PE Geotechnical flashcards

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

SPT sampler dimensions

Standard split-spoon sampler is 51 mm outside diameter and 35 mm inside diameter, driven 450 mm with a 63.5 kg hammer falling 760 mm; blows per 150 mm are recorded.

N60 correction

N60 equals measured N times energy ratio divided by 60; corrections for overburden, rod length, sampler, and borehole diameter are applied before design use.

RQD definition

Rock quality designation is the sum of core pieces 100 mm or longer divided by total run length times 100 percent; below 50 percent is poor rock.

CPT friction ratio

Cone penetration test measures tip resistance qc and sleeve friction fs; friction ratio Rf = fs/qc × 100 helps classify soil, with high Rf indicating clay.

Shelby tube area ratio

Thin-walled tube area ratio below 10 percent minimizes sample disturbance and is used to obtain relatively undisturbed samples of soft to stiff cohesive soil.

Overburden correction CN

CN equals the square root of 100 divided by effective vertical stress in kPa, capped near 2, and normalizes SPT N-values to a reference stress.

SPT seating blows

The first 150 mm of SPT drive is seating and is discarded; the N-value is the blows required for the next 300 mm, recorded in 150 mm increments.

Plasticity index

PI = LL − PL; it quantifies the range of water content over which soil remains plastic and correlates with clay compressibility and swell potential.

Liquidity index

LI = (w − PL)/PI; values near 1 indicate soil near the liquid limit with low strength, while values near 0 indicate stiff soil.

Modified Proctor energy

Modified Proctor compaction applies about 2700 kN·m/m³ in five layers with a 4.54 kg hammer; Standard Proctor applies about 600 kN·m/m³.

Effective stress principle

Effective vertical stress equals total stress minus pore water pressure; soil strength and compressibility depend on effective stress rather than total stress.

Mohr-Coulomb criterion

Shear strength equals cohesion plus effective normal stress times the tangent of friction angle; drained parameters c' and φ' apply for long-term conditions.

Coefficient of consolidation

cv is determined from the consolidation curve using root-time or log-time construction; it controls the rate of primary consolidation settlement.

Darcy's law

Discharge velocity equals hydraulic conductivity times hydraulic gradient; constant-head tests suit coarse soils, while falling-head tests suit fine soils.

Terzaghi bearing capacity

Ultimate bearing capacity for a strip footing is cNc + qNq + 0.5γBNγ; the terms represent cohesion, surcharge, and soil self-weight resistance.

Eccentric footing pressure

For eccentric load, maximum and minimum pressures equal Q/A times (1 ± 6e/B); e greater than B/6 causes tension and partial contact.

Pile shaft resistance clay

Alpha method: shaft resistance equals adhesion factor α times undrained shear strength times shaft area; α generally decreases as undrained strength increases.

Beta method sands

Pile shaft resistance in sand equals β times effective vertical stress, where β relates to friction angle and lateral earth pressure coefficient.

Pile group efficiency

Group capacity may be less than the sum of individual pile capacities because stress zones overlap; block failure and group settlement must be checked.

Static pile load test

Test load is typically applied to twice the design load; measured settlement defines failure using Davisson or 10 percent pile diameter criteria.

Rankine active pressure

Active earth pressure coefficient equals tan²(45 − φ/2); pressure acts on a wall when the wall moves away from the soil, reducing horizontal stress.

Rankine passive pressure

Passive coefficient equals tan²(45 + φ/2); large wall movement toward the soil is required to mobilize passive resistance.

At-rest pressure K0

For normally consolidated soil, K0 is approximately 1 − sinφ; for overconsolidated soil, multiply by OCR raised to about 0.5.

Infinite slope factor safety

For seepage parallel to a slope, factor of safety equals cohesion term plus tanφ' divided by tanβ; cohesion term is c'/(γz sinβ cosβ).

Retaining wall overturning

Factor of safety against overturning is resisting moments about the toe divided by driving moments; a minimum value of 2.0 is typical.

Bishop simplified method

Method of slices satisfying moment equilibrium; it assumes interslice forces are horizontal and is used to compute slope factor of safety for circular failure surfaces.

Liquefaction triggering

Compare cyclic stress ratio from the earthquake to cyclic resistance ratio from SPT or CPT; a factor of safety below 1 indicates liquefaction triggering.

Expansive soil indicator

High plasticity index, typically above 25, and free swell tests indicate expansive clay; lime treatment reduces swell potential and plasticity.

Frost-susceptible soil

Silts and fine sands with capillary rise and freezing temperatures cause ice lens growth and heave; clean gravelly soils are generally not frost susceptible.

Wick drain purpose

Prefabricated vertical drains shorten the drainage path to accelerate consolidation of soft clay under embankment or surcharge loading.

Stone column function

Vibro-replacement stone columns improve bearing capacity, reduce settlement, and accelerate drainage in soft cohesive and loose granular soils.

Site class from Vs30

ASCE 7 site classes A through F are assigned from average shear wave velocity in the top 30 m; softer sites amplify ground shaking.