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Historic claim

Letters

The news story 'Essex makes drilling history' (Ground Engineering May 2000) prompts me to write claiming a similar if not longer history for the Celtic fringe.

Since the closure of Cardiff Bay tidal exclusion barrage on November 4, 1999, control of groundwater levels and drainage in parts of south Cardiff has been provided by four deep horizontal drains drilled under streets and other available open space.

These drains are up to 200m long and the first one was drilled, installed and developed by Allen Watson in early 1998.

Three further drains were constructed by Stockton Pipelines during the summer of 1999.

The drains were drilled from the surface through made ground and soft clays to drain a thin continuous layer of gravels at a depth of 6m to 10m below the surface.

Holes were drilled in 120mm to 150mm diameter and subsequently reamed to 305mm.

The gravel sections are lined with 140mm outside diameter slotted wire wound stainless steel well screen provided by Johnson Well Screens.

Since impounding, the drains have been pumped at rates close to 10 litres/s and surrounding groundwater level responses are consistent with pre-impoundment model predictions.

The use of directional drilling to install deep drains in a crowded urban environment minimises disruption during both drain and outfall construction.

In appropriate circumstances this may provide an economically preferable solution to more conventional designs with higher operating and maintenance costs.

Stuart Sutton, Associate Director, Entec UK

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