INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 4, 2011

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1 Undrained response of mining sand with fines contents Thian S. Y, Lee C.Y Associate Professor, Department of Civil Engineering, Universiti Tenaga Nasional, Malaysia ABSTRACT This paper describes an experimental study on mining sand containing plastic fines to investigate pre failure and failure characteristics of suc mixtures under monotonic undrained compression triaxial tests. The results reveal that undrained shear strength, soil modulus and pore pressure decrease when clay content was increased. It was also found that the tested materials were overconsolidated by the fact that normalized shear strength and soil modulus depends on initial confining stress. Keywords: Mining sand, fines, shear strength, pore pressure, soil modulus 1. Introduction The investigation of the response of clean sands have been extensively studied under laboratory and field conditions. These types of sand include Ticino, Ottawa and Monterey #0 sands (Hardin and Richart, 1963, Chung et al. 1986, Bolton 1986, Lo Presti 1987 and Lo Presti 1992). However, natural soil consists of both sand and fines in various proportions. The characteristics of shear strength could be affected by the inclusion of fines in sand. Shen et al. (1977) indicated that the fines between sand to sand contacts increases the soil resistance to shearing. Several studies have shown that as fines content increases, the steadystate strength at the same void ratio decreases initially, followed by an increase in shear strength with further increase in fines content of more than 30% (Pitman et al.1994, Zlatovic and Ishihara, 1995 and Thevanayagam et al.1996). The objective of this paper is to investigate the pre failure and failure characteristics of mining sand containing plastic fines under monotonic undrained triaxial tests. The effects of fines on the shear strength, soil modulus and pore pressure are presented and discussed. 2. Materials and Method 2.1 Soil Constituents Selected In this experimental study, mining sand was selected as the coarser grain matrix, while the finer grain matrix was either kaolin clay or silt. Figure 1 shows the grain size distributions of mining sand, silt and kaolin clay used in this study. The sand was retrieved from a mining area with specific gravity of It has coefficient of uniformity of 2.08 and coefficient of curvature of 1.27 and it is classified as poorly graded. The mining sand has a minimum dry density of 1413kg/m 3 and maximum dry density of 1565 kg/m 3. The kaolin clay used has a liquid limit of 61%, plastic limit of 34%, plastic index of 27%, specific gravity of 2.69, optimum moisture content of 38% and maximum dry density of 1350kg/m 3. The silt used in this study is also defined as high purity crystalline quartz filler, 844

2 with specific gravity of 2.65 and ph of 5.6 ~ 7.5. This is a nonplastic silt that is composed of SiO 2 (99.8%), with Al 2 O 3 (0.05%) and Fe 2 O 3 (0.01%) as secondary components. Figure 1: Sieve analysis for mining sand, silt and Kaolin clay 2.2 Soil Sample Preparation and Testing Procedures The sand with different fines contents were reconstituted in the laboratory by mixing oven dried mining sand with either silt or clay fines. The mining sand and fines were mixed manually in a dry state thoroughly until the mixtures were observed to be visually homogeneous. The soil specimens were prepared by dry tamping method, and it was performed by compacting the mixture in three equal layers to a required relative density. The diameter and height of every soil sample were measured to be 50mm and 100mm, respectively. The mining sand was mixed with different amounts (10% ~ 40%) of fines to obtain different mixtures. Saturation of soil specimens was achieved by applying cell and back pressures and full saturation was assumed to have taken place when Skempton s B parameter was greater than Soil mixtures were isotropically consolidated under confining pressures of 200kPa after the saturation stage had completed. The specimens were then sheared under undrained condition (CIU) at the rate of 0.4 mm/min up to an axial strain of 25%. The testing procedures and data acquisition were performed by using GDSLAB software and GDS Data Acquisition System. 3. Test Results Figure 2 shows the stress strain and pore pressure behaviour of mining sand with 10% silt and clay fines. The sample with 10% silt fines content has higher deviator stress than sample with 10% clay fines content, which indicates that silt fines is enhancing the dilatancy and shear strength of sand when sand particles are in close contact during shearing process. On the other hand, sample with 10% clay fines content exhibits higher positive excess pore pressure than sample with 10% silt fines content, which means that the behaviour of sand with clay fines is more compressive. 845

3 a. Stress strain curves b. Pore pressure curves Figure 2: CIU results of mining sand with 10% fines content Figure 3 shows that the shear strength of sand silt and sand clay decrease when fines content increases from 10% to 20%. The strain softening behaviour in sand silt is more significant than in sand clay. When silt content is considerably low (20% or less), the silt particles occupy spaces in between sand particles and increase particle interlocking, which causes the soil to be dilative (Salgado et al. 2000). However, the sand particles do not have contacts with each other when clay fines is present as static stresses are not effectively transferred through clay fines (Carraro et al. 2009). Sample with 20% clay fines content behave in a more contractive manner as indicated by the increase in positive excess pore pressure. Sample with 10% silt fines content also exhibit contractive behaviour during initial shearing and followed by slight dilation. Both the clay and silt fines affect the excess pore pressure development in samples. a. Stress strain curves b. Pore pressure curves Figure 3: CIU results of mining sand with 20% fines content Stress strain and excess pore pressure curves for samples with 30% clay and silt fines content are illustrated in Figure 4. As fines content increases in mining sand, sample with silt content indicates contractive behaviour with only slight dilation. A primarily floating fabric develops 846

4 when silt fines content is greater than 15% is present in sand, causing the dilatancy in soil sample to be suppressed (Salgado et al. 2000). This could be explained by the soil fabric that gets progressively weaker when silt particles keep the sand particles from each other. According to Salgado et al.(2000), silt fines begins dominate the behaviour of soil samples when silt content is 15%, but silt fines start to dominate the soil response when silt content is 20%. a. Stress strain curves b. Pore pressure curves Figure 4: CIU results of mining sand with 30% fines content Figure 5 shows the deviator stress decreases considerably when fines content increases to 40%. However, it is interesting to note that the mining sand with 40% fines content only generates positive pore pressures due to contractive soil response. The strain softening behaviour in sample with 40% silt fines content is suppressed with only positive excess pore pressure built up in soil sample. a. Stress strain curves b. Pore pressure curves Figure 5: CIU results of mining sand with 40% fines content 847

5 The soil friction angle and cohesion of mining sand with different fines content are shown in Figure 6. The soil friction angle of samples with silt fines content is higher than that of samples with clay fines content and it decreases as fines content increases from 10% to 40%. As expected, the soil cohesion of sample with clay fines content is higher than that of sample with silt fines content. The soil cohesion increases with increasing fines contents. a. Soil friction angle b. Soil cohesion Figure 6: Soil parameters for mining sand with fines content Figure 7 shows the Skempton s parameter, A for sample with clay fines content increases significantly as clay fines increase from 10% to 40%, implying it has major effect in pore pressure development. On the other hand, the silt fines in soil sample appear to have less effect on the A values as silt fines increases from 10% to 40%. Figure 7: Variation of Skempton s parameters, A with fines content The strain dependent soil stiffness is our important pre failure property which controls the soil deformation characteristics. The soil secant modulus at 50% shear strength, E 50, is used to illustrate the deformation characteristics of the soil mixtures. The secant modulus is usually used in elastic plastic models of soil materials. Figure 8 shows the normalised secant modulus for sample with clay fines content that are subjected to different preconsolidation pressures. The normalised secant modulus of sample with clay fines content increases with 848

6 increasing overconsolidation ratios (OCR) because high preconsolidation pressure causes the soil particles to be densely packed together and therefore stiffer. However, the normalised secant modulus decreases as clay fines increases. This could be the result of more compressible clay fines that is entrapped between sand particles, and they deform and reshape themselves during isotropic compression, which causes sand particles to be further apart. Therefore, the static stresses are not effectively transferred through clay fines (Carraro et al. 2009). Figure 8: Normalised secant modulus for mining sand with different OCR and clay fines content (Thian and Lee, 2010) Figure 9 shows the variation of normalised peak strength of soil samples with OCR and clay fines content. The normalised peak strength decreases as clay fines and OCR increase. However, the declining rate of normalised peak strength decreases when clay fines content is more than 20% and OCR is greater than 3. The normalised peak strength is essential to be analysed because it is associated with initiation of flow deformation (Murthy et al and Shelly et al. 1997). Figure 9: Normalised peak strength for mining sand with different OCR and clay fines content (Thian and Lee, 2010) 849

7 Figure 10 shows the effect of clay fines on the normalised maximum excess pore pressure generation. The normalised maximum negative excess pore pressure developed in samples with less than 10% clay fines indicates dilatant response. Positive excess pore pressure developed in sample with clay fines content implies contractive behaviour when the clay fines content is more than 20%. Maximum positive excess pore pressure generation increases as clay fines content increases in soil sample. The inclusion of clay fines weakens the soil structure because clay fines acts as lubricant and there is an increase in sand skeleton void ratio (Thian and Lee, 2010). It also appears that the soil samples have lower normalised maximum excess pore pressure generation when OCR increases. Its declining rate decreases when OCR is greater than 2., Figure 10: Variation of normalised maximum excess pore pressure with OCR and clay fines content (Thian and Lee, 2010) 4. Conclusion Several conclusions may be drawn based on this experimental study: 1. Sand silt exhibits higher deviator stress than sand clay with fines content ranges from 10% to 40%. 2. Sand clay tends to exhibit contractive behaviour, while dilative behaviour is more significant for sand silt. 3. Clay content in sand has significant effect on Skempton s parameter, A, while silt content appears to have less effect on the A values. 4. Soil friction angle of sand silt is higher than that of sand clay at all fines content, and both decreases with increasing fines content. As expected, the soil cohesion increases with increasing fines content and the effect is more significant in sandclay. 850

8 5. References 1. Bolton, M.D. (1986) The strength and dilatancy of sands, Geotechnique 36(2), pp Carraro, J.A.H., Prezzi, M. and Salgado, R. (2009) Shear strength and stiffness of sands containing plastic or nonplastic fines, J. Geotech. Geoenviron. Engrg., 135(9), pp Chung, R.M., Yokel, F.Y. and Drnevich, V.P. (1984) Dynamic properties cement treated soils, Highway Research Record 379, pp Hardin, B.O. and Richart, F.E. Jr. (1963) Elastic wave velocities in granular soils, Journal of Soil Mechnanics and Foundations Divisions, Proceedings of the American Society of Civil Engineers, 89, (SMI), pp Lo Presti D.C.F., Pedroni S. and Crippa V. (1992) Maximum dry density of cohesionless soils by pluviation and by ASTM D : A comparative study, Geotechnical Testing Journal, 15(2), pp Ovando Shelley E. and Perez B.E. (1997) Undrained behaviour of clayey sands in load controlled triaxial tests, Geotechnique, 47(1), pp Pitman, T.D., Robertson, P.K. and Sego, D.C., Influence of fines on the collapse of loose sands, Canadian Geotechnical Journal, 31, pp Salgado, R., Bandini, P. and Karim, A. (2000) Shear strength and stiffness of silty sand, Journal Geotechnical and Geoenvironment Engineering., 126(5), pp Shen, C.K., Vrymoed J.L. and Uyeno C.K. (1977) The effect of fines on liquefaction of sands, Proceedings of the 9 th International Conference on Soil Mechanics and Foundation Engineering, Tokyo, 2, pp Thevanayagam, S., Ravisbankar, K. and Mohan, S. (1996) Steady sate strength, relative density and fines content relationship for sands, Transportation Research Record: Journal of the Transportation Research Board 1547/1996, pp Thian, S.Y. and Lee, C.Y. (2010) Effect of plastic fines on over consolidated mining sand, ARPN Journal of Engineering and Applied Sciences, 5(11), pp Zlatovic, S. and Ishihara, K. (1995) On the influence of nonplastic fines on residual strength, Proc. IS TOKYO 95, 1 st Int. Conf. On Earthquake Geotechnical Engineering, K. Ishihara, ed., A.A. Balkema, Rotterdam, The Netherlands, pp