Foundations on RockFoundations on RockDuncan C. WyllieDuncan C. Wyllie

Wyllie & Norrish Rock EngineersWyllie & Norrish Rock Engineers

ASCEASCERock Mechanics Short CourseRock Mechanics Short Course

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

AgendaAgenda1.1. Bearing capacityBearing capacity –– allowable allowable

bearing pressurebearing pressure2.2. SettlementSettlement –– layered formationslayered formations3.3. StabilityStability –– foundations of bridges foundations of bridges

and dams subject to tensile and/or and dams subject to tensile and/or inclined loadsinclined loads

Worked examples: Worked examples: -- settlementsettlement-- stabilitystability

Rock Foundation Characteristics Rock Foundation Characteristics ––Pacific NorthwestPacific Northwest

Strong rock with high allowable bearing Strong rock with high allowable bearing capacitycapacityRock contains persistent discontinuitiesRock contains persistent discontinuitiesCanyons often contain steep, glacialCanyons often contain steep, glacial--cut cut channelschannelsSteep rock faces are relaxed, and possibly Steep rock faces are relaxed, and possibly unstableunstableWeathering can cause deterioration of Weathering can cause deterioration of rock strengthrock strengthSeismic ground motions can cause Seismic ground motions can cause displacement and instabilitydisplacement and instability

Effects of Geology on Foundation Effects of Geology on Foundation StabilityStability

Persistent, planar joint dipping downstream

Gully cut by glacial outwash channel

Stability Deterioration with TimeStability Deterioration with TimeGrowth of tree roots

Ice and water pressures

Tension crack at crest of steep

rock face

Tension cracks at crest of steep rock face

1. Bearing Capacity1. Bearing Capacity

Bearing capacity design issuesBearing capacity design issuesAllowable bearing capacity based Allowable bearing capacity based on past experienceon past experienceBearing capacity related to rock Bearing capacity related to rock quality and geologic structurequality and geologic structureRock quality can deteriorate with Rock quality can deteriorate with time due to weatheringtime due to weatheringBearing capacity can usually be Bearing capacity can usually be adjusted by increasing footing sizeadjusted by increasing footing sizeMost difficult bearing capacity Most difficult bearing capacity problems in problems in karstickarstic terrain terrain

Allowable Bearing CapacityAllowable Bearing Capacity

Bearing Capacity in Bearing Capacity in KarsticKarstic TerrainTerrain

Preferential solution on joints

Sinkhole

Solution of limestone Solution of limestone occurs preferentially occurs preferentially

along geologic structurealong geologic structure

Examples of construction Examples of construction procedures for spread procedures for spread

footings on footings on karstickarstic terrainterrain

Influence of Influence of karstickarstic structure on pile supportstructure on pile support

1. Long, supported pile;2. Pile bent and wedged in

crack3. Pile tip damaged on sloping

rock surface4. Pile bearing on pinnacle 5. Pile bent and not supported6. Short, supported pile

Drill probe hole at each pile

AgendaAgenda1.1. Bearing capacityBearing capacity –– allowable allowable

bearing pressurebearing pressure2.2. SettlementSettlement –– layered formationslayered formations3.3. StabilityStability –– foundations of bridges foundations of bridges

and dams subject to tensile and/or and dams subject to tensile and/or inclined loadsinclined loads

2. Settlement of Foundations2. Settlement of Foundations

Microsoft Equation 3.0

Spread footing bearing Spread footing bearing on very weak, massive on very weak, massive

claystoneclaystone

Allowable Bearing CapacityAllowable Bearing Capacity

2

Settlement of Settlement of Foundations on Foundations on Layered RockLayered Rock

Flow contacts in basalt Flow contacts in basalt form low form low

strength/compressible strength/compressible seams seams

Worked Example 1Worked Example 1

Settlement of foundation on homogeneous or layered rock

B

Ε1, ν1

Q

Ε2, ν2

Ε1, ν1

H1

H2

∞

Rock mass properties

Calculate settlement of footing with width B and load Q bearing on homogeneous

rock, and layered rock.

Modulus of deformationModulus of deformationRock mass rating, RMR:•Intact rock strength•RQD•Joint spacing•Condition of joints•Ground water•Joint orientation

Settlement calculations Settlement calculations –– shape shape factors, factors, CCdd

δv = Cd q B(1 – υ2)/E

AgendaAgenda1.1. Bearing capacityBearing capacity –– allowable allowable

bearing pressurebearing pressure2.2. SettlementSettlement –– layered formationslayered formations3.3. StabilityStability –– foundations of bridges foundations of bridges

and dams subject to tensile and/or and dams subject to tensile and/or inclined loadsinclined loads

3. Foundation Stability3. Foundation Stability

a)a) Steel arch bridge Steel arch bridge –– landslide, erosion gullylandslide, erosion gullyb)b) Steel truss bridge Steel truss bridge –– toppling, planar slidingtoppling, planar slidingc)c) Tension cable bridge, Argentina Tension cable bridge, Argentina –– wedge wedge

slidingslidingd)d) Cantilevered bridge Cantilevered bridge –– compression, tension compression, tension

foundationsfoundationse)e) Single span bridge Single span bridge –– weak seams, slope weak seams, slope

stabilitystabilityf)f) Transmission tower Transmission tower –– sheet jointssheet jointsg)g) Cableway tail tower Cableway tail tower –– planar sliding on silt planar sliding on silt

filled jointsfilled jointsh)h) Spillway foundation, Sri Lanka Spillway foundation, Sri Lanka -- wedgeswedges

Mechanisms for Mechanisms for foundation stabilityfoundation stability

1. Planar2. Wedge3. Wedge4. Circular 5. Buckling6. Settlement

Stability of three Stability of three dimensional dimensional foundation blockfoundation block

a) Steel Arch Bridge Foundationsa) Steel Arch Bridge Foundations

Arch bridge, south abutment – slope excavated to remove landside

Arch bridge, south abutment – landside excavation

Arch bridge, north abutment – buried channel excavated to create bearing surface on sound rock

Arch bridge abutment – potential modes of instability and movement

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 )

a

c

Figure 6Figure 6

Truss bridge, north abutment – foundation containing sheet joints reinforced with tensioned cable anchors

Truss bridge, south abutment – concrete buttress and rock bolts supporting retaining wall foundation

Truss bridge, south retaining wall –foundation containing sheet joints. Cavity filled with dental concrete and rock reinforced with rock bolts

Truss bridge, south abutment

c) Tension Cable Bridgec) Tension Cable Bridge

Wedge in abutment formed by foliation and orthogonal faults in weathered gneiss

Face

Foliation

BenchFault F2

Fault F1

Line of Intersection

Tensioned BridgeCables, Q

MAGNITUDE AND DIRECTION OFEXTERNAL FORCES ON WEDGE

T

av.g.W(vertical down)

a .g.WH

W (vertical down)

Q

Plan View Section View

W

TQ

av.g.W

a .g.WHup)

Magnitude and direction of external forces acting on wedge

Stability of three Stability of three dimensional dimensional foundation blockfoundation block

Abutment secured with tensioned multi-strand anchors inclined at 45°

Tensioning strand anchor, with dial gauges to measure strain

d) Spread footings on basaltd) Spread footings on basalt

New bridge adjacent to existing bridge

Stability of footing bearing 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 kPaphi = 45 degs

c = 5 kPaphi = 30 degs

BASALT

Intact rock strength:c = 75 kPaphi = 40 degs

Vertical joint strength:c = 1 kPaphi = 40 degs

Fill load = 100 kPa Existing 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

DRAFT

August 16, 2006

Stability analysis of existing bridge to determine rock mass strength parameters

Stability analysis of reinforced foundation

Foundation reinforced with fully grouted steel bars

e) Cantilevered Bridgee) Cantilevered Bridge3850 kips @ -12°

5080 kips @ 42°

More Canyon -south abutment of cantilever bridge

Tension foundation – a) design of cable anchors; b) rock reaction block

a)

b)

f) Transmission Tower Foundationf) Transmission Tower Foundation

Figure 2Figure 2

Transmission tower founded on strong granite containing persistent sheet joints dipping at 40° out of slope

Figure 3Figure 3

Reinforcement of foundation with multi-strand cable anchors, with drain holes

g) Revelstoke Dam g) Revelstoke Dam –– cableway cableway tail tower foundationtail tower foundation

Cableway tail tower on Cableway tail tower on arc bench above left arc bench above left abutmentabutment

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

Foliation planes in foundation contain dense silt infilling

h) Spillway Foundationh) Spillway Foundation

Wedges formed by foliation dipping downstream

Spillway Spillway -- dynamic load dynamic load condition with gate opencondition with gate open

Spillway foundation containing foliation planes dipping downstream. Foundation treatment comprises grout curtain, drain holes and tensioned rock bolts

Foundation StabilizationFoundation Stabilization

Tensioning rock bolts, with dial gauge to measure elongation

Worked Example 2Worked Example 2

Stability of foundation supporting inclined loads

Resolution of forces to determine normal, N and shear, S components of forces on potential sliding surface

)forces_sliding()forces_resisting(FS

ΣΣ

=

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

ΣφΣ

=

Forces acting on foundation containing planar Forces acting on foundation containing planar discontinuity dipping out of facediscontinuity dipping out of face

A

(-) direction

(+) direction

Q2 Q1

ψp

ψQ2ψQ1

NU = sin(ψU – ψp)

SU = cos(ψU – ψp)

Uψu

Calculate factor of safety against sliding of foundation block, and direction of sliding,

up-slope or down-slope

Relationship between friction angle and cohesion based on back analysis of rock slopes

The endThe end