12.6 Reviewing Grading Design: Error Detection & Constructability
Key Takeaways
- Grading review proceeds in a fixed sequence — datum and benchmark, contour integrity, drainage continuity, slope compliance, accessibility, tie-in, and earthwork balance — because errors found late are errors already propagated.
- A contour that crosses, splits, or ends within the drawing interior is a drafting error, and an unlabeled contour interval change is the single most common source of downstream elevation mistakes.
- Every low point must have a designed outlet; an enclosed contour with no inlet, no overflow, and no positive outfall is a designed pond and is the most frequent defect on a grading plan.
- Positive drainage minimums are approximately 1% on pavement and 2% on turf, while accessible routes cap running slope at 5.00% and cross slope at 2.08%, so a compliant accessible route occupies a narrow band between ponding and non-compliance.
- Proposed contours must tie to existing contours at the limit of work at the same elevation, and a mismatch at the tie-in means the grading does not close.
1. The Task the Exam Tests Directly
Review Grading Design (e.g., review grading alternatives, evaluate for inconsistencies) is an enumerated task within Grading and Earthwork, which carries 44% of the Grading, Drainage and Stormwater Management section — the heaviest single content area on that exam section. It is also the task best matched to CLARB's advanced item types: a hot spot item that asks you to click the location of the error on a grading plan is literally a grading review exercise.
Reviewing is a different cognitive act from designing. Designing asks "what should this be?" Reviewing asks "what here cannot be true?"
2. The Seven-Step Review Sequence
Errors found late have already propagated, so review follows a fixed order.
Step 1 — Datum, benchmark, and contour interval
- Is a benchmark cited with description, elevation, and datum (NAVD88 or NGVD29)?
- Is the contour interval stated, and is it constant across the sheet?
- Do existing and proposed contours use distinguishable line types (existing dashed and lighter, proposed solid and heavier)?
A contour interval that changes between sheets without an explicit note is the most common source of downstream elevation errors, because a reader interpolating at the wrong interval produces spot elevations that are silently wrong.
Step 2 — Contour integrity
Check each proposed contour against the rules that cannot be violated:
| Rule | Violation signature |
|---|---|
| Contours never cross | Two contours intersecting (only valid at a vertical face such as a wall) |
| Contours never split or branch | A single contour forking into two |
| Contours close on themselves or run off the sheet | A contour that simply stops mid-drawing |
| Contours never repeat an elevation adjacently except at a summit or depression | Two adjacent lines labeled the same without a summit or hachured depression |
| Contour spacing reflects slope | Abrupt spacing change with no corresponding wall, curb, or slope break |
Step 3 — Drainage continuity
Trace every drop of water from every high point to a designed outlet. The essential question at each low point: where does the water go when this fills?
A closed contour with no inlet, no overflow weir, and no positive outfall is a designed pond. This is the most frequent defect on a student and professional grading plan alike, and it is what a hot spot item most often targets.
Also verify:
- Inlets are placed at low points, not near them
- Swales carry a continuous downhill gradient with no reverse grade
- Ridge lines between catchments actually separate flow
- Runoff is not directed onto adjacent property or onto a building
Step 4 — Slope compliance
| Condition | Typical standard |
|---|---|
| Minimum pavement slope | ~1.0% (0.5% achievable only on well-controlled concrete) |
| Minimum turf / planted slope | ~2.0% |
| Maximum mowable turf slope | 3:1 (33%), with 4:1 preferred |
| Maximum planted slope without stabilization | 2:1 (50%) |
| Maximum accessible route running slope | 5.00% (1:20) |
| Maximum ramp running slope | 8.33% (1:12) |
| Maximum cross slope on accessible route | 2.08% (1:48) |
| Maximum slope in any direction on a landing | 2.08% (1:48) |
Step 5 — Accessibility verification
This is where grading review most often fails, because the compliant band is narrow. An accessible walk must drain (at least about 1%) and must not exceed 5.00% running slope and 2.08% cross slope. Verify the actual computed slope between the given spot elevations rather than trusting the label.
Checks:
- Compute running slope between consecutive spot elevations along the route
- Compute cross slope perpendicular to the direction of travel
- Confirm ramp runs do not exceed 30 inches of rise between landings
- Confirm landings are at least 60 inches long and level within 2.08% in all directions
- Confirm accessible parking stalls and access aisles do not exceed 2.08% in any direction
- Confirm the accessible route connects the accessible parking, the site arrival point, and the entrance without a step
Step 6 — Tie-in at the limit of work
Proposed contours must meet existing contours at the same elevation at the limit of disturbance. A proposed 104 contour that ends adjacent to an existing 106 contour at the property line means the grading does not close and a retaining structure or additional grading is required but not shown.
Also verify grades at fixed points that cannot move: existing building finished floors, existing tree root collars, adjacent sidewalks, curb returns, manhole rims, and property lines.
Step 7 — Earthwork balance and constructability
- Does the plan produce a gross cut/fill imbalance that requires import or export?
- Has topsoil stripping and respreading been accounted for separately?
- Is there equipment access to every graded area?
- Do cut slopes daylight within the property, or do they run off site?
- Are grades achievable with the specified equipment, or does the plan require hand grading in inaccessible areas?
3. Error Signatures Worth Memorizing
| What you see | What it means |
|---|---|
| Closed contour, no inlet, no overflow | Water trapped; designed pond |
| Contour crossing itself or another | Drafting error or undocumented vertical face |
| Proposed contour ending mid-sheet | Incomplete grading; does not close |
| Adjacent same-value contours with no hachures | Ambiguous summit or depression |
| Spot elevation inconsistent with surrounding contours | Spot governs, so the contours are wrong (or the spot is a typo) |
| Walk with 2.5% cross slope | Exceeds 2.08%; not compliant |
| Walk with 0.0% or reverse cross slope | Ponds and ices; fails positive drainage |
| Ramp run longer than 30 ft at 8.33% | Exceeds 30 in rise between landings |
| Swale with a flat reach | Standing water and mosquito habitat |
| Turf slope at 2:1 | Not mowable; exceeds 3:1 maximum |
| Proposed grade above an existing tree root collar | Root suffocation; tree loss |
| Runoff directed toward a building or off site | Liability and likely code violation |
4. Comparing Grading Alternatives
Where the task is to evaluate two or more grading schemes rather than to find an error, compare on a fixed set:
- Earthwork balance — which scheme minimizes import and export?
- Tree and resource preservation — which scheme keeps grade changes outside critical root zones?
- Accessibility — which scheme achieves the accessible route with the fewest ramps and landings?
- Drainage — which scheme produces the shortest, simplest conveyance with fewest structures?
- Wall quantity — retaining walls are the most expensive way to resolve a grade change; the scheme with fewer wall feet usually wins
- Constructability and phasing — which scheme can be built in stages with usable access?
5. Exam Traps & Pitfalls
- Trusting the slope label. Compute the slope from the spot elevations; the label may be stale.
- Missing the trapped low point. Every low point needs an inlet and an overflow.
- Checking running slope and forgetting cross slope. Cross slope at 2.08% fails more accessible routes than running slope does.
- Ignoring the tie-in. Proposed must meet existing at the same elevation at the limit of work.
- Accepting a flat walk as compliant. Zero slope satisfies accessibility and fails drainage; the walk must do both.
- Overlooking existing fixed elevations. Finished floors, curb returns, and root collars cannot move to suit the plan.
While reviewing a grading plan for a corporate courtyard, a landscape architect finds a series of closed contours labeled 246, 245, and 244 from outside to inside, with no hachure marks, no inlet symbol, and no overflow shown. What does this condition represent?
A grading plan shows an accessible walkway with spot elevations of 214.60 at one end and 215.35 at the other, with a scaled horizontal distance of 18.0 feet between them. The plan labels this segment "2.0% max." What does the review find?
During review of a grading plan, the landscape architect observes that proposed contours along the eastern limit of work terminate adjacent to existing contours bearing different elevation values, with no retaining wall, slope, or transition shown. What does this indicate?
A landscape architect is comparing two grading alternatives for a sloping campus site. Scheme A balances cut and fill on site, preserves the existing tree grove, and requires 340 linear feet of retaining wall. Scheme B requires exporting 4,200 cubic yards, removes the tree grove, and requires no retaining wall. What analytical framing should govern the comparison?