High-Strength Geogrids Bridging a Sinkhole: First Project Worldwide Including Renewed Sinkhole Activity
In 1993 a critical huge sinkhole funnel in a karstic area on the German Federal Highway B180 near Eisleben was bridged and secured for the first time in Germany using extremely high-strength low-strain geogrids. Philosophy, design and construction of the high-strength geogrid solution are described. In October 2001 the sinkhole funnel re-opened. The geogrid system hold the road for over one hour, which was enough to stop the traffic. The solution proved to be successful in preventing disasters of this type. It is the first case known when a geogrid sinkhole-bridging was tested by real life.
Upon the reopening of the sinkhole, the first sinkhole bridging system incorporating geosynthetic reinforcement to be conceived, designed and constructed in Germany functioned better than predicted: it bridged a sinkhole that was larger than the design requirements for longer than was required. The 1993 system had been correctly planned, designed and constructed. The events proved the philosophy of a "ductile failure with warning", upon which the project had been based. The bridging of an oval funnel with one-way spanning reinforcement is possible and functions well with proper design and implementation. A flat soil-geogrid bridging system, laid close to the road surface can be feasible and will function correctly with suitable choice of design and reinforcement. The first project in the world to incorporate an Aramid® geogrid (in this case with a short-term strength of 1200 kN/m) was proven in practice. The method of analysis and design of sinkhole bridging systems in accordance with (BSI 1995) is sufficiently correct at least for relatively thin bridging layers using non-cohesive soils; the same applies for the "elastic rope / membrane theory". If reliance is placed on warning systems, it should be the aim to eliminate human involvement technically or logistically as much as possible. And: the project forming the subject of this paper considers a very rare event - the occurrence of the "worst case design scenario" (somewhat like the 1 in 100-year earthquake). As far as is known, this is the first time a sinkhole bridging solution has been tested in real life.