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Subglacial ploughing and drainage patterns in a glaciated valley (Andorra, Southeastern Pyrenees)
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Subglacial ploughing and drainage patterns in a glaciated valley (Andorra, Southeastern Pyrenees)


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  • 1. SUBGLACIAL PLOUGHING AND DRAINAGE PATTERNS IN A GLACIATED VALLEY (ANDORRA, SOUTHEASTERN PYRENEES) Valenti TURU (1) & Geoffrey S. BOULTON (2) (1)  Marcel Chevalier Foundation (Andorra) (2)  School of Geosciences, University of Edinburgh: 36 slides 1
  • 2. •  1) The Andorra glaciated valley •  Setting •  Pressuremeter tests•  2) Rehology •  Stress/Strain diagrams –  Type 1 P/V curves: Elastoplastic –  Type 2 P/V curves: Hyperplastic –  Type 3 P/V curves: Hyperelastic-hypoplastic•  3) Data •  Site 1: La Closa –  Consolidated layers and stratigraphy –  Prandtl penetration keel –  14C data and ploughing •  Site 2: P. del Roure •  Site 3: P. de les Oques•  4) Conclusions 36 slides 2
  • 3. The Andorra glaciated valley Geomorphology of the main valley and position of the glaciers at the last glacial advance from the Upper Pleistocene(1) fluvial network, (2) alluvial cone, (3) debris cone and scree, (4) mountain peak, (5) glacial cirques, (6) hummocks, (7) subglacial gorge, (8) morainic ridge, (9) reconstructed glacier margins, (10) till, (11) alluvium, (12) colluvium, (13) glacier front. Red circle main examples 36 slides 3
  • 4. Geomechanical data •  Glacial sediments produced during Quaternary glacial periods are widespread in both mountainous and lowland zones and influence many construction projects. •  Understanding the stratigraphy of the glacial loaded sediments of Andorra is particularly important for civil engineers. •  One of the characteristics of such sediments is the great variability and unpredictability of the consolidation state and accurately geotechnical and geophysical surveys are needed. Investigation data from Andorra main va Fondation Marcel Chevalier -5 -10 -15 Depth (m) -20 -25 Borehole (1596 m) -30 Carottage (385 m) Intact samples (195) -35 0 10 20 30 40 50 % Main valley, view upward, at Escaldes-EngordanyMain valley, view downward through Acquired geotechnical data at the main valley through the Valira d’Orient and Madriu confluence 36 slides 4
  • 5. IN SITU geotechnical data Shear test Oedometric test Void ratio 2 1 Po 1 Pressure ( Void ratio P/V diagram Po’ 250 (Example) 2 ’ 1 200 Strain (Volume cm3)Tests 150 (Po) 2 h (Po’) 100 h* *g Pressure ( Terrain normally consolidated. 50Bore-hole + 0 1 2 Pressuremeter test = 0 1 2 3 4 5 6 7 8 Stress (x 100 KPa) Oedometric + Shear test 36 slides 5
  • 6. Anomalous preconsolidation values have been observed at shallow depth As previously stated, this test has been performed in boreholes, introducing the cell at depths between 5 and 25 meters which, in the best scenario, implies ground pressures acquired according to a gravitational gradient between 0.1 to 0.5 MPa. However, with pressuremeter tests, overconsolidation pressures up to ten times greater than these have been obtained, implying that glacial sediments may be strongly consolidated. A-040.11.97 1200 Pressure 0 0 1000 h=-9m h =-4m Deformation (volume) 800 3Depth 600 6 400 Po’ 10 200 Po 1) Gravitational weight of s 2) Consolidation data (Po) 0 0 1 2 3 Pressure (MPa) 36 slides 6
  • 7. Stress/Strain analysis, the pressuremeter data Stress/strain data (pressuremeter P/V data) obtained permit us distinguish basically three types of charts: Type 1: P/V evolution with a single yield point Type 2: P/V evolution with various yield point Type 3: P/V evolution without any apparent yield point and strain rebounds are observed (ratcheting)Type 1 diagram Type 2 diagram Type 3 diagram 250 800 600 700 Po’ (4) 200 Po’ (3) 500Strain (Volume cm3) Strain (Volume cm3) Strain (volume cm3) 600 Extensive ratcheting, tooth-like stress-strain diagram 150 500 400 Po’ (2) Po’ 400 100 Po’ (1) 300 300 FEDA ERT S4-P3 (-3,4 m 200 Po’ (1) 50 200 Po’ (2) Load-Unload 100 cycles La Closa S3b-P3 (-21,6 m Po’ (3) BM/BI S1-P1 (-4,5 m Po’ (4) 0 0 100 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 9 10 1 12 13 14 15 0 10 20 30 40 50 60 70 Stress (x 100 KPa) Stress (x 100 KPa) Stress (x 100 KPa) 36 slides 7
  • 8. Type 1 P/V evolution is that which is most commonly describedin the literature, a linear stress/strain behaviour from elasticdomain is observed until a yield point is reached where startnon-linear stress/strain behaviour from the plastic domainuntil reaching the Coulomb failure valueMore than one yield point is observed in that type of diagramson the pseudoelastic domain (hyperplastic behaviour), untilthe greatest Yield pressure value is reached that closes theexternal hyperplasticity envelope. Far away the plasticity fieldis reached (drawn) until the Coulomb failure criteria (not drawn).Type 3 curves have lost their tensional history correspond to anevolution toward the hyperelasticity and hypoplasticity (HEHoP)of type 2 curves.Hyperelasticity can explain easily the behaviour of dense packing soilsfor small strains, where the stress is transferred through the porousmedia and small intergranular strain occurs without newrearrangement of grains, so the strain can be considered asreversible.For extreme stress ubiquitous ratcheting effects may be possible andit’s observed in type 3 stress/strain diagrams. Typical saw-tooth-likestress-strain diagrams are obtained in the vicinity of yield stresspredicted by the hypoplasticity models until is exceeded (HoPPpressure). 36 slides 8
  • 9. Pressuremeter data summary The hyperelastic and hypoplastic behaviour of type 3 Type 1, 2 and 3 stress/strain evolution with curves derive from previous hyperplastic behaviour from type 2 curves, while hyperplasticity of type 2 in 900 La Closa S3b-P3 (-21,6 turn derive from the elastic behaviour of type 1 curves. La Closa S3b-P2 (-17,6 Type 1 800 La Closa S3b-P1 (-13,6 The principal mechanism to that evolution is due to La Closa S3a-P3 (-11,8 m load-unload (L-UL) cycles, producing stiffening andStrain (volume cm3) 700 La Closa S3a-P2 (-10,2 Type 2 kinematic hardening of the subglacial sediment. La Closa S3a-P1 (-8,6 m 600 Stiffening The evolution from type 2 to type 3 soil behaviour 500 should start with a critical state consolidation (HoPP Type 3 yield), wile the HEHoP (Hyperelastic-Hypoplastic) yield 400 point appear when the soil is led to a dense packing by 300 further fine grain cleaning and rearrangement of grains. Yield locu 200 migration Between both, type 2 expansion of the yield curve due Kinematic hardening 100 to plastic hardening by load-unload cycles derive to ratcheting in type 3 diagrams by extensive 0 accumulation of deformation by those cycles. 0 10 20 30 40 50 60 70 Stress (x 100 KPa) Load-Un Load cycles are produced by the melting dynamics of the glacier. Could be diurnal, seasonal or climatic range in function of the subglacial possition. 36 slides 9
  • 10. Three geomechanical evidences Located sites36 slides 10
  • 11. P. del Roure 2 SITE 21 3 SITE 1: La Closa S2c S1b S2d L1 S6 S4c S5 S3b S4b S4a T1 SITE 3 P. de les Oques 36 slides 11
  • 12. Resistivity profile SITE 2 ? SITE 1: La Closa S2c S1b S2d L1 S6 S4c S5 S3b S4b S4a T1 SITE 3Soundings at la Closa La Closa Site 36 slides 12
  • 13. SITE 2 ? SITE 1: La ClosaType 2 Type 3: Hypoplastic S2c S1b Type 1 S2d L1 S6 S4c S5 S3bType 2 S4b S4a Type 3: Hyperelastic T1 SITE 3 Geomechanical behaviour 36 slides 13
  • 14. Laminated sands and silts Holocene 1a 1b La Closa sediments Striated gravels Massive sands and siltsStriated gravels Laminated sands and silts 36 slides 14
  • 15. S-N Hyperplastic Elastoplastic Hyperplastic Hyperplastic Hypoplastic Unconsolidated Hyperelastic Upwelling zoneIn the drilling-sampling-in situ tests process has been observed a very weak sand layer that collapses ina siphoning process, coutting all the stratigraphy and should be consider out of the sequence. 36 slides 15
  • 16. S-N Hyperplastic Elastoplastic Hyperplastic Hyperplastic Hypoplastic Unconsolidated Hyperelastic Glacier base L-UL cycles Pervasive shear stress Glacier base Accretionary pile up till Sheared Hyperplastic gravels Pile up till (striated) 1 3 Hyperelastic Hypoplastic Prandtl matrix pore pres logaritmic increase arroun loop /2- the gravel surfThe L-UL cycles produce substratum hardening and an stiffening effect that could locally overload the bearing capacity of the underlying layers.If that happen two main types of ground collapse can happen according to the substratum compacity. If weak a punching failure occour, if heavilydense then a general failure process starts in wich a more or less large Prandtl logaritmic loop failure produced accordinly to the frictional angle. 36 slides 16
  • 17. S-N 14C Data Hyperplastic Elastoplastic Hyperplastic Hyperplastic Hypoplastic Unconsolidated Hyperelastic 3 3 3 3The Load and Unload cycles at the Glacier base L-UL cyclesbottom of the glacier and produces 1 Plane Pervasivean accretionary pile up of till in wich shear stress 1 1 1the organic matter is incorporated into Accretionary 3 3 3 Clast ploughing 3 3the till matrix,being older on bottom and pile up till Sheared 3 3 3younguer on top. gravels (striated) 1 3 Failure 1 1 1 plane 1 1We observe that the age of the sandy matrix pore pres 1 1 1layer between hypoplastic-hyperelastic increase arroun a bad pore water dissipation can produce a the gravel surf failure plane on till by diggingtill layers is the same as the overlyingtill layer (age from the same till at site 2). Subglacial clasts are dragged through the sediment by the L-UL cycles producing pore pressures inThe geomechanical behavieur of both till excess that could weaken the sediment downtill from ploughing clasts producing a failure plane.layers are related with the same process, Here we observe that once the failure plane formed an decouppling till-substratum effect happenmeaning that once were the same till layer (glacier flotation?) that slides the infill of sands and silts on the space between tills. 36 permits 17an it has been separate by a failure plane.
  • 18. WNW-ESEThe same weak sand layer that collapses is present on that profile an seems to berelated to the 1a layer, the younguest subglacial consolidated layer 36 slides 18
  • 19. WNW-ESE Failure planeThe anomalous growth of layer 1a close to the weak sandy layer is interpreted as anaccommodation failure, in a piling up synsedimentary process 36 slides 19
  • 20. S-N And the same for the previous profile…. But some lateral contacts can’t be explained with a displacement above them, for that reason we needto invoke a lateral facies contact or a previous failure contact. Sedimentary lateral facies contact ispossible but not a horizontal variation from Type 2 to Type 3 geomechanic behaviour in a so short space(about 20 m), only in vertical direction sharp changes in the geomechanical behavieur are observed. 36 slides 20
  • 21. S-N PPK PPK Prandtl Penetration KeelBeing coherent with the geomechanical data we suggest a lateral mechanical contact.Such contact is related with a glacier overload structure, similar to what happen in a generalfailure under a shallow foundation when it exceed the bearing capacity of the soil beneath it:a Prandtl Penetration Keel is espected to be present on the Andorra glacial valley floor. The following slides shows the sedimentary and deformation sequence >> 36 slides 21
  • 22. S-NHypoplastic/Hyperelastic 36 slides Keel and general failure Prandtl 22
  • 23. S-N 36 slidesHigh water pressures produces till decoupling following the previous failure plane 23
  • 24. S-NSedimentation of the following subglacial till with a Hyperplastic behaviour (type 2) 36 slides 24
  • 25. S-NThe sand-silt layer formed before acts as a slides 36 detachment layer. The Prandtl keel is now inactive 25
  • 26. S-NA general glacier retreat permit to fill the slides floor, firstly with glaciolacustrine deposits26 36 valley
  • 27. S-NProglacial outwash infill sequence 36 slides 27
  • 28. S-NA general glacier readvance consolidate the 36 slides layers and a reactivation of the failure happen previous 28
  • 29. S-NFinal glacial retreat, fluvial infill and Holocene landslides invade the valley bottom 36 slides 29
  • 30. Site 2: P.del Roure Other resistive bodies are close to the la Closa ones Next >>Depth (m) 36 slides 30 Resistive bodies Distance (m)
  • 31. Site 2: Prat del Roure Prandtl penetration Keel (PPK) Possible PPK Holocene Hyperplastic Elastoplastic 1a Hyperplastic 1b Elastoplastic Hyperplastic 2aDepth (m) Hyperelastic Hypoplastic Elastoplastic PPK Hypoplastic ? Distance (m) 36 slides 31
  • 32. Site 3: P. de les OquesOn the lateral side of the Andorra valley is common to observe bouldery layers overlying sand and gravelslayers with load structures. Those layers have been consolidated after deposition. 36 slides 32
  • 33. Consolidations state of the deposits on the latereal side of the valley glacier “Décollement” Til l Til l Til l Til l Til l Granulometry KPa 0 10 20 C s S S’S" G B Light Sandy till with brown deformed water tractive structur Imbricated sand a Dark gravels. Horizont brown bedding. Silt and sand wit Brown some gravel beds Matrix supported and load casts. Light Silty till with brownTesting the shear strenght with a simple pocket vane apparatus is possible to see that thesilty-sandy layers show a decreasing pattern from top to bottom. The shear strenght are directlyrelated with the apparent cohesion and thus with its consolidation state. The only way to keep alow consolidation value is the presence of high water pressure in porous media that balance theoverlying glacier pressure. So at the lateral sides of the glaciated valley high water pressures should be common. 36 slides 33
  • 34. Combining field observations, geophysical data and pressuremeter data we can speculateabout the continuity of the ploghing PPKs (Prandtl penetration keels) on the Andorra glacialvalley floor, see the figure on next slide: 36 slides 34
  • 35. ConclusionsSubglacial tunnel Subglacial tunnel Subglacial tunnel Site 3: P. de les Oques Site 2: P. del Roure Site 1: La Closa 1a 1b Highly 2a Poorly consolidated 3a consolidated2b layers: layers 3b1a 680 m/s Holocene 606 m/s 1a 879 m/s 1a2a 2a 1174-977 m/s 1b 2a 1b1252 m/s 1b 2a 3100 2a m/s 2b 4 3a 2b 3 3b 3 4 3 3a Prandtl penetration keel 4 3b 4 at glacial stage 1 ? Prandtl penetration keel at glacial stage 3 5 5 Prandtl penetration keel at glacial stage 3? Roca Substratum Resistivity (ohms m) Bottom valley pressuremeter type 3 diagrams are related with hyperelastic/hypoplastic PPK’s Bottom valley high resistivity domains are related with the subglacial drainage plumbing Both (resistivity and geomechanical behaviour) are related on the bottom valley Small and large scale structures are related with ploughing process 1: Weaken heavily tills 2: Prandtl Penetration Kell 36 slides 35
  • 36. Thank you 36 slides 36
  • 37. GEOMECHANICAL ANNEX (If needed) 36 slides 37
  • 38. Subglacial plumbing Hypothetical glacier height (c) 100 m A B (b) Moulins Valley glac Static water ta (a) Dynamic water ta Snout H 2O Crevasses 0 Lateral eskeAquifer 3 4 5 3 Tunnel R 6 Depth Equipotentials 7 (m) 6 Flow lines 8 9 10 10 11 (e) (d) LATERAL POSITION CENTRAL POSITION TO THE TUNEL, A WITH REGARD TO THE TUNNEL, B Pressure (100 x KPa) Pressure (100 x KPa) 0 5 1 1 0 5 10 15 (f) Effective pressures (100xKPa) 2 0 0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 3 Lateral Depth (m) 3 Depth (m) 2 1+3 Tunnel effective 3 3 1 pressure 1+3 6 6 6 6 Depth (m) Esker Lateral 10 10 10 3 10 1) Gravitational weight of sediment effective 1 3 2) Dynamic water pressure pressure Lines join together 3) Gravitational weight of Both lines can no ice 36 slides Glacier flotation condition at the lateal of the tunnel 38 1 + 3 join, no flotati exist beneath the Effective pressure: 1 + 3 - drainage tunnel 2 Water pressure = Sediment weight + Ice weight
  • 39. 1 Type 2 diagram: 1 q q Hyperplasticity p’ p’Pervasive shear --> 0 Pervasive shear --> 0 Eventual "d collement" BOULTON & ZATSEPIN (2006) 2 q shown that the progressive atenuation of diurnal, seasonal and annual frequencies reflected in pressure p’ fluctuations at the ice-bed. At - + the inner part of the glacier only Pervasive shear large cycles are transmited to the subglacial bed (climatic cicles). q Load-Unload Stiffening Multiple yield field by load-unload 3 cycles CSL q4 4 - + p’ q 16 8 Pervasive shear Shape of q16 ESP a heavily consolidate clay in an undrained 6 TSP q 7 consolidation1 2 state Y3 3 m 4 ESP Dranaige Pervasive q with consta shear stress pervasive - + p’ drop to zero Beginning 36 slides of 5 q=0 1 p’ shear strai 39 Pervasive shear a L-UL cycle p’>0
  • 40. 18/28 Type 3 diagram Hyperelasticity - HypoplasticitySome particularities should be taking in account whenpervassive subglacial shear stress is present. GlacierThe zone of till where the available shear strength is less than - + Consolidationthe constant pervasive subglacial shear stress imposed by theoverlying glacier ice, undergoes critical state consolidation.Small load-unload hydrological cycles (follow the numbers on Pervasive shefigure below) produce that the stress state of the subglacialsediment moves away or close from the critical state line (CSL).Such consolidation is known as critical state consolidation Not sheared sedimentand can be more than 1.8 times greater than the isotropicconsolidation. 600 HEHoP HoPP Non-linear behaviour 500 Ratcheting non-linear behaviour (Hypoplasticity) (Plastic domain) Critical State Consolidation CSLStrain (Volume cm3) q q16 16 400 Extensive ratcheting, tooth-like stress-strain diagram Pervasive TSP shear stress Dranaige drop to zero with constan 300 Beginning of q4 8 pervasive Strain rebound a L-UL cycle 6 4 shear strain Strain rebound 7 200 Strain rebound 2 3 Strain rebound La Closa S3b-P3 (-21,6 m 1 Kinematic hardening La Closa S2d-P3 (-16,2 m m q ESP 100 0 10 20 30 Hyperelasticity field Stress (x 100 KPa) 40 50 60 70 36 slides 5 q=0 p’>0 1 p’40
  • 41. Compact cubic grain packing Resistivity and presuremeter data in a perpendicular profile to glacier flow. Type 1 diagrams are located on low resistivity facies. Type 3 diagrams are located on high resistivity facies. Type 2 diagrams in between. Andorra glaciated valley y = 1257.5 * 10^(-3.8756e-2x) R^2 = 0.838 10000 Correspondence between electrical resistivity and fine grained content Toward hyperelasticity Legend La Margineda 1000 Santa Coloma Roysa Santa Coloma Riberayg Ohms X m Escaldes Prat del Rou La Comella 100 10 0 5 10 15 20 25 30 35 40 45 50 55 Silt and Clays content (<0,08 mm) %Hyperelastic terrains acts like a dense packing (cubic or hexagonal grains packing) material. The dynamic shear modulus(P and L waves) with the pressuremeter (static) shear modulus are very nearer (ratio ≈ 1). Resistivity values suggest thathyperelastic and hypoplastic terrains seems to be cleaned of clays and silt by the groundwater flow through the subglacialdrainage tunnels. 36 slides 41
  • 42. Resistivity and hyperelasticity/hyperplasticityTunnel Tunnel Tunnel Type 3 diagram The consolidation of the subglacial sediments close to hydraulic singular points (subglacial Andorra glaciated valley tunnel drainage), are subject to an intense flow y = 1257.5 * 10^(-3.8756e-2x) R^2 = 0.838 10000 of water due to being situated near the place Correspondence between electrical resistivity and fine grained content of drainage where there is a high hydraulic drop. The idea of an high water flow through Legend porous media that produces a fine grain 1000 La Margineda Santa Coloma Roysa cleaning is supported by soil analysis and Santa Coloma Riberayg Ohms X m Escaldes Prat del Rou geophysical data. Such process combinate La Comella with pervasive subglacial shear stress and the 100 L-UL cycles rearrange the sediment grains to a dense packing (close to hexagonal or a cubic simetry). The soil will appear to be undergoing consolidation when its stress state is close to 10 critical state and loses it’s stress/strain history. 0 5 10 15 20 25 30 35 40 45 50 55 Silt and Clays content (<0,08 mm) % 36 slides 42
  • 43. 25/28 Prandtl penetration Keel (PPK) Central Lateral Esker Tunnel Lateral Esker - + - + consolidated- + - - + - - consolidated Prandtl logaritmic loop The overbunden pressure from the glacier weight plus the subglacial water drainage via porous media through the central tunnel, following Load-Un Load cycles (diurnal/seasonal/climatic cycles) promote a Penetration critical state of consolidation Keel and produce that the terrain becomes harder and stiffer than the sorrounding terrain σ1 (hyperelastic-hypoplastic), overloading the σ3 σ3 bearing capacity of the terrain and breaking Glacier load it following a Prandtl logaritmic loop. If the Lateral penetration keel is coupled to the glacier Lateral Esker Esker basal motion then a ploughing effect on the Tunnel middle of the glacial valley is possible. - + -- + - - + - Hyperelastic and hypoplastic Keel 36 slides 43
  • 44. Appendix: The pressuremeter 36 slides 44
  • 45. The pressuremeter device G 063.06.01 Controller “Push in” with a penetrometer Gas (Nitrogen) 36 slides 45
  • 46. The pressuremeter test P/V diagram Intact soil 250 200 Strain (Volume cm3) 150 Push in, soil 100 plastification ring Non linear behavieur (disturbed soil) 50 0 0 1 2 3 4 5 6 7 8 Stress (x 100 KPa) P/V diagram 250 Elasto-Plastic Yield point (Po’) 200 Pressuremeter test, Strain (Volume cm3) Non interpretable (disturbed soil) Linear behaviour (Elastic domain) cilindrical deformation 150 Non-linear behavio (Plastic domain 100 ˘p ˘v 50 0 0 1 2 3 4 5 6 7 8 Test end, soil Stress (x 100 KPa) recover parcially 36 slides 46
  • 47. Geomechanical data, pressuremeter testsG 085.12.02 A 151.10.00 G 093.09.03 G 040.11.97 Driller The most frequent problems: “Push in” Gravels between the slotted tube and the pneumatic cells Slotted tube Pinch out of the pneumatic cells 1 to 1,5 m metallic tubes by gravels or coarse sands Pneumatic cells Slotted tube braked & broken by big boulders or even Hydro-pneumatic conduit deformation of the slotted tube 36 slides 47