GLE/CEE 330: Soil Mechanics Introduction to Foundation Engineering Geological Engineering University of Wisconsin-Madison

Learning Objectives Define key terms in foundation engineering Discuss types of shallow and deep foundations Describe basic foundation design philosophy

Key Terms Foundation: structure that transmits loads to underlying soils. (Coduto) Shallow Foundations (Df/B < 2.5-4.0) Deep Foundations (Df/B > 4.0) Df = Embedment Depth B = Characteristic Length

Key Terms Bearing Capacity (q): Pressure that a soil can support (soil property) Ultimate bearing capacity (qult): Max. pressure soil can support (determined by analysis) Net bearing capacity (qnet): Max. pressure soil can support above current overburden pressure (account for embedment effect) Allowable bearing capacity (qall): Design soil bearing pressure F.S. = Factor of Safety (2.5 – 3.5) (Allowable Stress Design)

ASD vs. LRFD Allowable Stress Design (ASD) Historical Approach in Geotechnical Engineering “Blanket” factor of safety More conservative (More Cost) Less conservative (More Risk)

ASD vs. LRFD Load and Resistance Factor Design (LRFD) Modern Approach in Geotechnical Engineering Factored Load (Q) < Factored Resistance (R) gi are “load factors” for different types of loads (g ≥ 1.0) f are “resistance factor(s)” for nominal resistance (f ≤ 1.0) Effectively accounts for uncertainty in a quantitative way Structures may be designed for a specific amount of risk

Types of Foundation Loads Normal Loads Shear Loads Moment Loads Torsion Loads Load Sources: Dead Loads Snow Loads Earth Pressure Fluid Loads Earthquake Loads Wind Loads 8) Impact Loads 9) Temperature Induced Loads 10) Stream/Ice Loads 11) Centrifugal/Braking 12) Shear Loads (Coduto)

Shallow Foundation Types Spread Footing (column loads) (Alex Mead)

Shallow Foundation Types Strip Footing (wall loads)

Shallow Foundation Types Mat (Raft) Foundation (floor loads)

Deep Foundations Insufficient soil “bearing capacity” near surface Excessive settlements on shallow foundations Differential settlements on shallow foundations Constructability issues (e.g., shallow GWT) Lateral loads, uplift forces (wind, waves, earthquakes, earth pressure)

Deep Foundation Types Piles Timber Steel (H-piles, tubes, mandrels) Reinforced Concrete Cast-in-place concrete Characteristics/Issues: “Displacement” vs. “Non-displacement” Driving stresses Vibration/Noise H-Pile Concrete Pile

Deep Foundation Types Drilled Shafts B = 2-30 ft (0.6-9 m) L < 300 ft (91 m) Large Lateral Load Resistance (Hayward Baker)

Pile Load Transfer (Single Pile) Qf = “shaft friction” or “side shear” Qb = “end bearing” If Qf >0.8Qb then “friction pile” If Qb = 0 (design assumption) then “floating pile”

Pile Groups

Foundation Design Philosophy Limit State = “condition beyond which a component/member of a foundation or other structure ceases to satisfy the provisions for which the component/member was designed” Strength Limit State (bearing capacity analysis) Service Limit State (settlement analysis) Extreme Event Limit State Fatigue Limit State