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New Wendlingen–Ulm line: Track sealing on the Alb plateau – A construction site experience report

Project overview

The Stuttgart–Ulm rail project consists of the Stuttgart 21 sub-project and the new line from Wendlingen to Ulm. Stuttgart 21 refers to the complete reorganisation of the Stuttgart rail hub, with the new underground station at its heart and the line to Stuttgart Airport and Wendlingen. The subsequent new line will enable high-speed rail traffic to Ulm and bypass the Filstalbahn, where tight radii currently prevent trains from travelling at speeds suitable for modern long-distance transport (Fig. 1).

Fig. 1: Overview map of the entire Stuttgart–Ulm project (source: Stuttgart–Ulm project)

Planning approval section (PFA) 2.3 extends across the plateau of the Swabian Alb between Hohenstadt (NBS km 53.811) in the west and Dornstadt (NBS km 75.250) in the east. The section is characterised by its predominantly open route running parallel to the A8 motorway (see Fig. 2), which is being expanded from four to six lanes at the same time. The new line runs largely through Zone III water protection areas.

Fig. 2: Overview map of PFA 2.3 Alb plateau (source: DB Projekt Stuttgart–Ulm)

The section on the Alb plateau is characterised by heavy earthworks and rock excavation for the construction of cuttings and embankments as well as the excavation pits for the engineering structures.

Due in particular to the karstified subsoil, geosynthetics are also being used as part of the foundation and sealing of the high-speed track to protect the groundwater in the Swabian Alb.

Plastic sealing membranes as part of the sealed track

Cutting and embankment slopes are covered with a mineral sealing layer (clay coating), which extends at least 2 m above the top of the rails in the cutting area and also includes the drainage trough at the foot of the embankment and the drainage bodies under the railway side ditches in the embankment area. The central area of the slab track and the edge areas of the new track between the hydraulically bound base course HGT of the slab track and the embankment shoulder or the railway side ditch are also sealed. A system diagram of the track drainage with sealing is shown in Fig. 3.

The railway drainage then flows to decentralised infiltration basins with upstream rainwater clarification basins, see Fig. 3.

Fig. 3: System diagram of drainage and sealing scheme (source: DB Projekt Stuttgart–Ulm)

Neither the sealed drainage system nor the decentralised infiltration via open basins and the infiltration within WSG zone III are provided for in the DB regulations (Ril 836). Therefore, internal company approval (UiG) and approval in individual cases (ZiE) from the Federal Railway Authority were required.

According to the ZiE, profiled PE sealing membranes must be used to seal the edge path areas on both sides in accordance with application case 3.12, Sealing elements in earthworks (plastic sealing membranes), DBS 918 039 with a manufacturer-related product qualification (HPQ) from DB AG. The sealing membranes must be protected on both sides by protective fleece. If broken, sharp-edged material is used for the levelling or frost protection layer, the requirements for the protective fleece must be confirmed by suitability certificates.

Fig. 4: System drawing of standard cross-section of cut (source: DB Projekt Stuttgart–Ulm)

Materials from Naue were used: Carbofol 406 f/f DB plastic sealing membrane with texture on both sides and Secutex R901 geotextile, PP-white.

In accordance with the specifications, the sealing membranes must be integrated into the filter (cut layer) or led over the embankment shoulder (dam layer) to ensure sufficient overlap with the clay packing on the carriageway side. The arrangement of the sealing membranes in the edge path area is shown in Fig. 5.

Construction site experience report

Special boundary conditions for the implementation planning

The three sections of the Alb plateau (each approx. 7 km, see Fig. 2) were planned by the client with a continuous design plan, but were awarded to different contractors in succession in a competitive tender for execution. These in turn awarded the specialist waterproofing work (also in competition) to one and the same subcontractor: Bausanierung & Dichtungsbau H. Berger.

As the implementation planning was thus the responsibility of separate general contractors, different technical solutions were developed in detail, in particular different variants of building connections/sealing.

Installation in incision and dam layers

After creating the installation conditions in accordance with the guidelines of the manufacturer and the German Welding Society (DVS), the sealing components were rolled out along the route (Fig. 6). The reason for this was to deliberately minimise the number of joints in the area, thereby reducing the amount of welding and testing required, even though longitudinal cuts and welds were necessary due to varying installation widths.

All welding work and seam inspections were carried out and recorded in accordance with the applicable DVS guidelines.

A minimum upstand of 20 cm had to be created on fixtures and structures. This was achieved by raising the sealing membrane to the appropriate rounding radii or by using moulded parts.

Fig. 6: Waterproofing construction site

The general contractors installed either PEHD pipes or concrete pipes or concrete shaft rings at the foundations for overhead line and signal masts (Fig. 7). The PEHD plate collar, which serves as a link between the waterproofing membrane of the surface and the vertical pipe, was welded directly to the pipe or fitted with a PEHD sleeve. The sleeves, in turn, were made either from a finished pipe end or from PEHD plates. The resulting annular spaces (Fig. 8) between the concrete pipes and the seal were filled with bentonite granulate.

Fig. 7: Variant of waterproofing connection to OL foundation with concrete slip-on pipe

Fig. 8: Annular space between concrete slip-on pipe and sealing sleeve

In the area of the connections to tunnel portals, overpasses/underpasses, cable ducts and block foundations, fillets were created, the sealing membrane was deflected and flanged using VA rails. Alternatively, PEHD angles were used for the upstand, to which the sealing membrane was welded (Fig. 9).

Fig. 9: Connection detail for cable duct

PEHD angles were also used for the upstands in the area of the inspection chambers, where the seepage packing is interrupted, but there were other design variants in this case as well. Before the laying work began, a strip of sealing membrane was laid on a hollow moulding, set in concrete and later welded to the surface sealing, or halved moulded parts were used in the same way as for the overhead line mast foundations.

Dealing with weather conditions

Various factors caused difficulties with regard to the weather. The large-scale installation of the sealing membrane caused enormous waviness when the black plastic was exposed to direct sunlight. This was due to the high expansion coefficient of the sealing membrane and temperature differences of over 60 Kelvin on sunny summer days. This problem was solved by covering the membrane immediately with white geotextile and welding the cross and connection seams in the early morning or late evening hours. The wind conditions, gusts in dam areas and suction in the incisions made it necessary to secure the membrane against the wind immediately, at least in certain areas. This was extended to the entire surface immediately after complete welding.

Sudden rain showers, sometimes limited to a few hundred metres, as well as high humidity in autumn, spring and also in the summer mornings, led to interruptions in the work processes. Enclosing the welding areas and, if necessary, heating them helped to create the necessary environmental conditions.

To reduce the risk of adverse weather conditions, welding times on site were minimised by carrying out prefabrication and quality assurance in Berger's workshop facilities wherever possible. Around 2,000 metres of PEHD angles were hot-bent, around 1,300 weld collars were cut to size and more than 800 of these were welded to the sleeves to form finished moulded parts.

Summary and concluding remarks

Special measures are required for the construction of the new line in the Swabian Alb due to the karstified subsoil. This report describes the use of geosynthetics to seal the track and associated ancillary facilities.

The waterproofing is carried out with plastic sealing membranes, which are laid under the frost protection layer and drain the water directly into the closed drainage system. It is then transported via collection pipes to clarification and infiltration basins. For reasons of groundwater protection, the system is continuous across the entire Alb plateau.

Construction work on the project is now complete on two of the three sections of the route, and the track has been handed over to the subsequent railway equipment contractors.

Literature

1. ARGE Wasser Umwelt Geotechnik. Major project Stuttgart 21 – Wendlingen-Ulm. PFA 2.3 Alb plateau. "Real-case forecast of karst phenomena" as a basis for the development of earthworks and foundation engineering concepts in karst mountains as part of the design planning. Version dated 04.08.2009 (unpublished).
2. Kielbassa S., Prischmann F., Beer, N.: Stuttgart–Ulm railway project, karst exploration and remediation measures for the high-speed railway line in the Swabian Alb; Geomechanics and Tunnelling, issue no. 2, April 2015, pages 129–145.
3. Kielbassa S.: Exploration of karst on the Alb plateau during construction; Rock Mechanics Day, Weinheim 2015.
4. Deutsche Bahn AG: UiG with technical statement TM 2007-1309 I.NVT(K) Ril 836.0509: ABS/NBS Stuttgart-Augsburg Project NBS Wendlingen-Ulm km53.838 to 72.250; sealed railway drainage and infiltration in the water protection area dated 17 September 2007 (including application documents).
5. Federal Railway Authority: ZiE "Special design of railway drainage (sealed system) in karst area with infiltration within the water protection area, NBS Wendlingen – Ulm, PFA2.3, Alb plateau km 53.838 – km 75.250" dated 12 December 2011, including amendments 1 to 3.
6. Raithel M., Kielbassa, S., Baumbusch J.: Construction of the new Wendlingen-Ulm line in karstified ground. EI-Eisenbahningenieur 2015.
7. Raithel M., Kielbassa S., Baumbusch J.: Stuttgart–Ulm railway project; new Wendlingen–Ulm line: Geosynthetics as part of the foundation and sealing of the high-speed railway line to protect groundwater in the Swabian Alb; 10th NAUE Geosynthetics Colloquium on 17 February 2017.