Transcript Bearing Capacity - CE Meeting

Bearing Capacity
 foundations are designed to transmit
load from the structure they support to
the soil
 foundations are generally grouped into
two categories:
A.
Shallow Foundations
B.
Deep Foundations
Shallow Foundations
 the most common (and cheapest) type
of shallow foundations are
SPREAD FOOTINGS
 square spread
footings to support
individual columns
(also circular)
McCarthy, 6th Ed.
 Strip Footings to support wall loads
McCarthy, 6th Ed.
 Rectangular and Trapezoidal Footings for two
columns (combined footing) or machine base
McCarthy, 6th Ed.
RAFT or MAT Foundations
McCarthy, 6th Ed.
 To lower the bearing pressure and reduce
differential settlement on soils with low bearing
capacity or erratic or variable conditions
FLOATING Foundations
McCarthy, 6th Ed.
 where deep deposits of compressible, cohesive
soil are present and piles are impractical
 building’s substructure is a combination mat
and caisson to create a rigid box
 weight of earth displaced by foundation is
equal to total weight of structure, thereby
minimizing settlement from consolidation
Deep Foundations
 used when soil near surface has poor
load-bearing capacity
loose soil
bedrock
 they transmit load through weak soil
strata (overburden) to stronger, loadbearing stratum (eg., bedrock, dense
sand and gravel, etc.)
Types of Deep Foundations
PIERS
 where load-bearing stratum
no more than 5 m deep
 not used much any more
McCarthy, 6th Ed.
CAISSONS
McCarthy, 6th Ed.
 where overburden no more
than 8 - 9 m
thick
 replacing piers
PILES
 deep over-burden
more than 8 - 9 m
thick
 Various types and
placement
methods
Craig, 6th Ed.
Structural Requirements
1. Factor of Safety against General Shear Failure
of supporting soil is normally required to be in
the range 2.5 – 3.0
2. Tolerable amount of settlement; in particular,
differential settlement should not cause
significant damage to structure nor interfere
with function
3. Secondary to these, during construction, there
should be no adverse affect on adjacent
structures or services
Ultimate Bearing Capacity, qf
The least pressure that would cause shear
failure of supporting soil immediately below and
adjacent to a foundation
Craig, 6th Ed.
modes of failure:
General Shear Failure
 on low compressibility (dense or stiff) soils
 plastic equilibrium throughout support and
adjacent soil masses
 heaving on both sides of foundation
 final slip (movement of soil) on one side only
causing structure to tilt
Local Shear Failure




on highly compressible soils
only partial development of plastic equilibrium
only slight heaving on sides
significant compression of soil under footing
but no tilting
Punching Shear Failure
 on loose, uncompacted soils
 vertical shearing around edges of footing
 high compression of soil under footing, hence
large settlements
 no heaving, no tilting
Terzaghi’s Theory
Craig, 6th Ed.
 strip footing of infinite length and width B
 uniform surcharge, q0 on surface of isotropic,
homogeneous soil
 Rankine active wedge, ABC: forces 
 Passive zones, ADE () & BGF ()
Craig, 6th Ed.
 transition between & : ACD & BCG (zones
or radial shear or slip fans)
 above EDCGF: plastic equilibrium
 below EDCGF: elastic equilibrium
 the more general case is a footing at depth D
Craig, 6th Ed.
 Neglecting the shear strength of the soil above
depth D implies that this soil is a surcharge:
q0 = gD
 Terzaghi’s general equation:
qf = 0.5gBNg + cNc + gDNq
Contribution of:
Soil Self
Weight
Shear
Surcharge
Strength
Bearing Capacity Factors
 Ng, Nc and Nq are bearing capacity factors and
are derived from various sources
Craig, 6th Ed.
General Shear Failure of Footings (Ultimate
Bearing Capacity)
 theory was developed
for strip footings
 to adapt to square,
circular and
rectangular shapes,
Terzaghi & Peck
developed shape
factors here which
are still widely used
today:
q f  0.5γB( Ng Sg )  c( N c Sc )  gDNq
N q  e tan( ) tan2 (45  2 )
N c  ( N q  1) cot( )
Ng  ( N q  1) tan(1.4 )
FOOTING
TYPE
Sγ
Sc
Strip
1.0
1.0
Square
0.8
1.2
Circular
1.6
1.2
Rectangular
( BL )
1  0.2
( BL )
1  0 .2
Allowable Bearing Capacity
 the allowable bearing capacity, qa is the value
used in the design of footing size
 in North America, a factor of safety against
general shear failure, F is applied to the
ultimate bearing capacity, qf:
qa 
qf
F
 in Britain, F is not applied to the surcharge:
0.5gB( S g N g )  c( S c N c )
qa 
 gDN q
F
Skempton’s Nc Values
 if undrained shear
strength parameters
are used for the
design then a special
case arises:
 since u = 0, Nq = 1
and:
q f  cu N c  gD
 values of Nc are
acquired from
Skempton’s Chart
Craig, 6th Ed.