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Numerical and experimental study of stiff wave barriersfor the mitigation of railway induced vibrations
P. Coulier1, A. Dijckmans1, V. Cuellar2, A. Ekblad3, A. Smekal3, G. Degrande1, G. Lombaert11 KU Leuven, Department of Civil Engineering, Kasteelpark Arenberg 40, B-3001, Leuven, Belgiume-mail: [email protected]
2 CEDEX, Laboratoria de Geotecnia, Alfonso XII 3, 28014 Madrid, Spain
3 Trafikverket, 405 33 Goteborg, Sweden
AbstractRailway induced vibrations are an important source of annoyance in the built environment, causing malfunc-tioning of sensitive equipment and nuisance to people. Within the frame of the EU FP7 project RIVAS, miti-gation measures on the transmission path between source (railway track) and receiver (surrounding buildings)have been investigated. This paper reports on the numericaland experimental study of stiff wave barriersas efficient vibration reduction measures. Numerical simulations have demonstrated that the wave impedingeffect of such barriers depends on the stiffness contrast between the surrounding soil and the barrier, as wellas on the barrier’s depth. Findings from the numerical studies are verified by means of two field tests. InEl Realengo (Spain), a continuous barrier has been created close to an existing railway track using overlap-ping jet grout columns, while a sheet pile wall has been installed along a track in Furet (Sweden). At bothsites, geophysical and geotechnical tests were carried outprior to the installation of the mitigation measuresfor the determination of the dynamic soil characteristics.Measurements of train passages before and afterinstallation of the barriers are compared to numerical simulations in order to assess the vibration reductionefficiency. In El Realengo, additional measurements have been performed at a reference site adjacent to thetest site in order to correct for variations of track, train,and soil characteristics in time. It is shown that bothbarriers are effective and result in vibration reduction from 8Hz (El Realengo) and4Hz (Furet) on, respec-tively; the largest reduction is obtained immediately behind the barriers. This ability to solve low frequencyvibration problems is a unique feature compared to most other vibration mitigation measures for existingrailway lines.
1 Introduction
During the past decades, a lot of research has been performedto develop efficient and cost–effective vibrationcountermeasures for reducing the levels of railway inducedbuilding vibration [1, 2]. Measures can either betaken at the source [3], on the propagation path between source and receiver [4], or at the receiver [5]. Anadvantage of interventions on the propagation path is that no modifications of the track are required, whilemultiple buildings can be shielded simultaneously from vibration. Furthermore, this type of measures canrelatively easily be implemented in existing situations. Typical examples are vibration isolation screens [6],buried wall barriers [7], and wave impeding blocks [8].
Within the frame of the EU FP7 project RIVAS [9], several mitigation measures on the propagation pathbetween source and receiver have been investigated in detail. This paper reports on the numerical andexperimental study of so–called stiff wave barriers (i.e. barriers consisting of material that is stiffer thanthe surrounding soil) as efficient vibration reduction measures. In particular, two types of such barriers areconsidered: a jet grouting wall and a sheet pile wall. Both measures have been studied in detail using a
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coupled finite element – boundary element approach, while field tests have been carried out to verify thepredicted performance of these mitigation measures.
This paper describes the results of both field tests and provides a comparison with results from numericalsimulations. This allows for a physical interpretation of the experimental results and also reveals to whatextent numerical simulations can be used for designing these mitigation measures. The outline of the paperis as follows. Section 2 presents the numerical and experimental results for the jet grouting wall at the site ofEl Realengo (Spain). The test with the sheet pile wall at the site of Furet (Sweden) is subsequently consideredin section 3. Final conclusions are summarized in section 4.
2 Jet grouting wall
Numerical simulations have demonstrated that a stiff wave barrier adjacent to a railway track can hinder thetransmission of waves and act as an effective wave impeding barrier. The underlying physical mechanismhas been revealed in [10], highlighting how the performancedepends on site specific characteristics such asthe dynamic soil characteristics. These insights have beenused to design a jet grouting wall for a field test inEl Realengo (Spain). This section discusses the implementation of such a wall and compares the numericalsimulations with the experimental results.
2.1 Description of the test site and the mitigation measure
A suitable site for testing a stiff wave barrier has been identified in El Realengo (south–east of Spain) alongthe conventional railway line between Murcia and Alicante.Previous geotechnical studies indicated thepresence of soft soil layers at this site, which is a situation in which stiff barriers are expected to be veryeffective. At the site, a test section as well as a reference section have been identified. The jet grouting wallis implemented along the test section; the aim of the reference section is to control changing track, train, andsoil conditions over time.
Geophysical tests have been performed in April 2012 for the determination of the dynamic soil character-istics [11]. This includes Spectral Analysis of Surface Waves (SASW) tests, seismic piezocone down–holetests (SCPTU), and seismic refraction tests. These tests have allowed for the identification of a simplifiedhorizontally layered soil model, as summarized in table 1 (layer thicknessh, shear wave velocityCs, dilata-tional wave velocityCp, material damping ratiosβs andβp in both deviatoric and volumetric deformation,densityρ). The soil density values given in the table are those determined from undisturbed samples retrievedfrom the boreholes drilled in previous geotechnical studies. The identified soil profile confirms the presenceof a soft layer of silty clay with a thickness of±10m (layers 2 and 3) that overlies hard alluvial soil.
Layer h Cs Cp βs βp ρ[m] [m/s] [m/s] [-] [-] [ kg/m3]
1 0.30 270 560 0.123 0.123 18002 1.20 150 470 0.112 0.112 17503 8.50 150 1560 0.014 0.014 17504 10.00 475 1560 0.010 0.010 19005 ∞ 550 2030 0.010 0.010 1900
Table 1: Dynamic soil characteristics at the site in El Realengo.
The stiff wave barrier is designed as a jet grouting wall composed of overlapping grout columns (figure 1).Based on the preliminary design [11], a jet grouting wall with a depth of7.5m, a width of1m, and a lengthof 55m has been constructed adjacent to the railway track. The diameter of the individual columns is1.5m.For safety reasons, the jet grouting wall has been constructed at a distance of16.2m from the track.
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(a) (b)
Figure 1: (a) Sketch of the stiff wave barrier consisting of overlapping jet grout columns and (b) the stiffwave barrier after construction at the site of El Realengo.
During installation of the jet grouting wall, multiple testsamples have been taken out to verify its strength andstiffness. Laboratory tests (unconfined compression tests, non dispersive P-S sonic tests, dispersive benderelement tests) have been performed on these samples; the best estimate of the dynamic characteristics (twomonths after construction) is given in table 2. The lower part of the barrier is saturated, resulting in a variationof the dilatational wave velocityCp with depth.
Layer h Cs Cp βs βp ρ[m] [m/s] [m/s] [-] [-] [ kg/m3]
1 1.50 600 1150 0.03⋆ 0.03⋆ 14002 6.00 600 1650 0.03⋆ 0.03⋆ 1400
Table 2: Dynamic characteristics of the jet grouting wall. Estimated values are indicated by a star.
The track at the test site is a classical a ballasted track with bi–block reinforced concrete sleepers supportingRN 45 rails with Spanish gauge. The ballast layer with a height of 0.50m is supported by an embankment0.50m high. The reader is referred to [12] for a detailed description of the test site.
2.2 Experimental evaluation of the mitigation performance
Extensive measurement campaigns have been carried out before (October 2013) and after (December 2013)construction of the jet grouting wall. Since some time was needed for the grout to harden out, vibrationmeasurements after the construction of the jet grouting wall were performed one month after construction.Transfer functions as well as train pass–bys have been measured, both at the test section and at the referencesection.
This paper only reports on the experimental results obtained during train pass–bys. A total of 28 and 30 trainpassages have been recorded before and after construction,respectively, of three different train types: S592commuter trains, S599 medium distance trains, and long distance Talgo VI trains. In the following, resultsobtained during the passage of S592 trains are discussed, assimilar trends are revealed for the other traintypes. The S592 commuter train is a short train consisting ofthree carriages. Each carriage has two bogies,one in the front and one in the back. Each bogie is supported bytwo axles. The train has a total length of65m between the first and last axle. Each axle has an estimated unsprung mass of2000 kg.
As only a single track is present at the site, both train passages from Murcia to Alicante and vice versa arerecorded. The train velocities are estimated based on strain measurements on the rails at the reference andtest site. The train speed varies between112 km/h and122 km/h for the S592 commuter trains (with an
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average speed of117 km/h). Free field vertical vibration velocities were measured bymeans of geophonesalong a line perpendicular to the track, at10m, 14m, 18m, and32m from the outer rail, both at the referenceand test site. The receiver locations at18m and32m are situated behind the jet grouting wall.
The vibration velocity levelLv(f) during a train passage is defined as the one–third octave bandspectrumof the stationary part of the vibration velocity. The stationary part of the measured response during a trainpassage can be selected using the German DIN standard [13]. Figure 2 shows the measured vibration velocitylevelsLv(f) during the passages of all S592 commuter trains before and after construction of the jet groutingwall, where different train velocities are indicated with adifferent shade (a lighter shade indicates a highervelocity). It is observed that the vibration velocity levelLv(f) decreases with increasing distance from thetrack; especially the high frequency components are attenuated due to material damping in the soil. Beforeconstruction of the jet grouting wall, a clear peak ofLv(f) near25–30Hz can be distinguished at10m,14m, and18m from the track. This peak is more pronounced at the test site than at the reference site.
The vibration reduction efficiency of the jet grouting wall is quantified through the vertical insertion lossILz(f). The measured insertion loss is defined as:
ILz(f) =(Ltest,beforev (f)− Ltest,after
v (f))−
(Lref,beforev (f)− Lref,after
v (f))
(1)
The first bracketed term in equation (1) characterizes the reduction of vibration levels at the test site (com-paringLv(f) before and after construction of the jet grouting wall), while the second term is a correctionfor possible variations in time (based on measurements at the reference site). The resulting insertion lossvalues obtained from the passage of S592 commuter trains areshown in figure 3. Immediately behind thewall (at 18m), insertion loss values of about5 dB are already obtained from8Hz on, with a peak of about12 dB at 25Hz, which is also the frequency range where the highest vibration levels are found during trainpass–bys. This corresponds to a reduction by a factor of four. As expected [10], the insertion loss valuesdecrease further away from the jet grouting wall (at32m), although it still reaches almost8 dB at 25Hz (areduction by a factor of 2.5). A slight amplification of vibration levels is observed at the opposite side of thebarrier (at10m and14m).
The results in figures 2 and 3 indicate that the jet grouting wall is able to reduce the vibration levels duringtrain passages from relatively low frequencies, which is a unique feature compared to most other vibrationmitigation measures described in the literature.
2.3 Comparison with simulations
The experimental results are compared to numerical simulations that have been performed by means of a2.5D coupled FE–BE methodology [10, 14]. The dynamic axle loads depend on the unevenness experiencedby the wheels at the wheel–rail interface. No information isavailable about the unevenness at the site in ElRealengo, however; an unevenness profile corresponding to apoor track quality has been employed in thenumerical simulations, using an FRA track class 1 defined by the Federal Railroad Administration (FRA).Consequently, it is impossible to provide a fair comparisonof measurements and predictions in terms of thevibration velocity levelsLv(f). Therefore, a comparison of the insertion lossILz(f) is presented. Due tocomputational limitations, the simulations are limited to72Hz.
Superimposed on figure 3 are predicted insertion loss valuesILz(f) for the passage of S592 trains. Areasonable qualitative agreement between the measurements and predictions is observed, as the same trendsare revealed. A slight amplification is predicted in front ofthe barrier, and a peak in the insertion lossnear25Hz is predicted at18m. The predicted insertion loss also decreases at larger distances from thebarrier. The observed deviations between measurements andpredictions of±5 dB are of the same order ofmagnitude as the common uncertainty in the prediction of railway induced vibrations [15]. This indicatesthat state–of–the–art numerical prediction models can be used for the reliable design of stiff wave barriers.
The simulations tend to underestimate the measured insertion loss values behind the wall. A possible ex-planation for the observed discrepancy could be the fact that the soil surrounding the jet grouting wall has
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Figure 2: Measured vibration velocity levelsLv(f) in one–third octave bands at the reference (left) and thetest site (right) during the passage of S592 commuter trainsbefore (grey) and after (red) construction of thejet grouting wall. The average train speed is117 km/h. The jet grouting wall is situated at16.2m from thetrack center.
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Figure 3: Measured (black line) and calculated (dashed line) vertical insertion loss valuesILz(f) in one–thirdoctave bands for the passage of S592 commuter trains at an average speed of117 km/h. The jet groutingwall is situated at16.2m from the track center.
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been stiffened during the installation of the jet grout columns. As a result, it is likely that a wave barrierwith a larger width than assumed in the calculations has beencreated in reality, which results in a better per-formance of the barrier. The discrepancies between the measurements and predictions could also be causedby simplifications introduced in the numerical model, such as the assumption that the jet grouting wall is ofinfinite length and the fact that a simplified vehicle model isused (i.e. only the unsprung axle masses areaccounted for, which is inaccurate at low frequencies).
3 Sheet pile wall
3.1 Description of the test site and the mitigation measure
At Furet, a district in Halmstad, Sweden, vibration problems occurred in several buildings close to a railwaytrack. In 2002 it was decided that residents should have lessthan1.0mm/s frequency weighted RMS at night(10pm-07am). Measurements indicated that the vibration levels exceed this value in at least eight buildings.The highest vibration levels were measured in the frequencyrange4 − 5Hz. Mitigation measures installedin 2006, including installation of under sleeper pads at thetrack closest to the buildings and leveling of thetracks, were only partially successful in reducing the vibration levels in the houses. To further reduce thetrain induced vibrations at the site, a sheet piling wall wasinstalled next to the track in 2011. Measurementsafter installation of the sheet pile wall have showed sufficient reduction of vibration levels, except for a smallhouse with wooden structure at approximately40m distance from the track.
Geotechnical and geophysical tests performed for characterization of the soil have shown that the site has arelatively firm layer of sand with a shear wave velocity of about 150 m/s and a thickness of 2 m on top ofa softer clayey silt layer with a shear wave velocity of about120 m/s and a thickness of 10 m [16]. Table 3provides a summary of the soil parameters (layer thicknessh, shear wave velocityCs, dilatational wavevelocityCp, densityρ, material damping ratiosβs andβp in both deviatoric and volumetric deformation) foreach layer used in the simulations.
Layer h Cs Cp βs βp ρ[m] [m/s] [m/s] [-] [-] [ kg/m3]
1 2 154 375⋆ 0.025⋆ 0.025⋆ 18002 10 119 290⋆ 0.025⋆ 0.025⋆ 18503 ∞ 200 490⋆ 0.025⋆ 0.025⋆ 1710
Table 3: Dynamic soil characteristics at the site in Furet.
Figure 4a shows a sketch of the final design of the sheet pile wall. The depth of the sheet piles is12m withevery fourth pile extended to18m. The sheet piles installed are VL 603-K profiles (figure??b). The totallength of the sheet pile wall is100m. The wall is interrupted at four places due to the presence ofpipes andcables in the ground. The distance from the center of the nearest track to the center of the sheet piling wallis approximately5.60m. Adjacent sheet piles were welded together over the top30 cm (figure??c). Afterinstallation, the sheet pile wall was covered with about half a meter of soil. During installation of the sheetpiles, the track geometry was controlled once a day. Furthermore, the vibration levels were monitored in allbuildings close by during installation. The measured vibration levels were lower than the limiting values forvibration damage to buildings. This was confirmed by the examination of the nearest buildings before andafter installation of the sheet pile wall, where no changes were detected.
3.2 Experimental evaluation of the mitigation performance
The insertion loss of the sheet pile wall has been evaluated by comparing the vibration levels due to trainpassages before and after installation of the sheet pile wall. Vibration measurements were performed on a
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(a) (b)
Figure 4: Sheet pile wall: (a) sketch of the final design and (b) installation. wall
Measurement line
Tri-axial sensors
Vertical sensors
Vertical sensors on sleeper
Sheet pile wall
Figure 5: Measurement setup for the vibration measurementsat the test site.
measurement line perpendicular to the track, close to the middle of the sheet pile wall (figure 5). Geophoneswere placed at8m, 16m, 32m and64m from the center of the track. Geophones were also placed at8mand16m at the opposite side of the track to verify whether there is noincrease in vibration levels. Twogeophones were placed on the sleepers to measure the track stiffness.
Vibration measurements during train passages were performed before installation in October 2011 and afterinstallation in December 2011. The measurements lasted an entire week to include all types of traffic andto cover differences between working days and weekend days.At the test site, a total of 112 train passagesoccur per 24 hours, of which 29 are freight trains.
The vertical insertion lossILz(f) has been determined according to:
ILz(f) = Ltest,beforev (f)− Ltest,after
v (f) (2)
Here,Ltest,beforev (f) is the vibration level before installation of the measure atthe test section andLtest,after
v (f)is the vibration level after installation of the measure at the test section.
To obtain an accurate estimate of the insertion loss [17], atleast 10 pass-byes with a maximum variationof ±5% of the average speed have to be measured per train category defined by the train type and the trainspeed. For freight trains, at least 20 pass-byes per category are needed. Similar trains with similar velocitieshave to be measured before and after installation of the wall. Three categories of trains have been used in
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Category Speed Receiver distance[ km/h] 8m 16m 32m 64m
1 Passenger trains before 77± 5% 10 10 / /type X31 after 72± 5% 10 10 / /
2 Passenger trains before 81± 5% 9 9 / 9type X31 after 81± 5% 6 7 / 7
3 Freight trains before 63± 5% 5 5 5 5after 63± 5% 4 4 4 4
Table 4: Number of trains used for each train category
the determination of the insertion loss. The first category consists of passenger trains of type X31 with anaverage speed of77 km/h ± 5% before installation and72 km/h ± 5% after installation respectively. Thesecond category consists of passenger trains of type X31 with an average speed of81 km/h± 5%. The thirdcategory consists of freight trains with an average speed of63 km/h ± 5%. It was not possible to find asufficient number of train pass-byes in each category, especially for the freight trains. Furthermore, not allmeasurement results were available for all the geophones, probably due to cable disruption. Table 4 lists thenumber of trains used in the determination of insertion lossfor the three train categories.
The one-third octave band spectra of the vibration velocityhas been calculated for each train pass-by accord-ing to the procedure defined in deliverable D1.2 [17]. The velocity data from the geophones were filteredfrom 1 to 250Hz using a digital Chebyshev type I bandpass filter of order 4. The velocity signals weretransformed via FFT and the vibration level spectra were calculated by
Lv(f) = 20 log10v(f)
v0+ 10 log10
Ta
Tp
[dB] (3)
with v0 = 5.10−8 m/s. The analysis timeTa is taken equal to the duration of the measured velocity signals(10, 20 or 30 s). The passage timeTp is estimated from the measured data. The beginning and end ofthepassage time is taken equal to the time when the relative amplitude reaches about 25% of the maximum level.
Figure 6 shows the measured vibration velocity levelsLv for the passenger trains type X31 with averagespeed of75 km/h before (black) and after (red) construction of the sheet pile wall. The average backgroundvelocity level is plotted in green. A clear peak ofLv near30− 40Hz can be observed at8m and16m fromthe track which may be associated with resonance of the unsprung mass on the track stiffness. The peak ismore pronounced after construction of the sheet pile wall. The resulting insertion loss values obtained fromthese train passages are shown in figure 7a. Immediately behind the wall (at8m), the sheet pile wall starts toreduce vibrations from4Hz on. At 16m from the track, the reduction in vibration levels starts at8Hz. Upto a frequency of16Hz, the measured insertion loss values generally increase with frequency and decreasewith distance from the sheet piling wall. The negative insertion loss values around31.5Hz and at higherfrequencies as found from these measured train pass-byes are not observed in the other measurements or anyother predictions made for stiff wave barriers and are therefore believed to have other causes.
Figure 8 shows the measured vibration velocity levelsLv for the passenger trains type X31 with averagespeed of81 km/h before (black) and after (red) construction of the sheet pile wall. The velocity levels stillexhibit a peak around30− 40Hz but this peak is less pronounced than for the passenger trains with averagespeed of75 km/h. Furthermore it can be observed that the vibration velocitylevel decreases with increasingdistance from the track; especially the high frequencies are attenuated due to material damping in the soil.The velocity levels at64m and at high frequencies are disturbed by background noise. The insertion lossvalues corresponding to these train passages are shown in figure 7b. Positive insertion loss values are foundabove4Hz like in the other measurement results. The highest reduction is measured right behind the sheetpile wall wall while no reduction is seen at64m. Above31.5Hz, the insertion loss is decreased again butgenerally remains positive.
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Figure 8: Vibration level spectra for passenger trains of type X31 at an average speed of81 km/h before(black) and after (red) installation of sheet pile wall. Average background vibration level spectrum is indi-cated in green.
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Figure 9: Vibration level spectra for freight trains at an average speed of63 km/h before (black) and after(red) installation of sheet pile wall. Average background vibration level spectrum is indicated in green.
Figure 9 shows the measured vibration velocity levelsLv for the freight trains with average speed of63 km/hbefore (black) and after (red) construction of the sheet pile wall. Higher vibration levels are observed com-pared to the results for passenger trains, especially at lowfrequencies. At8m and16m, the vibration levelshows a broad peak around3−10Hz and a peak around30−40Hz. At larger distances, the vibration levelsat high frequencies are strongly attenuated and the maximumvibration levels are observed in the frequencyrange3− 10Hz. The insertion loss values for these freight trains are shown in figure 7c. High insertion lossvalues of up to5− 10 dB are obtained in a broad frequency range, already at very low frequencies. The gen-eral trend of decreasing insertion loss with increasing distance behind the sheet pile wall is observed. Theseresults should however be interpreted with care due to the limited amount of train pass-byes considered forthe freight trains and the relatively large spread between the individual pass-by spectra.
It can be concluded from the measurement results for the three train categories that, as a general trend,positive insertion loss values are found from4 − 6Hz on. Up till 20 − 30Hz, the measured insertion lossvalues generally increase with frequency. Finally, the effectiveness of the sheet pile wall generally decreaseswith distance from the sheet pile wall.
3.3 Comparison with simulations
The sheet piling wall was analyzed by means of 2.5D calculations. Since this requires assuming the geometryof the sheet piling wall to be longitudinally invariant, separate calculations have been made for depthsd of12m and18m. The actual sheet piles have a depth of12m, with every fourth sheet pile extended to18m.
The dynamic axle loads depend on the unevenness experiencedby the wheels at the wheel–rail interface. Noinformation is available about the unevenness at the site inFuret, however. Consequently, it is impossible toprovide a fair comparison of measurements and predictions in terms of the vibration velocity levelsLv(f).Therefore, a comparison of the insertion loss is presented only.
To reduce the computational cost, the presence of the track was disregarded in the models. This is possiblebecause only insertion loss are compared. Furthermore, preliminary comparison has shown that the influenceof the presence of the track on the predicted insertion loss values is limited. A vertical unit harmonic pointforce is applied directly at the surface at a distance of5.60m from the sheet pile wall. This corresponds with
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the distance from the center of the nearest track to the center of the sheet piling wall at the test site in Furet.To simulate the efficiency of the sheet piling wall for train induced vibrations, the velocity response due toa number of uncorrelated forces applied (referred to as a line load in the following) and the correspondinginsertion loss are also calculated. For the line load, the positions of the point forces are determined from thepositions of the axles of a79m long X31 passenger train. The insertion loss values for a longer line load,representative for the freight trains, are very similar andtherefore not shown here for brevity.
In the frequency range of interest, the bending wavelengthλb in the sheet pile wall is much larger than therepetition distancedr = 1200mm of the sheet pile wall. The profiling of the plate can thus be disregardedand the sheet pile wall can be modelled as an equivalent plate. When computing the vibration reductionfor a train passage, it is important to take into account the orthotropic behaviour of the sheet pile wall in themodel [16]. An isotropic model of the sheet pile wall leads toa strong overestimation of the bending stiffnessin the longitudinal direction and an overestimation of the insertion loss for a train passage. An equivalentorthotropic plate model of the sheet pile wall was thus adopted in the calculations. The thickness, density,moduli of elasticity, and Poisson’s ratios of the equivalent orthotropic plate are chosen such that the platehas approximately the same mass, bending stiffness in two directions as well as vertical axial stiffness as theVL 603-K profile. The sheet pile wall was modeled with 2-noded2.5D orthotropic shell elements. Theseelements were coupled to a conforming BE mesh for the surrounding soil. The element dimensions werechosen to ensure at least eight elements per minimal shear wavelength. Calculations have been performedup to50Hz and30Hz for the12m and18m deep sheet pile wall, respectively.
Figure 10 shows the predicted insertion loss values in one-third octave bands for receiver positions behindthe sheet piling wall (8m, 16m, 32m, 64m), both for a point load and a line load. The calculated insertionloss values are also compared with the measured insertion loss values (see section 3.2). At very low fre-quencies, the insertion loss values are negligible meaningthe wall has little effect on vibration transmission.The predicted results for receiver points behind the sheet piling wall indicate that the sheet piling wall caneffectively reduce the vibration levels at higher frequencies. For a point load, insertion loss values of5 dBand more are predicted above20Hz for the12m deep sheet piling wall (figure 10a). It must be reminded,however, that the frequency range4 − 5Hz is targeted. Below20Hz, the insertion loss is limited to2 dBfor the 12m deep wall. Only for small distances behind the wall (8m), the insertion loss can reach up to4 dB below 10Hz. For a line load, representative for a train passage, the insertion loss is on average1 to2 dB larger (figure 10b). The results indicate that the increase in depth to18m has a favorable effect atlow frequencies. The insertion loss is increased by2 dB in the one-third octave bands between4Hz and16Hz, both for a point load and a line load. Above20Hz, the insertion loss is not improved. The measuredand predicted insertion loss values show clear differences, but share some general trends. The sheet pilingwall results in vibration reduction from4Hz on. Up to a frequency of16Hz, the predicted and measuredinsertion loss values generally increase with frequency and decrease with distance from the sheet piling wall.The predicted insertion loss values are generally lower than the measured values. A possible explanation forthis difference could be the fact that the soil surrounding the sheet pile wall has been stiffened during theinstallation of the sheet piles. This increased stiffness will result in a better performance of the barrier. Thediscrepancies between the measurements and predictions could also be caused by simplifications introducedin the numerical model, such as the assumed uniform depth andinfinite length. The negative insertion lossvalues as found from the measured train pass-byes are not observed in any predictions made for stiff wavebarriers and are therefore believed to have other causes (see section 3.2).
An accurate prediction of railway induced ground vibrationis very difficult [15]. Even when the differencesbetween the measured and predicted insertion loss values are not fully understood, the observed discrepanciesbetween measured and predicted insertion loss values are ofthe same order of magnitude as the commonuncertainty in the prediction of railway induced vibrations. This indicates that state–of–the–art numericalpredication models can be used for the reliable design of wave barriers like sheet piling walls. It is concludedfrom the field test in Sweden that the sheet piling wall offerspotential for vibration reduction in soft soilconditions, provided it is built sufficiently deep.
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(a) Calculations point load (b) Calculations line load (c) Measurements
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Figure 10: Calculated vertical insertion loss in the case of(a) a point load and (b) a line load for the12mdeep (solid lines) and18m deep (dashed lines) sheet piling wall. (c) Measured vertical insertion loss forpassenger trains of type X31 at an average speed of75 km/h (blue), passenger trains of type X31 at anaverage speed of81 km/h (red) and freight trains at an average speed of63 km/h (green).
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4 Conclusions
This paper describes two field tests that have been performedto evaluate the performance of mitigationmeasures on the transmission path in the soil. At the test site in El Realengo, a jet grouting wall with a widthof 1m, a depth of7.5m, and a length of55m has been installed. The experimental results show that the jetgrouting wall is very effective. Immediately behind the wall, insertion loss values of about5 dB are alreadyobtained from8Hz on, with a peak of about12 dB at 25Hz, which is also the frequency range where thehighest vibration levels are found during a train pass–by. As predicted, the insertion loss values decreasefurther away from the jet grouting wall. The measured insertion loss values are consistently higher thanthe predicted values, which is believed to be caused by the fact that the soil surrounding the wall has beenstiffened by the installation of the jet grout columns.
At the test site in Furet, a sheet pile wall with a length of100m has been installed. The depth of the sheet pilewall is 12m, with every fourth pile extended to18m below the ground level in order to improve the vibrationreduction at low frequencies. Although the measured and predicted insertion loss values show some cleardifferences, the general trends are the same. The sheet pilewall results in vibration reduction from4Hz on.Comparing the predicted insertion loss values for the two depths of the wall confirms this is largely thanks tothe sheet piles reaching down to18m below the ground level. Up to a frequency of16Hz, the predicted andmeasured insertion loss values generally increase with frequency and decrease with distance from the sheetpile wall. Similar to the case of the jet grouting wall, the measured insertion loss values are consistentlyhigher than the predicted values; this may be due to the fact that the soil is assumed to be unaffected by theinstallation of the sheet pile wall in the numerical simulations. It is concluded from the field test in Furetthat the sheet pile wall offers potential for vibration reduction in the soft soil conditions, provided it is builtsufficiently deep. Numerical simulations show that the sheet pile wall is only effective when the depth of thesheet piles is sufficiently large compared to the Rayleigh wavelength in the soil.
Both field tests show that stiff wave barriers are able to solve low frequency vibration problems, providedthey are properly designed and sufficiently stiff compared to the soil in which they are installed. This abilityto reduce low frequency vibration is a unique feature, distinguishing these vibration mitigation measuresfrom most other measures reported in the literature. The observed differences between the experimental andnumerical results are reasonable in view of the significant uncertainty in the prediction of railway inducedvibrations, indicating that state–of–the–art numerical prediction models can be used for design of stiff wavebarriers.
Acknowledgements
The results presented in this paper have been obtained within the frame of the EU FP7 project RIVAS(Railway Induced Vibration Abatement Solutions) [9] undergrant agreement No. 265754. The first andsecond author are fellows of the Research Foundation Flanders (FWO). The financial support is gratefullyacknowledged.
References
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