Design and construction of backfills for railway track

Paixão, A., Fortunato, E., & Calçada, R. (2015) Design and construction of backfills for railway track transition zones. Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit, 229(1), pp 58-70 doi:10.1177/0954409713499016 Journal article published by SAGE on behalf of the Institution of Mechanical Engineers Accepted Author Manuscript Copyright © 2013 The Authors, Institution of Mechanical Engineers Reprinted by permission of SAGE Publications.

Design and construction of backfills for railway track transition zones André Paixão*†

, Eduardo Fortunato‡

, Rui Calçada§

Abstract: The railway track initial geometry degrades throughout the life-cycle. Changes in the track alignment give rise to dynamic axle load variations, accelerating the track degradation, with consequences in maintenance and availability of the line. This behaviour is particularly evident at some critical locations that are associated with abrupt changes in track vertical stiffness, such as transitions to bridges or other structures. In order to mitigate the problem, careful design and construction is required, for which several recommendations have been suggested. However, studies based on the maintenance records of existing high-speed lines have shown that track degradation associated with stiffness variations are far from being solved. This paper presents a short review on the design of transition zones. The design and construction of a case study in a new Portuguese railway line is analysed. Results of conventional laboratorial and cyclic load triaxial testing on granular materials and in situ mechanical characterization of the layers are presented. Relevant aspects regarding the construction are addressed and discussed. The obtained results seem to indicate that the design of the transition case study was successful in minimizing settlements and achieving a gradual stiffness increase as approaching a bridge, measured at the substructure level.

Keywords: railway transition zones, backfill design and construction, vertical stiffness, track degradation, cyclic load triaxial tests, deformation modulus, performance-based tests

*

Corresponding author

†

Transportation Department, National Laboratory for Civil Engineering (LNEC) Av. do Brasil 101, 1700-066, Lisboa, Portugal; Tel: (+351) 218443460; [email protected] ‡

Transportation Department, National Laboratory for Civil Engineering (LNEC) Av. do Brasil 101, 1700-066, Lisboa, Portugal; Tel: (+351) 218443460; [email protected] §

CONSTRUCT - LESE, Faculty of Engineering (FEUP), University of Porto R. Dr. Roberto Frias 4200-465, Porto, Portugal; Tel: (+351) 225081901; [email protected]

Please cite as: Paixão, A., Fortunato, E., & Calçada, R. (2015). Design and construction of backfills for railway track transition zones. Proceedings of the Institution of Mechanical Engineers, Part F: Journal of Rail and Rapid Transit, 229(1), 58-70. doi:10.1177/0954409713499016

1. INTRODUCTION The experience of the railway industry shows that in normal ballasted railway tracks, resting on good foundation conditions and after the initial stabilization of the supporting layers, the ballast is the layer that contributes most significantly to the degradation of the geometric quality 1, 2. When necessary, and in response to this track behaviour, railway infrastructure managers (RIM) carry out maintenance operations to restore adequate quality levels of track geometry. To enable more efficient rail operation management, studies have been developed and several empirical relationships using simplistic approaches have been proposed over the years for predicting the track degradation behaviour 3, 4. The experience regarding the operation of conventional or high-speed railway lines (HSL) points out that, at some locations, the degradation process of the track is clearly faster than normal and the application of empirical degradation relationships to predict settlements becomes pointless. At these locations, maintenance operations are required more often to re-establish proper geometric quality of the track and to maintain adequate passenger comfort and safety levels. Moreover, these locations usually correspond to specific singularities in the track, which in general evidence vertical stiffness discontinuities associated with changes in the track superstructure (rail, fastening system, sleeper and ballast) or substructure 5, 6. Such occurrences have been reported in studies developed in Spain on the analysis of the track geometry measurements carried out between 1992 and 2002 in the Madrid-Seville HSL 7. The analysis of the results indicate that maintenance works at transitions to bridges or to box culverts can be up to three or six times higher, respectively, when compared with open track. Earlier European studies point out that maintenance frequency at these locations may be five times higher and unit costs twice as high, because standard maintenance procedures cannot be applied 8. This paper presents a study on the design and construction of backfills at bridge approaches and aims to contribute to understand and minimize problems related to the substructure that arise at transition zones. To that end, a literature review was carried out and the design and construction of a particular transition zone is presented as a case study. The results of the study suggest that the adopted design for the case study was successful in limiting track settlements and achieving a gradual vertical stiffness increase of the backfill as approaching the bridge.

2. A REVIEW ON TRANSITION ZONES FOR RAILWAY TRACKS 2.1 General Aspects Already in the 70's some European authors addressed the negative aspects of track behaviour at transition zones 9. The causes were reported to be related to variations in the track vertical stiffness, to displacements and rotations of the bridge deck and to differential settlements at the transition. It was also noted that all these aspects contributed to the increment of the dynamic load of the trains 9. More recently, as referred in the literature and as evidenced by field measurements, important amplifications of the dynamic loads of the trains have been identified at these zones, in particular, vertical wheel-rail interaction forces 5, 10, 11. Severe cases of accelerated degradation can be caused by various factors and amplified by their combination, as observed recurrently at transitions between embankments and structures. Among the causes that contribute to the rapid track degradation, the following can be highlighted: i) abrupt variations in track stiffness, due to different mechanical characteristics of the materials in the superstructure or substructure (Fig. 1a), cause differential elastic track deflections which result in increasing dynamic loads that can trigger localized degradation; ii) important differential settlements of the backfill and its foundation (Fig. 1b), considering that no significant settlements are expected for the abutments or other structures, also increase the dynamic loads and cause further damage. This factor can lead to severe cases of geometry defects at bridge approaches with the deterioration of support conditions for a few sleepers at the transition, as depicted in Fig. 1b. In fact, unsupported sleepers can also be regarded as localized abrupt reduction of track stiffness, thus aggravating even more the difference between the track stiffness on embankment and on bridge, and further increasing the track degradation rate 12, 13.

2


Lower track stiffness sleeper

Higher track stiffness

increased track degradation rate

rail

soft substructure

ballast

fastenings

stiff substructure

a)

Lower vertical stiffness initial rail position voids

b)

differential settlements (backfill + foundation)

backfill

Higher vertical stiffness unsupported sleepers

bridge/box culvert

Fig. 1: Schematic representation of causes for rapid track degradation at transition zones.

Several railway entities have identified these problems and have suggested constructive measures to control the rapid degradation of the track, to smooth the abrupt stiffness transitions and to minimize differential settlements. The use of wedge shaped backfills and transition slabs, or track resilient components, like soft rail pads or under sleeper pads, are among the proposed measures 5. The former Japanese National Railways (JNR) was one the first to incorporate wedge-shaped transition zones in the backfills of bridge abutments for HSL. In the 70’s, French railways recommended wedges built with cement bound granular mixtures. Around that time, soil-cement mixtures were also applied in the top layers of the transition zones in the Roma-Florence HSL, for lengths of about 20 m on the backfill. Many studies have followed over the last decades, focusing on understanding the causes and suggesting solutions to minimize the negative aspects of transitions. In the late 90’s the former European Rail Research Institute (ERRI) promoted a joint project to carry out a study on the state of the art and eventually to prepare guidelines for design, construction and maintenance of transition zones between embankments and bridges 8. The study focused on various aspects of transition zones, from the point of view of railway, bridge and geotechnical engineering. A review of the technical specifications and current practices in construction was discussed and examples of transition designs applied by various railway infrastructure managers (RIM) were presented and compared. In recent studies, some authors suggest that the design of transition zones should be based primarily on minimising potential track faults and permanent deformation of the substructure in the transition zone rather than just simply providing a smoother track stiffness variation 11. In fact, problems related to the substructure behaviour are hard to identify and can be far more difficult to solve than the alternative approach of frequently lifting the rails and adding more ballast. Currently, it is worldwide common practice to construct wedge-shaped backfills at approaches to bridge or to box culverts in HSL. In general, the construction of these wedges-shaped structures follows specific geometries and comprises well-compacted layers of selected granular materials. In addition to reducing differential settlements, the wedge shaped backfills materialize a gradual transition of vertical stiffness between the embankment, built with traditional geomaterials, and the structure. With this objective, the materials applied in the wedges usually show higher deformation moduli and are less sensitive to plastic deformations than the geomaterials generally applied in embankments. For that reason, layers of unbound granular material (UGM) and cement bound granular mixtures (CBGM) are frequently applied in backfills 8. In some countries, these structures are called "transition wedges" or "technical blocks" 14. Despite the efforts that were undertaken in the past to minimize the negative effects that arise in these locations and although it is widely recognized the need to include a stretch that provides a smooth stiffness transition, in general, 3


transition zones continue to exhibit poor performance. Actually, at present days, a great amount of maintenance effort is still spent at these locations, with negative consequences in terms of costs, track availability and restrictions in train operations. Various transition designs have already been proposed by the railway industry to solve this problem. Frequently, the poor performance of these structures results from faulty execution of the backfill, in particular, the use of less suitable materials, inadequate compaction of the layers and poor drainage conditions 15. This situation is common in both old and new railway lines, aimed at high performance. With regard to road infrastructures, severe situations are also commonly reported, though repair operations seem to be easier to carry out in general. Several methods for rehabilitation and improvement of the performance of these zones have been proposed to address specific problems. Some of the suggested solutions are primarily to improve the track foundation, particularly to increase its bearing capacity and to control settlements 8, 16-19: i) replacement of existing geomaterials in the backfill with other natural material with better mechanical properties; ii) application of geosynthetics (geotextiles, geogrids, geomembranes or geocells); iii) soil stabilization or reinforcement of the trackbed layers with binders (lime, cement or bitumen); iv) construction of reinforced concrete transition slabs under the track; v) execution of piles (steel, reinforced concrete, gravel or wooden); vi) injection of cement grout into the backfill. Recent experimental studies have been developed in order to understand the problem and contribute to the minimization of maintenance efforts at these zones 20, 21. Numerical models have also been intensively used in other studies to address various aspects of transition zones 11, 13, 22-25. However, considering the high occurrence of sharp stiffness transitions along railway lines and the limitations of the solutions that have been implemented, some authors highlight that further research is still required to better understand the problems at transition zones 11.

2.2 Wedge-shaped backfills for transition zones Studies regarding measurements of track modulus at transition zones with a train loading vehicle evidenced that CBGM material seems to provide better gradual stiffness variations at bridge approaches, as compared to hot-mix asphalt or geocell trackbed reinforcements 15. In general, the current practice for transitions to bridge abutments or to box culverts in HSL comprises the construction of wedge shaped backfills. Normally, the backfill is composed of two main parts (Fig. 2): i) a wedge with layers of cement bound granular mixture (CBGM) next to the structure walls; ii) a wedge with layers of selected unbound granular material (UGM). Some designs also comprise treated/reinforced layers and/or increment of thickness of the upper layers at the approach to the structure. Other materials are also suggested for the abutment backfill base (AB) under the wedge-shaped CBGM.

HCBGM

H

structure (abutment/ box culvert)

LUGM LCBGM

1

sleepers

rail

1

2

2

3

3

- ballast layer - sub-ballast layer - reinforcement layers

h CBGM

h

v

UGM

v

embankment

AB backfill foundation

Fig. 2: Schematic design of wedge-shaped transition zones for various RIM (see Table 1).

Over the last decades, designs for transition zones of HSL have evolved. Table 1 provides a comparison between transition zones designs for which the authors were able to find relevant information. 4


Although not discussed here in detail, some RIM establish slightly different geometries for the transition zones depending 26-28: on the height of the backfill (H); if the structure was constructed before or after the embankment; on the type of structure (open or closed abutment, shallow or buried box culvert). Table 1: Comparison of different examples of transition zones for high-speed railway tracks (see Fig. 2). Aspects of the transition zone

France14, 26, 29

backfill foundation aspects

stripped or treated natural ground

Germany*30 EV2 ≥ 45MPa

Italy27

Spain14, 28, 31

Japan32 33

China34

Portugal+

N/I

specific geometry of the relief

depends on design

concrete or gravel at the abutment base Evd ≥ 30MPa

N/I

same as CBGM

same as CBGM

same as CBGM

≥2

3-5

3

H

H

H

1.15:1 4 ≥95 OPM PS: 0/37.5 Rc > 2 MPa geogrids with 0.3m spacing

2:1 to 5:1 3 ≥95 OP

1:1 5 ≥98 OPM

inverted wedge is used Evd ≥ 50 MPa

PS: 0/31.5 Rc > 1 MPa Rit > 0.25 MPa

N/I N/I N/I

N/A N/A N/A

20 3:2 ≥95 OPM

N/I

N/A

PS: 0/31.5

if H < 3 m: only CBM AB 3 < H < 10 m: if H≥4: (material UGM below UGM; between 3m same as CBGM same as CBGM backfill H > 10 m: else: foundation and selected same as CBGM CBGM wedge) material PS: 0/100 below 10m Aspects of the wedge-shape fill using cement bound granular mixtures (CBGM): LCBGM (m) 1 ≥1.5 1 3 HCBGM (m)

3

≥3

if H>4; 3 m else; H

H

≥1:1 1:1 1:1 2.5-3 3-5 >3 ≥98 OP ≥95 OPM ≥95 OPM PS: 0/32 prepared in prepared in the top 0.5m is plant only other aspects PS: 0/31.5 plant only UGM Rc > 3-7 MPa EV2 ≥ 160MPa EV2 ≥ 80MPa Rit > 0.2 MPa Aspects of the wedge-shape fill using selected unbound granular material (UGM): LUGM (m) 5 ≥20 ≥8 20 slope (h:v) 3:2 ≥2:1 2:1 3:2 Dc (%) N/A ≥100 OP ≥98 OPM ≥95 OPM EV2 ≥ 150MPa EV2 ≥133MPa PS: 0/20 to 63 other aspects PS: 0/45 EV2≥80MPa CBR ≥ 50 d/s ≥ 80% Aspects of the upper layers: CBGM e = 20 reinforcement UGM frost protection + layer (e.g. e = 40 to 60; no layer over UGM e = 20 PS: 0/31.5 capping, frost capping on top embankment + BC = 3% protection) of CBGM CBGM e = 20 for a length of 10 m stabilized with sub-ballast bituminous binder over e = 30 layer layer; e = 12 10 m with e=50 former TGV +5 cm +15% thickness transition zones thickness transition along ballast layer included N/I transition along 3 m, over the variation in 3 m, at the backfill thickness CBGM slope (h:v) BC (%) Dc (%)

1:1 3 N/A

N/I

N/I

N/I

treated granular material with BC=5% Dc=97% OP Evd ≥ 55MPa

N/I

same as CBGM or UGM, respectively

e = 30

e = 30

Notes: *different geometries for the transition are established for lines with slower speeds; +case study addressed to in Section 3; EV2 - deformation modulus obtained in the 2nd load of the plate load test 35, 36; Evd – dynamic deformation modulus 37; N/A - not applicable; N/I - no information; H and L - see Fig. 2; BC - Binder content; Dc - Degree of compaction; OP and OPM - Optimum Proctor and Optimum Proctor Modified test compaction conditions; PS – min./max. particle size in mm; Rc and Rit – compressive and indirect tensile strength of CBGM; CBR – Californian Bearing Ratio; e layer thickness in cm.

The transition designs analysed share some similarities: i) a backfill with layers of selected materials stretching out up to about 20 m from the structure wall and ending in a wedge-shaped fill with slope of 3:2 to 2:1 (h:v), thus providing a gradual transition to the embankment; ii) next to the structure walls, a wedge-shaped fill with 1:1 slope using layers of CBGM is generally applied with binder content (BC) of about 3-5%; iii) well-compacted layers of UGM are usually 5


applied in the wedge-shape fill between the CBGM wedge and the embankment; iv) drainage elements between the CBGM wedge and the structure. The Japanese innovative transition solution comprises geogrid reinforcements mostly due to seismic risk 33. Some RIM establish localized cemented/treated layers of sub-ballast or capping at the approach to the structure, or bituminous sub-ballast layer on top of CBGM layers interposed with UGM layers, as suggested in Italy 27. Additionally, some specifications comprise designs for transition zones with excessive skew 14, 26, 28. Most RIM establish minimum reference values of deformation modulus measured in situ during construction, on top of the layers, as performance based requirements. The EV2 modulus, obtained in the second load of the plate load test 35, 36 , or the Evd dynamic modulus 37, obtained with light falling weight deflectometer, are commonly used as reference parameters. It is assumed that higher deformation modulus values are related to good performance of the layers, both in terms of resilient behaviour and susceptibility to permanent deformation. It should be noted that the measured moduli (EV2 and Evd) depend on the resilient modulus of the materials comprising the various underlying layers. Thus, it is clear that the application of selected materials in different sections of transition zones, complying with established requirements, must be specified precisely. Moreover, the engineering disciplines involved in the railway project ought to think through the design of transition zones together with the aim to: i) facilitate execution of earthworks, for example assuring that the geometry of abutment simplifies compaction of the backfill layers, avoiding situations as depicted in Fig. 3a; ii) provide a smooth stiffness transition considering the train speeds, the deformability of the materials applied in the backfills and the stiffness of the track resilient elements; iii) minimize long-term settlement resulting from the behaviour of both the substructure and the superstructure of the track. Regarding the construction of the transition zone layers (embankment soils, UGM and CBGM) it is necessary to implement quality assurance and quality control procedures. Good compaction of the layers is required taking into account the thickness, the water content during compaction of the materials, the compaction equipment used plus the number of roller passes. Adequate compaction equipment is crucial bearing in mind constraints to use heavy compaction near the structure walls and less accessible places of the backfill (Fig. 3). Aspects of the CBGM need to be establish in detail, such as the BC and mix in place or mix in plant, regarding the need to achieve a good homogeneity of the mixture. It is noted that mix in place is rejected by some RIM.

a)

b)

Fig. 3: Construction aspects of transition zones: a) example of abutment geometry providing very limited access to compaction equipment; b) compaction of the CBGM inside the abutment with 11t single drum roller.

3. CASE STUDY OF TRANSITION ZONE CONSTRUCTION 3.1 General aspects of the transition zone case study The study presented herein regards the construction of the transition zone at the south end of a railway bridge over Sado River in Portugal that is part of a new 30 km rail bypass. The line allows mixed traffic with maximum speeds of 220 km/h for passenger tilting trains. The track has Iberian gauge (1.668 m) and comprises UIC60 rail resting on concrete monoblock sleepers, also prepared for UIC gauge. 6


In Fig. 4, a general drawing for the design of the transition case study is presented. Some characteristics of the transition were presented previously in Table 1. It comprises a backfill with a smaller wedge-shaped transition made of CBGM layers, followed by larger wedge of UGM layers providing the transition to the embankment. The design did not establish changes in the track superstructure at the transition zone. It is worth noting that the abutment type is open, about 9 m high, with two central columns supporting the cap beam, where the end of the bridge deck rests. The abutment has two relatively large wingwalls on the sides, both featuring one counterfort in the backfill. The abutment rests on a large footing covering all its structure which is supported on pile foundations. 20 m 1

3m rail

sleepers

1

2

2

3

3

- 30 cm ballast layer - 30 cm sub-ballast layer - 20 cm capping layer

9m

bridge deck

abutment

CBGM

1 1

embankment UGM

3

2

backfill foundation

Fig. 4: Transition zone design applied to the case study.

A well graded crushed limestone aggregate was selected both for the wedge-shape UGM and capping layers. The sub-ballast layer was made of well graded crushed granite aggregate with slightly more demanding requirements than those for the materials applied in the UGM or capping layers, namely the Los Angeles resistance to fragmentation. Grading envelopes and some of the required properties for these aggregates are presented in Fig. 5. Regarding the CBGM, a mix in plant of the previous limestone aggregate was used with BC = 5% by weight (Portland cement). As mentioned in Table 1, a minimum compressive strength (Rc) of 1 MPa was required for specimens after curing for 7 days. The minimum indirect tensile strength (Rit) required after curing for 60 days was 0.25 MPa. Laboratorial tests performed showed that the mixture complied with the requirements: average Rc values of 9.7 MPa and Rit values of 1.3 MPa were obtained.

3.2 Characterization of the backfill materials using cyclic load triaxial testing In general, unbound granular materials applied in roads and railways show non-linear resilient behaviour, depending on the applied stress level, the degree of compaction and water content, among other factors 38. Aiming at characterizing the materials for a mechanistic design approach 39, laboratorial tests have been developed, in particular cyclic load triaxial tests, to assess the resilient and long term behaviour of these materials for different state conditions and stress paths 40. Such tests also allow ranking the aggregates based on their resilient modulus (Er) and susceptibility to permanent deformation. In the present study, cyclic load triaxial tests were performed on large specimens 41, following the European Norm EN 13286-7 42. Although in service the aggregates undergo cyclic loads with relatively low stress levels, during construction they suffer much severe conditions that result from transport to the site, compaction with heavy rollers and traffic by heavy construction vehicles. Thus, in order to assure that the materials are not severely altered, they were characterized using the “high stress level” procedure established in the European Norm 42. In Table 2, some aspects of the tested specimens are presented, including the respective difference between the water content (w) and the optimum water content (wOPM), the degree of compaction (Dc) and the resilient modulus obtained for the ranking stress level of (𝜎1 − 𝜎3 ) = 500 kPa and (𝜎1 + 2𝜎3 )/3 = 250 kPa. Examples of best-fit curves for the material’s Er modulus are presented in Fig. 6, evidencing the typical non-linear behaviour of these materials with the stress level. The resilient modulus values in Table 2 seem to evidence the increase of Er with the reduction in water content. Earlier studies 7


reported similar conclusions and resilient modulus ranging from 300 to 1000 MPa were observed for well graded crushed aggregates, adequate for unbound layers, in similar stress paths and state conditions 43. According to the European Norm42, both materials are ranked as class C1, denoting excellent performance 41. It should be mentioned that it was not possible to carry out laboratorial dynamic tests on the materials for frequencies similar to those resulting from train passages. However, some studies on similar materials have concluded that the frequency of the load cycles does not seem to constitute important factor in reversible behaviour of granular materials, up to about 20 Hz 44, 45. 100

Percent passing by weight

90

Parameter

Requirement

Liquid Limit (%)

≤ 25

Plasticity Index (%)

≤6

80

Sand equivalent value (%)

 40

70

Methylene Blue, in g/kg, of material