PE Civil: Water Resources & Environmental — free theory

Project Sitework — PE Civil (Water Resources & Environmental)

• PE Civil WRE
• Construction

Project Sitework is the PE WRE area where design meets dirt: moving earth, grading a site so water goes where it should, keeping excavations safe and stable, and stopping construction from washing the site's soil into the nearest stream. The exam questions here are practical — volumes of cut and fill, which control goes in first, which trench needs a protective system, how a sediment basin works. If you can do earthwork arithmetic and name the right control for the situation, this area is straightforward marks.

Earthwork quantities: the average-end-area method

Most earthwork on the exam is average-end-area volume between two cross-sections a distance L apart: V = (A₁ + A₂) / 2 × L. Divide cubic feet by 27 for cubic yards. The common miss is forgetting the division by 2 (using the sum of the two areas) or forgetting the ft³-to-CY conversion. On a real project the sections come from a grid or from contour-difference methods, but the exam gives you the two areas directly.

Then comes the material balance:

• Cut and fill balance — compare total cut volume against total fill volume. Excess cut is waste (hauled off or stockpiled); a shortfall needs borrow (imported material).
• Swell and shrinkage — excavated soil occupies more volume than it did in the ground (swell, a loose-state increase), and compacted fill occupies less than the loose material placed (shrinkage). If a fill needs V_c cubic yards compacted and the soil shrinks s (as a decimal), the bank (in-place) volume required is V_c / (1 − s) — you need more bank material than the compacted volume. The trap answer is always V_c × (1 + s), which is close but wrong: shrinkage applies to the bank volume, not the compacted volume.

(Try the dedicated earthwork calculator for the arithmetic.)

Site grading and layout

Grading shapes the ground so the site drains by gravity to its outlets:

• Finished grades fall away from structures so surface water does not pond against foundations — a minimum slope of about 2% (2 ft of fall per 100 ft of run) away from a building is a common rule of thumb.
• Slopes are described as horizontal:vertical (e.g., 3:1). Cut slopes are steeper than fill slopes for the same soil; the steeper the slope, the more vulnerable it is to erosion, which is why long slopes get broken by benches or swales.
• Low points become inlets, swales, or ponds — grading and drainage design are the same exercise. A grading plan that creates a trapped low point with no outlet is a design error, not a detail.

Excavation safety and support

Excavation questions on the PE exam test a small set of bright-line rules:

• An excavation 5 ft (1.5 m) or deeper requires a protective system — sloping, benching, shoring, or shielding (trench box) — unless it is entirely in stable rock.
• A competent person must inspect the excavation daily and after rain or other events.
• Excavations 4 ft or deeper where a hazardous atmosphere could exist must be tested before workers enter (the 4-ft rule is about atmospheres, not the protective system).
• Spoil piles must be kept at least 2 ft from the excavation edge — the weight of spoil near the edge is a classic cave-in trigger. (Don't confuse the 2-ft setback with the 5-ft protective-system trigger.)

Below the safety rules sit the support methods: sheeting and shoring (vertical members with horizontal walers and struts across a trench), soldier piles with lagging, and trench boxes that shield workers rather than hold the soil. Struts and walers carry lateral earth pressure plus any surcharge (traffic, spoil, equipment) beside the cut. Dewatering is often part of the plan — you cannot inspect, test, or compact in a flooded excavation — and it lowers pore pressures that would otherwise push on the support.

Erosion and sediment control in construction

Construction strips the land bare, and bare soil plus rain equals sediment leaving the site. The control logic is always the same:

• Perimeter and access controls go in first, before any clearing: silt fence or fiber rolls downgradient of the disturbed area, and a stabilized construction entrance so trucks don't track mud onto public roads. Putting clearing before controls is the sequence the exam penalizes.
• Minimize and phase disturbance — clear only what is needed, stabilize (seed, mulch, blankets) any area that will sit idle, and work in phases so the exposed area stays small.
• Sediment controls trap soil after it erodes: silt fence for sheet flow from small areas (respect its drainage-area limit — roughly a quarter acre per 100 ft of fence), sediment traps and sediment basins for larger areas. A basin works by slowing water so particles settle — its effectiveness depends on surface area and detention time, and it needs regular cleanout.
• Inlet protection keeps sediment out of the storm drain system during construction; check dams in swales slow concentrated flow so it drops sediment instead of scouring.
• Erosion controls prevent detachment in the first place: mulch, erosion-control blankets, and turf reinforcement on slopes and channels. In RUSLE terms (A = R·K·LS·C·P), these practices lower the C factor — try the USLE calculator to see how much a mulch cover changes predicted soil loss.

The regulatory frame: land disturbance of 1 acre or more generally needs coverage under the construction general permit with a SWPPP (stormwater pollution prevention plan). (See Stormwater BMPs for the permanent controls that replace the temporary ones.)

Site drainage

Once graded, the site sheds water through gutters, inlets, and storm sewers:

• Street and gutter flow is checked against spread criteria — how far water may encroach into the travel lane during the design storm.
• Inlets (grate, curb-opening, combination) intercept that flow and drop it into the storm sewer; inlet capacity falls as the gutter grade flattens or bypass flow grows.
• Storm sewer pipes are sized with Manning's equation for the design flow, usually the Rational Method peak (Q = C·i·A) for the contributing area — the Rational Method calculator handles the peak, and the Manning calculator the pipe capacity. Pipes run part-full in design (free surface), with minimum cover and minimum slopes to keep velocities self-cleansing.