Every building needs a foundation, and the foundation options range from a $15,000 slab to a $2,000,000 deep-foundation system. The selection follows a decision tree. Here's the one engineers use.
Step 1: What does the ground offer?
The geotechnical report's boring logs and lab data answer the first question: at what depth is competent bearing, and how much will it settle? Shallow competent soil with tolerable settlement → shallow foundations. Deep competent bearing, thick soft layers, or intolerable settlement → deep foundations or ground improvement.
The red flags that force the expensive options: undocumented fill (cannot support foundations — bypass or remove), thick organics (compress forever — bypass), high water table with weak soils (dewatering plus deep foundations), and liquefiable layers (mitigate or bypass).
Step 2: What do the loads demand?
Light loads on decent soil → spread footings. Heavy column loads → mats or piles. The settlement analysis (not just bearing capacity) often governs: a soil strong enough for the pressure may still settle too much differentially.
Step 3: What's the water doing?
High groundwater complicates everything: dewatering for the excavation, buoyancy on the finished foundation, waterproofing as a structural system. Sometimes the water table alone pushes the decision (a mat doubling as the waterproof slab, or deep foundations avoiding the dewatering).
Step 4: What does the site allow?
Vibration limits near existing structures (driven piles out, drilled shafts in), access constraints (micropiles where rigs can't go), contaminated soils (minimize disturbance), and the construction schedule (driven piles are fast, drilled shafts are slow).
Step 5: What does the money say?
Shallow foundations: $5-$15/sq ft of footprint. Ground improvement: the middle path (30-60% of deep foundations). Deep foundations: 3-10x shallow. The geotechnical alternatives analysis — with costs — is the deliverable that lets the owner decide with eyes open.
The honest truth
Foundation selection is the highest-leverage decision in structural engineering: it determines the project's risk profile, a major cost center, and the construction schedule. Spend the investigation money ($10,000-$50,000) to inform the million-dollar decision. The cheapest foundation is the one the ground actually wants.