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Platform pulls pylon back from the cliff edge

CONSULTANT GIFFORD Geotechnical has helped save a threatened chair lift pylon sitting precariously on a retreating cliff edge.

The pylon forms part of a principal tourist attraction at Alum Bay on the Isle of Wight, carrying visitors between the Needles Park to a beach below the cliff face.

Monitoring the stability and retreat of the cliff edge began in 1994 after failures at the top of the steep slopes of Alum Chine, immediately to the north, when the Health & Safety Executive served a deferred probation notice.

Inclinometers installed deep in the Alum Chine showed that uncontrolled groundwater flows down the cliff were causing deep-seated ground movements.

Preliminary stabilisation consisted of placing coarse rock fill at the base of the rapidly eroding slope.This allowed the chair lift to operate safely, subject to monthly monitoring of inclinometers and daily inspections by park staff.

However, last year's wet winter accelerated the retreat of the cliff prompting further action.

Gifford chose to pull the support to the pylon 6m further back from the cliff edge, without actually moving it. This was done on a skewed cantilevered platform supported by two rows of CFA piles acting in compression and tension.

Site investigations showed that because of closely spaced vertical strata, ground conditions for the two rows of piles were completely different: one of the rows of piles was in sand the other in clay.

The new foundation platform, nicknamed 'the diving board' and the improvements to the cliff top, made by upgrading surface water controls, is expected to prolong the chair lift's life by up to 25 years.

The chair lift re-opened for Easter this year and improved safety means the expensive monitoring regime can now been scaled down.

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