preconsolidation in glacial sediments: the case of andorra

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

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

The largest glacial extention (MIS 3)

a

Pont Trencat

Andorra

La Massana

Ordino

Osde

Civis

Pal

Erts

Arinsal

Madriu

Claror Perafita

ElsCortals

Ari¸ge

Canillo

Encamp

Escaldes Engolasters Grau Roig

Incles

ArcalisRialb

Sorteny

Angonella

St Juli�de L�ria

Ransol

Juclar

Siscar—

St Josep

Montaup

Soldeu

Vall del Riu

Cosmogenic 21Ne59.100 ± 5.170 Ka

AND 6

Fontargente

Cerdanya

43 Km

SPAIN

France

ANDORRA

Widespreat retreat of the glaciers in the MIS 2 onset

< 13 Km

aa

Balma Margineda C1013.763±257 cal BP

(875 m)

El Forn13.380±380 cal BP

-124014(1540 m)

El Cedre(Charred material)13.585±275 cal BP

(875 m)

Glaciofluvial

Glacier advance: Main subglacial till deposition on H3

aa

Conflu¸ncia

Front a Sant Juli� de L�ria32.789 ± 1.187

OSL

Cal Tolse

Sorn�s23.530 ± 130 BP

28.432 ± 335 Cal BP-203440La Massana

24.770 ± 170 BP29.803 ± 334 CalBP

-115016 La Aldosa24.350 ± 150 BP

29.109 ± 451 Cal BP-115016

Escaldes27.010 ± 170 BP

31.770 ± 149 Cal BP-203458

37,5-35 Km

Layer 3a/3b

Glacial retreat and glaciofluvial sedimentation

on the main valley

<20 Km

Layer 2

aa


(875 m)


-124014(1540 m)


(875 m)

Northern

glaciers

Eaestern

glaciers

Glaciofluvial

Glacier advance in H2 - LGM: Glaciotectonite formation

33 - 31,5 Km

Layer 2a/2b

a

La Massana Lake

Sispony20.875 ± 322 cal BP

-115016

Aldosa

La Margineda

Entremesaig�es

AND 9

18.077±1.309 Be10

Engolasters

Els Vilars 19.824 ± 298 cal BP

Canolich Landslide18.230 - 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

Glacial retreat and again glaciofluvial sedimentation

on the main valley

<20 Km

Layer 1

aa


(875 m)


-124014(1540 m)


(875 m)

Valira del Nord

glaciers

Valira d’Orient

glaciers

Glaciofluvial

Glacier advance in H1, Glaciotectonite formation

31-29 Km

Layer 1a/1b

aaa

Sorn�s14.645±355 cal BP

(1300 m)

?

Santa ColomaCharred material

14.645±375 cal BP(975 m-16,4 m)

AndorraRossell

.. and recession

Widespread glacier retreat in the Bölling/Alleröd

13 Km

aa


(875 m)


-124014(1540 m)


(875 m)

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

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

Case 1: La Comella lateral moraine

Image previous to the final excavation

On top grounded micropiles (not visible),

bounded with concrete on top

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


Sampled layer


“Tova” sample

…… silts with some oxidated lenses (water seepage)

Fe+ 3


“Dura” sample

Highly compacted and shared, sandy core acting as intraclast boulder

clivage

Granulometry of “Dura”sample

Sands 79% - Silts 20 % - Clay 1 %

Shear box test “TOVA” sample

Consolidated and Undrained (CU)

Shear box “Dura” sample

Consolidated and Undrained (CU)

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

– 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

Site outcrops across the valley

Glaciotectonite 1a/1b

1b

1a Holocene

Antropogenic

Underground car parking excavation in la Closa site

Glaciotectonite 2a/2b

3 m

Massive sands and silts

Striated gravels

Striated gravels Laminated sands and silts

Laminated sands and silts

La Closa sediments

Layer 3: Lodgement till

Lateral valley eskers

ON NEXT SLICE

Sedimentary accretion is produced in lateral subglacial channels

(eskers)

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

aa

C s S S' S" G B0 10 20

KPa

Lightbrown

Darkbrown

Brown

Lightbrown

Granulometry

Sandy till withdeformed watertractive structures

Imbricated sand andgravels. Horizontalbedding.

Silt and sand withsome gravel beds.Matrix supportedand load casts.

Silty till with boulders

a

Til lTil l

Til l Til l

Til l

“Décollement”

G 063.06.01

Gas (Nitrogen) “Push in” with a penetrometer

Controller

In-Situ Soil Mechanichs: The pressuremeter device

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

Borehole data and preconsolidation values across the valley

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 zone Lower zone

Vertical scale > Horizontal scale

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)

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

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

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).

Type 3

Type 1

Type 2

Type 2

Type 3

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

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 Tunnel Eastern Tunnel

meters

meters

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

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.