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Impacts of glaciers on engineering geology: examples ancient and modern
Geoffrey Boulton
University of Edinburgh
Quaternary Research Association
Durham, December 2016
sediment pressure pswater
pressure pw effective pressure pe
Pressure
Depth
Effective pressure with depth
• Shear strength – pe tanΦ
• Shear stress at glacier base ≈ 10-110 kPa
• Till tanΦ = 0.35 - 0.55
• Critical effective pc for failure = 18 – 314 kPa
• Equivalent to 1.8 – 31.4m of water
Pe > pc
pc
Stable
Consolidation
No basal melting Basal meltingGlacier loading & shearing
No surface load No surface loadLOADING HISTORY
PRESSURE HISTORY ATDEPTH “D”
TIME0
sediment pressure
water pressure
DENSITYCHANGE
+ve
-veshear dilation
normal consolidation
overconsolidation
Consolidation History
Rutford ice stream, West Antarctica+ Giorgos PapageorghiouAndy Smith, Emma Smith
Breidammerkurjökull, Iceland+ Sergei Zatsepin
La Gran Valira, Andorra+ Valenti Turu
Examples from:
Ancient
Modern
!2100m&
!2150m&
!2200m&
!2250m&
!2300m&
!2350m&
Rutford
Ice Stream
E
l
l
s
w
o
r
t
h
M
o
u
n
t
a
i
n
s
020
km
Project location
o
77S
o
80W
o
85W
o
78S
Fletcher
Promontory
Grounding
Line
Ic
e
f
lo
w
WAIS
Antarctic
Peninsula
Rutford
Ice Stream
05
km
Fig$1$
5 km
Ice stream flow
Rutford ice stream bedIce thickness: 2 km
Passiveseismicemissionsfromtheice/bedinterface
Evidence of deforming / non-deforming zones
b) Acoustic impedance
Low = Deforming
High = Stable
a) Active Seismicity
c) Radar reflectivity at ice/bed interface& derived effective pressure from AVO
Aseismic soft deformation
Stick-slip atIce/bedinterface
SafetyFactor=1(Pcrit =35kPa≈3.5mwater)
Deforming
ProgradingleeEroding
stoss
Lineofsection
D SS D
Mobilestreamlinedbedform
• Veryloweffectivepressures(highwaterpressures)attheice/bedinterface
• Dilatantbehaviour iswidespread
• Drainageofmeltwater fromthebedisafundamentaldeterminantofshearingbehaviour andconsolidation
• Highwaterpressureconditionsinfluencesedimentmobility
Conclusions
Breidamerkurjokull, Iceland: monitoring changes due to glacier loading
Trench
Advancingkinematicwave
Terminusadvancesovertrench
15m
+20m tobasement
AQUIFER
TILLGLACIER
Stratigraphyattheglaciermargin
Samplingwaterpressurechanges
Pressure-kPa
Days
Pressuresattransducersites
0
1.0
2.0
40 80 120 160 200Pressure- kPa
Pi
Pw
waterflow
waterflow
Impactsofdownwarddrainageintoanaquifer
Depthofshearingonday105.75
Depthofshearingonday106.25
ICE
TILLTILL
AQUIFER
Drainage
WaterpressurefallsIcepressureincreases
Pi+Ps
Evidence of shear displacement during the mini-surge
Tunnelmouth
Groundwaterdominatestunnelwaterflux
Groundwaterheadseasonalfluctuation
Watertable&Inferredgroundwaterflow
Trajectoryoftunnelmouthretreat=esker
Heavierconsolidationnearesker
Lineofsection
Groundwaterflow– subglacial tunnel- esker
Depthbelow
surface-m
Till
Aquifer
1.0
2.0
0
Pw
Pi+Ps
Pressure
Upwarddrainagetoatunnel
TILL
Streamtunnel
AQUIFER
ICE
Conclusions
• Diurnal,annualandweathereventsinfluencewaterfluxanddrainage
• Theyinfluenceconsolidationstateandshearingbehaviour
• Thedirectionofdrainage(up/down)determinestheconsolidationpatterns
• Meltwater tunnelsplayamajorroleindeterminingdrainagegeometry
Fig. 2. Geomorphological map of the ablation zone of the main valley in Andorra. The different positions of glacier fronts and moraines correspond to the MIE stage and tosuccessive post-MIE glacial stages. Legend: 1, runoff; 2, alluvial fans and talus cones; 3, debris-flow fans or slipped masses; 4, peak; 5, glacial cirque; 6, lateral (single solidblack line) and frontal moraine (double solid black line); 7, bedrock step; 8, valley-floor sediments (alluvial, glaciofluvial, till); 9, kame; 10, dashed ornaments: undifferentiatedtill (basal and lateral); 11, past ice-front position at different glacial stages (Tills 0–5).
WU
RM
IAN
GL
AC
IAL
EV
OL
UT
ION
OF
AN
AN
DO
RR
AN
PA
LA
EO
LA
KE
Glacialretreatphases:GranValira d’Andorra
Maximum elevation of the glacier surface
Hydraulic headat glacier sole
875
2250 kPa
1500 kPa 2000 kPa
2000
Modelling subglacial groundwater flow - Andorra
T5
T4
T3
T2
SantaCo
lomaiceload
-T5
LaM
argine
da–T4
SanJulia-T
3
T4
T3
T5
Gran-Valira –compositestratigraphy&pre-consolidation
Data
600 1000 1400 1800
1aEvent
2aEvent
3aEvent
Unit 1Unit 2
Unit 3
Overconsolidation - kPa
0
20
40
60
80
Dep
th -
met
res
Unit 4
Modelling consolidation events
Model Data
T5T4
T3
Singlesimulation
Simulation1
Simulation2
Simulation3
Conclusions
• Majorroleofsubglacial streamsincontrollingdrainagegeometry
• Eachglacialphasesuperimposesitsownconsolidationimprint
• Broadpatternsofvariationarepredictableandshouldbeembeddedinsiteinvestigations
• Inwinter,thesystemdrainedfully
• SYSTEMANDSEDIMENTDRAINAGEGEOMETRYARETHEKEYSTOVARIATION
sediment pressure pswater
pressure pw effective pressure pe
Pressure
Depth
Shearing behaviour
• Shear strength – pe tanΦ
• Shear stress at glacier base ≈ 10-110 kPa
• Till tanΦ = 0.35 - 0.55
• Critical effective pc for failure = 18 – 314 kPa
• Equivalent to 1.8 – 31.4m of water
Pe > pc
pc
pe< pc
Deforming
Stable