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Fouling and water content influence on the ballast deformation properties
Construction and Building Materials 190 (2018) 881–895
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Fouling and water content influence on the ballast deformation properties Hamed Faghihi Kashani a,⇑, Carlton L. Ho b, James P. Hyslip a a b
HyGround/Loram, Williamsburg, MA, USA 28 Marston Hall, University of Massachusetts, Amherst, MA, USA
h i g h l i g h t s Increase in breakdown fouling does not result in the loss of strength at constant water content. With the increase in breakdown fouling, the friction angle increases non-linearly. The effect of fouling on the relationship between ballast strength and volumetric strain rate during dilation is significant. Ballast elastic modulus decreases linearly with an increase in water content. An increase in confining pressure results in an increase in ballast breakage.
a r t i c l e
i n f o
Article history: Received 20 October 2017 Received in revised form 23 August 2018 Accepted 10 September 2018
Keywords: Railroad Track Ballast Fouling Moisture Triaxial test Deformation properties
a b s t r a c t This paper discusses the drained static triaxial testing on granite ballast material with different amount of breakdown fouling and water content. Large-scale, 10 in., triaxial equipment was used for this testing program at the University of Massachusetts, Amherst. These tests were performed to study the effect of fouling and its inseparable associate, water content, on the strength properties and degradation characteristics of railroad ballasted track. Ballast with three different fouling percentages (defined as the ratio of dry weight material passing 3/800 (9.5 mm) sieve to dry weight of the total ballast sample) from clean to highly fouled ballast (<5%, 15% and 30%) and four water contents from dry to field capacity were tested under three different confining pressures (5, 10 and 15 psi). The results show that although increase in breakdown fouling at dry conditions increases the strength of the ballast, by an increase in water held by fouling material the ballast starts degrading. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Infiltration of fines into the ballast layer resulting from breakdown of granular particles, or breakdown fouled ballast creation, is a common track damage that leads a track to roughness problems and losing its acceptable ride quality and safety standards. Fouling and water held by fouling gradually deteriorates ballast by reducing hydraulic conductivity and strength properties [10,11,14,25]. This situation leads a track to require maintenance. Increase in train traffic and load carrying capacity will result in more ballast breaking down and consequently higher maintenance costs. Railroads spend tens of millions of dollars for ballast and ballast related maintenance in the U.S. [8]. A considerable proportion of these costs are related to geotechnical problems of a track as the ⇑ Corresponding author at: 29 Petticoat Hill Road, Williamsburg, MA 01096, USA. E-mail address: [email protected] (H. Faghihi Kashani). https://doi.org/10.1016/j.conbuildmat.2018.09.058 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
railroad foundation. Therefore, studying geotechnical deformation properties of ballast and understanding its performance at different situations will help to reduce these costs. Also, numerical analysis and computer modeling used for predicting ballast behavior under different loading conditions need to be verified by deformation and indices properties through laboratory tests. Many static and dynamic triaxial lab tests have been performed on different types of fouled ballast with different grain size distributions [1,6,9,12,18,20,21,26,28,29] but there is still a need to evaluate the effect of different breakdown fouling percentages at different moisture conditions on deformation properties of ballast. Accordingly, a large-scale laboratory testing program was performed on clean, moderately and highly fouled ballast contaminated by breakdown fouling. Each of these mixtures was tested under different water contents. The tests were also conducted under three confining pressures including 5, 10 and 15 psi. Material and experimental set up, specimen’s preparation and results evaluating
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the effects of different fouling percentages, water contents and confining pressures have been discussed and presented.
clean and fouled ballast including diameters of the grains at 10%, 30%, 50%, 60% percent passing, the coefficients of uniformity and curvature of tested ballast. The Grain Size Distribution tests were performed in general accordance with ASTM D6913-04 [2].
2. Material 2.2. Water content
2.1. Ballast and fouling In this study, ballast samples were granite crushed stone obtained from a quarry in Connecticut. The different sizes of crushed stone were mixed to meet American Railway Engineering and Maintenance-of-Way Association (AREMA) #4 gradation [3]. For simulating breakdown fouling, abraded stone dust from the same quarry was selected. Abraded ballast contributes to a large percentage of actual fouling [25]. The tests were conducted on clean (<5% Fouling), moderately fouled ballast (15% fouling) and highly fouled ballast (30% fouling) to determine the effects of fouling and moisture held by fouling on deformation properties of ballast. F0, F15 and F30 symbols have been used for ballast with <5%, 15% and 30 fouling, respectively. Percent fouling is the ratio of fine particles by weight passing 3/800 sieve to the total weight of the fouled ballast [25]. The particle size distribution plot has been presented in Fig. 1 for all samples, where the clean sample meets the AREMA#4 ballast and fouled samples exclude from the AREMA #4 gradation. Table 1 summarizes gradation characteristics of the
Fig. 1. Grain size distribution plot.
To evaluate the effect of water content on deformation properties of ballast, four stages of water content were tested including dry, field capacity and two points between the extremes. Field capacity is defined as the water content of ballast immediately after saturation, at which the material maintains the maximum amount of water that can be held in place by capillary tension after excess moisture has been drained by gravity. This simulates a heavy rainfall condition as the ballast surface is soaked in water but the water has moved under gravity out of the soil skeleton. To define the amount of water needed to be added to the ballast triaxial samples, reconstituted samples with the same grain size distributions were built at the same room temperature. The reconstituted samples were saturated and drained immediately followed by water content sampling to find the field capacity water content. The ⅓ and ⅔ portions of distance between zero (dry) and field capacity were defined as intermediate water contents (w1 and w2). To verify the actual water content of the triaxial samples in accordance with the defined water contents, water contents of whole samples were measured after breaking down. Water content sampling was conducted in general accordance with ASTM D2216-05 standard [2]. For intermediate water contents, water was introduced to the samples from the top; for field capacity conditions water was added from the bottom and drained immediately. The water in all moist samples was added after building the samples and applying 2 psi (13.8 kPa) confining pressure. To allow water to equally distribute through the sample, the intermediate water content samples sat for 16 h prior to the start of the consolidated drained triaxial test, ASTM D7181-11 [2]. The samples with field capacity water content were loaded immediately after complete drainage. Table 1 shows the average of defined and actual water content for clean and fouled ballast. Note that for clean ballast only one intermediate water content (w1) was tested. The field capacity for clean ballast samples was less than 1% and due small range in water content from dry to field capacity it was hard to distinguish between intermediate water contents.
Table 1 Grain Size Distribution (GSD) Test Results, Density Values, Targeted and Average Measured Water Contents of the Samples. Parameters
F0
F15
F30
Average Fouling (%) D10 (mm) D30 (mm) D50 (mm) D60 (mm) Cu Cc
2.0 16.7 21.2 24.0 25.2 1.5 1.1
15.7 1.2 19.5 22.7 24.2 20.2 13.1
30.0 0.2 13.0 21.0 23.0 100.0 31.9
Dry density (kg/m3)
1561
1958
2133
Targeted Water Content from reconstituted samples (%)
Dry w1 w2 Field Capacity
0.0 0.3 0.5 0.8
0.0 1.1 2.1 3.2
0.0 1.6 3.2 4.8
Measured Water Content from Traixial Samples (%)
Dry w1 w2 Field Capacity
0.1 0.5 – 0.8
0.3 1.7 2.4 3.2
0.6 1.7 3.0 4.5
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Fig. 2. Components of triaxial load apparatus.
Fig. 3. Stress-strain behavior during CIDC triaxial tests at different confining pressures and water contents (a) clean ballast (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
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3. Specimen preparation and experimental set up The test specimen used for ballast triaxial test had a nominal diameter of 10 in. (25.4 cm) and nominal height of 20 in. (50.8 cm) corresponding to an aspect ratio of H/D = 2. In accordance with Fig. 1, more than 98% of particles are smaller than 40 mm (1.57 in.), therefore the sample size ratio, defined as the diameter of the triaxial specimen divided by the maximum particle dimensions, is equal to 6.35. This ratio is more than 6 which means that the sample size effects can be eliminated on the test results in accordance with ASTM D7181-11 [2]. The triaxial apparatus used in this study is illustrated in Fig. 2. Specimens were prepared by compacting ballast in 8 layers with equal thickness using a steel rod and a hand tamper. Layers were compacted in a mold with densities close to the measured densities in the real track [24] by careful control on thickness and weight of material in each layer. In the fouled specimens, the weight of the ballast was the same as clean specimens and the needed amount of fouling was spread over each layer followed by compaction. The specimen was enclosed by a double-latex membrane with total thickness of 1.68 mm. Table 1 shows the average of the specimen densities for clean and fouled ballast. The circumferential displacement was measured by three string potentiometers mounted at ¼, ½ and ¾ of specimen height. To minimize friction between the membrane and the potentiometer cable, the cable was encased by small pieces of Teflon tubing.
The triaxial chamber was placed in a load frame and loaded statically by using a MTS servo-controlled hydraulic system with a 110 kN (24 kips) capacity actuator. Isotropically consolidated drained triaxial compression tests were performed (CIDC) on clean and fouled ballast with 5 psi (34.5 kPa), 10 psi (68.9 kPa) and 15 psi (103.4 kPa) confining pressures with an axial strain rate of 0.04 in/ min [4]. 4. Results presentation and discussion 4.1. Stress and strain behavior The deviator stress versus axial strain of CIDC triaxial tests on samples with different fouling percentages, water contents and confining pressures are shown in Figs. 3 and 4. The peak deviator stress (r1 r3)P increases with increase in confining pressure as expected. It can be seen in Fig. 3 that by increasing the water content at constant fouling percentage and confining pressure, the peak deviator stress decreases. Although there is some scatter at intermediate water contents, which might be as the result of imperfect water distribution in the whole sample, the total trend is clear. Fig. 4 also indicates that an increase in fouling percentage at constant confining pressure and water content, increases the peak deviator stress. This is the result of better gradation and internal packing of fouled ballast in comparison with clean ballast in the same moisture and confining pressure conditions. Fine particles
Fig. 4. Stress-strain behavior during CIDC triaxial tests at different confining pressures and fouling percentages (a) dry condition (b) intermediate water content condition (c) field capacity water content condition.
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within the fouled ballast matrix will help to gain a higher density and better packing [29].
4.2. Shear strength 4.2.1. Effect of water content and confining pressure Figs. 5 and 6 show the effect of water content on the maximum shear strength at different fouling percentages and confining pressures. These figures show that with an increase in water content, the maximum shear strength decreases linearly. The rate of this decrease is influenced by confining pressure and fouling percentages. Fig. 5 indicates that the rate of this reduction decreases from clean to fouled ballast. Although, fouling percentages do not have a significant effect on the rate of this reduction from moderately to highly fouled ballast. This can be as the result of capillary tension among wet fine particles in fouled ballast and lesser effect of water on contact area of ballast aggregates or the void structure of fouled ballast that has not been completely filled with finer particles and existence of some aggregate to aggregate contact maintained to carry the load. Regarding the results of this study, a 1% increase in water content, results in a 25%, 6% and 4% decrease in maximum shear strength of clean, moderately and highly fouled ballast, respectively. Under constant confining pressure, 15% increase in fouling will result in about 75% decrease in the rate
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of shear strength reduction versus water content from clean to moderately fouled ballast. This reduction is negligible from moderately to highly fouled ballast. The reduction rate of maximum shear strength versus water content, increases with the increase in confining pressure (Fig. 6). This behavior indicates that the greater contact area and force between particles attributed to higher confining pressure, the greater the effect of moisture on decreasing shear strength of ballast. Based on the results, an increase in confining pressure from 5 to 10 psi can double or triple the rate of shear strength reduction versus water content; 10–15 psi increase in confining pressure, increases the rate of shear strength reduction versus water content by approximately 45% at a constant fouling condition.
4.2.2. Effect of fouling percentages and confining pressure Fig. 7 is a three-dimensional view showing the effect of confining pressure and fouling on the maximum shear strength of ballast. This figure shows that regardless of the amount of moisture, with an increase in fouling and confining pressure the maximum shear strength of ballast increases. To evaluate this behavior more precisely, the effect of individual variables of Fig. 7 are presented in two dimensional views in Figs. 8 and 9. Interestingly, the increase in breakdown fouling does not result in strength loss for a constant water content. Water held by fouling
Fig. 5. Changes of max shear strength vs. water content at different fouling conditions (a) 5 psi confining pressure (b) 10 psi confining pressure (c) 15 psi confining pressure.
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Fig. 6. Changes of max shear strength vs. water content at different confining pressures (a) clean ballast (F0) (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
is the main reason for ballast degradation as discussed in the previous section. Increase in shear strength of ballast with an increase in breakdown fouling is the result of a better gradation, higher density and compaction that helps ballast matrix achieve higher shear strength [7,22,29]. Fig. 8 indicates that shear strength increase as the result of fouling increase is a non-linear behavior and increase in confining pressure increases the maximum shear strength as expected [12,26]. Fig. 9 presents the same data shown in Fig. 7 highlighting the effect of water content. This figure also shows a non-linear behavior of increase in maximum shear strength as the result of increase in breakdown fouling percentage. The confining pressure and fouling do not have a significant effect on the rate of shear strength changes versus breakdown fouling (Figs. 8 and 9). On the average, a 15% increase in fouling percentage will result in a 10% increase in maximum shear strength from clean to moderately fouled ballast while this value is 40% from moderately to highly fouled ballast in a constant confining pressure. 4.3. Friction angle Fig. 7. Effect of fouling and confining pressure on the maximum shear strength of clean and fouled ballast.
In the field of granular mechanics, the deviatoric stress invariant, q, and the hydrostatic stress invariant, p, parameters are com-
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Fig. 8. Effect of fouling on the maximum shear strength of ballast at different confining pressures and constant water content (a) dry (b) w1 (c) w2 (d) field capacity. Note: w2 has two studied points in moderately and highly fouled ballast.
monly used because they are more appropriate for describing frictional failure of granular materials under three dimensional stress conditions. Therefore stress path analyses have been used in this study instead of principal stresses [26,27]. The effect of water content and fouling percentage on friction angles of fouled ballast are discussed in following sections.
4.3.1. Effect of water content To study the effect of water content on the frictional failure of ballast, the stress paths for clean ballast at different water contents are presented in Fig. 10. Clearly, the friction angle decreases with the increase in water content. This is the result of better ballast interlocking under dry conditions than under moist conditions. Fig. 11 shows friction angle changes versus water content in all samples of this study. In clean ballast, reduction of friction angle by increase in water content is clear from dry to field capacity water contents. In fouled ballast, the friction angle is decreasing from dry to field capacity conditions by a small amount. The reduction rate of friction angle versus water content decreases from clean to fouled ballast although, this rate is not changing significantly in fouled ballast with different fouling percentages. Approximately 1% increase in water content results in 12% reduction of friction angle in clean ballast and less than 2% reduction of friction angle in fouled ballast from dry to field capacity moisture conditions.
4.3.2. Effect of fouling Fig. 12 presents the effect of breakdown fouling on friction angle and stress conditions at failure and at constant water content conditions. This figure clearly shows that an increase in breakdown fouling results in an increase in friction angle and shear strength of ballast. Although fouling is one of the main reasons for degradation of ballast under constant water content, the friction angle increases non-linearly as a function of the increase in breakdown fouling percentages (Fig. 13). In accordance with the effect of water content on strength properties of ballast, discussed in previous sections, it becomes clearer that the main degradational influence on fouled ballast is moisture as fouling increases its effect by holding moisture in the ballast layer. Water content does not have a defined effect on the rate of friction angle changes versus fouling percentages according to this study. Approximately 15% increase in fouling from clean to moderately fouled ballast increases the friction angle by 5% and from moderately fouled ballast to highly fouled ballast by 15% in constant moisture conditions. 4.4. Elastic modulus To look at the effect of breakdown fouling and water content on secant modulus of clean and fouled ballast, the secant modulus of elasticity at 50% of drained shear strength (E50) was evaluated. The effect of water content, confining pressure and fouling percentage on E50 of fouled ballast are discussed in following sections.
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Fig. 9. Effect of fouling on the maximum shear strength of ballast at constant confining pressure and different water contents (a) 5 psi confining pressure (b) 10 psi confining pressure (c) 15 psi confining pressure. Note: w2 has two studied points in moderately and highly fouled ballast.
Fig. 10. Stress paths for clean ballast at different water content conditions. Fig. 11. Changes of friction angle vs. water content at different fouling conditions.
4.4.1. Effect of fouling and confining pressure Indraratna et al. [12], conducted CIDC triaxial tests on coarse and fine particles. They reported that coarser particles result in a considerably higher axial strain upon loading, attributed to larger void ratios and resulting in a smaller deformation modulus [12]. They also reported that an increase in confining pressure results in the initial elastic modulus increase and for confining pressure
more than 180 kPa (26.1 psi) the changes in elastic modulus is insignificant. A three-dimensional view plot of the effect of confining pressure and fouling on E50 is shown in Fig. 14. Regardless to moisture condition of ballast, by increase in breakdown fouling, the elastic modulus increases. This figure does not show a defined effect of confining pressure on the elastic modulus. Fig. 15 shows
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a two-dimensional view of Fig. 14 at constant water content. This figure indicates that the effect of confining pressure on the elastic modulus in the range used in this study is not significant although the effect of breakdown fouling is significant and clear. With the assumption of a linear relationship between E50 modulus and fouling percentage, with a 15% increase in breakdown fouling percentage, the modulus increases by 30% regardless of the confining pressure and water content conditions.
Fig. 12. Stress paths for different fouling conditions at field capacity water contents.
4.4.2. Effect of water content Previously, it was shown that the effect of confining pressure on E50 modulus is not significant, therefore the effect of water content has been shown at constant confining pressure in Fig. 16. Elastic modulus decreases linearly with the increase in water content (Fig. 16). This is the result of greater strain upon loading in moist ballast in comparison with dry ballast. The rate of this reduction is higher in clean ballast in comparison with fouled ballast, while it is hard to distinguish between the degradation rate of moderately fouled ballast (F15) from highly fouled ballast (F30). This is likely be the result of capillary tension among wet fine particles in fouled ballast and a lesser effect of water on contact area of ballast aggregates. Regarding to the results of this study, with a 1% increase in water content, E50 decreases 60%, 12% and 8% in clean, moderately and highly fouled ballast, respectively. Under a constant confining pressure, a 15% increase in fouling will result in approximately an 80% decrease in the rate of E50 reduction versus water content from clean to moderately fouled ballast and a 25% decrease in that rate for moderately to highly fouled ballast. 4.5. Dilatancy
Fig. 13. Changes of friction angle vs. fouling percentages at different water contents.
Fig. 14. Effect of fouling and confining pressure on the elastic modulus of clean and fouled ballast.
Previous studies have shown that the rate of volume change at failure decreases with an increase in confining pressure [5]. Fig. 17 shows a typical plot of volumetric strain versus axial strain for the clean ballast of this study at different water content and confining pressures. This figure clearly shows that regardless of the moisture and fouling conditions, by an increase in confining pressure the rate of volume strain versus axial strain decreases as expected. To study the effect of breakdown fouling and water content on the rate of dilation quantitatively, the slope of the line starting from the peak volume strain and ending at axial strain corresponding to failure has been evaluated at volume strain versus axial strain plots (Fig. 17) 4.5.1. Effect of water content Charles and Watts [7], reported that the larger dilation rates of granular materials are associated with the maximum principal stress ratios [7]. Also Indraratna et al. [12] used a hyperbolic fit to describe the relationship between the ballast dilatancy factor defined as Dp ¼ 1 ðdev =dea Þ [22] and the maximum principal stress ratio [12]. Although the dilatancy rate (dilatancy versus axial strain slope) used in this study, is increasing non-linearly with an increase in maximum principal stress ratio, it is difficult to distinguish between different water contents from this view (Fig. 18). Fig. 19 clearly shows the effect of the confining pressure increase on the volume strain rate reduction, which agrees with literature [5], and presents a better view of the effect of water content on the volume strain rate at constant fouling conditions. In clean ballast with the increase in water content, the dilatancy volumetric strain rate clearly decreases and the effect of confining pressure on the volumetric strain rate becomes stronger in samples with higher water contents (Figs. 17 and 19a). Water decreases the interlocking friction of ballast thereby affecting the dilatancy strain rate. In fouled ballast, the increase in water content from dry to
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Fig. 15. Effect of fouling on E50 modulus of ballast at different confining pressures and constant water content (a) dry (b) w1 (c) w2 (d) field capacity. Note: w2 has two studied points in moderately and highly fouled ballast.
field capacity conditions decreases the dilatancy volumetric strain rate with some scatter in the intermediate water content conditions (Fig. 19b and c). There is no evidence of a defined effect of water content on the volume strain rate-confining pressure relationship based on the results of this study (Fig. 19).
4.5.2. Effect of fouling Fig. 20 presents the change of maximum principal stress ratio versus dilatancy volumetric strain rate (%) for different fouling condition independent of water content conditions. Unlike water content (Fig. 18), fouling is significantly affecting the strengthdilatancy relationship. The best hyperbolic fit in Fig. 20, presents a higher dilatancy volumetric strain rate associated with a higher maximum principal stress ratio as the result of higher fouling conditions. Note that this relationship is independent of water content conditions. In comparison with Fig. 18 it can be concluded that the effect of fouling is more significant on the strength-dilatancy relationship than the water content in ballast. Regarding to Fig. 21, dilatancy volumetric strain rate increases with an increase in breakdown fouling percentages. This is the result of smaller void ratio of fouled ballast in comparison with clean ballast that leads the sample to higher rate of dilation after the initial compression. There is not a defined effect of breakdown
fouling percentage on the volume strain rate -confining pressure relationship based on the results of this study (Fig. 21).
4.6. Breakage degree There have been different methods used for quantifying ballast particle breakage degree using percent passing a single sieve size or changes in grain size distribution [12,13,19]. In order to evaluate the effect of water content, fouling percentage and confining pressure on breakage degree of ballast particles, both grain size distribution and passing a single sieve size methods are used. It is obvious that with greater breakage, there is a shift of the grain size distribution curve toward smaller particle sizes. Fig. 22 shows the effect of confining pressure on the shift of grain size distribution for clean and fouled ballast. Qualitatively, this figure shows that under a constant fouling condition, an increase in confining pressure will increase the shift in the grain size distribution curve toward smaller particle sizes, as expected [13,23]. These plots also indicate that by increase in breakdown fouling percentages the grain size distribution shift increases. This is as the result of higher density and more particle-to-particle contact area in ballast with higher fouling percentages that results in more particle split and corner degradation.
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Fig. 16. Changes of E50 modulus vs. water content at different fouling conditions (a) 5 psi confining pressure (b) 10 psi confining pressure (c) 15 psi confining pressure.
Fig. 17. Changes of volume strain versus axial strain for clean ballast.
Fig. 18. Maximum principal stress ratio versus dilatancy volumetric strain rate at different water contents.
It can also be seen that most of the breakage in grain size distribution in this study is taking place at grain size range between 1 and 20 mm (Fig. 22b and c). This is outside of the scope of this research but might be in agreement with the concept of studies correlating breakage to shape and sizes of particles [17].
The grain size distribution plots of moderately fouled ballast and highly fouled ballast also show that confining pressure changes between 5 psi (34.5 kPa) and 10 psi (68.9 kPa) has more effect on breakage degree than 10 psi (68.9 kPa)–15 psi (103.4 kPa) in this study.
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Fig. 19. The dilatancy volumetric strain rate versus confining pressure at different water contents and constant fouling conditions (a) clean ballast (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
To study the effect of water content on the rate of particle breakage, percent passing 3/800 sieve versus confining pressure at different water contents and constant fouling conditions was plotted (Fig. 23). The 3/8 in. sieve size was selected as the separator of fouling from ballast based on the fouling percentage definition [25]. Fig. 23 indicates that with the increase in water content the rate of breakage versus confining pressure decreases. This is as the result of reduction in particle-to-particle contact stress and frictional interlock with the increase in water content. Note that Fig. 23 only has plots for fouled ballast samples. That is because clean ballast samples cannot hold enough water on the particles surface to affect the particle-to-particle contact area. By 1% increase in water content, the rate of breakage versus confining pressure decreases in an average of 25–30% in moderately and highly fouled ballast. Fig. 20. Maximum principal stress ratio versus dilatancy volumetric strain rate at different fouling conditions.
The ranges of confining pressure showing increase in breakage degree in this study is also in agreement with the confining pressure ranges for Optimum Degradation Zone (ODZ) and Compressive, Stable Degradation Zone (CSDZ) defined by Indraratna et al. [13].
5. Results verification At low confining pressure, high principal stress ratios have been observed in granular materials. This behavior has been attributed to greater frictional interlock of particles at lower confining pressure [7,12,15,16]. Indraratna et al. [12] presented the following non-linear equation for the relationship between principal stress ratio and confining pressure:
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Fig. 21. The dilatancy volumetric strain rate versus confining pressure at different fouling conditions and constant water content (a) dry (b) w1(c) w2 (d) field capacity. Note: w2 has two studied points in moderately and highly fouled ballast.
Rp ¼ aðr3 Þb
ð1Þ
where Rp is the maximum principal stress ratio, a is the maximum principal ratio at r3 = 1, and b is a constant ratio related to degree of particle degradation. The changes of maximum principal stress ratio versus confining pressure of this study have been compared with the results from the other research on granular materials for verification (Fig. 24). The trend of the stress ratio changes versus confining pressure is in good agreement with the literature and shows a high R squared fitting Eq. (1) regardless of the moisture conditions. Fig. 24 also indicates that by increase in breakdown fouling, regardless of the water content from dry to field capacity condition, the maximum principal stress ratio increases. This is as the result of better packing of particles and was discussed in the previous sections. The changes of peak friction angle versus confining pressure for this study and other studies are presented in Fig. 25. This plot clearly shows that with an increase in confining pressure, the peak friction angle decreases. The results from this study is in good agreement in trend and magnitude with other studies. Fouling percentage was not found to have a significant effect on the reduction rate of friction angle by an increase in the confining pressure. The higher values of the friction angle at lower confining
pressure is as the result of interparticle contact stresses being well below the crushing strenght of granular materials [12]. 6. Conclusion Large scale static drained triaxial tests were performed on clean and breakdown fouled ballast at different water content conditions to evaluate the effect of these two parameters on deformation properties of ballast. Based on the triaxial test results, the effects of water content, fouling percentage and confining pressure on ballast deformation properties were discussed and evaluated qualitatively and quantitatively. Based on this study the following conclusions can be drawn: By increasing the water content, the maximum shear strength of ballast decreases linearly and the rate of this decrease is affected by confining pressure and fouling percentage. Interestingly, the increase in breakdown fouling does not result in the loss of strength at constant water content and water held by fouling is the main reason for ballast degradation. Independent from the confining pressure and water content conditions, approximately a 15% increase in breakdown fouling percentage results in a 30% increase in elastic modulus of ballast.
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Fig. 24. Effect of confining pressure on the maximum principal stress ratio for various granular materials and this study [12]. Note: The upper band is representing values obtained from tests on Basalt.
Fig. 22. Effect of confining pressure on the particle degradation curve at different fouling conditions (a) clean ballast (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
Fig. 25. Effect of confining pressure on the peak friction angle for various granular materials and this study [12].
Fig. 23. Percent passing 3/800 sieve versus confining pressure at different water contents and constant fouling conditions (a) moderately fouled ballast (F15) (b) highly fouled ballast (F30).
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With the increase in breakdown fouling, the friction angle increases non-linearly and water content does not have a significant effect on the rate of this behavior. By increasing water content, the friction angle decreases and the rate of this reduction decreases by the increase in breakdown fouling. In the other words, the higher the fouling percentage, the lower slope of linear correlation between friction angle versus water content. Elastic modulus decreases linearly with an increase in water content. The rate of this reduction is higher in clean ballast in comparison to that for fouled ballast. The effect of fouling on the relationship between ballast strength and volumetric strain rate during dilation is more significant than water content. An increase in confining pressure results in an increase in ballast breakage. The higher the water content the less the rate of breakage versus confining pressure. Conflict of interest None. References [1] J.E.D. Alva-Hurtado, A methodology to predict the elastic and inelastic behavior of railroad ballast Ph D Dissertation, University of Massachusetts, 1980. [2] ASTM, Annual Book of Standards, Volume 04.08, Soil and Rock (II): D5714 – Latest, ASTM, Philadelphia, PA, 2008. [3] AREMA, Manual for Railway Engineering, American Railway Engineering & Maintenance-of-way Assoc, 2015. [4] B. Aursudkij, G.R. McDowell, A.C. Collop, Cyclic loading of railway ballast under triaxial conditions and in a railway test facility, Granular Matter 11 (6) (2009) 391–401, https://doi.org/10.1007/s10035-009-0144-4. [5] J. Billam, Some aspects of the behaviour of granular materials at high pressures, in: Paper Presented at The Proceeding of the Roscoe Memorial Symposium, Cambridge, 1971. [6] S.F. Brown, Repeated load testing of a granular material, J. Geotech. Eng. Div. 100 (GT7) (1974) 825–841. [7] J.A. Charles, K. Watts, The influence of confining pressure on the shear strength of compacted rockfill, Geotechnique 30 (4) (1980) 353–367. [8] S.M. Chrismer, in: Considerations of Factors Affecting Ballast Performance, American Railway Engineering Association, 1986, p. 87. [9] A. Ebrahimi, J.M. Tinjum, T.B. Edil, Large-Scale Cyclic Triaxial Testing of Rail Ballast, in: Paper presented at the Proceedings of the AREMA Annual Conference, Orlando, Florida, 2010.
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[10] E. Fortunato, A. Pinelo, M.M. Fernandes, Characterization of the fouled ballast layer in the substructure of a 19th century railway track under renewal, Soils Foundations 50 (1) (2010) 55–62. [11] X. Han, E. Selig, Effects of Fouling on Ballast Settlement, in: Paper Presented at the Proceedings of the 6th International Heavy Haul Railway Conference, 1997. [12] B. Indraratna, D. Ionescu, H. Christie, Shear behavior of railway ballast based on large-scale triaxial tests, J. Geotech. Geoenviron. Eng. 124 (5) (1998) 439– 449. [13] B. Indraratna, J. Lackenby, D. Christie, Effect of confining pressure on the degradation of ballast under cyclic loading, Geotechnique 55 (4) (2005) 325– 328. [14] B. Indraratna, W. Salim, Mechanics of Ballasted Rail Tracks: A Geotechnical Perspective, CRC Press, 2005. [15] N.D. Marachi, C.K. Chan, H.B. Seed, Evaluation of properties of rockfill materials, J. Soil Mech. Foundations Div. 97 (SM1) (1972). [16] R.J. Marsal, Large-scale testing of rockfill materials, J. Soil Mech. Foundations Div. 93 (2) (1967) 27–43. [17] G.R. McDowell, M.D. Bolton, D. Robertson, The fractal crushing of granular materials, J. Mech. Phys. Solids 44 (12) (1996) 2079, https://doi.org/10.1016/ S0022-5096(96)00058-0. [18] D. Mishra, H. Kazmee, E. Tutumluer, J. Pforr, D. Read, E. Gehringer, Characterization of railroad ballast behavior under repeated loading results from new large triaxial test setup, Transp. Res. Rec. (2374) (2013) 169–179, https://doi.org/10.3141/2374-20. [19] N. Miura, O. Hara, S., Particle-crushing of a decomposed granite soil under shear stresses, Soils Foundations 19 (3) (1979) 1–14. [20] Y. Qian, D. Mishra, E. Tutumluer, Y.M. Hashash, J. Ghaboussi, Moisture Effects on Degraded Ballast Shear Strength Behavior, in: Paper presented at the Joint Rail Conference, Columbia, SC, 2016. [21] Y. Qian, E. Tutumluer, Y.M.A. Hashash, J. Ghaboussi, Effects of Ballast Degradation on Permanent Deformation Behavior from Large-Scale Triaxial Tests, in: Proceedings of the Asme Joint Rail Conference, 2014. [22] P.W. Rowe, The Stress-Dilatancy Relation for Static Equilibrium of an Assembly of Particles in Contact, in: Paper presented at the Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 1962. [23] W. Salim, B. Indraratna, A new elastoplastic constitutive model for coarse granular aggregates incorporating particle breakage, Can. Geotech. J. 41 (4) (2004) 657–671, https://doi.org/10.1139/T04-025. [24] E. Selig 90 (Bulletin No. 723), in: Compilation of Field Mearsurements on Ballast State of Compactness, American Railway Engineering Association, 1989, pp. 382–391. [25] E.T. Selig, J.M. Waters, Track Geotechnology and Substructure Management, T. Telford, 1994. [26] A.S.J. Suiker, E.T. Selig, R. Frenkel, Static and cyclic triaxial testing of ballast and subballast, J. Geotech. Geoenviron. Eng. 131 (6) (2005) 771–782, https://doi. org/10.1061/(Asce)1090-0241(2005) 131:6(771). [27] J. Sulem, I.G. Vardoulakis, Bifurcation Analysis in Geomechanics, CRC, 2004. [28] V.N. Trinh, A.M. Tang, Y.-J. Cui, J.-C. Dupla, J. Canou, N. Calon, O. Schoen, Mechanical characterisation of the fouled ballast in ancient railway track substructure by large-scale triaxial tests, Soils Foundations 52 (3) (2012) 511– 523. [29] E. Tutumluer, Y. Qian, D. Li, C. Basye, Ballast degradation characterization through trixial testing, Technol. Digest (2015) 1–4.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Fouling and water content influence on the ballast deformation properties Hamed Faghihi Kashani a,⇑, Carlton L. Ho b, James P. Hyslip a a b
HyGround/Loram, Williamsburg, MA, USA 28 Marston Hall, University of Massachusetts, Amherst, MA, USA
h i g h l i g h t s Increase in breakdown fouling does not result in the loss of strength at constant water content. With the increase in breakdown fouling, the friction angle increases non-linearly. The effect of fouling on the relationship between ballast strength and volumetric strain rate during dilation is significant. Ballast elastic modulus decreases linearly with an increase in water content. An increase in confining pressure results in an increase in ballast breakage.
a r t i c l e
i n f o
Article history: Received 20 October 2017 Received in revised form 23 August 2018 Accepted 10 September 2018
Keywords: Railroad Track Ballast Fouling Moisture Triaxial test Deformation properties
a b s t r a c t This paper discusses the drained static triaxial testing on granite ballast material with different amount of breakdown fouling and water content. Large-scale, 10 in., triaxial equipment was used for this testing program at the University of Massachusetts, Amherst. These tests were performed to study the effect of fouling and its inseparable associate, water content, on the strength properties and degradation characteristics of railroad ballasted track. Ballast with three different fouling percentages (defined as the ratio of dry weight material passing 3/800 (9.5 mm) sieve to dry weight of the total ballast sample) from clean to highly fouled ballast (<5%, 15% and 30%) and four water contents from dry to field capacity were tested under three different confining pressures (5, 10 and 15 psi). The results show that although increase in breakdown fouling at dry conditions increases the strength of the ballast, by an increase in water held by fouling material the ballast starts degrading. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Infiltration of fines into the ballast layer resulting from breakdown of granular particles, or breakdown fouled ballast creation, is a common track damage that leads a track to roughness problems and losing its acceptable ride quality and safety standards. Fouling and water held by fouling gradually deteriorates ballast by reducing hydraulic conductivity and strength properties [10,11,14,25]. This situation leads a track to require maintenance. Increase in train traffic and load carrying capacity will result in more ballast breaking down and consequently higher maintenance costs. Railroads spend tens of millions of dollars for ballast and ballast related maintenance in the U.S. [8]. A considerable proportion of these costs are related to geotechnical problems of a track as the ⇑ Corresponding author at: 29 Petticoat Hill Road, Williamsburg, MA 01096, USA. E-mail address: [email protected] (H. Faghihi Kashani). https://doi.org/10.1016/j.conbuildmat.2018.09.058 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
railroad foundation. Therefore, studying geotechnical deformation properties of ballast and understanding its performance at different situations will help to reduce these costs. Also, numerical analysis and computer modeling used for predicting ballast behavior under different loading conditions need to be verified by deformation and indices properties through laboratory tests. Many static and dynamic triaxial lab tests have been performed on different types of fouled ballast with different grain size distributions [1,6,9,12,18,20,21,26,28,29] but there is still a need to evaluate the effect of different breakdown fouling percentages at different moisture conditions on deformation properties of ballast. Accordingly, a large-scale laboratory testing program was performed on clean, moderately and highly fouled ballast contaminated by breakdown fouling. Each of these mixtures was tested under different water contents. The tests were also conducted under three confining pressures including 5, 10 and 15 psi. Material and experimental set up, specimen’s preparation and results evaluating
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the effects of different fouling percentages, water contents and confining pressures have been discussed and presented.
clean and fouled ballast including diameters of the grains at 10%, 30%, 50%, 60% percent passing, the coefficients of uniformity and curvature of tested ballast. The Grain Size Distribution tests were performed in general accordance with ASTM D6913-04 [2].
2. Material 2.2. Water content
2.1. Ballast and fouling In this study, ballast samples were granite crushed stone obtained from a quarry in Connecticut. The different sizes of crushed stone were mixed to meet American Railway Engineering and Maintenance-of-Way Association (AREMA) #4 gradation [3]. For simulating breakdown fouling, abraded stone dust from the same quarry was selected. Abraded ballast contributes to a large percentage of actual fouling [25]. The tests were conducted on clean (<5% Fouling), moderately fouled ballast (15% fouling) and highly fouled ballast (30% fouling) to determine the effects of fouling and moisture held by fouling on deformation properties of ballast. F0, F15 and F30 symbols have been used for ballast with <5%, 15% and 30 fouling, respectively. Percent fouling is the ratio of fine particles by weight passing 3/800 sieve to the total weight of the fouled ballast [25]. The particle size distribution plot has been presented in Fig. 1 for all samples, where the clean sample meets the AREMA#4 ballast and fouled samples exclude from the AREMA #4 gradation. Table 1 summarizes gradation characteristics of the
Fig. 1. Grain size distribution plot.
To evaluate the effect of water content on deformation properties of ballast, four stages of water content were tested including dry, field capacity and two points between the extremes. Field capacity is defined as the water content of ballast immediately after saturation, at which the material maintains the maximum amount of water that can be held in place by capillary tension after excess moisture has been drained by gravity. This simulates a heavy rainfall condition as the ballast surface is soaked in water but the water has moved under gravity out of the soil skeleton. To define the amount of water needed to be added to the ballast triaxial samples, reconstituted samples with the same grain size distributions were built at the same room temperature. The reconstituted samples were saturated and drained immediately followed by water content sampling to find the field capacity water content. The ⅓ and ⅔ portions of distance between zero (dry) and field capacity were defined as intermediate water contents (w1 and w2). To verify the actual water content of the triaxial samples in accordance with the defined water contents, water contents of whole samples were measured after breaking down. Water content sampling was conducted in general accordance with ASTM D2216-05 standard [2]. For intermediate water contents, water was introduced to the samples from the top; for field capacity conditions water was added from the bottom and drained immediately. The water in all moist samples was added after building the samples and applying 2 psi (13.8 kPa) confining pressure. To allow water to equally distribute through the sample, the intermediate water content samples sat for 16 h prior to the start of the consolidated drained triaxial test, ASTM D7181-11 [2]. The samples with field capacity water content were loaded immediately after complete drainage. Table 1 shows the average of defined and actual water content for clean and fouled ballast. Note that for clean ballast only one intermediate water content (w1) was tested. The field capacity for clean ballast samples was less than 1% and due small range in water content from dry to field capacity it was hard to distinguish between intermediate water contents.
Table 1 Grain Size Distribution (GSD) Test Results, Density Values, Targeted and Average Measured Water Contents of the Samples. Parameters
F0
F15
F30
Average Fouling (%) D10 (mm) D30 (mm) D50 (mm) D60 (mm) Cu Cc
2.0 16.7 21.2 24.0 25.2 1.5 1.1
15.7 1.2 19.5 22.7 24.2 20.2 13.1
30.0 0.2 13.0 21.0 23.0 100.0 31.9
Dry density (kg/m3)
1561
1958
2133
Targeted Water Content from reconstituted samples (%)
Dry w1 w2 Field Capacity
0.0 0.3 0.5 0.8
0.0 1.1 2.1 3.2
0.0 1.6 3.2 4.8
Measured Water Content from Traixial Samples (%)
Dry w1 w2 Field Capacity
0.1 0.5 – 0.8
0.3 1.7 2.4 3.2
0.6 1.7 3.0 4.5
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Fig. 2. Components of triaxial load apparatus.
Fig. 3. Stress-strain behavior during CIDC triaxial tests at different confining pressures and water contents (a) clean ballast (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
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3. Specimen preparation and experimental set up The test specimen used for ballast triaxial test had a nominal diameter of 10 in. (25.4 cm) and nominal height of 20 in. (50.8 cm) corresponding to an aspect ratio of H/D = 2. In accordance with Fig. 1, more than 98% of particles are smaller than 40 mm (1.57 in.), therefore the sample size ratio, defined as the diameter of the triaxial specimen divided by the maximum particle dimensions, is equal to 6.35. This ratio is more than 6 which means that the sample size effects can be eliminated on the test results in accordance with ASTM D7181-11 [2]. The triaxial apparatus used in this study is illustrated in Fig. 2. Specimens were prepared by compacting ballast in 8 layers with equal thickness using a steel rod and a hand tamper. Layers were compacted in a mold with densities close to the measured densities in the real track [24] by careful control on thickness and weight of material in each layer. In the fouled specimens, the weight of the ballast was the same as clean specimens and the needed amount of fouling was spread over each layer followed by compaction. The specimen was enclosed by a double-latex membrane with total thickness of 1.68 mm. Table 1 shows the average of the specimen densities for clean and fouled ballast. The circumferential displacement was measured by three string potentiometers mounted at ¼, ½ and ¾ of specimen height. To minimize friction between the membrane and the potentiometer cable, the cable was encased by small pieces of Teflon tubing.
The triaxial chamber was placed in a load frame and loaded statically by using a MTS servo-controlled hydraulic system with a 110 kN (24 kips) capacity actuator. Isotropically consolidated drained triaxial compression tests were performed (CIDC) on clean and fouled ballast with 5 psi (34.5 kPa), 10 psi (68.9 kPa) and 15 psi (103.4 kPa) confining pressures with an axial strain rate of 0.04 in/ min [4]. 4. Results presentation and discussion 4.1. Stress and strain behavior The deviator stress versus axial strain of CIDC triaxial tests on samples with different fouling percentages, water contents and confining pressures are shown in Figs. 3 and 4. The peak deviator stress (r1 r3)P increases with increase in confining pressure as expected. It can be seen in Fig. 3 that by increasing the water content at constant fouling percentage and confining pressure, the peak deviator stress decreases. Although there is some scatter at intermediate water contents, which might be as the result of imperfect water distribution in the whole sample, the total trend is clear. Fig. 4 also indicates that an increase in fouling percentage at constant confining pressure and water content, increases the peak deviator stress. This is the result of better gradation and internal packing of fouled ballast in comparison with clean ballast in the same moisture and confining pressure conditions. Fine particles
Fig. 4. Stress-strain behavior during CIDC triaxial tests at different confining pressures and fouling percentages (a) dry condition (b) intermediate water content condition (c) field capacity water content condition.
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within the fouled ballast matrix will help to gain a higher density and better packing [29].
4.2. Shear strength 4.2.1. Effect of water content and confining pressure Figs. 5 and 6 show the effect of water content on the maximum shear strength at different fouling percentages and confining pressures. These figures show that with an increase in water content, the maximum shear strength decreases linearly. The rate of this decrease is influenced by confining pressure and fouling percentages. Fig. 5 indicates that the rate of this reduction decreases from clean to fouled ballast. Although, fouling percentages do not have a significant effect on the rate of this reduction from moderately to highly fouled ballast. This can be as the result of capillary tension among wet fine particles in fouled ballast and lesser effect of water on contact area of ballast aggregates or the void structure of fouled ballast that has not been completely filled with finer particles and existence of some aggregate to aggregate contact maintained to carry the load. Regarding the results of this study, a 1% increase in water content, results in a 25%, 6% and 4% decrease in maximum shear strength of clean, moderately and highly fouled ballast, respectively. Under constant confining pressure, 15% increase in fouling will result in about 75% decrease in the rate
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of shear strength reduction versus water content from clean to moderately fouled ballast. This reduction is negligible from moderately to highly fouled ballast. The reduction rate of maximum shear strength versus water content, increases with the increase in confining pressure (Fig. 6). This behavior indicates that the greater contact area and force between particles attributed to higher confining pressure, the greater the effect of moisture on decreasing shear strength of ballast. Based on the results, an increase in confining pressure from 5 to 10 psi can double or triple the rate of shear strength reduction versus water content; 10–15 psi increase in confining pressure, increases the rate of shear strength reduction versus water content by approximately 45% at a constant fouling condition.
4.2.2. Effect of fouling percentages and confining pressure Fig. 7 is a three-dimensional view showing the effect of confining pressure and fouling on the maximum shear strength of ballast. This figure shows that regardless of the amount of moisture, with an increase in fouling and confining pressure the maximum shear strength of ballast increases. To evaluate this behavior more precisely, the effect of individual variables of Fig. 7 are presented in two dimensional views in Figs. 8 and 9. Interestingly, the increase in breakdown fouling does not result in strength loss for a constant water content. Water held by fouling
Fig. 5. Changes of max shear strength vs. water content at different fouling conditions (a) 5 psi confining pressure (b) 10 psi confining pressure (c) 15 psi confining pressure.
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Fig. 6. Changes of max shear strength vs. water content at different confining pressures (a) clean ballast (F0) (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
is the main reason for ballast degradation as discussed in the previous section. Increase in shear strength of ballast with an increase in breakdown fouling is the result of a better gradation, higher density and compaction that helps ballast matrix achieve higher shear strength [7,22,29]. Fig. 8 indicates that shear strength increase as the result of fouling increase is a non-linear behavior and increase in confining pressure increases the maximum shear strength as expected [12,26]. Fig. 9 presents the same data shown in Fig. 7 highlighting the effect of water content. This figure also shows a non-linear behavior of increase in maximum shear strength as the result of increase in breakdown fouling percentage. The confining pressure and fouling do not have a significant effect on the rate of shear strength changes versus breakdown fouling (Figs. 8 and 9). On the average, a 15% increase in fouling percentage will result in a 10% increase in maximum shear strength from clean to moderately fouled ballast while this value is 40% from moderately to highly fouled ballast in a constant confining pressure. 4.3. Friction angle Fig. 7. Effect of fouling and confining pressure on the maximum shear strength of clean and fouled ballast.
In the field of granular mechanics, the deviatoric stress invariant, q, and the hydrostatic stress invariant, p, parameters are com-
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Fig. 8. Effect of fouling on the maximum shear strength of ballast at different confining pressures and constant water content (a) dry (b) w1 (c) w2 (d) field capacity. Note: w2 has two studied points in moderately and highly fouled ballast.
monly used because they are more appropriate for describing frictional failure of granular materials under three dimensional stress conditions. Therefore stress path analyses have been used in this study instead of principal stresses [26,27]. The effect of water content and fouling percentage on friction angles of fouled ballast are discussed in following sections.
4.3.1. Effect of water content To study the effect of water content on the frictional failure of ballast, the stress paths for clean ballast at different water contents are presented in Fig. 10. Clearly, the friction angle decreases with the increase in water content. This is the result of better ballast interlocking under dry conditions than under moist conditions. Fig. 11 shows friction angle changes versus water content in all samples of this study. In clean ballast, reduction of friction angle by increase in water content is clear from dry to field capacity water contents. In fouled ballast, the friction angle is decreasing from dry to field capacity conditions by a small amount. The reduction rate of friction angle versus water content decreases from clean to fouled ballast although, this rate is not changing significantly in fouled ballast with different fouling percentages. Approximately 1% increase in water content results in 12% reduction of friction angle in clean ballast and less than 2% reduction of friction angle in fouled ballast from dry to field capacity moisture conditions.
4.3.2. Effect of fouling Fig. 12 presents the effect of breakdown fouling on friction angle and stress conditions at failure and at constant water content conditions. This figure clearly shows that an increase in breakdown fouling results in an increase in friction angle and shear strength of ballast. Although fouling is one of the main reasons for degradation of ballast under constant water content, the friction angle increases non-linearly as a function of the increase in breakdown fouling percentages (Fig. 13). In accordance with the effect of water content on strength properties of ballast, discussed in previous sections, it becomes clearer that the main degradational influence on fouled ballast is moisture as fouling increases its effect by holding moisture in the ballast layer. Water content does not have a defined effect on the rate of friction angle changes versus fouling percentages according to this study. Approximately 15% increase in fouling from clean to moderately fouled ballast increases the friction angle by 5% and from moderately fouled ballast to highly fouled ballast by 15% in constant moisture conditions. 4.4. Elastic modulus To look at the effect of breakdown fouling and water content on secant modulus of clean and fouled ballast, the secant modulus of elasticity at 50% of drained shear strength (E50) was evaluated. The effect of water content, confining pressure and fouling percentage on E50 of fouled ballast are discussed in following sections.
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Fig. 9. Effect of fouling on the maximum shear strength of ballast at constant confining pressure and different water contents (a) 5 psi confining pressure (b) 10 psi confining pressure (c) 15 psi confining pressure. Note: w2 has two studied points in moderately and highly fouled ballast.
Fig. 10. Stress paths for clean ballast at different water content conditions. Fig. 11. Changes of friction angle vs. water content at different fouling conditions.
4.4.1. Effect of fouling and confining pressure Indraratna et al. [12], conducted CIDC triaxial tests on coarse and fine particles. They reported that coarser particles result in a considerably higher axial strain upon loading, attributed to larger void ratios and resulting in a smaller deformation modulus [12]. They also reported that an increase in confining pressure results in the initial elastic modulus increase and for confining pressure
more than 180 kPa (26.1 psi) the changes in elastic modulus is insignificant. A three-dimensional view plot of the effect of confining pressure and fouling on E50 is shown in Fig. 14. Regardless to moisture condition of ballast, by increase in breakdown fouling, the elastic modulus increases. This figure does not show a defined effect of confining pressure on the elastic modulus. Fig. 15 shows
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a two-dimensional view of Fig. 14 at constant water content. This figure indicates that the effect of confining pressure on the elastic modulus in the range used in this study is not significant although the effect of breakdown fouling is significant and clear. With the assumption of a linear relationship between E50 modulus and fouling percentage, with a 15% increase in breakdown fouling percentage, the modulus increases by 30% regardless of the confining pressure and water content conditions.
Fig. 12. Stress paths for different fouling conditions at field capacity water contents.
4.4.2. Effect of water content Previously, it was shown that the effect of confining pressure on E50 modulus is not significant, therefore the effect of water content has been shown at constant confining pressure in Fig. 16. Elastic modulus decreases linearly with the increase in water content (Fig. 16). This is the result of greater strain upon loading in moist ballast in comparison with dry ballast. The rate of this reduction is higher in clean ballast in comparison with fouled ballast, while it is hard to distinguish between the degradation rate of moderately fouled ballast (F15) from highly fouled ballast (F30). This is likely be the result of capillary tension among wet fine particles in fouled ballast and a lesser effect of water on contact area of ballast aggregates. Regarding to the results of this study, with a 1% increase in water content, E50 decreases 60%, 12% and 8% in clean, moderately and highly fouled ballast, respectively. Under a constant confining pressure, a 15% increase in fouling will result in approximately an 80% decrease in the rate of E50 reduction versus water content from clean to moderately fouled ballast and a 25% decrease in that rate for moderately to highly fouled ballast. 4.5. Dilatancy
Fig. 13. Changes of friction angle vs. fouling percentages at different water contents.
Fig. 14. Effect of fouling and confining pressure on the elastic modulus of clean and fouled ballast.
Previous studies have shown that the rate of volume change at failure decreases with an increase in confining pressure [5]. Fig. 17 shows a typical plot of volumetric strain versus axial strain for the clean ballast of this study at different water content and confining pressures. This figure clearly shows that regardless of the moisture and fouling conditions, by an increase in confining pressure the rate of volume strain versus axial strain decreases as expected. To study the effect of breakdown fouling and water content on the rate of dilation quantitatively, the slope of the line starting from the peak volume strain and ending at axial strain corresponding to failure has been evaluated at volume strain versus axial strain plots (Fig. 17) 4.5.1. Effect of water content Charles and Watts [7], reported that the larger dilation rates of granular materials are associated with the maximum principal stress ratios [7]. Also Indraratna et al. [12] used a hyperbolic fit to describe the relationship between the ballast dilatancy factor defined as Dp ¼ 1 ðdev =dea Þ [22] and the maximum principal stress ratio [12]. Although the dilatancy rate (dilatancy versus axial strain slope) used in this study, is increasing non-linearly with an increase in maximum principal stress ratio, it is difficult to distinguish between different water contents from this view (Fig. 18). Fig. 19 clearly shows the effect of the confining pressure increase on the volume strain rate reduction, which agrees with literature [5], and presents a better view of the effect of water content on the volume strain rate at constant fouling conditions. In clean ballast with the increase in water content, the dilatancy volumetric strain rate clearly decreases and the effect of confining pressure on the volumetric strain rate becomes stronger in samples with higher water contents (Figs. 17 and 19a). Water decreases the interlocking friction of ballast thereby affecting the dilatancy strain rate. In fouled ballast, the increase in water content from dry to
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Fig. 15. Effect of fouling on E50 modulus of ballast at different confining pressures and constant water content (a) dry (b) w1 (c) w2 (d) field capacity. Note: w2 has two studied points in moderately and highly fouled ballast.
field capacity conditions decreases the dilatancy volumetric strain rate with some scatter in the intermediate water content conditions (Fig. 19b and c). There is no evidence of a defined effect of water content on the volume strain rate-confining pressure relationship based on the results of this study (Fig. 19).
4.5.2. Effect of fouling Fig. 20 presents the change of maximum principal stress ratio versus dilatancy volumetric strain rate (%) for different fouling condition independent of water content conditions. Unlike water content (Fig. 18), fouling is significantly affecting the strengthdilatancy relationship. The best hyperbolic fit in Fig. 20, presents a higher dilatancy volumetric strain rate associated with a higher maximum principal stress ratio as the result of higher fouling conditions. Note that this relationship is independent of water content conditions. In comparison with Fig. 18 it can be concluded that the effect of fouling is more significant on the strength-dilatancy relationship than the water content in ballast. Regarding to Fig. 21, dilatancy volumetric strain rate increases with an increase in breakdown fouling percentages. This is the result of smaller void ratio of fouled ballast in comparison with clean ballast that leads the sample to higher rate of dilation after the initial compression. There is not a defined effect of breakdown
fouling percentage on the volume strain rate -confining pressure relationship based on the results of this study (Fig. 21).
4.6. Breakage degree There have been different methods used for quantifying ballast particle breakage degree using percent passing a single sieve size or changes in grain size distribution [12,13,19]. In order to evaluate the effect of water content, fouling percentage and confining pressure on breakage degree of ballast particles, both grain size distribution and passing a single sieve size methods are used. It is obvious that with greater breakage, there is a shift of the grain size distribution curve toward smaller particle sizes. Fig. 22 shows the effect of confining pressure on the shift of grain size distribution for clean and fouled ballast. Qualitatively, this figure shows that under a constant fouling condition, an increase in confining pressure will increase the shift in the grain size distribution curve toward smaller particle sizes, as expected [13,23]. These plots also indicate that by increase in breakdown fouling percentages the grain size distribution shift increases. This is as the result of higher density and more particle-to-particle contact area in ballast with higher fouling percentages that results in more particle split and corner degradation.
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Fig. 16. Changes of E50 modulus vs. water content at different fouling conditions (a) 5 psi confining pressure (b) 10 psi confining pressure (c) 15 psi confining pressure.
Fig. 17. Changes of volume strain versus axial strain for clean ballast.
Fig. 18. Maximum principal stress ratio versus dilatancy volumetric strain rate at different water contents.
It can also be seen that most of the breakage in grain size distribution in this study is taking place at grain size range between 1 and 20 mm (Fig. 22b and c). This is outside of the scope of this research but might be in agreement with the concept of studies correlating breakage to shape and sizes of particles [17].
The grain size distribution plots of moderately fouled ballast and highly fouled ballast also show that confining pressure changes between 5 psi (34.5 kPa) and 10 psi (68.9 kPa) has more effect on breakage degree than 10 psi (68.9 kPa)–15 psi (103.4 kPa) in this study.
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Fig. 19. The dilatancy volumetric strain rate versus confining pressure at different water contents and constant fouling conditions (a) clean ballast (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
To study the effect of water content on the rate of particle breakage, percent passing 3/800 sieve versus confining pressure at different water contents and constant fouling conditions was plotted (Fig. 23). The 3/8 in. sieve size was selected as the separator of fouling from ballast based on the fouling percentage definition [25]. Fig. 23 indicates that with the increase in water content the rate of breakage versus confining pressure decreases. This is as the result of reduction in particle-to-particle contact stress and frictional interlock with the increase in water content. Note that Fig. 23 only has plots for fouled ballast samples. That is because clean ballast samples cannot hold enough water on the particles surface to affect the particle-to-particle contact area. By 1% increase in water content, the rate of breakage versus confining pressure decreases in an average of 25–30% in moderately and highly fouled ballast. Fig. 20. Maximum principal stress ratio versus dilatancy volumetric strain rate at different fouling conditions.
The ranges of confining pressure showing increase in breakage degree in this study is also in agreement with the confining pressure ranges for Optimum Degradation Zone (ODZ) and Compressive, Stable Degradation Zone (CSDZ) defined by Indraratna et al. [13].
5. Results verification At low confining pressure, high principal stress ratios have been observed in granular materials. This behavior has been attributed to greater frictional interlock of particles at lower confining pressure [7,12,15,16]. Indraratna et al. [12] presented the following non-linear equation for the relationship between principal stress ratio and confining pressure:
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Fig. 21. The dilatancy volumetric strain rate versus confining pressure at different fouling conditions and constant water content (a) dry (b) w1(c) w2 (d) field capacity. Note: w2 has two studied points in moderately and highly fouled ballast.
Rp ¼ aðr3 Þb
ð1Þ
where Rp is the maximum principal stress ratio, a is the maximum principal ratio at r3 = 1, and b is a constant ratio related to degree of particle degradation. The changes of maximum principal stress ratio versus confining pressure of this study have been compared with the results from the other research on granular materials for verification (Fig. 24). The trend of the stress ratio changes versus confining pressure is in good agreement with the literature and shows a high R squared fitting Eq. (1) regardless of the moisture conditions. Fig. 24 also indicates that by increase in breakdown fouling, regardless of the water content from dry to field capacity condition, the maximum principal stress ratio increases. This is as the result of better packing of particles and was discussed in the previous sections. The changes of peak friction angle versus confining pressure for this study and other studies are presented in Fig. 25. This plot clearly shows that with an increase in confining pressure, the peak friction angle decreases. The results from this study is in good agreement in trend and magnitude with other studies. Fouling percentage was not found to have a significant effect on the reduction rate of friction angle by an increase in the confining pressure. The higher values of the friction angle at lower confining
pressure is as the result of interparticle contact stresses being well below the crushing strenght of granular materials [12]. 6. Conclusion Large scale static drained triaxial tests were performed on clean and breakdown fouled ballast at different water content conditions to evaluate the effect of these two parameters on deformation properties of ballast. Based on the triaxial test results, the effects of water content, fouling percentage and confining pressure on ballast deformation properties were discussed and evaluated qualitatively and quantitatively. Based on this study the following conclusions can be drawn: By increasing the water content, the maximum shear strength of ballast decreases linearly and the rate of this decrease is affected by confining pressure and fouling percentage. Interestingly, the increase in breakdown fouling does not result in the loss of strength at constant water content and water held by fouling is the main reason for ballast degradation. Independent from the confining pressure and water content conditions, approximately a 15% increase in breakdown fouling percentage results in a 30% increase in elastic modulus of ballast.
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Fig. 24. Effect of confining pressure on the maximum principal stress ratio for various granular materials and this study [12]. Note: The upper band is representing values obtained from tests on Basalt.
Fig. 22. Effect of confining pressure on the particle degradation curve at different fouling conditions (a) clean ballast (b) moderately fouled ballast (F15) (c) highly fouled ballast (F30).
Fig. 25. Effect of confining pressure on the peak friction angle for various granular materials and this study [12].
Fig. 23. Percent passing 3/800 sieve versus confining pressure at different water contents and constant fouling conditions (a) moderately fouled ballast (F15) (b) highly fouled ballast (F30).
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With the increase in breakdown fouling, the friction angle increases non-linearly and water content does not have a significant effect on the rate of this behavior. By increasing water content, the friction angle decreases and the rate of this reduction decreases by the increase in breakdown fouling. In the other words, the higher the fouling percentage, the lower slope of linear correlation between friction angle versus water content. Elastic modulus decreases linearly with an increase in water content. The rate of this reduction is higher in clean ballast in comparison to that for fouled ballast. The effect of fouling on the relationship between ballast strength and volumetric strain rate during dilation is more significant than water content. An increase in confining pressure results in an increase in ballast breakage. The higher the water content the less the rate of breakage versus confining pressure. Conflict of interest None. References [1] J.E.D. Alva-Hurtado, A methodology to predict the elastic and inelastic behavior of railroad ballast Ph D Dissertation, University of Massachusetts, 1980. [2] ASTM, Annual Book of Standards, Volume 04.08, Soil and Rock (II): D5714 – Latest, ASTM, Philadelphia, PA, 2008. [3] AREMA, Manual for Railway Engineering, American Railway Engineering & Maintenance-of-way Assoc, 2015. [4] B. Aursudkij, G.R. McDowell, A.C. Collop, Cyclic loading of railway ballast under triaxial conditions and in a railway test facility, Granular Matter 11 (6) (2009) 391–401, https://doi.org/10.1007/s10035-009-0144-4. [5] J. Billam, Some aspects of the behaviour of granular materials at high pressures, in: Paper Presented at The Proceeding of the Roscoe Memorial Symposium, Cambridge, 1971. [6] S.F. Brown, Repeated load testing of a granular material, J. Geotech. Eng. Div. 100 (GT7) (1974) 825–841. [7] J.A. Charles, K. Watts, The influence of confining pressure on the shear strength of compacted rockfill, Geotechnique 30 (4) (1980) 353–367. [8] S.M. Chrismer, in: Considerations of Factors Affecting Ballast Performance, American Railway Engineering Association, 1986, p. 87. [9] A. Ebrahimi, J.M. Tinjum, T.B. Edil, Large-Scale Cyclic Triaxial Testing of Rail Ballast, in: Paper presented at the Proceedings of the AREMA Annual Conference, Orlando, Florida, 2010.
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