QRA (Quaternary Research Association) 2017 Annual Discussion Meeting by Geoffrey Boulton, University of Edinburgh

1. Impacts of glaciers on engineering geology:
examples ancient and modern
Geoffrey Boulton
University of Edinburgh
Quaternary Research Association
Durham, December 2016

2. 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

3. No basal melting Basal melting
Glacier loading & shearing
No surface load No surface load
LOADING
HISTORY
PRESSURE
HISTORY AT
DEPTH “D”
TIME
0
sediment pressure
water pressure
DENSITY
CHANGE
+ve
-ve
shear dilation
normal
consolidation
over
consolidation
Consolidation History

4. Rutford ice stream,
West Antarctica
+ Giorgos Papageorghiou
Andy Smith, Emma Smith
Breidammerkurjökull,
Iceland
+ Sergei Zatsepin
La Gran Valira,
Andorra
+ Valenti Turu
Examples from:
Ancient
Modern

5. !2100m&
!2150m&
!2200m&
!2250m&
!2300m&
!2350m&
Rutford
IceStream
EllsworthMountains
020
km
Projectlocation
o
77S
o
80W
o
85W
o
78S
Fletcher
Promontory
Grounding
Line
Iceflow
WAIS
Antarctic
Peninsula
Rutford
IceStream
05
km
Fig$1$
5 km
Ice stream
flow
Rutford ice stream bed
Ice thickness: 2 km

6. Passive seismic emissions from the ice/bed interface

7. 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 at
Ice/bed
interface
Safety Factor = 1 (Pcrit = 35kPa ≈ 3.5m water)
Deforming

8. Prograding
leeEroding
stoss
Line of
section
D SS D
Mobile streamlined bedform

9. • Very low effective pressures (high water pressures) at
the ice/bed interface
• Dilatant behaviour is widespread
• Drainage of meltwater from the bed is a fundamental
determinant of shearing behaviour and consolidation
• High water pressure conditions influence sediment mobility
Conclusions

10. Breidamerkurjokull, Iceland: monitoring changes due to glacier loading
Trench
Advancing
kinematic
wave
Terminus
advances
over trench

11. 15m
+20m to
basement
AQUIFER
TILL
GLACIER
Stratigraphy at the glacier margin

12. Sampling water pressure changes

13. Pressure -kPa
Days
Pressures at transducer sites

14. 0
1.0
2.0
40 80 120 160 200
Pressure - kPa
Pi
Pw
water flow
water flow
Impacts of downward drainage into an aquifer
Depth of shearing
on day 105.75
Depth of shearing
on day 106.25
ICE
TILLTILL
AQUIFER
Drainage
Water pressure falls
Ice pressure increases
Pi + Ps

15. Evidence of shear displacement during the mini-surge

16. Tunnel mouth
Groundwater dominates
tunnel water flux
Groundwater head
seasonal fluctuation
Water table &
Inferred
groundwater
flow
Trajectory of tunnel
mouth retreat = esker
Heavier consolidation
near esker
Line of
section
Groundwater flow – subglacial tunnel - esker

17. Depth below surface -m
Till
Aquifer
1.0
2.0
0
Pw
Pi+Ps
Pressure
Upward drainage to a tunnel
TILL
Stream
tunnel
AQUIFER
ICE

18. Conclusions
• Diurnal, annual and weather events influence water flux and drainage
• They influence consolidation state and shearing behaviour
• The direction of drainage (up/down) determines the consolidation patterns
• Meltwater tunnels play a major role in determining drainage geometry

19. 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 to
Glacial retreat phases: Gran Valira d’Andorra

20. Maximum elevation of the glacier surface
Hydraulic head
at glacier sole
875
2250 kPa
1500 kPa 2000 kPa
2000
Modelling subglacial groundwater flow - Andorra

21. T5
T4
T3
T2
Santa Coloma ice load -T5
La Margineda–T4
San Julia-T3
T4
T3
T5
Gran-Valira –
composite stratigraphy
& pre-consolidation

22. Data
600 1000 1400 1800
1a 2a 3a
Unit 1
Unit 2
Unit 3
Overconsolidation - kPa
0
20
40
60
80
Depth-metres
Unit 4
Modelling consolidation events
Model Data
T5
T4
T3
Single
simulation
Simulation 1
Simulation 2
Simulation 3

23. Conclusions
• Major role of subglacial streams in controlling drainage geometry
• Each glacial phase superimposes its own consolidation imprint
• Broad patterns of variation are predictable and should be embedded
in site investigations
• In winter, the system drained fully
• SYSTEM AND SEDIMENT DRAINAGE GEOMETRY ARE THE KEYS TO VARIATION

24. 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