Shallow vs. Deep Foundations
The first fundamental decision in foundation design is whether to use shallow or deep foundations. Shallow foundations (spread footings, strip footings, mat foundations) bear on soil or rock near the surface, transmitting building loads to the soil at relatively shallow depths (typically less than the footing width below grade). They are appropriate when competent soil with adequate bearing capacity exists near the surface and expected settlements are within acceptable limits. Deep foundations (driven piles, drilled piers, auger-cast piles) extend through weak near-surface soils to bear on deeper competent strata or develop their capacity through skin friction along their length. They are required when weak soils, high loads, or strict settlement limits preclude shallow foundations.
The geotechnical report (boring logs, laboratory test results, and engineering recommendations) is the foundation for all foundation design decisions. Structural engineers must read the geotechnical report carefully to understand soil conditions at the site, not just the allowable bearing pressure recommendation. Understanding the basis for the recommendation -- what soils are present, what test results were used, what assumptions were made -- is essential for applying the recommendation correctly and identifying conditions that may require special attention.
Bearing Capacity Theory
Terzaghi (1943) developed the foundational general bearing capacity equation for shallow footings: qu = c x Nc + q x Nq + 0.5 x gamma x B x N_gamma. Here qu is the ultimate bearing capacity (in units of stress, e.g., psf or kPa); c is soil cohesion (shear strength component independent of normal stress, from undrained shear strength Su for clays or from effective cohesion c for sands); q is the overburden pressure at the footing base level (gamma x Df, where gamma is soil unit weight and Df is footing depth below grade); B is footing width; gamma is soil unit weight below the footing; and Nc, Nq, N_gamma are dimensionless bearing capacity factors that are functions of the soil friction angle phi. For phi = 0 (undrained condition in saturated clay), Nc = 5.14, Nq = 1, N_gamma = 0, giving the well-known qu = 5.14 x Su for a continuous footing.
Meyerhof (1963) and Hansen (1970) extended Terzaghi's equation with correction factors for footing shape (square vs. strip vs. circular), load inclination, ground surface slope, and footing base tilt. Modern practice typically uses the generalized bearing capacity equation with Meyerhof or Hansen correction factors. The ultimate bearing capacity qu is divided by a factor of safety (typically 2.5-3.0 for ASD, accounting for uncertainty in soil properties, load estimation, and the consequences of failure) to obtain the allowable bearing capacity qa used for footing design.
Settlement Analysis
A footing may have adequate bearing capacity (no shear failure) but still produce unacceptable settlements. Settlement is often the governing criterion for shallow foundations on compressible fine-grained soils. Total settlement has three components: immediate (elastic) settlement occurs during construction from elastic compression of soil; consolidation settlement occurs over months to years in saturated clays as excess pore water pressure dissipates (water slowly drains from the loaded clay); and secondary compression (creep) occurs over years to decades in organic soils and highly plastic clays at essentially constant effective stress.
Immediate settlement for sands and unsaturated soils is estimated using elastic theory: delta_i = q x B x (1-nu^2) / Es x I_f, where q is the net foundation pressure, nu is Poisson ratio, Es is the elastic modulus of the soil (estimated from SPT N-values or CPT tip resistance), and I_f is a shape and rigidity factor. Consolidation settlement in clays is calculated from the compression index (Cc) and recompression index (Cr) measured in oedometer tests: delta_c = H x Cc/(1+e0) x log(sigma_f/sigma_0) for normally consolidated clays, where H is the consolidating layer thickness, e0 is initial void ratio, sigma_0 is initial effective vertical stress, and sigma_f is final effective vertical stress. Differential settlement (the difference in settlement between adjacent footing locations) is as important as total settlement -- angular distortion (differential settlement divided by column spacing) above 1/300 can crack cladding and above 1/150 can affect structural frames.
Foundation Types and Selection
Spread (isolated) footings support individual columns. They are the simplest and most economical foundation type when soil conditions are adequate. Footing size is determined by dividing the column load (including self-weight of footing and soil overburden) by the allowable bearing capacity. Combined footings support two or more columns when column spacing is too close for individual spread footings. Strap footings (cantilever footings) connect an interior column footing to an eccentrically loaded column near a property line using a rigid strap beam to balance moments and equalize soil pressure.
Strip footings (continuous wall footings) support bearing walls. They are typically sized to provide 1,000-2,000 psf bearing pressure for lightly loaded residential walls and may require wider footings on weaker soils. Mat foundations (raft foundations) cover the entire building footprint with a single reinforced concrete slab, distributing column and wall loads to the soil at reduced pressure and stiffening the foundation against differential settlement. Mats are appropriate when individual spread footings would cover more than 50-60% of the building footprint (overlapping), when soil conditions are variable, or when heavy loads require very large footing areas. The mat is designed as an inverted floor slab with soil pressure as the "load" and column reactions as the "supports."
Special Conditions: Expansive Soils and Frost Heave
Expansive soils (predominantly CH and MH classification with swell pressure measured above 1,000 psf) can exert enormous pressures on foundations as they absorb water and expand. Damage from expansive soils costs billions of dollars annually in the United States, particularly in Texas, Colorado, Oklahoma, and California. Foundation options for expansive soils include: drilled piers below the active zone of moisture fluctuation (typically 8-15 feet deep) with a void space under grade beams to allow soil to heave without loading the structure; post-tensioned concrete slabs with perimeter beams designed to span across heaving soil; and chemical soil stabilization (lime or cement treatment) to reduce swell potential before construction.
Frost heave occurs when water in soil freezes and expands, lifting footings. Frost depth (the depth to which soil freezes in a 100-year extreme event) ranges from negligible in warm climates to 5-6 feet in northern Minnesota and New England. All footings must bear below the frost depth to prevent frost heave from lifting the structure. This requirement often governs minimum footing depth in cold climates regardless of bearing capacity requirements.