Failure analysis of concrete sleepers in heavy haul railway tracks

Engineering Failure Analysis 15 (2008) 90–117 www.elsevier.com/locate/engfailanal

Failure analysis of concrete sleepers in heavy haul railway tracks ´ lvarez-Ferna´ndez a, A. Mene´ndez-Dı´az C. Gonza´lez-Nicieza a, M.I. A ´ lvarez-Vigil c, F. Ariznavarreta-Ferna´ndez a A.E. A

b,*

,

a b

Department of Mining Engineering, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Department of Construction Engineering and Manufacturing, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain c Department of Mathematics, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Received 13 October 2006; accepted 20 November 2006 Available online 11 January 2007

Abstract Many industrial plants have railway lines that must support low-speed heavy haul freight traffic. This type of special haulage, such as pig iron torpedo ladles or heavy cisterns, may cause substantial settlement of the track foundations and require suitable sleeper and ballast design that allows these elements to uniformly transmit axle loads to the ground. In this study, we develop a failure analysis of a railway track used for transporting heavy haul industrial freight. The aim of the study is to describe the method with which this type of failure should be analyzed. We develop a specific case, establishing the causes of failure and offering guidelines for improving the design and upkeep the sleepers and ballast on which the tracks are laid. The ultimate aim is to offer guidance to the forensic engineer on the tests and variables to analyze with regard to the failure of railway track foundations. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Concrete sleeper; Sleeper/ballast interaction; Railway track substructure; Ballast material; Heavy axle loads

1. Introduction Designs for improved railroad track structure systems will be increasingly needed to ensure system safety, reliability and profitability as the railroads strive to compete with other means of transport for the movement of freight and passengers. Design improvements will focus primarily on accommodating increased vehicle weights, faster operating speeds, and reduced track maintenance cycles. On the other hand, due to changes in the railway industry, new types of vehicles are being introduced and higher operating speeds and axle loads are being proposed [1]. It is therefore of particular importance to be able to assess the effects of these vehicles when deciding on levels of track maintenance and on the design of new infrastructure. For instance, special freight wagons are used in heavy haul railway operations, and *

Corresponding author. Tel.: + 3498 510 4266; fax: + 3498 510 4265. E-mail address: [email protected] (A. Mene´ndez-Dı´az).

1350-6307/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2006.11.021

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an understanding of the probable levels of expected track damage and comparison with existing freight vehicles and locomotives is clearly useful. The increase in transport capacity has been stimulated by the growing industrial need for long-distance freight conveyance. The fact that freight transportation requires considerably heavier trains than passenger transportation can be illustrated by comparing the static axle load. The static axle load, which reflects the gravity loading of the train, normally does not exceed 170 kN for passenger trains, while for freight trains it may vary between 250 kN and 350 kN [2]. All these loads are ultimately transmitted to the ground through the system formed by the rail, sleepers and ballast. Inappropriate design of the railway track may thus be best appreciated in these elements [3–5]. On the other hand, it is precisely in these structural elements that failure first appears when the track has not been correctly designed. Therefore, greater control over the materials and design of railway track foundations is needed. In recent years, a great deal of research has gone into studying high-speed track failure as well as the design of lines that must support heavy hauls [6–9]. Basically, the structural elements to take into consideration in track foundations are the rail, the sleepers, the ballast and the track substructure. This paper reports on the methods that have been employed in a project to analyze the causes of damage resulting from track settlement. We start out from the characteristics of a heavy haul railway track in which traditional wooden sleepers have been substituted by new, pre-stressed concrete sleepers. After a short period of activity, the sleepers on this track presented a high degree of damage, with numerous fissures and cracks. We study the effects of the ballast and the concrete sleeper with different ground characteristics. The resulting values are not intended to be absolute indicators of damage but can be used in a comparative way to assess the effects of different concrete sleeper and ballast designs. 2. Components under study and description of track failure Many design improvements have almost invariably been achieved by starting out from failure analysis, subsequently presenting alternatives that reduce or completely remove said failure or its underlying causes. Hence the interest in studying failure using a methodology that allows the forensic engineer to obtain detailed knowledge of all the factors that trigger it, above all when said failure may give rise to important risks to users, as is the case of railway transport. We shall thus focus our study on the failure analysis of the concrete sleepers forming part of railway track foundations. We shall first study the materials that make up each element of the track foundations (rail, sleepers, ballast and substructure) to then describe the detected failure. Finally, we shall determine the causes that have triggered this failure. 2.1. Description of the railway track Our study case corresponds to 2 km of railway track linking two industrial plants shown in Fig. 1a. This line is subjected to heavy haul transport of ‘‘torpedo’’ wagons Fig. 1b.

Fig. 1. Layout of the track affected by the damage. (a) Layout of the railway track. (b) Heavy haul torpedo wagons.

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The railway track is made up of rail, sleepers, ballast, subballast, geotextile and the underlying ground (see Fig. 2). In the present study, the ground between and including the subballast and the geotextile material will be called the subgrade. This ground is highly heterogeneous, since it has been formed by repeated refilling throughout the life of the track. Ballast is situated on top of this subgrade, above which lie the pre-stressed concrete sleepers and the rail. There is a drainage device on the right of the track. The track has a layer of ballast that is a mixture of quartzite materials of variable thickness, though always greater than 50 cm. This value was estimated on the basis of the anticipated loads and traffic for the line. The ballast must transmit the axle load on the sleepers to the subgrade in the most uniform way possible. Pre-stressed concrete sleepers are employed on this railway line. These are of the DW-60 type with a space of 600 mm between each sleeper, the characteristics of which are given in the technical specification E.T.03.360.561.9l drawn up by RENFE (Spanish National Railways) [10]. These sleepers substituted the old wooden sleepers with the aim of reducing costs as a result of a decrease in track maintenance cycles. Standard UIC-60 rail (International Union of Railways) was used on the track. The separation between rails corresponds to the Iberian gauge of 1668 mm. It should be noted that these tracks support two types of freight that may be clearly differentiated by their load and their speed of transit. With special traffic (torpedoes, cisterns, etc.), the track must support high axle loads (370 kN per axle) at speeds of around 20 km/h. When the traffic is made up of coal wagons, axle loads of 200 kN at speeds of 80 km/h must be supported. The rolling stock that usually travels on the line is made up of two 1050 CV General Electric U10B DIESEL locomotives , each weighing 64 tons, suitable for circulating on RENFE track, adapted to the manoeuvring service and able to circulate in either direction. These engines can pull a train composed of 12 hopper wagons, each weighing 60 tons, or the equivalent 24 RENFE wagons, each weighing 30 tons. For special heavy haul freight, two of these engines must be used in tandem, since each torpedo wagon weighs 200 tons and can carry a load of 250 tons. As two torpedoes usually circulate in a convoy, the total load is 900 tons. As the topology of sleeper failure was observed to be directly related to torpedo wagon traffic, our study shall focus on this type of wagon. The typical configuration of one of these convoys is given in Fig. 3, which shows two torpedo wagons. These wagons are supported by two assemblies of six wheels with a separation of 7200 mm. Each assembly is made up of two bogies with a centre separation of 1350 mm. Each bogie has three wheels with a centre separation of 1200 mm. The locomotive has two bogies with a centre separation of 9880 mm made up of three wheels with a centre separation of 2118 mm. Having described the fundamental features of the railway track and of the rolling stock, we shall now describe the failure detected in this track.

Sleeper

Rail

Ballast

Subballast Subgrade

Geotextile Old Subballast Bedrock

Fig. 2. Typical cross-section of railway track.

Fig. 3. Schematic diagram of the locomotive with two torpedoes.

Drainage

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2.2. Description of track failure Torpedo wagon traffic was initially carried out on track consisting of wooden sleepers that were periodically removed in accordance with track maintenance cycles. When these sleepers were substituted by concrete ones, fracture failure of the sleepers was detected. In order to monitor this failure, visual examination of the state of the sleepers was carried out on two different occasions (October 2003 and February 2004). The track was divided into seven sections, obtaining the percentages of cracked sleepers in each section shown in Table 1. As can be seen, the increase in the number of sleepers affected by cracking over a short period of time is quite considerable, thus substantially conditioning the feasibility and safety of this railway line. Although an increase in the degree of cracking is barely appreciable in sections T2, T5, T6 and T7, it is so in the rest, especially in sections T3 and T4. It should be noted that the broken sleepers continue to work despite being cracked, and must only be replaced when the cracks grow until becoming a clear fracture. The geometry of the cracks may be seen in Fig. 9. It can be observed that these run vertically down through the centre of the sleeper for 2/3 of its height (see Fig. 4a), at which moment they tend towards the horizontal to then run parallel to the sleeper tendons. Many of the damaged sleepers present tensile fracture (see Fig. 4b) in their upper central segment. The fracture then spreads throughout the central segment of the sleepers in an ‘‘x’’ shape until they clearly fracture (see Fig. 4). Apart from the fractured sleepers, extrusion of the baseplates was observed at some points, these being forced out of their position little by little. Fig. 5 shows the lamination that is produced in the baseplates due to the loads transmitted by the rails situated on these. In this initial visual inspection of the track, it was observed that there is no direct relation between the characteristics of the subgrade and the presence of cracked sleepers. Failure appears both in areas whose subgrade is made up of bedrock, as well as in areas of the subgrade formed by repeated refilling with material. The subgrade was therefore ruled out as a determinant cause of sleeper failure, arriving at the apparent hypothesis that failure of the sleepers may have two causes:  The ballast does not uniformly transmit the load to the ground, resulting in failure of the sleepers.  If the ballast performs correctly, the sleepers are not designed to support such heavy loads, either due to defects in the concrete or as a result of their deficient prestressing. The goal of this study is to analyze which of these two causes of failure is the most likely, with the aim of providing the forensic engineer with the necessary basis for judgment so as to hence correct such failure. To do so, the following work was carried out.    

Visual inspection of and identification tests on the materials. Analysis of the subgrade and of the ballast via in situ and laboratory testing. Analysis of the sleepers, studying the concrete and prestressing steel of which they are made. Instrumentation and measurement of the displacements suffered by the sleepers and of the pressures transmitted to the ballast during the passing of the torpedo wagons.

Table 1 Percentage of damaged sleepers in each section of track Section

Date laid

No. of sleepers in total

No. of cracked sleepers (23/10/2003)

No. of cracked sleepers (12/02/04)

T1 T2 T3 T4 T5 T6 T7

VI/2003 VI/2003 VI/2003 VI/2003 IX/2004 IX/2004 IX/2004

143 393 1211 162 39 54 200

21 30 558 58 13 25 10

26 32 581 71 13 26 10

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Fig. 4. Damage suffered by the concrete sleepers. (a) Initial fracture of the sleeper. (b) Detail of a damaged sleeper. (c) Detail of a failed sleeper.

Fig. 5. Laminated baseplate.

 Analysis of the displacements and stresses in the track by means of 3D finite-difference modelling, which enables its behaviour to be ascertained under different loads and characteristics of the ballast and of the subgrade. The introduction carried out in this section has allowed us to present an initial description of the elements affected by the failure. In the following sections, we shall present the data supplied by laboratory and in situ tests, we shall discuss the records obtained by means of instrumentation and measurement of displacements and pressures, and finally we shall analyze the results of the numerical modelling carried out. 3. Analysis of the track ballast We shall now describe the laboratory and in situ tests needed to analyze the materials that form the railway track. We shall first study those affecting the subgrade, then the ballast, and finally the concrete of the sleepers and the steel of their pre-stressed tendons. 3.1. Analysis of the subgrade Four types of tests were employed to characterize the subgrade: identification tests, Proctor tests (Standard Proctor Test), CBR tests (California Bearing Ratio), and in-situ Borros dynamic penetration tests of the subgrade [11]. The results obtained for each of these tests are given below. 3.1.1. Identification tests Identification tests allow the ground forming the subgrade to be classified according to its different properties: physical, chemical and status (which depend on external factors, such as moisture). In the present case, the following parameters were determined on the samples obtained from the subgrade:

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Granulometric analysis by sieving. Atterberg limits. Presence of sulphates. Carbonate content. Organic matter content. Density of solid particles. In-situ apparent density

The results obtained from a total of eight samples are shown in Table 2. The granulometric curves of the subgrade materials were likewise determined by calculating the percentages of gravel, sand and fines (clay and silt). The Atterberg limits of the fines of the analyzed materials were obtained in accordance with the standard ASTM D 4318 [12]. Organic matter is present at around 2.1%, with a maximum value of 3.9% and a minimum of 1.6%. There is evident contamination of the subgrade due to spillage of the coal from the wagons on many sections of the track. The analyzed ground may be classified using the USCS (unified soil classification system) ASTM D 2487-83 [13] and on the basis of its granulometry and the Atterberg limits, a great variety of materials being found, from well graded sand (SW) to clayey sand (SC). 3.1.2. Proctor, CBR and borros tests Tests were performed to estimate the compaction of the subgrade employing the Proctor test ASTM D 1557 [14], which measures the optimum compaction moisture content of a soil, and the CBR test ASTM D 1883-73 [15], which measures the shear strength under controlled conditions of moisture and density. Dynamic penetration tests (the Borros test) were likewise carried out. The values of optimum moisture content varied between 33.1% and 24.1%, the mean optimum moisture content being 24.1%. Maximum density reached its highest value at 18.4 kN/m3 and its lowest value at 1.73 kN/m3. The maximum CBR index was 3.1 and the minimum 0.5, which indicates a subgrade with a reduced bearing capacity (Spanish standard for roads [16] requires values greater than 5). The dynamic penetration test carried out was of the Borros type [11]. This consists in driving a blunt conical spike into the ground by blows with the aim of measuring the resistance to penetration along a depth. In the present case, trials of this type were conducted on the subgrade once the ballast had been removed. The heterogeneity of the results obtained is worth highlighting. The strength values of the subgrade ranged between 14.51 and 0.98 kp/cm2. Fig. 6 shows the strength values of the ground in 20-cm sections obtained in one of the dynamic penetration tests. These tests must be interpreted with caution. As they consisted of penetration tests conducted with a small spike, it is possible that very high strength values were obtained at certain points, and even rebound, due to the presence of boulders or stones. 3.2. Analysis of the ballast Ballast is the coarse grain arid into which the sleepers fit. The aim of ballast is to provide a stable drainage base in order to maintain the alignment of the track with minimum maintenance. It must dampen the loads transmitted to the subgrade and must elastically absorb the deformations induced by the sleepers. The sleepers rest on the points of the angular fragments of ballast, which break or degrade depending on the applied load. The capacity of the ballast to support these loads depends on its thickness, its purpose being to bear the intermittent loads produced by railway traffic. Both the dynamic effects of the load and high values of the said load are damaging if the line is used to transport special heavy haul freight. The standards N.R.V. 3-4-0.0. [17] and UNE-EN 13450 [18] stipulate the characteristics that the ballast used by RENFE must have. The ballast on the track under study comprises of an average thickness of granular materials, more or less cubic in shape, with a particle size initially ranging between 25 and 50 mm. The thickness of the ballast layer increased at specific points in those areas in which the subgrade presents a low bearing capacity.

96

Sample

Moisture content (%)

LL*

PL*

PI*

Organic matter

Presence of sulphates

Carbonate content (%)

Apparent density (tons/m3)

Solid particle density (tons/m3)

Dry density (tons/m3)

Optimum moisture content (%)

Maximum density

M1 M2 M3 M4 M5 M6 M7 M8

14.27 12.84 13.73 11.96 2.96 20.15 27.96 19.59

40.02 57.91 46.29 47.26 52.38 55.42 44.98 41.68

27.54 30.32 29.67 26.86 32.78 34.51 27.55 26.03

12.48 27.59 16.62 20.40 19.60 20.91 17.43 15.65

3.91 2.51 1.61 1.69 2.10 2.22 – –

No No No No No No – –

No No 2.03 No No No – –

1.29 1.47 1.52 1.69 2.31 1.47 2.15 2.03

2.73 2.58 2.72 2.50 2.63 2.55 – –

1.13 1.31 1.33 1.51 2.25 1.23 1.68 1.69

20.6 30.3 20.3 30.2 18.4 33.1 25.4 13.7

1.75 1.75 1.78 1.80 1.73 1.82 1.83 1.84

LL, Liquid limit; PL, Plastic limit; PI, Plasticity index.

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Table 2 Results of tests carried out on the samples

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Dynamic strength (kp/cm2) 0

50

100

150

200

250

300

0-20 20-40 40-60 60-80 80-100 100-120 Depth of the section 120-140 (cm) 140-160 160-180 180-200 200-220 220-240 240-260 260-280 280-300

Fig. 6. Table of dynamic penetration trials.

On the track under study, A-type ballast [17] is employed. This is composed of white quartz sand belonging to the ‘‘Barrios Quartzite’’ Formation [19], of the Lower Ordovician period. Its SiO2 content varied between 93% and 99%, while its content in Fe2O3 varied between 0.11% and 2.35%. Laboratory tests were used in its characterization to determine the granulometric curve of the ballast, its wear resistance, freeze resistance and the ballast coefficient via in situ load plate tests. The results obtained for each of these different tests are given below. 3.2.1. Granulometric analysis of the ballast Although recently laid ballast for the track complies with the technical specifications established by RENFE Standard A [17] (see Fig. 7a), granulometric analysis of the samples taken on the track indicates that said ballast degrades rapidly. The main characteristic of the sample ballast is its low granulometry. Simple visual observation reveals a high content in sizes of less than 2 cm, especially under the sleepers (see Fig. 7b). The ballast can also be seen to present a high content in fines and is soiled with clay particles and remains of coal at many points.

Fig. 7. Appearance of the ballast particle size. (a) Detail of recently laid ballast. (b) Fines in the area of ballast under the sleepers.

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98 100 80 60

% That 40 passes 20 0 0.01

0.1

1

10

100

Opening of mesh (mm) Fig. 8. Granulometric curve for a ballast sample.

Fig. 8 shows the granulometric curve obtained for a sample of quartzite ballast (red plot1), in which the major deviations with respect to the stipulations of the standard NRV 3.4.0.0 (black plot) can be observed. In view of the above findings, it was concluded that the ballast had degraded to such a extent on the track that neither its granulometry nor its status were in keeping with the desirable characteristics of this type of foundation. 3.2.2. Wear resistance tests The wear resistance test (Los Angeles test [20]) measures the impact wear resistance of the ballast. This datum is important due to the fact that the continuous passing of trains produces impacts between the ballast particles, generating fines that progressively reduce the elasticity of the ballast. Product specification PRV 3-4-0-0 accepts Los Angeles coefficient values lower than 19% as valid in quartzite ballast, and lower than 22% in limestone ballast. All the analyzed ballast samples present wear within the limits required by the standard with values of below 19%, although these values are very close to the limit, varying between 15.8% and 18.8%. 3.2.3. Freeze resistance The aim of the standard UNE-EN 1367-5 [21] is to describe the procedure to be followed to determine the freeze resistance of ballast. Tests were conducted on six different samples, a weight loss of 0.4% and 0.1% only being observed in two of the analyzed samples. According to the standard, samples may undergo a weight loss equal to or less than 8%. The samples hence comply with the standard in terms of freezability. 3.2.4. In situ load plate tests Load plate tests were conducted on the ballast situated under the sleepers with the aim of estimating the ballast coefficient. For these tests, load plates were employed below both fractured and stable sleepers, as well as at different positions on these (A-North, B-Centre and C-South), obtaining the values shown in Table 3. The obtained values of the ballast coefficient are characterized by their heterogeneity, both within and between sleepers. The tests carried out in the areas in which there are a substantial number of fractured sleepers constitute an illustrative example. Fig. 9 shows the characteristic curves of one of these load plate tests. The ballast coefficient (K) for the test carried out in the ballast close to the Centre of the sleeper is 500 kp/cm3, in the one situated below the North rail it is 583 kp/cm3, while in the South segment, the value of K is only 13 kp/cm3. Two phenomena are observed in the load plate tests of the fractured sleepers:  In sleeper E1, great variability between the ballast coefficient of one end of the sleeper and the other, passing from 583 to 13 kp/cm3.  In sleeper E2, the values of the ballast coefficient are very low, indicating a high degree of deformability, which is also accompanied by substantial residual settlement. 1

For interpretation of the references to colour in Fig. 8, the reader is referred to the web version of this article.

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Table 3 Load plate test summary Sleeper

Status

Position

Maximum load (kp/cm2)

E1

Fractured

Centre North South

7.00 5.00 4.00

0.20 0.12 8.15

0.20 0.01 0.82

500 583 13

E2

Fractured

Centre North South

5.00 1.75 3.00

14.99 5.15 10.60

12.96 4.08 –

5 4 9

E3

Stable

Centre North South

5.00 1.00 5.50

13.92 0.47 5.83

12.51 0.41 4.49

6 21 10

E4

Stable

Centre North South

5.00 3.50 4.00

6.07 6.89 1.20

4.24 5.87 0.2

10 20 28

Maximum settlement (mm)

Residual settlement (mm)

Ballast coefficient (kp/cm3)

Pressure (kp/cm2) 0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.0 A B C 0.1

A

B

C 1.0

A - North B - Centre C - South 10.0

Settlement (mm) Fig. 9. Load plates in the ballast.

This would explain the tensile cracks that appear in the Centre of the sleepers, since it is this central segment that is subject to notable bending stress. However, it is necessary to study whether the concrete and the tendons of the pre-stressed sleepers can support such settlement of the ballast. We shall carry out this study in the next section. 3.3. Analysis of the sleepers Below we discuss the different tests aimed at obtaining the strength properties of the sleepers and at verifying whether they comply with the respective technical requirements UNE-EN 13230-2 [22]. Tests were carried out on the concrete and the steel of the prestressing tendons. Tests were also carried out to verify whether the tendons were properly pre-stressed. 3.3.1. Tests on the concrete The tests performed to determine the strength properties of the concrete of which the sleepers are made were:

100

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 Simple compression plus measurement of deformation: Eleven tests of this type were carried out. The compressive strength results varied between 33.16 MPa and 65.99 MPa, with a mean value of 51.44 MPa. The obtained longitudinal deformation moduli have a mean value of 54 GPa, Poisson’s modulus being 0.3.  Indirect tensile strength (Brazilian test): Five tensile strength trials were carried out on the same number of concrete samples. The obtained strengths varied between 5.0 MPa and 6.5 MPa.  Flexotraction: Three flexotraction trials were conducted, obtaining an average strength of 7.45 MPa. On the basis of these results, the conclusion was reached that the sleeper concrete complies with the technical requirements established by the standard UNE-EN 13230-2 [22]. 3.3.2. Prestressing verification tests First, the strength of the steel of the pre-stressed tendons was studied. Tests were performed on several samples of steel, obtaining tensile strength values of between 1645.2 MPa and 1681.8 MPa, while the elastic modulus varied between 1.15 GPa and 1.14 GPa. On the other hand, the tensile stress to which the tendons of the sleepers are subjected due to prestressing was measured by means of an extensometric test. Fig. 10a shows the two tested sleepers. To carry out the test, a longitudinal extensometric strip was placed on the central segment of the two tendons that make up the upper armature of the sleepers (see Fig. 10b) after removing a small area of the concrete that cover the tendons. Once the extensometric strips had been placed in position, the steel tendons were cut one at a time (using a circular saw), thus removing the stress to which they are subjected due to prestressing (see Fig. 10c). Removal of this stress provokes displacements of the steel that are measured via the extensometric strips (see Fig. 10d). Taking this displacement into account in addition to the value of the previously determined elastic modulus, it was found that the prestressing achieves values of 961.7 MPa and 871.6 MP, i.e. within the prestressing values established in the sleeper design for working under good conditions.

Fig. 10. Prestressing analysis of the sleepers.

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4. In situ pressure and settlement instrumentation All the aforementioned tests and analyzes focussed on the sleepers. However, it is necessary to study how these elements perform jointly in situ and to analyze the real effect of the passing of the convoys over the rail– sleeper–ballast–subgrade system. The railway track was accordingly instrumented in order to obtain the vertical displacements and pressures transmitted by the sleepers to the ballast. These measurements were carried out using continuous register sensors while the convoys circulated on the track. The area chosen for instrumentation consisted of 10 adjacent sleepers, the majority of which presented incipient fissures and cracks, although none had yet fractured completely. Due to the interest of this test, Fig. 11 shows the position of the cracks in each sleeper and Table 4 presents a short summary of the characteristics of each of the sleepers numbered 1–10. Once these sleepers had been chosen, three displacement sensors were placed on each sleeper to measure its settlement (one under each rail and another in the central segment). Three hydraulic stress cells were also installed on sleeper 6 (one under each rail and the other in the central segment). Fig. 12a shows the position of a displacement sensor in the central segment of a sleeper, Fig. 12b the location of a pressure cell under one of the rails, and Fig. 12c the test carried out when one of the wagons is passing. Fig. 13 shows the displacement and stress sensors used to instrument the track. The displacement sensor is a linear resistance sensor with a standard resistance range of 5 K, independent linearity of 1%, and hysteresis of below 0.2%. The stress cell consists of a pressure pad connected to the transducer via a hydraulic line. The internal cavity between the two steel plates of the pressure pad and the hydraulic line are filled in the laboratory with special

South

North

1

Crack in Sleeper 6

Cracks in Sleeper 7

10

Fig. 11. Sleepers 1–10, which were instrumented, plus their cracks.

Table 4 General description of the characteristics of the chosen sleepers Sleeper no.

Observations

Sleeper Sleeper Sleeper Sleeper Sleeper Sleeper

Presents a single crack, which is quite deep, at its North end Presents two cracks, both situated to the North although one of these, the deepest, is slightly centred Presents a small incipient crack, situated in the Centre–North Presents a relatively insubstantial crack, situated towards the Centre–North Presents a deep crack, situated close to the Centre at its North end Presents a very deep crack close to the Centre, though slightly displaced towards the North. A small step in the sleeper can even be seen Two very deep cracks can be observed, situated at the North end and in the Centre–North, respectively Presents a single crack, which is quite thin and relatively insubstantial, situated in the Centre–North Presents two cracks, one situated in the Centre–South and the other in the Centre–North, the latter being the deeper of the two No cracks observed

1 2 3 4 5 6

Sleeper 7 Sleeper 8 Sleeper 9 Sleeper 10

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Fig. 12. Instrumented area and location of the displacement and stress sensors. (a) Displacement sensor. (b) Stress cell. (c) Location of the sensors on the track.

Fig. 13. Sensors used to instrument displacement and stress. (a) Displacement sensor. (b) Stress sensor.

oil that has been de-aired under high vacuum so as to obtain minimum oil compressibility. The rigidity of the cell is even assured by the transducer, which has a ceramic diaphragm with negligible volume displacement in comparison with the cell cavity. Its measuring range is 0–20 MPa (200 Bar) with a resolution of 0.01 MPa. Fig. 14 represents the settlement values of sleeper 4 when a convoy of two loaded torpedo wagons passes over it at a speed of 20 km/h. At the bottom of the figure, there is a schematic diagram of the configuration of the convoy to aid in the interpretation of the obtained curves.

North

2.5

South

2.0 1.5 1.0 0.5 0.0 -0.5

Center

70.0

60.0

50.0

40.0

30.0

20.0

10.0

Position of the train in metres Fig. 14. Settlement plot for sleeper 4.

0.0

Settlement mm

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103

100

South 90 85 80 75 70

Pressure N/cm2

65 60

North

55

70.0

60.0

50.0

40.0

30.0

20.0

10.0

0.0

Position of the train in metres Fig. 15. Pressure plot for sleeper 4.

Maximum settlements are produced when the torpedo wagon wheel bogies pass over the sleeper, those of the South rail being much greater than those of the North rail. This is due to the fact that the ballast coefficients of the South segment are much lower. A bending effect is also noticed in the sleepers that results in a relative swelling of their Central segment with respect to the ends, although the general movement is decreasing. On the other hand, it is worth noting that negative subsidence is produced in the North and Centre segments when the locomotive passes over the sleeper as a result of the high subsidence produced by the passing of the first torpedo. Fig. 15 represents the pressures transmitted to the ballast, which were measured by means of stress cells. The difference in pressures transmitted between the passing of the locomotive and that of the loaded wagons can be appreciated. The rail situated in the South segment supports pressure of around 90 N/cm2, while the pressure on the rail situated in the North segment does not reach 78 N/cm2. The increases in pressure due to the wheel assemblies can be perfectly distinguished, and even the different wheels in each assembly can be distinguished. This finding is interesting, since it allows us to verify that the transmission of loads is not perfect, as the wheels are either not balanced or at a different height. Measurements were also taken in sleepers 6 and 7 of the horizontal unit deformations that are produced with the passing of the convoy in order to verify whether they were large enough to produce fracture of the concrete. To do so, three extensometric gauges were bonded to each sleeper as shown in Fig. 16. The unit deformation was registered during the passing of the locomotive and of the first bogey composed of three wheels of the convoy torpedo, but from that moment on the extensometric gauges broke and it was

Fig. 16. Position of the extensometric gauges on the sleepers.

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104

160.0 140.0 120.0

Centre

100.0 80.0

Unit deformation (μm/m) 20.0 60.0

South

40.0 North

0.0 -20.0 -40.0

70.0

60.0

50.0

40.0

30.0

20.0

10.0

0.0

Position of the train in metres Fig. 17. Horizontal unit deformation for sleeper 7.

Table 5 Unit deformation values measured by means of extensometric gauges Sleeper

Position

Maximum microdeformation (lm/m)

Minimum microdeformation (lm/m)

Sleeper 6

North Centre South

60 134 95

13 17 8

Sleeper 7

North Centre South

51 150 96

5 22 10

not possible to continue registering the produced deformations. Fig. 17 shows the horizontal unit deformations for sleeper 7. It can be observed that the greatest microdeformations were obtained in the Centre segment of the sleeper, reaching values of 150 lm/m. In this case, the behaviour of the areas close to the ends of the sleeper in the North and South segments is similar. Table 5 gives the maximum and minimum registered values in the two sleepers. This value of 150 lm/m of horizontal unit microdeformation was taken as the reference value to decide whether the sleeper is in the process of failing or not. Sleeper 7 presents two very deep fissures and it may be stated that it is practically in the process of failing. 5. Finite difference numerical modelling The purpose of the finite-difference numerical modelling employed in this study was to integrate all the available information obtained from laboratory trials, in situ tests and instrumentation in order to identify more precisely the causes of track failure. The field data, and especially that obtained from the load plates, served to define the properties of the materials (ballast and subgrade) to be used as input for the numerical model. The properties of the sleepers and of the steel of the rail were obtained from both laboratory and field tests. The track instrumentation results (measurement of displacements and pressures on the ballast) served to calibrate the models and to test their goodness-of-fit. Due to the complexity of the numerical models, a simplification of the model was carried out. First, a study of only 1 sleeper was performed (see Fig. 18a) and then a series of 10 sleepers was studied (see Fig. 18b).

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Fig. 18. Geometric models generated using ALMEC for: (a) 1 sleeper, and (b) 10 sleepers.

The 3D modelling of the rail–sleeper–ballast–subgrade system for both the 1-sleeper model and the 10-sleeper model was performed with the aid of the ALMEC computer program (the name is a Spanish acronym for Lagrangian analysis of continuous media). This software, belonging to the set of computer programs based on the finite difference method, was developed by the Ground Engineering Group at the University of Oviedo. Starting out from the aforementioned geometric model, the ALMEC program generates discrete elements by means of an irregular hexahedral mesh. A mesh of nodes and areas is thus formed to which scalar and vectorial magnitudes are applied that define the behaviour of the model, i.e. strength and deformation properties and initial stress state. The program then initiates an iterative calculation process on each element, recalculating the displacements in each step as a function of deformations and stresses until equilibrium is reached. We shall next analyze the peculiarities of the calculation mesh, the properties of the materials under study and the actions to which they are subjected. To finish, we shall present the results obtained using ALMEC for the two numerical models comprising one and 10 sleepers. 5.1. Geometric mesh of the model using interfaces As may be deduced from the complexity of the problem, the numerical model requires geometrically discretizing at least 10 sleepers in order to take into account the different materials and applied loads. Constructing a mesh of the entire model with the same degree of detail is not feasible, as the number of nodes and areas that are generated render the numerical model inoperative. Therefore, we decided to construct a different mesh for each structural element, using geometric interfaces to bond each segment of the mesh. From the geometric point of view, an interface is a surface that connects two segments of a hexahedral mesh with distinct mesh densities and which allows the values of one segment of mesh and those of the other segment to be interpolated while maintaining the continuity of the model. It may be seen as a structural element that appropriately distributes the stresses on both meshes in order to maintain the homogeneity of the 3D model.

Fig. 19. Calculation of displacements in the same model: (a) without an interface, and (b) with an interface.

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Fig. 20. Definition of an interface between two different meshes.

We illustrate this concept in Fig. 19 with a simple case. The mesh on the left is a conventional mesh that we might use to model by means of finite differences, while the mesh on the right corresponds to the same model but with different densities, greater in the top half than in the bottom half. To obtain the same result in both cases, in the model on the right we define the interface between the two meshes in addition to the hexahedrons of the 3D mesh. Fig. 20 represents the surface of this interface, slightly separated from the hexahedrons of the lower face on which it is actually situated. This figure also shows the hexahedrons and most characteristic nodes of the geometric discretization. Seeing that in our case the intermediate element that permits the loads on the rail to pass to the ground is the sleeper, the anchored nodes on which we have defined the interfaces are those corresponding to the nodes of the sleeper. The contact nodes correspond to the lower part of the rail and the upper part of the ballast. Fig. 21 shows the interfaces between the rails and sleepers (R–S interface) and between the sleepers and the ballast (S–B). Thus, although node-to-node correspondence of the mesh is not produced, the interfaces allow stresses to be transmitted from the rails to the sleepers and from the sleepers to the subgrade. 5.2. Definition of materials and their properties The following materials were considered in the 3D models (see Fig. 22):  Material A: The rail was assumed to behave elastically, with a deformation modulus of 2100 GPa.  Material B: The sleepers were also assumed to behave elastically, with a deformation modulus of 54 GPa.

Fig. 21. Detail of the contact surfaces where interfaces are considered.

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Fig. 22. Materials considered in the modelling.

 Material C: The ballast situated at the sides of the track, with a thickness of 50 cm, and with common properties in all the models (Young’s modulus of 130 MPa and Poisson’s modulus of 0.2). This material corresponds to the original, recently laid ballast, which has still not been affected by the transit of the torpedo wagon loads.  Three additional materials were considered for the ground formed by the subgrade and the ballast located under each sleeper, which did suffer the effect of the torpedo wagons. The conducted load plate tests made it evident that the properties of this ground are different under the North rail, in the Centre of the track and under the South rail. We have Material D for the North rail, Material E for the Centre, and Material F for the South rail. The properties of each material are shown in Table 5, which specifies whether said material is situated under a fractured sleeper or under a stable sleeper. The properties in Table 6 were obtained from the load plates situated in the ground, obtaining the ballast coefficient (Kc) of the joint ballast-subgrade. Young’s modulus (E) may be estimated from the ballast coefficient, and assuming a Poisson modulus (v) of 0.2, we obtain the bulk modulus (K) and the shear modulus (G). Considering the different properties according to whether the sleeper is fractured or stable will allow us to simulate the two models; one in which failure occurs and another in which failure of the sleepers is not produced. By comparing the two models, we may thus better determine the causes of sleeper failure. 5.3. Loads applied in the model In this case, the equilibrium stress state of the entire system is altered by the existence of two types of actions:  The prestressing of the sleepers, which was simulated as a series of horizontal loads applied to the edge of the sleepers, with a compressive effect. The value of these loads, the application of which was shared by the two faces of the corresponding sleeper, was calculated to give a resultant of 27 tons (the minimum permissible value for prestressing). Table 6 Properties of the ballast and subgrade material under the stable and fractured sleepers Material Material Material Material Material Material Material

D E F D E F

Location

Status

Kc (kp/cm3)

E (MPa)

K (MPa)

G (MPa)

North Centre South North Centre South

Stable Stable Stable Fractured Fractured Fractured

20 10 27 583 500 13

211 107 292 6121 5250 139

117 60 162 3401 2917 77

88 45 121 2551 2187 58

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Fig. 23. Weight of the wagons and prestressing loads in the models of (a) 1 sleeper, and (b) 10 sleepers.

 Fig. 23 offers a magnified representation of the effect of these loads on the set of 10 sleepers.  The load produced by the torpedo wagons, which in this case was simulated as a vertical force representing the weight transmitted by each of the torpedo wheels, was assumed to be equal to 18.5 tons.  In Fig. 23a, the weight of one axle is considered for the single sleeper model.  While Fig. 23b considers the weight of three axles seeing that 10 sleepers were simulated. 5.4. Results of the numerical models The calculation method consisted in determining the deformations and stresses that are produced for the one-sleeper model and the 10-sleeper model, considering two cases: stable sleepers and fractured sleepers. Depending on each case, appropriate values were taken for the properties of Materials D, E and F indicated in Table 5. By analyzing the differences in results between one case and the other, it is possible to interpret the real cause of sleeper failure more precisely. The obtained results are summarized below, starting with the one-sleeper model and ending with the 10sleeper case. 5.4.1. One-sleeper model If we consider the properties of materials that do not produce failure of the sleeper, we obtain a distribution of displacements as shown in Fig. 24. Each colour represents a stress or displacement interval; the legend for which can be seen on the right of the figure. The units employed as those of the International System, in Pa for stresses and in meters for displacements. Negative pressure values correspond to compression, while positive values correspond to traction. When representing displacements, negative values indicate settlement, while positive values indicated swelling. The maximum settlements obtained for the model in Fig. 24 are 2.90 mm in the North segment, 3.20 mm in the Centre of the sleeper and 4.60 mm in the South segment. It can be seen that greater settlement exists in the South segment than in the North segment for the same loads, due to the fact that the ballast coefficient in the former is practically half that in the latter. In addition to vertical displacements, this figure also shows the deformation of the sleeper. It should be noted that all the deformations that are shown are magnified or exaggerated for ease of viewing. XX deformations are shown in Fig. 25. The greatest microdeformations due to traction that appear are in the order of 158 lm/m, a value that falls within the permissible limits for concrete. These values appear in the Centre segment of the sleeper, as shown in the figure. The XX stresses measured in the concrete are of great interest from the point of view of stress as they give an idea of the traction phenomena to which the sleepers are subjected. Fig. 26 shows the XX stresses in the sleeper, in which an area exists that is subject to very slight traction or compression. The lower part of the sleeper is subject to compressions of up to 21 MPa.

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Fig. 24. Sleeper with vertical displacements and deformation.

Fig. 25. XX deformations in the sleeper.

As regards the vertical stresses that the sleeper transmits to the ballast, these are different in the North, Centre and South segments of the sleeper, as can be seen in Fig. 27. In the South rail, the stresses are lower (6–7 kp/cm2) because there are greater deformations due to a lower ballast coefficient, just the opposite of the stresses in the North rail, in which values of up to 8.5 kp/cm2 are obtained. The stress that appears in the Centre segment is much lower, around 2.6 kp/cm2 (0.26 MPa). These are the maximum values that the ground must support, but it can be seen that lower values rapidly appear in concentric form. Therefore, this decrease must be taken into account when defining the average value of the transmitted stresses. Thus, the average value in the North segment will be approximately 7.5 kp/cm2, and in the South segment around 5.5 kp/cm2. If we now consider the properties of the materials that produce sleeper failure, we shall have a distribution of displacements as shown in Fig. 28. The settlements that are produced are practically null in the North and

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Fig. 26. XX stresses in the sleeper.

Fig. 27. ZZ stresses in the sleeper.

Centre segments of the sleeper, while those produced in the South segment of the sleeper present a value of 1.3 mm. That is, settlement of the sleeper is not produced as in the previous case at the two ends, but rather all the settlement is produced in the South segment of the sleeper. Fig. 29 shows the XX stresses in the fractured sleeper, with the aim of observing the possible tractions to which it is subjected. Slight tractions can be observed with values of below 45 kp/cm2 (4.5 MPa). In this model, the greatest microdeformations that are produced are in the order of 203 lm/m, which is greater than the value that defines cracking for concrete, thus confirming why the sleepers fail (Fig. 30). As regards the ZZ stresses in the ballast, maximum compression occurs in the limit between the Centre and South materials, due to the effect of the deformation of the sleeper which is not displaced in the Centre, while

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Fig. 28. Vertical displacements and deformations.

the South segment sinks 1.3 mm. As a result, compressions of up to 15.5 kp/cm2 (1.5 MPa) appear in this area of transition, as can be seen in Fig. 31. 5.4.2. Ten-sleeper model Following the same methodology as in the previous case, the displacements and stresses were calculated in the 10-sleeper model, considering the materials that maintain the sleepers stable and those that produce fractured sleepers. The sleepers were identified by numbering them from 1 to 10 starting from the front of the model, as shown in Fig. 32. The North segment is on the left and the South segment on the right.

Fig. 29. XX stresses in the sleeper.

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Fig. 30. XX deformations in the sleeper.

Fig. 33 shows the vertical displacements suffered by a series of 10 sleepers with the properties of the materials of the subgrade–ballast corresponding to the areas of stable sleepers. The greatest settlement is suffered by sleepers 5 and 6. Major differences are not observed between the settlements of the North segment and the South segment, being in the order of 1.71 mm. The lowest settlements are produced in the Centre segment, around 1.10 mm. Fig. 34 shows the magnified deformation of the series of 10 stable sleepers. The microdeformations due to traction are in the order of 107 lm/m, and are located in the Centre segment of the sleepers. Fig. 35 shows the vertical stress that the sleepers produce on the ballast. Sleepers 5 and 6 transmit a stress of 7.5 and 12 kp/cm2, respectively. Sleepers 3 and 8, which are very close to the wagon loads, present maximum pressure values of 9.5 kp/cm2, while the remaining sleepers transmit a clearly lower stress. It can also be observed that the stresses on the North side are double those on the South side.

Fig. 31. ZZ stresses under the sleeper.

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Fig. 32. Identification of the sleepers.

Fig. 33. Vertical displacements of the 10 stable sleepers.

Fig. 34. Magnified deformation of the 10 stable sleepers.

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Fig. 35. Vertical stresses on the ballast of the 10 stable sleepers.

Let us know compare these results with those obtained for the case of 10 sleepers situated on materials that correspond to areas with fractured sleepers. Fig. 36 shows the vertical displacements suffered by the 10 fractured sleepers. The greatest settlement is suffered by sleepers 5 and 6. Major differences are not observed in this case between the settlements of the North segment (practically null) and the South segment (around 0.69 mm). That is to say, the settlement produced in the South segment is not accompanied by subsidence in the Centre or North segments. This is due to the fact that the rigidity of the ballast in the Centre and North segments is not capable of transmitting the stresses uniformly. Fig. 37 shows the magnified deformation of this model (the viewpoint in this figure is from the South side so as to better appreciate the settlement that is produced in this area). The microdeformations due to traction are in the order of 194 lm/m, which are slightly higher values than the strength limit of the concrete.

Fig. 36. Vertical displacements of the 10 fractured sleepers.

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Fig. 37. Magnified deformation of the 10 fractured sleepers.

Fig. 38 shows the vertical stress that the sleepers transmit to the ballast. Sleepers 5, 6 and 8 transmit the greatest stresses, since they are closest to the loads applied by the torpedo wagons, presenting values of 13.4 kp/cm2. Summarizing the results of the 3D models analyzed using the ALMEC program, the existence under the same sleeper of very different properties leads to the appearance of microdeformations in the sleeper of more than 150 lm/m, i.e. the value above, which cracks, would start to appear in the concrete. This phenomenon also takes place when the materials situated under the sleeper have very low ballast coefficients (lower than 10 kp/cm3) due to the major difference in settlement between the centre and ends of the sleepers. Whereas the deformations of the South side are practically absorbed by the settlement of the ballast, in the case of the North side, the rigidity and compactness of the ballast, which under normal conditions would be optimum, prevents the existence of sufficient deformation in the substructure of the track to compensate that produced by the passing of the loaded torpedo wagons. Therefore, it is precisely in this North segment in which the cracks appear that lead to the final failure of the sleepers.

Fig. 38. Vertical stresses on the ballast of the 10 fractured sleepers.

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6. Causes of failure and recommendations The main conclusion to be drawn from this study is that no single cause can be found for the cracking of the sleepers, but rather that a series of factors exist that would not be decisive in isolation but which are so when acting in conjunction. More precisely, we may conclude that:  Analysis of the sleepers allows us to rule out anomalies in their manufacture or in their technical characteristics. However, it is clear that these sleepers were not designed for the axle loads transmitted by the torpedo wagons and thus work at their strength limit and do not suffer mass failure thanks to their manufacturing safety factors.  Seeing that the sleepers must work under limit conditions, which are imposed by the heavy freight loads, the status of the ballast–subgrade system is determinant in the appearance of cracks. Thus, any deficiency in one of these two elements may bring about failure of the sleepers. Throughout this study, faults and anomalies were detected in both the ballast and the subgrade.  The ballast presents a lower grain size than recommended. A high content in particles smaller than 20 mm was observed, which was produced by degradation of the ballast originally laid on the track.  The subgrade presents deficiencies at some points on the track, with CBR indexes below 5. According to the recommendations of different authors for these types of loads, this value should be higher than 20. The penetration tests carried out show considerable heterogeneity in the bearing capacity of the subgrade.  The problems of the ballast in conjunction with those of the subgrade condition the existence of a great variety of properties in the ballast–subgrade system, as shown by the load plate tests. In these tests, it was observed that the ballast coefficients at the sides under one and the same sleeper may vary between 13 and 580 kp/cm3, while under another sleeper situated 5 m from the previous one these coefficients were lower than 10 kp/cm3.  The results obtained via instrumentation of the track enabled us to observe the evolution of the displacements of the sleepers and the pressures on the ballast as the train passes. The maximum measured settlement, around 2.6 mm, occurs under the South rail. Settlement is lower under the Centre of the sleepers, except when the sleepers are fractured, in which case these may rise off the ground. The maximum measured pressures are in the order of 8 kp/cm2, also under the South rail.  As can be appreciated in the 3D models constructed using the ALMEC program, the existence under one and the same sleeper of very different properties leads to the appearance of microdeformations in the sleeper of more than 150 lm/m, i.e. the value above which cracks would start to appear in the concrete. On the basis of the observed problems, the following recommendations are made:  Increase the CBR of the subgrade and standardize this at least transversally, thus making the North and South segments behave similarly.  Improve the strength of the fragility of the ballast in order to reduce its degradation, which would be achieved by using another type of quartzite that is stronger, or even granite.  Improve the design of the concrete sleepers, increasing their cross-section and their prestressing, as a result of which they would take longer to fracture.  The strength characteristics of the baseplates should be improved owing to the fact that they laminate as a result of heavy freight loads.  The frequency of ballast maintenance on the track should be increased so as to increase, as far as possible, the life of each one of the strength elements that depend on the ballast, especially the sleepers.

Acknowledgements The authors gratefully acknowledge the assistance of Paul Barnes in the preparation of this paper in English. The authors also wish to express their gratitude to the management and staff of Aceralia for allowing in situ experiments.

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