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Porosity/cement index to evaluate geomechanical properties of an artificial
cemented soil
Le paramètre porosité/ciment pour
l’évaluation des propriétés
géomécaniques d'un sol cimenté
artificiellement
Rios S., Viana da Fonseca A.
Faculty of Engineering of the University of Porto
ABSTRACT: This paper highlights the importance of the porosity/cement index on the evaluation of the geomechanical properties of
soil-cement mixtures as a contribution to analyse these materials. This index is defined as the ratio between porosity and volumetric
cement content combining the degree of compaction with the cement content. The relevance of these two parameters is defined by an
exponent to the volumetric cement content which changes with the type of soil. This paper results from a broad experimental program
with unconfined compression tests, indirect tensile tests, triaxial tests and oedometer tests, which were all analysed by this index
adjusted by a specific exponent value. The (tensile and compression) strength, the (elastic and initial tangent) stiffness, as well as the
compressional behaviour are conveniently represented by this index and a different behaviour is observed when this index is changed.
RÉSUMÉ
: L’importance du paramètre porosité/ciment dans l’évaluation
des propriétés géoméchaniques des mélanges sol-ciment est
présentée dans cet article comme une contribution pour
l’analyse de ces matériaux. Ce paramètre est défini comme le r
apport entre la
porosité et la teneur volumique
en ciment. L’importance relativ
e entre la porosité et la teneur en ciment est introduite en introduisant
un exposant à la teneur volumique en ciment dépendant du type de sol. Les r
ésultats d’un
vaste programme expérimental incluant
essais de compression simples, essais de traction indir
ect, essais triaxiaux et essais œdométriques sont présentés et analysés par ce
paramètre ajusté par un exposant spécifique. La résistance à la compression et à la traction, la rigidité élastique et tangente initiale,
ainsi que le comportement en compression sont bien représentés par
l’intermédiaire de ce paramètre
et un comportement différent est
observé si le paramètre est modifié.
KEYWORDS:soil-cement, porosity/cement index, tensile strength, compression strength, compressional behaviour.
1 INTRODUCTION
Soil-cement mixtures are very interesting for the construction of
road and railway platforms, especially in the noble layers of
subgrade as well as in transition zones between embankment
and concrete structures, where good mechanical properties are
required. This solution, not only concurs to improve those
characteristics, but also leads to a significant reduction in the
economic and environmental costs of these works. Despite these
advantages this method has not a generalized application in
Portugal due to the lack of design methodologies based on
mechanical parameters.
There are several factors affecting the behaviour of cemented
soils, such as the type of cement and cement content, the curing
time and stress, the water content and porosity. Seeking for a
ratio that would reflect the influence of some of these
parameters Consoli et al. (2007) presented an index property
defined as the ratio of porosity to the volumetric cement
content, called porosity/cement ratio (n/C
iv
). Some previous
attempts have been made, such as the degree of cementation
proposed by Chang and Woods (1992) that concerns the
percentage of voids filled with cement, being this parameter
developed for sands. Lorenzo and Bergado (2004) have also
presented the ratio of the after curing void ratio to the cement
content (e
ot
/A
w
) proving to be quite interesting for clay mixtures
with high values of water and cement content.
Another available parameter is the water/cement ratio used
for concrete. However, soil-cement mixtures for road or railway
platforms are usually cured in a non saturated condition, which
makes the previous ratio inadequate in the analysis of these
mixtures behaviour. The main difference between soil-cement
mixtures and concrete (besides the cement content) is that
during the curing of concrete all voids are completely full of
water and therefore concrete stress-strain behaviour is not
dependent on the void ratio but on the water content. In
opposition, soil-cement mixtures currently executed in
embankments and transport platforms have curing water content
lower than the saturation water content and so their
compressibility will be related to its porosity. Moreover, while
concrete has an almost linear behaviour for a wide range of
deformations, soil-cement mixtures have a clear non-linear
behaviour since very small strains as a result of the progressive
degradation of the cemented structure. Therefore, even if the
soil-cement mixture is saturated after the maximum strength has
been achieved (i.e. after curing) the curing void ratio still has a
very important role on the mechanical behaviour of the mixture.
The influence of the porosity/cement ratio on strength and
stiffness parameters is described in Consoli et al. (2012)
providing the comparison between two different materials
mixed with Portland cement: well graded Porto silty sand and
uniform Osorio sand. An advance analysis on the compression
and shearing behaviour of cemented Porto silty sand through
this parameter is described in Rios et al. (2012).
This paper summarizes some geomechanical properties of
cemented Porto silty sand through this index in terms of
strength (unconfined, tensile and triaxial), stiffness (initial
tangent and unload-reload) and one-dimensional compression.
2 MATERIALS AND SPECIMEN PREPARATION
A well graded soil, classified as silty sand (SM) in the unified
classification system (ASTM, 1998) was used in this study. The
soil is derived from weathered Porto granite which is abundant
in Northern Portugal (Viana da Fonseca et al., 2006). Its particle
specific gravity is 2.72, and it contains around 30% fines,
