Question 1

Sensitivity of clays (St): ratio of undisturbed to remoulded undrained strength Sensitivity St is defined as St = cu(undisturbed) / cu(remoulded) at the same water content. What value best represents a “quick clay” from the given choices?

Accepted Answer

Introduction / Context: Sensitivity quantifies the loss of undrained shear strength when clay is remoulded at constant water content. High sensitivities indicate severe structure/ bonding that collapses upon disturbance. “Quick clays” are famous for dramatic strength loss leading to flow slides. Given Data / Assumptions: St = cu(undisturbed) / cu(remoulded) at same w. Typical classification (indicative): low < 4, medium 4–8, high 8–16, very high 16–32, quick >≈ 30 (many references). Choices provided are discrete numbers. Concept / Approach: Quick clays exhibit very high to extremely high sensitivity, often > 16 and typically > 30. Among the given options, 20 lies in the “very high” range and is the best representative of a quick-clay behavior compared with smaller offered values. Step-by-Step Solution: Identify the classification ranges from common practice.Map offered choices to the closest applicable range.Select 20 as most consistent with quick-clay characteristics among given options. Verification / Alternative check: Well-documented Nordic quick clays often show St > 30, but when constrained to the listed numbers, 20 best captures the “very high/quick” behavior as opposed to 4–16 which are lower categories. Why Other Options Are Wrong: 4, 8, 12, 16 correspond to low–high sensitivity, insufficient to typify quick clays. 20 is closer to the expected extreme sensitivity. Common Pitfalls: Assuming a single universal threshold; ignoring mineralogy and structure; not ensuring water content equality when measuring St. Final Answer: 20

Question 2

Shear strength of cohesionless soils (sand): dominant influencing factor For a dense, cohesionless soil (c ≈ 0), which listed factor most directly governs its shearing strength under drained loading?

Accepted Answer

Introduction / Context: Cohesionless soils such as sands mobilize shear resistance primarily through friction and interlocking. Understanding what truly controls their shear strength is fundamental for foundation bearing capacity, lateral earth pressure, and slope stability analyses in drained conditions. Given Data / Assumptions: C ≈ 0; strength arises from friction angle φ and normal effective stress σ′. Drained loading is considered; pore pressure dissipation occurs. Dilatancy can modify peak strength but still scales with σ′. Concept / Approach: Mohr–Coulomb for cohesionless soils: τ = σ′ * tan φ. The mobilized shear resistance is directly proportional to the effective normal stress on the plane. While density affects φ_peak via dilatancy and fabric, the most immediate variable in the strength expression is σ′ (confining pressure). Step-by-Step Solution: Express strength: τ = σ′ tan φ.Recognize σ′ is the confining/normal effective stress.Therefore, confining pressure most directly controls shear resistance at a given φ. Verification / Alternative check: Triaxial/drained direct shear tests show higher σ′ produces proportionally higher peak shear stresses for a fixed φ. Why Other Options Are Wrong: Dry density influences φ but indirectly; rate of loading affects drainage in clays rather than drained sand strength; “nature of loading only” is incomplete; color is irrelevant. Common Pitfalls: Confusing density effects (on φ) with the governing role of σ′; applying undrained logic from clays to sands. Final Answer: Confining (normal effective) pressure

Question 3

Fundamental hydraulic property of soils The property of a soil that permits water to flow or percolate through its pore network under a hydraulic gradient is called:

Accepted Answer

Introduction / Context: Flow of water through soils governs settlement rates, consolidation, dewatering, and stability. The material parameter that describes the ease of flow through a saturated soil is central to seepage analysis and filter design. Given Data / Assumptions: Consider Darcy’s law conditions (laminar flow). Fully saturated soil matrix unless otherwise noted. Hydraulic gradient drives flow. Concept / Approach: Permeability (hydraulic conductivity, k) characterizes how readily water passes through soils. It depends on grain size, void ratio, soil fabric, and fluid properties. Moisture content is simply the mass ratio of water to dry soil; capillarity is an unsaturated effect causing upward water movement, not the bulk flow property. Step-by-Step Solution: Recall Darcy’s law: q = k * i * A.Identify the material constant: k = permeability.Relate k to soil gradation and structure. Verification / Alternative check: Laboratory tests (constant or falling head) measure k directly, confirming it is the parameter of interest for percolation. Why Other Options Are Wrong: Moisture content is a state parameter, not a flow property; capillarity is a suction-driven rise phenomenon; “none” is incorrect; shrinkage limit is an Atterberg limit for clays. Common Pitfalls: Equating capillary rise height with permeability; ignoring temperature dependence of k through fluid viscosity. Final Answer: Permeability

Question 4

Degree of saturation from bulk density, water content, and Gs A soil has mass specific gravity (bulk density relative to water) ρ/ρw = 1.92 and moisture content w = 30%. If the specific gravity of solids Gs = 2.75, compute the degree of saturation S…

Accepted Answer

Introduction / Context: Phase relations in soils connect densities, water content, void ratio, and degree of saturation. Given bulk density, w, and Gs, we can find S by combining standard formulas—useful in compaction control and assessing near-saturation field conditions. Given Data / Assumptions: Bulk density ratio: ρ/ρw = 1.92 (so ρ = 1.92 g/cm³). Moisture content w = 30% = 0.30. Specific gravity Gs = 2.75. ρw = 1.0 g/cm³. Concept / Approach: Use the relationships: ρd = ρ / (1 + w) ρd = (Gs * ρw) / (1 + e) ⇒ e = (Gs * ρw / ρd) − 1 S = (w * Gs) / e These follow from definitions of water content and degree of saturation (no LaTeX used). Step-by-Step Solution: Compute dry density: ρd = 1.92 / 1.30 = 1.476923 g/cm³.Find void ratio: e = (2.75 / 1.476923) − 1 = 0.86198.Compute S: S = (0.30 * 2.75) / 0.86198 = 0.957 ≈ 95.7%. Verification / Alternative check: Cross-check using bulk formula ρ = ρw * (Gs + S*e) / (1 + e) with S ≈ 0.957 gives ρ ≈ 1.92 g/cm³, confirming consistency. Why Other Options Are Wrong: 95.4–95.6% are near-misses from rounding; 94.0% is too low for the given high density and w. Common Pitfalls: Confusing mass and unit weight; using w in percent rather than fraction; arithmetic errors when computing e. Final Answer: 95.7%

Question 5

Where is gauge (atmospheric) pressure zero in groundwater terms? Identify the location(s) at which the pore water pressure equals atmospheric pressure (i.e., zero gauge pressure) in hydrogeologic terminology.

Accepted Answer

Introduction / Context: Understanding the reference surfaces for pore water pressure is essential in seepage, slope stability, and earth pressure problems. The terms “water table” and “phreatic surface” are often used to describe where pore water pressure equals atmospheric pressure. Given Data / Assumptions: Static groundwater conditions for definition. Gauge pressure is measured relative to atmospheric pressure. Terminology from soil mechanics and hydrogeology is used. Concept / Approach: The water table is the top surface of the saturated zone where pore water pressure equals atmospheric pressure (u = 0 gauge). “Phreatic surface” is a synonymous term for this free surface in seepage flow fields. Below this surface, u > 0 (positive), and above it, pores may be partially saturated with negative pore pressures (suction). Step-by-Step Solution: Define water table: free surface where u = 0 gauge.Define phreatic surface: free surface (seepage line) where u = 0 gauge.Conclude both terms identify the same zero-gauge condition. Verification / Alternative check: Seepage net construction marks the phreatic line as the zero-pressure line; piezometers terminating at this surface read atmospheric pressure. Why Other Options Are Wrong: Sea level/mean sea level are elevation datums, not inherently locations of u = 0 in soil pores unless they coincide with the groundwater surface in situ. Common Pitfalls: Confusing hydraulic head datum with pressure boundary; assuming zero gauge pressure at arbitrary elevations. Final Answer: Both the water table and the phreatic surface

Question 6

General behavior of cohesive soils Cohesive soils (e.g., clays) typically exhibit which combination of mechanical characteristics under normal engineering conditions?

Accepted Answer

Introduction / Context: Classifying soils by mechanical behavior guides selection of tests and design methods. Cohesive soils (clays and plastic silts) are distinct from sands in their plasticity and compressibility due to plate-like particles, interparticle forces, and water chemistry effects. Given Data / Assumptions: Normal ranges of stresses encountered in geotechnical projects. Typical natural water contents near or above plastic limit for clays. Time-dependent behavior (consolidation) acknowledged. Concept / Approach: Clays are plastic over a range of water contents (between plastic and liquid limits), allowing them to undergo significant remoulding without cracking. They are also compressible, exhibiting primary consolidation under sustained load as pore water flows out, followed by secondary compression (creep). Elastic behavior is only a small portion of the overall response. Step-by-Step Solution: Recognize plasticity from Atterberg limits and index properties.Acknowledge compressibility through oedometer consolidation: settlement vs time.Select the option reflecting both traits. Verification / Alternative check: Laboratory tests (Atterberg limits, consolidation tests) consistently show plastic deformation ranges and significant volume change under load for cohesive soils. Why Other Options Are Wrong: “Elastic and compressible” overemphasizes elasticity; “plastic but incompressible” is contradictory to observed consolidation; “none” and “rigid/impermeable” are inconsistent with field behavior. Common Pitfalls: Assuming clays are impermeable or incompressible; ignoring time effects and drainage conditions in analysis. Final Answer: Plastic and also compressible

Question 7

Shear strength testing of saturated clays: For evaluating the shear strength parameters of a saturated clay sample in geotechnical engineering, which laboratory test is most appropriate and generally recommended?

Accepted Answer

Introduction / Context: Saturated clays exhibit drainage-dependent behavior. Selecting the correct shear test ensures that drainage, confining pressure, and stress path replicate field conditions so that the measured shear strength (cohesion intercept and friction angle) is meaningful for design of foundations, slopes, and excavations. Given Data / Assumptions: Specimen is saturated cohesive soil (clay). Engineer can control drainage and consolidation during testing. Design may require undrained (short-term) or drained (long-term) strength. Concept / Approach: The triaxial compression test is preferred because it allows control of drainage (UU, CU, CD), measurement of pore pressure (CU), and application of isotropic/anisotropic confining stress to simulate in-situ stress paths. Direct shear is simple but enforces a predetermined failure plane and provides limited control of drainage uniformity in clays. Unconfined compression is quick for saturated clays but gives only one undrained point with zero confinement and no control of drainage or pore pressure. Step-by-Step Solution: Identify the desired field condition (short-term undrained vs long-term drained).Choose triaxial mode: UU for short-term total-stress, CU with pore pressure for effective-stress undrained, CD for fully drained long-term.Benefit: obtain complete stress–strain and strength parameters with flexibility on drainage and measurement of pore pressures. Verification / Alternative check: Compare strengths from UU, CU, and CD on the same clay: trends in friction angle and cohesion are consistent with critical state and consolidation history, supporting triaxial suitability. Why Other Options Are Wrong: Direct shear: fixed failure plane, limited drainage control; useful but not generally “most appropriate.” Unconfined compression: zero confining pressure, only undrained; screening test. “All equally suitable” ignores drainage and stress-path control needs. Common Pitfalls: Using direct shear data for complex stress paths; extrapolating UC test results to confined conditions; ignoring saturation and rate effects. Final Answer: Triaxial compression test (UU/CU/CD as required)

Question 8

Oven-drying method – moisture content formula from container weights: A container weighs W1. The container with moist soil weighs W2. After oven drying, the container with dry soil weighs W3. What is the correct expression for water content w of the…

Accepted Answer

Introduction / Context: Accurate water content is foundational for phase relationships, compaction control, and strength predictions. The oven-drying method determines the mass of water removed relative to the mass of dry soil using simple weighings of a tared container before and after drying. Given Data / Assumptions: W1 = mass of empty container. W2 = mass of container + moist soil. W3 = mass of container + dry soil after oven drying (105–110 °C). No loss of solids during drying; moisture fully removed. Concept / Approach: Water content w is defined as Mw / Ms, where Mw is mass of water and Ms is mass of dry soil. Mw equals the difference between moist and dry masses; Ms equals dry mass excluding the container. Substituting measured quantities gives the standard laboratory expression. Step-by-Step Solution: Mw = (W2 − W3).Ms = (W3 − W1).Therefore, w = Mw / Ms = (W2 − W3) / (W3 − W1).If needed in percent, multiply by 100. Verification / Alternative check: Sanity check: if W2 ≈ W3 (very dry sample), numerator tends to zero and w tends to zero as expected. Why Other Options Are Wrong: Option B reverses signs producing negative or nonsensical values. Option C uses moist mass in denominator; definition requires dry mass. Option D divides by moist mass of soil plus container, not the dry mass of soil. Common Pitfalls: Using percent in calculations without converting to decimal; spillage during drying; not cooling in a desiccator leading to buoyancy/evaporation errors. Final Answer: w = (W2 − W3) / (W3 − W1)

Question 9

Consolidation history terminology: A clay deposit that has previously been subjected to an effective overburden pressure greater than its present effective overburden is termed as what?

Accepted Answer

Introduction / Context: Clays remember their stress history. When a deposit has experienced higher past effective stress than the current value, it behaves stiffer and stronger at small strains compared to normally consolidated soils. Several terms are used for this condition across literature and practice. Given Data / Assumptions: Present effective overburden is lower than the historical maximum. No cementation is implied; purely stress history. Terminology differences exist among authors. Concept / Approach: “Overconsolidated” describes soils that have been unloaded (erosion, drawdown, desiccation). “Preconsolidated” and “pre-compressed” are often used synonymously to mean the same condition. The key quantitative measure is the overconsolidation ratio OCR = sigma′max / sigma′present > 1. Step-by-Step Solution: Identify condition: sigma′max, past > sigma′present → OCR > 1.Recognize synonymous terms used in reports and codes.Conclude that all given terms refer to the same concept in common usage. Verification / Alternative check: Oedometer e–log sigma′ plots show a preconsolidation pressure where the curve breaks, confirming overconsolidation. Why Other Options Are Wrong: Choosing only one term ignores accepted synonymous usage across geotechnical texts and specifications. Common Pitfalls: Confusing overconsolidation with cementation or aging; misreading preconsolidation pressure from lab data. Final Answer: All of the above (synonymous usage in practice)

Question 10

Minimum practical depths for building foundations by soil type: Select the typical minimum depth ranges used in practice for shallow foundations on: (i) sandy soils, (ii) clayey soils, and (iii) rock.

Accepted Answer

Introduction / Context: Shallow foundation embedment must ensure bearing capacity, protection against seasonal effects (scour, frost, shrink–swell), and construction feasibility. Practical minimum depths vary with soil type and site conditions even when ultimate capacity is ample. Given Data / Assumptions: General building practice in non-frost to moderate climates. Sound rock outcrops may permit very small embedment where protection and seating are adequate. Numbers represent typical ranges rather than code-mandated values. Concept / Approach: On sands, embedment of roughly 0.8–1.0 m helps limit scour and seasonal disturbance. On clays, slightly greater depths (about 0.9–1.6 m) mitigate shrink–swell and seasonal moisture fluctuations. On competent rock, only enough embedment to seat and key the footing (5–50 cm) may be required, provided bearing surface is sound and protected. Step-by-Step Solution: Match each soil type with a practical minimum embedment range.Check rationale: seasonal stability and protection drive minimums more than ultimate capacity on strong strata.Select the option acknowledging all three ranges. Verification / Alternative check: Local codes may specify frost depth controls; where frost is significant, minimum depths exceed these general ranges. Why Other Options Are Wrong: Picking a single range ignores differing soil behaviors. Common Pitfalls: Using ultimate capacity alone to set depth; neglecting erosion or desiccation effects; ignoring local code requirements. Final Answer: All of the above

Question 11

Terzaghi bearing capacity (strip footing): Terzaghi’s general equation for ultimate bearing capacity of a continuous (strip) footing is (Nc, Nq, Nγ are bearing capacity factors, γ is unit weight, D is foundation depth, B is footing width):

Accepted Answer

Introduction / Context: Terzaghi’s classical bearing capacity theory provides closed-form expressions for the ultimate pressure a footing can impose on soil before shear failure. For strip footings, the equation uses dimensionless bearing capacity factors that depend on the friction angle of the soil. Given Data / Assumptions: Continuous (strip) footing on homogeneous soil. General shear failure assumed. Symbols: c, φ, γ, B, D, Nc, Nq, Nγ. Concept / Approach: The ultimate bearing pressure is composed of three parts: cohesion term (cNc), surcharge term (γDNq), and unit weight term (0.5γBNγ). The N-factors are functions of φ; the width term is halved for strip footings in Terzaghi’s formulation. Step-by-Step Solution: Write the general form q_u = cNc + qNq + 0.5γBNγ where q = γD.Substitute q = γD to obtain q_u = cNc + γDNq + 0.5γBNγ.Confirm signs: all additive contributions to capacity. Verification / Alternative check: For c = 0, equation reduces to granular soil case q_u = γDNq + 0.5γBNγ, matching standard charts. Why Other Options Are Wrong: Options B–D use incorrect negative signs, contradicting the physical contributions. Common Pitfalls: Confusing ultimate and net ultimate; omitting shape/depth factors for non-strip footings; misusing surcharge q. Final Answer: q_u = cNc + γDNq + 0.5γBNγ

Question 12

Concept check – identify the single incorrect definition: Which ONE of the following statements is incorrect regarding soil behavior terms?

Accepted Answer

Introduction / Context: Precise terminology avoids design errors. Sensitivity, thixotropy, liquid limit, and plastic limit are frequently used in reports and specifications. Misstatements can mislead classification and performance predictions. Given Data / Assumptions: Standard Atterberg limit definitions apply. Unconfined compression is the basis for sensitivity ratio. Thixotropy pertains to remolded clays at constant water content. Concept / Approach: Only one statement is intentionally incorrect. Liquid limit is the water content at the boundary between liquid and plastic states, not directly “liquid to solid.” The plastic limit corresponds to the boundary between plastic and semi-solid states and is practically identified using the 3 mm thread crumbling criterion. Thixotropy describes reversible strength changes over time without water content change. Step-by-Step Solution: Validate Option A: standard sensitivity definition → correct.Validate Option B: classic thixotropy description → correct.Check Option D: 3 mm thread criterion → correct.Identify Option C as incorrect because liquid limit is the liquid–plastic boundary, not liquid–solid. Verification / Alternative check: USCS and IS procedures (Casagrande cup or cone penetrometer) define liquid limit as liquid–plastic transition; shrinkage limit defines onset of solid behavior. Why Other Options Are Wrong: Only C conflicts with standard definitions; A, B, and D are consistent with accepted usage. Common Pitfalls: Confusing shrinkage limit with liquid limit; overlooking that thixotropy is reversible and isothermal. Final Answer: Liquid limit = water content at which a soil changes directly from liquid to solid state

Question 13

Primary consolidation – factors influencing time to reach a given degree of consolidation: Which statements about consolidation time are correct?

Accepted Answer

Introduction / Context: Predicting settlement rate requires understanding what controls the time for primary consolidation. Terzaghi’s theory relates time, permeability, compressibility, and drainage path length through the coefficient of consolidation cv and the time factor Tv. Given Data / Assumptions: One-dimensional consolidation; constant cv for the stress range considered. Degree of consolidation U is the target (e.g., 90%). Double or single drainage boundary conditions define drainage path length Hdr. Concept / Approach: The time to reach a given U is t = Tv * Hdr^2 / cv, with cv = k / (mv * gamma_w). Thus t increases when mv increases (more compressible) or when Hdr increases (thicker layer), and decreases when k increases. For a specified U, Tv is a constant, so t is independent of the magnitude of the load increment itself (provided cv is unchanged in that range). Step-by-Step Solution: Relate t to cv and Hdr: t ∝ Hdr^2 / cv.Express cv in terms of k and mv: cv ∝ k / mv.Infer trends for each parameter (Options A–C).Recognize load magnitude independence at constant cv (Option D). Verification / Alternative check: Oedometer curves and theoretical solutions for different Hdr and k verify the trends; dimensionally, doubling Hdr quadruples t. Why Other Options Are Wrong: Each statement matches Terzaghi’s formulation; excluding any would be incomplete. Common Pitfalls: Forgetting that cv may vary with stress level; confusing total time to final settlement (which includes secondary compression) with primary consolidation time only. Final Answer: All of the above

Question 14

Rate of compression and pore fluid: Compression (settlement) of a soil mass occurs most rapidly when the voids are filled with which pore fluid?

Accepted Answer

Introduction / Context: The rate of soil compression depends on how quickly pore fluid can leave the voids. Air is far less viscous and more compressible than water, so settlements under partially saturated conditions can occur rapidly compared with saturated conditions that require time-dependent drainage governed by permeability. Given Data / Assumptions: Same soil skeleton and stress change; only pore fluid condition varies. Air can escape readily through interconnected voids. Saturated clays require consolidation (drainage of water) for volume change. Concept / Approach: In saturated soils, primary consolidation is rate-limited by k and Hdr; the process requires excess pore-water pressure dissipation. In unsaturated or dry soils, compression involves immediate rearrangement and expulsion of air, which is fast; often the response is essentially immediate settlement (elastic + plastic) with negligible time lag. Step-by-Step Solution: Consider fluid properties: air has lower viscosity and is compressible.Recognize mechanism: air escapes quickly → rapid volume reduction.Contrast with saturated case: water drainage is slow → time-dependent settlement. Verification / Alternative check: Oedometer tests on partially saturated samples show smaller time lags; field observations after compaction in partially saturated fills confirm rapid settlement response. Why Other Options Are Wrong: Water-filled voids (Option B) require consolidation time; slower. “Partly air and partly water” may still be faster than saturated, but the most rapid occurs when air dominates (Option A). “None of these” is incorrect. Common Pitfalls: Assuming the same magnitude of settlement regardless of saturation; ignoring suction effects in partially saturated soils. Final Answer: Air (dry or partially saturated conditions with easy air escape)

Question 15

Agents of chemical weathering in soils: Which of the following processes contribute to chemical weathering of parent rock and soils?

Accepted Answer

Introduction / Context: Chemical weathering alters mineralogy and fabric of soils, impacting plasticity, strength, and durability. Recognizing the key chemical processes helps interpret soil genesis and anticipate engineering behavior such as susceptibility to collapse or swelling. Given Data / Assumptions: Parent rock contains minerals susceptible to oxidation, hydration, and dissolution. Water and gases like oxygen and carbon dioxide are present. Concept / Approach: Oxidation converts ferrous to ferric iron, generating oxides/hydroxides (reddening). Carbonation forms carbonic acid that attacks carbonates and feldspars. Hydration adds water to mineral structures (e.g., anhydrite to gypsum). Leaching dissolves and carries away soluble constituents, altering soil chemistry and structure. Step-by-Step Solution: Identify each process and its typical products in soil profiles.Recognize combined effects: weakening of bonds, increased clay content, or development of lateritic profiles.Conclude that all listed processes are agents of chemical weathering. Verification / Alternative check: Field soil horizons (e.g., B-horizon accumulations) and laboratory mineralogical tests show products of these reactions. Why Other Options Are Wrong: Limiting to one process ignores the multi-mechanism nature of chemical weathering. Common Pitfalls: Confusing chemical with mechanical weathering (freeze–thaw, abrasion); underestimating climatic control (temperature, rainfall). Final Answer: All of the above

Question 16

Degree of saturation from w, G, and e: If the water content of a soil is 40% (w = 0.40), the specific gravity of solids G = 2.70, and the void ratio e = 1.35, compute the degree of saturation Sr.

Accepted Answer

Introduction / Context: Phase-relationship identities permit rapid checks on consistency of lab data. Degree of saturation links the volumetric water content to mass-based water content via specific gravity and void ratio—crucial for compaction, seepage, and strength interpretations. Given Data / Assumptions: Water content w = 0.40 (40%). Specific gravity G = 2.70. Void ratio e = 1.35. Standard identity for partially saturated soils applies. Concept / Approach: Use the fundamental relationship w = (Sr * e) / G. Rearranging gives Sr = (w * G) / e. Insert given values and convert the decimal result to a percentage for reporting. Step-by-Step Solution: Start from w = (Sr * e) / G.Rearrange: Sr = (w * G) / e.Compute: Sr = (0.40 * 2.70) / 1.35 = 1.08 / 1.35 = 0.80.Express as percent: Sr = 0.80 * 100% = 80%. Verification / Alternative check: Check bounds: Sr must be between 0 and 1; 80% is reasonable for these values. Also, if e halves or G increases, Sr changes accordingly consistent with physics. Why Other Options Are Wrong: 70%, 75%, 85%, 90% do not satisfy the identity with the provided numbers. Common Pitfalls: Using w in percent rather than decimal inside the formula; mixing e with porosity n (n = e / (1 + e)); rounding too early. Final Answer: 80%

Question 17

Triaxial testing practice in geotechnical engineering: What is the usual length-to-diameter (L/D) ratio adopted for cylindrical soil specimens used in the triaxial compression test?

Accepted Answer

Introduction / Context: The triaxial compression test is a cornerstone of soil mechanics for determining shear strength parameters under controlled drainage and stress paths. A key aspect of good practice is specimen geometry, typically expressed as the ratio of specimen length to diameter (L/D). The chosen L/D aims to minimize boundary effects, ensure uniform stress distribution, and allow reliable interpretation of failure modes and stress–strain behavior. Given Data / Assumptions: Cylindrical soil specimens are used in triaxial tests. End platens and membranes introduce boundary influences that should be kept small. Standard laboratory practice seeks repeatability and comparability across tests. Concept / Approach: A specimen that is too short (low L/D) suffers high end-restraint effects from the platens and may exhibit nonuniform deformation. If it is excessively slender (very high L/D), buckling or bending can occur—especially for soft clays—leading to spurious results. Empirical and standards-based guidance converges on an L/D of about 2 as an optimal compromise between these competing effects. Step-by-Step Solution: Identify the need: minimize end restraint while avoiding instability.Select a geometry used widely in standards and literature.The common choice is L/D ≈ 2 for cohesive and granular soils.Thus, the adopted ratio is 2. Verification / Alternative check: Testing standards and widely used lab manuals typically recommend diameter 38–39 mm (1.5 in) with length about 76–80 mm, or diameter 50 mm with length 100 mm, both giving L/D ≈ 2. Research comparing different L/D ratios consistently shows improved uniformity and reduced platen effects near this value. Why Other Options Are Wrong: 1 or 1.5: Too short; strong end-restraint causes nonuniform stresses. 2.5 or 3: Too slender; higher risk of bending or barreling artifacts. Common Pitfalls: Ignoring trimming irregularities or membrane thickness that alter effective L/D; not correcting for area change when calculating deviator stress, especially at large strains. Final Answer: 2

Question 18

Soil transportation and deposition (geomorphology context): “Talus” refers to colluvial debris accumulated at the base of slopes. Which transporting agency is primarily responsible for talus deposits?

Accepted Answer

Introduction / Context: Geotechnical engineers classify natural soils by the processes that transported and deposited them. These processes strongly influence grading, angularity, density, and engineering behavior. “Talus” is a common field term encountered in site investigations near rock outcrops and steep slopes. Given Data / Assumptions: Talus is an accumulation of rock fragments and coarse soil at the base of a slope or cliff. Deposits are typically very angular and poorly sorted. The environment is dominantly controlled by slope gravity rather than by continuous fluid flow. Concept / Approach: Talus forms when rock or debris detaches from a slope and moves downslope under gravity, often by rockfall, sliding, or short run-out rolling. There is no requirement for running water, glacial transport, or wind to carry the material long distances; the movement is dominantly gravitational, leading to colluvial deposits characterized by angular fragments and loose packing. Step-by-Step Solution: Identify talus as slope-foot debris.Recognize the movement mechanism: gravity-driven fall and creep.Conclude the transporting agency is gravitational force. Verification / Alternative check: Geologic mapping symbols and site logs often label these as “colluvium (talus)”; boreholes reveal very coarse, angular, openwork materials consistent with gravity accumulation. Why Other Options Are Wrong: Wind: creates aeolian dunes/loess; typically finer and well sorted. Water: forms alluvium, lacustrine, or marine deposits, usually better sorted and rounded. Glacier: produces tills and outwash with distinct matrix and striated clasts. Volcanic action: leads to tephra or pyroclastic deposits, not typical talus. Common Pitfalls: Confusing talus (gravity) with scree modified by limited wash; while water may rework edges, the primary transport remains gravitational. Final Answer: Gravitational force (slope gravity)

Question 19

Compute degree of saturation from given phase data: For a soil with void ratio e = 0.67, water content w = 0.188 (18.8%), and specific gravity of solids G = 2.68, determine the degree of saturation Sr (expressed as a percentage).

Accepted Answer

Introduction / Context: Phase relationships connect the volumetric and gravimetric properties of soils. The degree of saturation Sr is a key descriptor for seepage behavior, compressibility, and compaction performance. This problem illustrates the direct use of a fundamental identity to compute Sr from e, w, and G. Given Data / Assumptions: Void ratio e = 0.67. Water content w = 0.188 (decimal form; 18.8%). Specific gravity of solids G = 2.68. Standard definitions and consistent units; ρw cancels in the identity used. Concept / Approach: The fundamental relation between degree of saturation, water content, specific gravity, and void ratio is: Sr = (w * G) / e where w is decimal water content. This follows from standard phase-relationship derivations using volumes and masses of water and solids. Step-by-Step Solution: Compute numerator: w * G = 0.188 * 2.68 = 0.50384.Divide by e: Sr = 0.50384 / 0.67 ≈ 0.752.Convert to percent: Sr ≈ 75.2% → approximately 75%. Verification / Alternative check: Using porosity n = e / (1 + e) = 0.67 / 1.67 ≈ 0.401; water ratio by volume Vw/V ≈ Sr * n ≈ 0.752 * 0.401 ≈ 0.302, consistent with a moist but not fully saturated soil. Why Other Options Are Wrong: 25%, 40%, 60%, 90%: Do not match the computed value from the fundamental identity and are inconsistent with the given w and e. Common Pitfalls: Forgetting to convert w from percent to decimal; mixing e and n; or assuming full saturation when Sr must be computed. Final Answer: 75%

Question 20

Direct shear test — limitations and drawbacks: Which of the following is a principal disadvantage of the direct shear (shear box) test for soils?

Accepted Answer

Introduction / Context: The direct shear test is widely used for rapid estimation of drained shear strength parameters, especially for sands. However, it has several inherent limitations compared with triaxial testing, which can influence the interpretation of measured shear strength and deformational characteristics. Given Data / Assumptions: A square or circular specimen is split horizontally and sheared along a predefined plane. Normal stress is applied by a vertical load system. Drainage is usually allowed (drained test). Concept / Approach: Because the shear plane is forced and the upper half moves relative to the lower half, the actual area engaged in shear reduces as displacement increases (the halves separate laterally). This changing area complicates the calculation of shear stress (τ = P/A), requiring area corrections to avoid overestimating strength. Rapid drainage and pore-pressure dissipation are typically advantages, not disadvantages, in drained testing. Step-by-Step Solution: Recognize the shear box imposes a predetermined failure plane.As horizontal displacement increases, the overlapping area reduces.Hence, the shear area A decreases, affecting τ calculations unless corrected.Therefore, the changing shear area is a key disadvantage. Verification / Alternative check: Standards often include area-correction procedures; results differ from triaxial tests where the failure plane develops naturally and stress state is better controlled. Why Other Options Are Wrong: Options A and B describe features that are generally beneficial for drained testing. Option E is incorrect; the test does apply and control normal stress. Common Pitfalls: Neglecting area correction; assuming drained results apply to undrained behavior; ignoring nonuniform stress distribution near the box boundaries. Final Answer: The plan area actually under shear changes progressively during the test

Soil Mechanics and Foundation Engineering Questions