Geotechnical Science

Soil Compaction

• Definition: The mechanical process of increasing soil density by pressing air voids out between soil particles using weight, vibration, or impact.

• Purpose: It reduces future settlement, increases load-bearing strength, and decreases water seepage.

• Methods: Commonly achieved using smooth-wheel rollers, sheep-foot rollers, vibratory compactors, and dynamic compaction heavy weights.

Bearing Capacity of Soil

• Definition: The maximum load or contact pressure that soil can safely support from a structure without experiencing a shear failure or excessive settlement.

• Key Types:

  • Ultimate Bearing Capacity (qu): The theoretical maximum pressure that triggers a sudden shear failure in the soil.

  • Allowable / Safe Bearing Capacity: The ultimate capacity divided by a factor of safety (typically 3 for shallow foundations).

• General Formula (Terzaghi’s Equation):

• (Where c' is cohesion, y is unit weight, D is foundation depth, B is footing width, and (N_c, N_q, N_y) are bearing capacity factors).

• Typical Safe Values: Ranges from soft clay (< 75 kPa) up to dense gravel or hard rock (200 to over 2000 t/m²). Learn more via the Tensar Guide on Bearing Capacity.

Soil Liquefaction

• Definition: A phenomenon where saturated, loose, cohesionless soils (such as fine sands and silts) temporarily lose their shear strength and stiffness during dynamic or seismic loading, behaving like a viscous liquid.

• Mechanism: Rapid shaking causes pore water pressure to rise sharply until it equals or exceeds the grain-to-grain contact stress, floating the particles apart.

• Common Effects: Building tipping, structural sinking, sand boils, lateral spreading, and utility line damage.

• Prevention & Mitigation: Ground improvement techniques include vibratory compaction, stone columns, dynamic compaction, and compaction grouting.