LRFD Design of Shallow Foundations

Nominal Geotechnical Resistances ASD Failure Modes Overall Stability Bearing Capacity Settlement Sliding Overturning

Nominal Geotechnical Resistances LRFD Service Limit State Overall Stability Vertical (Settlement) and Horizontal Movements LRFD Strength Limit State Bearing Resistance Sliding Eccentricity Limits (Overturning)

Service Limit State Global Stability Stabilize Destabilize

Global Stability Factor of Safety – Method of Slices WT WT N tan f N tan f cl l cl l T N T WT N a WT a T T

Resistance Factors LRFD ASD Factors of Safety Soil/Rock Parameters and Ground Water Conditions Based On: Slope Supports Abutment or Other Structure? Yes No In-situ or Laboratory Tests and Measurements 1.5 1.3 No Site-specific Tests 1.8 LRFD

Stability Wrap-Up Unfactored loads Applied stress must be limited Service Limit State Applied stress must be limited Footings supported in a slope f ≤ 0.65 (FS ≥ 1.5) Stress criteria for stability can control footing design

Service Limit State Design – Settlement Cohesive Soils Evaluate Using Consolidation Theory Cohesionless Soils Evaluate Using Empirical or Other Conventional Methods Hough Method

Impact on Structures

Settlement of Granular vs. Cohesive Soils Relative importance of settlement components for different soil types Elastic Primary Consolidation Secondary Settlement (Creep)

Settlement of Granular vs. Cohesive Soils Structural effects of settlement components Include Transient Loads if Drained Loading is Expected and for Computing Initial Elastic Settlement Transient Loads May Be Omitted When Computing Consolidation Settlement of Cohesive Soils

Hough Method Settlement of Cohesionless Soils

Stress Below Footing Boussinesq Pressure Isobars

Nominal Bearing Resistance at Service Limit State For a constant value of settlement Rn Bf

Eccentricity of Footings on Soil eB = MB / P eL = ML / P

Effective Dimensions for Footings on Soil B′ = B – 2eB L′ = L – 2eL

Applied Stress Beneath Effective Footing Area

Stress Applied to Soil Strip Footing

Footings on Rock Trapezoidal Distribution

Footings on Rock Triangular Distribution

Use of Eccentricity and Effective Footing Dimensions Service Limit State Nominal Bearing Resistance Limited by Settlement Strength Limit State Nominal Bearing Resistance Limited by Bearing Resistance Prevent Overturning All Applicable Limit States

Strength Limit State Bearing Resistance

Strength Limit State Design – Bearing Resistance Footings on Soil Evaluate Using Conventional Bearing Theory Footings on Rock Evaluate Using CSIR Rock Mass Rating Procedure

Bearing Resistance Mechanism Ground Surface sv =  Df Df B 3 b’ 1 b 3 B>Df 2 2 d’ d a e = C + s’ tan f Soil Shear Strength Pp c b a I b’

METHOD/SOIL/CONDITION Table 10.5.5.2.1-1 Resistance Factors for Geotechnical Resistance of Shallow Foundations at the Strength Limit State METHOD/SOIL/CONDITION RESISTANCE FACTOR Bearing Resistance b Theoretical method (Munfakh, et al. (2001), in clay 0.50 Theoretical method (Munfakh, et al. (2001), in sand, using CPT Theoretical method (Munfakh, et al. (2001), in sand, using SPT 0.45 Semi-empirical methods (Meyerhof), all soils Footings on rock Plate Load Test 0.55 Sliding  Precast concrete placed on sand 0.90 Cast-in-Place Concrete on sand 0.80 Cast-in-Place or precast Concrete on Clay 0.85 Soil on soil ep Passive earth pressure component of sliding resistance

Footings on Rock Service Limit State – use published presumptive bearing Published values are allowable therefore settlement-limited Procedures for computing settlement are available

Footings on Rock – Strength Limit State Very little guidance available for bearing resistance of rock Proposed Specification revisions provide for evaluating the cohesion and friction angle of rock using the CSIR Rock Mass Rating System

CSIR Rock Mass Rating System CSIR Rock Mass Rating developed for tunnel design Includes life safety considerations and therefore, margin of safety Use of cohesion and friction angle therefore may be conservative

LRFD vs. ASD All modes are expressly checked at a limit state in LRFD Eccentricity limits replace the overturning Factor of Safety

Width vs. Resistance - ASD Shear Failure controls Settlement controls 800 Bearing Pressure (kPa) 600 400 0.0 1.0 2.0 3.0 4.0 5.0 Footing width, B (m) Allowable Bearing Capacity, FS = 3.0 Bearing Pressure for 25-mm (1in) settlement

Settlement vs. Bearing Resistance

Width vs. Resistance - LRFD 35 25 Nominal Bearing Resistance (ksf) 15 5 0 4 8 12 16 20 Effective Footing width, B’ (m) Strength Limit State Service Limit State

Recommended Practice For LRFD design of footings on soil and rock; Size footings at the Service Limit State Check footing at all other applicable Limit States Settlement typically controls!

Summary Comparison of ASD and LRFD for Spread Footings Same geotechnical theory used to compute resistances, however As per Limit State concepts, presentation of design recommendations needs to be modified

METHOD/SOIL/CONDITION Strength Limit State Resistance Factors METHOD/SOIL/CONDITION RESISTANCE FACTOR Bearing Resistance  All methods, soil and rock 0.45 Plate Load Test 0.55 Sliding t Precast concrete placed on sand 0.90 Cast-in-Place Concrete on sand 0.80 Clay 0.85 Soil on soil ep Passive earth pressure component of sliding resistance 0.50