Design of geosynthetics over areas prone to subsidence

Sinkholes can progress due to natural or human related activities causing problems for local communities, business and rail and road infrastructure. The use of high strength geosynthetics instead of traditional solutions could offer considerable sustainable and economic benefits and protection against unexpected collapse caused by subsidence. A comparison of the tension and strain requirements predicted by BS 8006 and EBGEO (RAFAEL method) indicated that BS 8006 predicted slightly higher tensions but that the trend was similar. Both design methods also showed that the required tension in the geogrid reinforcement increased exponentially while the design strain reduced as the void width increased relative to the height of the embankment. The reduction in the design strain was attributed to the need to restrict surface deformations to a pre-defined limit. The optimal embankment height to void width was found to be close to unity in both methods. The analysis also found that there was no advantage in using a high stiffness geogrid with a low ultimate strain in this application. The optimal design strain was found to be 3%-6%, corresponding to an ultimate strain in the geogrid of 8%-14%. Two recent UK case studies were geogrid reinforcement was used over areas prone to subsidence are also presented. The first from 2017, was a housing development over a former clay workings, while the second was a housing development over chalk solution features. The design philosophy and the final solutions adopted in each case study are presented and discussed.