1. EAG346-2015 Lesson 4 Foundation

2. Bearing Capacity Terms
Bearing capacity is ability of a soil to support a foundation
Ultimate bearing capacity is loading per unit area that just causes soil’s shear failure (qult)
Allowable bearing capacity is loading per unit area the soil able to support safely (qall)
Ultimate bearing capacity is also total loading for the foundation area that just causes soil’s shear failure (Qult)

3. Bearing Capacity Analysis
B = 2b W Pp C

4. Terzaghi’s Equations – General Shear Failure

6. Terzaghi’s Equations – Local Shear Failure

7. For Granular Soils

8. For Cohesive Soils

9. Pile Foundation

10. geotechnical strength vs. structural strength of pile
Geotechnical strength is strength of surrounding material in holding the pile
Structural strength is strength of structural pile in holding the load

11. Geotechnical Strength =
strength from skin friction +
strength from tip resistance

12. Skin Friction (in Sand) vs. Depth Curve
Total Skin Friction is area under this curve sv
sh = Ksv
Ksv(tand)
Ksv(tand)(pD) H
Critical Depth
10D-20D

13. Skin Friction of Pile in Sand
1. Critical depth is shallow for loose sand, deep for dense sand
2. K is Coefficient of lateral earth pressure at rest
3. K low for loose sand, high for dense sand
4. K varies ; If you have to pick a value, use 0.8 or 0.9

14. Coefficient of friction (tan d) between Sand and Pile Material
Concrete
Wood
Steel (smooth)
Steel (rough, rusted)
Steel (corrugated)
0.45
0.4
0.2
Use tan f of sand

15. End Resistance of Pile in Sand

16. Nq* vs f

17. Skin friction of pile in Clay
Skin friction is a function of adhesion. Adhesion is a function of cohesion.
Adhesion changes with time after pile driving but cohesion does not.
Adhesion is normally less than but can also becomes equal to cohesion.

18. Skin Friction (Clay) vs. Depth Curve
Total Skin Friction is area under this curve H
acpD

19. Adhesion (Bustamante and Gianaselli, 1982)
Nature of Soil
Adhesion, a
Qt. MPa
Soft clay and mud
.90
<1
Moderately compact clay
.40
1 to 5
Compact to stiff clay and compact silt
.60
>5
Adhesion from European CPT (de Ruiter and Beringen, 1979)
Nature of Soil
Adhesion, a
Normally Consolidated Clay
1.0
Over-consolidated Clay
0.5

20. End Resistance of Pile in Clay
Therefore qtip = 9c
C = qu /2
qu = Qt/(15 to 20 for clay)

21. End Resistance of Pile resting on Rock
qtip = qu(Nf+1)
Where
Nf= tan2(45+f’/2)
qu = unconfined compressive strength of rock
f’=drained angle of riction
Furthermore, due to scale effect:
qu(design) = qu(lab) /5
Therefore,
Qtip = [qu(design)(Nf+1)] Atip
FOS

22. Typical values of unconfined compressive strengths and angle of frictions
Rock
Compressive Strength, MN/m2
Sandstone
Limestone
Shale
Granite
Marble
70-140
35-70
60-70
Rock
Angle of Friction, degrees
Sandstone
Limestone
Shale
Granite
Marble
27-45
30-40
10-20
40-50
25-30

23. END