6.2 Surveying, Site Layout, and Excavation
Key Takeaways
- Use the 3-4-5 rule (or 6-8-10, 9-12-15 multiples) to verify square building corners during layout.
- Height of Instrument = known elevation + backsight; unknown elevation = HI − foresight; a higher rod reading means lower ground.
- OSHA Subpart P requires a protective system at 5 ft depth and an engineered design at 20 ft depth.
- Maximum allowable slopes: Type A = 3/4:1, Type B = 1:1, Type C = 1.5:1 (horizontal:vertical).
- Keep spoil piles at least 2 ft from the trench edge and provide egress within 25 ft in trenches 4 ft or deeper.
Surveying, Site Layout, and Excavation
Site layout translates the drawings to the ground. The contractor establishes a benchmark (a fixed point of known elevation) and a baseline or control point from the survey, then lays out building corners using batter boards set back from the excavation. Layout accuracy is verified by the 3-4-5 rule for square corners: a triangle with legs 3 and 4 and a hypotenuse of 5 forms a perfect right angle. Multiples such as 6-8-10 or 9-12-15 are used for larger buildings.
Elevations, grades, and the leveling instrument
Elevations are read with an automatic level or laser level and a stadia/leveling rod. Two terms are tested constantly:
- Backsight (BS) — the rod reading taken on a point of known elevation (the benchmark).
- Foresight (FS) — the rod reading on a point of unknown elevation.
- Height of Instrument (HI) = Known elevation + BS.
- Unknown elevation = HI − FS.
Worked example: benchmark elevation 100.00 ft, backsight reads 4.50 ft, so HI = 104.50 ft. A foresight on the footing point reads 6.20 ft, so that point sits at 104.50 − 6.20 = 98.30 ft. A higher rod reading always means a lower ground elevation — a common reversal trap.
Excavation classification and protective systems
Excavation is governed by OSHA 29 CFR 1926 Subpart P. A competent person must classify the soil and select a protective system.
| Item | Rule |
|---|---|
| Protective system trigger | Trenches 5 ft deep or more (1926.652) |
| Engineer required | Any excavation 20 ft deep or deeper needs a registered professional engineer design |
| Spoil pile setback | Keep at least 2 ft from the trench edge (1926.651(j)) |
| Daily inspection | Competent person inspects before each shift and after rain |
| Access/egress | Ladder, ramp, or stairway within 25 ft of lateral travel in trenches 4 ft+ deep |
Protective options are sloping, benching, shoring, or a trench box (shield).
Maximum allowable slopes by soil type
OSHA Subpart P, Appendix B lists slope ratios (horizontal:vertical) for excavations up to 20 ft:
| Soil type | Max allowable slope (H:V) | Angle |
|---|---|---|
| Stable rock | Vertical | 90° |
| Type A | 3/4 : 1 | 53° |
| Type B | 1 : 1 | 45° |
| Type C | 1.5 : 1 | 34° |
A Type C trench 10 ft deep must be sloped back 1.5 × 10 = 15 ft on each side, widening the top of the cut by 30 ft total. Misreading the ratio (using 1:1.5 instead of 1.5:1) is a frequent error in takeoff and safety questions alike.
A benchmark is at elevation 250.00 ft. The backsight rod reading is 5.25 ft, and a foresight on a proposed slab corner reads 7.80 ft. What is the slab corner elevation?
Under OSHA 1926 Subpart P, a trench excavated in Type C soil to a depth of 8 ft must be sloped back how far horizontally on each side?
Datums, Benchmarks, and Stationing
Site layout starts from a benchmark (BM) — a fixed point of known elevation referenced to a datum. Horizontal control uses a baseline and stationing (1 station = 100 ft; station 2+50 = 250 ft). Property corners, setbacks, and the building offset are staked from these controls. The surveyor sets batter boards and offset stakes clear of excavation so the building lines can be re-established after digging.
Cut, Fill, and Grade Calculations
Cut removes soil; fill adds it. An elevation higher than existing grade needs fill; lower needs cut. Read contour lines: closely spaced = steep, widely spaced = flat. Worked example: existing grade 100.0 ft, finish grade 96.5 ft → cut = 3.5 ft. Finish floor elevation (FFE) is the reference for the building. Balanced earthwork (cut volume ≈ fill volume on site) avoids costly import/export of soil.
Instruments and Layout Methods
A builder's level/transit or modern total station/GPS establishes horizontal and vertical control. The 3-4-5 rule (a 3-4-5 triangle forms a perfect 90°) squares foundation corners by hand; checking that diagonals are equal confirms a layout is rectangular. Differential leveling carries elevation from a benchmark across the site: Backsight + known elevation = Height of Instrument; HI − Foresight = new elevation.
Common Exam Traps
- Trap: A finish grade below existing grade needs fill. Lower = cut.
- Trap: 1 station = 1,000 ft. A station = 100 ft.
- Trap: Closely spaced contours mean flat ground. They mean steep.
- Trap: Confusing Backsight/Foresight in leveling (BS adds to known elevation, FS subtracts).
Existing grade at a point is 102.0 ft and the required finish grade is 105.5 ft. What earthwork operation is needed and how much?
Excavation Types and Safe Bank Slopes
Sitework excavation splits into mass/bulk excavation (large-area cut to subgrade), trench excavation (utilities, footings), and structural excavation (for foundations). Carry the cut to the subgrade elevation, then verify with the surveyor before placing fill or concrete. Tie back to OSHA Subpart P (Ch. 5): the bank slope must be safe for the soil type, and a competent person inspects daily. Over-excavation is corrected with engineered/structural fill compacted to spec, not loose backfill, so the foundation bears on competent material.
Worked Leveling Example
Example: A benchmark elevation is 100.00 ft. The instrument reads a backsight of 4.20 ft on the BM, so the Height of Instrument = 100.00 + 4.20 = 104.20 ft. At a foundation corner the foresight reads 6.75 ft, so that corner's elevation = 104.20 − 6.75 = 97.45 ft. Comparing 97.45 to the required subgrade tells the crew whether to cut or fill — the exact backsight-adds, foresight-subtracts logic the exam tests in differential leveling problems.