CIE 338 - Soil Mechanics and Foundations II

Soil Properties and Site Investigation

Shallow Foundation Design

Deep Foundation Design

Retaining Structures and Slope Stability

Building Codes

Local and national codes guide practice.

Design must conform to code or

Departures require approval

Compliance does not assure safety or economy

Df

B

Footing

Ground Surface

Col

um

n

P

Shallow Foundation Df < 4B

A = total building foundation plan area

Af = Individual footing area

If Af => 0.5 A then consider a mat foundation

Mat

Column Loads

Df

Shallow (Mat) Foundation Df < 4B

B

Firm Soil

Deep Foundation - Piles

Hard Soil / Rock

Hammer Hammer

Friction Pile End Bearing Pile

Pile

Hammer

Shaft

Pre bored hole

Poured in place fill

Deep Foundations

Df < 4B Df > 4B

Af < 0.5A Af > 0.5A Driven Cast in Place

CaissonPierBored Pile

A = plan area of structure

Af =sum of footing areas Df = depth of cover

B = footing width

Earth Retaining Systems

Wall System (external)

Stabilized Earth System(internal)

Slope StabilityHow steep can the slope be?

What is the factor of safety (FS)?

How can we improve the FS?

Foundations transfer loads to subsurface

DL - dead loads consist of the structure load

usually well known

LL - live loads are service loads, wind, earthquake

usually involve large uncertainty

Types of Loads

• Normal Loads, P

• Shear Loads, V

• Moment Loads, M

• Torsion Loads, T (usually negligible)

z

y

z

y

P Normal loadimportant for buildings

z

y

Vy

Shear loadimportant for retaining walls

Vy, Vx

z

y

MxMoment loadimportant for retaining walls and buildings

Mx, My

z

y

T

Torsion loadusually not significant

Applied loads induce:

Failure - (collapse/instability)

Design with a generous factor of safety

Movement - (settlement/deformation)

Design to a performance criteria

Factor of Safety (FS) against failureor for bearing capacity

FS = Resistance/Driving

FS for buildings ~ 3

FS for retaining structures and slopes ~1.5

Movement / Settlement / Deformation

Uniform settlement - least critical

Even tilting - can be critical

Distortion - potentially troublesome

Uniform settlement,

Even tilting, (limit < 1/250)

As built foundation

D

s s

= maximum total settlement

D= maximum differential settlement

S = column spacing

= distortion = D/ S

Da = a * S

a, denotes allowable

a = allowable rotation (Table 2.2, Coduto)

Type of structure a

Frame warehouse 1/200

Steel and reinforced concrete buildings 1/600

Unreinforced masonry 1/2500

Consult Fig. 2.9 and 2.10 (Coduto)

Upper limit of D/ for foundations on sand = 1

Upper limit of D/ for foundations on clay ~ 0.3

The allowable total settlement is set to limit D

Example 2.1

A steel frame building without diagonal bracing

S = 20 ft on clay foundation, what are the allowable total and differential settlements?

Da = a * S obtain a = 1/500 (Table 2.2)

Da = 0.5 in obtain D/ = 0.8 (Fig 2.9)

a = Da * ( /D) = 0.5 / 0.8 = 0.6 in

Frost Heave

Ground swell due to water volume expansion on freezing regular and < 50 mm (minor)

Water rise by capillary action and formation of ice lenses irregular and of the order of 300 mm (major)

Surface down thawing leads to super saturationfoundation becomes very weak until drained

Conditions for Frost Heave

Freezing temperaturesusually natural, artificial also

Source of waterground water table

Frost susceptible soil (Table 2.3, Coduto)silts and fine sands - F4 soils

Measure to mitigate frost heave

Insulate - rare

Remove / replace - not common

Place foundation below depth of heave, Df

most common

Df is depth of frost penetration

Varies by geographic location (Fig. 2.12, Coduto)

Syracuse ~ 1.4 m

Minnesota ~ 2.5 m

California and Southern States < 0.3 m

Consult local practice, building codes

Other water related problems

Scour (armour, riprap)

Corrosion (coating, cathodic protection)

Sulfate attack (special cement, low w/c ratio)

Decay, insects and fire (creosote pressure treat)

Soils

75 - 5 mm Gravel

5 - 0.075 Sand

< 0.075 Silt and Clay

Coarse

Fine

#200#200

Grain size vs plasticity and liquid limit

minerals

water

air

Soil

mass volume

voids

solids

Va

Vw

Vs

Ma

Mw

Ms

Coarse grained soils

e = void ratio = Vv/Vs

emax and emin

Dr = relative density

= {(e-emin)/(emax-emin)}*100

Fine grained soils

Water content = w = (Mw / Ms)*100

SL = shrinkage limitPL = plastic limitLL = liquid limitw = natural water content

Coarse grained soils

Fine grained soils

eminemaxe

PL LLw

0

10

20

30

40

50

60

70

80

90

100

0.0010.010.1110Grain Size (mm)

% P

as

sin

g b

y W

eig

ht

Gravel Sand Silt Clay

Dewatering

0

10

20

30

40

50

60

70

80

90

100

0.0010.010.1110Grain Size (mm)

% P

as

sin

g b

y W

eig

ht

Gravel Sand Silt Clay

Gravity G Wellpoint V Wellpoint Electro-osmosis?

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100Liquid Limit

PI