Use of monitoring method in accordance with DIN 1054

The construction of the four sections of the new A26 motorway from Stade to the interchange at the A7 at Hamburg, is intended to relieve traffic on the B73 trunk road and the local road network. Work commenced on the first section from Stade to Horneburg in Autumn 2001. The remaining three sections are in the design stage. The first section of new road is founded on up to 12 metres of soft Holocene deposits (silty clays and peat), with very low bearing capacities. Partial or full excavation and replacement was not an option, so the solution of surcharging and draining the soils to consolidate them was adopted.

The surcharging and draining works involved the following additional measures:

1. The incorporation of a non-woven geotextile on the surface of the existing ground to act as a separation layer between the fine-grained foundation soils and the coarse-grained embankment fill, and to create a working surface that could support site traffic loads (layer thickness 0.6 m to 0.8 m).

2. The installation of vertical drains to accelerate the consolidation of the low bearing capacity foundation soils (the installation depth of the vertical drains from the top of the working surface went down to 2m above the Pleistocene sands, with a drain spacing of 1.25 m). Before the vertical drains were installed, penetration soundings were taken to ensure that there would be a drainage layer at least 2 m thick above the Pleistocene sands, so that water under pressure would escape through the multiple aquifers formations of the Pleistocene soils and not from the ground surface.

3. The installation of high tensile reinforcement, in the form of Stabilenka® 200/45, 400/50 and 600/50, below the underside of the embankment. The geotextile was placed after the installation of the vertical drains. The geosynthetics were made from high-modulus polyester yarn with low creep properties. In the ramp between the lower level embankment on soft soils and the higher level sand soils ("Geest"), two layers of the geotextile reinforcement Stabilenka® 600/50 were used. The special double rolls required for this part of the works were prefabricated by manufacturer HUESKER Synthetic and installed in accordance with the placement plan drawn up by the German Federal Highways Office (the Bundesanstalt für Straßenwesen, or BASt).

The design and use of this geotextile reinforcement is based on over 20 years of experience.
The monitoring method given in DIN 1054 was used as the basis for geotechnical instrumentation in compliance with the requirements of the earthworks design certificate produced by the BASt and as agreed with the Stade highways department.

Geotechnical instrumentation

Geotechnical instruments and systems that had already proved themselves in comparable projects, or are standard equipment for monitoring in accordance with DIN 1054 were used to fulfil the diverse range of monitoring requirements. Automatic systems read and transmit the information from all the instruments.

More details of the geotechnical measuring instruments:

• Electrical settlement gauges, operating using the hydrostatic tube principle, constantly monitor the deformation behaviour of the railway embankment at the transition zone between soft soils and sand soils ("Geest").
• Fixed chain inclinometers for continuous automatic measurement of horizontal deformation of the road embankment and at the high-voltage pylons.
• Mobile inclinometers to measure the horizontal deformation at the embankment toe for ensuring the safety of the pylons and watercourses.
• Pore water pressure gauges (pneumatic) for obtaining readings at various depths below the embankment fill and for recording changes in ground water levels.
• Settlement gauges (pneumatic) on the ground surface.
• Earth pressure gauges (pneumatic) for recording the surcharge loading (height of fill).
• Earth pressure sensors (pneumatic) for recording the horizontal stresses at the sheet pile walls for the high voltage pylons.
• Extensometers (electrical) for recording the extension of the Stabilenka® geotextile.

These geotechnical measurements allow the expected deformations and changes in stress in the foundation soils and their progression with time, both laterally and vertically, to be monitored, checked and controlled. The automatically-collected readings are made available very shortly afterwards by telemetry to BASt for assessment and the results passed to the companies carrying out the work (ArGe Bunte/Möbius) by the local site supervisory staff and the Stade highways department. The readings allow each filling operation approval to be managed in the most effective way, taking into account the particular circumstances relating to the section of embankment, the thickness of the fill layer and rate and sequences of filling.

This approach ensures the safety of the earthworks and the structures in the embankment slope and allows the works to progress in an economically efficient way. Above all it was shown that by using the DIN 1045 monitoring method for the first time in this type of construction work, the first section of the A26 motorway could be built and be taking traffic off the local roads in the shortest possible time.

The results so far confirm the success of the concept, materials and procedures applied to the construction works. Already it can be said that the logical use of the monitoring method in accordance with DIN 1054 has more than proved itself on these works. The success of the project to date is in no small part due to the excellent relationship and cooperation between all project participants.