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Shallow Foundations Bearing Capacity The problems of soil mechanics can be divided into two principal groups - stability problems and elasticity problems - Karl Terzaghi, 1943 Slide 2 Karl Terzaghi (1883-1963) Father of modern soil mechanics Born in Prague, Czechoslovakia Wrote Erdbaumechanick in 1925 Taught at MIT (1925-1929) Taught at Harvard (1938 and after) Slide 3 Karl Terzaghi at Harvard, 1940 Slide 4 Bearing Capacity Failure Slide 5 Transcosna Grain Elevator Canada (Oct. 18, 1913) West side of foundation sank 24-ft Slide 6 Stability Problem Bearing Capacity Failure Chapter 6. Bearing Capacity Analysis How do we estimate the maximum bearing pressure that the soil can withstand before failure occurs? Slide 7 Bearing Capacity Failures Types/Modes of Failure general shear failure local shear failure punching shear failure Slide 8 General Shear Failure Slide 9 Punching Shear Failure Slide 10 Model Tests by Vesic (1973) Slide 11 General Guidelines Footings in clays - general shear Footings in Dense sands ( > 67%) -general shear Footings in Loose to Medium dense (30%< < 67%) - Local Shear Footings in Very Loose Sand ( < 30%)- punching shear Slide 12 Bearing Capacity Formulas Slide 13 Terzaghi Bearing Capacity Formulas Slide 14 For Square foundations : For Continuous foundations : For Circular foundations : Slide 15 Terzaghi Bearing Capacity Factors Slide 16 Bearing Capacity Factors Slide 17 Terzaghi Bearing Capacity Formulas D B No sliding between footing and soil soil: a homogeneous semi-infinite mass general shear failure footing is very rigid compared to soil Slide 18 See Extra Handout Further Developments Skempton (1951) Meyerhof (1953) Brinch Hanson (1961) De Beer and Ladanyi (1961) Meyerhof (1963) Brinch Hanson (1970) Vesic (1973, 1975) Slide 19 Vesic (1973, 1975) Formulas Shape factors.Eq. 6.14, 6.15 and 6.16 Depth Factors .Eq. 6.17, 6.18 and 6.19 Load Inclination Factors . Eq. 6.20, 6.21 and 6.22 Base Inclinations factors.. Eq. 6.25 and 6.26 Ground Inclination Factors.Eq. 6.27 and 6.28 Bearing Capacity Factors .Eq. 6.29, 6.30 and 6.31 Slide 20 Vesic Formula Shape Factors Slide 21 Vesic Formula Depth Factors Slide 22 Bearing Capacity of Shallow Foundations 6.3 Groundwater Effects 6.4 Allowable Bearing Capacity 6.5 Selection of Soil Strength Parameters 6.6 Local & Punching Shear Cases 6.7 Bearing Capacity on Layered Soils 6.8 Accuracy of Bearing Capacity Analyses 6.9 Bearing Capacity Spreadsheet Slide 23 Groundwater Table Effect Slide 24 Groundwater Table Effect; Case I 1.Modify zD 2.Calculate as follows: Slide 25 Groundwater Table Effect; Case II 1.No change in zD 2.Calculate as follows: Slide 26 Groundwater Table Effect; Case III 1.No change in zD 2.No change in Slide 27 Allowable Bearing Capacity .. Allowable Bearing Capacity F . Factor of safety Slide 28 Factor of Safety Depends on: Type of soil Level of Uncertainty in Soil Strength Importance of structure and consequences of failure Likelihood of design load occurrence Slide 29 Minimum Factor of Safety Slide 30 Selection of Soil Strength Parameters Use Saturated Strength Parameters Use Undrained Strength in clays ( S u ) Use Drained Strength in sands, Intermediate soils that where partially drained conditions exist, engineers have varying opinions; Undrained Strength can be used but it will be conservative! Slide 31 Accuracy of Bearing Capacity Analysis In Clays ..Within 10% of true value (Bishop and Bjerrum, 1960) Smaller footings in Sands. Bearing capacity calculated were too conservative but conservatism did not affect construction cost much Large footings in Sands Bearing capacity estimates were reasonable but design was controlled by settlement Slide 32 Accuracy; Bearing Capacity Analysis Slide 33 Bearing Capacity Spreadsheet Can be downloaded from http://www.prenhall.com/coduto See Appendix B (page 848) for further instructions