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INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 4, 2011 © Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN 0976 – 4399 968 Effect of particle size and shape on the grainsize distribution using Image analysis Seracettin Arasan 1 , Suat Akbulut 1 , A.Samet Hasiloglu 2 1 – Department of Civil Engineering, Engineering Faculty, Ataturk University, Erzurum, Turkey 2 – Department of Computer Engineering, Engineering Faculty, Ataturk University, Erzurum, Turkey [email protected] ABSTRACT Grainsize distribution that is one of the base properties of soils and soil classification systems gives idea about engineering properties of soils. Determination of grainsize distribution derived from mechanical method (sieving) is time consuming and difficult. Hence, the image analysis methods for determination of grainsize distribution have been also investigated by several researchers. In that research, the grainsize distributions for ten soil samples with different sizes and shapes are determined with image analysis methods. The effects of particle shape (i.e., elongated, flat, spherical) and volume computation techniques (cube, cylinder, ellipsoid, and modified ellipsoid) on grainsize distribution were also researched. The results of the research indicated that the shape of particles significantly affected grainsize distribution. Additionally, modified ellipsoid method is determined as the best volume computation method. Keywords: Grainsize distribution, image analysis, soil, particle shape 1. Introduction The grainsize distributions (GSD) are one of the basic and most important properties of soil. It is primarily used for soil classification and provided a firstorder estimate of other soil engineering properties such as permeability, shear strength, and compressibility. In practice, the GSD of the full size spectrum of soil grains is determined by integrating data obtained from two inherently dissimilar tests, mechanical sieving for the coarse grained soil fraction and hydrometer tests for the fine fraction. In sieve analysis, the particle size is characterized by a single linear dimension representing the minimum square sieve aperture that which the particle just passed through (Ghalib and Hryciw, 1999). The results of sieving are dependent upon the shape of the particles (Mora et al., 1998; Kwan et al., 1999; Mora and Kwan, 2000). Particles passing through a sieve can actually have one dimension that is larger than the size of the sieve apertures. From Fig. 1(a), it can be seen that an elongated particle that has its length greater than the aperture size can pass through the sieve without any difficulties. Therefore, the sieve aperture size is a measure of the lateral dimensions of the particles only. A relatively flat particle can pass through the sieve aperture, which is square in shape, diagonally as shown in Fig. 1(b). As a result, the breadth of a particle passing through a sieve can also be greater than the sieve size,

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

© Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN 0976 – 4399

968

Effect of particle size and shape on the grain­size distribution using Image analysis

Seracettin Arasan 1 , Suat Akbulut 1 , A.Samet Hasiloglu 2 1 – Department of Civil Engineering, Engineering Faculty,

Ataturk University, Erzurum, Turkey 2 – Department of Computer Engineering, Engineering Faculty,

Ataturk University, Erzurum, Turkey [email protected]

ABSTRACT

Grain­size distribution that is one of the base properties of soils and soil classification systems gives idea about engineering properties of soils. Determination of grain­size distribution derived from mechanical method (sieving) is time consuming and difficult. Hence, the image analysis methods for determination of grain­size distribution have been also investigated by several researchers. In that research, the grain­size distributions for ten soil samples with different sizes and shapes are determined with image analysis methods. The effects of particle shape (i.e., elongated, flat, spherical) and volume computation techniques (cube, cylinder, ellipsoid, and modified ellipsoid) on grain­size distribution were also researched. The results of the research indicated that the shape of particles significantly affected grain­size distribution. Additionally, modified ellipsoid method is determined as the best volume computation method.

Keywords: Grain­size distribution, image analysis, soil, particle shape

1. Introduction

The grain­size distributions (GSD) are one of the basic and most important properties of soil. It is primarily used for soil classification and provided a first­order estimate of other soil engineering properties such as permeability, shear strength, and compressibility. In practice, the GSD of the full size spectrum of soil grains is determined by integrating data obtained from two inherently dissimilar tests, mechanical sieving for the coarse grained soil fraction and hydrometer tests for the fine fraction. In sieve analysis, the particle size is characterized by a single linear dimension representing the minimum square sieve aperture that which the particle just passed through (Ghalib and Hryciw, 1999). The results of sieving are dependent upon the shape of the particles (Mora et al., 1998; Kwan et al., 1999; Mora and Kwan, 2000). Particles passing through a sieve can actually have one dimension that is larger than the size of the sieve apertures. From Fig. 1(a), it can be seen that an elongated particle that has its length greater than the aperture size can pass through the sieve without any difficulties. Therefore, the sieve aperture size is a measure of the lateral dimensions of the particles only. A relatively flat particle can pass through the sieve aperture, which is square in shape, diagonally as shown in Fig. 1(b). As a result, the breadth of a particle passing through a sieve can also be greater than the sieve size,

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although it has to be smaller than the diagonal length of the sieve aperture (Mora et al., 1998).

(a) (b) Figure 1: (a) An elongated particle passing through a sieve aperture. (b) Plan view of a

flat particle passing through a sieve aperture. (Kwan et al., 1999)

The determination of grain­size distribution with mechanical method (sieving) is difficult and it takes long time. Hence, image analysis has been used by several researchers to determine the size distribution, particle shape and surface texture of aggregates, both in two and three dimensions (Mora et al., 1998; Kwan et al., 1999; Mora and Kwan, 2000; Maerz and Lusher, 2001; Rao et al., 2001; Taylor, 2002; Maerz, 2004; Fernlund, 2005a,b,c; Tutumluer et al., 2005; Lira and Pina, 2007). Although soil particles are 3­D in nature, most of the image analysis techniques treat particles as two­dimensional objects because only the two­dimensional projection of the particles is captured and measured. However, for obtaining 3D images, many researchers used laser scanners (Lanaro and Tolppanen, 2002; Tolppanen et al., 2002; Lee et al., 2007) and flatbed color scanner (Lira and Pina, 2007), some are also used autofocus microscope (Chandan et al., 2004; Masad et al., 2005) and optical interferometer (Alshibli and Alsaleh, 2004), and also the others used two different perspective images (Kuo et al., 1996; Rao and Tutumluer, 2000; Maerz, 2004; Fernlund, 2005a). These methods analyzed only one particle at a time, however, and significantly increased the complexity compared to two­dimensional analysis.

When plotting grain­size curves by sieve analysis, the particle size is presented with respect to the percent cumulative mass of particles. However, image analysis could not measure particle mass so results could be only presented in percent of particles (Fernlund, 1998; Andriani and Walsh, 2002; Fernlund, 2005a, b, c) or percent of area (Mora et al., 1998; Mora and Kwan, 2000; Lira and Pina, 2007). Several researchers have also claimed that more accurate volume and mass determinations are necessary for accurate plotting of grain­size curves (Taylor, 2002; Maerz, 2004; Tutumluer et al., 2005; Fernlund et al.,

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2007). A great number of studies dealing with volume computations of particles are based on stereology, to convert data from areas (Barbery, 1991; King, 1984; Lin et al., 1987; Vallebuona et al., 2003). These methods are applied for obtaining parameter estimates (mean, variance and skewness) in three dimensions from two­dimensional data. Bozdag and Karpuz (1997) also estimated the 3D size distribution of soil particles from 2D measures such as length and area by using the method proposed by King (1984) and the moment method. The other studies are focused on geometrical computations (Mertens and Elsen, 2006; Lira and Pina, 2007; Fernlund et al., 2007).

The aims of this paper are to investigate the effect of particle size and shape (i.e., elongated, flat, spherical) on the GSD, to compare the image analysis methods (top view, front view, and volume) and to determine the more accurate method for volume computations. For this purpose, ten soil samples with different shapes and sizes are used. The results from the research were compared with those of the mechanical analysis.

2. Materials and Methods

2.1. Soil Samples

In this research, four different size fractions (i.e., 2mm­4,75mm, 4,75mm­8mm, 2mm­ 8mm, and 8mm­16 mm) of a river soil were used to determine grain­size distribution. Additionally, 4,75mm­8mm and 8mm­16mm size fractions were used for investigation of particle shapes effect on GSD. For this purpose, particles were selected by hand as flat, elongated, spherical, and mixed as mentioned below section. The classification of soils is also mentioned in Table 1 according to the Unified Soil Classification System­USCS.

Table 1: The size fractions and classes of the soils used in this study

Size Fraction

Soil Classification

Particle Shape

S1 2 mm­4,75 mm SP Mixed S2 2 mm­8 mm SP Mixed S3 4,75 mm­8 mm GP Mixed S4 8 mm­16 mm GP Mixed

S5 4,75 mm­8 mm GP Elongated S6 4,75 mm­8 mm GP Flat S7 4,75 mm­8 mm GP Spherical

S8 8 mm­16 mm GP Elongated S9 8 mm­16 mm GP Flat S10 8 mm­16 mm GP Spherical

2.2. Particle Shapes

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Quantification of form (particle shape) requires the measurement of the length (L), breadth (I) and thickness (S) of a particle. A number of different identifications have been used historically. Most previous authors have firstly identified the dimension L, defined as the maximum caliper dimension, and have then measured I and S dimensions orthogonal to L (Blott and Pye, 2008). Zingg (1935) indicated that the particles classified four base classes as mentioned in Figure 2(a) with using above measurement system. Also, Sneed and Folk (1958) considered Zingg’s diagram to be inadequate in that it contained only four form classes which divided the field of form variation very unequally. They suggested that only three end­members limit the system of dimensional variation: a prolate spheroid with one long axis and two short ones (L > I = S), an oblate spheroid with two long axes and one short one (L = I > S), and a sphere with all axes equal (L = I = S). A number of different notations have been used historically to refer to these three classes, including, rod, disk and cube (Krumbein, 1941), columnar, flat and spherical (Blott and Pye, 2008). Sneed and Folk (1958)’s three classes are also used in this study with different notation as elongated, flat, and spherical, respectively (Figure 2b).

(a) (b) Figure 2: (a)Four different particle shapes (Zingg, 1935) (b) Three different particle

shapes (Sneed and Folk, 1958)

2.3. Imaging System

A high quality image of the particles is needed before any image processing can be performed. In this study, the equipment for taking pictures of particles is set up by mounting a camera on a photographic stand as shown in Figure 3(a), adjusting the height of the camera to obtain a sufficiently large measurement area on the sample tray, and adjusting the light sources so that there is no shading of any object placed on the sample tray. A white cotton cloth is laid on the sample tray before the particles are put in for obtaining better contrast between the particle and the background pixels. A Nikon D80 Camera and Micro 60 mm objective manufactured by Nikon were used for taking photos.

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The tests were performed in a dark room. Four fluorescent light sources were positioned on the base plane to make the borders of the particles more visible for digital measurements. The flashlight of the camera was not also utilized during image acquisition. After the initial projected image (top view image) of the particles is captured and measured, the photographic stand with camera is rotated 90 degrees so that the particles are now perpendicular to their original orientation and another projection of the particles (front view image) is captured and measured (Figure 3b).

(a) (b)

Figure 3: Schematic of the image analysis systems (a) for top view (b) for front view.

Figure 4 shows 3­D view of a regular­shaped solid, a rectangular box. Clearly, both the top and front views of the solid are identical rectangles. Using a 2­D image analysis setup consisting of only top camera and capturing an image of each solid would not be effective in distinguishing the 3­D shapes of the solids (Rao and Tutumluer, 2000).

Figure 4: Three­dimensional views of two regular­shaped solid: rectangular box.

Top View

Front View

S

L

I

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In this research, the particles were placed to the sample tray in their stable position and two images from top view (longest portion of particle) and front view (thickest portion of particle) were obtained by imaging system for capturing the each length of particle (i.e., small, intermediate, and long) as showed in Figure 5. Images of the top (Figure 5a) and front views (Figure 5b) were used for the maximum and minimum projected areas of the particles, respectively. The position of particles was not changed when the two images were captured.

(a) (b) Figure 5: (a)Top views of particles, (b) Front views of particles

2.4. Image Processing

Digital­image processing consists of converting camera pictures into digital form and applying various mathematical procedures to extract relevant information from the picture. In other words, digital image processing and analysis is concerned with the transformation and analysis of picture by a computer. The process starts with the capture of a digital image (usually referred to as imaging) followed by its storage, transmission, processing, and analysis of the image by computer to extract information or features of interest (Masad and Sivakumar, 2004).

This section describes the image analysis method used in this study. The output of camera was a 3872­2592 pixel, 32­bit digital image of RGB color. The particles had to be identified prior to analysis. ImageJ was used as the image analysis program. Threshold gray intensity therefore had to be chosen. Thresholding determines the outline of the aggregate particle in a captured image. Clearly, the particle should have a sharp contrast against the background to accurately delineate the actual boundaries. A threshold value of pixel gray level is typically specified, and the actual image is converted to a binary image. This binary image has only black or white (gray level 0 or 255) pixels to clearly identify the particle against its background. The gray intensity measured on a given point was compared to the threshold value. Then, the initial gray image was converted into a binary image in which the aggregate particles that have lower gray intensity than the threshold value were set to black while the background was set to white. Applying a global threshold value for all the image worked well only if the objects of interest (aggregate particles) had uniform interior gray level and rested upon a background of different, but uniform, gray level. This was made possible in this study by placing particles on white background. The original image (32­bit digital image of RGB) (a), 8­bit 256 gray scale

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image (b), 1­bit binary image (c) and output of ImageJ image analysis program (d) are shown in Figure 6.

Figure 6: Image processing steps

2.5. Grain­Size Distribution with Mechanical and Image Analysis

Mechanical (sieve) analysis depends on measuring mass of particles; whereas image analysis depends on measuring the number of particles or a single morphological parameter (i.e., area, length, breadth, equivalent area diameter (dA), mean Feret's diameter (dF)) of particles. Hence, it is not obvious that the results of these two methods are comparable (Fernlund et al., 2007). Because image analysis produces area gradation and uses a particle size definition different from the square sieve size used in mechanical sieving, direct comparison is not possible, unless the gradation basis and particle size definitions of the two methods are aligned. For this purpose, a simple method of converting the area gradation to mass gradation was proposed in literature (Mora et al., 1998). Moreover, a size correction factor is used to convert the particle sizes measured by image analysis to equivalent square sieve sizes so that comparison between the image and mechanical analysis results can be made.

In this research, grain­size distribution of soil samples determined by both mechanical (sieve) and image analysis methods. In order for the sample to be representative, approximately 1 kg of each soil sample was taken for mechanical analysis. For image analysis, the number of particles that can be placed within the measurement area without

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touching or overlapping each other ranges from 15 to about 100, depending on the size and shape of the particles as shown in Figure 6. Because the soil samples contain more than 500 particles, all of the particles could not be placed onto the sample tray at same time. Therefore, the soil samples have to be divided into several sub­samples whose images are taken successively so that the image of all particles can be captured.

The image processing steps explained above section were applied to all captured images and the output of image processing program (ImageJ) was obtained. The output of the ImageJ is area, perimeter, long dimension (length or Feret’s diameter) values in unit of millimeter, and quantity of particles. These data are transferred to Excel, Microsoft software. Similar to Lira and Pina (2007), to construct grain­size distribution curves of the particles, the area gradation was used as described at equation (1). To make comparison of mechanical and image analysis results, the area gradation points were transformed to sieve size by extracting the square root of area. Due to the square sieve openings, this simple method was used. After then, the results are sorted independently and cumulative curves of the axial relationships plotted. In order to compare results of top and front views, the images of both views were separately analyzed and plotted.

( )

+ + + + − = ⋅ ⋅ 100 *

... 100 sin 3 2 1

Area Total Area Area Area Area

X Area g Pas Percent X (1)

2.6. Volume Computation of Particles

Image analysis does not measure mass of particles. However, several researchers have claimed that more accurate volume and mass determinations are necessary for accurate construction of grain­size curves (Taylor, 2002; Maerz, 2004; Tutumluer et al., 2005). Three­dimensional representation of particles is considered very important to be able to accurately determine the volume (Fernlund et al., 2007). In this sense, a study was undertaken on gravel soils (S8, S9, and S10). Elongated (S8), flat (S9), and spherical (S10) particles in gravel soil (8 mm­16 mm) were selected by hand and used for determination of their volumes. 200 particles were selected for each particle shape. The particles were also placed three by three within the sample tray for capturing the top and front images as seen in Figure 5. These images were processed in ImageJ then output data of ImageJ was transferred to Excel. The output data also contain length (L), breadth (I), thickness (S), top area and front area of each particle.

Five different methods for calculating volume and hence mass have been tested: Volume 1 is the product of the axial dimensions (cube: V=L*S*I); Volume 2 is the product of the area of top view and the mean thickness (cylinder1: V= Atop*S); Volume 3 is the product of the front view area and the longest dimension (cylinder2: V= Afront*L); Volume 4 is the product of the ellipsoid (V=4/3*Π*((L/2)*(I/2)*(S/2)); Volume 5 is the product of the modified ellipsoid (V=4/3* Atop*(S/2)). The mass of analyzed particles was determined with specific gravity. The mass was also calculated by multiplying the specific gravity of

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soil (Gs=2,58) and volume. The grain­size distribution curve was also plotted and compared with the other results.

3. Results and Discussion

In the following sections, the results of image analysis and mechanical analysis of particles are presented. The findings of the experimental tests are compared with those of other studies in the literature and discussed in detail.

3.1. Comparison of mechanical (MA) and image analysis (IA) on GSD

Grain­size distribution of S1 (2mm­4,45mm), S2 (2mm­8mm), S3 (4,75mm­8mm), and S4 (8mm­16mm) soil samples were determined by using mechanical and image analysis. The GSD curves are given in Figure 7. It is seen that the GSD curves obtained by IA and MA do not quite agree with each other, with the curve obtained by MA being consistently lower than that by IA. The difference between the results of IA and the results of MA could be attributed that the particle size definitions used in the two analyses are different.

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1 10 100 Particle Size (mm)

Percent Passing

(%)

S1­MA S1­IA S2­MA S2­IA S3­MA S3­IA S4­MA S4­IA

Figure 7: GSD curves of four soil samples with different sizes obtained by mechanical and image analysis (The results of S1, S2, and S3 are also mentioned in Arasan and

Akbulut (2008))

In this study, grain­size distributions (GDS) of soils were estimated by the area distribution of particles method used by King (1984) and Vallebuona et al. (2003) and mechanical analysis is also performed by using the mass of particles. Hence, the difference is obtained from analysis as indicated by Mora et al (1998) and Kwan et al. (1999). Mora et al. (1998) presented a conversion factor, between 0.81 and 0.89 times the

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length of the intermediated axis and obtained exactly same curves from IA and MA by using these conversion factors.

On the other hand, some particle properties of soil samples were obtained from the curves. Table 2 shows these results such as: D10, D30, D50, D60, Cu, Cc, and soil classification according to the Unified Soil Classification System­USCS. It can be also seen from Table 2 that D10, D30, D50, and D60 values of IA greater than that MA. However, the classifications do not change. The soil classification is more important than GSD curve in soil mechanics. Thus, it could be said that the difference between two analyses results are insignificant and the size of particles do not significantly affect the GSD.

Table 2: Some particle properties of soils used in mechanical and image analysis (The results of S1, S2, and S3 are also mentioned in Arasan and Akbulut (2008))

S1 S2 S3 S4 MA IA MA IA MA IA MA IA

D10 2,3 2,8 2,5 2,9 5,3 6,3 8,8 11,5 D30 2,8 3,3 3,25 3,7 5,9 7,5 9,8 12,9 D50 3,4 4 3,8 4,5 6,3 8,2 11,5 14,1 D60 3,6 4,7 4 5,5 6,6 8,7 12,0 14,5 Cu 1,57 1,68 1,6 1,89 1,25 1,38 1,36 1,26 Cc 0,95 0,83 1,06 0,86 1,00 1,03 0,91 1,00

Soil Classification SP SP SP SP GP GP GP GP

3.2. Influence of volume calculations on GSD

Flat, elongated, and spherical particles in gravel soil (8 mm­16 mm) were selected by hand as mentioned above and analyzed for determination of their volumes. Five different methods for calculating volume and hence mass have been tested. The comparison of real total weight with image analysis results is given in Figure 8. Particle shape did not significantly affect the weight calculation. However, there are great differences between the volume calculation methods. It is clearly seen that all of the estimated weights by volume calculation are higher than real weight and also the modified ellipsoid method give the best results. The total weights of the S4 (mixed), S8 (elongated), S9 (flat), and S10 (spherical) were measured manually to be 176gr, 205gr, 126gr, and 143gr, respectively, while the calculated value by the image analysis approach using a Gs of 2,58 gr/cm 3 were 177gr, 212gr, 132gr, and 144gr, respectively for modified ellipsoid method. These differences in total weights of about 1%, 3%, 5%, and 1% are insignificant.

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0

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300

400

500

Flat Elongated Spherical Mixed

Particle Shape

Total W

eigh

t (gr)

Real Weight Modified Ellipsoid Ellipsoid Cylinder 1 Cylinder 2 Cube

Figure 8: The comparison of volume computation type on the total weight of particles

Similarly, Rao et al. (2001) and Tutumluer et al. (2005) proposed a method based on assuming that the particle is an ellipsoid of revolution for volume calculation. Fernlund et al. (2007) used a method based on assuming that the particles are ellipsoidal, then the volume is two­thirds the product of the largest projected area times the thickness. Furthermore, Rao and Tutumluer (2000) showed that the weights of the aggregates estimated from the image analysis system agree quite well with the physically measured values and the errors are clearly biased towards the positive side. They also advocated that this is expected; as the volumes are slightly overestimated because the cameras cannot capture information about hollow portions cannot seem by the cameras. The results of the study described herein demonstrate that the proposed image analysis method is a promising method for 3D analysis of soil particles.

3.3. Comparison of area (from top and front views) and volume methods

To study the affect of camera positions on the estimated distributions, the results of the profile area approach applied to two types of analysis (top and front view) were compared and the results were shown in Figure 9. By comparing the estimated size distribution when the camera was positioned in the two different positions, it was found that the top view analysis had the worst results (Figure 9). The reason may due to direction of particle passing which had the most influence in the image analysis as indicated previous researchers (Mora et al., 1998; Fernlund, 1998; Kwan et al., 1999). Fernlund (1998) reported that the least cross­sectional area is most important and usually the longest dimension of a particle has little effect on sieve results. In this study, the maximal and least areas of a particle are obtained from top view and front view, as illustrated in Figure 4 and 5, respectively. In this sense, it could be said that front views

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give more accurate results than top views in image analysis used for grain­size distribution.

On the other hand, it is clearly seen from Figure 9 difference between the GSD curves of all methods is insignificant when soil classification results put in consideration. The GSD curve of soils obtained from volume calculation is similar to the GSD curves of soils obtained from 2D views (i.e., top and front view) and mechanical analysis. Similarly, Al­ Thyabat et al. (2007) investigated the size distribution of particles moving on a conveyer belt. They also indicated that profile views give worst results but top and free falling views are closely to the mechanical views. Consequently, it could be said that using particle area (2D views) is adequate for obtaining GSD, when contrast using particle volume/mass (3D view).

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1 10 100 Particle Size (mm)

Percent Passing

(%)

S4­MA S4­IA(Area­Front View) S4­IA(Area­Top View) S4­IA(Volume/Mass­M.El lipsoid)

Figure 9: The comparison of different views’ image analysis results on GSD curves

3.4. Influence of particle shape on GSD

Particle shape (elongated, flat, and spherical) is an important issue on soil mechanics. For this reason, effect of particle shape on the GSD was investigated. Size fractions of 4.75 mm­8 mm (S5, S6, S7) and 8 mm­16mm (S8, S9, S10) in soil were selected by hand as flat, elongated, and spherical and performed the image analysis for grain­size distribution. Figure 10 shows the grain­size distributions of the samples have 4.75mm­8mm size fraction. Figure 11 and 12 also show the GSD of samples have 8mm­16mm size fraction from top and front views, respectively. It is clearly seen that the particle sizes do not affect the results. The curves from IA in spherical and elongated particles are the nearest and the farthest to the curve of the mixed particles from MA, respectively (Figure 10, 11).

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Similar to Figure 7, the GSD curves obtained from IA and MA are not compatible with each others.

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1 10 100 Particle Size (mm)

Percent Passing

(%)

v

MA­Mixed Particles IA­Mixed Particles IA­Elongated Particles IA­Flat Particles IA­Spherical Particles

Figure 10: GSD curves of gravel soil (4.75mm­8mm) with different shapes (Arasan and Akbulut, 2008)

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1.0 10.0 100.0

Particle Size (mm)

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IA­Flat Particles IA­Elongated Particles IA­Spherical Particles IA­Mixed Particles MA­Mixed Particles

Figure 11: GSD curves of gravel soil (8 mm­16mm) with different shapes by using top views of particles

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1.0 10.0 100.0 Particle Size (mm)

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IA­ Flat Particles IA­Elongated Particles IA­Spherical Particles IA­Mixed Particles MA­Mixed Particles

Figure 12: GSD curves of gravel soil (8 mm­16mm) with different shapes by using front views of particles

On the other hand, the curve obtained by MA consistently gave the higher particle size than that of IA for front views. The GSD curve of spherical and mixed particles obtained by IA are the nearest curve from the curve of mixed particles obtained by MA. Also, the GSD curve of flat particles obtained by IA is the farthest curve from the curve of mixed particles obtained by MA. The difference between the results of top and front views analysis is explained by the passing mechanism of particles through sieve aperture. As indicated previously, a particle passes through a sieve by longest dimension. In this sense, it could be said that the particle form is more effects than particle size to the sieve results. Similarly, Fernlund (1998) reported that the particle form influences the sieve results. Taylor (2002) also points out that sieving does not separate particles according to volume. There is a variation in the volume of particles retained on a sieve due to the varying shapes of the particles.

It is previously mentioned that the results of sieving are dependent upon the shape of the particles (Mora et al., 1998; Kwan et al., 1999; Mora and Kwan, 2000; Arasan and Akbulut, 2008). Similarly, the results of this study showed that the shape of the particles affected the GSD obtained from image analysis. In this sense, it could be said that the particle shape is the most important factor on the GSD of soils.

4. Conclusions

The following results are concluded based on the results and on the discussion presented in this research:

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1. The classification of soils according to the USCS obtained from image and mechanical (sieve) analysis do not change and the difference between results of two analyses is insignificant

2. The size of particles does not significantly affect the grain­size distribution.

3. Particle shape does not significantly affect the weight calculation. However, there are great differences between the volume calculation methods. All of the estimated weights by volume calculation are higher than real weight and also the modified ellipsoid method gives the best results.

4. The front views give more accurate results than top views in image analysis used for grain­size distribution. The difference between the results of top and front views analysis is explained by the passing mechanism of particles through sieve aperture.

5. For determination of grain­size distribution, 2D view image analysis is adequate rather than using volume/mass calculation.

6. The shape of particle significantly affects the grain­size distribution.

In this sense, the image analysis is considered to be a significant technological advancement towards grain­size distribution of soils with images obtained from digital cameras from taken photos. It should also be pointed out that further studies on the determination of index properties of soils by using image analysis are needed to make more reasonable judgments for utilization of image analysis in geotechnical engineering.

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