Dynamic behaviour of transition zones in railwaysB. E. Z. Coelho1

Delft University of Technology, Delft, the Netherlands

P. Hölscher Deltares, Delft, the Netherlands

F. B. J. Barends Delft University of Technology and Deltares, Delft, the Netherlands

ABSTRACT

Transitions between the free track and fixed structures such as bridges and culverts often require substantial additionalmaintenance to preserve line and level and ride quality. This extra maintenance not only increases costs but causes delays. Despite its importance for railway infrastructure owners, the fundamental cause of the poor performance of transition zones isnot fully understood. To gain a better insight into the physical mechanisms that are involved, an extensive field investigation hascommenced on a known problematic transition zone in the Netherlands. The transition zone chosen is typical of those found onDutch Railways, consisting of two reinforced concrete slabs, which span between the free track and a concrete culvert. The freetrack consists of 4 m high embankment, initially made from sand, on top of a peat layer 7 m thick. At a depth of 11 m below thetrack there is a sand layer, in which the piles that support the culvert are founded. In this paper the dynamic behaviour of thetransition zone in response to routine passenger trains will be presented, together with the numerical analysis. Keywords: railway, transition zones, dynamic behaviour, static behaviour

1 TNO-Bouw, Van Mourik Broekmanstraat 6, 2628 VK Delft, Netherlands. bruno.coelho@tno.nl

1 INTRODUCTION

Preservation of track quality is one of the main concerns for railways infrastructures managers, in line with safety regulations and passenger comfort. Because of increasing train speed and axle loads, standards are becoming more restrictive regarding the admissible limits for track quality. This leads to an increase in maintenance (given current design and construction techniques). Recent studies [1,2] have shown that track maintenance represents between 40 to 70% of the total budget spent for railway maintenance.

Transition zones are among the most demanding locations. These link the free track to (or from) a structure, such as bridges, tunnels or

culverts. Despite its importance, very little research has been undertaken to understand the physical causes of the track degradation phenomena.

At present there are only limited experimental data on the behaviour of transition zones. Most experimental work has been performed on normal track, with attention focussed on wave propagation effects [3-7]. Kolisoja and Mäkelä [8] evaluated transition behaviour from a structural point of view, but obtained no significant information about the soil/track behaviour. Jenks [9] and Lundqvist et al. [10] presented track stiffness profiles, and determined relationships between stiffness and track degradation; both papers stated that the problems with transitions are due to the abrupt stiffness

variation between the normal track and that on the structure.

Thus to overcome this shortfall in knowledge, and therefore improve the design and maintenance of transition zones, an extensive monitoring program has been initiated in the Netherlands, over one complete year. The main goal was to gain a better insight into the physical mechanisms that occur at a transition zone, not only during individual train passages, but also the cumulative effect these have on track behaviour and the influence of the maintenance cycles.

2 SITE OVERVIEW

The site chosen for the monitoring is located in the centre of the Netherlands, and consists of a transition zone between an embankment and a piled concrete culvert. The track is composed by UIC54 rails connected to timber sleepers, on top of a ballast layer. The transition structure itself comprises two reinforced concrete slabs, each of 4 m length, on both sides of the culvert. The culvert is a 2 m square box to allow for water flow between the two sides of the railway track and is a typical solution used in the Netherlands.

The soil profile comprises a 4 m embankment on top of a 7 m peat/clay layer, with a sand layer in between. At about 11 m depth a stiffer sand layer is observed, in which the piles that support the culvert are founded.

Figures 1 and 2 show the results of a Vertical Seismic Cone Penetration Test (VSPT) carried out at the site. The discrepancies within and between the traces indicate that the embankment is quite heterogeneous. The very low cone resistance of the soft peat/clay layer together with the low shear wave velocity, confirms that its mechanical properties are poor, and there is a thin sand layer at about 8 m depth.

The ballast thickness was determined by means of trial pit excavations and Ground Penetrating Radar (GPR). The initial ballast thickness (according to the design drawings) was 30 cm, but as shown in Figure 3, on top of the culvert the ballast is now about 80 cm thick. Above the free end of the transition slab the

ballast thickness is about 1.2 m - much more than on the embankment. This is the result of years of maintenance, primarily comprising tamping and re-ballasting.

Figure 1. Tip resistance.

Figure 2. Shear wave velocity.

3 MONITORING PLAN

The dynamic measurements of displacement, velocity and acceleration were undertaken during scheduled train services. In total, 14 train passes were recorded with the types of trains shown in Table 1. Nine uniaxial accelerometers (A), three triaxial accelerometers (T), eleven geophones (G), and 1 video camera were used to measure accelerations, velocities and displacements of the track and soil, respectively.

Figure 3. Ballast thickness and approach slab position from

Ground Penetration Radar

Figure 4 shows the locations of the geophones and accelerometers. Geophones were mounted both on the sleepers and within the soil; all of the accelerometers were located within the soil. Transducer G8 measured in the horizontal direction parallel with the track. Transducers T1, T2 and T3 were triaxial accelerometers, measuring in three mutually perpendicular directions (vertical, lateral and horizontal) at each of the three locations. Table 1. Train information

Train type Number of train passages

Range of speeds

Intercity double deck (IDD)

3 114-130

Intercity single deck train set (ISD)

4 98-129

locomotive with carriages (ILC)

2 126-130

Local train (LT) 5 89-114

4 STATIC BEHAVIOUR

The results from the static vertical displacement of the track are shown in Figure 5. These results cover the time period between October 2008 and June 2009, which corresponds to a complete maintenance cycle (first levelling was performed directly after maintenance had been performed). Between the measurements of October and November significant settlement of the track occurred. This is most likely due to ballast densification during train traffic after maintenance. Afterwards, the points located on top of the culvert show no settlement, but the points located above the approach slab and embankment continues to settle due to consolidation or secondary compression. This becomes clear in Figure 6, where the track settlement rate is shown. After an initial fast settlement, the settlement is approximately constant. The pattern found for the track settlement follows the ones observed in the literature [11,12].

Figure 4. Setup overview.

From a comparison between the left/east and

the right/west side of the culvert it can be observed that, though the behaviour seems different when considering the total vertical displacement (Figure 5), a close look to the displacement rate shows that the results on the two sides of the culvert are quite similar (Figure 6). The differences on the total displacement are due to maintenance procedures.

Figure 5. Track settlement during one maintenance cycle.

Figure 6. Settlement rate during one maintenance cycle.

5 DYNAMIC BEHAVIOUR

Dynamic track displacements calculated from geophone velocities are presented in Figure 6 for three different sleepers (G1, G3 and G7 in Figure 7). The calculation of displacements from geophones or accelerometers requires a single or double integration procedure, respectively. For the geophones data, the procedure described in [13] was used, while for the accelerometers the procedure proposed in [14] was followed.

The results show that the maximum downwards displacement occurs when the wheel is above the monitored sleeper, while the maximum upwards motion is obtained in between the passage of the bogies of a coach. It is observed that the peak to peak displacement in geophone G3, located in the middle of the transition slab, has a significant higher amplitude (~6 mm) than geophone G7 (~1.5 mm), located on the free track, and than geophone G1 (~0.7 mm), on top of the concrete culvert. The minimum displacement amplitude is obtained, as expected, for the culvert and is approximately half of that recorded on the free track (G7). Unexpectedly, the greatest result is found for the geophone placed on top of the approach slab

(G3), for which the displacement amplitude is about eight times greater than on the free track (G7). This indicates that, instead of a smooth reduction of the displacement when the train is moving towards the culvert, the displacement reaches a maximum value over the approach slab.

Figure 8 presents the average amplitude of the vertical displacement for all the geophones, regarding four different train types (according to Table 1). The sleeper located at the beginning of the approach slab consistently produces higher displacement. A similar pattern and magnitude of displacements is found for all the different trains. The results exhibit that the apparent dynamic stiffness reaches a minimum over the approach slab, a maximum recorded on top of the culvert, and varying along the free track. This apparent stiffness demonstrates that the design requirements of having a smooth stiffness increase between the free track and the culvert (as suggested in [15-17]) are not realistic.

Figure 7. Displacement during train passage of an IDD train

at 114 km/h.

Figure 8. Average displacement amplitude along the

transition zone for different train types.

6 NUMERICAL ANALYSIS

In the present analysis, the Tochnog Professional FEM package is used [18]. An elastic material formulation coupled with groundwater flow is adopted. In the current analysis, a three–dimensional model is used to evaluate the behaviour of the system. All the components of the track and soil are modelled by means of low–order isoparametric brick elements (8 nodes). The maximum element size is chosen to be 0.40 m which is in agreement with the limit of ten elements per wavelength, as defined in [19], assuming a maximum frequency of interest of 10 Hz, and a minimum shear wave velocity of 40 m/s (Figure 2). Smaller element sizes are often used in the model due to geometrical constraints. The maximum aspect ratio of an element is 5, which is enough to avoid errors due to artificial stiffening [20]. The time–step used for the analysis is 0.0075 s, smaller than the limit of Tmin/10, where Tmin represents the minimum period of interest [21].

For the model layering and parametrisation, data from Section 2 are used. The shear modulus G corresponds to the dynamic value determined through the VSPT data, by the following relation:

2

sVG ρ= (1) where Vs represents the shear wave velocity. Further details about the model are available in [22]. An overview of the model is presented in Figure 9.

The behaviour assessment of the initial configuration of the structure is made by considering the passage of constant axle loads (180 kN) with distances that produce the IDD train configuration (for further details about train characteristics the reader is referred to [22]).

Figure 10 presents the vertical strain σzz due to the train passage for point P6, located in the ballast at the start of the approach slab. The result shows that the assumption of small strain is admissible, since the maximum recorded strain is about 0.08 o/oo. The strain magnitude is in agreement with the experimental findings of [8].

Figures 11 and 12 show the time response for both displacements and stresses for point P6, located in the ballast. It follows that while for the displacement (Figure 11) the problem is governed by the z (vertical) direction, the stresses (Figure 12) have significant components in all three directions. For the stresses each wheel is identified in the time response while for the displacements only the bogies are visible.

Figure 9. 3D mesh.

Figure 10. Vertical strain for a point located in the ballast.

This shows that stresses are more pronounced

at higher frequencies than displacements. The stresses change during the passage of the trainload. This causes a cyclic rotation of the principal stresses, which might have consequences on the long-term deformation of the soil.

Figure 11. Displacement for a point located in the ballast.

Figure 13 shows the stress path for point P6

located on the ballast, in the p−q diagram, whereas:

Figure 12. Stress for a point located in the ballast.

3'''

' zzyyxxpσσσ ++

=

( ) ( ) ( )

( )] .3

2''''''

21222

222

xyxzyz

yyxxxxzzzzyyq

τττ

σσσσσσ

+++

+

−+−+−= (2)

As the first wheel approaches P6 (Figure 13)

the stress path moves from O to A. The stress path then unloads from A to B, and returns to A as the second wheel crosses P6. As the first bogie crosses the point of analysis, the stress path returns to the origin O. Again, as the second bogie crosses point P6 the stress path goes from O to A. Therefore, two different stress path are identified. From O–A, related with the loading and unloading due to the each single bogie, and A–B due to the unloading reloading caused by the wheel loads of the bogie.

Figure 13. Stress path p'-q for a point located in the ballast.

7 CONCLUSIONS

This paper has reported a field investigation into the dynamic behaviour of a transition zone during regular train services on a section of railway track in the Netherlands, together with a numerical analysis of the structure. The experimental results included the current position and make-up of the track bed and the approach slab, as well as the soil dynamic response (accelerations, velocities and displacements). The numerical part focused on the stress analyses of the ballast.

Significant track degradation after maintenance operation due to ballast compaction was observed, as well a long term degradation of the vertical track alignment.

The results have demonstrated that instead of the desired increase in stiffness across the approach slab, running from the normal track to the culvert, over the approach slab the stiffness was much less than either the normal track or the culvert. The reduction in track support stiffness was considerable, with observed vertical displacements being about eight times greater on the approach slab than on the normal track.

Two stress paths were identified for the ballast and embankment, one with higher amplitude in volumetric stress related with the passage of the bogies, and another with smaller amplitude related with the passage of the wheels of the bogies.

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