Free PE Geotechnical Exam Flashcards

ASTM D1586 drives a split-barrel sampler 18 in. with a 140-lb hammer falling 30 in.; N is the blow count for the second and third 6-in. increments, discarding the seating blows. Report N60, energy-corrected to 60% of the theoretical free-fall hammer energy, before applying published correlations, because modern automatic hammers deliver far more energy than the rope-and-cathead rigs those correlations were built on.

An electronic piezocone (ASTM D5778) pushed at a constant rate records tip resistance, sleeve friction, and pore pressure behind the tip, producing a near-continuous profile and a friction ratio used to infer soil behavior type. It recovers no sample, so pair soundings with borings whenever you need Atterberg limits, gradation, or consolidation testing.

RQD is the summed length of intact, sound core pieces at least 100 mm (4 in.) long divided by the total core run length, expressed as a percent; below 25% is very poor rock and above 90% is excellent. Low RQD signals closely fractured rock, which lowers rock-socket side resistance, increases groundwater inflow, and changes excavation, rippability, and rock-slope stability assumptions.

Under ASTM D2487 a soil is coarse-grained when more than 50% is retained on the No. 200 sieve (0.075 mm); within the coarse fraction, gravel and sand divide at the No. 4 sieve (4.75 mm). That split selects the whole classification path: coarse-grained symbols come from gradation coefficients, while fine-grained symbols come from the plasticity chart.

A thin-walled tube pushed into cohesive soil (ASTM D1587) recovers relatively undisturbed samples suitable for consolidation, undrained strength, and permeability testing. A split-barrel SPT sampler is driven, so its samples are disturbed and useful only for classification, moisture content, and index testing - which is why a boring log full of SPT samples alone cannot support a settlement analysis.

Drilling fluid, short observation time, and low-permeability soil mean the borehole reading may never have equilibrated. Design should rest on stabilized piezometer or observation-well readings over time, because perched water, confined artesian pressure, and seasonal swings change effective stress, buoyant uplift, and the dewatering system you have to build.

Effective stress equals total stress minus pore water pressure, and soil strength, stiffness, and volume change respond to effective stress alone. That is why dewatering, rapid drawdown, or a rising water table can change bearing capacity and trigger settlement with no change whatsoever in the applied structural load.

Dry unit weight is total unit weight divided by (1 + water content), and the fundamental identity S x e = w x Gs links degree of saturation, void ratio, water content, and specific gravity. Together they let you move from a measured moist unit weight and moisture content to the void ratio, porosity, and saturation needed for settlement, compaction, and seepage work.

PI = LL - PL, and the A-line on the Casagrande plasticity chart is PI = 0.73(LL - 20). Points above the A-line classify as clays and points below as silts or organic soils - the same split that drives swell potential, frost susceptibility, and which empirical strength correlations legitimately apply.

Undrained strength controls short-term loading of saturated fine-grained soil, such as an embankment placed quickly over soft clay or a footing at end of construction. Drained parameters control long-term conditions, and they normally govern cuts and excavated slopes in clay, where negative pore pressures dissipate over time and the slope becomes less stable years after it is built.

OCR is preconsolidation pressure divided by current effective overburden stress; OCR = 1 means normally consolidated. Loading that stays below the preconsolidation pressure follows the flat recompression index and produces small settlement, while loading past it follows the much steeper compression index - so misjudging the preconsolidation pressure can misstate settlement by an order of magnitude.

Tv = cv x t / (drainage path)^2, with Tv about 0.197 at 50% average consolidation and 0.848 at 90%. The drainage path is the longest distance water must travel to a pervious boundary, so a clay layer that drains to both top and bottom uses half the layer thickness and consolidates roughly four times faster than the same layer draining one way.

Standard Proctor (ASTM D698) applies about 12,400 ft-lbf/ft3 of compactive effort; Modified Proctor (ASTM D1557) applies about 56,000 ft-lbf/ft3. The higher effort yields a higher maximum dry unit weight at a lower optimum moisture content, so a spec calling for '95% compaction' sets very different field targets depending on which reference test it cites.

The sand cone (ASTM D1556) is a direct volume-replacement measurement and serves as the usual reference standard; the nuclear gauge (ASTM D6938) is fast and non-destructive but must be calibrated and correlated against a reference test. Nuclear moisture readings read high in soils containing organics or other hydrogen-bearing material, which can make an acceptable fill appear wet of optimum and under-compacted.

A protective system is required for excavations 5 ft or deeper unless made entirely in stable rock, and any protective system for an excavation deeper than 20 ft must be designed by a registered professional engineer. Appendix B maximum allowable slopes are 3/4:1 for Type A, 1:1 for Type B, and 1-1/2:1 for Type C soil, so reclassifying soil after rain, vibration, or seepage forces flatter slopes or shoring.

Inclinometers track lateral movement of walls and slopes, piezometers track pore pressure, settlement plates and extensometers track vertical movement, and vibration monitors track ground motion from driving or blasting. Instrumentation only protects a project when threshold and action levels - and the response to exceeding them - are written down before construction begins.

Site class is assigned from the upper 100 ft of profile using shear wave velocity (preferred), SPT resistance, or undrained shear strength, and it sets the coefficients that amplify mapped ground motions - softer profiles generally amplify more. Check which edition your exam supplies: the April 2024 specification provides ASCE 7-16 with classes A through F, while the design standards effective with the April 2027 exam move to ASCE 7-22, which adds the intermediate classes BC, CD, and DE.

Liquefaction requires saturated, loose to medium-dense cohesionless soil - clean sands and non-plastic silts are the classic candidates - shaken hard enough to generate excess pore pressure faster than it can drain. As excess pore pressure approaches the initial effective stress, strength approaches zero, producing bearing failure, lateral spread, buoyant uplift of buried tanks and vaults, and post-shaking downdrag on piles.

The factor of safety is the cyclic resistance ratio divided by the cyclic stress ratio. The demand side comes from design peak ground acceleration, total and effective overburden stress, and a depth reduction factor; the resistance side comes from corrected penetration resistance such as clean-sand-equivalent corrected SPT or CPT values, so a sloppy energy or overburden correction moves the answer directly.

A horizontal seismic coefficient converts shaking into a static inertial force applied at the centroid of the sliding mass, and that coefficient is normally taken as a fraction of the design peak ground acceleration when limited permanent displacement is acceptable. A computed factor of safety slightly below 1.0 predicts movement, not collapse, so the follow-up is a displacement estimate such as a Newmark sliding-block analysis.

The Ordinary Method of Slices and Bishop's Simplified Method satisfy moment equilibrium for circular surfaces only; Spencer and Morgenstern-Price satisfy both force and moment equilibrium for arbitrary surfaces. Use a rigorous method when the critical surface is non-circular, when anchors, reinforcement, or seismic forces are applied, or when a thin weak seam controls the failure geometry.

End of construction is an undrained case governed by the foundation clay's undrained strength; long-term steady seepage is a drained case using effective-stress parameters with the phreatic surface in place; rapid drawdown removes the stabilizing water load from the upstream slope faster than pore pressures can dissipate. Each case uses different strength and pore-pressure assumptions, so the critical failure surface can move between them.

Wick drains replace a long vertical drainage path with short radial flow to closely spaced drains, which can cut consolidation of a thick soft clay from years to months. Adding surcharge above the final design load lets you reach the design settlement early and remove part of the long-term secondary compression before the structure or pavement is built.

Vibro-compaction and dynamic compaction densify clean granular soils; soft fine-grained soils generally need reinforcement or replacement - stone columns, aggregate piers, rigid inclusions, or deep soil mixing - or accelerated consolidation with drains. If the real problem is that the load is too large rather than the ground too weak, lightweight fill such as EPS geofoam reduces the load instead of strengthening the soil.

A filter geotextile must retain soil while passing water: retention is checked by comparing the apparent opening size against the soil's D85, and flow capacity is checked by permittivity relative to the soil's hydraulic conductivity. Too open and the filter pipes fines and creates voids; too tight and it clogs, builds pore pressure behind the wall or drain, and quietly defeats the drainage design.

Flexible pavement design uses subgrade CBR or resilient modulus, while rigid pavement design uses the modulus of subgrade reaction. Weak, wet, frost-susceptible, or expansive subgrade either forces a thicker structural section or requires treatment - lime or cement stabilization, geosynthetic separation and reinforcement, or undercut and replacement with free-draining material.

For a flow net drawn with curvilinear squares, seepage per unit width equals hydraulic conductivity times total head loss times the ratio of flow channels to equipotential drops. The same net also gives pore pressure anywhere in the section, which is exactly what you need for uplift on a cutoff wall, exit gradient at the toe, and effective stresses inside a dam or excavation.

The critical gradient equals buoyant unit weight divided by the unit weight of water, or (Gs - 1)/(1 + e), which is near 1.0 for most soils. When upward seepage at an excavation bottom approaches that gradient, effective stress goes to zero and the soil boils or pipes - so braced cuts in granular soil are checked for exit gradient and heave, then fixed by deepening the cutoff, lowering the head, or loading the base with a graded filter.

Open sumps suit shallow cuts in stable, permeable ground; wellpoints handle sands and silty sands but lower water only about 15 ft per stage, so deeper cuts need multiple stages or deep wells; eductor systems handle deeper cuts in low-permeability silts. Lowering the water table also raises effective stress outside the excavation, so consolidation settlement of adjacent structures must be evaluated and is often mitigated with recharge wells or a cutoff.

High-plasticity clays containing smectite minerals swell when wetted and shrink when dried; percent swell and swell pressure are measured in a one-dimensional oedometer test (ASTM D4546). Light structures suffer most because slab and footing pressures are often smaller than the swell pressure, so mitigation relies on moisture control, deepened or pier-and-beam foundations, non-expansive select fill, or lime treatment rather than on structural load.

Loess and similar low-density, low-moisture soils held together by weak clay or salt bonding lose that bonding when wetted and collapse under stresses they had already been carrying, so settlement occurs with no increase in load at all. Wetting-type oedometer testing (ASTM D4546) quantifies collapse potential; mitigation includes prewetting, dynamic compaction, undercut and replacement, or extending foundations below the collapsible zone.

Frost heave needs a frost-susceptible soil (silts and silty sands are worst because they wick water quickly), freezing temperatures penetrating the ground, and a water supply feeding growing ice lenses - remove any one and heave stops. That is why designs place footings below the local frost depth and replace frost-susceptible subgrade with free-draining material; the springtime thaw of those ice lenses is what leaves a saturated, very low-strength subgrade under pavements.

For a smooth vertical wall with level cohesionless backfill, the active coefficient is tan^2(45 - phi/2) and the passive coefficient is tan^2(45 + phi/2), while the at-rest coefficient for normally consolidated soil is Jaky's 1 - sin(phi'). Walls that cannot move - basement walls, braced excavation walls, integral abutments - must be designed for the at-rest condition, which is substantially larger than active.

The active state develops after only a small outward movement of the wall, but full passive resistance requires movement roughly an order of magnitude larger, often a few percent of wall height. Because that movement is usually unacceptable next to existing structures, passive resistance is commonly reduced by a large factor of safety or neglected, and the passive block is also checked against scour, future excavation, or utility trenching.

If backfill saturates, the wall must carry effective earth pressure plus full hydrostatic pressure, which can more than double total thrust and raise the resultant on the stem. Weep holes, chimney or blanket drains, drainage composites, and free-draining backfill with proper filtration keep water out of the backfill, and blocked or omitted drainage is a leading cause of retaining wall failure.

Check sliding along the base, overturning or eccentricity of the base resultant, bearing capacity of the foundation soil, and global stability of the whole slope containing the wall. In allowable stress design the resultant is normally kept within the middle third of the base so the base stays entirely in compression - and global stability is the check most often skipped, yet it is what fails walls built on slopes or over soft ground.

External stability treats the reinforced mass as a gravity block and checks sliding, eccentricity, bearing, and global stability; internal stability checks reinforcement rupture, pullout beyond the assumed failure surface, and facing connection strength. FHWA and AASHTO practice sets a minimum reinforcement length on the order of 0.7 times wall height, and long-term durability requires screening backfill for corrosivity with steel reinforcement or chemical compatibility with geosynthetics.

Terzaghi-and-Peck-style apparent pressure envelopes are empirical envelopes back-calculated from measured strut loads; they are tools for sizing struts and walers, not a prediction of the true pressure distribution at any single stage. Braced cuts also need bottom stability checks - basal heave in soft clay, exit gradient and piping in sand - that the strut envelope says nothing about.

Ground anchors are prestressed against the wall face, so they load the ground actively, each one is proof- or performance-tested, and the unbonded free length must extend beyond the potential failure surface. Soil nails are passive and closely spaced, mobilizing tension only as the ground deforms, so nailed walls tolerate - and depend on - somewhat more movement, which matters where adjacent structures are settlement-sensitive.

In qult = c x Nc + q x Nq + 0.5 x gamma x B x N-gamma the terms are the cohesion contribution, the surcharge from soil above founding level, and the width-and-friction contribution; Nq and N-gamma rise steeply with friction angle. In saturated clay loaded undrained the factors reduce to Nc = 5.14, Nq = 1, N-gamma = 0, so widening the footing adds almost nothing and only more embedment or ground improvement helps.

Meyerhof's approach replaces the real footing with an equivalent one of reduced dimensions, subtracting twice the eccentricity from each side so the equivalent footing is centered under the resultant, and bearing pressure is checked over that reduced area rather than the gross area. If eccentricity exceeds one-sixth of the width, the base tends to lift on one side and contact pressure concentrates sharply over a smaller area.

Vertical stress increase decays with depth, and the simple 2:1 method spreads the load over an area of (B + z) by (L + z) as an approximation to Boussinesq theory. Because the influence depth scales with loaded width, a compressible layer that is harmless below a small isolated footing can govern settlement beneath a mat or a closely spaced footing group - so compute stress at the depth of the soft layer, not just at founding level.

On sands and stiff soils, allowable pressure is usually limited by tolerable settlement long before shear failure, so a footing with a factor of safety of 3 against bearing capacity can still settle unacceptably. Design is therefore checked against total settlement and, more critically, differential settlement and angular distortion between adjacent supports, since that is what cracks the structure.

Nominal axial compressive capacity is side (shaft) resistance plus base (toe) resistance, adjusted for the element's own weight. For drilled shafts, side resistance is fully mobilized at very small displacement - well under one percent of shaft diameter - while base resistance typically needs on the order of 5% of base diameter, so the two components rarely peak together and the usable capacity depends on the displacement the structure can accept.

When the surrounding soil settles more than the pile - because of new fill, dewatering, or consolidation of a soft layer - side friction reverses direction and drags the pile downward, adding axial load while removing that length from useful resistance. Mitigation includes bitumen coatings, isolation casing, preloading the site before installation, or designing for the added load and locating the neutral plane; downdrag is fundamentally a load and settlement problem, not a soil strength failure.

A group's capacity can be less than the sum of its piles because of overlapping stress zones and, in clay, potential block failure of the pile-and-soil mass. Group settlement is almost always larger than single-pile settlement at the same load per pile because the group stresses soil to a far greater depth - so a compressible layer below the tips can govern even when every individual pile tests satisfactorily.

Lateral response is normally analyzed with p-y curves that model soil as nonlinear springs along the shaft, and it is dominated by the strength and stiffness of the upper several diameters of soil. Short elements rotate as rigid bodies while long ones fail by forming a plastic hinge, and any loss of near-surface support - scour, liquefaction, an adjacent excavation, or soft fill - sharply reduces capacity and increases deflection.

The static axial compressive load test (ASTM D1143) is the benchmark measurement of capacity and generally supports the most favorable design resistance. High-strain dynamic testing with a pile driving analyzer (ASTM D4945), interpreted by signal matching, is far faster and cheaper, reports hammer energy and driving stresses, and separates shaft from toe resistance - so it is used to test many piles, often calibrated against one static test.

Low-strain impact integrity testing (ASTM D5882) is quick and needs no preinstalled hardware but loses resolution in long or large-diameter shafts. Crosshole sonic logging (ASTM D6760) resolves defects between access tubes far better, but those tubes must be tied to the reinforcing cage before concrete placement - so the decision to use it has to be made before construction, not after a suspect pour.

Driving displacement piles densifies loose granular soil but remolds sensitive clay, which loses strength during driving and regains it as excess pore pressure dissipates - the setup that makes restrike testing more meaningful than end-of-drive testing. Dense sands, silts, and some shales can instead relax and lose capacity with time, while drilled shafts avoid vibration but depend on borehole stability, slurry management, and base cleanout to deliver the assumed resistance.