Foundation Design: Key Considerations and Best Practices
Foundation Design: Key Considerations
Safety Factors for Clay Foundations
Clay exhibits greater variability in behavior and resistance. Therefore, a higher safety factor is crucial to prevent failures.
Preventing Foundation Failure Due to Scour
Investigate the potential scour depth, especially for foundations exposed to temporal or permanent surface runoff.
Foundation Types for Different Soil Compressibility
- Low Compressibility Soil: Spread footing
- Heavy Loads (Footing Area > 50% of Building Footing): Foundation slab
- Medium, High, Very High Compressibility Soils: Compensated foundations (if economical). Otherwise, use friction piles and tip piles.
Design Theories for Improved Safety
- Load Resistance Factor Design (LRFD)
- Allowable Stress Design (ASD): Based on elastic theory. Most codes use ASD for geotechnical design.
Selecting the Factor of Safety
Consider the following factors when selecting the factor of safety for foundation design:
- Soil type
- Site characterization
- Soil variability
- Importance of the structure
- Consequences of failure
- Likelihood of design load occurring
Construction Aspects for High-Quality Cast-in-Place Foundations
Closely supervise the following construction aspects:
- Quality of the framework
- Dimensions of the cross-sectional area
- Concrete compressive strength (f’c)
- Steel yield strength (Fy)
- Area of steel
- Minimum concrete cover
- Concrete vibration
- Rebar chair placement
Relativity Based Design
This method calculates the probability of failure related to a safety factor.
Methods to Determine Internal Forces in a Slab
- Rigid method
- Flexible method
- Numerical methods (finite elements and finite differences)
Forces Acting on Footings
Footings are subjected to:
- Shear (one-way or two-way)
- Bending
- Compression (pedestal)
Shallow Foundation Design Process
Bearing capacity determination is iterative. Input data includes:
- Loads
- Soil properties and conditions
- Geometric limitations
Determine:
- Dimensions
- Structural and constructive details
Factors Influencing Foundation Depth
Foundation depth depends on:
- Freezing depth
- Active expansion zone
- Scour depth
- Settlements
Factors Influencing Bearing Capacity
Bearing capacity depends on:
- Location
- Shape
- Rigidity of the foundation
- Water conditions
Soil Failure Modes
Failures due to soil conditions:
- General shear (stiff soil)
- Local shear (medium soil)
- Punching shear (loose soil)
Equation for Cohesive Soils with Varying Width/Length Ratios
Alec Skempton’s equation.
Shallow Foundation Factors
Nc, Ny, Nq are factors used in bearing capacity calculations for shallow foundations.
Vesic’s Correction Factors
Vesic studied bearing capacity modification errors and proposed corrections for:
- Foundation shape
- Foundation depth
- Rigidity of the soil
- Load inclination
- Ground slope
What is Scour?
Scour is the removal of sediment (sand, rocks) around a structure due to swiftly moving water. It can create scour holes, compromising structural integrity.
Foundation Slab Design
- Bearing capacity for slabs can be calculated using bearing capacity equations.
- Stress increments reach greater depths compared to footings.
- Settlements are usually more critical.
- Typically, greater safety factors are required (minimum 3 for sands and clays).
- Slab dimensions are typically dictated by the building’s geometry.
Slab Internal Forces Methods
There are three main methods to determine internal forces in a slab:
- Rigid Method: Assumes uniform ground deformation.
- Flexible Method: Simulates the ground with springs (Winkler model). Spring constants are determined from the modulus of subgrade reaction (k), typically estimated in the field with a plate load test.
- Numerical Methods:
- Finite elements (Plaxis, Abaqus, Ansys, Midas, etc.)
- Finite differences (FLAC)