Module 3 Bearing Capacity 4u3m3v

Bearing Capacity  foundations are designed to transmit load from the structure they 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 individual columns (also circular) McCarthy, 6th Ed.

 Strip Footings to 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 ing 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 ing 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 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   ive 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 ( N g S g )  c( N c S c )  gDN q

N q  e tan( ) tan 2 (45  2 ) N c  ( N q  1) cot( )

N g  ( 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.