PE Civil common mistakes
PE Civil · Understand the traps and how to avoid them.
Multiplying earthwork quantities by a swell or shrinkage factor in the wrong direction, or multiplying bank-volume cycle rates by truck capacity without converting to a common volume basis.
The problem statement supplies cut volume, truck struck capacity, and a percent swell or shrinkage, and the candidate grabs the first factor that looks relevant. Candidates who do not track whether a number is in bank, loose, or compacted cubic yards default to multiplying everything.
What to do instead
Before touching the calculator, write a one-line ledger: 'Given: 12,000 BCY cut; 25% swell; truck = 12 LCY.' Convert every quantity to one basis (usually loose for hauling, compacted for fill) and label each intermediate number with its unit. If a computed haul count is not larger than the bank volume divided by truck capacity when swell is positive, the factor was applied backwards.
Treating float as a fixed property of an activity instead of recomputing the forward and backward pass after a duration change; missing that consuming float on one activity can make a parallel path critical.
Most candidates memorize 'critical path = longest path' and then reason qualitatively about delays. Total float is shared along a chain, so shortening one activity can shift the critical path and change the answer.
What to do instead
On every schedule question, even ones that look trivial, redraw the network and perform the forward and backward pass explicitly, listing ES, EF, LS, LF, and TF for each activity. When a duration changes, redo the pass rather than subtracting days from the original float. If two paths have equal total duration, state that both are critical.
Using the total (moist) unit weight below the groundwater table instead of the buoyant unit weight when computing vertical effective stress, or omitting pore pressure entirely.
The unit weight is the number printed on the soil profile, and with a saturated soil the difference between total and effective stress is invisible unless pore pressure is subtracted deliberately.
What to do instead
Sketch the profile to scale, draw the water table, and build a three-column table at each layer boundary: total stress (sum of gamma times thickness), pore pressure (gamma_w times depth below the water table), and effective stress (difference). Below the water table, use gamma' = gamma_sat - 62.4 pcf. Check that effective stress at the water table equals total stress there.
Selecting shear strength parameters by habit rather than by drainage condition, e.g., using c' and phi' for an end-of-construction stability check on a saturated clay, or using undrained shear strength for a long-term condition.
Textbook problems are often phrased without an explicit time frame, and candidates default to effective stress parameters because they are used in the majority of practice calculations.
What to do instead
Before choosing parameters, write one sentence naming the drainage condition and time frame implied by the problem: end of construction or rapid loading of saturated fine-grained soil means undrained, total stress, phi = 0 with s_u = c; long-term or drained loading means effective stress with c' and phi'. Match the analysis method (total vs effective stress) to that sentence and verify the factor of safety uses consistent units.
Checking only the maximum-gravity load combination and skipping the minimum-gravity combination (for example 0.9D + 1.0W) that governs uplift, overturning, and net tension in anchors or hold-downs.
Gravity combinations produce the largest axial forces and moments in most members, so a single check appears sufficient, and the 0.9 dead load case is easy to forget because it looks like it should be inconsequential.
What to do instead
For any member or foundation resisting lateral load, list all applicable combinations and explicitly compute both the maximum and minimum dead-load cases. Plot or tabulate net axial force and net overturning moment for each so the sign reversal is visible. Report the governing case by name, not just the number.
Mixing factored loads with service-level deflection or vibration limits, or skipping the serviceability check entirely because the strength check passed.
Deflection questions often appear at the end of a problem where the candidate has already built up factored moments, and it is tempting to reuse those numbers rather than returning to unfactored loads.
What to do instead
Structure every solution in two labeled blocks: 'Strength' using factored combinations and design strengths, and 'Serviceability' using unfactored loads and allowable deflections such as L/360 or L/240. Compute immediate and long-term deflection separately where required. At the end, state in one line whether the controlling limit state was strength or serviceability.
Mixing arc-definition and chord-definition degree of curve formulas, or substituting radius in feet into a formula that expects stations (or vice versa) when computing tangent length, curve length, or stationing.
Both definitions circulate in reference material with formulas that look nearly identical, and the 100-ft station convention makes length in stations numerically close to length in feet, so unit errors survive a plausibility check.
What to do instead
At the top of the problem, state the definition being used: arc definition, R = 5729.58/D; chord definition, R = 50/sin(D/2). Compute the curve length as L = R times Delta in radians (arc) or by the chord-definition formula, then convert to stations by dividing by exactly 100. Verify that the final station equals the PC station plus the computed length, and confirm tangent length is shorter than curve length for the same Delta.
Getting the sign of the algebraic grade change wrong on a vertical curve, or selecting a K value from the wrong column (crest vs sag, or the wrong design speed) when checking stopping sight distance.
A = g2 - g1 changes sign with the order grades are listed, and the crest and sag tables sit side by side in most references, so a misread row or column still produces a plausible-looking K.
What to do instead
Write g1 and g2 with explicit signs and direction arrows before computing A, and note whether the curve is crest (A negative) or sag (A positive). Compute the required K from the SSD relation, then compare against the tabulated K for the stated design speed in the correct crest-or-sag column, and state the minimum length L = K times |A| with the sign of A already resolved.
Applying a single runoff coefficient to a composite drainage area, or using an intensity from an IDF curve whose duration does not match the computed time of concentration for the design point.
Subareas are given in a table and it is faster to average the coefficients than to area-weight them; separately, IDF curves are usually read at a convenient duration rather than at the tc that was just computed, especially when tc is short.
What to do instead
Compute C as the area-weighted value, sum of (C_i times A_i) divided by total area, and show the subarea table. Compute tc from the longest flow path, verify the intensity read off the IDF curve is at duration equal to tc and at the correct return period, and check that peak flow is in the expected units for the given drainage area (for 10 acres and C near 0.5, a peak of several thousand cfs indicates a unit error).
Confusing solids retention time with hydraulic retention time in biological treatment calculations, or dropping the 8.34 lb/(MG times mg/L) conversion when moving between concentration and mass loading.
Both are called 'retention time' and are computed as a volume divided by something, so the formulas look interchangeable; the concentration-to-mass factor is treated as a constant to memorize rather than a unit conversion to track.
What to do instead
Label the numerator and denominator every time: SRT = mass of solids in the system divided by solids leaving per day; HRT = aeration tank volume divided by influent flow. For mass loading, write the full unit chain, flow in MGD times concentration in mg/L times 8.34 equals lb/day, and carry the units through to the answer. Confirm that the two times differ by roughly the ratio of MLSS to influent suspended solids, which is typically a factor of several.