11. Foundation in Soil Mechanics

 

What are Foundations in Soil Mechanics?

In civil engineering and soil mechanics, a foundation (or footing) is the structural element that transfers loads from a building, bridge, or other structure to the underlying soil or rock. Its primary functions are:

• Distribute loads over a sufficient area to prevent excessive settlement or tilting.
• Ensure the soil's bearing capacity is not exceeded, avoiding shear failure.
• Minimize differential settlement for structural stability.
• Resist uplift, overturning, and lateral forces (e.g., from wind or earthquakes).

Foundations are classified into two main types based on depth: shallow and deep. The choice depends on soil properties, load magnitude, groundwater conditions, and economic factors.

 

Types of Foundations

Shallow Foundations

These are used when competent soil exists near the surface (typically depth < width, Df/B < 1-2). They spread loads over a large area.

Common types:

• Spread/Pad Footing: Isolated square or rectangular base for columns.
• Strip Footing: Continuous strip under walls or closely spaced columns.
• Raft/Mat Foundation: Large slab supporting multiple columns or the entire structure, used for poor soils or high loads to reduce differential settlement.
• Combined Footing: For two or more columns when space limits individual footings.

 

 

Deep Foundations

Used when surface soils are weak, and loads must transfer to deeper, stronger layers (Df/B > 4-10). They rely on end-bearing, friction, or both.

Common types:

• Pile Foundations: Slender columns (concrete, steel, timber) driven or bored into soil. Types include end-bearing, friction, or combined.
• Pier Foundations: Large-diameter drilled shafts (similar to piles but thicker).
• Caissons: Watertight boxes sunk into soil, often for bridges in water.

 

 

 

Ultimate Bearing Capacity (UBC)

The ultimate bearing capacity (qu) is the maximum gross pressure the soil can sustain before shear failure. Failure modes include:

• General Shear Failure: Complete failure surface, heaving on both sides (dense sands).
• Local Shear Failure: Partial heaving, common in medium soils.
• Punching Shear Failure: Vertical shearing, no heaving (loose sands or soft clays).

Terzaghi's theory (1943) for strip footings:

qu = c Nc + γ Df Nq + 0.5 γ B Nγ

Where:

• c = cohesion
• γ = unit weight of soil
• Df = depth of foundation
• B = width
• Nc, Nq, Nγ = bearing capacity factors (functions of soil friction angle φ).

 

 

Later theories (Meyerhof, Hansen, Vesic) include shape, depth, inclination, and base tilt factors for more accuracy.

Net Ultimate Bearing Capacity = qu - γ Df Safe Bearing Capacity = Net UBC / Factor of Safety (typically 2.5-3) Allowable Bearing Pressure considers both strength and settlement.

Settlement of Foundations

Even if bearing capacity is safe, excessive settlement can damage structures. Total settlement (S) = Immediate (Si) + Consolidation (Sc) + Secondary (Ss).

• Immediate Settlement: Elastic/plastic deformation under undrained loading (common in sands).
• Primary Consolidation: Time-dependent in saturated clays (as discussed previously).
• Secondary Consolidation: Creep in organic or highly plastic soils.

Allowable settlement limits: 25-50 mm total for buildings, less for sensitive structures.

Design Considerations in Soil Mechanics

1. Site investigation (borings, SPT, CPT) to determine soil profile and parameters.
2. Calculate bearing capacity and settlement.
3. Apply factors of safety.
4. Consider groundwater effects (reduces capacity in submerged soils).
5. Eccentricity and inclined loads.
6. Seismic and dynamic loads.

Proper foundation design ensures long-term stability and prevents failures like excessive tilting (e.g., Pisa Tower) or collapse. It's a core application of soil mechanics principles in geotechnical engineering.