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Combined system proves answer for marginal soil

PARADRAIN, GEOSYNTHETICS manufacturer Terram's combined geogrid and drainage system, was recently launched in the UK but has already proved effective on a project to stabilise a failed slope in Taiwan.

The system allows marginal quality fill material to be used in reinforced soil solutions.

The geogrid is profiled to form a drainage channel and covered with a non-woven geotextile filter that allows controlled drainage of pore water, while retaining fine soil particles to prevent clogging.

Marginal fill typically has a high silt and/or clay content which, when loaded, generates excess pore water pressure in the backfill.

This reduces the soil's effective strength and the bond between reinforcement and soil.

However, marginal material can only be used in reinforced soil structures if adequate drainage is provided, says Terram consultant Geoff Kempton.

Kempton led the research and development of the ParaDrain system. He believes engineers should be able to design reinforced soil structures using marginal, poorly drained fills, without the need to import granular material, which has both cost and environmental implications.

This can be achieved using the ParaDrain system, because as well as providing additional strength, it allows dissipation of pore water pressure generated during construction. This results in early soil strength gain and improved bond between the reinforcement and the fill, says Kempton.

The geogrid was recently used to rebuild a collapsed slope on a hill road at Wan Li, near Taipei, the capital of Ta iw an .

The country suffers from slope failures every year during the typhoon season. At Wan Li, failure was caused by a build-up of pore water pressure and poor drainage in the slope.

Because of the seasonal storms and poor soil conditions, any remediation using geogrids is usually restricted to the summer months. Using ParaDrain allowed work to be undertaken for much longer periods, avoiding work delays and reducing disruption because of road closure.

Soil layers were supported at the face by open-weave bags, filled with pre-seeded soil, wrapped around with ParaDrain 100/15.

Because pore water pressure was rapidly dissipated, contractor Fu-Yi Construction Company, working with consultant Johnson Engineering Company for the Taipei County Government, completed the 12m high slope in only three weeks. Terram claims that up to 200m 2of slope can be built in a day using ParaDrain.

The system encourages rapid vegetation growth: after six months, the vegetation on the slope had recovered, ensuring a permanently stable structure.

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