1: Soil types and bearing capacity

Section 1: Foundations - Soil Types and Bearing Capacity

The foundation is the critical interface between a structure and the ground it rests upon. Understanding soil types and their bearing capacity – the maximum pressure the soil can support without excessive settlement or shear failure – is fundamental to safe and efficient foundation design. Different soil types exhibit vastly different load-bearing characteristics and behaviors under stress.

Soils are broadly classified based on particle size, distribution, and mineral composition:

• Granular Soils (Cohesionless): Sands and gravels. Characterized by individual particles (visible to the naked eye) that rely primarily on inter-particle friction for strength. Key properties:
  • Good Drainage: Water flows through easily.
  • High Permeability: Reduces risk of frost heave and water pressure buildup.
  • Strength: Generally have high bearing capacity when compacted. Strength increases with density and particle angularity. Loose sands can be prone to liquefaction under vibration.
  • Settlement: Settlement under load is typically rapid (occurring during/soon after construction) and relatively small if properly compacted.
• Cohesive Soils: Silts and Clays. Characterized by fine particles (often microscopic) and significant electrochemical attraction (cohesion) between particles. Key properties:
  • Poor Drainage: Water moves very slowly through the tiny pore spaces.
  • Plasticity: Can be molded when wet; strength and volume change significantly with moisture content.
  • Strength: Bearing capacity is highly sensitive to water content. Dry clays are strong, but saturated clays can be very weak (low bearing capacity). Sensitive clays can lose strength drastically when disturbed.
  • Settlement: Exhibit long-term, time-dependent settlement (consolidation) as water is slowly squeezed out under load. This settlement can be large and uneven, posing significant risk to structures.
• Organic Soils: Peat, muck, humus. Contain significant decomposed plant/animal matter. Key properties:
  • Very Low Strength: Extremely low bearing capacity due to high compressibility and water content.
  • High Compressibility: Prone to large, long-term settlements.
  • Unsuitable: Generally avoided for direct foundation support without extensive soil improvement or deep foundations bypassing the layer.

Bearing Capacity Factors: The ultimate bearing capacity ($q_{ult}$) of a soil is determined by its inherent shear strength and influenced by:

1. Soil Type & Properties: Cohesion ($c$), angle of internal friction ($\phi$), density, and moisture content are primary determinants.
2. Foundation Geometry: Width, depth, and shape of the footing. Wider and deeper foundations generally have higher capacity.
3. Groundwater Level: Saturation reduces effective stress and shear strength, lowering bearing capacity, especially in granular soils. Hydrostatic pressure (uplift) must also be considered.
4. Loading Conditions: Magnitude, eccentricity, and inclination of the applied load.

Determining Bearing Capacity: Geotechnical investigations involving soil sampling (disturbed and undisturbed), Standard Penetration Tests (SPT), Cone Penetration Tests (CPT), and laboratory tests (triaxial shear, consolidation) are essential. Results feed into bearing capacity equations (e.g., Terzaghi's, Vesić's) which incorporate soil strength parameters ($c$, $\phi$), foundation geometry, and depth to calculate allowable bearing pressure ($q_{all}$), applying a factor of safety to $q_{ult}$ for design. The allowable bearing pressure must safely support structural loads while limiting settlement to acceptable levels.