impacts of glaciers on engineering geology: examples ancient and modern

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