INFRASTRUCTURE | SWITZERLAND
Photos: Ingenieurbureau Heierli AG
Keeping it quiet below the streets of Zürich
TRACK DESIGN Engineers designing the cross-city Durchmesserlinie project in Zürich chose to use LVT high attenuation ballastless track to ensure adequate vibration and noise protection for the urban districts above the Weinberg tunnel. Dipl-Ing Thomas Rubi, Tobias Gerber, Marco Trovato, Anabel Hengelmann, Peter Laborenz and Dr Armin Ziegler*
t the heart of the Durchmesserlinie, the cross-city link now nearing completion between Altstetten and Oerlikon in Switzerland’s financial capital, lies the 4·8 km Weinberg tunnel. Fitting out began on July 2 after completion of the bore in June, and work is now progressing on cabling and the installation of fire suppression equipment. Described in RG 11.07 p711, the Durchmesserlinie includes a low-level through station below the existing Hauptbahnhof that will be known as Löwenstrasse. From there, the Weinberg tunnel takes the new route east and then north to Oerlikon, describing a long, wide S-shape beneath several 44
Railway Gazette International | August 2012
heavily populated districts. To be used by both inter-city and SBahn trains, the route runs at varying depths — not just below residential areas but also under vibration-sensitive buildings such as the ETH Swiss Federal Institute of Technology, the university hospital and the DRS radio broadcasting studio. All these buildings exhibit varying degrees of sensitivity to vibration and structure-borne sound, so the track and tunnel design teams recognised at a very early stage that it was essential for residents and occupants of the various buildings not to be affected by the frequent trains passing below. Specifically developed system
Costly measures were needed to reduce the impact of vibration and structure-borne noise, even during the
A Vibro-scan vibration generator was used to generate accurate readings in buildings above the tunnel.
construction phase. One requirement was to predict the level of nuisance as accurately as possible; this prediction could then be verified upon completion of the tunnel shell so that any necessary modifications could be made. Now that the original projections have been confirmed, it has become clear that two factors are decisive for the design of noise and vibration reduction measures. The first is a detailed knowledge of the geological strata in the affected area. This allowed accurate prediction of the likely impact. Second, the type of ballastless track to be used must be known in advance. Tracklaying is all set to start in early 2013 — we are using the LVT lowvibration track system, which is now standard for tunnels in Switzerland. Over the last two decades, many refinements have been made to the original design, thanks to a process
SWITZERLAND | INFRASTRUCTURE of continuous development by the engineering design firm Heierli AG. The result is a trackform that offers excellent track geometry and a long, low-maintenance life. But for the first time in Switzerland we are using the high-attenuation version (LVT HA), which has been specifically developed to mitigate vibration and structureborne sound. The trackform has so far been chosen for more than 1 000 km of track around the world on high speed lines, metro networks and routes used by heavy trains. Well-known applications include the Channel Tunnel, the Lötschberg Base Tunnel and the Gotthard Base Tunnel (RG 7.11 p44). LVT consists of single concrete blocks with rail fastening systems that are embedded on top of resilient pads in rubber boots and then cast in unreinforced concrete. Vertical rail deflection is achieved through the resilient pad below the block, which acts in a similar way to the ballast in ballasted track. One notable feature of LVT, distinguishing it from other ballastless track systems, is the dual-level elasticity.
* Dipl-Ing Thomas Rubi is Technical Project Manager for Track & Railway Engineering, Zürich Durchmesserlinie, Swiss Federal Railways; Tobias Gerber is Project Manager for Track, IG Züri-BT; Marco Trovato is Deputy Project Manager for Track, IG-ZüriBT; Anabel Hengelmann is Head of the Slab Track Division, Vigier Rail AG; Peter Laborenz is Project Manager for Slab Track, Vigier Rail AG; Dr Armin Ziegler is Proprietor, Ziegler Consultants. IG-Züri-BT is an engineering consortium formed of Ernst Basler + Partner AG, Ingenieurbureau Heierli AG, Elbas Schweiz AG, AWK Engineering AG, Ziegler Consultants and Vigier Rail AG.
Fig 1. The Vibra-2 software package from Ziegler Consultants was used to compute vibration and sound levels affecting the buildings above the alignment.
First, the rail pads dampen the action of the wheel-rail contact on the concrete blocks, thereby reducing wear. At the second level of elasticity, the vertical deflection of the block serves to attenuate vibrations in the lower frequency range. At the same time, isolation of the concrete blocks from the track slab through the rubber boots serves to reduce the loads on the slab, avoiding the ‘pump effect’, while also dampening vibration transmission. To improve the attenuation of vibration, LVT HA uses blocks up to 30% larger than those for the standard system in conjunction with more resilient pads. Thanks to the greater mass of the concrete blocks and higher resilience of the pads, LVT HA exhibits a natural frequency in the order of 30 Hz — comparable to that of a light massspring system. The Durchmesserlinie application includes the use of LVT with 16 sets of points on the approach ramps and in the tunnel along a 4·6 km doubletrack section — this is the first time LVT is being used in pointwork in Switzerland. Predicting nuisance levels
Image: Ziegler Consultants
In line with standard practice for all rail tunnel projects, the projected
nuisance from vibration and structure-borne sound in the Weinberg tunnel was the subject of detailed investigation at all stages of the planning process. During the design development phase, the Vibra-2 software package was used to predict the resulting nuisance for the buildings above the tunnel. Fig 1 outlines the procedure adopted for the Vibra-2 computations: a source spectrum, in the form of a one-third octave band spectrum, was generated for the Weinberg tunnel on the basis of measurements for comparable rail tunnels. The transfer spectrum for attenuation between the tunnel and a free-field point above the tunnel was also derived from measurements on comparable tunnels. The so-called ‘coupling effect’ (from subsoil to building foundation) entails an attenuation of vibration in the medium frequency range (10 to 100 Hz). This was factored in through the transfer spectrum for coupling. The free vibration of the structural floor, on the other hand, leads to an amplification at the natural frequency. This resonance was modelled by the transfer spectrum between foundation and structural floor. Finally, the floor vibration and structure-borne sound were determined by multiplying the
Fig 2. Crosssection and long section of measurement point layout.
Railway Gazette International | August 2012
INFRASTRUCTURE | SWITZERLAND Table I. Comparison of dynamic parameters for train/P23 vibration generator Parameter
Unsprung wheelset mass/ excitation mass
3 505 kg
2 000 to 4 000 kg
Maximum excitation force
Load on formation/ bed plate
about 140 to 225 kN
Maximum surface load
≤ 19 N/cm2
≤ 15 N/cm2
Frequency band/ spectrum
1 to 250 Hz
3 to 230 Hz
Source: Ingenieurbureau Heierli AG
source spectrum with all the relevant transfer spectra. The results were then used to establish what measures would be required in the different tunnel sections. Upon completion of the tunnel shell, it was possible to refine the prediction by inducing excitations in the actual structure. The Vibra-2 procedure was again adopted for the prediction of vibration nuisance based on the Vibroscan measurements. Instead of the spectrum in the tunnel, however, the Vibro-scan spectra measured in the buildings were applied as the source spectrum. These required some minor adjustments, as the frequency content and amplitude of the Vibro-scan excitations do not precisely match the vibrations produced by real trains. Schaffhausen
Hardbrücke Letzigraben viaduct
Durchmesserlinie SBB lines SZU lines Tunnels
© Railway Gazette 2012
Railway Gazette International | August 2012
Table II. Key data for the heavy mass-spring system used in the Weinberg tunnel
To verify the assumptions underlying the computations, propagation measurements were taken on the basis of artificial excitations using lorry-mounted Vibro-scan equipment. Here, the vibrations generated were recorded in 20 buildings, with readings taken at 20 independent MR2002 measurement stations in each building. Each station was programmed to take 90 sec readings every 20 min. An equivalently programmed measurement station — acting as a sort of pulse generator — was carried through the tunnel by a technician as an economical means of ensuring that the measurements in the buildings were synchronised with the excitations in the tunnel. Fig 2 shows the basic configuration of the measurement points in the buildings and tunnel. Comparison of the technical data with the corresponding train data confirmed that Vibro-scan allows realistic simulation of train-induced excitations (Table I). Four sweeps, each of 20 sec, were performed at each excitation point. During these sweeps, the excitation frequency was steadily raised from 10 Hz to 125 Hz. This allowed us to measure the propagation of vibration in the tunnel tube, its transmission from the tunnel invert to the freefield point at the surface, the coupling effect from subsoil to building, and transmission between building foundation and structural floor.
The adjustments were based on measurements in and above the existing Zürichberg tunnel.
The newly computed prediction based on the Vibro-scan excitations allowed the design team to draw the following conclusions: • the computations based on the
Left: The 4·8 km Weinberg tunnel describes an S-shape between the Hauptbahnhof and Oerlikon. Right: Fig 3. Heierli AG has refined the design of LVT ballastless track over the last two decades.
Structure-borne sound reduction
< 10 Hz ~ -80%
~ -20 dB
Vibro-scan measurements confirmed the results predicted with the Vibra-2 model; • the measures planned to reduce vibration and structure-borne sound nuisance would be adequate; • the measures would ensure compliance with the sound and vibration thresholds for all the residential buildings. Special clarification work was needed for the ETH Zürich buildings, the university hospital and the DRS broadcasting studio, but in each case the results complied with the specified threshold. Mass-spring systems
Our predictions showed that additional measures to mitigate vibration and structure-borne sound propagation would be needed in certain sections of the Weinberg tunnel. Here we will install a resiliently-mounted floating-slab track using a massspring system. This vastly reduces the natural frequency of the track system and, accordingly, the transmission of vibration and structure-borne sound. The Durchmesserlinie includes several sections using light or medium mass-spring systems and one further section where a heavy mass-spring system was needed. The mass-spring system sections are jointless, without provision for expansion, and are tied to the tunnel shell at each end by means of horizontal force bearings. The rails are continuously welded along the entire length of the tunnel. One 550 m section with a medium mass-spring system is incorporated at
SWITZERLAND | INFRASTRUCTURE Top: Fig 4. The unreinforced version of LVT ballastless track as developed by Heierli AG is being installed in the Weinberg tunnel. Below: Fig 5. This heavy mass-spring system was used below the DRS broadcasting studio.
the south portal. A second 515 m section at the north portal has a heavy mass-spring system with a natural frequency below 10 Hz, as this lies below the DRS radio broadcasting studio. The heavy mass-spring system separates the track from the tunnel shell both vertically and horizontally by individual, high-performance resilient bearings. When a train passes over it, the system undergoes vertical deflections of up to 15 mm. Ample transition zones ensure that the deflection will not be perceived by the train passengers. Part of the heavy mass-spring section is located close to a set of points and in a curve with a cant of 40 mm. The mass-spring systems should all be in place by the end of this year ready for tracklaying to start early in 2013.
For applications subject to particularly stringent vibration control requirements that necessitate the use of mass-spring systems with low natural frequencies, LVT can be readily integrated into the track slab of the massspring system, maintaining a uniform and continuous track design. LVT HA thus bridges the gap between the standard LVT and mass-spring systems. It caters for applications with clearly defined vibration control requirements while limiting the use of massspring systems to those cases where they are really essential. On many projects, significant savings are
Image: Ingenieurbureau Heierli AG
therefore achievable through the use of LVT HA technology. We are sure that the trackforms chosen for the Weinberg tunnel will provide the necessary protection against unwanted vibration and
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structure-borne noise along the alignment. Post-completion measurements will be performed on the finished structure to verify the effectiveness of the vibration and structure-borne sound attenuation measures. l
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Railway Gazette International | August 2012