CIE Soil Mechanics and Foundations II èSoil Properties and Site Investigation èShallow Foundation Design èDeep Foundation Design èRetaining Structures and Slope Stability

Building Codes 4Local and national codes guide practice. 4Design must conform to code or 4Departures require approval 4Compliance does not assure safety or economy

DfDf B Footing Ground Surface Column P Shallow Foundation D f < 4B

A = total building foundation plan area A f = Individual footing area If  A f => 0.5 A then consider a mat foundation

Mat Column Loads DfDf Shallow (Mat) Foundation D f < 4B B

Firm Soil Deep Foundation - Piles Hard Soil / Rock Hammer Friction PileEnd Bearing Pile

Pile Hammer Shaft Pre bored hole Poured in place fill Deep Foundations

D f < 4BD f > 4B A f < 0.5AA f > 0.5ADrivenCast in Place Caisson Pier Bored Pile A = plan area of structure A f =sum of footing areas D f = depth of cover B = footing width

Earth Retaining Systems Wall System (external) Stabilized Earth System (internal)

Slope Stability How 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 load important for buildings

z y Vy Shear load important for retaining walls Vy, Vx

z y Mx Moment load important for retaining walls and buildings Mx, My

z y T Torsion load usually 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 failure or 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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 warehouse1/200 Steel and reinforced concrete buildings1/600 Unreinforced masonry1/2500

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 * Sobtain  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 saturation foundation becomes very weak until drained

Conditions for Frost Heave Freezing temperatures usually natural, artificial also Source of water ground 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, D f most common

D f 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 mmGravel Sand < 0.075Silt and Clay Coarse Fine #200 Grain size vs plasticity and liquid limit

minerals water air Soil massvolume voids solids Va Vw Vs Ma Mw Ms

Coarse grained soils e = void ratio = Vv/Vs e max and e min Dr = relative density = {(e-e min )/(e max -e min )}*100

Fine grained soils Water content = w = (Mw / Ms)*100 SL = shrinkage limit PL = plastic limit LL = liquid limit w = natural water content

Coarse grained soils Fine grained soils e min e max e PLLL w

GravelSandSiltClay

GravelSandSiltClay GravityG WellpointV WellpointElectro-osmosis?