Foundations on Rock Presentation

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Foundations on RockFoundations on RockDuncan C. WyllieDuncan C. Wyllie

Wyllie & Norrish Rock EngineersWyllie & Norrish Rock Engineers

ASCE ASCERock Mechanics Short CourseRock Mechanics Short Course

Seattle, WASeattle, WAJanuary 12, 2007January 12, 2007

Agenda Agenda

1.1. Bearing capacityBearing capacity – – allowableallowable

bearing pressurebearing pressure

2.2. SettlementSettlement – – layered formationslayered formations

3.3. StabilityStability – – foundations of bridgesfoundations of bridgesand dams subject to tensile and/or and dams subject to tensile and/or

inclined loadsinclined loads

Worked examples:Worked examples: -- settlementsettlement

-- stabilitystability

Rock Foundation CharacteristicsRock Foundation Characteristics – –

Pacific NorthwestPacific NorthwestQQ Strong rock with high allowable bearingStrong rock with high allowable bearing

capacitycapacity

QQ Rock contains persistent discontinuitiesRock contains persistent discontinuities

QQ Canyons often contain steep, glacialCanyons often contain steep, glacial--cutcut

channelschannelsQQ Steep rock faces are relaxed, and possiblySteep rock faces are relaxed, and possibly

unstableunstable

QQ Weathering can cause deterioration of Weathering can cause deterioration of rock strengthrock strength

Seismic ground motions can causeSeismic ground motions can causedisplacement and instabilitydisplacement and instability

Effects of Geology on FoundationEffects of Geology on Foundation

StabilityStability

Persistent, planar joint

di in downstream

Gully cut by glacial

outwash channel

Stability Deterioration with TimeStability Deterioration with Time

Growth of tree roots

Ice and water pressures

Tension crack atcrest of steep

rock face

Tension cracks at crest of steeprock face

1. Bearing Capacity1. Bearing Capacity

Bearing capacity design issuesBearing capacity design issues

QQ Allowable bearing capacity based Allowable bearing capacity basedon past experienceon past experience

QQ Bearing capacity related to rockBearing capacity related to rockquality and geologic structurequality and geologic structure

Rock quality can deteriorate withRock quality can deteriorate withtime due to weatheringtime due to weathering

QQ Bearing capacity can usually beBearing capacity can usually be

adjusted by increasing footing sizeadjusted by increasing footing sizeQQ Most difficult bearing capacityMost difficult bearing capacity

problems inproblems in karstickarstic terrainterrain

Allowable Bearing Capacity Allowable Bearing Capacity

Bearing Capacity inBearing Capacity in KarsticKarstic TerrainTerrain

Preferential solution

on joints

Sinkhole

Solution of limestoneSolution of limestone

occurs preferentiallyoccurs preferentially

along geologic structurealong geologic structure

Examples of constructionExamples of construction

procedures for spreadprocedures for spreadfootings onfootings on karstickarstic terrainterrain

Influence of Influence of karstickarstic structure on pile suppostructure on pile suppo

1. Long, supported pile;

2. Pile bent and wedged in

3. Pile t ip damaged on sloping

rock surface4. Pile bearing on pinnacle

5. Pi le bent and not supported

6. Short, supported pile

Drill probe hole at each

Agenda Agenda

2. Settlement of Foundations2. Settlement of Foundations

Microsoft

Equation 3.0

Spread footing bearingSpread footing bearingon very weak, massiveon very weak, massive

claystoneclaystone

All bl B i C itAll bl B i C it

Allowable Bearing Capacity Allowable Bearing Capacity

Settlement of Settlement of Foundations onFoundations on

Layered RockLayered Rock

Flow contacts in basaltFlow contacts in basalt

form lowform low

strength/compressiblestrength/compressible

seamsseams

Worked Example 1Worked Example 1

Settlement of foundation on

homogeneous or layered rock

Ε1, ν1

Ε2, ν2

Ε1, ν1

Rock mass

properties

Calculate settlement of footing with width

B and load Q bearing on homogeneousrock, and layered rock.

Modulus of deformationModulus of deformation

Rock mass rating, RMR:

•Intact rock strength

•RQD

•Joint spacing

•Condition of joints•Ground water

•Joint orientation

Settlement calculationsSettlement calculations – – shapeshape

factors,factors,

δv = Cd q B(1 – υ2)/E

Agenda Agenda

3. Foundation Stability3. Foundation Stability

a)a) Steel arch bridgeSteel arch bridge – – landslide, erosion gullylandslide, erosion gully

b)b) Steel truss bridgeSteel truss bridge – – toppling, planar slidingtoppling, planar slidingc)c) Tension cable bridge, ArgentinaTension cable bridge, Argentina – – wedgewedge

slidingsliding

d)d) Cantilevered bridgeCantilevered bridge – – compression, tensioncompression, tensionfoundationsfoundations

e)e) Single span bridgeSingle span bridge – – weak seams, slopeweak seams, slope

stabilitystabilityf)f) Transmission tower Transmission tower – – sheet jointssheet joints

g)g) Cableway tail tower Cableway tail tower – – planar sliding on siltplanar sliding on siltfilled jointsfilled joints

h)h) S illwa foundation, Sri LankaSpillway foundation, Sri Lanka -- wed eswedges

Mechanisms for Mechanisms for foundation stabilityfoundation stability

1. Planar

2. Wedge

3. Wedge

4. Circular

5. Buckling6. Settlement

Stability of threeStability of threedimensionaldimensional

foundation blockfoundation block

a) Steel Arch Bridge Foundationsa) Steel Arch Bridge Foundations

Arch bridge, south

abutment – slope

excavated to removelandside

Arch bridge, southabutment – landsideexcavation

Arch bridge, north

abutment – buried

channel excavated tocreate bearing surface

on sound rock

Arch bridge abutment

– potential modes of

instabili ty andmovement

b) Steel Truss Bridgeb) Steel Truss Bridge

Concrete buttresses

Truss bridge, north abutment – foundation containing

sheet joints reinforced with tensioned cable anchors (a)

and concrete buttress (c )

Fi 6Fi 6

Figure 6Figure 6

Truss bridge, north

abutment – f oundation

containing sheet jointsreinforced with

tensioned cable

anchors

Truss bridge, south

abutment – concrete

buttress and rockbolts supporting

retaining wall

foundation

Truss bridge, south

retaining wall –

foundation containing

sheet joints. Cavity

fil led with dental

concrete and rockreinforced with rock

Truss bridge, south

abutment

c) Tension Cable Bridgec) Tension Cable Bridge

Wedge in abutment formed byfoliation and orthogonal faults in

weathered gneiss

Foliation

Fault F2

Fault F1

Line of Intersection

Tensioned BridgeCables, Q

MAGNITUDE AND DIRECTION OF

EXTERNAL FORCES ON WEDGE

av.g.W(vertical down)

a .g.WH

W (vertical down)

Plan View Section View

av.g.W

a .g.H

Magnitude and direction of external forces actin

on wedge

Stability of threeStability of threedimensionaldimensional

foundation blockfoundation block

Abutment secured with tensioned multi-strandanchors inclined at 45°

Tensioning strand

anchor, with dial

gauges to measurestrain

d) S d f ti b ltd) S d f ti b lt

d) Spread footings on basaltd) Spread footings on basalt

New bridge

adjacent to

existing bridge

Stability of footingbearing on columnar

basalt with flow

contact below water surface

FEA - displacement

vectors showing

movement into lake

FEA – displacement

vectors of foundation

reinforced with fully

grouted dowels

FEA – section showing loading from both

bridges and displacement into lake

Basalt, RMR = 55, E = 13 GPa

Metadiorite, RMR = 67, E = 27 GPa

Flow Contact, RMR = 40, E = 6 GPa

c = 200 kPa

phi = 45 degs

c = 5 kPa

phi = 30 degs

BASALT

Intact rock strength:

c = 75 kPa

phi = 40 degs

Vertical joint strength:

c = 1 kPa

phi = 40 degs

Fill load = 100 kPaExisting bridge load

= 220 kPa

Back-analysis of rock shear strengths for a FOS = 1.3 under static conditions with

no rapid drawdown.

EXISTING BRIDGE CONDITIONS

Basalt, RMR = 55, E = 13 GPaRapid drawdown

condition

D R A F T

A u g u

s t 1 6

, 2 0 0

Stability analysis of

existing bridge to

determine rock mass

strength parameters

Stability analysis of reinforced foundation

Foundation reinforced with

fully grouted steel bars

) C til d B ide) Cantile ered Bridge

e) Cantilevered Bridgee) Cantilevered Bridge

3850 kips @ -12°

5080 kips @ 42°

More Canyon -south abutment of

cantilever bridge

Tension foundation – a) design

of cable anchors; b) rockreaction block

f) Transmission To er Fo ndationf) Transmission Tower Foundation

f) Transmission Tower Foundationf) Transmission Tower Foundation

Figure 2Figure 2

Transmission tower

founded on strong

granite containing

persistent sheet jointsdipping at 40° out of

Figure 3Figure 3

Reinforcement of foundation

with multi-strand cableanchors, with drain holes

g) Revelstoke Damg) Revelstoke Dam cablewaycableway

g) Revelstoke Damg) Revelstoke Dam – – cablewaycableway

tail tower foundationtail tower foundation

Cableway tail tower onCableway tail tower onarc bench above leftarc bench above leftabutmentabutment

Tail tower arrangement showing external load onfoundation, geologic structure and backfill surcharge

Foliation planes in foundation

h) Spillway Foundationh) Spillway Foundation

Wedges formed by

foliation dippingdownstream

SpillwaySpillway -- dynamic loaddynamic load

condition with ate o en

Spillway foundation containing foliation planes

dipping downstream. Foundation treatmentcomprises grout curtain, drain holes and tensioned

Foundation StabilizationFoundation Stabilization

Tensioning rock bolts, with dial gauge to measure elongation

Stability of foundation supporting

inclined loads

Resolution of forces to determine normal, N andshear S components of forces on potential

shear, S components of forces on potential

sliding surface

)forces _ sliding(

)forces _ resisting(FS

)S,forces.driving()tanN,forces.resisting(FS

Σ φ Σ

Forces acting on foundation containing planar Forces acting on foundation containing planar

discontinuity dipping out of facediscontinuity dipping out of face

( - ) d i r e c t i o n

( + ) d i r e c t i o n

NU = sin(ψU – ψp)

SU = cos(ψU – ψp)

Calculate factor of safety against sliding of

foundation block, and direction of sliding,up-slope or down-slope

Relationship between

friction angle andcohesion based on back

analysis of rock slopes

The endThe end

Foundations on Rock Presentation

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