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2589 Porosity/cement index to evaluate geomechanical properties of an artificia

2589 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 rapport entre la porosité et la teneur volumique en ciment. L’importance relative 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 résultats d’un vaste programme expérimental incluant essais de compression simples, essais de traction indirect, 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/Civ). 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 (eot/Aw) 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, 2590 Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013 although a low plasticity index was obtained (IP=wL-wP=34%- 31%=3%). From the particle size distribution curve presented in Figure 1 an average diameter D50 equal to 0.25 mm was obtained, as well as uniformity and curvature coefficients of 113 and 2.7 respectively.A high strength Portland cement (CEM I 52,5R) of grain density equal to 3.15 was used as the cementing agent in order to speed up the laboratory tests. The experimental program is performed with specimens made by the mixture of silty sand, Portland cement and tap water that is compacted statically in three layers in a stainless steel mould. For each specimen, a quantity of fines equal to the weight of cement to be introduced was removed from the soil, in order to have the same grain size distribution curve in the mixture of soil-cement as in the soil itself. Following this procedure the dry density of the soil was also constant throughout the study even though the cement content changed. The specific gravity of the cement-soil mixture was calculated as a weighted average of those of the soil (Gs=2.72) and of the cement (Gs=3.15), and thus it was different for different cement contents. 0.0001 0.001 0.01 0.1 1 10 Grain size (mm) 0 10 20 30 40 50 60 70 80 90 100 100 90 80 70 60 50 40 30 20 10 0 ASTM sieves series (mm) Figure 1. Grain size distribution curve 3 STRENGTH PARAMETERS 3.1 Unconfined compression strength Strength properties of the cemented sand were evaluated in different ways by means of unconfined compression tests, indirect tensile tests, as well as triaxial tests. First, several specimens moulded to have different cement contents (2%, 3%, 5% and 7%) and dry unit weights (16.4, 17.2, 18.0 and 18.8 kN/m3) were tested in unconfined compression in a total of 16 tests. In these tests, the water content was kept equal to 12%. The representation of the unconfined compression strength (UCS) and the ratio of porosity to the volumetric cement content (n/Civ) revealed that some adjustment was needed and therefore, an exponent was added to Civ. This exponent was defined as the value that provides the best correlation coefficient with the data, which, for this material, was found to be 0.21 – Eq. (1). UCS (kPa) = 4E+09 (n/Civ 0.21)-4.296 (1) This exponent seems to depend on the type of soil as other authors have found different coefficients when working with different soils (Consoli et al., 2007, 2011): an exponent of 0.28 was found in a residual soil from sandstone (Botucatu soil), while a value of 1.0 was found in an uniform sand (Osorio sand). Based on this parameter, named adjusted porosity/cement ratio (n/Civ 0.21), the results of different tests were analysed. 1.1 Tensile strength Taking into account the possibility of shrinkage in cemented materials, the evaluation of the tensile strength is of utmost importance. In that sense, indirect tensile tests following the standard EN 13286-42 (CEN, 2003) were performed on similar specimens whose results were plotted against n/Civ 0.21 for which Eq. (2) was obtained, Rtb (kPa) = 2E+09 (n/Civ 0.21)-4.719 (2) The results showed that the indirect tensile strength (Rtb) was about 11% of the UCS. In Figure 2 both Rtb and UCS are plotted against n/Civ 0.21 in different scales for comparison. It is clear that both trends are very similar (except for the absolute values) corroborating the convenience of the adjusted porosity/cement ratio. In Consoli et al. (2011), where the data from these tests is plotted together with data from other two soils, it is shown that for the three soils a decrease in porosity promotes an uploads/Voyage/ porosity-cement-index-to-evaluate-geomechanical-properties-of-an-artificial-cemented-soil-by-fonseca-2013.pdf