Soils
Introduction to Foundations
Definition of foundation: Something that supports a structure or load.
Foundation types: Shallow (below superstructure) and deep (extends into the earth).
Focus of chapter: Shallow foundations, primarily footings.
Types of Shallow Foundations
Individual Footing: Supports a single column.
Combined Footing: Supports two or more columns by one large footing.
Continuous or Wall Footing: Extended to support long structures like walls.
Strap Footing: Multiple footings connected by a beam (strap).
Mat or Raft Foundation: Large slab supporting multiple columns not aligned.
Design Criteria for Foundations
Positioning: Correct vertical and horizontal placement to avoid outside influences.
Bearing Capacity: Must prevent failure due to excessive loads.
Settlement: Must prevent excessive settlement.
Steps for Footing Design
Calculate loads on the footing.
Obtain soil profiles and test results.
Determine depth and location of the footing.
Evaluate soil bearing capacity.
Determine footing size.
Compute footing contact pressure and stability checks.
Estimate total and differential settlements.
Design the footing structure.
Loads Acting on Foundations
Dead Load: Permanent weight (e.g., structure materials).
Live Load: Temporary applied weights (e.g., people, furniture).
Wind Load: Forces from wind acting on structures.
Snow Load: Weight from accumulated snow on surfaces.
Earth Pressure: Lateral forces from soil substructure.
Water Pressure: Forces from moisture in soil, potentially causing uplift.
Earthquake Forces: Seismic forces acting on the structure.
Specific Loads Explained
Dead Load: Calculated based on known material weights, requires initial estimates during design.
Live Load: Generally specified by building codes (e.g., 100 lb/ft² for restaurants).
Wind Load: Varies with building height; calculated per local regulations (e.g., 15 lb/ft² for <30 ft high).
Snow Load: Varies by region; averages around 6 lb/ft³.
Earth Pressure: Acts laterally against the foundation; can be treated like dead load.
Water Pressure: May cause hydrostatic uplift and must be managed to avoid soil strength reduction.
Earthquake Forces: Consult building codes for local specifications.
Foundation Depth and Location Considerations
Frost Action: Prevent movement caused by soil frost by placing footings below frost line (4+ ft in some areas).
Soil Volume Change: Clay soils may shrink or swell; design to mitigate this below 5-10 ft depth.
Adjacent Structures: New footings must not disturb old structures or violate property lines; maintain safe distance.
Groundwater: Preferably locate above groundwater to avoid problems like hydrostatic pressure and reduced soil strength.
Underground Defects: Avoid constructing on or near faults, caves, and man-made structures like sewers.
Bearing Capacity Analysis
Bearing Capacity: Refers to soil's ability to support a structure without shear failure.
Ultimate Bearing Capacity (𝑞_{ult}): Max load per area causing shear failure.
Allowable Bearing Capacity (𝑞{a}): Load per area supportable without movement, given by [ 𝑞{a} = \frac{𝑞_{ult}}{FS} ] where FS is the factor of safety (usually 2.5-3).
Evaluation Process: Review shear resistance under load, evaluate potential slip surfaces to find critical failure load.
Calculating Bearing Capacity
Terzaghi's Bearing Capacity Equations: Use factors based on soil friction and cohesion for analysis in both cohesive and cohesionless soils.
**Methods to Determine Soil Strength: **
Unconfined compression tests for clay; cohesion measured by [ c = \frac{1}{2} q_{u} ].
Corrected SPT values to determine angle of internal friction (φ).
Effect of Water Table on Bearing Capacity
Adjust calculations when the water table affects soil unit weight.
Use submerged unit weight if the water table is at or above the footing.
Find effective unit weight linearly between submerged weight at ground surface and full weight at depth.
Contact Pressure
Defined as pressure between the footing base and soil.
Computed using flexural formula for load conditions. Variations appear in the contact pressure distribution due to uneven loading and soil characteristics.
Total and Differential Settlement
Calculate total settlement after obtaining footing size to check for excessive movement.
Permissible settlement is largely dictated by the structure type and requirements.
Structural Design of Footings
Post-load calculations are the responsibility of a structural engineer, who uses information from soil engineers on contact pressures and shear distribution to finalize designs.