Impacts of glaciers on engineering geology: examples ancient and modern
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QRA (Quaternary Research Association) 2017 Annual Discussion Meeting by Geoffrey Boulton, University of Edinburgh
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Impacts of glaciers on engineering geology: examples ancient and modern
- 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
- 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
- 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
- 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
- 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
- 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