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7/27/2019 Foundations on Rock Presentation
http://slidepdf.com/reader/full/foundations-on-rock-presentation 1/77
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
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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
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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
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Effects of Geology on FoundationEffects of Geology on Foundation
StabilityStability
Persistent, planar joint
di in downstream
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Gully cut by glacial
outwash channel
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Stability Deterioration with TimeStability Deterioration with Time
Growth of tree roots
Ice and water pressures
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Tension crack atcrest of steep
rock face
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Tension cracks at crest of steeprock face
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1. Bearing Capacity1. Bearing Capacity
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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
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Allowable Bearing Capacity Allowable Bearing Capacity
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Bearing Capacity inBearing Capacity in KarsticKarstic TerrainTerrain
Preferential solution
on joints
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Sinkhole
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Solution of limestoneSolution of limestone
occurs preferentiallyoccurs preferentially
along geologic structurealong geologic structure
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Examples of constructionExamples of construction
procedures for spreadprocedures for spreadfootings onfootings on karstickarstic terrainterrain
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Influence of Influence of karstickarstic structure on pile suppostructure on pile suppo
1. Long, supported pile;
2. Pile bent and wedged in
crack
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
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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
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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
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Allowable Bearing Capacity Allowable Bearing Capacity
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Settlement of Settlement of Foundations onFoundations on
Layered RockLayered Rock
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Flow contacts in basaltFlow contacts in basalt
form lowform low
strength/compressiblestrength/compressible
seamsseams
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Worked Example 1Worked Example 1
Settlement of foundation on
homogeneous or layered rock
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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 homogeneousrock, and layered rock.
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Modulus of deformationModulus of deformation
Rock mass rating, RMR:
•Intact rock strength
•RQD
•Joint spacing
•Condition of joints•Ground water
•Joint orientation
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Settlement calculationsSettlement calculations – – shapeshape
factors,factors,
CC
dd
δv = Cd q B(1 – υ2)/E
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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
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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
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Mechanisms for Mechanisms for foundation stabilityfoundation stability
1. Planar
2. Wedge
3. Wedge
4. Circular
5. Buckling6. Settlement
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Stability of threeStability of threedimensionaldimensional
foundation blockfoundation block
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a) Steel Arch Bridge Foundationsa) Steel Arch Bridge Foundations
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Arch bridge, south
abutment – slope
excavated to removelandside
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Arch bridge, southabutment – landsideexcavation
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Arch bridge, north
abutment – buried
channel excavated tocreate bearing surface
on sound rock
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Arch bridge abutment
– potential modes of
instabili ty andmovement
b) Steel Truss Bridgeb) Steel Truss Bridge
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b) Steel Truss Bridgeb) Steel Truss Bridge
Concrete buttresses
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Truss bridge, north abutment – foundation containing
sheet joints reinforced with tensioned cable anchors (a)
and concrete buttress (c )
a
c
Fi 6Fi 6
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Figure 6Figure 6
Truss bridge, north
abutment – f oundation
containing sheet jointsreinforced with
tensioned cable
anchors
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Truss bridge, south
abutment – concrete
buttress and rockbolts supporting
retaining wall
foundation
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Truss bridge, south
retaining wall –
foundation containing
sheet joints. Cavity
fil led with dental
concrete and rockreinforced with rock
bolts
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Truss bridge, south
abutment
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c) Tension Cable Bridgec) Tension Cable Bridge
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Wedge in abutment formed byfoliation and orthogonal faults in
weathered gneiss
Face
Foliation
Bench
Fault F2
Fault F1
Line of Intersection
Tensioned BridgeCables, Q
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MAGNITUDE AND DIRECTION OF
EXTERNAL FORCES ON WEDGE
T
av.g.W(vertical down)
a .g.WH
W (vertical down)
Q
Plan View Section View
W
T
av.g.W
a .g.H
up)
Magnitude and direction of external forces actin
on wedge
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Stability of threeStability of threedimensionaldimensional
foundation blockfoundation block
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Abutment secured with tensioned multi-strandanchors inclined at 45°
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Tensioning strand
anchor, with dial
gauges to measurestrain
d) S d f ti b ltd) S d f ti b lt
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d) Spread footings on basaltd) Spread footings on basalt
New bridge
adjacent to
existing bridge
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Stability of footingbearing on columnar
basalt with flow
contact below water surface
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FEA - displacement
vectors showing
movement into lake
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FEA – displacement
vectors of foundation
reinforced with fully
grouted dowels
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FEA – section showing loading from both
bridges and displacement into lake
FT
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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
6
Stability analysis of
existing bridge to
determine rock mass
strength parameters
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Stability analysis of reinforced foundation
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Foundation reinforced with
fully grouted steel bars
) C til d B ide) Cantile ered Bridge
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e) Cantilevered Bridgee) Cantilevered Bridge
3850 kips @ -12°
5080 kips @ 42°
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More Canyon -south abutment of
cantilever bridge
a)
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Tension foundation – a) design
of cable anchors; b) rockreaction block
a)
b)
f) Transmission To er Fo ndationf) Transmission Tower Foundation
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f) Transmission Tower Foundationf) Transmission Tower Foundation
Figure 2Figure 2
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Figure 2Figure 2
Transmission tower
founded on strong
granite containing
persistent sheet jointsdipping at 40° out of
slope
Figure 3Figure 3
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Figure 3Figure 3
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Reinforcement of foundation
with multi-strand cableanchors, with drain holes
g) Revelstoke Damg) Revelstoke Dam cablewaycableway
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g) Revelstoke Damg) Revelstoke Dam – – cablewaycableway
tail tower foundationtail tower foundation
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Cableway tail tower onCableway tail tower onarc bench above leftarc bench above leftabutmentabutment
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Tail tower arrangement showing external load onfoundation, geologic structure and backfill surcharge
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Foliation planes in foundation
h) Spillway Foundationh) Spillway Foundation
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h) Spillway Foundationh) Spillway Foundation
Wedges formed by
foliation dippingdownstream
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SpillwaySpillway -- dynamic loaddynamic load
condition with ate o en
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Spillway foundation containing foliation planes
dipping downstream. Foundation treatmentcomprises grout curtain, drain holes and tensioned
Foundation StabilizationFoundation Stabilization
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Foundation StabilizationFoundation Stabilization
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Tensioning rock bolts, with dial gauge to measure elongation
Worked Example 2Worked Example 2
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Worked Example 2Worked Example 2
Stability of foundation supporting
inclined loads
Resolution of forces to determine normal, N andshear S components of forces on potential
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shear, S components of forces on potential
sliding surface
)forces _ sliding(
)forces _ resisting(FS
Σ
Σ =
)S,forces.driving()tanN,forces.resisting(FS
Σ φ Σ
=
Forces acting on foundation containing planar Forces acting on foundation containing planar
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discontinuity dipping out of facediscontinuity dipping out of face
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A
( - ) d i r e c t i o n
( + ) d i r e c t i o n
Q2 Q1
ψp
ψQ2
ψQ1
NU = sin(ψU – ψp)
SU = cos(ψU – ψp)
U
ψu
Calculate factor of safety against sliding of
foundation block, and direction of sliding,up-slope or down-slope
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Relationship between
friction angle andcohesion based on back
analysis of rock slopes
The endThe end
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The endThe end