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Bearing 2

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Bearing 2

  1. 1. BEARING CAPACITY OF SHALLOW FOUNDATIONS ‫قدرة تحمل التربة‬ ‫للساسات الضحلة‬April 5, 2012 Bearing Capacity 1
  2. 2. BEARING CAPACITY OF SOILS Bearing capacity is the ability of soil to safely carry the pressure placed on the soil from any engineered structure without undergoing a shear failure with accompanying large settlements. ‫قدرة تحمل التربة هى مقدرة التربة على أن تتحمل‬ ‫بأمان الضغط الواقع عليها من أى منشأ هندسى بدون‬ .‫حدوث انهيار بالقص مصحوبا بهبوط كبير‬April 5, 2012 Bearing Capacity 2
  3. 3. Terzaghi B/C AssumptionsThree zones do exist: 1- Active zone, just below the foundation. .‫1- القطاع الفعال: أسفل الساسات مباشرة‬ 2- Transition zone, between the active and passive zones. ‫2- القطاع النتقالى: بين القطاع الفعال والقطاع‬ .‫المقاوم‬ 3- Passive zone, near the ground surface, just beside the foundation. ‫3- القطاع المقاوم: بالقرب من سطح الرض بجانب‬ .‫الساسات مباشرة‬ passive active TransitionApril 5, 2012 Bearing Capacity 3
  4. 4. Terzaghi Bearing Equation for Strip Footing qu net = c Nc + γ 1 D (Nq - 1) + 0.5 B γ 2 Nγ Overburden ‫وزن عمود التربة‬ γ1 D B Failure Zone (depth ≈ 2B) Generalized soil strength : c,φ Soil unit weight : γ 2 ‫معاملت القص للتربة‬ ‫كثافة التربة أسفل الساسات‬April 5, 2012 Bearing Capacity 4
  5. 5. Terzaghi Bearing Equation qult = Cohesion Term qult = c Nc ‫الجزء الخاص بالتماسك‬ Above F.L.qult = c Nc + γ 1 D Nq ‫أعلى منسوب التأسيس‬ ‫أعلى منسوب التأسيس‬qult = c Nc + γ 1 D Nq + 0.5B γ 2 Nγ Below F.L.April 5, 2012 Bearing Capacity ‫أسفل منسوب التأسيس‬ 5
  6. 6. Terzaghi Bearing Equation Nc, Nq, Nγ are Terzaghi B/C Coefficients, f(φ ) .‫معاملت ترزاجى لقدرة تحمل التربة، دالة فى زاوية الحتكاك الداخلى‬C, φ are the soil shear strength parameters ‫(.معاملت القص للتربة )التماسك وزاوية الحتكاك الداخلى‬April 5, 2012 Bearing Capacity 6
  7. 7. Bearing Capacity Factors qult = c Nc + γ 1 D Nq + 0.5 B γ 2 N γ Nc Nq Nγ 40 Ø – in Degrees 30 20 10 0 70 60 50 40 30 20 10 0 10 20 40 60 80 Nc and Nq 5.7 1.0 NγApril 5, 2012 Bearing Capacity 7
  8. 8. Terzaghi Bearing Equation B L PApril 5, 2012 Bearing Capacity 8
  9. 9. Foundation Shape Factorsqult = c.Nc λ c + γ 1 D Nq λ q + 0.5B γ 2 Nγ λ γ For Rectangular or Square Footing λ c = 1.0 + 0.30 B/L λ q = 1.0 Egyptian Code of Practice λ γ = 1.0 – 0.30 B/L For Circular Footing λ c = 1.30 λ q = 1.00 Egyptian Code of Practice λ γ = 0.70April 5, 2012 Bearing Capacity 9
  10. 10. Bearing Capacity of Clay, φ = 0 qult = c Nc + γ 1 D Nq + 0.50 B γ 2 N γ For Clay: Nc = 5.70, Nq = 1.0, Nγ = 0.0 qult = 5.70 cu + γ 1 D qult net = 5.70 cu qall net = 1.90 cu cu = qu/2 qu Unconfined compressive strengthApril 5, 2012 Bearing Capacity 10
  11. 11. Bearing Capacity of Sand, cu = 0 qult = c Nc + γ 1 D Nq + 0.50 B γ 2 N γ For Sand: Nc, Nq, Nγ are determined from curve, and cu = 0, then: qult = γ 1 D Nq + 0.50 B γ 2 N γApril 5, 2012 Bearing Capacity 11
  12. 12. Gross and Net Bearing Capacity qult qall = Gross allowable bearing capacity F .S . qult = qult net + γ 1 D Gross ultimate B/C qult net qall = + γ1 D Gross allowable B/C F .S . qult net qall net = Net allowable B/C F .S . γ 1 D is the overburden pressureApril 5, 2012 Bearing Capacity 12
  13. 13. Example A strip footing of 1.8 m width is located at a depth of 1.5 m below the ground surface, in an over- consolidated clay layer. The groundwater level is 10 m below the ground surface. The unconfined compressive strength of that clay is 120 kPa, γ bulk = 18 kN/m3, and γ sat = 20 kN/m3. Determine the net allowable bearing capacity, assuming a factor of safety of 3.April 5, 2012 Bearing Capacity 13
  14. 14. Solution qult = c Nc + γ 1 D Nq + 0.5B γ 2 Nγ For Clay: Nc = 5.7, Nq = 1.0, Nγ = 0.0 qult net = c Nc + γ 1 D (Nq -1) qu = 120 kPa, cu = 60 kPaApril 5, 2012 Bearing Capacity 14
  15. 15. qult net = 60 x 5.7 = 342.00 qult net 342 qall net = = = 114.00 kN/m 2 F .S . 3 qall net = 11.40 t/m2 = 1.14 kg/cm2April 5, 2012 Bearing Capacity 15
  16. 16. . Bearing capacity factors for differentφ φo Nc Nq Nγ 0 5.0 1.0 0.0 5 6.5 1.50 0.0 10 8.5 2.5 0.50 15 11.0 4.0 1.0 20 15.0 6.50 2.0 22.50 17.50 8.0 3.0 25.0 20.50 10.50 4.50 27.50 25.0 14.0 7.0 30.0 30.0 18.0 10.0 32.50 37.0 25.0 15.0 35.0 46.0 33.0 23.0 37.50 58.0 46.0 34.0 40.0 75.0 64.0 53.0 42.50 99.0 92.0 83.0April 5, 2012 Bearing Capacity 16

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