bearing-capacity- dr. tarek nagib.pptx

bearing-capacity- dr. tarek nagib.pptx

1. 4 th Year Civil FOUNDATION ENGINEERING BEARING CAPACITY OF SHALLOW FOUNDATIONS Dr. Tarek Nageeb Salem Associate Professor April 5, 2012 Bearing Capacity 1

2. DEFINITION OF SOIL Soil is a mixture of irregularly shaped mineral particles of various sizes containing voids between particles. The particles are a by-product of mechanical and chemical weathering of rock and described as gravels, sands, silts, and clays.. Any manmade structure should, one way or another, rest and/or transmit its load to the underlying soil April 5, 2012 Bearing Capacity 2

3. 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. Therefore, settlement analysis should generally be performed since most structures are sensitive to excessive settlement. April 5, 2012 Bearing Capacity 3

4. Bearing Capacity of Shallow Foundations Soil Bearing Capacity is Controlled by: Bearing Capacity Analysis: » Terzaghi ’ s Theory (1943), based on Prandtl theory (1920). » General B.C. Equation. Settlement Analysis: » Immediate Settlement » Consolidation Settlement April 5, 2012 Bearing Capacity 4

5. Types of Foundations Shallow F o u n d a t i o ns Deep F o u n d a t i o ns S p r e ad Mat or Raft Friction Piles Belled Pier April 5, 2012 Bearing Capacity 5

6. Failure Modes for Shallow Foundations General Shear Failure, Zones I, II, III, Dense Sand Local Shear Failure, Zones I, II, Medium Dense Sand April 5, 2012 Bearing Capacity 6

7. Fa i l u r e M o d es , C on t i nu ed Punching Failure, Zone I Only, Loose Sand and Soft Clay April 5, 2012 Bearing Capacity 7

8. Terzaghi ’ s Assumptions Zone I, Active. Zones II, Transition. Zones III, Passive. April 5, 2012 Bearing Capacity 8

9. Terzaghi B/C Assumptions Three zones do exist: Active zone , just below the foundation. Transition zone , between the active and passive zones. Passive zone , near the ground surface, just beside the foundation. passive ac t i v e Transition April 5, 2012 Bearing Capacity 9

10. Ap ril 5 , 20 12 B ea r i n g C a pa c i ty 1 Terzaghi Bearing Equation for Strip Footing q u net = c N c +  1 D ( N q - 1) + 0.5 B  2 N  Generalized soil strength : c,  (drainage as applicable) Soil unit weight :  2 (total or effective as applicable) Overburden  1 D B Failure Zone ( depth  2B )

11. Terzaghi Bearing Equation q ult = q ult = c N c q ult = c N c +  1 D N q Cohesion Term Above F . L. q ult = c N c +  1 D N q + 0.5 B  2 N  Below F.L. April 5, 2012 Bearing Capacity 11

12. Terzaghi Bearing Equation N c , N q , N  are Terzaghi B/C Coefficients, f(  ) C,  are the soil shear strength parameters April 5, 2012 Bearing Capacity 12

13. Bearing Capacity Factors N  Ø – in Degrees 4 3 2 1 N q N c N  q ult = c N c +  1 D N q + 0.5 B  2 N  70 60 50 40 30 20 10 10 20 40 60 80 N c and N q 5.7 1.0

14. Terzaghi Bearing Equation L B P April 5, 2012 Bearing Capacity 14

15. Foundation Shape Factors  q = 1.0 q ult = c.N c  c +  1 D N q  q + 0.5 B  2 N    For Rectangular or Square Footing  c = 1.0 + 0.30 B/L   = 1.0 – 0.30 B/L For Circular Footing  c = 1.30  q = 1.00   = 0.70 Egyptian Code of Practice Egyptian Code of Practice April 5, 2012 Bearing Capacity 15

16. Terzaghi ’ s Equation for Different Foundation Shapes Continuous (Strip) Footing: q u = c N c +  1 D N q + 0.5 B   N  q u-net = c N c +  1 D (N q - 1) + 0.5 B   N  Square Footing: q u-net = 1.3 c N c +  1 D (N q - 1) + 0.35 B   N  Circular Footing: q u-net = 1.3 c N c +  1 D (N q - 1) + 0.35 B   N  N c , N q , N  = B.C. Factors April 5, 2012 Bearing Capacity 16

17. Footings with inclined loads  P P April 5, 2012 Bearing Capacity 17 Inclined Load Factors  ci ,  qi ,  gi

18. Inclined Loads Correction Factors,  ci ,  qi , and   i are empirically determined from experiments April 5, 2012 Bearing Capacity 18  ci =  qi = (1.0 -  / 90  2   i = (1.0 -  /  ) 2

19. Inclined Load Factors q ult = c N c  c  ci +  1 D N q  q  qi + 0.5 B  2 N      i Inclined Load Factors:  ci =  qi = (1.0 –  /90) 2   i = (1.0 –  /  ) 2 April 5, 2012 Bearing Capacity 19

20. Bearing Capacity of Clay,  = q ult = c N c +  1 D N q + 0.50 B  2 N  For Clay: N c = 5.70, N q = 1.0, N  = 0.0 q ult = 5.70 c u +  1 D q ult net = 5.70 c u q all net = 1.90 c u c u = q u /2 April 5, 2012 Bearing Capacity 20 q u Unconfined compressive strength

21. Bearing Capacity of Sand, c u = q ult = c N c +  1 D N q + 0.50 B  2 N  For Sand: N c , N q , N  are determined from curve, and c u = 0, then: q ult =  1 D N q + 0.50 B  2 N  April 5, 2012 Bearing Capacity 21

22. Gross and Net Bearing Capacity F . S . q a l l  q ult q ult = q ult net +   D q a l l F . S . 1  q ult net Gross allowable bearing capacity  D Gross allowable B/C Gross ultimate B/C F . S . April 5, 2012 Bearing Capacity 22 q all net  q ult net Net allowable B/C  1 D is the overburden pressure

23. Effect of Water Table on B/C April 5, 2012 Bearing Capacity 23

24. Effect of Water Table on B/C Case (a):  1 D =  b (D f -d) +  sub d,  2 =  sub q ult = c N c +  1 D N q + 0.5B  2 N  Case (b):  1 D =  b D f ,  2 =  sub Case (c):  1 D =  b D f , April 5, 2012 Bearing Capacity 24 If , d ≤ B, then  2 =  sub If , d > B, then  2 =  b  b = Bulk unit weight  sub = Submerged Unit weight,  sub =  sat -  water

25. Settlement Analysis Allowable bearing capacity, as calculated from the settlement analysis, usually controls the soil bearing capacity, especially in clay and silt. The maximum allowable settlement is set first, then the stress (bearing pressure) that will induce that settlement will be the allowable bearing capacity. For stratified soils, 2:1 stress distribution is used to determine the stresses at the top of each layer. April 5, 2012 Bearing Capacity 25

26. Bearing Capacity in Stratified Deposits soft clay stiff clay or dense sand B 1 q 1 q 2 P P = q 1 A 1 = q 1 B 1 L 1 B April 5, 2012 Bearing Capacity 26 2 P = q 2 A 2 = q 2 B 2 L 2 q is the allowable 1 bearing capacity for layer 1 2 B q is the allowable 2 bearing capacity for layer 2, and so on The allowable bearing capacity as determined from either the shear strength parameters or the settlement analysis.

27. Settlement Analysis Total Settlement = Immediate Settlement + Primary Consolidation Settlement + Secondary Consolidation Settlement In Sand: Total Settlement ≈ Immediate settlement In Silts, Stiff and Medium Clays: Total Settlement ≈ Immediate Settlement + Primary Consolidation Settlement April 5, 2012 Bearing Capacity 27

28. Settlement Analysis In Soft Clays, Silts and Organic Soils: Total Settlement = Immediate Settlement + Primary Consolidation Settlement + Secondary Consolidation Settlement April 5, 2012 Bearing Capacity 28

29. E xa mp l e A footing 1.8 m x 2.5 m is located at a depth of 1.5 m below the ground surface, in an over- consolidated clay layer. The groundwater level is 2 m below the ground surface. The unconfined compressive strength of that clay is 120 kPa,  bulk = 18 kN/m 3 , and  sat = 20 kN/m 3 . Determine the net allowable bearing capacity, assuming a factor of safety of 3. April 5, 2012 Bearing Capacity 29

30. S o l u t i on q ult = c N c  c +  1 D N q  q + 0.5 B  2 N    For Clay: N c = 5.7, N q = 1.0, N  = 0.0 For Rectangular Footing:  c = 1.0 + 0.30 B/L = 1.0 + 0.30 x 1.8/2.5 = 1.22  q = 1.0 q ult net = c N c  c +  1 D (N q -1)  q q u = 120 kPa, c u = 60 kPa April 5, 2012 Bearing Capacity 30

31. q ult net = 60 x 5.7 x 1.22 = 417.20 F . S . 3 April 5, 2012 Bearing Capacity 31 all net q  q ult net  417.2  139.08 kN/m 2 q all net = 13.91 t/m 2 = 1.39 kg/cm 2

32. April 5, 2012 Bearing Capacity 32