Every forensic engineer has a mental catalog of foundation failures, and the striking thing is how repetitive it is. The same causes, the same crack patterns, the same preventable mistakes. Here are the seven.
1. Expansive soils
Clay soils that swell when wet and shrink when dry move foundations seasonally — the classic stair-step brick cracks and sticking doors that worsen every year. Prevention: soil testing before design, pier foundations to stable depth (not shallow footings on active clay), and moisture maintenance (consistent watering, no leaks) around the foundation. Post-tensioned slabs designed for the soil's movement are the standard in expansive-soil regions.
2. Poor drainage and water management
Water is the universal accelerant: it saturates bearing soils (losing strength), erodes soil from beneath footings, and drives hydrostatic pressure against basement walls. The failures look like settlement, but the cause is water. Prevention: gutters discharging away from the foundation, positive grading (6 inches of fall in 10 feet), and subsurface drainage where groundwater is high. Most 'foundation problems' are drainage problems wearing a foundation's clothes.
3. Inadequate bearing capacity
Footings sized for the wrong soil strength settle excessively — the geotechnical report said 2,000 psf, the design assumed 3,000, or nobody tested at all. Prevention: actual borings with laboratory testing (not the neighbor's anecdote), footings sized from the report's recommendations, and proof-rolling or inspection of the bearing surface before concrete. The bearing check happens once, before the pour — there is no second chance.
4. Fill placed without compaction
Structures built on uncompacted or undocumented fill settle as the fill consolidates — sometimes for years. The addition on fill drops while the original house on native soil stays put, tearing the connection apart. Prevention: engineered fill (specified material, placed in lifts, tested for compaction) or deep foundations through the fill to native soil. 'It looked compacted' is not a compaction test.
5. Plumbing leaks under slabs
A leaking water or sewer line beneath a slab-on-grade erodes the supporting soil, creating voids the slab then spans — until it cracks and drops. The failure appears as localized settlement with no soil explanation, because the soil washed away. Prevention: pressure-test plumbing before the pour, sleeve penetrations, and investigate localized settlement with leak detection before assuming a soils problem.
6. Trees and vegetation
Large trees near foundations extract moisture from clay soils (differential settlement as one side dries) and roots physically displace footings and slabs. The pattern: settlement on the tree side, seasonal movement tracking rainfall. Prevention: root barriers, species selection, and setback distances scaled to the tree's mature size. Removing the tree after decades of growth can cause heave — the soil rebounds when the water thief leaves.
7. Adjacent construction and excavation
Excavating next door removes the lateral support your foundation relied on — shoring failures, dewatering that draws down your water table, and vibration from pile driving all damage neighboring foundations. Prevention: pre-construction surveys of adjacent structures (document existing cracks before the neighbor's shoring goes in), monitoring during excavation, and shoring designed by a geotechnical engineer — not the excavator's judgment.
The through-line
Six of the seven causes are water, soil knowledge, or construction quality — not structural design errors. The foundation design is rarely the problem; the ground it sits on and the water around it are. Spend the money on the geotechnical investigation, the drainage, and the compaction testing. The foundation itself is the cheap part.