Grube Hoffnung” open-cast mine residual pit
HPC Harress Pickel Consult (Merseburg)
Autumn 2006 - Spring 2007
The former clay pit “Grube Hoffnung”, located south of Helmstedt in Saxony-Anhalt, served as a hazardous waste landfill after its closure. Once closed, the pit filled with rainwater, in some places up to more than two metres above the mud surface. The sludge itself turned out to be thixotropic: i.e., under load, the viscosity changes and the sludge liquefies.
Due to the variable nature of the subsoil, it was decided to use two HUESKER products: Firstly, a nonwoven geotextile was laid as a separator at the higher points of the pit giving workers access to this area, and allowing some of the nonwoven sheets to be installed by hand.
On top of this, in the areas with solid soil, a single-layer biaxial reinforcement geogrid was laid. However, the situation was different in the deeper part of the mine, where the soil was much softer. For this reason, a two-layer reinforcement system, with biaxial geogrids Basetrac 60/60 and Basetrac 80/80, was installed. Additionally, the softer, southern area was divided by retaining dams into four smaller basins to be covered. Both designers and contractor took advantage of the cold weather and laid some of the geotextile and geogrid on frozen ground. In addition, the geotextile rolls were unrolled laterally using guide ropes to allow accurate laying and overlapping. After anchoring the reinforcement around the edge, the base course material was pushed over the reinforcement in a controlled fashion with an initial layer thickness of only 10 cm. By the late summer of 2006, the pit had already been extensively backfilled with base course and contouring material up to heights of 5 m above geogrid/silt level. Finally, 30 cm of thick clay was installed as a sealing layer. As a result of so much concerted effort, the “Grube Hoffnung” project was completed, without delay, after one and a half years of construction.