NICET Geotech flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Liquid limit (LL)

Water content at which a remolded soil closes a 12.7 mm groove over 25 blows in the Casagrande cup (ASTM D4318); a key input for USCS and AASHTO classification.

Plasticity index (PI)

PI = LL − PL. It quantifies the range of water content over which soil remains plastic; higher PI indicates more clay and greater compressibility and swell potential.

No. 200 sieve opening

The No. 200 sieve has 0.075 mm openings. Material passing it is fines (silt and clay); material retained is sand or gravel for classification purposes.

Coefficient of uniformity (Cu)

Cu = D60/D10 from the gradation curve. Cu ≥ 4 for gravels or ≥ 6 for sands, combined with 1 ≤ Cc ≤ 3, qualifies the soil as well graded (GW or SW).

Coefficient of curvature (Cc)

Cc = (D30)²/(D10 × D60). The value between 1 and 3, together with sufficient Cu, distinguishes well-graded from poorly graded gravels and sands.

Standard Proctor compaction

ASTM D698: 2.5 kg rammer, 305 mm drop, three layers, 600 kN·m/m³ compactive energy. The test yields maximum dry density and optimum moisture content for a soil.

Modified Proctor compaction

ASTM D1557: 4.54 kg rammer, 457 mm drop, five layers, 2700 kN·m/m³ energy. Higher energy yields greater maximum dry density and lower optimum moisture content than Standard Proctor.

Optimum moisture content

The water content at which a given compaction energy produces maximum dry density. Dry of optimum, soil is stiff and may swell; wet of optimum, it is more compressible and lower strength.

Relative compaction

Field dry density divided by the laboratory maximum dry density from the specified Proctor test, expressed as a percentage. Typical specification minimums are 95% Standard or 92% Modified Proctor.

Sand cone test

ASTM D1556 in-place density method: a known volume of calibrated sand fills the excavated hole, giving hole volume; wet mass divided by volume gives wet density, then dry density from water content.

Nuclear density gauge

ASTM D6938 field method using gamma-ray attenuation for density and neutron moderation for water content. Requires calibration and daily standard counts against a reference block.

Effective stress principle

σ′ = σ − u, where σ is total stress and u is pore pressure. Effective stress controls shear strength, consolidation, and deformation of saturated soils; it cannot be measured directly.

Mohr–Coulomb failure criterion

τ = c′ + σ′·tan φ′. Cohesion intercept c′ and friction angle φ′ define the shear strength envelope; the slope of the line is tan φ′.

Direct shear test

ASTM D3080: a soil specimen is sheared along a forced horizontal plane under normal stress. Peak or residual shear stress versus normal stress defines drained c′ and φ′.

UU triaxial test

Unconsolidated-undrained triaxial (ASTM D2850) shears a saturated specimen without drainage; results give total-stress parameters, typically φ = 0 and undrained shear strength su = (σ1 − σ3)/2.

Consolidation test

ASTM D2435 one-dimensional oedometer test: load increments are applied and settlement versus log time is recorded to obtain compression index Cc, recompression index Cr, and coefficient of consolidation cv.

Overconsolidation ratio (OCR)

OCR = preconsolidation stress σ′p divided by current effective vertical stress σ′v0. OCR = 1 indicates normally consolidated soil; OCR > 1 indicates overconsolidation from past loading or unloading.

Constant-head permeability test

For coarse soils: k = QL/(A·h·t), where Q is collected volume, L specimen length, A cross-sectional area, h head difference, and t time. Used when flow is measurable at steady head.

Falling-head permeability test

For fine-grained soils: k = (a·L/(A·t))·ln(h1/h2), where a is standpipe area and h1, h2 are initial and final heads. Preferred when flow is too small for constant-head measurement.

Liquid limit groove closure

In ASTM D4318 the cup is dropped 25 times at 2 drops per second; the groove must close over 12.7 mm. A multi-point plot of water content versus log blows is extrapolated to 25 blows.

USCS group symbols

G gravel, S sand, M silt, C clay, O organic; W well graded, P poorly graded, H high plasticity, L low plasticity. Example: CL = low-plasticity clay, SP = poorly graded sand.

AASHTO soil classification

Groups A-1 through A-7, with A-1 granular and A-7 clayey. Group index GI = (F−35)(0.2+0.005(LL−40)) + 0.01(F−15)(PI−10), using percent passing No. 200 (F).

Hydrometer analysis

ASTM D7928 sedimentation test based on Stokes' law; particle diameter is proportional to the square root of settling velocity. It extends the gradation curve below the No. 200 sieve.

Specific gravity of solids

ASTM D854: Gs = mass of solids divided by mass of an equal volume of water. Typical values are 2.65 for quartz sand and 2.70–2.80 for clay minerals; used in phase relationships.

Void ratio and porosity

Void ratio e = Vv/Vs; porosity n = Vv/Vt. They relate as n = e/(1+e) and e = n/(1−n). Dry unit weight γd = Gs·γw/(1+e).

Sieve analysis procedure

ASTM D6913: a dried, weighed sample is shaken through stacked sieves; mass retained on each sieve is recorded and cumulative percent passing is computed and plotted versus sieve opening on a log scale.

Liquid limit one-point method

A single blow count near 25 with its water content can estimate LL using the slope factor from a prior multi-point test; the multi-point method remains the reference procedure.

Shrinkage limit

ASTM D427: water content below which further drying does not reduce volume. The shrinkage limit is the lower boundary of the plastic range and indicates swell potential of clays.

Relative density of sand

Dr = [(emax − e)/(emax − emin)] × 100%. It describes the compactness of cohesionless soil from loose (Dr near 0%) to dense (Dr near 100%).

Unconfined compression test

ASTM D2166: a cylindrical specimen is axially loaded with no confining pressure; the peak stress is qu = 2·su. Applicable to cohesive soils that can stand unsupported.

Swell potential of clays

Expansive clays with high PI and LL (CH) can swell on wetting and shrink on drying. Free-swell and swelling-pressure tests quantify the risk before foundation or pavement design.

Standard penetration test (SPT)

ASTM D1586: a 63.5 kg hammer falling 760 mm drives a split spoon; the blow count for the second and third 150 mm increments is N, corrected for overburden and energy.