PE Geotechnical common mistakes
PE Geotechnical · Understand the traps and how to avoid them.
Treating the stratigraphy drawn between two borings as if it were observed continuously-carrying a clay layer straight across a gap where the second boring shows sand, and then designing a foundation or slope on that assumed continuous layer.
Candidate sketches a clean cross-section in the first five minutes and then answers every later question from that sketch, forgetting that between-boring contacts are interpolated, not measured. The exam figure looks authoritative, so the assumption goes unnoticed.
What to do instead
Before solving, redraw the profile yourself and mark each layer contact as 'observed' or 'interpolated.' Where a contact is interpolated, state the assumption in your scratch work and check whether the answer choices bracket both possible conditions (e.g., clay continuous vs. sand seam).
Plugging raw field SPT blow counts directly into empirical correlations (bearing capacity, liquefaction triggering, settlement, phi correlations) without converting to N60 and (N1)60.
The problem statement hands over an N-value and the correlation formula is memorized with 'N' in it; hammer energy, rod length, sampler type, and overburden corrections feel like extra steps that the problem did not explicitly ask for.
What to do instead
Adopt a fixed routine: write N_field, apply the energy ratio to get N60, apply the overburden correction to get (N1)60, and only then enter the correlation. If the problem gives hammer type and rod length, corrections are being tested-treat the raw N as an intermediate, never a final input.
Computing effective vertical stress using the total (moist) unit weight for soil below the water table, or forgetting to subtract pore pressure at all.
Total unit weight is the number given in the boring log and the one used in the layer table, so it gets carried down the whole profile by habit; the water table is a single line on the log that is easy to skim past.
What to do instead
Build an explicit table with a row for every interface: depth, total unit weight used, total stress, pore pressure, effective stress. Below the water table enter gamma_sat and subtract u = gamma_w times depth of submergence at each row. Check that the effective stress increment below the water table is smaller than the total stress increment.
Selecting shear strength parameters from a test whose drainage condition does not match the field case-e.g., taking phi from a UU triaxial test, or using peak strength for a remolded/creeping slope, or using drained parameters for a rapid load on saturated clay.
The problem supplies a test name and a friction angle, and the natural move is to use the number that is given; the link between test drainage, field drainage, and design case is a reasoning step that gets skipped under time pressure.
What to do instead
Before writing any strength value, state the field drainage condition in one line: short-term undrained, long-term drained, or rapid drawdown. Then select the matching test (UU/total stress, CU with pore pressures, CD/effective stress) and check whether the design case (end of construction, long-term, seismic) permits peak or requires residual strength.
Comparing a gross allowable bearing pressure from the bearing capacity equation against a net applied footing pressure (or vice versa), and omitting the weight of the footing and the excavated soil from the net pressure calculation.
The classic bearing capacity equation returns a gross ultimate value, and the applied load is usually reported as a column load; the conversion between gross and net, and the subtraction of overburden at the footing base, is where the two get mismatched.
What to do instead
Write both sides in the same basis before dividing by the factor of safety: net ultimate = gross ultimate minus overburden at base; net applied pressure = (column load + footing weight) / area minus overburden removed. Always confirm which basis the answer choices are in.
Sizing a pile group by multiplying single-pile capacity by the number of piles without checking block failure in clay, group efficiency in sand, and group settlement.
Single-pile capacity is the calculation that has just been practiced, and multiplying by n is a one-line step that feels complete; the group check is a separate failure mode that is easy to overlook when the question asks only for 'capacity.'
What to do instead
For every pile group, compute both the sum of individual capacities and the block capacity (perimeter times length times su plus base), and take the smaller. Then check group settlement against the single-pile value, and state which mechanism governs.
Using active earth pressure for a wall that cannot move enough to mobilize it-basement walls, walls with restrained tops, tied-back walls-and omitting water pressure or surcharge from the lateral load sum.
Rankine active is the formula drilled in review courses, so it is applied reflexively; the wall's movement condition and the drainage/surcharge details are described in the problem narrative but not in the equation.
What to do instead
Classify the wall first: free to rotate or translate (active), restrained (at-rest), or being pushed into the soil (passive). Then build the pressure diagram as a stack-surcharge, soil, water-and use the appropriate K for each, keeping water pressure separate from effective soil pressure.
Running the same factor-of-safety calculation for every slope case-using drained effective-stress strength for a rapid drawdown or end-of-construction case, or applying the resistance factor to the wrong side of the equation.
The method of slices procedure is memorized as one recipe, and the design case is treated as a label rather than a change in the strength and pore pressure inputs.
What to do instead
For each slope question, write the design case first (end of construction, long-term, rapid drawdown, seismic) and list which strengths and pore pressures apply: undrained su for short-term saturated clay, drained c-phi with appropriate ru for long-term, and residual strength for slides on pre-existing surfaces. Then confirm the factor of safety is defined as resisting over driving on the same basis as the answer choices.
Designing only the final, in-service condition and never checking temporary construction stages-excavation base heave, uplift on a mat or empty tank, dewatering-induced settlement, or backfill compaction pressures on a wall.
The problem statement describes the completed structure, so the analysis stops there; the construction sequence is mentioned in one clause or implied, and its governing case is not obvious from the final geometry.
What to do instead
Before finalizing, list the stages: excavation, dewatering, construction, backfill, service. Run a quick check on each temporary stage (base heave factor, uplift safety factor with the structure empty, compaction-induced lateral pressure) and identify which stage controls.
Losing track of units and magnitudes-mixing psf with ksf, pcf with pci, kips with tons, or accepting an answer of 40,000 psf bearing pressure or 0.5 ft of settlement without a sanity check.
Unit conversions are done mentally between steps, and the exam's multiple-choice format rewards picking the number that appears in the calculation rather than the number that is physically plausible.
What to do instead
Carry units through every equation and write the result with units before comparing to the choices. Add a 10-second plausibility test: typical allowable bearing pressures are 1–10 ksf, footing settlements are fractions of an inch to a couple of inches, and unit weights are 100–140 pcf. If the result is off by 1000x, a ksf/psf conversion was missed.