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ANCHORS AWAY

SPAIN - Use of ultra high 1800kN working load soil anchors reduced the number required to support a deep diaphragm wall in Spain.

Albacete in east Spain is getting a new El Corte Inglés outlet as the giant superstore chain continues its expansion in the country and large Spanish-based geotechnical contractor Terratest Cimentaciones has won the contract for all of the basement work.

The 125m by 90m and 17m deep cofferdam will allow the construction of a four level basement and a multi-storey shopping centre above.

Its diaphragm wall thickness varies but ranges from 0.8m to 1.2m with the wall going down 30m.

To retain the basement walls, Terratest Cimentaciones turned to Keller Terra, which is the licensee of the single bore multiple anchor system in the country.

Client El Corte Inglés needed the extensive cofferdam to be restrained by a strut-less support system and ground anchor support systems ensure a complete open excavation allowing ease and rapid progress of subsequent building work.

The system's designer is UK-based consultant Tony Barley of company Single Bore Multiple Anchor, nicknamed 'the anchorman' for his many years of work and published papers in the field. He explains that four levels of anchors are required to retain the 17m free standing height of the wall while the upper level retention used conventional post grouted anchors with working loads of 850kN. This load was considered to be the maximum safe working limit of the conventional anchors for the site's ground conditions.

The key to choosing these anchors was that at lower levels, where earth pressure loading on the wall is much higher, the installation of three or four conventional anchors per 5m panel width would require many anchors at unusually close spacing.

The problem here is that packing them in so close together could result in anchor interaction. But using these high capacity anchors avoided such tightly packed spacing.

The anchors work on the principle that a standard tensioned anchor will only be working under normal loading in its upper end nearest the surface. This can be demonstrated by holding a rubber band tightly in a fist, leaving an end sticking out. When one end is pulled, the tension can only be felt where it emerges from the fist. The entire length will only be working at the point of failure where the whole rubber band is pulled from the hand.

By using multiple anchors with staggered bonded working lengths, each anchor is contributing to a collective working length all the way down the borehole. The bonded working length of each strand is called the unit anchor encapsulation length and this section transfers the load from the grout to the surrounding soil.

The tendon supplier is Dywidag Sistemas Constructivos, which is licensed to fabricate the special multiple and removable anchor tendons for Keller Terra.

Barley says the unique quality of the system allows the safe use of 900kN to 1800kN working load in cohesive soils, and with such loads there is only the need for two anchors per panel. This in turn allowed the number of anchors in lower rows to be only half of the number that would have been needed if only conventional anchors were available.

A number of anchors are installed within one borehole in the ground creating financial benefits as well as saving programme time. This is because only half the number of holes are drilled in the diaphragm wall and only half the number of anchor heads are required. There is the added benefit of a 35% reduction in the overall drilled length and half the number of anchor testing and loading operations.

Also, Barley claims that reducing the number of anchors by 35% from the original estimate of 1000 meant a time saving of almost two months on the installation, stressing and excavation works. Site workers founded all anchors in stiff clay with some sand or silt content.

Engineers performed all anchor testing in accordance with the European Code of Practice of Anchors EN 1537:2000 which demands a proof loading of 1.25 times the working load. The proof load of 2250kN applied to these anchors is more than twice that which can normally be sustained by conventional anchors in the same cohesive soils.

Throughout the excavation and the anchor works, engineers monitored the inward movement of the top of the diaphragm wall at a variety of locations. But Barley claims that owing to the success of the cofferdam design and anchor performance the wall movement was less than 10mm at all locations. This is less than one third of the rule of thumb consideration that wall movement may equal 0.1% of its height.

The anchors installed to temporarily support the diaphragm wall will become redundant when internal floor slabs are completed in the basement area. But the steel tendons of temporary anchors often remain in-situ.

This anchor system also demonstrates the installation, stressing and complete removal of all prestressing strands.

Removal of tendons from temporary anchors has become mandatory in some of the world's large cities. Singapore, for example, has demanded removal of several thousand anchor tendons over recent years during the cities transportation developments.

Barley says the trials in Albacete demonstrated that a system using strand and loop allows withdrawal of the entire length of strand from the complete anchor. The contractor used muck shifting plant to pull strands from the anchor borehole.

With the correct system control the pull out forces are less than 3t per strand. This allows a number of strands to be removed simultaneously from a single anchor hole.

The main phase of anchor installation for the Albacete contract completed at the end of last year and is the fourth that Keller Terra has completed in Spain using these high capacity anchors, within the last 18 months.

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