Guest author: Alan Bamforth, managing director of ABG Ltd.
Railway infrastructure has retaining walls and bridge abutments. Inherent in the design of these structures is the requirement for adequate water pressure relief behind the wall. This is provided by a drainage layer at the rear of the wall that safely directs water from the retained ground to a collection pipe and/or weep-holes at the base of the wall.
Traditionally, the drainage layer is constructed using granular crushed stone, typically 500mm thick. Technical studies of highway structures for the Transport Research Laboratory in the 1980s identified that this traditional drainage was often failing due to:
- Cracks in the face of masonry retaining walls, often identified by tell-tale calcium staining;
- Clogging of the drainage stone due to infiltration by the backfill soil;
- Poorly graded drainage stone;
- Damage to the water proofing on the rear of the wall by the sharp angular drainage stone.
Introducing geosynthetics
Innovation since 1990 has led to the proven use of geosynthetic drainage for these back-of-wall applications. At the forefront in the UK is ABG, manufacturer of the Deckdrain geocomposite range, which consists of a cuspated high-density polyethylene (HDPE) high-strength core bonded to a specific non-woven geotextile. It has a BBA (British Board of Agreement) certificate of Fitness for Purpose for structural drainage.
ABG Deckdrain was installed recently on the Bermondsey dive-under for Thameslink – a project that achieved a CEEQUAL Excellent rating.
This project was particularly interesting as it involved both existing Victorian brick arches and new concrete structures. Installing traditional crushed-stone drainage to the rear of structures that were six metres high and 100 metres long would normally require 600 tonnes of crushed stone. Instead, principal contractor Skanska chose to use ABG Deckdrain that was supplied in ten rolls weighing, in total, just 600kg.
The reduction of 30 deliveries to just one was not only a great logistical benefit but also saved over 54 tonnes of CO2 . In addition, the fast installation of the ABG Deckdrain also led to a safer working environment.
Proven performance
The real innovation, however, is the development of test methods and research by ABG to provide the reassurance that engineers need to trust in something they have not used before. ABG Deckdrain is manufactured from HDPE, a polymer with a life expectancy in excess of 120 years. To ensure that Deckdrain can withstand the long-term ground pressure, extensive SIM (Stepped Isothermal Method) creep testing at pressures of 250kPa has demonstrated 80 per cent retained performance after 114 years.
The drainage flow performance is also accurately tested to BS ISO12958, using soft foam in contact with the Deckdrain to replicate the soil backfill and ensure that the geotextile surface of the Deckdrain is loaded exactly as it would be when installed in the structure.
Being UK based and specialising in geosynthetic drainage, with a 30 year history, ABG has a focus on engineering for engineers, which means ABG have a great deal of experience and are always willing to help.
