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Analysis of ettringite attack to stabilized railway bases and embankments
Analyse de l’attaque chimique par ettringite de remblais et plateformes ferroviaires stabilisées
Ramon A.
Department of Geotechnical Engineering and Geosciences, UPC, Barcelona, Spain
International Center for Numerical Methods in Engineering (CIMNE), UPC, Spain
Alonso E.E.
Department of Geotechnical Engineering and Geosciences, UPC, Barcelona, Spain
ABSTRACT: Two cases of massive sulphate attack to cement treated embankments and track bases in a high speed railway line are
described. In the first case, two large access embankments to a railway viaduct were treated with jet–grouting columns. In the second
case a compacted cement treated soil was placed over a rigid concrete caisson. The treated layer, 1.5 m thick, expanded at a
continuous rate of 0.9-1.3 mm/month. In the two cases the soil was excavated from nearby cuts in gypsiferous Tertiary (Oligocene)
claystones. Ettringite and thaumasite crystals were found within the expanding levels. The chemical evolution of an interface between
a cement treated body and a compacted soil is presented. Solutions adopted to remediate the created problem are briefly described.
RÉSUMÉ : On décrit dans cet article deux cas d’attaque par sulfates, la première dans un remblai traité au ciment et la deuxième dans
une plateforme ferroviaire de train à grande vitesse. Plus précisément, le premier cas traite de deux grands remblais d’accès à un
viaduc de chemin de fer qui avaient été stabilisé avec des colonnes de jet-grouting. Le second cas considère une couche de sol
compacté traitée au ciment et reposant sur des caissons rigide en béton. Dans les deux cas, le sol était le matériau provenait
d’excavations proches dans des argilites tertiaires (Oligocène) gypsifères. La couche traitée 1,5 m d’épaisseur a gonflé à un taux
continu de 0.9 à 1.3 mm / mois. Des cristaux d'ettringite et thaumasite ont été trouvés dans les niveaux gonflants. L’article présente
l'évolution chimique qui se produit à l’interface entre un corps traité au ciment et un sol compacté. Les solutions adoptées pour
remédier au problème sont brièvement décrites.
KEYWORDS: Swelling, ettringite, gypsum, sulphate attack, embankments
1 INTRODUCTION
Sulphate attack in cement and lime treated soils has been often
reported in stabilised road bases and subbases when the soil has
some proportion of gypsum, or the treated soil is exposed to
sulphated waters. Sulphate attack results in loss of strength and
significant heave (Sherwood, 1962; Mitchell & Dermatas, 1992;
Puppala et al., 2003; Rajasekaran, 2005). Some of these studies
discuss the minimum sulphate content which triggers the attack.
Sherwood (1962) described an unconfined compressive strength
reduction of 24% of treated soil when the sulphate content was
as low as 0.25%.
Sulphate attack leads to the development of ettringite
(Ca
6
[Al(OH)
6
]
2
(SO
4
)
3
·26H
2
O). This mineral crystallises in
bundles of elongated filaments. The development of ettringite
implies a destruction of the strength of the cement paste and a
substantial
swelling.
Another
mineral,
thaumasite
(Ca
6
[Si(OH)
6
]
2
(CO
3
)
2
(SO
4
)
2
·24H
2
O), develops also as a
consequence of sulphate attack. In both minerals, the presence
of water is remarkable. The development of ettringite and
thaumasite follows a complex process which has been described
by Mitchell & Dermatas (1992) and Mohamed (2000). The
highly basic environment (pH in excess of 12) created by the
hydration of cement’s calcium oxide is capable of dissolving the
clay minerals and releasing Al and Si ions. High pH also favors
the dissolution of sulphate minerals, which provides Ca
++
and
SO
4
--
ions. Ettringite precipitates when aluminum released from
clays, calcium from cement or lime and sulphates combine with
water molecules. Carbonic acid, present in the pore water and
the dissolution of calcite leads to precipitation of thaumasite,
once ettringite is present. Crystals develop in the pore solution.
Most of the geotechnical literature on sulphate attack
concerns the stabilisation of compacted road bases and sub-
bases. In those cases the treatment is applied to relatively thin
layers and the sulphate attack results in surface heave and
reduction of soil strength. In contrast, the two cases affected by
sulphate attack described here concern larger soil masses in
railway embankments. Field observations, laboratory tests and
remedial measures are described below.
2 PALLARESSOS EMBANKMENTS
Thaumasite and ettringite crystal growth is at the origin of an
intense expansion that affected two embankments, 18 meters
high, located in the Madrid-Barcelona high speed railway. The
case of sulphate attack to Pallaressos embankments is described
in detail in Alonso and Ramon (2012).
The embankments were made of compacted sulphated
Tertiary claystone. The embankments material belongs to the
same geologic formation where Lilla tunnel (Alonso et at.,
2012) and Pont de Candí bridge (Alonso and Ramon, 2012),
have experienced severe heave problems due to gypsum crystal
growth.
Pallaressos embankments give access to a bridge 196 meters
long. A transition wedge was built next to abutment structures
in both embankments in order to provide a progressive change
of stiffness when trains approach the rigid bridge structure.
Cement treated soil was used for the construction of both
wedges (Figure 1).
Heave of the surface of embankments, near the abutments,
was detected at an early time after the end of the embankments
construction during the track levelling monitoring carried out
periodically by the railway administration. Afterwards a grid of
jet grouting columns was executed on both embankments to
stabilize the embankment material. However, heave rate did not
stop after the jet grouting treatment. Continuous extensometers
installed in boreholes through the embankments showed that
strains were developing in the upper 8-10 m of the
embankments (Figure 2).
