls: Assuming quicksand is a soil type; overlooking that clays rarely “boil” like sands; forgetting that upward gradients from improper well-point spacing can trigger boiling. Final Answer: All of the above

Question 12

Critical Hydraulic Gradient for Quicksand – Compute Using Gs and Porosity For a sand with specific gravity of soil solids Gs = 2.696 and porosity n = 44% (loose state), determine the critical hydraulic gradient i_c for the onset of quick (boiling) c…

Accepted Answer

Introduction / Context: The critical hydraulic gradient i_c marks the threshold for boiling (quicksand) in cohesionless soils under upward seepage. Designers use i_c to check against piping at exits beneath hydraulic structures and to plan safe dewatering around excavations below the water table. Given Data / Assumptions: Specific gravity of solids Gs = 2.696. Porosity n = 44% = 0.44. Water unit weight is reference; formula based on submerged weight balance. Concept / Approach: For upward seepage, the critical gradient is i_c = (Gs − 1) / (1 + e), where e is void ratio. Convert porosity to void ratio by e = n / (1 − n). If the applied gradient equals or exceeds i_c, the seepage force cancels the submerged unit weight, making σ′ → 0 (boiling condition). Step-by-Step Solution: Compute void ratio: e = 0.44 / 0.56 = 0.785714.Compute numerator: Gs − 1 = 2.696 − 1 = 1.696.Compute denominator: 1 + e = 1 + 0.785714 = 1.785714.i_c = 1.696 / 1.785714 ≈ 0.9496 ≈ 0.95. Verification / Alternative check: Check reasonableness: For typical sands with e around 0.7–0.9 and Gs near 2.65, i_c usually lies near 1.0; the computed value 0.95 fits expectations. Why Other Options Are Wrong: 0.91–0.94 underestimate the ratio slightly given the stated porosity and Gs; 0.95 best matches the computed i_c. Common Pitfalls: Using n instead of e directly in the denominator; forgetting to convert percent to fraction; confusing Gs with unit weight ratio γ_s/γ_w (they are consistent here since γ_w is the reference). Final Answer: 0.95

Question 13

Darcy’s Law – Conditions for Validity in Seepage Through Soils Darcy’s law for laminar flow through porous media is applicable provided the soil and flow satisfy which conditions?

Accepted Answer

Introduction / Context: Darcy’s law (q = k * i * A) is fundamental for analyzing seepage, filter design, and dewatering. However, it holds under specific conditions related to soil fabric and flow regime. Recognizing these ensures correct application and interpretation of permeability tests and flow nets. Given Data / Assumptions: Steady, laminar flow through a saturated porous medium. Constant fluid properties over the domain. Representative elementary volume concept is valid. Concept / Approach: Darcy’s law presumes linear proportionality between seepage velocity and hydraulic gradient. This linearity is typically satisfied when the medium is homogeneous and isotropic at the scale of interest, and when the soil skeleton does not deform significantly (incompressible framework) under the hydraulic load. At high gradients in very coarse materials, non-Darcian (Forchheimer) behavior can arise. Step-by-Step Solution: Identify soil conditions supporting linear q–i relation: homogeneity and isotropy.Ensure structural changes are negligible: incompressibility assumption.Conclude that all three listed conditions collectively justify Darcy’s applicability. Verification / Alternative check: Laboratory constant-head and falling-head tests demonstrate linear head loss with discharge within laminar regime; deviations occur in gravels or cracked clays where assumptions fail. Why Other Options Are Wrong: Any single condition alone is insufficient; the combination in (d) is the safest practical statement. Option (e) contradicts established seepage theory. Common Pitfalls: Applying Darcy’s law to turbulent flows in rockfill or to anisotropic stratified clays without directional permeability corrections. Final Answer: All of the above

Question 14

Principal Stresses – Plane of Maximum Normal Stress in a Loaded Soil Element At a given point within a loaded soil mass, on which principal plane is the maximum normal (principal) stress acting?

Accepted Answer

Introduction / Context: Understanding principal stresses and planes is essential for interpreting failure criteria, earth pressure states, and stress paths. Principal planes are mutually orthogonal planes on which shear stress is zero and normal stress is extreme (max, min, intermediate). Given Data / Assumptions: Continuum mechanics applies; stress tensor symmetric. Three principal stresses σ1 ≥ σ2 ≥ σ3 and corresponding principal planes exist. Concept / Approach: The major principal plane is the plane on which the major principal stress σ1 acts normal to the plane, with zero shear. By definition, σ1 is the maximum normal stress at the point. Hence, the maximum normal stress acts on the major principal plane, not on the intermediate or minor ones. Step-by-Step Solution: Define principal stresses σ1, σ2, σ3 and associate them with their orthogonal planes.Recall: shear stress on principal planes is zero; normal stress equals the corresponding principal value.Therefore the plane carrying the maximum normal stress is the major principal plane. Verification / Alternative check: Mohr’s circle shows σ1 at the far right; the corresponding plane orientation yields zero shear and maximum normal stress, confirming the selection. Why Other Options Are Wrong: Minor and intermediate principal planes carry σ3 and σ2 respectively, which are lower than σ1. Equal normal stress on all planes occurs only under hydrostatic conditions, not general loading. Common Pitfalls: Confusing “plane of σ1” with “direction of σ1”; mixing up shear-free condition with failure plane orientation. Final Answer: Major principal plane

Question 15

Terzaghi’s Bearing Capacity – Key Assumptions in Classical Analysis Which of the following are assumptions in Terzaghi’s classical bearing capacity theory for shallow foundations on soil?

Accepted Answer

Introduction / Context: Terzaghi’s bearing capacity theory remains a cornerstone for preliminary design of shallow footings. It uses an idealized soil model and failure mechanism to derive ultimate bearing capacity in terms of c, φ, and γ with bearing capacity factors N_c, N_q, and N_γ. Given Data / Assumptions: Shallow, strip footing on weighty soil mass. Soil treated as homogeneous and isotropic near the footing. Failure mechanism with defined plastic and elastic regions. Concept / Approach: In Terzaghi’s mechanism, a central active wedge under the footing is bounded by planes at angles ψ = φ from the horizontal, flanked by passive Rankine zones, with radial shear zones outside. The failure surface is assumed not to extend above the footing base. These idealizations enable closed-form expressions for ultimate capacity. While actual soils may deviate, the theory provides practical baseline values modified by shape, depth, and load inclination factors. Step-by-Step Solution: Match each statement with Terzaghi’s mechanism and assumptions.(a) Homogeneous, isotropic soil – standard simplification.(b) Straight boundaries at ψ = φ for the elastic region; plastic zones assumed fully developed at failure.(c) Failure region limited below the base plane – a modeling assumption of the original derivation.Therefore, (d) All of the above is correct. Verification / Alternative check: Modern formulations (e.g., Meyerhof, Hansen, Vesic) adjust Terzaghi’s assumptions but keep the core mechanism concept, confirming the statements’ relevance to the classical approach. Why Other Options Are Wrong: Choosing any single item omits other essential assumptions; 'None of the above' is incorrect. Common Pitfalls: Applying Terzaghi’s theory without correction factors; ignoring groundwater and footing shape effects. Final Answer: All of the above

Question 16

Index Parameters – Find Void Ratio from a_v and m_v Relationship If the coefficient of compressibility a_v = 6.75 × 10^-2 and the coefficient of volume change m_v = 3 × 10^-2 (in consistent units), determine the void ratio e of the soil sample.

Accepted Answer

Introduction / Context: The small-strain compressibility of saturated soils in one-dimensional loading is commonly expressed using two related parameters: the coefficient of compressibility a_v and the coefficient of volume change m_v. Their relationship involves the void ratio e and allows back-calculation of e from laboratory test data when direct measurement is unavailable or needs checking. Given Data / Assumptions: a_v = 6.75 × 10^-2 (units consistent with stress increment). m_v = 3 × 10^-2 (same unit system). Small strain, saturated conditions; standard one-dimensional framework. Concept / Approach: The definitions are: a_v = Δe / Δσ′ and m_v = (Δe / (1 + e)) / Δσ′. Eliminating Δe / Δσ′ gives the relation m_v = a_v / (1 + e). Hence (1 + e) = a_v / m_v and e = (a_v / m_v) − 1. Step-by-Step Solution: Compute the ratio a_v / m_v = (6.75 × 10^-2) / (3 × 10^-2) = 0.0675 / 0.03 = 2.25.Therefore 1 + e = 2.25.Hence e = 2.25 − 1 = 1.25. Verification / Alternative check: Dimensional consistency is maintained because a_v and m_v share the same stress denominator; the ratio is dimensionless and equals (1 + e) as derived from definitions. Why Other Options Are Wrong: 1.10–1.20 underestimate 1 + e; 1.30 slightly overestimates; only 1.25 satisfies the exact ratio. Common Pitfalls: Reversing a_v and m_v in the relation; mixing units; attempting to use compression index Cc (a different parameter) in place of a_v. Final Answer: 1.25

Question 17

Rankine Passive Earth Pressure – Functional Dependence on φ For a horizontal backfill (no wall friction), the passive earth pressure coefficient Kp under Rankine theory varies with the angle of internal friction φ as:

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Introduction / Context: Earth pressure coefficients link in-situ stress states to backfill friction angle under limiting equilibrium. For Rankine conditions (no wall friction, planar ground), passive pressure corresponds to the largest lateral resistance mobilized by compressing the soil against the wall. Given Data / Assumptions: Planar ground surface, no surcharge. Wall friction neglected (Rankine assumption). Soil is homogeneous and isotropic with internal friction angle φ. Concept / Approach: Rankine’s coefficients are K_a = tan^2(45° − φ/2) for active, and K_p = tan^2(45° + φ/2) for passive. These follow from Mohr–Coulomb stress circles at limiting states relative to the principal stress directions. Step-by-Step Solution: Recall: Kp must increase rapidly with φ because higher friction gives more lateral resistance.Identify the correct functional form: Kp = tan^2(45° + φ/2).Hence choose the option with tan^2(45° + φ/2). Verification / Alternative check: For φ = 0°, Kp = tan^2(45°) = 1, which is consistent; for φ = 30°, Kp = tan^2(60°) = 3, a standard textbook value. Why Other Options Are Wrong: (a) and (c) correspond to K_a (active), not K_p. (b) uses φ instead of φ/2. (e) is not the Rankine expression. Common Pitfalls: Mixing active and passive forms; forgetting the half-angle; misreading units (degrees vs radians when coding calculators). Final Answer: Kp ∝ tan^2(45° + φ/2)

Question 18

Flow Net Applications – What Can Be Determined from a Properly Drawn Net? A correctly constructed flow net for seepage through a homogeneous, isotropic soil can be used to determine which of the following quantities?

Accepted Answer

Introduction / Context: Flow nets graphically solve Laplace’s equation for two-dimensional, steady, incompressible flow through porous media. They are extensively used in the analysis of earth dams, sheet piles, and cutoff walls to estimate uplift, piping potential, and seepage quantities. Given Data / Assumptions: Soil is homogeneous and isotropic in the plane of analysis. Steady-state flow; boundary conditions defined by impervious and constant-head lines. Flow net composed of orthogonal flow lines and equipotentials forming curvilinear squares. Concept / Approach: Once the net is drawn, the discharge per unit thickness is q = k * (N_f / N_d) * Δh, where N_f is number of flow channels and N_d is number of potential drops. Pore pressure at any point follows from the potential head at that equipotential. Seepage pressure equals γ_w times the head loss per distance along the flow line. The exit gradient is the head loss across the last potential drop divided by the distance to the outlet, read from the net geometry. Step-by-Step Solution: Use the net to count N_f and N_d → compute discharge.Read potential at a point → compute pore pressure u = γ_w * h.Determine hydraulic gradient locally along a flow line → seepage pressure = γ_w * i.Assess gradient at downstream exit → compare with critical gradient to check piping. Verification / Alternative check: Comparisons with finite element seepage models show good agreement when the net satisfies curvilinear-square quality. Why Other Options Are Wrong: Each item is a legitimate outcome of a valid net; omitting any would be incomplete, so 'All of the above' is correct. Common Pitfalls: Drawing distorted elements that are not near squares; miscounting flow channels or potential drops; applying the method to anisotropic soils without transforming axes. Final Answer: All of the above

Question 19

Grain-Size Parameters – Acceptable Range of Coefficient of Curvature (C_c) for Well-Graded Soils For a soil to be considered well graded (in addition to meeting the uniformity requirement), the coefficient of curvature C_c should typically fall with…

Accepted Answer

Introduction / Context: Grain-size distribution parameters help classify soils for filter design, compaction, and drainage. Besides the uniformity coefficient C_u, the coefficient of curvature C_c gauges the shape of the gradation curve near the median size and helps identify well-graded materials. Given Data / Assumptions: C_c is defined as C_c = (D30^2) / (D10 * D60). Conventional thresholds: well-graded sands/gravels require both adequate C_u and appropriate C_c range. Concept / Approach: Standard practice considers a soil well graded if its coefficient of curvature lies roughly between 1 and 3 along with sufficient uniformity (e.g., C_u > ~6 for sands, > ~4 for gravels, depending on the standard). Values far outside this range often indicate humps or gaps in the gradation curve (poor gradation or gap gradation). Step-by-Step Solution: Recall formula for C_c and interpret physically as curvature near median size.Apply the typical acceptance band: 1.0 ≤ C_c ≤ 3.0.Hence, select the range '1.0 to 3.0'. Verification / Alternative check: Classification manuals and filter design guides consistently use this band in conjunction with a suitable C_u threshold. Why Other Options Are Wrong: Ranges below 1 or above 3 usually reflect kinked or gap-graded curves; very high values suggest an overly pronounced hump. Common Pitfalls: Judging well-graded solely on C_c without checking C_u; misreading D-sizes from the cumulative percent passing chart. Final Answer: 1.0 to 3.0

Question 20

Vane Shear Test – In-situ Measurement of Undrained Strength The field vane shear test is primarily used for in-situ determination of the undrained shear strength of which intact, fully saturated soil type?

Accepted Answer

Introduction / Context: The vane shear test provides a quick in-situ measure of undrained shear strength without the need for large sampling disturbance. It is especially useful for soft to medium clays where sampling for laboratory shear tests may be problematic or unrepresentative. Given Data / Assumptions: Undrained loading conditions apply (short-term behavior). Soil is intact (not remolded before testing) and fully saturated. Field apparatus consists of a cruciform vane pushed and rotated in the ground. Concept / Approach: In clays, undrained shear strength governs short-term stability. The vane test mobilizes soil resistance on cylindrical surfaces formed during rotation and correlates torque to undrained strength. Cohesionless soils like sands and gravels cannot be reliably tested by vanes in situ because they lack stability under undrained torque—grains rearrange and drainage prevents undrained conditions. Step-by-Step Solution: Identify soil types amenable to undrained, torque-based measurement → clays.Recognize limitations: sands/gravels drain rapidly and do not maintain a coherent undrained response.Select 'Clays' as the correct application. Verification / Alternative check: Field correlations between vane strength and unconfined compressive strength of cohesive soils support the method; repeat tests (remolded strength) also yield sensitivity estimates. Why Other Options Are Wrong: Sands and gravels require different in-situ tests (e.g., CPT, SPT). Highly organic soils may give erratic vane results due to fibrous structure and anisotropy. Common Pitfalls: Testing fissured or highly anisotropic clays without correction; ignoring rate effects or temperature dependence on torque readings. Final Answer: Clays

Soil Mechanics and Foundation Engineering Questions