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Reservoirs repaired

Norwest Holst Soil Engineering recently completed two remedial grouting contracts on earthfill embankments at reservoirs in Yorkshire and Lancashire.

The March Haigh reservoir is 3.5km west of Marsden, near Huddersfield, Yorkshire, at the head of the Colne Valley in an area of isolated moorland is owned by British Waterways. It was built in the 1830s to supply water to the Huddersfield narrow canal. A reservoir outlet pipe passes through a 300m long, 23m high earth embankment in a 30m long valve tunnel that emerges at the downstream toe.

Leakage into the tunnel meant that fine material was being transported from the embankment. To prevent further damage, it was decided to carry out remedial grouting.

Two rows of rotary percussive boreholes were drilled with air flush and 90mm diameter steel casing installed down to bedrock - at depths of between 19m to 27.8m. These were filled with a weak mix of 2:1 bentonite cement. Plastic tubes-a-manchette with one way ports at 0.5m centres were then inserted and held in position until the annulus grout cured.

Stage grouting of the embankment used 1:1 bentonite cement grout. Grouting on each pass continued until 50 litres had been injected or when the limiting injection pressure - equivalent to the overburden pressure - had been attained and held for five minutes. The sequence, designed by British Waterways, involved primary and secondary boreholes and ports and re-injection was carried out as needed. Over 2,400 injections were carried out, using 100,000 litres of grout. Once grouting was finished, the TAMs were backfilled with grout, capped off and the embankment reinstated.

Access was difficult, with a steep stone track built for the last 2km and small dumpers and tracked vehicles used to transport equipment and materials to site. A wooden bridge spanning the outfall needed temporary bracing.

Remedial grouting work at Anglezarke reservoir near Chorley in Lancashire was carried out for North West Water. Scheme designer was Bechtel with Mott MacDonald as panel engineer.

Heapy embankment carries a road across the head of the reservoir and leakage had been observed in the downstream face. Norwest Holst drilled 51 boreholes along and across the dam core and installed tubes-a-manchette in the same way as at March Haigh. Stage grouting used a 1:1 cement bentonite mix in the clay core and an enriched 3:1 mix in the rock. Around 1,603 injections were carried out using over 35,000 litres of grout.

Precise levelling monitored ground heave. Ancillary work included building French drains and flow monitoring stations in leakage areas on the downstream face, installation of permanent levelling stations, construction of dry stone walls and debris clearance from the reservoir rip rap.

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