Degradation of compacted marls. A microstructural investigation.
Rafaela Cardoso
High Technical Institute, Lisbon, Portugal
XVIII European Young Geotechnical Engineers’ Conference
Portonovo, Ancona (Italy) - June 17-20, 2007
PORTUGUESE GEOTECHNICAL
SOCIETY
INTRODUCTION
This paper presents a micromechanical study where the evolution of Abadia marls (Arruda dos Vinhos, Portugal) is simulated.
Concepts of unsaturated soil mechanics are used since alternate wetting and drying cycles (strong suction changes) are the main cause of degradation.
Marls are classified as hard- soil/ soft rock and exhibit a typical evolutive behaviour due to weathering processes.
INTRODUCTION
Embankments built with marls result in an agglomerated structure of rock fragments.
These particles evolve and result in major changes in the overall behaviour of the aggregate.
Settlements and loss of strength in time are the main concerns in practice.
Suitable constitutive and computational models are required to predict these phenomena.
Hydro-mechanical behaviour of Abadia marls
Evolution phenomena observed in one wetting drying cycle:
marl matrix
uniform-size particles (9mm≥D>4.75mm) initial water content w=9% (initial suction s=10MPa)
0
10
20
30
40
50
60
70
80
90
100
0.010.1110100
Size, D (mm)
% m
ater
ial p
assi
ng (c
umul
.)
before the cyclesafter 1 cycle (Rd=16%)after 2 cycles (Rd=49%)after 3 cycles (Rd=60%)after 4 cycles (Rd=68%)after 5 cycles (Rd=70%)after 6 cycles (Rd=72%)after 7 cycles (Rd=74%)after 8 cycles (Rd=78%)
Hydro-mechanical behaviour of Abadia marls
• Abadia Formation (Upper Jurassic, Arruda dos Vinhos, Portugal)
• LL=49%, IP=25%
• γs =27.4kN/m3
• Mineralogy analysis showed mainly:
chlorite gypsumquartzCaCl2mica
Hydro-mechanical behaviour of Abadia marls
e= 0.012w + 0.378
0.30
0.40
0.50
0.60
0.70
0 2 4 6 8 10 12 14 16 18 20
water content, w (%)
eMore characteristics:
• volume dependence on water content
• kint =8×10-21 m2 • ksat =8×10-14 m/s
in situ porosity=37%
win situ = 17% (Sr=77%)
κs =0.020
Hydro-mechanical behaviour of Abadia marls
• Water retention curve, WRC
0.10
1.00
10.00
100.00
1000.00
2 4 6 8 10 12 14 16 18water content, w (%)
Tota
l suc
tion
(MP
a)
Drying _ BlockWetting _ BlockWRC-DryingWRC-Wetting
λ
λλ −
−
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛ −+=
11
PPP
S lge
Van Genuchten’s expression:
drying branch
P=0.3MPa
λ= 0.20
wetting branch
P=0.9MPa
λ= 0.20
Hydro-mechanical behaviour of Abadia marls
0
2
4
6
8
10
12
14
16
0 100 200 300 400Vertical stress (kPa)
swel
ling
stra
in (%
) s=4.8MPa
s=11.0MPa
s=135.9MPa
Suction (MPa)
Vertical stress (kPa)52 158 290
135.9
11.04.8
initial suction:
• swelling strain dependence on initial suction and vertical stress
Hydro-mechanical behaviour of Abadia marls
Compacted samples
Suction controlled tests on compacted aggregates of marl, uniform-size particles (9mm≥D>4.75mm)
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
1.10
1 10 100 1000 10000vertical tension (kPa)
Void
ratio
, e
s=230MPas=38MPas=12MPas=3MPaSaturated test
Drying (σv=50kPa)
Saturation (σv=600kPa)
Suction of the test (MPa)
Compressibility index, Cc
Volume decrease due to drying (%)
Collapse due to full wetting (%)
230 0.095 3.9 20.438 0.379 3.4 15.712 0.394 1.3 13.93 0.536 -- 9.7
dry samples: rockfill behaviour
Hydro-mechanical behaviour of Abadia marls
saturated samples: behaviour similar to the one of a clayey soil
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1 10 100 1000 10000vertical tension (kPa)
Nor
mal
ized
voi
d ra
tio e
/eo
s=3MPa
Saturated test
Reconstituted(w=1.3LL)
Cc =0.141
Hydro-mechanical behaviour of Abadia marls
Microstructural analysis of the compacted samples:
Compacted samplesbehaviour
single particlebehaviour
volume decrease caused by drying
volume decrease of each particle when dried
collapse when fully saturated Particle breakage and rearrangement of the broken
fragmentsSaturated compressibility = reconstituted compressibility
degradation suffered by the smallest fragments
DEGRADATION MECHANISM
s=0 MPa (saturated in the border)
sinitial
differential deformation due to swelling (proportional to the differential of suction)
Wet zone (sinitial<s<0)
Dry zone(suction=sinitial)
Tension development (cracking)
suction differential inside the fragment
differential swelling deformations
stress development (tension and shear)
cracking and destructuration.
NUMERICAL MODEL
Analysed cases
C
H
G
F
AB
C D
E
C
AG
F
individual fragments of rock (D=9mm) saturated from the exterior perimeter
HM analysis with CODE_BRIGHT®
• constitutive mechanical model: BBM
• conductive flux: Darcy´s law
• diffusive and dispersive fluxes: Fick´s law
• intrinsic permeability: Kozeny’s model
• WRC: Van Genuchten’s expression
Case 1 Case 2 Case 3
calibration with experimental
results
deformable porous media
a) 15 minutes after wetting.
σI Psδε
b) 5 hours after wetting.
σI Psδε
RESULTS
AB
C D
E
• initial suction=10MPa
• 15 minutes for wetting
• full saturation reached
Cracking mechanism
-4
-2
0
2
4
6
8
0.00 0.25 0.50
time (hours)
max
. prin
cipa
l stre
ss (k
Pa)
AE
B D
C
C
B D
Not saturated Saturated
Com
pres
sion
Te
nsio
n
tension
compression
tension
compression
RESULTS
• initial suction=10MPa
• 15 minutes for wetting
Influence of confinementC
H
G
F
A B
C D
E
C
AG
F
Case 1 Case 2 Case 3
dmax=0.18 mm dmax=0.44 mm dmax=0.47 mm
displacements
RESULTS
Plastic deformationsp
vδε psδε
Case 1
Case 2
Case 3
C
H
G
F
AB
C D
E
C
AG
F
Case 1 Case 2 Case 3
05
10152025303540
0.00 0.25 0.50 0.75 1.00
time (h)
poro
sity
(%)
Case 3
Case 1 Case 2
Point C
not saturated fully saturated
initial value: 27%
Hardening occurs in case 3.
RESULTS
s (kPa)
p (kPa)
q (kPa)
100
0
50
100
50
-100 -50
0
0 2000 4000 6000 8000
12000 10000
yielding
0
10
20
30
40
50
60
70
80
90
-10 0 10 20 30 40 50 60 70 80 90
net mean stress, p (kPa)
devi
ator
ic s
tress
, q (k
Pa)
Case 3Case 1
Case 2
Point Cs=10MPa
saturated
saturated curve after hardening (Case 3)CSL
(saturated)
Yielding point at maximum shear stress, near saturation.
Case 1 – Point C
AB
C D
E
Very complex stress paths.
RESULTS
Effect of wetting-drying cycles
Drying does not lead to relevant increment of the plastic deformations
The plastic volumetric deformations increase with the number of wettings.
The most severe changes occur after the first wetting.
pvδε
psδε
pvδε
psδε
pvδε
psδε
pvδε
psδε
1st wetting 2nd wetting 3rd wetting Legend
CONCLUSIONS
The patterns of tension and plastic deformation during wetting and drying processes allowed the identification of the degradation mechanism of fragments of marl.
This mechanism is mainly due to the suction differential inside the fragment developed along wetting.
Tension development and cracking are due to the differential swelling deformations caused by this suction differential.
CONCLUSIONS
In the study of the individual rock fragment:
• The suction differential increases with the suction change rate.
• Dryer samples exhibit more severe damage due to larger values of suction differential.
• Increasing confinement decreases the swelling displacements but increases tension and plastic shear deformation.
• Hardening occurs when compression due to the restrain of swelling displacements occur.
• The first wetting causes most severe degradation.
• Wetting is more penalizing than drying.
CONCLUSIONS
The numerical results from individual rock fragments provided a mechanical explanation for the overall behaviour of aggregates (compacted material) observed in experimental tests:
• Cracking development due to saturation leads to fragment size reduction. The collapse observed results from the rearrangement of the broken fragments.
• Large collapse deformations are due to high suctions before wetting. This can be explained because the material degradation is proportional to the value of the suction differential.
FUTURE WORK
A possible explanation of the cracking mechanism was found, based on the mechanics of unsaturated soils.
The cracking mechanism explains the changes in the amplitude of the collapse observed but does not explain the degradation observed.
There is a need of a suitable constitutive model able to simulate the transition from rockfill behaviour (dry samples) to the one of a clayed soil (fully saturated samples).
ACKNOWLEDGEMENTS
THE END
Thank you
Professor Emanuel Maranha das NevesProfessor Eduardo E. Alonso,
supervisors of the work developed,for the help in the preparation of this paper
andDr Sebastiá Olivella
for his useful comments concerning the use of Code Bright.
Portuguese Geotechnical Society, SPGfor the financial support provided for this conference.
Portuguese Foundation for Science and Technology, FCT (Ref. SFRH/BD/25846/2005 and POCTI/ECM/59320/2004) for the financial support that allowed this study.
