Types

• Adobe
  Sun-dried earth brick of clay, silt, sand, and straw, used for thick thermal-mass walls in dry climates.
• Bedrock
  Solid rock beneath loose surface material, the strongest foundation bearing stratum when found at reachable depth.
• Clay
  Fine-grained earth, fired into brick, tile, and terra cotta, used for masonry walls, veneer, paving, roofing, and sanitary ware.
• Clay Soil
  Plastic, fine-grained soil finer than 0.002 mm that holds water, swells and shrinks, and serves as a barrier material.
• Crushed Stone
  Mechanically crushed and graded rock, used as aggregate base, drainage stone, concrete and asphalt aggregate, and fill.
• Gravel
  Loose rock fragments larger than sand, used as drainage stone, base course, fill, and concrete aggregate.
• Lightweight Aggregate
  Low-density kiln-expanded clay, shale, or slate granules used in lightweight concrete, fill, drainage, and growing media.
• Peat
  Partially decayed vegetation accumulated in waterlogged ground, highly compressible and unsuitable as foundation support.
• Sand
  Granular mineral particles, mostly quartz, between gravel and silt in size, used in concrete, mortar, glass, and fill.
• Silt
  Fine, non-plastic soil between sand and clay, weak when wet, requiring careful treatment under foundations and slabs.
• Structural Fill
  Engineered, compacted soil or aggregate placed in lifts under slabs, footings, and pavements to a specified density.
• Topsoil
  Organic-rich surface soil layer, about 5 to 10 inches deep, used for landscaping, planting beds, and finish grading.

Code reference

Always verify the adopted code edition and local amendments for the project jurisdiction.

Governing editions

Foundation soils and fill

• IBC 2024, Section 1809.2: footings bear on undisturbed soil, compacted fill, or controlled low-strength material (CLSM)
• IBC 2024, Section 1809.3: footing slopes up to 10 percent; steeper slopes are stepped
• IBC 2024, Section 1809.5: frost protection: footings below the frost line, per ASCE 32, or on rock
• IBC 2024, Section 1807.2.2: lateral soil loads per Section 1610; SDC D, E, and F with more than 6 ft of backfill need seismic lateral earth pressure from a geotechnical investigation
• IBC 2024, Section 1807.2.3: retaining wall safety factor 1.5 against sliding and overturning, 1.1 with earthquake loads

Bearing values and soil classes

• IBC 2024, Section 1806.2 (Table 1806.2): presumptive vertical bearing pressure by class of material: crystalline bedrock 12,000 psf; sedimentary and foliated rock 4,000 psf; sandy gravel and gravel (GW, GP) 3,000 psf; sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC) 2,000 psf; clay, sandy clay, silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH) 1,500 psf (bedrock, gravel, sand, silt, clay soil)
• IBC 2024, Section 1806.2: mud, organic silt and clay (OL, OH), peat (Pt), and undocumented fill have no presumptive capacity unless supporting data are submitted (peat, structural fill)
• IBC 2024, Section 1806.3: lateral bearing and sliding resistance; for clay, silt, and similar soils sliding resistance may not exceed one-half the dead load (Section 1806.3.2)
• IBC 2024, Section 1803.5.3: soil is expansive only if all four apply: plasticity index 15 or greater (ASTM D4318), more than 10 percent passing the No. 200 sieve and more than 10 percent finer than 5 micrometers (ASTM D6913), and expansion index above 20 (ASTM D4829); the first three need not be tested if the expansion index is determined (clay soil)
• IBC 2024, Section 1803: geotechnical investigation per ASTM D420; borings continuous to 12 ft or 12 ft below the lowest floor or foundation, whichever is greater; report required unless waived (Section 1803.6)

Compacted fill and grading

• IBC 2024, Section 1804.6: fill more than 12 in. deep needs an approved geotechnical report; fill 12 in. or less may skip the report if in-place dry density is at least 90 percent of the maximum dry density at optimum moisture per ASTM D1557, with special inspection (structural fill)
• IBC 2024, Section 1804.4: ground slopes away from the foundation at least 1:20 for 10 ft; swales and impervious surfaces within 10 ft at least 2 percent; finished floor at least 6 in. above finished grade at the perimeter (topsoil, structural fill)

Moisture protection

• IBC 2024, Section 1805.2.2: dampproofing from the top of the footing to above ground level
• IBC 2024, Section 1805.3.2: waterproofing where hydrostatic pressure exists, from the bottom of the wall to 12 in. above maximum groundwater

Referenced standards

Sources

• UpCodes: 2024 IBC Chapter 18 soils and foundations, https://up.codes/viewer/department-of-defense/ibc-2024/chapter/18/soils-and-foundations