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PE Geotechnical practice questions

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Question 1Site Characterization and Subsurface Exploration

When standard penetration test blow counts are normalized to N60, the reference hammer energy ratio is:

  1. 60 percent Correct answer.
  2. 45 percent
  3. 75 percent
  4. 55 percent

Explanation

The N60 notation expresses the blow count normalized to a rod energy ratio of 60 percent of the theoretical free-fall energy, since the measured energy delivered by different hammer and anvil systems varies widely. The measured energy ratio is determined by calibration and applied as a multiplier of ER/60.

Question 2Site Characterization and Subsurface Exploration

In the rock quality designation, a core piece is counted only if it is intact and at least:

  1. 100 mm long Correct answer.
  2. 200 mm long
  3. 150 mm long
  4. 50 mm long

Explanation

RQD is the sum of the axial lengths of intact core pieces 100 mm (4 in) or longer divided by the total length of the run, expressed as a percentage. Pieces shorter than the threshold, and any drilling-induced breaks, are excluded.

Question 3Site Characterization and Subsurface Exploration

The N-value reported for a standard penetration test is the number of blows required to drive the sampler through the:

  1. final 12 inches of an 18-inch drive Correct answer.
  2. final 6 inches of a 12-inch drive
  3. full 18 inches of the drive
  4. first 6 inches of an 18-inch drive

Explanation

The sampler is driven 18 in in three 6-in increments; the first increment is the seating drive and is not counted. The N-value is the sum of the blows for the second and third increments, that is, the final 12 in.

Question 4Site Characterization and Subsurface Exploration

For a thin-walled tube to qualify for relatively undisturbed sampling of cohesive soil, its area ratio should generally not exceed:

  1. 5 percent
  2. 20 percent
  3. 1 percent
  4. 10 percent Correct answer.

Explanation

Area ratio is computed as the difference between the outside and inside cross-sectional areas divided by the inside area. Thin-walled tubes are fabricated with area ratios of about 10 percent or less to limit distortion of the sample.

Question 5Site Characterization and Subsurface Exploration

The groundwater level in a newly drilled borehole should be recorded:

  1. after the water level has stabilized Correct answer.
  2. at the start of the next boring on the same site
  3. only while drilling fluid is still circulating
  4. immediately after drilling stops

Explanation

The level observed immediately after drilling reflects the drilling fluid and transient heads imposed by the operation. The hole or standpipe is allowed to equilibrate, commonly overnight, before the stabilized level is read.

Question 6Site Characterization and Subsurface Exploration

A standpipe piezometer with a short screened interval sealed above and below with bentonite is installed to:

  1. determine the in-situ density of the sand stratum
  2. recover disturbed samples of the aquifer material
  3. measure pore water pressure at a specific depth Correct answer.
  4. determine the total unit weight of the surrounding soil

Explanation

The seals isolate a single response zone so that the water level in the riser reflects the piezometric head at that depth only. Monitoring the response to loading or excavation then gives pore pressures at that elevation.

Question 7Site Characterization and Subsurface Exploration

Seismic refraction surveying is most commonly used in site exploration to:

  1. measure the plasticity of near-surface clays
  2. locate buried metallic utilities
  3. determine the salinity of the groundwater
  4. estimate depth to bedrock and major layer boundaries Correct answer.

Explanation

Refraction interprets the first-arrival travel times of compressional waves and uses velocity contrasts to compute the depth of refractors such as the top of rock or a major stratum change. It requires velocity to increase with depth.

Question 8Site Characterization and Subsurface Exploration

Bentonite slurry is used as a drilling fluid mainly because it:

  1. dissolves soluble minerals and thereby speeds drilling
  2. increases the driving resistance of the sampler
  3. builds a filter cake and stabilizes the hole walls Correct answer.
  4. lubricates the rods so that casing is never required

Explanation

The slurry deposits a low-permeability filter cake on permeable walls and its column provides hydrostatic head that opposes formation and water pressures, limiting inflow and collapse. Casing is still required in some ground conditions.

Question 9Site Characterization and Subsurface Exploration

A sampling tube has an outside diameter of 3.00 in and an inside diameter of 2.875 in. Its area ratio is closest to:

  1. 17.8 percent
  2. 8.2 percent
  3. 4.4 percent
  4. 8.9 percent Correct answer.

Explanation

Area ratio equals the outside area minus the inside area, divided by the inside area, times 100. Here the areas are proportional to 9.000 and 8.266 in-squared, giving about 8.9 percent.

Question 10Site Characterization and Subsurface Exploration

An SPT in sand gave 20 blows for the final 12 in, and the hammer system has a measured energy ratio of 45 percent. The N60 value is:

  1. 9
  2. 27
  3. 15 Correct answer.
  4. 20

Explanation

The correction multiplies the measured blow count by the ratio of measured to reference energy, so N60 = 20 times 45/60, which equals 15. A low-energy hammer produces more blows per foot, and the correction reduces the count accordingly.

Question 11Site Characterization and Subsurface Exploration

A piezocone penetration test records continuous profiles of:

  1. unit weight, water content, and Atterberg limits
  2. tip resistance, sleeve friction, and penetration pore pressure Correct answer.
  3. shear wave velocity, damping, and Poisson's ratio
  4. blow count, sample recovery, and groundwater level

Explanation

The cone is pushed at a constant rate and its sensors log tip resistance, sleeve friction, and, in the piezocone, pore pressure at the filter location. No sample is recovered, so the output is a continuous index profile.

Question 12Site Characterization and Subsurface Exploration

The field vane shear test is best suited to determining the undrained shear strength of:

  1. stiff fissured overconsolidated clay
  2. dense gravel with cobbles
  3. soft to medium saturated clay Correct answer.
  4. weathered fractured rock

Explanation

The vane measures the torque needed to shear a cylindrical surface in saturated cohesive soil under undrained conditions, which is valid for soft to medium clays. Coarse or fractured materials and fissured clays give unreliable results.

Question 13Site Characterization and Subsurface Exploration

A water pressure test conducted between packers in a borehole is used to estimate:

  1. the unconfined compressive strength of the core
  2. the total unit weight of the overburden soil
  3. the permeability of the rock mass Correct answer.
  4. the depth to the regional water table

Explanation

Water is injected at controlled pressure into an isolated interval and the steady intake is measured, giving a Lugeon value or hydraulic conductivity governed largely by jointing. It is a standard test for rock mass permeability.

Question 14Site Characterization and Subsurface Exploration

A slug test performed in a monitoring well completed in a low-permeability silty clay is used to:

  1. estimate the formation hydraulic conductivity Correct answer.
  2. measure the elastic modulus of the clay
  3. measure the Atterberg limits of the formation
  4. determine the overconsolidation ratio of the clay

Explanation

An abrupt change in water level is imposed and the recovery is monitored; fitting the response curve yields the hydraulic conductivity of the material near the well. The method is practical where pumping tests would produce negligible yield.

Question 15Site Characterization and Subsurface Exploration

The most reliable way to obtain an undisturbed sample of loose, clean sand below the water table is to:

  1. push a thin-walled tube at a constant rate of penetration
  2. freeze the ground and core the frozen sand Correct answer.
  3. collect the cuttings from the auger flights
  4. drive a split-spoon sampler and correct the blow count

Explanation

Clean sand below the water table drains and loosens as it is sampled, so tubes and driven samplers seldom preserve density or structure. Artificial freezing immobilizes the pore water and allows a core to be cut without disturbance.

Question 16Site Characterization and Subsurface Exploration

When a borehole that penetrated a confined aquifer is no longer needed, it should be:

  1. left open so the groundwater level can still be observed
  2. backfilled loosely with the drill cuttings
  3. sealed with a grout plug over its full length Correct answer.
  4. capped at the surface with a steel plate

Explanation

An open or loosely backfilled hole forms a vertical pathway between aquifers and can allow artesian flow or cross-contamination. Grouting the full length, usually by tremie from the bottom up, restores the natural barrier.

Question 17Site Characterization and Subsurface Exploration

In a monitoring well, the sand pack placed around the screen is intended to:

  1. seal the annulus against vertical leakage between aquifers
  2. support the full weight of the riser pipe and pump
  3. act as a graded filter around the screen Correct answer.
  4. increase the artesian head acting at the screen

Explanation

A properly graded sand pack retains formation particles while permitting water to enter the screen, preventing the surrounding soil from collapsing or piping into the well. The hydraulic seal is provided separately by the bentonite above the pack.

Question 18Site Characterization and Subsurface Exploration

Sealed tube samples of cohesive soil should be stored and transported so that they are:

  1. allowed to drain freely to reach equilibrium
  2. protected from vibration and moisture change Correct answer.
  3. dried slowly in air before testing
  4. kept at temperatures above 40 degrees C

Explanation

Vibration, desiccation, and freezing all alter the structure and water content of a tube sample, so tubes are sealed, cushioned, kept near ambient temperature, and tested within a limited storage period.

Question 19Site Characterization and Subsurface Exploration

Total core recovery differs from rock quality designation in that total core recovery:

  1. counts only pieces longer than 100 mm
  2. reports core loss as zero by definition
  3. includes all recovered core pieces regardless of their length Correct answer.
  4. is measured only in rock whose RQD is below 25 percent

Explanation

Total core recovery is the recovered length of core divided by the length of the run, counting every recovered piece whatever its size. RQD applies the additional 100 mm intact-piece criterion and is therefore always equal to or less than total recovery.

Question 20Site Characterization and Subsurface Exploration

A downhole seismic survey performed in a cased borehole provides:

  1. the depth to the regional water table
  2. shear wave velocity versus depth Correct answer.
  3. the in-situ total unit weight profile
  4. the coefficient of permeability of each stratum

Explanation

A surface source is struck and a geophone lowered in the hole records arrival times, so interval travel times give shear wave velocity with depth. The result characterizes small-strain stiffness and is used for dynamic and liquefaction analyses.

Question 21Site Characterization and Subsurface Exploration

At a site, a survey is run over 3 m of loose sand with a compressional velocity of 400 m/s, above a stiff clay with a velocity of 1800 m/s, which in turn overlies a soft clay with a velocity of 900 m/s. The refraction survey will most likely:

  1. resolve the layers only if geophones are placed in a borehole
  2. resolve all three layers, since each has a distinct velocity
  3. detect the layer but not its thickness
  4. not detect the soft clay layer Correct answer.

Explanation

Refraction relies on refracted energy returning to the surface as first arrivals, which requires each deeper layer to be faster than the one above it. Arrivals from a layer beneath a higher-velocity layer are not first arrivals, so the softer clay is hidden.

Question 22Site Characterization and Subsurface Exploration

An SPT at a depth where the vertical effective stress is 50 kPa gave N60 of 18. Taking atmospheric pressure as 100 kPa and applying the standard overburden correction, N1,60 is closest to:

  1. 18.0
  2. 12.7
  3. 9.0
  4. 25.5 Correct answer.

Explanation

The overburden correction factor is the square root of atmospheric pressure divided by vertical effective stress, here the square root of 2, or about 1.41. Multiplying 18 by 1.41 gives approximately 25.5, since a shallow test at low confining stress is corrected upward.

Question 23Site Characterization and Subsurface Exploration

A borehole is advanced through thick clay into a confined sand aquifer and water rises in the casing to 2 m above ground surface. The level recorded represents:

  1. a static level fixed at the top of the clay layer
  2. a level 2 m below the top of the sand aquifer
  3. a piezometric head 2 m above ground level Correct answer.
  4. the water table of an unconfined aquifer, 2 m above ground

Explanation

In a confined aquifer, the water level in a standpipe equals the piezometric head of the aquifer, which can stand above ground level under artesian conditions. It is a pressure head rather than a phreatic surface.

Question 24Site Characterization and Subsurface Exploration

When SPTs are made through a hollow-stem auger below the water table in a clean sand, the blow counts may be too low because:

  1. the sampler is fitted with a core catcher that retains the sample
  2. gravel particles in the sand obstruct the sampler and raise the blow count
  3. the borehole is kept full of water while the sampler is driven
  4. sand heaves into the hole and loosens the soil being tested Correct answer.

Explanation

Unless a head of water or mud is maintained inside the hollow stem, clean sand can flow upward into the hole, loosening the surrounding soil so that the sampler meets less resistance. Keeping the hole full of water or mud is the usual remedy.

Question 25Site Characterization and Subsurface Exploration

The main advantage of cross-hole seismic testing over downhole testing is that cross-hole testing:

  1. needs only one hole
  2. resolves low-velocity layers Correct answer.
  3. needs no energy source
  4. averages a large volume

Explanation

In cross-hole testing the source is in one borehole and receivers at matching elevations in another, so the travel path is essentially horizontal and true interval velocities are measured. Velocity reversals that would be hidden from surface-source testing are therefore detected.

Question 26Site Characterization and Subsurface Exploration

The standard split-spoon sampler used in the standard penetration test has an inside diameter closest to:

  1. 50 mm (2.0 in.)
  2. 25 mm (1.0 in.)
  3. 35 mm (1.4 in.) Correct answer.
  4. 65 mm (2.6 in.)

Explanation

The standard split-spoon sampler is about 51 mm (2 in.) in outside diameter, approximately 35 mm (1-3/8 in.) in inside diameter, and 457 mm (18 in.) long with a 6-in. shoe. The inside diameter controls the size of the disturbed sample retained for classification and index testing.

Question 27Site Characterization and Subsurface Exploration

Which method is most suitable for obtaining a high-quality sample of soft to medium clay for laboratory strength testing?

  1. Driving a split spoon
  2. Pushing a thin-walled tube Correct answer.
  3. Scraping auger flights
  4. Catching return cuttings

Explanation

Thin-walled (Shelby) tubes are pushed with a continuous stroke and cause minimal distortion of the soil structure, so the sample is suitable for strength and consolidation testing. Driving a sampler or collecting cuttings remodels the clay and destroys the fabric that controls its behavior.

Question 28Site Characterization and Subsurface Exploration

Which geophysical method is best suited to locating a buried steel drum at a former industrial site?

  1. Resistivity profiling
  2. A magnetometer survey Correct answer.
  3. Seismic refraction
  4. Cross-hole seismic testing

Explanation

A magnetometer measures small variations in the earth's magnetic field and responds strongly to ferrous objects such as steel drums, tanks, and pipes. The anomaly is localized, so the method can pinpoint the target for test pit confirmation.

Question 29Site Characterization and Subsurface Exploration

Ground penetrating radar performs poorest in which ground condition?

  1. Massive, unweathered granite
  2. Saturated clean gravel
  3. Dry, clean sand
  4. Clay with a high dissolved-salt content Correct answer.

Explanation

Radar signal penetration drops sharply as the electrical conductivity of the ground increases; saline pore water in clay attenuates the signal within a very short distance. Low-conductivity materials such as dry sand, gravel, and intact crystalline rock allow much greater penetration.

Question 30Site Characterization and Subsurface Exploration

A thin-walled sampling tube has an inside diameter of 73.0 mm and a cutting-edge inside diameter of 72.5 mm. Its inside clearance ratio is closest to:

  1. 3.4 percent
  2. 0.35 percent
  3. 0.7 percent Correct answer.
  4. 1.4 percent

Explanation

The inside clearance ratio is Ci = (Di - De)/De x 100, where Di is the tube inside diameter and De is the cutting-edge inside diameter. Here (73.0 - 72.5)/72.5 x 100 = 0.69 percent, which is within the usual 0 to 1 percent target range.

Question 31Site Characterization and Subsurface Exploration

A thin-walled tube is pushed 600 mm and 450 mm of soil is recovered. The recovery ratio is closest to:

  1. 100 percent
  2. 75 percent Correct answer.
  3. 133 percent
  4. 67 percent

Explanation

Recovery ratio is the recovered length divided by the length pushed, expressed as a percentage: 450/600 x 100 = 75 percent. A value well below 100 percent indicates sample loss, compression, or disturbance of the soil in the tube.

Question 32Site Characterization and Subsurface Exploration

In a piezocone test, penetration is stopped and the decay of pore pressure with time is recorded. This procedure is used chiefly to estimate:

  1. the coefficient of consolidation of the soil Correct answer.
  2. the depth to bedrock
  3. the direction of groundwater flow
  4. the relative density of the sand

Explanation

The rate at which the induced excess pore pressure dissipates is controlled by the permeability and compressibility of the soil, which together give the horizontal coefficient of consolidation. The same record also shows the equilibrium pore pressure once the decay is complete.

Question 33Site Characterization and Subsurface Exploration

A pressuremeter test performed in a borehole provides a direct measurement of:

  1. the undrained shear strength alone
  2. the grain-size distribution of the soil
  3. the permeability of the soil
  4. in-situ lateral stress and deformation modulus Correct answer.

Explanation

The pressuremeter expands a flexible membrane against the borehole wall and records pressure versus volume change, giving the lift-off pressure that approximates the in-situ horizontal stress and the shape of the curve that yields the deformation modulus. The undrained shear strength can also be estimated, but it is not the only output.

Question 34Site Characterization and Subsurface Exploration

Exploration beneath a wide mat foundation must extend deeper than exploration beneath a narrow footing carrying the same contact pressure because:

  1. wide mats are normally founded directly on bedrock
  2. the depth of significant stress influence increases with foundation width Correct answer.
  3. groundwater is always deeper beneath large structures
  4. SPT correction factors change with the width of the loaded area

Explanation

The zone of soil that is significantly stressed by a loaded area extends to a depth that grows with the width of that area, roughly one to two times the width for a mat. Borings must therefore reach below the compressible zone so that settlement can be evaluated.

Question 35Site Characterization and Subsurface Exploration

A test pit excavated with a backhoe is generally preferred over a borehole when:

  1. samples must be recovered from far below the water table
  2. exploration must reach depths greater than 30 m
  3. the site is underlain by hard, massive bedrock
  4. soil and rock must be observed in place at shallow depth Correct answer.

Explanation

A test pit allows the engineer to see the soil in its natural position, measure layer thicknesses, and map defects, utilities, or boulders directly. The method is limited to the reach of the excavator and to depths above the water table where the pit can stay open safely.

Question 36Site Characterization and Subsurface Exploration

A braced excavation will extend 6 m below grade into a thick soft clay, with dense sand at 15 m depth. The borings should extend below the excavation base mainly to:

  1. allow base heave and bottom failure to be evaluated Correct answer.
  2. determine the weathering profile of the clay
  3. locate the water table for a dewatering permit
  4. measure the horizontal stress in the dense sand

Explanation

In soft clay, a deep excavation can fail by basal heave or by plastic flow of the clay beneath the base, and the geometry of the clay layer relative to a stiffer stratum controls that mechanism. Exploration must therefore penetrate the soft clay and establish the depth and character of the underlying bearing layer.

Question 37Site Characterization and Subsurface Exploration

Standard penetration test blow counts are least representative when the soil is:

  1. firm, low-plasticity silt
  2. gravelly sand containing cobbles and boulders Correct answer.
  3. loose, clean, fine sand
  4. soft, normally consolidated clay

Explanation

Coarse particles can be pushed aside by the spoon or jam against its shoe, so the blow count reflects the obstruction rather than the resistance of the matrix soil. Correlations developed for sand are unreliable in such deposits, and large-diameter samplers or test pits may be needed.

Question 38Site Characterization and Subsurface Exploration

A site is underlain by 3 m of dense sand, then 2 m of soft clay, then granite. A seismic refraction survey using a surface source will most likely:

  1. detect the clay but not the granite surface
  2. miss the low-velocity clay layer entirely Correct answer.
  3. resolve all three layers at their correct depths
  4. show a mirror image of the sand and granite depths

Explanation

Refraction interpretation relies on successively faster layers, because the first arrival comes from a head wave generated only at an interface where velocity increases. A lower-velocity layer buried below a faster one generates no first arrival, so the clay is invisible and the granite appears deeper than it is.

Question 39Site Characterization and Subsurface Exploration

A double-tube core barrel is used instead of a single-tube barrel mainly to:

  1. allow coring without any circulating fluid
  2. keep drilling fluid from washing the core Correct answer.
  3. increase penetration rate in soft shale
  4. recover core from cohesionless sand

Explanation

In a double-tube barrel the inner tube that receives the core does not rotate and the fluid circulates in the annulus between the tubes, so the core is not eroded by the flush. This greatly improves recovery and preserves the fabric of the rock, particularly in broken or weathered zones.

Question 40Site Characterization and Subsurface Exploration

In a borehole completed in stiff clay till, the water level measured immediately after drilling stands 3 m below the level measured one week later. The most likely cause is:

  1. a regional rise of the water table during the week
  2. leakage from a nearby sewer line
  3. the casing having been driven too deep
  4. slow equilibration with the low-permeability soil Correct answer.

Explanation

In a low-permeability deposit, water seeps into or out of the borehole very slowly, so the level read soon after drilling reflects drilling conditions rather than the formation pore pressure. The reading stabilizes only after the hole has had time to equilibrate, which is why water levels should be monitored over days in clay.

Question 41Soil Mechanics and Laboratory/Field Testing

Gravimetric water content of a soil specimen is defined as the:

  1. volume of water divided by the total volume of the specimen, expressed as a percent
  2. mass of water divided by the total mass of the moist specimen, expressed as a percent
  3. mass of water divided by the volume of the solids, expressed as a percent
  4. mass of water divided by the mass of oven-dry solids, expressed as a percent Correct answer.

Explanation

Water content is a gravimetric (mass-based) ratio in which the denominator is the mass of solids remaining after oven drying, usually at 110 ± 5 °C. Dividing by total mass gives a different, smaller ratio that is not the standard water content. Volumetric ratios describe saturation and porosity, not water content.

Question 42Soil Mechanics and Laboratory/Field Testing

The specific gravity of solids for a typical clean quartz sand is closest to:

  1. 3.10 to 3.30
  2. 1.85 to 2.05
  3. 2.20 to 2.35
  4. 2.65 to 2.68 Correct answer.

Explanation

Quartz is the dominant mineral in most sands and has a specific gravity of about 2.65, so measured values for clean sands cluster tightly around 2.65 to 2.68. Values near 2.2 indicate organic or lightweight material, and values above 3.0 indicate heavy iron-bearing minerals. An assumed value of 2.65 is standard when no test is run.

Question 43Soil Mechanics and Laboratory/Field Testing

In the Casagrande cup device, the liquid limit of a fine-grained soil is the water content at which:

  1. a standard groove in the soil closes over a length of 13 mm under 25 blows Correct answer.
  2. a rolled thread of soil crumbles at a diameter of about 3 mm
  3. the soil can no longer be remolded without crumbling
  4. the soil changes from the plastic state to the semisolid state

Explanation

The liquid limit is defined by the arbitrary blow-count criterion: a groove of standard dimensions closes over 13 mm (0.5 in) along its base at exactly 25 blows. The blow count is adjusted to 25 using the flow curve when tests are run at other counts. The plastic limit, not the liquid limit, is the rolled-thread criterion.

Question 44Soil Mechanics and Laboratory/Field Testing

A clay has a liquid limit of 45 and a plastic limit of 20. Its plasticity index is:

  1. 45
  2. 20
  3. 25 Correct answer.
  4. 65

Explanation

The plasticity index is the numerical difference between the liquid limit and the plastic limit, expressed as a percentage water content. Here PI = 45 − 20 = 25. The PI represents the range of water content over which the soil behaves plastically.

Question 45Soil Mechanics and Laboratory/Field Testing

Compared with the standard Proctor test, the modified Proctor test differs in that it uses:

  1. the same compactive energy applied to oven-dry soil rather than moist soil
  2. a heavier hammer dropped through a greater height, applying more compactive energy per unit volume Correct answer.
  3. the same hammer and drop height with a larger mold so that more soil is compacted
  4. a lighter hammer dropped through a greater height, applying energy more gradually

Explanation

The modified Proctor test raises both the hammer mass and the drop height, roughly quadrupling the compactive energy per unit volume of soil compared with the standard test. The result is a higher maximum dry unit weight at a lower optimum water content. Mold size and specimen preparation are similar in both procedures.

Question 46Soil Mechanics and Laboratory/Field Testing

In the standard penetration test, the reported N-value is the number of blows required to drive the sampler through the:

  1. final 150 mm of a 300 mm total drive
  2. final 300 mm of a 450 mm total drive Correct answer.
  3. full 450 mm of the total drive
  4. first 150 mm of a 450 mm total drive

Explanation

The sampler is driven 450 mm in three 150 mm increments, and the first increment is treated as a seating drive that is excluded from the count. The N-value is the sum of blows for the second and third increments, that is, the final 300 mm. Any refusal or unusually high blow count in the seating interval is recorded separately.

Question 47Soil Mechanics and Laboratory/Field Testing

The constant-head permeability test in a laboratory permeameter is best suited to:

  1. a highly plastic fat clay
  2. a silty clay of low permeability
  3. a clean gravel or coarse sand Correct answer.
  4. a fibrous peat

Explanation

The constant-head test measures a steady flow rate through a specimen, which requires a relatively high hydraulic conductivity so that measurable volumes pass in a reasonable time. Coarse-grained soils such as clean sands and gravels satisfy this requirement. Low-permeability soils are tested with a falling-head device instead.

Question 48Soil Mechanics and Laboratory/Field Testing

An unconfined compression test on a saturated clay specimen gives an axial stress at failure of 120 kPa. The undrained shear strength of the clay is:

  1. 60 kPa Correct answer.
  2. 120 kPa
  3. 240 kPa
  4. 30 kPa

Explanation

In an unconfined compression test the lateral total stress is zero, so the Mohr circle at failure has a diameter equal to the compressive strength qu. For a saturated clay with zero friction angle, the undrained shear strength is half of qu. Thus su = 120/2 = 60 kPa.

Question 49Soil Mechanics and Laboratory/Field Testing

A hydrometer analysis determines the particle-size distribution of the fine fraction of a soil by:

  1. measuring the water content of a saturated paste after various drying periods
  2. measuring the electrical conductivity of a dilute soil suspension
  3. measuring the mass of soil retained on each sieve after washing
  4. measuring the density of a soil-water suspension at a given depth and elapsed time and applying Stokes' law Correct answer.

Explanation

The hydrometer measures suspension density, which reflects how much soil of a given size or coarser is still in suspension at that depth and time. Stokes' law converts the settling velocity of a sphere into an equivalent particle diameter. Sieve analysis covers the coarser fraction and cannot resolve sizes below about 75 µm.

Question 50Soil Mechanics and Laboratory/Field Testing

In a mechanical sieve analysis, the percentage of fines in a soil is taken as the percentage of the total dry mass:

  1. passing the 75 mm sieve
  2. passing the No. 200 sieve Correct answer.
  3. retained on the No. 4 sieve
  4. retained on the No. 200 sieve

Explanation

The No. 200 sieve has 0.075 mm openings, and material finer than this is classified as silt and clay, collectively called fines. The percentage passing this sieve is the fines content used in classification systems. Material retained on the No. 200 sieve is the coarse fraction.