Types of Foundation Systems
The selection of a foundation system is one of the most critical decisions in building design, as the foundation must safely transfer all building loads to the soil without excessive settlement or failure. Shallow foundations are used when the load-bearing soil is located near the surface, typically within 10 feet of the ground level. The most common shallow foundation type is the spread footing, which distributes column loads over a wider area of soil to reduce the bearing pressure. Continuous wall footings serve the same function for load-bearing walls, spreading the wall load along the length of the footing. The minimum footing width for a simple one-story masonry wall on medium soil is typically 12 inches, while a two-story wall requires 16 to 20 inches depending on the wall load and soil bearing capacity.
Mat foundations, also called raft foundations, are large continuous footings that support the entire building as a single unit. They are used when soil bearing capacity is low or when column loads are heavy enough that individual footings would overlap. A mat foundation can reduce differential settlement by distributing the building load uniformly over the entire footprint. The thickness of mat foundations ranges from 18 inches for light residential structures to 6 feet or more for heavy commercial buildings. The reinforcement in mat foundations is typically heavy, with bars in both directions at the top and bottom of the slab to resist bending moments in both directions.
Deep foundations transfer building loads through weak surface soils to stronger soil or rock at greater depths. Pile foundations are the most common deep foundation type, consisting of long slender columns driven or drilled into the ground. End-bearing piles transfer load through the pile tip to a firm stratum below, while friction piles transfer load through skin friction along the pile shaft. The capacity of a pile foundation depends on both the end bearing and skin friction components, with the total capacity being the sum of the two. Typical pile capacities range from 20 tons for small timber piles to over 200 tons for large diameter drilled shafts.
Foundation Design Considerations
The bearing capacity of soil is the maximum pressure that can be applied without causing shear failure or excessive settlement. Allowable bearing pressures for different soil types are established through geotechnical investigation and laboratory testing. Dense sand and gravel can typically support 4,000 to 6,000 pounds per square foot, while soft clay may only support 1,000 to 2,000 psf. The footing size must be selected so that the applied bearing pressure does not exceed the allowable value. spread footing design for residential buildings. pile foundation capacity testing and verification. Douglas fir lumber strength properties. The net bearing pressure is calculated by dividing the total column load by the footing area, subtracting the weight of the footing itself.
Settlement analysis is equally important as bearing capacity in foundation design. Total settlement is the overall vertical movement of the foundation, while differential settlement is the difference in movement between different parts of the building. Differential settlement is more damaging than total settlement because it causes distortion of the structure. The allowable differential settlement for a typical framed building with masonry veneer is 1/2 inch. For structures with sensitive finishes or equipment, the allowable differential settlement may be as low as 1/4 inch. Consolidation settlement in clay soils can continue for years after construction, requiring careful analysis and sometimes surcharge loading to pre-compress the soil before building construction.
| Soil Type | Allowable Bearing Capacity (psf) | Settlement Potential | Typical Foundation |
|---|---|---|---|
| Hard Rock | 12,000+ | Negligible | Spread footings |
| Dense Sand/Gravel | 4,000-6,000 | Low | Spread footings |
| Medium Clay | 2,000-4,000 | Moderate | Mat or spread |
| Soft Clay | 1,000-2,000 | High | Pile foundation |
| Loose Sand | 1,500-3,000 | Moderate | Deep foundation |
Foundation Construction
Foundation construction begins with excavation to the required depth, which must extend below the frost line to prevent frost heave damage. In northern climates, the frost depth ranges from 36 to 48 inches below grade. The excavation bottom must be undisturbed soil or compacted fill that has been approved by the geotechnical engineer. Soft spots encountered during excavation must be over-excavated and replaced with compacted granular material. Groundwater encountered during excavation must be controlled by dewatering to maintain stable working conditions and prevent disturbance of the bearing soil.
Concrete for foundations should have a minimum compressive strength of 3,000 psi for residential applications and 4,000 psi for commercial structures. The concrete mix must be workable enough to flow around reinforcement and fill all corners of the forms. Reinforcement placement must comply with the design drawings, with proper cover maintained using concrete blocks or chairs. The minimum concrete cover over reinforcement is 3 inches for concrete cast against earth and 1.5 inches for concrete exposed to weather. Foundation walls must be waterproofed on the exterior and provided with drainage systems to prevent water accumulation against the wall.
The curing of foundation concrete is critical for achieving design strength and durability. Moist curing should continue for at least 7 days after placement. In cold weather, the concrete must be protected from freezing using insulation blankets or heated enclosures. In hot weather, the concrete should be placed during cooler periods and kept moist to prevent rapid drying that causes cracking. Backfilling against foundation walls should not begin until the concrete has reached at least 75 percent of its design strength, typically 7 to 14 days after placement depending on temperature conditions.
Waterproofing and Drainage
Foundation waterproofing protects the below-grade structure from water infiltration that can cause structural damage, mold growth, and indoor air quality problems. Exterior waterproofing systems include liquid-applied membranes, sheet membranes, and bentonite clay panels that are applied to the foundation wall before backfilling. The waterproofing must extend from the footing to the finished grade level and must be protected from damage during backfilling by a protection board or drainage mat. Interior waterproofing systems are applied to the inside of existing walls and are less effective than exterior systems because they do not prevent water from entering the wall structure.
Foundation drainage systems collect and remove water that accumulates around the foundation. A perforated drain pipe installed at the footing level around the entire foundation perimeter collects water and conveys it to a sump pit or daylight outlet. The pipe must be surrounded by washed gravel that provides a drainage path to the pipe while preventing soil from clogging the perforations. Geotextile fabric wrapped around the gravel layer prevents fine soil particles from migrating into the drainage system. The drain pipe must have a minimum slope of 1/8 inch per foot toward the outlet and must be accessible for inspection and cleaning through cleanout fittings at regular intervals.