Performance of Geogrid Reinforced Recycled Ballast under Dynamic Loading. Submitted to: Railway Maintenance Committee (AR060) Word Count:

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1 Performance of Geogrid Reinforced Recycled Ballast under Dynamic Loading Submitted to: Railway Maintenance Committee (AR060) Word Count: Number of Words in Text: 4245 Number of Figures: 10 Number of Tables: 3 Robert L. Parsons* Ph.D., P.E. Professor Civil, Environmental, & Architectural Engineering Department University of Kansas 2150 Learned Hall, 1530 W. 15 th Street Lawrence, KS rparsons@ku.edu (785) Milad Jowkar Geotechnical Engineer Berkel and Company Construction, Inc Marks Lane Austell, GA Miladjowkar@yahoo.com (913) Jie Han, Ph.D., P.E. Professor Civil, Environmental, & Architectural Engineering Department University of Kansas 2150 Learned Hall, 1530 W. 15 th Street, Lawrence, KS jiehan@ku.edu (785) *corresponding author August 1, 2012

2 ABSTRACT Railroad ballast consists of open graded crushed stone used to support railroad track and provide lateral stability. It provides drainable support for the track base and distributes the load to the weaker subgrade below. The issue of ballast performance has become more acute with increased traffic and heavier traffic. Over time ballast degrades and loses strength. Fouling of ballast is one cause of degraded performance and has been a major issue of railway engineering. Fouling can be caused by break down of ballast itself or intrusion of fines from below or from the environment. Fouling material can be removed by undercutting and the remaining ballast can be recycled back on to the rightof-way. It is possible to install a geogrid reinforcement layer in the ballast during an undercutting action. In this experimental study a full-scale railroad section five feet in length was constructed with and without geogrid reinforcement of recycled ballast. The track was dynamically loaded up to approximately 35 psi tie bearing pressure. The reinforced test section with geogrid placed 7 inches below the tie performed better than the unreinforced test section with regard to settlement and fouling of ballast. Settlement of the reinforced test section between the ties and geogrid was substantially less than the settlement of the same portion of the unreinforced test section. The percentage of rock dust and small diameter particles generated by ballast breakdown beneath the ties was observed to be less for the reinforced test section than for the unreinforced test section. Key Words: geogrid, recycled ballast, track settlement, cyclic loading, fouling 2

3 INTRODUCTION Railroad ballast consists of open graded crushed stone used as a drainable foundation element for track and to provide lateral stability for the track. Ballast performance has become a more important issue as heavier car loads and increased traffic place more demand on track structure than in the past. Over time ballast degrades from rounding and breakdown of ballast particles and from fouling. Fouled ballast degrades performance of the track section by reducing strength and inhibiting drainage. Five sources of fouling have been identified as follows: ballast breakdown; infiltration from the ballast surface; sleeper (tie) wear; infiltration from underlying granular layers; and subgrade infiltration (1). Major causes of ballast fouling include tamping, repetitive loading and vibration from trains, and contamination from both above and below the ballast level. Track maintenance operations are very time consuming and costly. Track maintenance can cost between 26 and 80 thousand dollars per mile of track (2). In addition, trains have become much heavier and faster than in the past, while the structural capacity of the majority of track sections has not been substantially improved. Minimization of maintenance actions would reduce railroad maintenance costs and impacts on traffic. Development of an effective method for extending the ballast life cycle would have significant economic value. The specific area of research addressed in this paper is extension of the life of recycled ballast by including a reinforcing layer during an undercutting maintenance action. During an undercutting action the upper portion of the ballast is removed, screened to remove fouling material, and returned to the right-of-way. The research in this paper focused on the benefits of reinforcing recycled ballast with geogrid to reduce fouling. Having a more stable 3

4 reinforced section would help reduce the number and frequency of maintenance actions required to keep the railroad in good repair. Geosynthetics are very durable polymeric products used for a wide range of civil engineering applications to provide reinforcement, layer separation, filtration, drainage, and containment functions (3). Use of geosynthetic materials has become more common in the past 40 years for a number of applications and they have the potential to reduce the cost of maintenance by increasing system design life. Geosynthetics can be categorized into eight different products as follows: geotextiles, geogrids, geonets, geomembranes, geosynthetic clay liners, geofoam, and geocells. The most common form of geosynthetic used for reinforcement is geogrid. Geogrids are a flexible polymeric product consisting of sets of parallel tensile ribs. The primary potential advantage of geogrids for the proposed application is extension of the maintenance cycle and life of the ballast through reinforcement of the ballast. Geogrid can also be used to reinforce the sub-ballast, which increases the bearing capacity of soft subgrade. Inclusion of geogrid has several potential benefits. It has the potential to more evenly and more widely distribute stresses to layers below. It has the potential to provide better confinement to ballast between the geogrid and the ties. Better confinement could improve stiffness and reduce ballast particle movement and breakdown. For sections with soft subgrades it also intercepts potential failure planes and could provide additional resistance to a bearing capacity type failure. These benefits would be reflected in reduced settlement of the ballast and subgrade and in a reduction of the rate of ballast breakdown. Some deformation is required to mobilize the strength of geogrid as it must be in tension to benefit the section. When compared with an unreinforced section, settlements for a reinforced 4

5 section will typically be similar initially, but as deformation continues strength of the geogrid will be mobilized and the reinforced section will outperform the unreinforced section. PREVIOUS WORK Degradation of ballast in railway engineering has long been an issue because it can lead to misalignment of the rails. Factors such as number of load cycles, gradation of aggregates, track confining pressure, and angularity and fracture of individual grains of ballast can cause ballast degradation and deformation. Research on ballast has been ongoing for a number of years. Selected recent findings are discussed in this section. Fouling of ballast in the railroad industry occurs as the voids within the ballast are filled with finer particles. Accumulation of coal dust in ballast is a primary concern for railroad engineers. Tutumluer (4) mixed ballast with coal dust at a series of different moisture contents and coal dust weight percentage. He reported when the ballast was fully fouled with coal dust content of 25 percent by weight and with a 35% moisture content, the friction angle of ballast was approximately the same as the friction angle of coal dust. Laboratory work by Grabe (5) shows a significant stress reduction on the subgrade for different depths to the top of treatment using several types of geosynthetics. Stress reductions observed for geogrids ranged from 34 to 66% for depths up to 12 inches (600 mm). Indraratna et al (6) conducted a series of tests to measure the settlement, vertical strain, and lateral strain of reinforced (geogrid-geotextile) recycled ballast in wet and dry conditions. The use of geogrid-geotextile resulted in an increase in the bearing capacity and resilient modulus of recycled ballast. The results also indicated a decrease in degradation and lateral movement of ballast. A finite element analysis was conducted to determine the optimum depth at which the 5

6 geosynthetic is the most effective. It was found that optimum depth for geosynthetics is in the range of 150 to 200 mm (6 8 inches) beneath the ties. Indraratna et al (7) also conducted series of tests using fresh ballast, recycled ballast, fresh ballast with geocomposite, and recycled ballast with geocomposite. The geocomposite was placed between the sub-ballast and ballast. A reduction in the average vertical deformations of recycled ballast for a large number of cycles when reinforcement was included was observed. Davis et al (8) tested biaxial geogrid in the ballast of mainline track beneath insulated joints at the Facility for Accelerated Service Testing (FAST) in Pueblo, Colorado. Their preliminary results showed that installing geogrid within fouled ballast reduced settlement to the amount observed for clean ballast, and reinforced clean ballast settled less than unreinforced clean ballast. TEST SECTION DESIGN Two full-scale railroad sections five feet in length were constructed. One contained geogrid reinforcement and one did not. Figure 1a illustrates how the lab test sections were constructed and Figure 1b shows the constructed section. A full-scale trapezoidal cross section of a railroad was built which consisted of a 2 ft deep subgrade covered with 2 feet of recycled ballast that was sieved to remove fouling material. These layers had a side slope of 1.5:1. For the reinforced section a layer of geogrid was installed 10 inches above the subgrade (7 inches below the bottom of the ties) within the recycled ballast. This depth was chosen because it was considered the optimal depth based on work by Indraratna (7). A track panel five feet in length and with three wooden ties (7 x 9 x 8.5 ) was used for the track section. Additional unsieved recycled ballast was placed on the side slopes such that the final side slopes were 2:1. 6

7 ballast (with fines) to achieve 2:1 slope 2 screened ballast subgrade geogrid (reinforced test) ballast (with fines) to achieve 2:1 slope Figure 1a. Railroad cross section Figure 1b. Test setup Subgrade The subgrade was constructed of locally available CH (highly plastic) clay. Properties of the clay are presented in Table 1. 7

8 Table 1. Subgrade soil properties Classification (Unified Soil Classification System, USCS) CH LL ( liquid limit) 52 PL (plastic limit) 21 PI (plasticity index) 31 % < 0.075mm (#200) 51 % % < 0.002mm 39 % Max dry unit wt lb/ft 3 Optimum Moisture 23 % Undrained shear moisture 630 psf Undrained shear moisture 1900 psf Undrained shear moisture 3300 psf The subgrade was placed with a skid loader in 6 inch lifts. Each lift was compacted with a vibratory plate compactor. Unreinforced and reinforced sections were compacted wet of optimum moisture with dry unit weights of 91.5 lb/ft 3 and 93 lb/ft 3 respectively. Instrumentation Tell-tales were used to measure vertical deformation in soil profiles at selected depths. As the load was applied to the section, the base plate and the interior pipe moved with the soil at the base plate elevation. The exterior pipe minimizes interference from the soil or ballast above the base plate. Two tell-tales were set at the top of the subgrade (1 and 2) and the other two (3 and 4) were set 7 inches below the ties (depth of the geogrid) for the unreinforced and reinforced tests. Tell-tale locations are shown in Figure 2. Five pressures cells (Geokon, Model 3515) were placed between the subgrade and ballast beneath the railroad ties as shown in Figure 2. 8

9 Top View Tie Telltales N E Rail Figure 2. Pressure cell and tell-tale locations Two string potentiometers (string pots) with a maximum range of 20 inches were used to measure the displacement of center ties in the railroad section. Four displacement transducers were placed at the ends of the rails for redundancy. Quality control tests Three sets of quality control tests (moisture/density, lightweight deflectometer or LWD, California bearing ratio or CBR) were performed before and after each test in order to verify the compaction quality, modulus, and CBR of subgrade. Drive tubes were used to determine the level of compaction of the subgrade. The average dry unit weight for the subgrade based on an average of four samples was 91.5 lb/ft 3 for the unreinforced test (92% of Proctor) and 93lb/ft 3 for the reinforced case (93% of Proctor). Moisture contents were 26 and 27 percent for the unreinforced and reinforced tests, respectively (9, 10). LWD is a test method for measuring deflection and compaction quality control during construction. LWD also measures the degree of compressibility and average settlement of the section. The LWD tests were carried out on the subgrade with the 30 cm (12 in) plate since this 9

10 plate is suitable for fine soil. This device has a 10 kg drop hammer with a drop height of 1 meter. For the LWD test the device hammer is released which hits the plate. Acceleration is measured with time and a modulus is calculated based on the force required to generate a given deflection for that soil type. The LWD was used before and after testing of both the reinforced and unreinforced test sections to find the modulus of elasticity of the subgrade section at six locations. The average modulus results for the six locations were consistent and are shown in Table 2. Table 2. Quality Control Data Measurement Unreinforced Reinforced Unit Weight Before Test (lb/ft 3 ) Number of Cycles Moisture Content Before Test Moisture Content After Test % Proctor Before Test 92% 93% LWD Modulus Before Test, E vd MN/m LWD Modulus After Test, E vd MN/m The DCP (dynamic cone penetrometer) was used to measure the in-situ strength of subgrade before and after each test. Results from DCP were then converted to CBR values using the following correlation (11): CBR% = 292/(DPI) 1.12 DPI units = mm/blow 10

11 CBR values were estimated from dynamic cone penetrometer data from six locations before and after the unreinforced and reinforced tests. The average CBR value was approximately 2.0 through most of the subgrade for all tests but was slightly higher near the surface. The slightly higher values near the surface may have been due to drying of the surface prior to completion of DCP testing. Ballast Recycled ballast was provided by the BNSF and came from track undergoing maintenance in Gardner, Kansas. This ballast was composed of heterogeneous igneous rock. It was sieved to remove fouling material and to meet BNSF gradation specifications. The grain size distribution after sieving is shown in Figure 3 with the gradation range in use by BNSF. All sizes were within the range gradation sizes recommended by BNSF except for the 0.75 inch material, which exceeded the limit by 1%. After review it was the judgment of the authors and BNSF that the sieved ballast was essentially consistent with their specifications and would be accepted for use. Geogrid Triaxial geogrid (TX190L) produced by Tensar International Corporation was used to reinforce the ballast section in the reinforced test. The grid was placed in the ballast 7 inches below the bottom of the ties. 11

12 Sieve Size(in) % KU Ballast 90% BNSF Lower Bound 80% BNSF Upper Bound 70% Percent Passing 60% 50% 40% 30% 20% 10% 0% Figure 3. Sieve analysis for ballast DYNAMIC LOADING RESULTS Two dynamic loading tests were conducted on full scale railroad sections to investigate the effects of triaxial geogrid on reducing settlement and ballast degradation. The first test was conducted on an unreinforced control section. The second test was conducted on a section with geogrid reinforcement. Figure 1b showed the test setup as it appeared for both tests. Loading plan Dynamic loading was conducted in five steps with total track panel loads of near 100,000 lb. Loading for the reinforced test stepped up in a similar manner but the actual loading levels were slightly higher for the reinforced case for all steps except for step 3. The highest loading level 12

13 was 5.9 percent greater for the reinforced test than for the unreinforced test. Step loading values are presented in Table 3 along with average tie bearing pressure values. Reported load and pressure values are averages for the load step. For this research the average bearing pressure was calculated by dividing the total load by the area of the ties. Actual pressures immediately beneath the ties may have varied with location; however these variations should not have affected one test section relative to the other. Figures 4a and 4b show the deformation with dynamic loading on the east side of the middle tie for the unreinforced and reinforced tests. Table 3. Loading information Load Step Cycles Loading Rate (Sec/Cycle) Unreinforced Test Target Supply Pressure (psi) Actual Average Total Load (lb) Tie Bearing Pressure (psi) (dry) (soaked) Load Step Cycles Loading Rate (Sec/Cycle) Reinforced Test Target Supply Pressure (psi) Actual Average Total Load (lb) Tie Bearing Pressure (psi) (dry) (soaked)

14 0 Pump Pressure (psi) Deformation(in) East String Pot Figure 4a. Middle tie settlement versus pump pressure (east string pot, unreinforced test) Pump Pressure (psi) Deformation (In) String Pot East Figure 4b. Middle tie settlement versus pump pressure (east string pot, reinforced test) 14

15 For a typical load step, more permanent deformation was observed for the early cycles and especially during the first loading cycle where there was a significant amount of unrecovered deformation. As the number of cycles increased, less permanent deformation occurred and a higher percentage of the deformation that did occur was elastic. Behavior after soaking Fifty gallons of water were added to the section by garden hose over period of 15 minutes, which was the equivalent of approximately 1inch of rainfall at the subgrade elevation. The section was left overnight for water to penetrate into the soil. One-hundred cycles were applied to the railroad section the following day. The average total accumulated deformation after 100 cycles was 6.10 in for the unreinforced section and 4.99 in for the reinforced section. As with the earlier steps, the beginning cycles experienced more permanent deformation per cycle. As the number of cycles increased a higher percentage of the deformation was elastic. Subgrade pressures and surface settlement Figure 5 shows the pressure cell response versus tie bearing pressure for the unreinforced test. Pressure cell 1 measured a maximum pressure of 19 psi, which was the highest vertical stress observed for the unreinforced test. When subgrade pressure at the location of pressure cell 1 reached 18 psi, the subgrade began to yield. Pressure cell 4 experienced mechanical failure at a pressure of 12 psi. The subgrade did not experience failure at the locations of pressure cells 2, 3, or 5 until it was soaked and reloaded. Pressure cell 3 experienced lower pressure due to the movement of the tie or the subgrade beneath the pressure cell. 15

16 Figure 6 is a similar figure for the reinforced test. Data was recorded for pressure cells 1, 2, and 3 throughout the test. Partial data was recorded for pressure cell 5 prior to its failure. For this test, unlike the unreinforced case, the subgrade did not fail at any location until it was soaked and higher maximum pressures were recorded during the reinforced test than during the unreinforced test for all three locations for which there was complete data. Pressure Cell Pressure (psi) Pressure Cell 1 Pressure Cell 2 Pressure Cell 3 Pressure Cell 4 Pressure Cell Tie Bearing Pressure (psi) Figure 5. Pressure at top of subgrade vs. tie bearing pressure in the unreinforced section Figures 7 and 8 show the settlement that occurred for the middle tie based on the number of cycles and with tie bearing pressure. Both of these figures show that settlements were similar for early cycles/low loading conditions, but as the demand on the structure increased the unreinforced section settled more. There was a particular change in settlement for the soaked step with the unreinforced case settling more than two inches during this step, while the reinforced section settled substantially less despite being subjected to a slightly higher load. 16

17 Pressure Cell Pressure (psi) Pressure Cell 1 Pressure Cell 2 Pressure Cell 3 Pressure Cell Tie Bearing Pressure (psi) Figure 6. Pressure at top of subgrade vs. tie bearing pressure in the reinforced section When considered in terms of traffic required to cause a specific settlement, the number of cycles required to cause 0.75 inches of settlement was essentially the same (100 cycles, Figure 7). However the number of cycles required to cause 3 inches of settlement was approximately 250 for the unreinforced case and 300 for the reinforced case, a 20% improvement. There is a slightly larger difference in total settlement between the two tests for load step 3 when compared with load step 4 in Figure 7; This is likely due to a slightly higher pump pressure for the unreinforced case compared with the reinforced case for Step 3 and a slightly lower pressure for the unreinforced case compared with the reinforced case for Step 4. 17

18 The observed settlement behavior was consistent with the theory of geogrid reinforcement, which normally requires some deformation (settlement) to occur before the strength of the geogrid is mobilized. The contribution of the geogrid should increase as deformations increase. Settlement (in) Number of Cycles Load for reinforced section was 4.4% lower than for the unreinforced section Load for reinforced section was 5.2% higher than for the unreinforced section Unreinforced East String Pot Reinforced East String Pot Figure 7. Number of cycles vs. track settlement (east string pot) 18

19 Track Settlement (in) Tie Bearing Pressure (psi) water applied prior to next step 6 7 unreinforced reinforced Figure 8. Deformation vs. actual tie bearing pressure at later cycles (average of east and west) Tell-Tales and Internal Sources of Settlement When the information for the inner tell-tale tubes is combined with the overall settlement information, the settlement within the different layers can be calculated. The settlements for the ballast above the geogrid depth, below the geogrid depth, and for the subgrade are shown in Figure 9, along with the strain experienced by each layer. Given the large particle sizes involved and minor variation in placement, some natural variation in measurements is to be expected; the general trends, however, are clear. Average settlement of the ballast between the base of the tie and the geogrid was 2.0 in., or 28% strain for the unreinforced section, which was substantially more than the strain in the reinforced section (18%). If the settlements of the east string pot only are considered, which was physically closer to tell-tales 3 and 4 (geogrid level), the settlements and strains between the base of the tie and the geogrid level were 1.8 inches (26 percent strain) for the unreinforced section and 0.6 inches (9 percent strain) for the reinforced section. These 19

20 strains have not been adjusted for the heavier loading on the reinforced section. If adjusted, the differences in strain would be slightly larger. For the lower portion of the section there was no observed benefit as the reinforced section settled slightly more than the unreinforced section. Settlement of the subgrade was 0.6 in. less for the reinforced section. This may be due to random variation, or the geogrid may have contributed to this reduction in settlement by distributing the load more widely; however, the authors believe this possibility requires more testing for confirmation. The subgrade for the reinforced test was also observed to have less waviness, or differential movement, when the reinforced test was dismantled. Reinforced Unreinforced 5.0" 18 % Strain 3.8" 6.1" 1.2" 7" 2.0" 28 % Strain upper ballast 4.1" 14 % Strain 1.4" 10" 1.1" 11 % Strain 2.4" lower ballast 3.0" 10% Strain 24" 13 % Strain subgrade Figure 9. Average settlements and strains by layer 20

21 A series of ballast samples was taken after each test from different locations in an attempt to determine if the presence of the geogrid reduced the breakdown of ballast. Effective sampling of the ballast material was challenging, as disturbance from the act of sampling caused some loss of fines to lower levels within the ballast. It is also possible that some segregation of material occurred during handling and placement of the ballast, particularly with respect to the ballast for the reinforced test, as there was additional handling of this material. These factors can be shown to have impacted some of the results, and therefore grain size distribution results (shown in Figure 10) should be viewed with caution. For example, the samples near the subgrade showed almost no small diameter material, which was not consistent with the visual observations. These distributions are not included. These results do show, however, that of all samples, the sample taken below the tie for the unreinforced test had the highest percentage of material smaller than ½ inch in diameter. Additionally, the percentage of material smaller than ½ inch located below the tie for the reinforced test was among the lowest of all the samples, which is consistent with the visual observation. These results suggest that the presence of the geogrid helped to lock the ballast structure in place and limited crushing and grinding action from particle movement that would generate rock fines. 21

22 % 90% 80% 70% Percent Passing 60% 50% 40% 30% 20% 10% 0% Sieve Size(in) Original material after sieve Unreinforced under tie Reinforced under geogrid Reinforced under tie Figure 10. Grain size distribution of ballast General Observations 1. Unevenness of the subgrade after both tests was observed, but, based on visual inspection, was greater after the unreinforced test. 2. During excavation after both tests, the ballast was damp from the subgrade level up to 8 in. below the tie, apparently due to the presence of fines generated by ballast breakdown that retained a portion of the water from the soaking step. 3. A higher percentage of small diameter material was observed beneath the ties after the unreinforced test than after the reinforced test. 4. The ballast penetrated the subgrade slightly in both tests (approximately 1 in). 5. The amount of fines that accumulated close to the subgrade level was substantial. 22

23 CONCLUSIONS Full scale testing of railroad sections constructed with unreinforced recycled ballast and reinforced recycled ballast yielded the following observations: The reinforced section settled less than the unreinforced section. Settlement of the ballast between the ties and geogrid was substantially less than the settlement of the equivalent portion of the unreinforced test section. The observed reduction in settlement was more significant for larger deformations, which is consistent with normal geogrid mobilization behavior. For settlements of more than 1 inch, more loading cycles (traffic) were required to cause the same amount of settlement. More ballast breakdown resulting in the generation of small particles and dust was observed for the ballast beneath the ties in the unreinforced test than in the reinforced test. The reinforced test section subgrade supported more load prior to subgrade failure than the unreinforced test section. Based on these observations it was concluded that ballast reinforcement provided benefits with regard to reduction of fines from the grinding and crushing of ballast and a reduction in the settlement of the ballast. These benefits became significant after some initial settlement that was required to mobilize the geogrid. For sections in service such benefits should result in a reduction in the frequency of maintenance activity. Additional research on the optimal location of reinforcement and benefits of different geogrids for track in service may yield additional benefits. 23

24 ACKNOWLEDGMENT The authors thank the Mid-America Transportation Center (MATC) for providing financial support for the research described in this paper, and Mr. Hank Lees of BNSF and Dr. Mark Wayne of Tensar International Corporation for providing materials and technical support for this research. Their support is greatly appreciated. REFERENCES 1. Selig, E.T. and Waters, J.M. (1994). Track Geotechnology and Substructure Management. Thomas Telford. London. 2. Zarembski, Allan M. and John F. Cikota, Jr. Estimating Maintenance Costs for mixed High-Speed Passenger and Freight Rail Corridors. TR News, 225, March-April 2008: Koerner, Robert M. Designing with Geosynthetics. Prentice Hall. Upper Saddle River, NJ Tutumluer, E., Dombrow, W., and Huang, H. (2008). Laboratory Characterization of Coal Dust Fouled Ballast Behavior. Draft Manuscript Submitted for the AREMA 2008 Annual Conference & Exposition September 21-24, 2008, Salt Lake City, UT. 5. Grabe, H. (2010). Geosynthetics for Improving Poor Track Formations. TRB 2010 Committee presentation. Web. No longer posted. 6. Indraratna, B., Shahin, M.A., Rujikiatkamjiron, C., Christe, D. (2006). Stabilization of Ballasted Rail Tracks and underlying soft formation soils with geosynthetic grids and drains. University of Wollongong

25 7. Indraratna, B., Rujikiatkamjorn, C. and Nimbalkar, S. (2011) Use of Geosynthetics in Railways including Geocomposites and Vertical Drains. GeoFrontiers 2011: Advances in Geotechnical Engienering. Proceedings of the Geo-Frontiers Conference, Dallas, Texas. Geotechncial Special Publication 211. ASCE. Reston, Virginia. pp Davis, D.D., J.P. Hyslip, C.L. Ho, and V.R. Terrill. (2008). Evaluation of Reinforced Ballast for Foundations in Rail Joints and Special Trackwork. Technology Digest. American Association of Railroads. TD pp. 9. Jowkar, M. (2011) Performance of Geogrid Reinforced Ballast under Dynamic Loading. Master s Thesis, University of Kansas. 117pp. 10. Parsons, R.L., M. Jowkar, and J. Han. (2012). Performance of Geogrid Reinforced Ballast under Dynamic Loading. Report No Mid-America Transportation Center University of Nebraska-Lincoln. 121pp. 11. Webster, S.L., R.H. Grau, T.P. Williams. (1992). Description and Application of Dual Mass Dynamic Cone Penetrometer. USAE Waterways Experiment Station, Vicksburg, MS, Instruction Report GL