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BAM Ritchies

Case Study: Gibraltar East Side Water Catchments

Location: Gibraltar

Client: Defence Estates

Engineer: Gifford & Partners

Date: 2000 - 2003

Edmund Nuttall Ltd was awarded the contract to decommission the East Side Water Catchments in Gibraltar for Defence Estates. As a consequence of the high geotechnical input to the contract, Nuttall’s specialist geotechnical division Ritchies managed the work.

The catchments were constructed between 1905 and 1908 to provide a fresh water supply to Gibraltar and comprised 140,000 m² of corrugated iron sheeting fixed to a wooden framework on a natural 35º sand slope. Two masonry and concrete drainage channels split the catchment horizontally and were used to collect and channel the water.

As the construction of a reverse osmosis plant has rendered the catchments redundant, the MOD wished to decommission and remove the sheets and return the slope to the Government of Gibraltar.

To leave the slope in a stable condition, the soft sand which overlies a more competent cemented sand, required stabilisation. The solution proposed by the Client’s designer involved the installation of 11,000 ground nails with a seeded coir matting.

Our alternative design, developed with our geotechnical engineer, Donaldson Associates, offered a more economical nail layout. The alternative design was confirmed by further ground investigation and the installation of trial nails which gave a much better understanding of the ground conditions.

Key to the successful completion of the contract and one of the principal innovations was the development of self-contained rig carriers, designed and manufactured by Ritchies to operate in the demanding circumstances. Eight purpose made rigs were used requiring complex man and material accesses.

Within a short period of time planned outputs were being achieved with a workforce comprising experienced Ritchies operatives supplemented by a sizeable proportion of local labour trained on the job.

Seeding of the coir matting has used seeds collected locally to ensure sympathetic plant growth.