PE Mining common mistakes
PE Mining · Understand the traps and how to avoid them.
Computes reserve tonnage as resource tonnage × grade × recovery and reports it as a reserve, omitting modifying factors entirely.
Coursework drills grade-times-tonnage arithmetic, and exam stems often give exactly the numbers needed to do that multiplication, so it feels like the intended path.
What to do instead
Before answering, classify the material: resource (geologic) versus reserve (mineable). Apply and state mining recovery, dilution, metallurgical recovery, and economic/legal factors. If a factor is not given, list it as an explicit assumption and carry it through the calculation rather than dropping it.
Selects a ground support system straight from an RMR or Q value without asking whether the failure mode is structurally controlled (wedge, plane sliding) or stress driven.
Classification tables get memorized as deterministic design answers, and a stem that supplies a single rock mass rating invites a look-up.
What to do instead
Treat RMR/Q as screening input only. Compare joint set orientation to the excavation face using kinematic/stereonet reasoning, and compare induced stress to rock strength. The support element chosen must address the actual mechanism, and the answer should say which mechanism it controls.
Computes slope factor of safety with total unit weight and no pore pressure even when the stem specifies a water table, rainfall, or a tailings beach.
The dry planar-wedge formula is the most heavily rehearsed equation, and the pore-pressure term is easy to leave out under time pressure.
What to do instead
Write FS in effective stress and subtract pore pressure u from normal stress on the failure plane. Check the drained case and the rapid-drawdown/undrained case separately, and sanity-check the result against the usual acceptance threshold for permanent versus temporary slopes.
Mixes explosive mass and rock volume bases in powder factor, for example dividing kilograms of explosive by loose cubic metres and then comparing the result to a pound-per-ton bench standard.
Powder factor has several legitimate unit conventions, and stems frequently give volume in one system and explosive quantity in another.
What to do instead
Convert to a single unit system first, fix the basis (bank cubic metres or metric tons of in-place rock) before dividing, and label the units on the answer. When comparing to a rule-of-thumb value, restate that rule of thumb in the same basis.
Checks blast vibration against the total charge in a hole or a pattern rather than the maximum charge per delay, or inserts distance and charge in mismatched units in scaled distance.
The total explosive weight is the prominent number in a blast design table, while charge per delay is a separate column that must be constructed from the timing diagram.
What to do instead
Compute scaled distance as D divided by the square root of W, where W is the largest charge detonating within any 8-millisecond window. Keep distance in feet with charge in pounds, or metres with kilograms, and state which pairing was used before reading the vibration curve.
Solves a ventilation problem by adding fan pressure directly to mine resistance figures, or finds the fan flow without intersecting the fan curve and system curve, or drops natural ventilation pressure.
Fans and airway resistance are usually studied in separate chapters, and the exam question is the first place they must be combined.
What to do instead
Construct the system curve as pressure equals resistance times flow squared, add natural ventilation pressure with its sign, and intersect that curve with the fan curve to obtain the operating point. For parallel splits, solve at equal pressure drop across branches; for series, sum resistance.
Reports concentrate grade when recovery was requested, or uses feed grade in the recovery expression, without setting up a two-product balance.
Recovery and grade move together and stems typically supply three of the four balance quantities, which invites a shortcut straight to the answer.
What to do instead
Write the mass balance explicitly: feed times feed grade equals concentrate times concentrate grade plus tailings times tailings grade, with feed mass equal to concentrate plus tailings. Solve for the unknown, then compute recovery as concentrate mass times concentrate grade divided by feed mass times feed grade, and verify the result is between feed and concentrate grade and does not exceed 100 percent.
Uses in-situ tonnage where the equipment actually handles loose volume, or omits swell/bulking when converting between bank and loose conditions for trucks, belts, or stockpiles.
Mass is conserved when rock is dug, so it feels as though nothing should change; the volume change is invisible in the tonnage figure.
What to do instead
Decide the basis the question asks for before computing. Bank volume equals loose volume divided by the swell factor, and tonnage is unchanged by digging. State the assumed in-place density and swell, and for conveyors confirm capacity at the specified surcharge angle and belt speed rather than at flat-belt conditions.
Discounts cash flows at the wrong point within the year, or combines escalated cash flows with a real discount rate, or buries capital, salvage, and tax inside the discount rate.
End-of-year discounting is the default taught in most courses, so mid-year conventions and the nominal-versus-real distinction get skipped when the stem implies them.
What to do instead
Lay out a time line and label each period starting at time zero. Apply mid-year factors only when the stem states them, pair nominal cash flows with a nominal market rate and real cash flows with a real rate, and treat capital, salvage, depreciation, and tax as separate line items in the cash flow rather than adjustments to the rate.
Answers a health and safety item from general-industry practice such as an OSHA permissible exposure limit or a state rule, or averages exposure samples arithmetically instead of computing a time-weighted average.
Most candidates' safety background is OSHA-based and state mining programs are assumed to govern, while a time-weighted average looks like an ordinary mean of the sample values.
What to do instead
Anchor on the Mine Act and the 30 CFR parts administered by MSHA, which apply to operators and contractors on mine property regardless of any state plan. For exposures, compute the time-weighted average as the sum of each concentration times its duration divided by eight hours, using full-shift samples, and compare against the MSHA limit.