Restoration works using plastic sheet piling
Within the context of plastic sheet piling, restoration works centres upon the creation of plastic piling dams to block drains and ditches, as in accordance with guidance of Appendix XVII Plastic Piling Dams produced by the Scottish Executive, and documents such as Drain-blocking techniques on blanket peat: A framework for best practice by A. Armstrong et al. These documents illustrate the high level of interest in this area, and the importance of standardisation, in terms of methods and associated merits.
In the Armstrong document it is stated that the principal rationale for drain blocking in UK varies between stakeholders, but in most instances it is water table recovery to encourage more peat forming species to establish. Other aims or added benefits are reduction of dissolved organic carbon flux and reduced sediment transfer and erosion.
Armstrong et al continue to state that regardless of the motivation for drain-blocking at a site, it is desirable to create a watertight dam: if there is no flow then the water table rises, there is no sediment loss, dissolved organic carbon loss is generally decreased (Armstrong et al., in preparation), and pool environments are created which are ecologically beneficial.
Plastic sheet piling is stated as a preferred method used on around 5% of all dams included in the survey, but in practice and as recommended by these reports, typically only used on worst case scenarios – as a means of creating a watertight seal on large drains width greater than 0.7m2 and those with steepness greater than 3 degrees.
The attempt at standardisation and the detailing of a decision tree, is exceptionally important, but do nothing to lend appreciation to the fact that there are now a wide range of plastic sheet piling available, some cheaper than others, other stronger, other more versatile and finally there is a variety of pile interlocks creating a range of water tightness.
This is a very important point as the main limiting factor in the use of plastic sheet piling is clearly cost related, many sources stating that plastic sheet piling is one of the more expensive means of drain blocking. Armstrong et al acknowledge that the creation of a watertight seal is the most effective method to reduce sediment transfer, this is often set a lower priority to cost.
Appendix XVII is based upon a much older document and hence considers plastic piling in terms of the two oldest commercially available options, profiles analogous to our EcoZ or Masterpile. Its methods are thereby limited to the limitations of earlier designs. The consequence of this is that many organisations seeking to use plastic piling will follow antiquated information as a standard rather than benefit from improvements now available. Further, despite the recent nature of the Armstrong et al publication, it only considers plastic piling as a single generic option. This does have implications for a given design, the limits and performance of the design and the cost of that design.
Armstrong et al state “block spacing should not exceed 4 m and minimum spacing’s could be derived as a function of gully depth. The target height of a gully block should be 45 cm while 25 cm should be a minimum height.