6.3 Foundation Types and Design

Foundation Types in Civil Engineering

Foundations transfer structural loads into the ground. Every building, bridge, and retaining wall depends on a foundation that matches the site's soil conditions, and picking the wrong type can mean excessive settlement, structural cracking, or outright failure. This section covers the two main categories (shallow and deep), how engineers design each one, and the bearing capacity and settlement analyses that drive those decisions.

Shallow and Deep Foundation Categories

Foundations fall into two broad categories based on how deep they sit relative to their width and how they transfer loads into the soil.

Shallow foundations transfer loads near the surface. They work when competent soil exists at or near grade. Common types include:

• Spread footings (also called isolated footings): support individual columns
• Strip footings (continuous footings): support load-bearing walls
• Combined footings: support two or more closely spaced columns
• Mat (raft) foundations: one large slab supporting the entire structure

Deep foundations transfer loads to stronger soil or rock well below the surface. They're used when near-surface soils can't handle the structural loads. Common types include:

• Driven piles: prefabricated elements hammered into the ground
• Drilled shafts (also called bored piles or caissons): large-diameter, cast-in-place concrete elements
• Micropiles: small-diameter drilled and grouted piles for tight access or underpinning

The depth-to-width ratio is a useful rule of thumb: if the foundation depth $D_f$ is less than or roughly equal to its width $B$, it's generally considered shallow. Deep foundations have $D_f / B$ ratios much greater than 1.

Foundation Selection and Design Considerations

Choosing between shallow and deep foundations depends on several factors working together:

• Soil conditions: What type of soil is present, and at what depth does competent bearing material exist?
• Groundwater table: High water tables complicate excavation and can reduce bearing capacity.
• Structural loads: Heavier loads (like those from high-rises or bridges) often require deep foundations.
• Site constraints: Adjacent structures, property lines, and underground utilities limit what's feasible.
• Economics: Shallow foundations are almost always cheaper if the soil can support them.

Geotechnical investigations (soil borings, lab testing of samples, in-situ tests like SPT and CPT) provide the data engineers need to make this decision. Without reliable subsurface data, foundation design is guesswork.

Two main design philosophies exist: Allowable Stress Design (ASD), which uses a single factor of safety, and Load and Resistance Factor Design (LRFD), which applies separate factors to loads and soil resistance. LRFD is becoming the standard in modern practice because it accounts for uncertainty more consistently.

Shallow Foundation Design

Spread Footing Design Process

Designing a spread footing means sizing it so the soil beneath isn't overstressed and settlement stays within acceptable limits. Here's the general process:

1. Gather soil data from the geotechnical report: soil classification, shear strength parameters (cohesion $c$ and friction angle $\phi$), compressibility, and groundwater depth.
2. Determine the loads the footing must carry: dead load, live load, and any lateral loads from wind or seismic forces. Combine these using the appropriate load combinations (from ASCE 7 or your governing code).
3. Calculate the required footing area. Divide the total service load by the allowable bearing pressure:

$A = \frac{P}{q_a}$

where $A$ is the footing area, $P$ is the total service load, and $q_a$ is the allowable soil bearing pressure.

1. Choose footing dimensions (length and width). For a square footing under a centered column, $B = \sqrt{A}$.
2. Check bearing capacity using Terzaghi's equation or one of the more advanced methods (covered below) to confirm the footing won't cause a shear failure in the soil.
3. Check settlement to make sure it stays within tolerable limits (typically around 25 mm for isolated footings, though this varies by structure type).
4. Design the structural reinforcement. The footing is a reinforced concrete element, so you need to check one-way shear, two-way (punching) shear, and flexural capacity, then detail the rebar accordingly.

Mat Foundation Design Principles

A mat foundation is essentially one big footing that supports the entire structure. Engineers turn to mats when:

• Individual spread footings would overlap or nearly overlap due to closely spaced columns
• Soil bearing capacity is low, so loads need to be spread over a larger area
• Reducing differential settlement (where one part of the building settles more than another) is a priority

Mat design involves analyzing the slab for flexure and shear under the column loads, while also considering how the soil and structure interact. A rigid mat assumption works for simple cases, but for larger or more flexible mats, engineers model the soil-structure interaction, often using a "beam on elastic foundation" approach or finite element analysis. The soil is sometimes represented by springs with a modulus of subgrade reaction $k_s$.

Deep Foundation Principles

Pile Foundation Design

Piles carry loads down to competent soil or rock through two mechanisms:

• Skin friction (shaft resistance): friction between the pile surface and the surrounding soil along its length
• End bearing (toe resistance): the pile tip resting on or in a strong layer

Most piles rely on some combination of both. A pile driven to bedrock is primarily an end-bearing pile, while a long pile in clay that doesn't reach rock is primarily a friction pile.

Pile materials and installation:

• Concrete piles: precast and driven, or cast-in-place in a drilled hole
• Steel piles: H-piles or pipe piles, good for driving through dense layers
• Timber piles: used for lighter loads; must be treated to resist decay

Capacity determination uses several approaches:

• Static analysis methods: The $\alpha$-method estimates skin friction in cohesive (clay) soils using undrained shear strength. The $\beta$-method works for cohesionless (sandy) soils using effective stress. The $\lambda$-method is a mixed approach.
• Dynamic formulas and wave equation analysis: estimate capacity based on driving resistance
• Load testing: the most reliable method. Static load tests apply a known load and measure settlement; dynamic load tests (like PDA testing) use hammer impact data.

Two additional considerations that often trip up students:

• Negative skin friction (downdrag): When soil around a pile consolidates (e.g., from fill placement or groundwater lowering), it drags down on the pile instead of supporting it. This adds load to the pile rather than providing resistance.
• Group effects: Piles in a group don't each carry their full individual capacity. The group efficiency depends on pile spacing, soil type, and arrangement.

Drilled Shaft Design Considerations

Drilled shafts are large-diameter (typically 0.5 m to 3 m), cast-in-place concrete foundations. They're a go-to choice when very large loads need to be carried to depth, or when vibration from pile driving is unacceptable.

Construction methods affect shaft quality and capacity:

• Dry method: used in stable soils above the water table; the hole stays open on its own
• Casing method: a steel casing keeps the hole open in caving soils
• Wet (slurry) method: bentonite or polymer slurry stabilizes the hole in loose or saturated soils

Like piles, drilled shafts resist loads through skin friction and end bearing. When a shaft is extended into bedrock, this is called rock socketing, and it can dramatically increase capacity.

Design must account for both axial loading (compression and sometimes tension/uplift) and lateral loading (from wind, seismic forces, or earth pressure). Lateral analysis often uses the p-y curve method, which models how the soil resists sideways movement at different depths.

Foundation Bearing Capacity and Settlement

Bearing Capacity Analysis Methods

Bearing capacity is the maximum pressure the soil can handle before it fails in shear. For shallow foundations, the classic starting point is Terzaghi's bearing capacity equation:

$q_u = cN_c + qN_q + 0.5\gamma BN_\gamma$

where:

• $q_u$ = ultimate bearing capacity
• $c$ = soil cohesion
• $q$ = overburden pressure at the foundation depth ($= \gamma D_f$)
• $\gamma$ = unit weight of soil
• $B$ = foundation width
• $N_c, N_q, N_\gamma$ = bearing capacity factors (functions of the soil friction angle $\phi$)

Terzaghi's equation assumes a strip footing with a rough base. For real-world conditions, Meyerhof's, Hansen's, and Vesic's methods add correction factors for foundation shape, depth, and load inclination.

The allowable bearing capacity divides the ultimate value by a factor of safety (commonly 2.5 to 3.0 for ASD):

$q_a = \frac{q_u}{FS}$

For deep foundations, capacity comes from the static analysis methods described above ($\alpha$, $\beta$), supplemented by empirical correlations from SPT blow counts or CPT tip resistance. Complex cases may call for finite element modeling.

Settlement Analysis Techniques

A foundation can be safe against bearing capacity failure but still settle too much. Settlement analysis has two main components:

• Immediate (elastic) settlement: occurs as soon as load is applied, mainly in sandy soils and the undrained response of clays. Calculated using elastic theory and stress distribution beneath the footing.
• Consolidation settlement: the slow, time-dependent compression of saturated clay as water is squeezed out of pore spaces. This can take months to years and is often the controlling factor for foundations on clay.

For deep foundations, settlement sources include elastic shortening of the pile/shaft itself, load transfer settlement at the tip, and group settlement effects (a pile group settles more than a single pile because the stress bulb extends deeper).

Engineers verify their analytical predictions with load testing:

• Static load tests: apply load incrementally and measure settlement directly
• Dynamic load tests: use pile driving analyzer (PDA) data
• Statnamic tests: apply a rapid load pulse; useful for high-capacity shafts

The final foundation design balances bearing capacity, settlement, constructability, and cost. A foundation that's technically perfect but wildly expensive isn't good engineering either.