THE EFFECT OF REALIGNMENT ON TRACK LATERAL STABILITY

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1 Paper prepared for presentation at the AREMA 1999 Track & Structures Annual Conference, September 12-15, 1999 in Chicago THE EFFECT OF REALIGNMENT ON TRACK LATERAL STABILITY Gopal Samavedam Foster-Miller, Inc., Waltham, MA Telephone: (781) FAX: (781) Andrew Sluz USDOT/Volpe Center, Cambridge, MA Telephone: (617) FAX: (617) Andrew Kish USDOT/Volpe Center, Cambridge, MA Telephone: (617) FAX: (617) ABSTRACT Maintenance of railroad track alignment, where continuous welded rail (CWR) is deployed, can impact track lateral stability, especially in areas where thermal track buckling is a potential problem. This paper documents the changes in rail neutral temperature and tie lateral resistance resulting from operations associated with realignment of a 1 curve. The measurements of rail longitudinal force changes demonstrated that the standard procedure used for the realignment produced a reduction in neutral temperature at all locations. The de-stressing procedures employed in these operations during cold winter temperatures, represented current accepted practice, but failed to develop the desired target neutral temperature of 95 Fahrenheit. The CWR-BUCKLE computer program developed by the John A. Volpe National Transportation Systems Center (Volpe Center) for the Federal Railroad Administration (FRA) was used to estimate the safety of the track against buckling after the realignment using actual measured track parameters. Track lateral resistance was measured using single tie push tests (STPTs) before maintenance, after tamping and lining and after dynamic track stabilization (DTS). The measurements showed that lateral resistance increased 4% from the post tamping value after dynamic stabilization. Based on the measured data, the test curve was determined to be within the safe region with respect to thermal buckling, but the drop in the factor of safety resulting from the maintenance could have created a potential hazard if parameters measured or assumed were slightly worse. The results discussed in this paper provide new information concerning the effect of realignment, de-stressing and dynamic track stabilization on CWR track, namely that curve realignment does change the neutral temperature, and post maintenance lateral strength restoration in conjunction with DTS was less than expected. The measurement and analysis were conducted as a cooperative effort with Amtrak under the FRA Track Systems Research Program. 1

2 1 Introduction Neutral temperature and track lateral resistance are the two primary parameters which determine the lateral buckling potential of continuous welded rail (CWR) track at a given maximum rail temperature. These parameters can change due to maintenance and service operations. Resurfacing, for example, can have a large impact on lateral resistance and rail neutral temperature [1], [2]. The Federal Railroad Administration (FRA) Track Research Division of the Office of Research and Development has sponsored the development of computer tools such as CWR-BUCKLE and CWR-INDY [3] to determine the buckling safety of specific track sections of known characteristics. Recently, a field test was conducted on an Amtrak curve near Foxboro, Massachusetts to determine the extent the neutral temperature was affected by curve realignment, and to determine whether the resulting drop in neutral temperature and lateral resistance affected the thermal buckling potential of the concrete tie track. Measurements of the change in rail axial force and track lateral resistance were made and CWR-BUCKLE was used to determine the change in buckling safety. A description of the work performed, results from the test measurements and conclusions as to the buckling safety of the track are presented below. 2 Background Most railroads impose speed restrictions to reduce risk on tracks after maintenance has caused substantial ballast disturbance. This is particularly true when the track has been tamped or realigned in the summer, and the possibility of thermal track buckling is high. The slow orders mitigate risk in two ways. The potential for a thermal buckle is less at a speed regime slower than normal service. Secondly, if a buckle were to occur, the damage caused by the derailment is likely to be less severe. Even when maintenance is performed in the winter, it is important to determine the post-maintenance neutral temperature to evaluate whether thermal buckling might be a problem when temperatures rise. For an assurance of safety, it is necessary to evaluate the buckling strength of CWR track directly after any maintenance. If the buckling strength is not adequate, speed restrictions may have to be imposed until the track gains adequate lateral strength through traffic or other means, e.g. de-stressing or ballast consolidation. It is also important to determine whether the neutral temperature is adequate for the expected high rail temperature in the coming year. If not, then the neutral temperature needs to be adjusted. The following are important issues to be addressed within the context of evaluations of track buckling strength after maintenance activity. i. What is a reliable methodology to determine the buckling strength of freshly maintained track? 2

3 ii. Does the CWR track need vehicle speed restrictions soon after the maintenance to minimize the damage in the event of potential buckling risk? iii. If speed restrictions cannot be imposed, what are the available means of increasing the buckling strength? This paper addresses the first issue on the methodology for the assessment of the buckling strength of fully maintained track based on certain field measurements and the software developed previously. It also interprets the data for this specific field test within the context of the other issues. 2.1 Methodology The critical measurement issues for this test were to establish the neutral temperature of the rail before maintenance and to determine the changes in axial force (and hence neutral temperature) at each stage of the maintenance and to determine the other parameters to apply the CWR-BUCKLE software. The primary measurable parameter for the program was the lateral resistance, which was measured using the standard single tie push test (STPT) developed by the Volpe Center and Foster-Miller for FRA [1]. Figure 1 shows the STPT fixture for use on concrete tie track. Figure 1. Single Tie Push Test for Concrete Ties The rail was instrumented with strain gages every two hundred feet. When it was determined where the rail was to be cut for de-stressing purposes, a strain gage was 3

4 installed within three ties of the proposed cut on each rail. When the rail was cut, the absolute neutral temperature of the rail was determined at these gage locations because the absolute axial force was known. Changes in neutral temperature could then be monitored. The lateral resistance of the track was quantified using the STPT procedure before realignment, after tamping and after dynamic stabilization. Up to seven ties were selected randomly for testing after each operation. Only four tests were made in each case because the scatter was within an acceptable range. The accepted range, based on previous experience, was defined as a standard deviation, σ, less than 20% of the observed mean Averaged plots from the tests are shown in Figure 2. This plot shows the pre-tamped, post-tamped and post-dts tie resistance. The critical input for the CWR-BUCKLE program is the peak resistance, F p, which can occur, based on previous experience, at displacements as small as 0.1 inches to more than 1 inches, depending on the particular track and ballast conditions tested. (In this connection, note that track panel-pull type tests for the direct measurement of lateral resistance is not acceptable due to it providing a track panel bending response, and not the individual tie non-linear spring characteristics required) Average Lateral Resistance (lbs) Pre-Tamping Post-Tamping Post DTS Lateral Displacement (inches) Figure 2. STPT Response for Amtrak Tests on Concrete Ties Using the computer program CWR-BUCKLE, the track buckling strength can be determined for the measured input parameters of lateral resistance and the rail neutral 4

5 temperature and other track and service characteristics (e.g., anticipated maximum rail temperature). The margin of safety can also be determined by the procedure schematically illustrated in Figure 3. For a discussion of the margin of safety concept see refer to Section 5. Measured Field Parameters Lateral Resistance (STPT) Neutral temperature change (strain gage) Other Parameters Rail maximum temperature Curvature Misalignment /track class Rail size Wheel loads CWR-BUCKLE Buckling temperature Margin of safety If margin of safety is not adequate* Speed restrictions Artificial consolidation Re-destress for a higher neutral temperature** * if less than 10 F ** if in-situ conditions permit Figure 3. Buckling Strength Evaluation Procedure 5

6 3 Application to Revenue Service Track To apply the methodology to concrete tie track in revenue service, a joint test program with Amtrak was implemented. In preparation for the upcoming high-speed passenger rail service on the Northeast Corridor, Amtrak has been active in several types of modifications of the existing track at a number of locations. One such location is near Foxboro, MA, where two curved tracks needed to accept higher speeds. At this location, the short spiral, or transition between the tangent and the curve limits the speed. Amtrak proposed to lengthen the spirals without changing the radius in the main body of the curve. The required effect was to move the curve towards the center. The alignment machine used for this purpose throws the track to achieve the new geometry of the track. As a result of the tamping, the ballast lateral resistance drops off substantially and the track buckling strength could be an issue, particularly when the operation is performed in the summer. As a result of track lateral movement, there would be an overall net change in the track length, altering the CWR neutral temperature. When curves are realigned inside towards the center of curvature, as is the case with the Foxboro site, the neutral temperature in the body of the curve will be lowered. This reduction in the neutral temperature coupled with the reduced lateral resistance from pre-tamping conditions could increase the risk of track buckling unless adequate precautions are taken to control the drop in the neutral temperature as well as improve the lateral resistance by mechanical means. Of the two tracks at the test site, the outside track is more vulnerable to buckling due to the curve tendency to buckle out. Since there is generally adequate ballast between the two tracks, the inside track is less likely to buckle outwards. In winter conditions, however, it is more likely for the inside track to move radially inwards. Adequate ballast must be provided on the field side of the inside curve to reduce the potential for pulling in winter. This study focuses on the buckling potential of the outside curve in summer conditions. The methodology will be illustrated on the basis of tests obtained on the outside track. 3.1 Objectives of Tests on Amtrak Site The overall objective is to assess the change in buckling strength due to realignment and tamping operations on the curves at the Foxboro site. Specific objectives are: Evaluate the CWR neutral temperature change in the curve and spirals due to the curve realignment. Evaluate the lateral resistance of the ties before and after tamping and after mechanical consolidation. Using CWR-BUCKLE, estimate the buckling strength after tamping. 6

7 To realize the above objectives, the following instrumentation was deployed: Single Tie Push Tests for measuring the tie lateral resistance prior to and after tamping and after mechanical consolidation. Strain gages to measure neutral temperature changes. Details of the instrumentation deployment and hardware, as well as test conduct are described below. 3.2 Work Site Description/Instrumentation Amtrak performed the realignment of Curve 22 located near Foxboro, MA, on the Northeast Corridor line. A schematic of the site can be seen in Figure 4. The line has two tracks at this location. The outside track has a curvature of The track has concrete ties with a spacing of 24 inches. The rail is 131RE CWR, and is fixed to the ties with Pandrol fasteners. The curve was moved inwards, preserving the curvature, but lengthening the spirals or transitions between the tangent and curved portions. The track was then tamped and a dynamic track stabilizer (DTS) was used for mechanical consolidation. The rails were cut in the middle and de-stressed. Instrumentation Strain gages, with 4-arm bridge configuration, were installed onto the rails to monitor the forces before and after each of the realignment operations. The location of the gages can be seen in Figure 4. Figure 4. Instrumentation Layout on Amtrak Curve 22 7

8 4 Test Data The following measurements were made on the track. 4.1 Lateral Resistance Using the Single Tie Push Test (STPT) device the lateral resistance of randomly selected ties was measured prior to and after realignment, tamping and after application of DTS. Four ties were tested prior to realignment, four ties at different locations were tested after tamping, and four ties were tested after DTS. Prior to realignment, the track was in a consolidated condition, exhibiting high lateral resistance, with a mean value of 3438 pounds per tie. After tamping, the mean value reduced to 1869 pounds per tie, giving a drop of 46% as shown in Table 1. The standard deviation is about 411 pounds for the pre-tamped and 293 pounds for the post-tamped measurements. All the results fell within approximately one standard deviation of the mean, which was the basis for considering four ties a sufficient sample. The improvement in the lateral resistance due to DTS was about 4% for the ties tested in the program. This is in line with Reference [4], which quotes an improvement of 6% after one DTS pass and 5% after a second pass in average track lateral resistance on Austrian high-speed track. Note that the above applies only to concrete tie track. European standards, issued by the Union of International Railways (UIC) attribute an increase in resistance equivalent to 75,000 to 100,000 tonnes (0.083 to 0.11 MGT) of traffic to dynamic track stabilization [5]. Table 1. Lateral Resistance Prior To Realignment After Tamping After DTS Peak Lateral Test Number Peak Lateral Test Number Resistance - Resistance - (Lbs.) Tie Number (Lbs.) Tie Number Test Number - Tie Number * Peak Lateral Resistance (Lbs.) Mean 3438 Mean 1869 Mean 1938 Standard Deviation 411 Standard Deviation * Ties from each test were randomly selected 293 Standard Deviation Neutral Temperature Neutral temperature changes were due to the track realignment. The change in the lateral displacement depended upon the location in the curve; the track moved inwards by as much as 9 at the transition locations and outwards by 0.42 at the center location. 8

9 As stated earlier, strain gages were fixed to both the inside and outside rails. The initial readings of the gages and the rail temperatures were taken. Cuts were made at the center on both the inside and outside rails. The center gages were zeroed with respect to these conditions. Both the rails were later de-stressed over the track segment covering the strain gages. This involved removal of the fasteners and heating the rails to freely expand to obtain the target neutral temperature for re-welding. Although de-stressing operations were performed, the rails were not re-welded, but instead joint bars were installed due to time considerations. Table 2 gives the rail cut data for the center location. The neutral temperatures in the rails prior to the cut were determined from the strain gage readings. Table 2. Rail Cut Data Rail Rail Force (kips) Rail Temperature ( F) Neutral Temperature ( F) (Inside rail) (Outside rail) The neutral temperatures for all the gage locations are shown in Table 3 below. Data at some of the locations could not be collected due to time constraints during the field tests. However, the average change in rail neutral temperature over the whole test segment was accurately represented by the limited data (analysis showed a possible error of no more than 1 ) so the average values were used in the buckling analysis. The outside rail had a lower neutral temperature prior to realignment. The average of the inside and the outside rails is used for buckling strength predictions. The average neutral temperature at the center prior to realignment is 66 F. After realignment, the change in neutral temperature was 3.4 F at the center. At location 12, the change in the neutral temperature due to realignment was 16.3 F, which was the maximum drop in the section. 5 Buckling Strength Evaluation The buckling strength is defined as the capacity of the track to resist forces causing sudden loss of lateral stability resulting in large misalignments. Analytically, it is quantified in terms of an allowable rail temperature increase above neutral, T all that the track structure can withstand before buckling. The buckling margin of safety (BMS) often used in buckling safety evaluations is the difference between the actual buckling strength, T all, and the buckling strength required which is based on the difference of the maximum rail temperature and the rail neutral temperature. For buckling safety, BMS>0, and the larger the BMS, the larger is the buckling safety. For additional discussion on buckling safety concepts, parametric behavior, and CWR-BUCKLE field test validation studies refer to [3], [6] and [7]. The track s buckling strength in the consolidated condition prior to realignment was calculated using the CWR-BUCKLE program using the parameters shown in Table 4. The buckling strength prior to realignment was found to be 112 F over the neutral temperature. For an anticipated maximum rail temperature of 140 F, this gives a margin 9

10 of safety of 42.5 F for the in-situ neutral temperature of 70.5 F. For the center location with a neutral temperature of 66 F, the margin of safety will be reduced by about 4.5 F. Table 3. Rail Neutral Temperature Data Gage Prior to realignment and tamping After realignment and tamping Number Outside Inside Rail Average Outside Inside Rail Average Rail Rail Center Average Table 4. Input Parameters to CWR-Buckle PARAMETER UNITS Rail Size 131 AREA Rail Neutral Temperature 70.5 F Lateral Resistance 3438 lbs. Track Tie Type Concrete Track Curvature 1-03 Ballast Type Granite Track Class 6 After realignment and tamping, the average neutral temperature was 61 F. The lateral resistance was also reduced to an average value of 1869 pounds per tie. CWR-BUCKLE computes a buckling strength of 99 F above neutral temperature for this condition. At 140 F rail temperature, this will provide a margin of safety of 20 F. Use of average lateral resistance values dictated the use of average rail neutral temperature through the section for the buckling analysis. The buckling strength results are presented in Table 5. 10

11 Track Condition Table 5. Buckling Strength Data Neutral Buckling Strength, Temperature ( F) T ( F) Margin of safety at rail temperature, T R =140 F Prior to realignment After tamping After DTS From Table 5, it is seen that the track tested has a good margin of safety prior to tamping, and that tamping reduced the margin of safety to 20 F. This is considered to be adequate based on the analyses and test experience [7]. For the segment tested, additional measures such as the speed restriction and mechanical consolidation were probably not required. Mechanical consolidation through DTS was in fact used for the segment, consistent with Amtrak practice, though it did not show marked improvement in lateral resistance, and consequently in the buckling strength for this specific test case. It should also be noted that although the post-tamping and post DTS margins of safety of 20 F may be adequate, this is true only for the parameters measured and used in CWR-BUCKLE estimates. Should track conditions degrade, (such as a reduction of 20 F in neutral temperature which would reduce the BMS to zero), a high buckling potential could result. 6 Conclusions A procedure has been presented for the evaluation of freshly realigned and tamped track s buckling strength based on actual field measurements and application of CWR-BUCKLE. Evaluation of buckling strength of CWR tracks after realignment and tamping should help the track engineer decide adequacy of track lateral strength, neutral temperature and slow order requirements. The need for mechanical consolidation after tamping using machines such as DTS can also be determined. The efficiency of mechanical consolidation can also be evaluated in the procedure. Specific field measurements for the determination of the two key parameters influencing buckling, namely lateral resistance and the rail neutral temperature, have been presented. The former can be measured using the STPT technique, while the latter can be determined using strain gage data prior to and after rail cutting operations. Test measurements indicate that realignment produced substantial reductions in rail neutral temperature, tamping reduced track lateral resistance by 46%, and post maintenance lateral strength restoration with DTS was less than expected. It should be noted that these results are applicable only to this one curve and to draw broader conclusions would require further studies. The tamped concrete tie segment tested and analyzed via CWR-BUCKLE had a buckling margin of safety of about 20 F, which is considered to be sufficient from a buckling strength point of view. However, care should be taken that this 20 F 11

12 margin of safety is not reduced further. Prior to tamping, the consolidated segment had a margin of safety of about 42.5 F, which is considered to be good. 7 References 1 Samavedam, G., A. Kanaan, J. Pietrak, A. Kish, and A.Sluz., Wood Tie Track Resistance Characterization and Correlation Study, DOT/FRA/ORD-94/07, Final Report, January Kish, A. and G. Samavedam, The Neutral Temperature Variation of Continuous Welded Rails, AREA Bulletin 712, Kish, Andrew and Gopal Samavedam, Risk Analysis Based CWR Track Buckling Safety Evaluations, AREMA Track and Structures Technical Conference, September Lichtberger, Bernhard, Better Measurement of Lateral Resistance, Railway Track and Structures, March UIC Leaflet #720R Laying and Maintenance of CWR Track, ERRI D202/RP 10, pp 10, April Kish, A. and G. Samavedam, Dynamic Buckling of CWR Tracks: Theory, Tests and Safety Concepts, Proceeds of TRB on Track Lateral Stability, May Samavedam, G., Kish, A., Purple, A., and Schoengart, J., Parametric Analysis and Safety Concepts of CWR Track Buckling, DOT/FRA/ORD-93/26, Final Report, December, ACKNOWLEDGEMENTS The authors would like to acknowledge the sponsorship and support of the Federal Railroad Administration s Office of Research and Development, notably Steven Ditmeyer, Director, and Magdy El-Sibaie, Chief, Track Research Division. Also thanks are due to AMTRAK, notably to Conrad J. Ruppert, Jr. and Chris Sheppard for the invaluable field test support. Thanks are also due to Ms. Susan B. MacPherson and Mr. John Kidd of Foster-Miller for performing the test conduct and data analysis. 12