Foundations and Geotechnical Considerations
Soil behavior
Foundations transfer loads to soil. Candidates must understand bearing capacity (strength against shear failure) versus settlement (serviceability), and that either may govern. Soil properties come from borings, standard penetration tests, and laboratory tests. Key concepts include allowable bearing pressure, differential settlement, and the effect of groundwater on effective stress.
Foundation types
Shallow foundations (spread footings, mats) suit competent soil near the surface. Deep foundations (piles, drilled shafts) transfer load by end bearing and skin friction to deeper strata. Traps include ignoring uplift on footings from high groundwater, forgetting that a mat can redistribute load but not eliminate settlement, and misreading a soil report's allowable pressure as a strength value rather than a serviceability-based number.
Lateral and special conditions
Retaining walls need stability checks for sliding, overturning, and bearing, plus drainage to prevent hydrostatic buildup. Expansive clays and frost-susceptible soils require special detailing. Candidates should know that a footing must extend below the frost line in cold climates.
Worked rule of thumb
For a square footing carrying P = 100 kip on soil with allowable bearing 2.5 ksf, required area is 100/2.5 = 40 ft², so a 6.3 ft square works; round up to 6.5 ft. Then check that the footing's own weight (roughly 0.15 kcf × volume) is added to P before final sizing, and verify settlement is acceptable.