shallow foundations bearing...
TRANSCRIPT
Shallow Foundations
Bearing Capacity
• The problems of soil mechanics can be
divided into two principal groups -
stability problems and elasticity
problems
- Karl Terzaghi, 1943
Karl Terzaghi (1883-1963)
• Father of modern soil mechanics
• Born in Prague, Czechoslovakia
• Wrote “Erdbaumechanick” in 1925
• Taught at MIT (1925-1929)
• Taught at Harvard (1938 and after)
Stability Problem
Bearing Capacity Failure
• Chapter 6. Bearing Capacity Analysis
• How do we estimate the maximum
bearing pressure that the soil can
withstand before failure occurs?
Bearing Capacity Failures
Types/Modes of Failure
general shear failure
local shear failure
punching shear failure
General Guidelines
Footings in clays - general shear
Footings in Dense sands ( > 67%)
-general shear
Footings in Loose to Medium dense
(30%< < 67%) - Local Shear
Footings in Very Loose Sand ( < 30%)-
punching shear
rD
rD
rD
Terzaghi Bearing Capacity Formulas
BNNNcq qzDcult 5.0
For Square foundations:
For Continuous foundations:
BNNNcq qzDcult 4.03.1
For Circular foundations:
BNNNcq qzDcult 3.03.1
Terzaghi Bearing Capacity Factors
07.5 whenNc
)2/45(cos2 2
2
aNq
tan)360/75.0(expa
1
cos2
tan2
pKN
0tan
1
when
NN
q
c
Terzaghi Bearing Capacity Formulas
D B
No sliding between footing and soil
soil: a homogeneous semi-infinite mass
general shear failure
footing is very rigid compared to soil
See Extra Handout
Further Developments
Skempton (1951)
Meyerhof (1953)
Brinch Hanson (1961)
De Beer and Ladanyi (1961)
Meyerhof (1963)
Brinch Hanson (1970)
Vesic (1973, 1975)
Vesic (1973, 1975) Formulas
Shape factors….… Eq. 6.14, 6.15 and 6.16
Depth Factors ……. Eq. 6.17, 6.18 and 6.19
Load Inclination Factors …. Eq. 6.20, 6.21 and 6.22
Base Inclinations factors .. Eq. 6.25 and 6.26
Ground Inclination Factors….Eq. 6.27 and 6.28
Bearing Capacity Factors …. Eq. 6.29, 6.30 and 6.31
gbidsBNgbidsNgbidsNcq qqqqqqzDccccccult 5.0
Bearing Capacity of
Shallow Foundations
6.3 Groundwater Effects
6.4 Allowable Bearing Capacity
6.5 Selection of Soil Strength Parameters
6.6 Local & Punching Shear Cases
6.7 Bearing Capacity on Layered Soils
6.8 Accuracy of Bearing Capacity Analyses
6.9 Bearing Capacity Spreadsheet
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
Selection of Soil Strength
Parameters
Use Saturated Strength Parameters
Use Undrained Strength in clays (Su)
Use Drained Strength in sands,
Intermediate soils that where partially
drained conditions exist, engineers have
varying opinions; Undrained Strength can
be used but it will be conservative!
and c
Accuracy of Bearing Capacity
Analysis
In Clays …..Within 10% of true value (Bishop
and Bjerrum, 1960)
Smaller footings in Sands…. Bearing capacity
calculated were too conservative – but
conservatism did not affect construction cost
much
Large footings in Sands … Bearing capacity
estimates were reasonable but design was
controlled by settlement