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Returning Rock to Gibraltar

Foundations : Soil nailing

A corrugated steel rainwater catchment system, which Gibraltar once relied on, is finally being decommissioned. Dan Simpson reports.

Decommissioning of a 99 year old rainwater catchment, once Gibraltar's only public water source, involves a complex slope stabilisation operation.

Edmund Nuttall's ground engineering division Ritchies is well into the £14.2M project to decommission the Ministry of Defence's east side water catchment in the colony.

Gibraltar has no springs or rivers and after its capture by Sir George Rooke in 1704, during the War of the Spanish Succession, the only public water supply was an old Spanish aqueduct and wells of brackish water. As the population increased it became necessary to incorporate underground tanks in barracks and homes to collect rainwater from the roofs.

Matters were made worse when many of the private wells were closed down in an effort to stem a cholera epidemic and the Spanish aqueduct was put out of use for public health reasons.

In 1863, a parliamentary commission on barrack and hospital improvement proposed the preparation of an area of rock as a catchment area for rainwater.

The first water catchment was built in 1903 on the sand slopes of the east side of Gibraltar, which has an average inclination of 35degrees.

Vegetation was stripped away and any cracks in the exposed rock filled with a cement sand mortar. The slopes were trimmed as evenly as possible and timber piles driven into them. Rafters were nailed into the piles and covered with corrugated galvanised iron sheets.

Two water channels running horizontally across the midpoint and base of the slope channelled the water to reservoirs excavated within the limestone rock behind the structure.

The system was extended and extra reservoirs were added periodically up to 1961. In all, 12 reservoirs were built, with a storage capacity of 72,727m 3. The catchment area was taken out of service in 1993, but the reservoirs are still used.

In 1991 a desalination plant complex at Waterport was expanded to include two reverse osmosis desalination plants, each producing 240m 3of fresh water per day, permanently solving the supply problem. The large upkeep costs of the water catchment were no longer justifiable and Defence Estates decided to decommission the 130,000m 2corrugated iron sheet structure and wooden framework and hand the land back to the Gibraltar government.

Consulting engineer for the project, Gifford & Partners, reported that the slope under the catchment structure was in a 'critical' state.

Geology of the site comprises wind-blown uncemented sand lying over variably cemented sand, limestone talus and limestone bedrock at depth.

Ritchies director and general manager Ian Dalgleish says: 'Gifford developed a stabilisation proposal, based upon the tender information, involving over 11,000 soil nails and use of seeded coir geotextiles.

'However, working with geotechnical consultant Donaldson Associates and using our own experience we reduced the number of soil nails down to 9,000, which has been further reduced as the project has progressed to about 8,200, making a considerable cost saving.'

He explains: 'The mode of failure would be a translational slide between the cemented sand and the uncemented sand.

The cemented sands therefore carry the loads transferred by the soil nails, which actually act as a hybrid between a soil nail and a rock anchor.'

Fully grouted galvanised Dywidag bars are being used as soil nails. These are 13m long and pressure grouted over a 5m fixed section within the cemented sands. The diameters of the bars vary from 20mm to 32mm, the larger diameter bars being used in areas where the thickness of the loose sand is the greatest.

More than 1,300 Mackintosh probe tests were carried out at the beginning of the project in March 2000 to determine the thickness of the blown sand.

Defence Estates recognised the logistical problems involved in the project early on. At the base of the site is a narrow road, while the top is bounded by steep limestone cliffs, which rise abruptly from the low sandy isthmus link to Spain to over 400m. A detailed methodology therefore had to be included as early as the tender stage.

Ritchies developed and built eight rigs to install soil nails on the steep slopes. The rigs are winched from the rock face, using the double rope method for safety. Traditional soil nailing rigs would require scaffold platforms to work on the steep slope but because of the size of the area this was not possible. The new rigs are configured to suit the slope, with a drilling mast mounted on the horizontal upper deck at the correct angle.

Each rig comprises a Boart Longyear power pack, drill mast and control panel mounted on a tracked unit that can ascend the 600m long slope at 3m a minute.

Four hydraulic lines attached to the top of the slope ensure failsafe operation.

The rig is operated by a crew of three and can install up to four anchors per day. The power pack and a grout batching plant is mounted on a second smaller deck below the main rig.

About 60% of the nails have now been installed. Almost a third of the corrugated iron sheets have been removed, allowing the coir geotextile to be laid down soon after.

'This maintains the stability of the slope during construction, ' says Dalgleish. 'The geotextile will biodegrade after five to 10 years to allow vegetation to become established. The coir is being seeded by a local botanical group, who will plant only native species.

'Before we started there was no objective monitoring on the slope so we installed eight down hole inclinometers which provide hourly monitoring data back to the site office. There are also surface mounted prisms, which act as an early warning system.'

Movement of the prisms is measured by an electronic distance measurement device mounted on a secure platform.

So far only very minor movements have been recorded on the slopes during construction, Dalgleish says.

To prevent surface flow destabilising the slope, a drainage channel was constructed at the crest. Ritchies is also installing catch fencing on the slopes. Designed and supplied by Austrian firm Geobrugg, the fences work dynamically by taking the energy out of the rocks as they hit them; internal breaks in the fence dissipate the energy.

The 7m high fences were positioned based on computer simulations of rock falls. They have been placed in the areas of highest risk and 15m to 30m below the interface of the rock face and the sand slope.

Ritchies is due to finish on the site in June 2002.

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