Shallow Foundations
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Transcript of Shallow Foundations
DAA2523GEOTECHNICAL ENGINEERING
SHALLOW FOUNDATIONS
PREPARED BY :
Youventharan Duraisamy
UNIVERSITI MALAYSIA PAHANG
TOPIC OUTLINE Introduction to foundation Bearing capacity Factor of safety & bearing capacity failure Bearing capacity analysis for shallow
foundation
1) Terzaghi’s equation
2) Skempton’s chart Effect of variable water table
BEARING CAPACITY The ultimate bearing capacity (qu) is the value of
bearing stress which causes a sudden catastrophic settlement of the foundation (due to shear failure)
The allowable bearing capacity (qa) is the
maximum bearing stress that can be applied to the foundation such that it is safe against instability due to shear failure and the maximum tolerable settlement is not exceeded. The allowable bearing capacity is normally calculated from the ultimate bearing capacity using a factor of safety (Fs).
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
qall = FS
qu The FS should be at least 3.0
BEARING CAPACITY FAILUREBearing capacity failure can occur in 3 different
mode : general shear failure local shear failure punching shear failure
TERZAGHI’S BEARING CAPACITY The qu of a foundation is calculated from an
equation that incorporates appropriate soil parameters (e.g. shear strength, unit weight) and details about the size, shape and founding depth of the footing.
Terzaghi (1943) stated the qu of a footing as a three-term expression incorporating the bearing capacity factors: Nc, Nq and N, which are related to the angle of friction ().
Terzaghi Bearing Capacity Formulas
BNqNNcq qcult 5.0
For square footing:
For strip footing:
BNqNNcq qcult 4.03.1
For circular footing:
BNqNNcq qcult 3.03.1
Where Nc, Nq and N referred to Table 15.1 attached
SKEMPTON’S BEARING CAPACITY Skempton's equation is widely used for undrained clay soils:
qu = su .Ncu + qo
where :
Su = undrained shear strength
Ncu = Skempton's bearing capacity factor, which can be obtained from chart
qo = D
Groundwater Table Effect; Case I ( 0 Dw D )
’ = sat - w
q = Dw + ( D - Dw ) ’ where
The value of q in the bearing capacity equation replace with:
and the value of has to be replaced with ’
D
Groundwater Table Effect; Case II ( 0 d B )
dB
q = D
and the value of has to be replaced with
The value of q in the bearing capacity equation replace with:
dBdBave '1
ave
For d B
For d B
Groundwater Table Effect; Case III ( d B )
d
In this condition, the water table has no effect on the ultimate bearing capacity