QRA (Quaternary Research Association) 2017 Annual Discussion Meeting

1. Preconsolidation in glacial
sediments: the case of Andorra
Like geotechnician I’m interested on:
Soil mechanics
Related with glaciogenic sediments
About its consolodation state of the soils
Like geologist I’m interested on:
The stress history of the sediments
To release glacial geology and geomorphology

2. Preconsolidation in glacial sediments: The case of Andorra
• Introduction to the last glacial cycle in Andorra
• Cases of study
– Case 1 : Soil mechanics in the lateral moraine
• Case 1 servers to introduce into case 2
– Case 2 : Soil mechanics of the overdeepened valley infill
• Major question marks

3. The largest glacial extention (MIS 3)
Pont Trencat
Andorra
La Massana
Ordino
Os
de
Civis
Pal
Erts
Arinsal
Madriu
Claror Perafita
Els
Cortals
Ari¸ge
Canillo
Encamp
Escaldes Engolasters Grau Roig
Incles
Arcalis
Rialb
Sorteny
Angonella
St Juliˆ
de Lˆria
Ransol
Juclar
Siscar—
St Josep
Montaup
Soldeu
Vall del Riu
Cosmogenic 21Ne
59.100 ± 5.170 Ka
AND 6
Fontargente
Cerdanya
43 Km
SPAIN
France
ANDORRA

4. Widespreat retreat of the glaciers in the MIS 2 onset
< 13 Km
Balma Margineda C10
13.763±257 cal BP
(875 m)
El Forn
13.380±380 cal BP
-124014
(1540 m)
El Cedre
(Charred material)
13.585±275 calBP
(875 m)
Glaciofluvial

5. Glacier advance: Main subglacial till deposition on H3
Conflu¸ncia
Front a Sant Juliˆde Lˆria
32.789 ± 1.187
OSL
Cal Tolse
Sornˆs
23.530 ± 130 BP
28.432 ± 335 Cal BP
-203440La Massana
24.770 ± 170 BP
29.803 ± 334 CalBP
-115016 La Aldosa
24.350 ± 150 BP
29.109 ± 451 Cal BP
-115016
Escaldes
27.010 ± 170 BP
31.770 ± 149 Cal BP
-203458
37,5-35 Km
Layer 3a/3b

6. Glacial retreat and glaciofluvial sedimentation
on the main valley
<20 Km
Layer 2
Balma Margineda C10
13.763±257 cal BP
(875 m)
El Forn
13.380±380 cal BP
-124014
(1540 m)
El Cedre
(Charred material)
13.585±275 calBP
(875 m)
Northern
glaciers
Eaestern
glaciers
Glaciofluvial

7. Glacier advance in H2 - LGM: Glaciotectonite formation
33 - 31,5 Km
Layer 2a/2b
La Massana Lake
Sispony
20.875 ± 322 cal BP
-115016
Aldosa
La Margineda
Entremesaigˆes
AND 9
18.077±1.309 Be
10
Engolasters
Els Vilars 19.824 ± 298 cal BP
Canolich Landslide
18.230 - 17.390 cal BP
-198808
Santa Coloma
La Comella moraine
Turu et al. (2016) Did Pyrenees glaciers dance to the beat of global climatic
events? Evidence from the würmian sequence stratigraphy of an ice-
dammed paleolake depocentre in Andorra.
In: Quaternary Glaciation in the Mediterranean Mountains (P.D. Hughes & J.C.
Woodward,Eds.), Geological Society of London, Special Publications.
Doi:10.1144/SP433.6

8. Glacial retreat and again glaciofluvial sedimentation
on the main valley
<20 Km
Layer 1
Balma Margineda C10
13.763±257 cal BP
(875 m)
El Forn
13.380±380 cal BP
-124014
(1540 m)
El Cedre
(Charred material)
13.585±275 calBP
(875 m)
Valira del Nord
glaciers
Valira d’Orient
glaciers
Glaciofluvial

9. Glacier advance in H1, Glaciotectonite formation
31-29 Km
Layer 1a/1b
Sornˆs
14.645±355 cal BP
(1300 m)
?
Santa Coloma
Charred material
14.645±375 cal BP
(975 m-16,4 m)
Andorra
Rossell
.. and recession

10. Widespread glacier retreat in the Bölling/Alleröd
13 Km
Balma Margineda C10
13.763±257 cal BP
(875 m)
El Forn
13.380±380 cal BP
-124014
(1540 m)
El Cedre
(Charred material)
13.585±275 calBP
(875 m)

11. The overdeepened infill of Andorra
Geomorphology of the main valley and position of the glaciers at the last glacial advance from the Upper Pleistocene
CASE 1 : on the lateral moraine – La Comella
CASE 2 : on the bottom of the valley – La Closa
(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
2
1

12. TURU, V., BOULTON, G.S; ROS, X.; PEÑA-MONNÉ, J.LL.; MARTÍ-BONO C.; BORDONAU, J.; SERRANO-CAÑADAS, E.; SANCHO-
MARCÉN, C.; CONSTANTE-ORRIOS, C.; POUS, J.; GONZÁLEZ-TRUEBA, J.J.; PALOMAR, J.; HERRERO, R. & GARCÍA-RUÍZ, J.M.
(2007). “Structure des grands bassins glaciaires dans le nord de la péninsule ibérique: comparaison entre les vallées d’Andorre
(Pyrénées Orientales), du Gállego (Pyrénées Centrales) et du Trueba (Chaîne Cantabrique)”; Quaternaire, 18 (4), 309-325
The overdeepened infill geophysics

13. Case 1: La Comella lateral moraine
Image previous to the final excavation
On top grounded micropiles (not visible),
bounded with concrete on top

14. La Comella lateral moraine
Final excavation fixed with projected concrete … but micropile spacing
was not enough tight, and a portion of the excavated slope fail.
Opportunity arise then to take a look into the supraglacial till.
Moraine
ridge

15. La Comella lateral moraine
Sampled layer

16. La Comella lateral moraine
“Tova” sample
…… silts with some oxidated lenses (water seepage)
Fe+ 3

17. La Comella lateral moraine
“Dura” sample
Highly compacted and shared, sandy core acting as
intraclast boulder
clivage

18. Granulometry of “Dura”sample
Sands 79% - Silts 20 % - Clay 1 %

19. Shear box test “TOVA” sample
Consolidated and Undrained (CU)

20. Shear box “Dura” sample
Consolidated and Undrained (CU)

21. Oedometric tests
The stress history is archived. On both samples a common preconsolidation
exist (supraglacial till formation), while multiple overconsolidations are
present on “Dura” sample (from a former subglacial till)
5MPa
1MPa

22. – Existent data
• One borehole until 60 and 90 m depth has been published (Miquel et al., 2011), but is difficult to consult
• Seismic reflexion profiles published (Teixidor, 2003), but is difficult to consult
– MC Earth Science Foundation data
• Mainly borehole observations come from the first 30 m depth
• In-situ testing (pressurometer tests and SPT or other dynamic penetration proves
• Pumping tests and slug tests
• Seismic refraction (P and S waves)
• Resistivity measurements, mostly VES soundings
• Shallow Nuclear Magnetic Resonance (SNMR)
Acquired geotechnical data at the main valley
Main valley, view upward, at Escaldes-Engordany
through the Valira d’Orient and Madriu confluence
Main valley, view downward through
CASE 2: Geomechanical data in the valley floor

23. Site outcrops across the valley

24. Glaciotectonite 1a/1b
1b
1a
Holocene
Antropogenic
Underground car parking excavation in la Closa site

25. Glaciotectonite 2a/2b
3 m

26. Massive sands and silts
Striated gravels
Striated gravels Laminated sands and silts
Laminated sands and silts
La Closa sediments
Layer 3: Lodgement till

27. Lateral valley eskers
ON NEXT SLICE

28. Sedimentary accretion is produced in lateral subglacial channels
(eskers)

29. In-Situ soil testing of the shear strength: Pocket vane test
The silty-sandy layer show a decreasing pattern from top to bottom.
The shear strength is directly related with the apparent cohesion and thus with its consolidation state.
So in the lateral sides of the glaciated valley of Andorra, former high water pressures were present
Strain state on the latereal subglacial channels
C s S S' S" G B0 10 20
KPa
Light
brown
Dark
brown

Brown
Light
brown
Granulometry
Sandy till with
deformed water
tractive structures
Imbricated sand and
gravels. Horizontal
bedding.
Silt and sand with
some gravel beds.
Matrix supported
and load casts.
Silty till with boulders
Till
Till
Till Till
Till
“Décollement”

30. G 063.06.01
Gas (Nitrogen)“Push in” with a penetrometer
Controller
In-Situ Soil Mechanichs: The pressuremeter device

31. 1
2 h
+
1
2
Tests
Bore-hole
Shear test Oedometric test
Sediment normally consolidated
Pressuremeter test =
Oedometric + Shear test
Po
Po’
IN SITU geotechnical data

32. Borehole data and preconsolidation values across the valley

33. Geomorphological interptretation
Hice – Hw ~ constant
small preconsolidation differences
Hice – Hw <> constant
Hw
Hice
>250 m
high pervasive shearing zone
(glacier confluence)
Low pervasive
Shearing zone
High
pervasive
zone
(close to the
bedrock)
Upper zoneLower zone
Vertical scale > Horizontal scale

34. 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 are up to ten times
greater, that strongly suggest that glacial sediments may be heavIly consolidated
Anomalous preconsolidation values have
been observed at shallow depth (intermediate unit)

35. Seeking out for plumbing paths
A) Glacier surpass a granular aquifer
B) Englacial and subglacial meltwaters
may be drained beneath the glacier
in the most efficient known form
(tunnels)
C) Flow paths under the central tunnel
D) Water and effective pressure in C
E) The same as D but further far from C
F) The shape of the effective pressure
beneath tunnels

36. 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 multiple yield point (case sample “Dura”)
Type 3: P/V evolution without any apparent yield point and strain rebounds are observed (ratcheting)
Extensive ratcheting, tooth-like stress-strain diagram
Stress/Strain analysis, the pressuremeter data
Turu (2007a,b) Pressurometer tests in glaciated valley sediments (Andorra, Southern Pyrenees);
Landform Analysis, 5, 89-99

37. Type 1 P/V evolution is that which is most commonly described
in the literature, a linear stress/strain behaviour from elastic
domain is observed until a yield point is reached where start
non-linear stress/strain behaviour from the plastic domain
until reaching the Coulomb failure value
More than one yield point is observed in that type of diagrams
on the pseudoelastic domain (hyperplastic behaviour), until
the greatest Yield pressure value is reached that closes the
external hyperplasticity envelope. Far away the plasticity field
is reached (drawn) until the Coulomb failure criteria (not drawn).
Type 3 curves have lost their tensional history correspond to an
evolution toward the hyperelasticity and hypoplasticity (HEHoP)
of type 2 curves.
Hyperelasticity can explain easily the behaviour of dense packing soils
for small strains, where the stress is transferred through the porous
media and small intergranular strain occurs without new
rearrangement of grains, so the strain can be considered as reversible.
For extreme stress ubiquitous ratcheting effects may be possible and
are observed in type 3 stress/strain diagrams. Typical saw-tooth-like
stress-strain diagrams are obtained in the vicinity of yield stress
predicted by the hypoplasticity models until is exceeded (HoPP
pressure).

38. Type 3
Type 1
Type 2
Type 2
Type 3

39. TURU, V. (1999) Aplicación de diferentes técnicas geofísicas y geomecánicas para el diseño de una
prospección hidrogeológica de la cubeta de Andorra, (Pirineo Oriental): Implicaciones paleohidrogeológicas en
el contexto glacial andorrano; ACTUALIDAD DE LAS TÈCNICAS GEOFÍSICAS APLICADAS EN
HIDROGEOLOGÍA, (M. Olmo Alarcón i J.A. López Geta, Eds.), ITGE, 203-210
Vcúbic = [ {81 E2 g z / (1 - u2)2 }P2 d2 ] 1/6 (SHERIFF y GELDART, 1991)
E = Dynamic Young modulus
v = Dynamic poisson ratio
d = Natural density
z = Depth
g = gravity
Type 3

40. The consolidation of the subglacial sediments
close to hydraulic singular points (subglacial
tunnel drainage), are subject to an intense flow
of water, situated beneath the central tunnel.
High water flow through porous media could
produce fine grain cleaning.
Such process combinate with pervasive
subglacial shear stress and the 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
critical state and loses it’s stress/strain history.
Type 3
diagram
Resistivity and hyperelasticity/hyperplasticity
Tunnel Western TunnelEastern Tunnel
meters
meters

41. The hyperelastic and hypoplastic behaviour of type 3
curves derive from previous hyperplastic behaviour from
type 2 curves, while hyperplasticity of type 2 in turn
derive from the elastic behaviour of type 1 curves.
The principal mechanism to that evolution is due to
load-unload (L-UL) cycles, producing stiffening and
kinematic hardening of the subglacial sediment.
The evolution from type 2 to type 3 soil behaviour
should start with a critical state consolidation (HoPP
yield), wile the HEHoP (Hyperelastic-Hypoplastic) yield
point appear when the soil is led to a dense packing by
further fine grain cleaning and rearrangement of grains.
Between both, type 2 expansion of the yield curve due
to plastic hardening by load-unload cycles derive to
ratcheting in type 3 diagrams by extensive accumulation
of deformation by those cycles.
Pressuremeter data summary
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.
Figure: courtesy from Geoffrey Boulton

42. Rather than conclusions
Question Marks
1) How can survive distinct consolidated layers to the glacier
overcomes?
Possible answer: Pore water pressure could not be dissipate quickly enough before the glacier
overcomes.
2) How can gravitational pressures exist beneath heavily consolidated
layers?
Possible answer: The deeper overpressured aquifer has been always in steady-state. The nature of
layer 3 (lodgement till) subdivide the aquifer. The upper part of the aquifer was enough
efficent to drain the lateral water inputs.
3) Can we infer the preconsolidation state to a former effective
pressure?
Possible answer: For type 3 curves not because consolidation is acquired close to the critical failure
state. Nevertheless geomorphology is always in the landscape to infer the maximum
effective pressure at each phase. For type 1 sure it might be related and for type 2 also.
However for type 2 terrains different consolidation states are found. Here the spatial
distribution of the values are relevant. Preconsolidation values are strongly site related.
Since now geotechnicians consider that the preconsolidation state of the sediments was a ramdom
distribution. But is not so, their relationship to the subglacial plumbing distribution make them
previsible.