Use of geosynthetics to aid construction over soft soils - successes and cautions
Geosynthetic reinforcement and prefabricated vertical drains (PVDs), both separately and in combination, have substantially improved the engineer's ability to cost-effectively construct embankments over difficult and soft soils. This paper explores the reason for the success and the benefits that can be achieved using geosynthetics, as a basal reinforcement, and PVDs to increase short-term stability of embankments and to accelerate consolidation and long-term strength gain. The paper examines both embankments on fibrous peats and a range of soft clays. It demonstrates that good geotechnical engineering is at least as important with the use of geosynthetics as it is without their use. For example, geosynthetic basal reinforcement can be extremely effective in allowing timely construction of embank· ment over peats and soft clays - however an understanding of the mechanism at work and the key engineering properties is essential for consistent success. Not all soils are the same and likewise not all geosynthetic reinforcement is the same. The paper discusses the key mechanism and engineering properties of both soil and reinforcement critical to success. It highlights not only the advantages but also the limitations of different types of reinforcement for different types of soil. It provides field examples, supported by numerical analysis, of great success in the use of geosynthetics - and failures. The paper discusses where both can be anticipated. Finally the paper explores the role of the rate·sensitivity of both reinforcement and soil on the long-term performance of embankments over difficult soils.