EFFECTS OF PARALLEL GRADATION ON STRENGTH PROPERTIES OF BALLAST MATERIALS. Domenica Cambio 1, and Louis Ge 2

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1 EFFECTS OF PARALLEL GRADATION ON STRENGTH PROPERTIES OF BALLAST MATERIALS Domenica Cambio 1, and Louis Ge 2 1 University of Naples Federico II, Department of Geotechnical Engineering, Via Claudio, , Napoli, Italy; domenicacambio@virgilio.it 2 University of Missouri-Rolla, Department of Civil, Architectural, and Environmental Engineering, 187 Miner Circle, Rolla, MO 6549; PH (573) ; FAX (573) ; geyun@umr.edu ABSTRACT: The railroad ballast is used to fill in irregular surface topology, distribute and transfer loads from a surface structure or system to the subgrade or subsoils as uniformly and widely as possible in order to provide stable and stiff longterm embankment support for railways. A typical ballast grain sizes range from 3 to 7 mm, which makes large-scale laboratory tests difficult to conduct. The parallel gradation technique is to preserve the particle shape, particle surface roughness, and particle mineralogy, and creates a parallel gradation of soil with a maximum particle size for the available apparatus. This paper presents the result of a series of monotonic direct shear tests for three ballast materials having parallel gradation curves, which are served as background study for the ongoing research on validating it under loading-unloading condition. INTRODUCTION Testing and modeling constitutive behavior of roadbeds and subgrade materials under traffic loading has been a challenging task for geotechnical engineers. A typical example include railroad ballast, usually comprised of highly coarse-graded gravel-size particles, such as crushed or fractured rock or aggregates, with grain sizes in the range of 3 to 7 mm. The ballast is used to fill in irregular surface topology, distribute and transfer loads from a surface structure or system to the subgrade or subsoils as uniformly and widely as possible in order to provide stable and stiff longterm embankment support for railways. While ballast is typically deposited or placed at variable packing densities, it is expected to behave elastically and exhibit minimal stiffness and strength degradation over long time periods, and a large number of repeated load cycles. The mechanical behavior of ballast or rockfill materials have been studied for decades (Marachi et al. 1972; Raymond and Diyaljee 1979; Janardhanam and Desai 1

2 1983; Indraratna et al. 1998). These material properties are found governed by the factors including particle size, particle shape, surface roughness, parent rock strength, particle crushing strength, particle size distribution, density, degree of saturation, confining pressure, load history, and number of load cycles (Indraratna and Salim 25). The ability of the parallel gradation physical analog model was investigated and validated by Jernigan (1998) for Swedish railroad ballast, which is linearly graded granular material, ranging from 32 to 64 mm in size. He concluded that the use of parallel gradation method is to preserve the particle shape, particle surface roughness, and particle mineralogy, and creates a parallel gradation of soil with a maximum particle size for the available apparatus. Varadarajan et al. (23) reported that there are four techniques used to reduce the size of the large-sized crushed rock materials, and the parallel gradation technique was found most suitable. All the previous work on parallel gradation technique was done under monotonic loading condition, but there are issues, such as attrition, and particle angularity, which have not been addressed under the circumstances of cyclic loading. This paper presents the result of a series of monotonic direct shear tests for three ballast materials having parallel gradation curves, which are served as background study for the ongoing research on validating it under loading-unloading condition. EXPERIMENTAL SETUP Direct Shear Apparatus The direct shear apparatus has both an upper and lower shear boxes, and the sample is sheared along the plane between them by pushing the lower shear box horizontally with a normal (vertical) load applied to it. The shear force is measured with a load cell that is attached between the normal load actuator and the top of the shear box. The test was conducted with two stages, consolidation and shear loading. The first one consists in the application, through normal actuator, of a normal stress to that one investigated. The duration of the consolidation stage was 1 minute. The second stage consists in the application, through shear actuator deformation, of a shear displacement with a rate of 1 mm/min. The duration of the shear load stage was defined by the maximum shear deformation which 15 minutes. Modeled Ballast Materials The ballast materials were shipped from the Iron Mountain Trap Rock Company, MO, which provides 3 and 4A mainline ballast to railroad industry. Smaller size of ballast materials were also available from the site and were used to manufacture 3 sets of materials (M1, M2, and M3) having parallel gradation curves, as shown in Figure 1, to the prototype ballast. Before manufacturing the M1, M2, and M3 materials, their parallel gradation curves were determined and chosen carefully so that as many as sieving pans can be used. This is critical for the material preparation to get a smooth and parallel gradation curve as the prototype ballast. A total of 18 direct shear tests are reported in the papers as listed in Table 1, where 3 constant vertical stress levels and 2 density states were chosen. The initial void ratios for dense and loose specimens were determined by trial and error. Also, the relative densities for both dense and loose specimens remain unknown due to the lack 2

3 of material information. The maximum and minimum void ratios will be determined in the next phase of the project. 1 CLAY SILT SAND GRAVEL ROCK 9 8 M1 7 % Passing M M Grain size [mm] FIG. 1 Grain size distribution curves for M1, M2, and M3 materials. TABLE 1. Direct shear testing program. MONOTONIC DENSE TESTS MONOTONIC LOOSE TESTS MATERIAL DENSE VERTICAL STRESS INITIAL VOID FILENAME MATERIAL LOOSE VERTICAL STRESS INITIAL VOID FILEMANE [-] [kpa] [-] [-] [-] [kpa] [-] [-] M1S2D M1S2S M1S4D M1S4S M1S8D M1S8S M2S2D M2S2S M2S4D M2S4S M2S8D M2S8S M3S2D M3S2S M3S4D M3S4S M3S8D M3S8S EXPERIMENTAL RESULTS There are 9 normalized shear stress versus shear displacement curves for dense material shown in Figure 2. The families of red, green, and blue curves represent M1, M2, and M3 materials, respectively. For each family of curves, the thicker line stands for the data with higher vertical stress. It is found that the stress ratio τ/σ reaches the peak at about 1.8 mm shear displacement for both M1 and M2 while 2.5 mm for M3. For each material, the lower the vertical stress, the higher the stress ratio at the peak. Figure 3 shows the normalized shear stress versus shear displacement 3

4 curves for loose materials. The same color scheme was adopted for M1, M2, and M3, respectively. It is also found that for each material, the lower the vertical stress, the higher the stress ratio at the ultimate state. These 18 tests were performed by using the fresh materials, whose gradation curves displayed in Figure 1. Three bowls (for M1, M2, and M3, respectively) were prepared to collect the material after each test. Three sieve analyses were then carried out to determine the grain size distribution curves. Figure 4 shows the gradation curves for the M1, M2, and M3 materials. The red curves denote the gradation curves before the direct shear tests while the blue curves represent the curves after the direct shear tests. As seen in Figure 4, the grain size distribution curves did not change much at all after the monotonic direct shear tests, and the curves for M1, M2, and M3 remain parallel. Table 2 summarizes the friction angles from the 18 tests. It is worth noting that the peak friction angles for the dense M1 and M2 are both about 36 o although their ultimate friction angle differs 4 o (17.4 o and 12.8 o ). This somehow proves the parallel gradation technique valid. However, the ultimate friction angle for loose M1 is 6 o lower than the angle for loose M2, which shows invalidation of the parallel gradation technique. Compared to M1 and M2 materials, M3 behaved quite differently in terms of peak and ultimate friction angles, which can conclude that the parallel gradation technique is not working well. 1.2 M1S2D 1 M2S2D τ/σ.8.6 M3S2D M1S4D M2S4D.4 M3S4D M1S8D.2 M2S8D δ T [mm ] M3S8D FIG. 2 Normalized shear stress versus shear displacement curves for dense materials. 4

5 1.2 1 M1S2S M2S2S τ/σ.8.6 M3S2S M1S4S M2S4S.4.2 M3S4S M1S8S M2S8S δ T [mm ] M3S8S FIG. 3 Normalized shear stress versus shear displacement curves for loose materials. 1 CLAY SILT SAND GRAVEL ROCK 9 M1 AFTER 8 7 M1 BEFORE % Passing M2 AFTER M2 BEFORE 3 2 M3 AFTER Grain size [mm] M3 BEFORE FIG. 4 Grain size distribution curves for M1, M2, and M3 (before and after the tests). 5

6 TABLE 2. Peak and ultimate friction angles for M1, M2, and M3 materials. Materials Dense Loose φ peak φ ultimate φ ultimate [-] [ ] [ ] [ ] M M M CONCLUSIONS A total of 18 monotonic direct shear tests were conducted to validate the parallel gradation technique. The materials, namely M1, M2, and M3, were prepared from the railroad ballast manufacturer. The gradation curves for M1, M2, and M3 are parallel to the prototype railroad ballast. From the test results, it is found that parallel gradation technique works well for M1 and M2 materials. M3 behaved differently than M1 and M2 although they all have parallel gradation curves. Angularity and particle crushing (attrition) due to loading condition are believed to be two major factors influencing the validity of the parallel gradation technique. A digital imaging technique has been proposed to re-visit the M1, M2, and M3 materials. A quantified measure will be defined to account for the particle angularity for the parallel gradation technique. Attrition can be assessed by comparing the gradation curves. Since the gradation curves did not change much from the test results, it is believed that the particle angularity is the main factor causing the parallel gradation technique not working with M3 material. REFERENCES Indraratna, B., Ionescu, D., and Christie, H.D. (1998). Shear behavior of railway ballast based on large-scale triaxial tests. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 124(5), Indraratna, B., and Salim, W. (25). Mechanics of Ballasted Rail Tracks A Geotechnical Perspective, Taylor & Francis. Janardhanam, R., and Desai, C.S. (1983). Three-dimensional testing and modeling of ballast. Journal of Geotechnical Engineering, ASCE, 19(6), Jernigan, R.L. (1998). The Physical Modeling of Soils Containing Oversized Particles. Ph.D. thesis, University of Colorado at Boulder. Marachi, N.D., Chan, C.K., and Seed, H.B. (1972). Evaluation of properties of rockfill materials. Journal of Soil Mechanics and Foundation Engineering, ASCE, 98(1), Raymond, G.P., and Diyaljee, V.A. (1979). Railroad ballast sizing and grading. Journal of Geotechnical Engineering, ASCE, 15(5), Sitharam, T.G., and Nimbkar, M.S. (2). Micromechanical modeling of granular material: effect of particle size and gradation. Geotechnical and Geological Engineering 18,

7 Varadarajan, A., Sharma, K.G., Venkatachalam, K., and Gupta, A.K. (23). Testing and modeling two rockfill materials. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 129(3),