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

FOUNDATION

SHALLOWDf B

DEEP Df B

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

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

Allowable Bearing Capacity

F

qq ult

a

….. Allowable Bearing Capacity F …. Factor of safety

aq

THE END

GENERAL SHEAR FAILURE

LOCAL SHEAR FAILURE

PUNCHING SHEAR FAILURE

Strip footing

Raft foundation