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Use of geosynthetics in motorway construction for waterproofing in accordance with RiStWag and in retention basins, using the current example of the A 72 motorway south of Leipzig

Two practical examples are used to explain waterproofing with plastic sealing membranes and the use of geosynthetics.

The new section of the A 72 motorway south of Leipzig runs over challenging subsoil with problematic geology. The area is a landscape left behind by lignite mining in the 20th century – surrounded by partially filled former open-cast mines.

The motorway runs through former mining areas with the Witznitz and Espenhain open-cast mines. The remaining holes were flooded after the mines were closed in the 1990s. This resulted in the creation of Lake Hainer and Lake Haubitzer.

The section of motorway described above is fraught with dangers in the landscapes left behind by mining. The spoil heaps are loose, susceptible to subsidence and unsuitable for building. There are also dangers posed by cavities in old drainage tunnels, which often occur as sinkholes.

The rise in groundwater levels is causing major problems, including waterlogging in towns and swamp areas. The existing residual holes with endangered embankments and landslide areas have been largely secured – but will be reactivated by the motorway construction.

Complex and expensive safety measures are necessary for motorway construction in such areas. For example, a dam up to 17 metres high was built on the north-east corner of Lake Hainer on a slope prone to landslides and interspersed with banded clay slip surfaces.

Stabilisation with large-calibre replacement boreholes, 28,000 metres drilled, 2 m diameter and up to 26 metres deep. In another area, a sinkhole area with old tunnels was first secured with dynamic intensive compaction and then covered with a bridging layer with high-tensile geogrid reinforcement.

Adjacent spoil areas were stabilised with gravel columns and thus prepared as a subgrade for the motorway.

Difficult and complicated subsoil conditions result in multiple areas of application for plastics: separation and filter layers in drainage and bridge construction, sinkhole protection with geogrid reinforcement, working levels and construction roads, separation function under foundation pads and horizontal drainage layers, use in slope support slabs and sealing in rainwater retention basins and water protection areas.

Taking the example of the RRB 3 rainwater retention basin near Rötha, geological exploration in the borehole profile shows carbonaceous sand, silty, organically black, dynamic probing with 1 to 3 blows, high and still rising groundwater, susceptible to settlement and at risk of liquefaction. The RRB 3 basin is unusually large, covering approximately 3 hectares. The basin consists of two parts – the retention basin and the upstream settling basin. The subsoil is very problematic.

To stabilise the subsoil, 6-metre-long gravel columns were vibrated into place, including in the embankment area. Above this, layers of gravel with separating fleece and geogrid reinforcement. Then the protective layers and the foil seal, and finally a protective fleece with fine and coarse gravel on top. The settling basin was lined with concrete and paving. The submerged wall with pipe penetrations was also sealed, especially the part of the pipe penetration.

Carbofol 406 2.0 mm with both a smooth (bottom area) and textured surface (embankment area) from NAUE was used as the sealing membrane. In the base area, a "Combigrid 20/20 Q1 GRK 4C" geogrid reinforcement was also installed below the sealing to even out the settlement.

The protective layer for the plastic sealing membrane was a Secutex non-woven fabric from NAUE with a weight of 800 g/m².

The detail for connecting the pipe outlets with a standard solution from NAUE, either with an applied seam or with stainless steel clamping rails on the concrete collar.

The PEHD sealing membrane "Carbofol" has DIBt and BAM approvals with regulations for factory-internal

production control with regular external monitoring, as well as specifications for planning, dimensioning and execution, right through to the use, maintenance and servicing of the membranes. "Carbofol" sealing membranes have been confirmed as having a high aggressive stress level for liquids. DVS 2225-4 is a special regulation for welding sealing membranes.

The product information and product advantages of "Carbofol" published by NAUE have been confirmed in practice. In particular, the structured surface of "Carbofol friction/friction" with a high friction angle is very well suited for embankment installations on sloping surfaces.

The PEHD material has an extremely long service life and is free of plasticisers.

The structuring of Friction is carried out in an extrusion process with embossed rollers. This results in a durable bond.

Other positive properties include UV resistance and optimal welding, made possible by a welding protection strip at the edge of the rollers and edge marking. The film is only removed immediately before the welding process, thus guaranteeing clean seams.

In the A72 project, almost 30,000 square metres of "Carbofol" were installed, both smooth and textured, with a thickness of 2 millimetres, and almost 60,000 square metres of protective fleece and geogrid.

The basin now has to prove its functionality. Since it was commissioned at the end of 2018, the water level has been at its permanent level.

The second topic deals with road construction in water protection areas according to RiStWag. Water protection zones 1 and 3 are present in the area of the A 72 section 5.1. This necessitates protective measures in the catchment area of the drinking water extraction points.

There are extensive regulations for water protection areas, such as the FGSV's Guideline for Structural Measures in Water Protection Areas [RiStWag, as of 2016]. Among other things, this guideline sets out requirements for the sealing system, with different variants and options such as:

• Sealing with mineral soils, i.e. cohesive, defined soils with low water permeability and high retention capacity,
• Asphalt waterproofing, which is used less frequently,
• Variant using sealing membranes, differentiated into plastic sealing membranes and geosynthetic clay sealing membranes.

When selecting the plastic sealing membrane, high demands were placed on its barrier effect in the critical subsoil conditions in the tip soil.

Requirements are set for chemical and mechanical resistance and must be verified. Mechanical mechanisms include, in particular, loads from traffic and compaction, but also from accidents and incidents.

Support and protective layers are required for gravelly and stony subsoil, and geotextile fleeces are particularly suitable for this purpose. A special issue is stability on embankment surfaces.

The KDB/plastic sealing membrane must not absorb any tensile stresses and must transfer all forces to the subsoil via friction forces only.

The sealing membrane must extend to below the asphalt carriageway. Above the seal, separate drainage with discharge into the RRB must be provided.

EBGEO [as of 2010], section 8, contains design and construction information, in particular verification procedures for stability calculations for inclined sealing systems.

There are variants for the possible anchors, e.g. via a trench solution or horizontal integration in the dam body.

The presentation shows photos of the laying of sealing membranes and protective nonwovens, in particular the use of special lifting techniques and crossbeams. Careful laying of the membrane is important for welding. The work is very detailed when it comes to shafts and pipes. Shaft applications had to be sealed over 400 times.

The hot summer and weather conditions in 2018 were problematic.

There, the welding work was often carried out in the early hours of the morning, as wrinkling of the

plastic sealing membrane due to material expansion as a result of heat input. The

Welded seams require a great deal of effort and must be checked for leaks. The extruded seams were carefully monitored and logged by qualified specialists.

Around 30,000 square metres of smooth and textured material were installed in the water protection areas along the A 72 motorway. Around 60,000 m² of "Secutex" protective non-woven fabric and 9,000 m² of "Secugrid" geogrid were used as slip protection in the embankment area.

The RiStWag also contains requirements for inspections and measures in the event of accidents, in particular the inspection of damage to coverings, even after accidents. Holes such as drill holes, guard rails, vegetation, trees and animal passages must be avoided.

The use of NAUE materials for sealing has proven very successful.