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Reinforcement of completely decomposed granite with discrete fibres
Renforcement de granite complètement décomposé avec des morceaux fibres
Madhusudhan B.N., Baudet B.A.
The University of Hong Kong, Hong Kong
ABSTRACT :The use of discrete fibres as reinforcing material for soils has been researched by many, e.g. Gray and Al-Refeai
(1986), Maher and Ho (1994), Crockford et al. (1993), Santoni et al. (2001), Consoli et al. (2009a), but these studies have been
generally done independently and have not always been consistent. Silva dos Santos et al. (2010) used data gathered through many
years of study to develop a framework of behaviour for a poorly graded quartzitic sand reinforced with polypropylene fibres. In Hong
Kong, the construction industry has used reinforcement with continuous fibres for some time, but it is mainly applied to landscaping
of otherwise stabilised slopes, for example as a green cover on an existing shotcreted slope. Using randomly distributed short discrete
fibres in Hong Kong completely decomposed granite (CDG) could help stabilise the soil while keeping the density low enough to
allow growth of vegetation. It is not guaranteed, however, that a well graded residual soil like CDG would behave in the same way as
sands used by previous researchers. Laboratory tests have been carried out on completely decomposed granite using short discrete
polypropylene fibres as a reinforcing material. The fibres are randomly distributed in the soil. It was found that the fibres increase the
unconfined compressive strength of the CDG prepared at its maximum dry density by up to tenfold for fibre contents less than 1%.
The behaviour of the fibre-CDG mixture during drained triaxial compression changed from dilative to compressive, with more effects
at low confining pressures. These tests seem to indicate that discrete fibres could be considered for improving the performance of
CDG.
RÉSUMÉ : L’utilisation de fibres pour renforcer les sols ont déjà fait l’objet de nombreux travaux de recherche e.g. Gray and Al-
Refeai (1986), Maher and Ho (1994), Crockford et al. (1993), Santoni et al. (2001), Consoli et al. (2009a), mais ces études ont été
généralement faites indépendamment et elles n’ont pas toujours été synthétisées. Silva dos Santos et al. (2010) ont utilisé les données
obtenues au cours d’années de recherche pour développer un modèle de comportement pour un sable quartzitique uniforme renforcé
avec des fibres en polypropylène. A Hong Kong, l’industrie de la construction a utilisé des fibres continues comme moyen de
renforcement depuis longtemps, mais l’application se limite à l’aspect paysager de pentes déjà stabilisées, par exemple pour la
plantation de surfaces de pentes recouvertes de béton projeté. L’utilisation de fibres courtes distribuées de façon aléatoire dans le
granite complètement décomposé de Hong Kong (CDG) pourrait aider à stabiliser le sol tout en gardant sa densité assez basse pour
permettre à la végétation de pousser. Il n’est pas garanti cependant qu’un sol résiduel a la distribution granulométrique bien calibrée
comme le CDG se comportera de la même façon que les sables utilises par les chercheurs précédents. Des essais de laboratoire ont été
faits sur du granite complètement décomposé en utilisant du béton projeté et des fibres courtes en polypropylène comme matériau de
renforcement. Les fibres sont distribuées de façon aléatoire dans le sol. On a trouvé que les fibres ont pour effet de multiplier par
presque dix fois la résistance en compression simple du CDG préparé à sa densité sèche optimale, pour une teneur en fibres de moins
de 1%. Le comportement du mélange CDG-fibres lors de l’essai triaxial drainé en compression es tpassé de dilatant a contractant avec
plus d’effet aux pressions faibles. Les essais paraissent indiquer que l’utilisation de fibres courtes pourrait être considérée pour
améliorer la performance du CDG.
KEYWORDS: laboratory tests ; reinforced soils ; residual soil
1 INTRODUCTION
Adding fibres to soil can be an effective way of strengthening it,
by providing tensile strength at high strains. The factors
influencing the effectiveness of the fibre-reinforced soils are a)
the type of soil and its deformation behaviour; b) the type of
fibre and its specifications (fibre length, fibre content and its
aspect ratio). A careful study of the mechanics of the fibre-
reinforced soil will help practising and design engineers to
understand better its behaviour under different loading
conditions.
Hong Kong is a modern city with growing population, so
that engineers are pressed to optimise land utilisation. The
topology of Hong Kong has led to urban development on
natural or man-made slopes. Conventional methods of
stabilising slopes such as shotcreting the whole face of the slope
(current practice) are neither cost effective nor environmentally
friendly and alternative sustainable methods are being sought
after.
Many researchers have produced a large body of research
on the performance of discrete fibres with soils (Gray and Al-
Refeai, 1986; Maher and Ho, 1994; Crockford et al., 1993;
Santoni et al., 2001; Consoli et al., 2009a), but these studies
have been generally done independently and they have not
always been consistent (Silva dos Santos et al., 2010). This
paper presents initial results from laboratory tests performed on
completely decomposed soil reinforced with discrete fibres.
2 MATERIALS AND METHODS
Completely decomposed granite was used as the host soil. It
originates from in-situ weathering of the parent igneous rock,
and is one of the most common geo-materials in Hong Kong.
The short discrete fibres used in the tests presented here were
similar to those used by Silva dos Santos et al. (2010).
2.1
Materials tested
The completely decomposed granite (CDG) host soil was
obtained from a construction site near Beacon Hill, Hong Kong.
Completely decomposed residual soils are well-graded in nature
as the tropical climate has weathered the parent rock to a
material comprising gravel and sand grains down to silt and
clay-sized particles. Coarser particles are usually of quartz
origin owing to its high chemical resistance while finer particles
are most likely other primary hydrous minerals, such as
