6.4 Foundations, Footings, and Site Drainage
Key Takeaways
- Exterior footings must bear below the frost line (IBC 1809.5) to prevent frost heave; interior heated footings are exempt.
- 1 cubic yard = 27 cubic feet; compute footing volume in cubic feet then divide by 27 and add waste.
- Lower water-cement ratio means higher strength; f'c is tested on cylinders per ASTM C39 at 28 days.
- Footing width = wall load (plf) ÷ allowable soil bearing (psf); weaker soil requires a wider footing.
- Slope finished grade 6 in within 10 ft away from the foundation; use waterproofing (not dampproofing) under hydrostatic pressure.
Foundations, Footings, and Site Drainage
Foundations transfer building loads into the soil. The footing spreads the load over enough bearing area; the foundation wall or pier carries it down to the footing. Footings must rest below the local frost line — IBC 1809.5 requires the bottom of exterior footings to extend below the frost depth (commonly 12–48 in depending on climate) to prevent frost heave. Interior footings in heated buildings are exempt from frost depth.
Concrete basics: yield, water-cement ratio, strength
Concrete quantities are measured in cubic yards (CY): 1 CY = 27 cubic feet. A footing 2 ft wide × 1 ft deep × 100 ft long = 200 cu ft ÷ 27 = 7.4 CY (order ~8 CY with waste).
Key relationships:
- Water-cement (w/c) ratio — lower ratio = higher strength and durability. A 0.45 w/c mix is far stronger than 0.60; excess water weakens concrete and causes bleeding.
- Compressive strength (f'c) — tested on cylinders per ASTM C39 at 28 days; residential footings often 2,500 psi, commercial slabs 3,000–4,000 psi.
- Slump — measured per ASTM C143; typical footing slump 3–5 in. Adding water on site to raise slump is the classic strength-killing trap.
Footing sizing and reinforcement
Footing width is set so soil pressure stays below the allowable bearing value.
Worked example: a wall delivers 6,000 lb per linear foot onto soil rated 2,000 psf. Required footing width = load ÷ bearing = 6,000 ÷ 2,000 = 3 ft wide. If the soil were only 1,500 psf, width grows to 4 ft.
Reinforcement notes tested on the exam:
| Item | Rule of thumb |
|---|---|
| Concrete cover (cast against earth) | 3 in (ACI 318) |
| Cover (formed, exposed to earth/weather) | 1.5–2 in |
| Footing thickness | ≥ width of projection, min 6 in residential |
| Anchor bolts | 1/2 in min, embed 7 in, max 6 ft o.c. (IRC/IBC) |
Site drainage and foundation moisture control
Water is the foundation's enemy. Code and good practice require moving it away:
- Final grade slope — IBC 1804.4 / IRC R401.3: slope ground away from the foundation a minimum of 6 in within the first 10 ft (a 5% slope).
- Foundation drain — perforated pipe in washed gravel at the footing, draining to daylight or a sump, required where the water table is high.
- Dampproofing vs waterproofing — dampproofing resists moisture vapor; waterproofing (IBC 1805) resists hydrostatic pressure and is required where a high water table exists.
- Backfill — compact in lifts and slope; never backfill before the first-floor framing braces the wall, or the wall can be pushed in.
A continuous footing is 2 ft wide, 12 in deep, and 90 ft long. How many cubic yards of concrete are required (before waste)?
Per IBC/IRC, what is the minimum required slope of finished grade away from a foundation within the first 10 feet?
Shallow vs. Deep Foundations
Choose the system to the soil and load. Shallow foundations spread load near the surface: spread (isolated) footings under columns, continuous (strip) footings under walls, combined footings, and mat/raft slabs for weak or uniform soils. Deep foundations carry load past weak strata: driven piles and drilled piers/caissons transfer load by end bearing on rock or skin friction along the shaft. Footing width is sized so soil pressure ≤ allowable bearing.
Frost Depth, Footing Sizing, and a Worked Example
Footings must bear below the frost line (the frost-depth varies by climate; e.g., 12 in in the South to 48+ in up North) to avoid frost heave. Worked example: A column carries 30,000 lb onto soil with 3,000 psf allowable bearing. Required area = 30,000 / 3,000 = 10 ft², so a square footing ≈ 3'-2" × 3'-2" (3.17 ft side). Reinforce per ACI; the footing also must extend below frost depth and onto undisturbed/compacted soil.
Foundation Drainage and Waterproofing
Keep water away from foundations: perimeter (footing) drains — perforated pipe in gravel wrapped in filter fabric, sloped to daylight or a sump — relieve hydrostatic pressure. Grade the finish soil to slope away from the building (code commonly 6 in fall within the first 10 ft, ~5%). Below-grade walls get dampproofing or waterproofing (Ch. 10). Standing water against a wall is the classic cause of basement leaks and the exam's go-to drainage failure.
Common Exam Traps
- Trap: Footings can bear above the frost line if reinforced. They must bear below frost depth.
- Trap: A pile only works by end bearing. It also carries load by skin friction.
- Trap: Grading flat at the foundation is fine. Slope away ~6 in / 10 ft.
- Trap: Footing drain without gravel/filter fabric — it clogs with fines.
A column load of 24,000 lb bears on soil with an allowable bearing pressure of 2,000 psf. What minimum footing area is required?
Foundation Wall Types and Anchorage
Foundation walls are poured concrete, CMU (block), or precast; they transfer building loads to the footing and resist lateral soil pressure on basements/retaining walls. Tall or backfilled-early walls can fail in flexure if the first floor (which braces the top) is not yet in place — backfill timing matters. The wood structure ties down with anchor bolts (commonly 1/2 in dia., embedded ~7 in, max 6 ft on center, within 12 in of each plate end) per IRC/IBC. A vapor retarder under the slab and proper reinforcement complete the system the exam expects you to recognize.