Site Work, Excavation, and Foundations
Reading the ground
Start with soil borings and classification. OSHA soil types: A (cohesive, strongest), B, C (granular, weakest, running). Allowable bearing capacity drives footing size directly. Compaction is specified as a percentage of maximum dry density from a Proctor test (95% Standard Proctor is common), placed in lifts and verified by nuclear gauge. Moisture content near optimum gives the densest result.
OSHA excavation rules (memorize these numbers)
- 5 ft or deeper (or any excavation showing potential for cave-in): a protective system is required - sloping, benching, shoring, or a shield.
- Maximum slopes (horizontal:vertical): Type A 3/4:1, Type B 1:1, Type C 1.5:1.
- Ladder or ramp within 25 ft of lateral travel when the trench is 4 ft or deeper.
- Spoil and equipment at least 2 ft back from the edge.
- A competent person inspects daily, after rain, and after any event; call 811 before digging.
Trap: questions deliberately swap the 4 ft ladder trigger with the 5 ft protective-system trigger, or the 2 ft setback with a 2 ft depth.
Foundations and water
Systems: spread footings, mat/raft, piles (friction vs end-bearing), drilled piers, grade beams, pile caps. Footings bear below frost depth on undisturbed soil. Provide footing drains and positive slope away from the structure. Dampproofing resists soil moisture; waterproofing resists hydrostatic pressure - never interchangeable on an exam.
Worked example
Column load 60,000 lb, allowable soil bearing 3,000 psf. Required area = 60,000 / 3,000 = 20 sf. Square footing side = square root of 20 = 4.47 ft, so specify a 4 ft 6 in square. Add footing self-weight at about 150 pcf only if the question supplies it.